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Manufacturability of AlSi10Mg overhang structures fabricated by laser powder bed fusion
Abstract
The main advantage of laser powder bed fusion (LPBF) is its use for directly manufacturing metal components with highly complex geometries. But the LPBF manufacture of overhang structures, also known as downward-facing surfaces, is a challenge because of the possibility of incurring distortion and dross defects. This paper presents a systematic examination of the manufacturability and structural integrity of AlSi10Mg overhang structures fabricated by LPBF using computational and experimental techniques. The experimental and simulation results indicate that the use of support structures facilitates the manufacturability and structural integrity of both full-circle and half-circle overhang structures. The influence of supports on circularity was found to be more beneficial as the diameter increased above 15 mm. The experiments also suggest that the use of supports plays a significant role in maintaining mechanical performance by successful fabrication of downward-facing surfaces free of dross defects. From a design perspective, small overhang features are preferable to large overhangs, especially when support removal is impractical. This study significantly contributes to design for metal additive manufacturing by providing an improved understanding of the manufacturability of overhang structures in applications intended for lightweight structural performance.
... During multi-track printing, these depressions and tails overlap with each other to form hills and valleys in the printed region, as shown in Fig. 9a. Consequently, the surface profile in the printing region is highly associated with the stability of thermodynamics during laser melting, and any variations in the profile will induce fluctuations in the printing process [72,73]. ...
... suggesting that the powder beds obtained at those conditions would negatively affect the laser printing process [72,74]. Especially, in the printing region formed by 80% recycled powder involved ( Fig. 9a and b), a necking where the melting track disappears can be observed due to the flow interruption caused by printing process instability, resulting in a significant altitude difference on both sides [54]. ...
... However, the results in this work demonstrate that the addition of 60% recycled powder would not significantly impair the LPBF process. Facing the reuse of recycled powder, one of the key issues to realize a stable LPBF is ensuring the packing fraction and uniformity of the powder bed during spreading [14,15,54,72,[74][75][76]. Therefore, the χ r is particularly important due to its direct impact on the powder spreading and subsequently melting process [14,15]. ...
The reuse of recycled powder in laser powder bed fusion (LPBF) has significant economic and practical value. While numerous investigations have proposed the negative effects of recycled powder in LPBF, further research is still needed to develop effective schemes for powder reuse, which plays a critical role in sustainable development of additive manufacturing. In this article, the spreading and subsequent selective melting of actual Ti-6Al-4V powder mixture with different mixing proportions of recycled powder (χr, weight percentage) were reproduced by using discrete element method (DEM) and computational fluid dynamics (CFD) simulations, where the influences of recycled powder content on the stability of LPBF process were systematically investigated/evaluated and corresponding mechanisms were explored. The results showed that for current cases, χr 60% can guarantee a stable LPBF process and desired powder bed/printing region quality. However, if χr > 60%, the LPBF process becomes unstable, which can result in not only the degraded powder bed but also the rough surface and internal defects in the printed region. The underlying mechanisms can be ascribed to: (1) during powder spreading, the excessive χr would exacerbate the interaction between particles to cause the unexpected particle scattered movement during deposition, thereby degrading the powder bed quality; (2) during laser melting, the formed loose powder bed would impede the heat absorption and dissipation, resulting in unstable liquid flow in the molten pool, thus deteriorating the quality of the printing region. Therefore, it can be confirmed that maintaining the powder bed quality in LPBF is crucial for achieving recycled powder reuse, and χr = 60% could be regarded as the threshold of current research cases.
... Another factor that can in uence the porosity and morphology of L-PBF printed parts is the heat transfer from the meltpool during part manufacturing [9][10][11][12][13]. Examples of geometric features that affect heat transfer in L-PBF parts include lattice structures [14], support structures [10], and unsupported overhangs [12]. ...
... Another factor that can in uence the porosity and morphology of L-PBF printed parts is the heat transfer from the meltpool during part manufacturing [9][10][11][12][13]. Examples of geometric features that affect heat transfer in L-PBF parts include lattice structures [14], support structures [10], and unsupported overhangs [12]. ...
... away from a meltpool generated in an overhang, may result in large thermal gradients, which can, in turn, destabilize the meltpool. A larger thermal gradient may impact both the level of porosity generated and the alloy microstructure of the layers immediately above an overhang [12]. A further example of this effect is a report by Scime et al. [13], who imaged laser meltpools, while printing unsupported overhangs using an off-axis camera. ...
Geometrically complex features, such as overhang structures, can be a challenge in laser powder bed fusion (L-PBF), as they can be associated with print defects, such as porosity. In this study, Ti-6Al-4V alloy overhang structures were fabricated using an L-PBF system. Differences were observed in the microstructure in the region around the overhang structures, compared with that observed for the bulk alloy. These included larger grain sizes and a less homogenous microstructure in the print layers closest to overhang structures. It is hypothesized that these microstructural changes are associated with the excess heat generated in the overhang region due to the decreased thermal conductivity of the powder immediately below the print layers, compared with solid alloy. Also observed was an increased level of porosity (up to 0.08%) in the overhang print alloy, compared with the corresponding <0.02% in the alloy bulk. During printing, in-situ process monitoring of the meltpool emissions was obtained in the near-infrared range and correlated with the properties of the printed parts.
This in-process data assisted in selecting optimal laser processing conditions in the first fifteen layers above the overhang to prevent meltpool overheating. By systematically controlling the laser energy during the printing of L-PBF overhang structures, the level of porosity was reduced to match that of the bulk alloy. There was also an associated reduction in the roughness of the overhang itself, with its Ra decreasing from 62.4 ± 7.3 µm to 7.5 ± 1.9 µm.
... These influences establish the design rules for the respective manufacturing technology, which can limit the design freedom for AM. These design rules are particularly challenging for novice designers who lack extensive experience with the AM design process [11,12]. The design rules provide instructions on various design features such as wall thickness, ledge size, circular holes, overhang angles, and the clearance between adjacent features ( Fig. 5(a)). ...
... To reduce the amount of support required and improve the quality of the component (e.g. surface quality or process-induced thermal distortion), the design rules for the overhang angle must be carefully considered [9,11,12,[51][52][53][54][55]. Specially, removing support structures from delicate compliant structures can damage or even defect the component [56]. ...
... However, the manufacturability of the surfaces up to γ ≥ 25° has not yet been reported in the literature and has restricted the findings of this paper. Notwithstanding its limitations, a building angle of γ ≥ 30°, as confirmed in the literature [11,12], resulted in an excluded range of 0° to 90° within ...
Additive manufacturing (AM) has the potential to revolutionize the design and fabrication of compliant mechanisms (CMs). CMs are monolithic mechanisms that use compliant structures to achieve the desired functional behavior. Their single-part nature is beneficial for various applications as it does not experience friction, wear, stick-slip effects and does not require lubrication. The layer-by-layer nature of AM provides significant design freedom, allowing for the creation of CM designs with “free complexity” in terms of shape, functionality, material, and hierarchy, resulting in added-opportunities (e.g. part consolidation, functional integration and customization) for CM applications.
Most mechanisms are produced using plastic AM technologies. Despite the increasing use of AM for metallic parts, there are still limited examples of metallic AM CMs available. This lack is due to challenges in both the metallic AM and the CM domains. These challenges include: the considerations of process specific design rules for metallic AM (CI), the complexities of CM in terms of its design synthesis (CII) and in term of its structural analysis (CIII). In addition, the limited structural flexibility of CM poses a further challenge (CIV). The challenges can be particularly difficult for novice designers who lack expertise in design approaches and methodologies for designing compliant parts using metallic AM.
Therefore, the goal of this research is to enable the design for AM CMs by addressing the main challenges (CI-CIV). The research objectives are to establish a process chain that encompasses the material, design, and fabrication aspects of AM. This process chain is then consecutively advanced by incorporating design synthesis approaches and structural analysis methods, in order to exploit the AM design freedom potentials and explore added-opportunities that focus on the force-displacement behavior and motion behavior of CM case studies.
These objectives lead to the contribution of three research studies (SI-SIII): The first study (SI) establishes a process chain based on a design synthesis approach for creating of metallic, self-supported 3D building blocks (BBs) for use in CMs. The approach is evaluated and validated numerically, and demonstrated through the manufacturability of the BBs and their hierarchical aggregation into several demonstrators.
The second study (SII) introduces a design approach and a numerical computational framework that utilizes self-supported BBs to customize the stiffness of metallic AM springs. Structural optimization methods are employed to synthesize springs with linear and non-linear stiffness curves, while also ensuring the desired level of stiffness performance during the manufacturing process.
The third study (SIII) presents customizable, miniaturized continuum mechanisms using micro-AM. These functional components find their relevance in the application of minimal-invasive surgical instruments. The study investigates key considerations for selecting process parameters in the micro-AM process to achieve micro-sized design features. The continuum mechanisms are synthesized analytically using micro-BBs, and their geometry and topology are automatically user programmed through the use of programmable design tools and automated algorithmic digital workflows. This allows for the automated design synthesis of continuum mechanisms for 2D planar and 3D spatial motion. The performance of the continuum mechanisms is validated with various fabricated miniaturized samples.
In conclusion, the creation of the BB design synthesis (using a bottom-up design approach) that considers the design rules of AM, has contributed to the facilitation of self-supported CM designs. These BBs are also preferred as a foundation for the design of CMs (using a top-down design approach). By doing that, the design focus is guided towards the added-opportunities of AM, such as the customization of functionalities for springs and miniaturized continuum mechanisms. Within these functional customizations, it is concluded that structural analysis methods are selected problem-specific, taking into account the strengths and weaknesses of different methods such as analytical-based digital workflows or numerical finite element analysis optimizations. Further conclusions are drawn regarding the structural flexibility of CMs. AM materials with a high modulus of resilience, such as precipitation hardening stainless steel 17-PH (H900), offer the desired compliance. In addition, the patterned topology of the CMs enhances their structural flexibility. Both the cross-sectional geometry of the BBs and their boundary conditions influence the compliance-increasing and compliance-decreasing properties within the CMs. This thesis also provides an outlook on relevant applications of AM springs and monolithic miniaturized continuum mechanisms, highlights potential future considerations for ensuring the structural performance quality of AM CMs, and possible future trends in the design freedom of “material complexity” for AM CMs.
... This strategy results in high-energy inputs on a small spatial scale, which may implicate overheating or heat accumulation, especially for structures with flat overhang angles [1,2]. As a result, thermal distortion frequently occurs in overhang areas [3]. Consequently, the manufacturing of overhang structures using PBF-LB/M is challenging. ...
... This reinforces the overhang structure and allows heat to be dissipated from the process zone. Designing and manufacturing support structures is a distinct field of research that includes investigating how to quickly dissipate heat [3,4] and how to easily remove the supports [4,5]. In powder bed fusion of metals using an electron beam, even contact-free support structures have been demonstrated [6]. ...
... The density of the cuboids is determined using the Archimedean method according to DIN EN ISO 3369. To determine the relative density ρ rel , the solid density of Ti6Al4V is assumed to be 4.43 g/cm 3 . In addition, the density of the cuboids is determined optically based on micrographs. ...
Several studies demonstrate the potential of pulsed exposure strategies for improving spatial accuracy, surface quality, and manufacturability of low-angle overhangs in laser-based powder bed fusion of metals. In this paper, those fundamental potentials are transferred to the support-free manufacturing of heat exchanger structures with partial horizontal overhangs made of Ti6Al4V. The pulsed exposure with pulse repetition rates of 20 kHz and pulse duration of 25 µs enabled the support-free manufacturing of these complex structures with densities of more than 99%. A comparison of the Archimedean density determination with optical density determination using micrographs indicate permeability of the specimens below an applied volume energy density of 30 J/mm³ due to open porosity. Furthermore, the pulsed manufactured structures show an improved flow behavior within the heat exchanger compared to specimens manufactured with continuous exposure strategies.
... AM components often require temporary support material to avoid collapse or warping during fabrication Blakey-Milner et al. 2021;Liu et al. 2018b;Mirzendehdel and Suresh 2016;Hussein et al. 2013;Strano et al. 2012;Calignano 2014;Hu, Jin, and Wang 2015;Langelaar 2016a;Cacace, Cristiani, and Rocchi 2017;Jiang, Xu, and Stringer 2018;Leary et al. 2019;Han et al. 2018). No matter how these support materials are removed chemically or mechanically, the use of sacrificial material increases total material usage, build time, and clean-up cost. ...
... Besides, the cost of support structure removal can make up for about 8% of the total product cost (Thomas and Gilbert 2014). Furthermore, research on support structures for MAM is important because support structures play a critical role in MAM by eliminating cracks, curls, sags, or shrinkages (Jiang, Xu, and Stringer 2018;Leary et al. 2019;Han et al. 2018). For example, in PBF, a high-power laser/electron beam selectively scans over metal powder to form a solidified metal layer to form parts layer-by-layer. ...
... In Figure 9, five types of support structures are shown: vertical strut-type, honeycomb, porous-type, contact-free, and topology-optimised supports. These temporary supports ensure a component does not collapse or warp during fabrication (Blakey-Milner et al. 2021;Liu et al. 2018b;Jiang et al. 2018;Leary et al. 2019;Han et al. 2018;Vouga et al. 2012). ...
Metal additive manufacturing is gaining immense research attention. Some of these research efforts are associated with physics, statistical, or artificial intelligence-driven process modelling and optimisation, structure–property characterisation, structural design optimisation, or equipment enhancements for cost reduction and faster throughputs. In this review, the focus is drawn on the utilisation of topology optimisation for structural design in metal additive manufacturing. First, the symbiotic relationship between topology optimisation and metal additive manufacturing in aerospace, medical, automotive, and other industries is investigated. Second, support structure design by topology optimisation for thermal-based powder-bed processes is discussed. Third, the introduction of capabilities to limit manufacturing constraints and generate porous features in topology optimisation is examined. Fourth, emerging efforts to adopt artificial intelligence models are examined. Finally, some open-source and commercial software with capabilities for topology optimisation and metal additive manufacturing are explored. This study considers the challenges faced while providing perceptions on future research directions.
... Figure 1 illustrates the overhang angle and support of a part fabricated via additive manufacturing. A small overhang angle may disrupt the part geometry owing to the gravitational load over the structure or the over-melting of regions during fabrication [6]. Specifically, down skin surface with stiff overhang angle is fused without a solid base in the fabrication process. ...
... Exceeding the critical overhang angle does not necessarily induce collapse. As demonstrated experimentally by Vora et al. [18], the fabrication of an overhanging part using a powder bed fusion method, i.e., anchorless selective laser melting (ASLM), yielded a partially overhanging part, although the fabrication was unsuccessful. of regions during fabrication [6]. Specifically, down skin surface with stiff overhang angle is fused without a solid base in the fabrication process. ...
... Figure 11 illustrates the second step, where values are received from two adjacent facets, is defined as the further propagation, and indexes of the facets and edges are numbered to aid discussion. The assigned value is determined by two values obtained using Equation (6). However, the value transferred across the edge 3 of facet 3 is defined as zero because the adjacent facet sharing the third edge is undefined, although the values across the edge 1 and edge 2 of facet 3 are defined. ...
Recent advances in additive manufacturing have provided more freedom in the design of metal parts; hence, the prototyping of fluid machines featuring extremely complex geometries has been investigated extensively. The fabrication of fluid machines via additive manufacturing requires significant attention to part stability; however, studies that predict regions with a high risk of collapse are few. Therefore, a novel algorithm that can detect collapse regions precisely is proposed herein. The algorithm reflects the support span over the faceted surface via propagation and invalidates overestimated collapse regions based on the overhang angle. A heat exchanger model with an extremely complex internal space is adopted to validate the algorithm. Three samples from the model are extracted and their prototypes are fabricated via laser powder bed fusion. The results yielded by the fabricated samples and algorithm with respect to the sample domain are compared. Regions of visible collapse identified on the surface of the fabricated samples are predicted precisely by the algorithm. Thus, the supporting span reflected by the algorithm provides an extremely precise prediction of collapse.
... Consequently, these partially melted powders accumulated on a slanted downward surface. The disparity between the upper and downward surfaces of SLMed parts has also been observed in previous studies [26,27]. ...
... The variation tendency of strut thickness is consistent with the variations observed in surface morphologies. This phenomenon is an inevitable consequence of the SLM characteristic [26] but can be diminished through post-processing to achieve a homogeneous surface with appropriate strut thickness. Figure 7a illustrates the average surface roughness at different parameters. ...
Selective laser melting (SLM) has gained great attention to manufacture cardiovascular stents given its potential of fabricating customized stents with complex shapes to satisfy clinical requirements. In this study, the surface characteristics of NiTi cardiovascular stents by SLM were explored. The effect of SLM machining parameters on surface morphology, geometry accuracy, phase composition, surface roughness, and contact angle were analyzed. The results demonstrated that the surface morphology of stent became more irregular, and the surface roughness was enhanced accompanied by the volume energy density (VED) increased. SLMed stents exhibited hydrophobic properties, and the rougher surface obtained a lower contact angle. The deviation of strut thickness was more than 200% than the nominal value under 194 J/mm3. The lowest VED displayed a strong cubic B2 structure with less content loss of Ni, satisfying the self-expand NiTi stent requirements. Then electrochemical polishing (ECP) process with green electrolytes distinctly improved the surface quality, providing smoother surfaces. The surface roughness reduced minimum to 0.45 μm from 6.64 μm for SLMed stent, and the average strut thickness was reduced to 230 μm at most. Finally, electrochemical test results revealed that SLM-ECPed stents showed a more obvious tendency to resist corrosion compared to SLMed stents.
... Among the different support structure geometries already explored in the literature, cylindrical supports with a truncated conical final part were chosen as starting point of the present optimisation study (Fig. 1). Han et al. previously exploited this support shape, demonstrating an excellent ability in anchoring overhang structures and an easy-to-model heat transmission [17]. Considering that the present study aims to develop an empirical method to optimise the support structure geometry to reduce residual stress and allow a proper sampleplatform adhesion, support height, diameters and their spacing were optimised using the cantilever approach. ...
... For the present study, the height of the support structures was set at 4 mm, and the optimisation of the two indices was achieved by varying the two characteristic diameters (di and ds) and the spacing between the pins (l). As starting support geometry, a compromise between the "fine geometry" and the "coarse geometry" of Han et al. work was selected [17]. The geometrical parameters related to the first attempt and the obtained results are reported in Fig. 3. ...
The Powder Bed Fusion-Laser Beam is a promising additive-manufacturing process that allows the production of complex-shaped functional components for many applications. However, the layer-by-layer scanning and high cooling rates result in a high thermal gradient (ΔT) and, thus, in thermally induced stresses that could lead to undesirable cracking and delamination phenomena in the final component. A strategy to reduce the ΔT and facilitate a correct heat flow is using support structures. However, the support geometry needs to be optimised, considering that the thermal resistance increases as the support-height increases and the contact cross-section decreases. Furthermore, it is essential to consider the anchoring function of the support structures. Based on these considerations, two geometric indices and a decision support matrix were developed in the present work for a quick and efficient setting of geometric parameters. The robustness of the developed approach was verified on two different alloys: AlSi10Mg and IN625.
... Because AM is reliant on support structures, any overhanging or downskin areas are prone to distortion [37,38]. To prevent this, all holes were filled to a 1.27 mm diameter to become pilot holes for later machining. ...
... The first step in designing the supports is determining the orientation of the part with respect to the build plate. Usually, this decision is based on minimizing part height to reduce build time and reducing overhanging areas to decrease the need of supports that can cause poor surface finishes and increased post-processing [37,38]. Supports for both the crankcase and cylinder head were designed within 3DXpert. ...
This effort investigates the use of metal additive manufacturing, specifically laser powder bed fusion (LPBF) for the automotive and defense industries by demonstrating its feasibility to produce working internal combustion (IC) engine components. Through reverse engineering, model modifications, parameter selection, build layout optimization, and support structure design, the production of a titanium crankcase and aluminum cylinder head for a small IC engine was made possible. Computed tomography (CT) scans were subsequently used to quantify whether defects such as cracks, geometric deviations, and porosity were present or critical. Once viability of the parts was established, machining and other post-possessing were completed to create functional parts. Final X-ray CT and micro-CT results showed all critical features fell within ±0.127 mm of the original equipment manufacturer (OEM) parts. This allowed reassembly of the engine without any issues hindering later successful operation. Furthermore, the LPBF parts had significantly reduced porosity percentages, potentially making them more robust than their cast counterparts.
... Fig. 5 schematizes stress (σ)-strain (ε) curves for cellular structures under compression [26][27][28]. The Gibson-Ashby equations [29] are as follows: ...
... At the final stage, the cellular samples are gradually compacted until the solid structure is fully in contact, with stress increased exponentially to failure [32]. The three stages of cellular structures under compression are consistent with those described by Gibson and Ashby [29]. ...
... Component-scale studies have also been performed to examine the influence of scanning strategy, geometry, build orientation, and heat buildup on defect populations [44,47,71,104,151,200]. These studies are tedious and expensive to perform; thus, conjugate computational models have been pursued to mirror experimentation [91,142,143,205]. ...
... In the overhanging region at the end of the link, heat accumulates as the path to the build plate is longer and shared by other points where heat is deposited on the top surface. The increased temperature and decreased cooling rates observed in overhanging regions are consistent with PBF fabrication experiments[71,104].Another important result of this study is that the rate of minimum temperature increase approaches zero after only a few thermal cycles. This allows for accurate predictions of residual heat accumulation at the beginning of each layer, with only the previous few layers requiring simulation. ...
... Yang Xiongwen et al. [18] has used 316L stainless steel powder resulted in the smooth formation of overhanging round holes with diameters greater than or equal to 0.5 mm, and the larger the diameter of the hole, the worse the shape accuracy. Quanquan Han et al. [19] studied the deformation, roughness and roundness of circular holes with and without internal support, and the results showed that the shape deviation of circular holes without internal support was greater and the inner surface was rougher than those with internal support, and the difference was more obvious when the diameter of circular holes was greater than 15 mm. ...
Selective laser melting has the potential to be applied into hydraulic pipelines manufacturing because it can realize the forming of flow channels with arbitrary direction and curvature. Due to the stacking of layers, selective laser melting still has many limitations while processing complex flow channels. In particular, the manufacturing of overhanging structures with circular cross sections needs to use internal supports to prevent surface collapse, which is challenging to be removed. Therefore, it is necessary to optimize the flow channel with a self-supporting ability, then systematically discussing its forming quality with the influence on fluid dynamics to compromise. In this paper, a simplified multi-channel structure with 1 inlet and 4 outlets is extracted from a hydraulic valve block of an aero-engine system, and the cross-section of its branch channel is re-designed to guarantee its self-supporting ability based on additive manufacturing optimization strategy. Numerical simulation was used to analyze the influence of different shape sections on the pressure loss and mass flow rate of multi-channel structure. The results show that the pressure loss and outlet flow of the 45° rhombus + ellipse section are the closest to the circular area. According to the maximum internal deformation of the three outlets, the 65° rhombus section has the better forming quality and the non-circular section is not the worst.
... The length of the overhanging surface in Figure 6F was significantly shorter than that in Figure 6G, and the inclination angle of the overhanging surface in Figure 6F was about 30°, which was obviously higher than the 0° inclination angle of the horizontal overhanging surface in Figure 6G. This not only contributed to a smaller volume of dross in Figure 6F than in Figure 6G, but also may lead to a smaller molten pool size of overhanging surface in Figure 6F compared with Figure 6G [52] . In Figure 6F, the molten pool was mainly supported by the underlying solidified layers, which possessed a higher thermal conductivity compared to the loose powder. ...
The heat dissipation structure used in modern airborne radar chassis not only requires lightweight, but also pursues better mechanical properties and heat dissipation performance. In this study, a stochastically porous pomelo peel-inspired gradient structure was fabricated by laser powder bed fusion using Al-Mg-Sc-Zr powder. This study focused on the formability, microstructure, mechanical properties, and heat dissipation performance of the biomimetic structure through experimental and finite element analysis approaches. The influence of volume fraction (VF) on structural mechanical properties, deformation modes, stress distribution, and heat dissipation performance was investigated. The results showed that the mechanical properties of the structure declined as the VFs decreased. The optimal mechanical performance was obtained at the VF of 45%, where the compressive strength, specific energy absorption (Ws), and specific compressive strength values were measured to be 63.47 MPa, 34.84 J/g, and 142.16 MPa/(g·cm-3), respectively. Moreover, the Ws of the structures was higher than that of the reported aluminum alloy structures at the same VF. The biomimetic structure exhibited improved heat dissipation performance as the VFs decreased, with Reynolds number ranging from 2700 to 13,400. The structure of 30% VF with a remarkable heat transfer efficiency index of 1.86 displayed the best heat dissipation performance. In addition, compared with the traditional fin structures, the bionic structure possessed better thermal resistance, heat transfer efficiency index, and temperature uniformity at the same VF. This study demonstrated notable potential of pomelo peel-inspired design for lightweight load-bearing applications capable of heat-dissipating performance, providing a novel perspective for design and fabrication of versatile structures in the aviation field.
... The geometry of the inverted frustum contributes to the severe heat up, as overhanging structures are prone to accumulation of heat. [31][32][33] In addition, the necking of the geometry disturbs the heat flux in the z-direction. The contribution of the surrounding powder to heat dissipation via heat conduction is rather limited, as the heat conductivity of the unmolten powder is much smaller than the heat conductivity of the bulk material. ...
The capability to produce complexly and individually shaped metallic parts is one of the main advantages of the laser powder bed fusion process. Development of material and machine specific process parameters is commonly based on the results acquired from small cubic test coupons of ∼10 mm edge length. Such cubes are usually used to conduct the optimization of process parameters to produce dense materials. The parameters are then taken as the basis for the manufacturing of real part geometries. However, complex geometries go along with complex thermal histories during the manufacturing process, which can significantly differ from thermal conditions prevalent during the production of simply shaped test coupons. This may lead to unexpected and unpredicted local inhomogeneities of the microstructure and defect distribution in the final part, and it is a root cause of reservations against the use of additive manufacturing for the production of safety relevant parts. In this study, the influence of changing thermal conditions on the resulting melt pool depth of 316L stainless steel specimens is demonstrated. A variation in thermographically measured intrinsic preheating temperatures was triggered by the alteration of interlayer times and a variation in cross-sectional areas of specimens for three distinct sets of process parameters. Correlations between the preheating temperature, the melt pool depth, and occurring defects were analyzed. The limited expressiveness of the results of small density cubes is revealed throughout the systematic investigation. Finally, a clear recommendation to consider thermal conditions in future process parameter optimizations is given.
... A fully circular (conical and cylindrical) configuration was selected as the basic geometry of the support structures. This geometric configuration has already been explored by Han et al. [33], revealing an excellent ability to anchor overhang structures and an easy-to-model heat transmission. The support heights and diameters were set according to the authors' previous optimization work performed on AlSi10Mg samples. ...
Over the past few years, several studies have been conducted on the development of Al-Si-Cu-Mg alloys for PBF-LB/M processing. The attention gained by these systems can be attributed to their light weight and strength provided by a solid solution in the as-built state and by precipitation after heat treatment. However, published studies have kept the copper content below its solubility limit in the Al-Cu binary system under equilibrium conditions (5.65 wt%). The present study aims to explore Al-Si-Cu-Mg systems with high copper content, starting with the well-known AlSi10Cu4Mg system, moving towards AlSi10Cu8Mg, and arriving at AlCu20Si10Mg, a system never before processed with PBF-LB/M. Through the SST approach, the production of bulk samples, advanced microstructural characterization by SEM and FESEM analysis, phase identification by XRD analysis, and preliminary investigation of the mechanical properties through Vickers micro indentations, the effects of copper quantities on the processability, microstructural properties, and mechanical behavior of these compositions were investigated. The obtained results demonstrated the benefits of the supersaturated solid solution and the fine precipitation resulting from the addition of high Cu contents. In particular, the AlCu20Si10Mg system showed a very distinctive microstructure and unprecedented microhardness values.
... Generally, features above 45 • from the base are considered self-supported, while those below 45 • are either supported or undergo part consolidation [12]. The support-free fabrication of the overhanging feature below 45 • is documented to suffer from massive distortion, high surface roughness, and dross formation on the downskin [12][13][14][15]. Therefore, external supports are typically used to overcome these challenges by anchoring the overhanging feature with the substrate plate or build section. ...
An internal support-free Inconel 625 (IN625) closed impeller of 10.25 cm, and 20.5 cm diameter at 30 μm and 60 μm layer thickness is fabricated successfully. The document identifies the zero-degree lowest angle feature as a critical section to base the downskin and bulk study discussion. A systematic investigation of the microstructure and mechanical properties of both the bulk and downskin sections is conducted. Impeller fabricated at 60 μm layer thickness reported lower residual stress (RS) (183 ± 10 MPa) compared to 30 μm impeller RS (227 ± 75 MPa). Scanning electron microscope (SEM) and electron backscattered diffraction (EBSD) confirm hierarchical microstructures consisting of cellular sub-grain structures inside columnar grains spread across the bulk region of the impeller. The impeller shows excellent tensile properties with a maximum yield stress of 790 MPa, a maximum ultimate tensile strength of 1040 MPa, and a maximum ductility of 45%. The downskin region near the critical section experiences high heat accumulation due to the poor thermal conductivity of the powder. As a result, the PDAS of the critical section is 6.67 times high than the bulk region and has a maximum drop of 20.7% in microhardness. The critical section of the 20.5 cm diameter Impeller at 30 μm and 60 μm layer thickness shows a maximum downskin thickness affected due to heat accumulation are 480 μm and 310 μm.
... In addition, a building platform preheating is often performed to slow down the heat flow and thus reduce the thermal-induced stresses [25][26][27]. Another successful alternative strategy to reduce the heat flow is using support structures between the cold building platform and the component to be built [14,[28][29][30][31]. ...
Inconel 625 (IN625) superalloys can be easily fabricated by the laser-based powder bed fusion (PBF-LB/M) process, allowing the production of components with a high level of design freedom. However, one of the main drawbacks of the PBF-LB/M process is the control over thermally induced stresses and their mitigation. A standard approach to prevent distortion caused by residual stress is performing a stress-relieving (SR) heat treatment before cutting the parts from the building platform. Differently from the cast or wrought alloy, in additively manufactured IN625, the standard SR at 870 °C provokes the early formation of the undesirable δ phase. Therefore, this unsuitable precipitation observed in the PBF-LB/M material drives the attention to develop a tailored SR treatment to minimise the presence of undesirable phases. This work investigates SR at lower temperatures by simultaneously considering their effects on residual stress mitigation, microstructural evolution, and mechanical properties. A multiscale approach with cantilever and X-ray technologies was used to investigate how the residual stress level is affected by SR temperature. Moreover, microstructural analyses and phase identifications were performed by SEM, XRD, EBSD, and DSC analyses. Finally, mechanical investigations through microhardness and tensile tests were performed as well. The results revealed that for the additively manufactured IN625 parts, an alternative SR treatment able to mitigate the residual stresses without a massive formation of δ phase could be performed in a temperature range between 750 and 800 °C.
... overhang and print orientation), and minimised the required support structure. This reduction decreased the post-processing effort (Han et al. 2018;Leutenecker-Twelsiek, Klahn, and Meboldt 2016). Post-processing of metallic AM is considered a design challenge as it is both cost-and timeconsuming (Feng et al. 2020;Calignano 2014). ...
Additive manufacturing (AM) facilitates the fabrication of compliant mechanisms through its free-form and design customisation capabilities. Specifically, the properties of kinetic mechanisms such as springs can be extended with regards to their inherent (non-)linear stiffness functions. This allows for the customisation of AM springs according to user preferences. By combining the design synthesis approach of building blocks with the structural optimisation approach for AM, it is possible to define and customise spring stiffness functionalities. The optimisation process employs an automated computational framework based on a genetic algorithm scheme, which has been demonstrated through randomised and reference case studies. This framework enables the attainment of linear, progressive (stiffening), and degressive (softening) stiffness curves. The manufacturability of the springs has been validated through laser powder bed fusion using stainless-steel material 17–4 PH (H900). The springs have resulted in an accuracy error of maximum 6.48% and precision error of maximum 5% through compression testing.
... Quanquan Han et al. investigated the manufacturing limit of the fully circular AlSi10Mg overhang structure, which was 15 mm in diameter without added support. When a longer diameter was formed, the overhang surface produced significant dross formation defects [21]. K.Q. ...
Warping and dross formation are the main defects of an overhang structure formed by laser powder bed fusion. In order to study these defects, a seven−shaped overhang structure with different lengths and heights of the overhang was printed. The influence of the temperature and stress field on the overhang structure was investigated using a 3D finite element (FE) model. The results of the simulation showed that the molten pool in the powder support zone was much larger than the molten pool in the solid support zone. The molten pool sank due to the actions of gravity and the capillary force. This led to the powder melting, which then formed a droplet−like dross formation on the lower surface. The temperature difference between the regions led to a large residual stress. When the residual stress exceeded the material strength, warping deformation occurred in the top area, affecting the subsequent powder−laying process. The warping zone was remelted when the next layer was processed. As the number of forming layers increased, the thermal conductivity and stiffness increased continuously, and the deformation of the top area gradually decreased. The experiment results showed that the longer the overhanging length was, the more serious the warpage was. When the overhanging length was below 3 mm, the warping of the top area continued to decrease to zero as the building process proceeded. Meanwhile, the dross formation appeared at the bottom of the overhanging area in all experimental groups. Studying the process of warping and dross formation was helpful to understand the defect change process in the manufacturing process of an overhang structure.
... The utilization of support structures is the solution for manufacturing of overhang features. Previous studies showed that unsupported overhang features have poorer dimensional accuracy, mechanical properties, and surface integrity [3][4][5]. According to a rule of thumb, features that make angles less than 45° with respect to the build direction (i.e., the Z direction) are self-supported; otherwise, they should be produced using support structures [6]. ...
The use of support structures is an essential requirement for powder-bed fusion additive manufacturing (AM) processes. Supports are responsible for fixing the component on the build plate, carrying the weight of the structure, providing heat dissipation from the component to the build plate and preventing distortion during the process. Support efficiency and performance can be evaluated through the ease of removability, strength, thermal management, cost-effectiveness, and material consumption. As the support structures are the waste material during manufacturing of metal AM components, their design has a significant impact on the productivity and cost of the manufacturing process. Due to lack of concentrated information on the effect of each mentioned support function, this paper aims to gather studies and innovations in support design and production, specifically for the powder-bed fusion methods. At first, the effect of support type and contributing geometrical parameters on the overall performance of support structures is discussed. Then, an in-detail approach is taken to categorize each key characteristics of metallic support structures and reinforce the discussion with related published papers. Finally, the role of topology optimization (TO) in designing optimum support geometry is presented. The overall conclusion is that unless there are several studies on design and manufacturing of support structures, achieving the best setup has not been guaranteed by the existing tools. The research trend is toward developing more cost-effective optimization methods based on genetic algorithms (GA) and multi-objective functions to generate automated and high-performance supports, especially for complex geometries. Furthermore, integrating AM constraints with GA and TO can be achieved through defining self-supporting index or coupling with multi-objective optimization methods, which leads to a more efficient solution.
... They conclude that a concave shape of the overhanging surface is more suitable for smaller angles, while large angles benefit from a convex shape. Circular shapes of the overhanging surface were part of the investigations by Han et al. (2018). They examined the printability of full and half circles with diameters between 5 mm and 30 mm at a 5 mm wall thickness. ...
Additive manufacturing (AM) is known for its high level of design freedom. Especially for the laser-based powder bed fusion of metals, support structures are often still required for printability reasons or increased process reliability. Currently, there is a lack of knowledge about support structure design, leading to the risk of build failures or increased material consumption.
The primary aim of this study was the development of an approach for the design of tree-like support structures to achieve the best mechanical behavior of an AM part. For this, experimental studies on the printability, geometrical accuracy and strength of thin struts were performed and an AM process simulation model was set up for four generic overhang geometries. This model was then used for parameter optimization to determine support structure design parameters, resulting in Pareto optimal support designs regarding the support volume and the part’s deformation. Measurements of printed samples were used to verify simulation results and the suitability of tree-like support. In addition, two case studies on the additive manufacturing of discontinued spare parts were used for an initial validation of the approach.
This research was the first to consider thermal and structural loads in a parameter optimization for the design of support structures with the objective of an increased mechanical behavior of the part. The potential of the presented approach to achieve a successful print on the first attempt was shown by a case study. Further, the presented approach forms the basis for fully automated support generation tools, which require little to no user input.
... Using aluminum and titanium alloys, they provided some printability limits for the investigated features [9]. Another piece of experimental evidence was given by [10], trying to assess the printability issues of parts having unsupported holes and base supports, the latter having also different sizes. On the other hand, according to point (ii), many strategies have been investigated and studied, giving, in any case, very useful suggestions and guidelines for designers. ...
In the context of the Design for Additive Manufacturing (DfAM), the elimination and/or reduction of support structures for the parts is a key issue for process optimization in terms of sustainability and surface quality. In this work, the assessment of the surface quality of overhanging thin walls and unsupported holes with different diameters (4, 6, 8 mm) was carried out through confocal microscopy, SEM-EDS analysis and CMM measurements. To this aim, two different types of AlSi10Mg alloy parts were produced with the L-PBF technology, having self-supporting features such as thin walls and holes with different overhang angles. The results showed that (i) unsupported, down-facing surfaces can be printed consecutively without supports up to a 30° overhang angle and with a surface roughness (Sa) ranging from 3 to 40 µm; (ii) unsupported holes can be produced as well, having a mean circularity tolerance ranging from 0.03 to 0.55 mm, regardless of the diameter value; (iii) density and microstructure analysis both revealed that the parts’ integrity was not affected by the design choices.
... This leads to a larger melt pool and to a correspondingly slower cooling rate. This hypothesis is supported by observations of an increase in melt pool when printing over deep powder as compared to bulk material [23] . These observations do not agree with our studies, especially as illustrated in Figs. 3 and 5 , indicating that a different mechanism is at play in the results reported here. ...
Metal lattices are an important class of cellular materials that offer great advantages by providing high-strength and lightweight structures as compared to bulk materials. Progress in additive manufacturing techniques has led to increased complexity in design and shape of produced objects and is greatly beneficial for the development of metallic lattice structures. However additive manufacturing of lattices suffers from unpredictable defect creation that can compromise mechanical integrity of lattice structure. Although post-build inspection techniques can provide quality assurance of the process, accurate assessment can be technically challenging, time consuming and costly. In this work, we investigate the use of high-speed measurements of thermal emission from the melt pool to identify defective individual struts formed with a missing bottom half in an otherwise fully built lattice structure produced with laser powder bed fusion. Surprisingly, results indicate lower photodiode signal, suggesting colder melt pool surface temperature, when printing struts with missing bottom half as compared to nominal struts. Additional thermographic imaging and multi-physics simulations reveal that the low photodiode signal is accompanied by presence of hot spatters carrying heat away from detection and continuous avalanche of powder on the melt pool. Based on these observations, a method was developed to identify defective individual struts with missing bottom half in full built lattices. This prediction approach provides valuable insights about part quality which are important for process qualification and illustrates the utility of melt pool thermal emission monitoring for identifying specific defects introduced by laser powder bed fusion.
... Among the different geometries examined in the literature, a fully circular (conical and cylindrical) configuration was the most suitable for the present application. Han et al. previously exploited this support shape, demonstrating an excellent ability in anchoring overhang structures and an easy-to-model heat transmission [34]. The optimisation of support heights and diameters was carried out using cantilever parts. ...
The Powder Bed Fusion-Laser Beam\Metals (PBF-LB\M) is a promising additive manufacturing process that can be used to directly produce functional components with a complex shape for a wide variety of applications. However, the layer-by-layer scanning and high cooling rates result in a high thermal gradient and thus, in thermally induced stresses. The stresses developed during the additive process could lead to undesirable cracking and delamination phenomena that can seriously affect the performance of the final component. The alloy composition can exacerbate crack and delamination formation, however, the need to expand the portfolio of high-strength materials processable for PBF-LB\M makes the resolution of these undesirable phenomena a primary challenge in the additive manufacturing field. This works aims to systematically investigate some strategies to make processable non-standard compositions. As no standard compositions, the promising pre-alloyed AlSi10Cu8Mg composition was chosen for the present work. Based on the results obtained from a condition of severe delamination, the synergetic use of appropriate process parameters and support structures can lead to crack-free and fully dense specimens also when platform heating is not allowed. The developed approach could also be applied to adapt other cracking-sensitive alloys for PBF-LB\M production.
... When considering internal channels, the use of support structures may be required for LPBF depending on the angle of the overhanging structure [8,12]. Overhangs were studied by Wang et al., among others [13][14][15][16]. A larger overhang length leads to increased sagging and curling. ...
High pressure die casting (HPDC) tools undergo several repairs during their life cycle. Traditional repair methods (e.g., welding) cannot always be applied on damaged tools, necessitating complete replacement. Usually, direct energy deposition (DED) is considered and applied to repair tools. In this study, the potential of laser powder bed fusion (LPBF) for HPDC tool repair is investigated. LPBF of the hot work tool steel 1.2343/H11 normally requires preheating temperatures above 200 °C to overcome cracking. Therefore, a process window for the crack-susceptible hot work tool steel 1.2343/H11 with no preheating was developed to avoid preheating an entire preform. Laser power, hatch distance, and scan speed are varied to maximize relative density. Since the correlation of LPBF process parameters and resulting build quality is not fully understood yet, the relationship between process parameters and surface roughness is statistically determined. The identification of suitable process parameters with no preheating allowed crack-free processing of 1.2343/H11 tool steel via LPBF in this study. The LPBF repair of a volume of ~2000 cm3 was successfully carried out and microstructurally and mechanically characterized. A special focus lays on the interface between the worn HPDC tool and additive reconstruction, since it must withstand the mechanical and thermal loads during the HPDC process.
... Cheng proposed a new support method which can effectively reduce the post-processing workload caused by adding support by means of simulation [16]. Quanquan Han and his team investigated that the molding limit of the fully circular overhang structure is 15 mm in diameter without adding support, and when a longer diameter is formed, the overhang surface will produce signi cant dross formation defects [17]. K.Q.Le studied the effect of laser energy density on the process of overhang structure by using a raytracing model. ...
Warping and dross formation are the main defects of overhang structure formed by Laser Powder Bed Fusion. In order to study the process of warping and dross formation, the “7” shape overhang structure with different lengths and heights of overhang was printed. The influence of temperature field and stress field on the forming quality of overhang structure was analysed by numerical simulation. The results of experiment and simulation showed that there were significant differences in the forming process of temperature field between solid support zone and powder support zone. Due to the poor thermal conductivity of powder, the molten pool in the powder support zone was much larger than that in the solid support zone. On one hand, the molten pool sank due to the action of gravity and capillary force, which lead to the melting of the powder outside its original shape and contour, formed a droplet like dross formation on the lower surface. On the other hand, the temperature difference between regions led to large thermal stress. When the thermal stress exceeded the material strength, warping deformation occurred on the top area and affected the subsequent powder laying process. The powder could not be spread on the warping zone so it was remelted when the next layer is processed. As the number of forming layers increased, the original powder area became solid after fusion and solidification, so the thermal conductivity and stiffness increased continuously, the variation of temperature tended to be stable and the deformation of the top area gradually decreases. The study of warping and dross formation process was helpful to understand the defect change process in overhang manufacturing process.
... This research identifies support structures as one of the most important factors influencing DMLS design and fabrication processes [46][47][48]. The support geometries added to the design by the production software helps to secure the part to the production platform by limiting the mechanical and thermal effects that may occur during the manufacturing process. ...
Unmanned aerial vehicles are expected to complete their missions even in adverse weather conditions. Overheating in the engine area may occur, particularly at high speeds due to using the full engine throttle. Another design factor that affects endurance duration and maximum takeoff weight is the weight of the unmanned aerial vehicle heat exchanger. In this paper, a new type of air-cooled fuel cooler heat exchanger is proposed to reduce the total weight of the heat exchanger by increasing cooling performance through the design flexibility offered by additive manufacturing. The printed circuit heat exchanger has a circular cross-section to prevent dead bends in the heat transfer surfaces and feed coolant air directly onto the fins and circulation pipes. Twisted fins and piping structure were adjusted so they could be produced without any support, generate turbulence, and reduce total weight. The fluid flow and heat transfer characteristics of novel and conventional heat exchangers were simulated in 3-D. The flow physics of both exchangers were studied to determine how the new design enhances heat transfer. Numerical results showed that the new heat exchanger has 55.6% more cooling capacity than the conventional one. The heat exchanger weight was decreased from 774g to 263g.
... In the last decade, the influence of processing parameters on distortion in the L-PBF process has been investigated by numerous researchers through experimental study and numerical modeling [17][18][19][20][21]. To numerically study the formation of residual stress and distortion in the L-PBF process, numerical models based on the finite-element method (FEM), including the thermomechanical model and the inherent strain method (ISM), are effective approaches. ...
The laser powder bed fusion (L-PBF) process is a powder-based additive manufacturing process that can manufacture complex metallic components. However, when the metallic components are fabricated with the L-PBF process, they frequently encounter the residual stress and distortion that occurs due to the cyclic of rapid heating and cooling. The distortion detrimentally impacts the dimensional and geometrical accuracy of final built parts in the L-PBF process. The purpose of this research was to explore and predict the distortion of Ti-6Al-4V components manufactured using the L-PBF process by using numerical modeling in Simufact Additive 2020 FP1 software. Firstly, the numerical model validation was conducted with the twin-cantilever beam part. Later, studies were carried out to examine the effect of component sizes and support-structure designs on the distortion of tibial component produced by the L-PBF process. The results of this research revealed a good agreement between the numerical model and experiment data. In addition, the platform was extended to predict the distortion in the tibial component. Large distortion arose near the interface between the tibial tray and support structure due to the different stiffness between the solid bulk and support structure. The distortion of the tibial component increased with increasing component size according to the surface area of the tibial tray, and with increasing thickness of the tibial tray. Furthermore, the support-structure design plays an important role in distortion reduction in the L-PBF process. For example, the maximum distortion of the tibial component was minimized up to 44% when a block support-structure design with a height of 2.5 mm was used instead of the lattice-based support. The present study provides useful information to help the medical sector to manufacture effective medical components and reduce the chance of part failure from cracking in the L-PBF process.
... This can cause high thermal gradients resulting in differing material expansion behavior in areas of raised or reduced temperatures during the process of fast melting and solidification. This causes thermal stress and distortion [5][6][7][8]. These thermal behaviors were, for example, studied in simulation and experiment of forming T-shape overhangs [9]. ...
Laser powder bed fusion (LPBF) is a promising technique used to manufacture complex geometries in a layer-wised manner. Radiation during the LPBF process is influenced by the part geometry, e.g., the overhang angle and the wall thickness. Locally varying radiation can cause deformation of the product after manufacturing. Thus, the prediction of the geometry-caused radiation before the manufacturing can support the evaluation of the design printability to achieve first-time-right printing. In this paper, we present a framework to predict the geometry-based radiation information using a deep learning (DL) algorithm based on the part geometry from computer-aided design (CAD). The algorithm was trained using data from an LPBF-print job consisting of parts with varying overhang angles. Image data, which include the information of radiation, were captured with an optical tomography (OT) camera system that was installed on a LPBF machine used in a laboratory environment. For the DL algorithm, a U-Net based network with mean absolute error (MAE) loss was applied. The training input was binarized OT data representing the contour of the designed geometry. Complementary, the OT data were used as ground truth for the model training. For the application, the design contours of multiple layers were extracted from the CAD file. The result shows the applicability to predict the OT-like radiation by its contour, which has the possibility to show the anomaly due to the part geometry.
... Therefore, specific designs are used as a benchmark tool to find the best process parameters such as critical overhang angle, process settings (Laser Power, Hatch distance, Layer Thickness), border thickness, etc. Knowledge of this angle is essential to position the supports correctly. Indeed, as we know, the critical overhang angle for Al alloys is 30 0, so we can conclude that the benchmark part is prone to failure at some point due to the absence of supports [200]. ...
In recent years, metal additive manufacturing has undergone a huge paradigm shift from prototyping to mass production due to its ability to produce complex parts. In addition, metal additive manufacturing offers additional degrees of freedom in terms of flexibility and design functionality. Today, machine suppliers are looking to improve the performance of commercial machines by instrumenting them to perform real-time in-situ measurements. This monitoring will improve the quality, reliability and repeatability of parts. The challenge is to be able to process the data captured in-situ and correlate it with the different stages of the process.This thesis presents a methodology to detect and identify anomalies during the manufacturing of a part by the laser-powder bed fusion (L-PBF) process using commercial in situ instrumentation. First, an in-depth study based on the microstructural and mechanical aspects of the AlSi7Mg0.6 alloy is performed, and the most suitable scanning strategy is identified. This study was used as a basis to perform correlations between laser/material interactions and the process outcome (melt pool signals).Different types of in-situ instrumentation such as infrared cameras and commercial photodiode-based systems are studied to better understand the laser-powder interaction. EOSTATE coaxial optical tomography based on a camera provided by EOS GmbH is exploited, and a case study based on data analysis techniques is proposed to identify potential drift layers. Similarly, a machine learning-based methodology is developed to extract critical features at global and local scales for SLM Solutions GmbH's in situ photodiode-based melt monitoring module. As we know, the laser-powder interaction depends not only on the process parameters but also on the spread of the powder bed. Critical characteristics of the powder bed are thus identified with the help of the layer control system and computer vision algorithms and a methodology is proposed to identify powder bed spreading anomalies and their influence on the melt signal. A case study is presented to investigate the effectiveness and robustness of the proposed methodology.
... Also, the mass distribution of material [136,137] in a part is well controlled and engineered in an AM design process. This new frontier of design poses its own challenges, including overhung structures [138,139]. ...
In this review, some of the latest applicable methods of machine learning (ML) in additive manufacturing (AM) have been presented and the classification of the most common ML techniques and designs for AM have been evaluated. Generally, AM methods are capable of creating complex designs and have shown great efficiency in the customization of intricate products. AM is also a multi-physical process and many parameters affect the quality in the development. As a result, ML has been considered as a competent modeling tool for further understanding and predicting the process of AM. In this work, most commonly implemented AM methods and practices that have been paired with ML methods along with their specific algorithms for optimization are considered. First, an overview of AM and ML techniques is provided. Then, the main steps in AM processes and commonly applied ML methods, as well as their applications, are discussed in further detail, and an outlook of the future of AM in the fourth industrial revolution is given. Ultimately, it was inferred from the previous papers that the most widely applied AM techniques are powder bed fusion, direct energy deposition, and fused deposition modeling. Also, there are other AM methods which are mentioned. The application of ML in each of the renowned techniques are reviewed more explicitly. It was found that, the lack of training data due to the novelty of AM, limitations of available materials to be applied in AM methods, non-standardization in AM data and process, and computational capability were some of the constraints of the application of ML in AM methods.
... Additive manufacturing (AM) of metals and alloys has gained popularity over the last two decades and is rapidly growing in terms of research and development, especially suited to the aerospace sector [1][2][3][4][5][6]. Amongst the various metal AM techniques, laser powder bed fusion (LPBF) is the most studied and versatile technique for precise near net shape geometry [7][8][9][10]. AlSi10Mg is a hypoeutectic Al alloy that has been extensively studied using the LPBF [11][12][13][14]. ...
Power-law creep behavior of laser powder bed fusion (LPBF) AlSi10Mg alloy is evaluated at 250 oC under uniaxial and bending test conditions. Uniaxial tests are performed in both tension and compression, and the cantilever samples are tested using digital image correlation (DIC)-augmented-bending test methodology, which is a way of establishing creep parameters in high throughput fashion using relatively smaller specimens. In each test condition, the applied normal stress is along the transverse direction relative to the build direction. The creep response of the alloy exhibits a tension-compression symmetry in terms of the steady-state and minimum creep rates. Correlations of uniaxial and high throughput bending creep tests reveal an excellent correspondence, thereby establishing the viability of evaluating creep behavior via bending, as a first of its kind, for additively manufactured Al alloys. Detailed electron microscopy of the crept specimens revealed the stability of microstructure during the prolonged exposures up to 500 h (during uniaxial creep) and the pinning of dislocations by intra-granular Si (20-50 nm) particles. A detailed discussion on the structure-property relationships is provided.
... Since the thermal conduction through the powder is much worse than through solid material, heat accumulates here, often leading to undesirable, residual stress-induced distortions [Kam19]. Support structures are often used to prevent these geometrical deviations and facilitate the part production, but they have to be removed after the process [Han18]. Since post-processing increases time and cost, it is generally desired to consider and counteract the possible distortions in the original design of the parts to ensure geometrical accuracy. ...
Laser Powder Bed Fusion (LPBF) is a generative manufacturing process, where a focused laser beam selectively melts a thin layer of powder. The three-dimensional component is created layer by layer by repeating the application of a new powder layer followed by laser exposure. LPBF is becoming increasingly widespread and is the subject of much research, as the resulting material properties and possible geometric freedom are significant advantages compared to conventional manufacturing processes. At the same time, the temperature gradients associated with focused energy input lead to process instabilities and quality problems. Together with the inherent cyclic heating and cooling, a complex thermal state is the result, leading to significant residual stresses, which in the worst-case lead to cracking and failure of the manufacturing operation. In addition to the formation of residual stresses, LPBF parts inherit a characteristic microstructure, the origin of which still needs further understanding.
This thesis aims to extend the fundamental process understanding regarding the origin and evolution of internal stresses, the characteristic microstructure, and the influence of process parameters on these properties.
These aims were achieved by performing in situ experiments with a customized LPBF system at the High Energy Materials Science beamline at the PETRA III synchrotron. The beamline's high energies of up to 100 keV allow experiments in transmission with a high temporal resolution to map the phenomena and mechanisms of action that occur. X-ray diffraction was used to track the evolution of lattice strains, stresses, texture, and phase composition during the LPBF process. Several underlying mechanisms of laser-matter interaction were identified by investigating a comprehensive set of process parameters and different materials.
Various thermal phenomena were demonstrated, including lateral heat accumulation in a single layer and vertical heat accumulation in the build-up direction. Additionally, a novel temperature estimation method was developed.
During the LPBF process, the stress state of the sample is changed by the laser until the very last layer. Depending on the laser parameters, a stress maximum occurs at a certain distance from the uppermost layer. This distance is increased by increasing the laser power and scanning speed. At the part edge, the stress changes from tensile to compressive with increasing distance from the top layer. The combination of low laser power and slow exposure speed resulted in a more homogeneous stress distribution than a high laser power and fast speed. It was also shown that the alpha to beta phase transformation in pure titanium occurs up to 400 µm below the surface during the repeated thermal cycling.
The influence of repeated heating and cooling also affects grain growth and texture, leading to in situ recrystallization in areas close to the substrate. After recrystallization, an increase in diffraction peak widths was observed and attributed to the formation of micro-strains. In addition, chemical segregation effects were derived from the change in peak shape and substantiated with subsequent transmission-electronic investigations.
This thesis's findings underline the benefits and necessity of in situ experiments at modern synchrotron light sources. Furthermore, the acquired diffraction data revealed several phenomena only observable during the process to extend the understanding of stress formation, microstructure evolution, and phase transformations in LPBF.
In this study, TiC/CM247LC nickel-based composite was successfully prepared by selective laser melting, then was heat treated at a solid solution temperature of 1260 °C and different aging temperature of 840 °C, 870 °C, 900 °C and 930 °C respectively. Effects of aging temperatures on the microstructures and mechanical properties were systematically studied. The results show that the microstructures of all the heat treated samples are composed of γ matrix, carbides and γ′ phase. The γ grains remain a columnar shape after treatments, but the size of γ′ phase grows up gradually with the increasing aging temperature. The composite treated at an aging temperature of 870 °C exhibits the best mechanical properties with the tensile strength of 1073 MPa, yield strength of 1004 MPa and elongation of 7.57%. The plastic deformation and strengthening mechanisms of heat treated composite were systematically investigated.
Selective laser melting (SLM) has gained great attention to manufacture cardiovascular stents given its potential of fabricating customized stents with complex shapes to satisfy clinical requirements. In this study, the surface characteristics of NiTi cardiovascular stents by SLM were explored. The effect of SLM machining parameters on surface morphology, geometry accuracy, phase composition, surface roughness and contact angle were analyzed. The results demonstrated that the surface morphology of stent became more irregular and the surface roughness was enhanced accompanied by the volume energy density (VED) increased. SLMed stents exhibited hydrophobic properties, and the rougher surface obtained a lower contact angle. The deviation of strut thickness was more than 200% than the nominal value under 194 J/mm ³ . The lowest VED displayed strong cubic B2 structure with less content loss of Ni, satisfying the self-expand NiTi stent requirements. Then electrochemical polishing (ECP) process distinctly improved the surface quality, providing smoother surfaces. The surface roughness reduced minimum to 0.45 µm from 6.64 µm for SLMed stent, and the average strut thickness was reduced to 230 µm at most. Finally, electrochemical test results revealed that SLM-ECPed stents showed a more obvious tendency to resist corrosion compared to SLMed stents.
The present study investigated the influence of interface shear strength on the support removal effort without deviating the dimensional accuracy of laser powder bed fusion processed gear-type parts. The finite element method simulations were carried out to evaluate the effect of different process parameters of support structures on the distortion of laser powder bed fusion processed parts. Additionally, the dimensional accuracy of manufactured parts was assessed using a non-contact three-dimensional white light scanning technique. Support structures are the inevitable features in the laser powder bed fusion process. A simple in-house technique is adopted to evaluate the interface shear strength of support removal using a mechanical torque wrench. The support structures were fabricated with four different laser power and scanning speed. The results showed that all the laser powder bed fusion parts had insignificant dimensional deviation compared to the computer-aided design model despite changing the process parameters (laser power and scanning speed). However, the results indicated that ∼60% interface shear strength could be reduced with optimized process parameters which are favourable for easy part removal after printing.
In this study, the microstructure and deformation mechanisms of selective laser melted GTD222 and TiC/GTD222 composite were studied. The results show that the TiC/GTD222 composite has finer grains and more precipitation phases. Meanwhile, TiC/GTD222 composite has higher yield strength both at room and high temperatures, which can be mainly attributed to the synergistic effect of the TiC strengthening and γ′ strengthening. The deformation mechanisms of TiC/GTD222 composite at 800 °C were identified as isolated stacking faults shearing the γ′ phase, continuous stacking faults shearing the γ and γ′ phases, dislocations cutting the γ′ phase, and dislocation slip within the γ matrix. This study provides insights for understanding the influence of TiC particles on the deformation mechanisms of additive manufactured nickel-based alloys.KeywordsNickel-based compositeMicrostructuresMechanical propertiesStrengthening mechanismsPlastic deformation mechanisms
Selective laser melting (SLM) is a metal additive manufacturing process that shows significant advantages in manufacturing lattice structures. In this paper, a novel surface-based square origami structure made of a nickel-based superalloy was fabricated using SLM. Three different wall thicknesses (50, 75 and 100 μm) were used to examine the manufacturability and corresponding compressive behaviour of the manufactured lattice components. Finite element analysis (FEA) was conducted to determine the uniaxial compression and then verified by quasi-static compression testing. The results showed that the components with thinner walls more easily folded and buckled than those with thicker walls, indicating that higher densification strains and energy absorption values may be achieved with thinner walls, although the thicker walls were stronger and could withstand larger loads. This research offers insights into the design and manufacture of advanced lattice structures by providing an improved understanding of the compressive behaviour of surface-based square origami structures.
It is important to study metal powder obtained from industrial waste given that its incorporation in the productive cycle is key to the sustainability of the cities of the near future, as well as to the use of sustainable manufacturing techniques. In this paper, an iron/iron oxide powder obtained by reduction of mill scale from hot deformation of low carbon steel was processed by two different techniques: die pressing and metal fused filament fabrication. Considering that microstructural features and the chemical composition are important factors in the mechanical properties of the final pieces, these two aspects were studied, both in the powders and in the final pieces. It was found that the presence of the polymer matrix during sintering of the piece fabricated by metal fused filament plays a key role in the process. This is because the hydrocarbons and H2 produced during its decomposition lead to a further reduction in the remaining oxides, which in turn leads to an important difference with the pieces obtained by die pressing. Moreover, the dislocation density of the pieces is different, because the compaction of the powder by die pressing causes dislocation to multiply, while in the fused filament fabrication process there is no stage that involves dislocation multiplication.
Many prior works demonstrate the potential of additive manufacturing (AM) for flow components. Examples include nozzles, flow distributors, hydraulic manifolds, and heat exchangers. Compared to conventional manufacturing methods, such as milling and drilling, AM offers a high degree of design freedom and enables the fabrication of organic-shaped and functionally optimized flow structures. Such parts can be produced without additional tooling at reduced lead times, allowing the rapid and iterative testing of many design variants. In addition, AM enables the cost-efficient production of customized parts that can be tailored to the individual needs of specific customers or applications.
Despite the potential of AM, a key challenge is to design complex parts for AM. In practice, a common approach is to manually create the 3D geometry of parts using computer-aided design (CAD) tools. However, a manual design process can lead to several challenges. First, it requires considerable expert knowledge of AM and skills with CAD tools, which can be a critical barrier for novice designers. Second, the design of organic-shaped parts can be time-consuming and require the creation of hundreds of design features. Third, designers must consider process-related restrictions of AM (e.g., prevention of critical overhangs) and may require several manual loops to analyze and modify design features for manufacturability (e.g., adaption of circular flow channels to droplet shapes). Fourth, the part development can involve many design changes, e.g., to include feedback from simulations and iterative tests, compare different production scenarios, and customize parts to specific needs. If such frequent design changes are performed manually, the manual effort, labor costs, and development time can significantly increase.
This work aims to automate the design of complex flow components fabricated using AM. For this purpose, this work follows a knowledge-based engineering approach and implements rule- and knowledge-based design algorithms for specific flow components. In particular, this work focuses on multi-flow nozzles and hydraulic manifolds produced using the AM process of laser powder bed fusion. This work presents three studies, each focusing on a specific design challenge.
Study I automates the design of complex AM multi-flow nozzles. The basic modeling idea is to decompose nozzles into a set of design elements that are used as the basic building blocks of nozzles and include recurring features, such as different cross-section shapes, flow channels, channel branches, guiding vanes, and reinforcement ribs. These design elements are organized using a hierarchical structure. This modeling approach allows to capture the hierarchical nature of complex nozzles and automate the design creation and nesting of multiple flow channels.
Study II focuses on the automated consideration of the AM overhang constraint during the design generation of AM parts, such as hydraulic manifolds. For this purpose, the study models the dependency between geometric parameters (e.g., inclination of flow channel cross-sections) and process-related parameters of AM (e.g., build direction, minimal build angle, and maximum diameter of horizontal cross-sections). Based on these relations, this study demonstrates how to automatically create flow channels without critical overhangs inside the channels by locally modifying the shape of circular cross-sections to adapted shapes (e.g., droplet). In addition, this study shows how to generate integrated and sacrificial supports. The result is a production-ready 3D part design that can be used to fabricate prototypes or conduct simulations.
Study III automates the routing of multiple flow channels for AM flow components, such as hydraulic manifolds. For this purpose, the study models flow channels as virtual cables defined by a chain of particles (= centerline of flow channels) and collision spheres (= required space of each flow channel). These cables are iteratively subjected to geometric-based constraints in order to impose different functional part requirements (e.g., minimizing the length of flow channels and preventing overlaps between different channels). In addition, the adaption of channel cross-sections for AM is taken into account by iteratively updating the radii of the collision spheres during the automated routing of flow channels.
Based on the presented studies, a key conclusion is that a rule- and knowledge-based approach can be applied successfully to automate the design of complex AM flow components, such as multi-flow nozzles and hydraulic manifolds. Potential future research directions include transferring the results to different applications, further simplifying the automated design process, and integrating machine learning techniques.
This work aims to propose a novel geometric modeling method to obtain lattice structures with internal walls and external skins that can be selectively activated. Internal walls can separate two adjacent cells, locally increase the stiffness of the component, and generate internal ducts; external walls are used to strengthen the entire structure and create a division from the outside. The proposed approach models a beam-based cellular structure with the introduction of internal walls according to an activation pattern that indicates whether a cell is communicating with the adjacent one through their connecting faces or not. The data structure describes the topology of the subdivision surface control polygon. The proposed method is then applied to a case study based on the hydraulic manifold applications. The possibility of building custom internal channels is exploited, with the advantage of obtaining smooth surfaces at the direction changes, with lower pressure drops, and a lightweight component due to the lattice structure that surrounds the channels. The resulting structure has a complex geometry that perfectly suits the manufacturing capabilities of additive manufacturing technologies.KeywordsAdditive manufacturingGeometric modelingLattice structuresClosed cellsHydraulic manifold
Laser powder bed fusion (LPBF) enables layer-by-layer manufacturing and provides distinguished microstructure compared with conventional processing methods. Microstructure controlling of LPBF parts requires deep insights into the solidification process. The aim of this article is to review the solidification fundamentals and microstructure characteristics during the LPBF process, and to serve as a guide on how to tailor microstructure for a given part. To clarify the distinguished microstructural characteristics of LPBF parts, casting is briefly discussed for comparison. The article begins with the introduction of solidification theory and microstructure features in LPBF and casting. Thereafter, the attempts to tailor microstructure in LPBF and casting are addressed and compared. The final part of this paper provides an outlook on the remaining challenges and potential research topics of microstructure controlling in LPBF with Al alloys.
Meta-biomaterials are applied to orthopedic implants to avoid stress shielding effects; however, there is no reason for the yield strength to be comparable to that of human bone. In this study, a composite unit cell was designed by combining the positive Poisson's ratio (PPR) and negative Poisson's ratio (NPR) unit cells, inspired by the second-phase strengthening theory. The purpose was to increase the strength while maintaining the elastic modulus. All structures were successfully fabricated from Ti-6Al-4V via selective laser melting. The relative density is between 0.08 and 0.24, which falls within the optimal range for bone growth. Mechanical tests indicated that the center of the inclined rod fractured in a stepwise fracture mode, which was consistent with the predictions of the Johnson–Cook model. The elastic modulus ranged from 0.652 ± 0.016 to 5.172 ± 0.021 GPa, and the yield strength varied from 10.62 ± 0.112 to 87.158 ± 2.215 MPa. An improved Gibson–Ashby law was proposed to facilitate the design of gradient structures. When the re-entrant angle was 40°, a hybrid body-centered cubic NPR structure was formed, resulting in a significant improvement in the mechanical properties. Importantly, the yield strength of the proposed composite structures increased by 43.23%, and the compression strength increased by 44.70% under the same elastic modulus. The strengthening mechanism has been proven to apply to other bending-dominated structures. Overall, this imparts unprecedented mechanical performance to auxetic meta-biomaterials and provides insights into improving the reported porous structures.
Statement of significance
: Auxetic meta-biomaterials exhibit auxetic properties that can improve the contact between the bone-implant interface and reduce the risk of aseptic failure. To avoid the stress shielding effect, the elastic modulus has traditionally been decreased by increasing the porosity. However, the strength is simultaneously reduced. Therefore, a composite unit cell was proposed to increase strength rather than modulus by combining the positive and negative Poisson's ratio unit cells, inspired by the second-phase strengthening theory. We observed a 43.23% increase in the yield strength of the composite structure without increasing the elastic modulus. This strengthening mechanism has been proven to apply to other bending-dominated structures. Our approach provides insights into improving other bending-dominated structures and broadening their applications for bone implantation.
In this paper the influence of a variation of the effective thermal conductivity of the powder bed on the properties and manufacturability of overhanging component areas in laser-based powder bed fusion (PBF-LB/M) of Ti-6Al-4V is investigated. The variation of the effective thermal conductivity of the powder bed is realized by contactless support structures under the over-hanging test specimens. The downskin angle between the overhanging test specimens and the base plate as well as the distance between the overhanging area and the test specimens are varied in a full factorial experimental design. The test specimens are examined with regard to the properties of relative density, surface roughness and geometric accuracy. In addition, a key figure is introduced which makes the results comparable with those obtained with other system configurations and parameters such as laser spot diameter, hatch distance and layer thickness.
In this paper, the dimensional accuracy of inclined structures formed through laser powder bed fusion (LPBF) was predicted by a developed theoretical model. The investigated dimension of an inclined structure in this study includes the dimensions in thickness direction and in length direction. The thickness direction is inclined to the substrate, while the length direction is parallel to the substrate. According to the model, the dimension in thickness direction is not only related to the track width and solidification shrinkage, but also to the stair effect and laser penetration effect. However, the dimension in length direction is only related to the track width and solidification shrinkage, and is hardly affected by the laser penetration effect. A batch of Ti6Al4V inclined samples was fabricated by LPBF to test this model. As a result, the experimental and predicted results correspond well. Furthermore, the effect of contour scanning strategy with different process parameters on the dimensional accuracy was assessed. The results show that the irregular contour scanning line is proved to be good for the dimensional accuracy in thickness direction, while the dimension in length direction is directly affected by the width of the contour scanning line. Finally, an optimization window of the process parameters for the contour scanning strategy is established. Under the optimal process parameters, the dimensional accuracy is improved.
In this research the Al 10 SiMg alloy were printed with different build direction (horizontal, vertical and inclined) using laser powder bed fusion (LPBF) method. The primary aim of this present study was to investigate the effect of build direction and surface finish condition on wear behavior of laser powder bed fusion printed AlSi 10 Mg alloy. The significance of printing direction and surface finish was revealed via wear behavior using pin-on-disc. According to the results the horizontally casted alloy shows lesser surface roughness. Subsequently the rough finished AlSi 10 Mg alloy on to the abrasive surface produced less wear loss. The fine finished alloy produced higher wear loss due to adhesion and erosion wear loss mechanism. Overall the horizontally casted cum rough finished AlSi 10 Mg alloy showed better wear resistance than other print direction and surface finish condition. The optical microscope and scanning electron microscope worn surface morphology confirmed the scar formation on fine finished AlSi 10 Mg alloy. These wear resistance improved AlSi 10 Mg alloy could be used as building material in aircraft and automobile applications where light weight and high wear resistance properties are required.
Additive manufacturing of metal parts was investigated in this research work, which connects to the models' supports. The connection between the support and the model has a high impact on the mechanical fixturing, the heat conduction, the removability of the support and the success of printing. In this research work, some support and tooth parameters such as the top length, the hatch distance, and Z offset were investigated to create a model-support connection strength map. This knowledge makes it possible to select the most appropriate connection type between the support and the model by the requirements specified in different cases. The strength of the support-model connection was characterized by the shearing properties using the torsion test. The results are illustrated and compared by different support parameters and materials. The 316L stainless steel and Ti6Al4V alloy were investigated. Different failure modes were determined depending on the chosen parameters.
In today’s modern era, DMLS for additive manufacturing is advance technology to manufacture components directly from powders. It has vast advantages over conventional process like minimization of loss of material and cost-effective. The quality of the product depends on temperature profile, which is govern by laser diameter, scanning seed, laser power, hatching distance, and scanning pattern. Here, temperature profile and molten pool dimensions were investigated for ALSi10Mg in COMSOL 5.4 platform where all parameters are kept constant, while scanning speed is varied. From the investigation, it is found out that peak temperature and molten pool depth decrease as scanning speed increases.
Direct Metal Laser Sintering (DMLS) process is a laser based additive manufacturing process, which built complex structures from powder materials. Using high intensity laser beam, the process melts and fuse the powder particles makes dense structures. In this process, the laser beam in terms of heat flux strikes the powder bed and instantaneously melts and joins the powder particles. The partial solidification and temperature distribution on the powder bed endows a high cooling rate and rapid solidification which affects the microstructure of the build part. During the interaction of the laser beam with the powder bed, multiple modes of heat transfer takes place in this process, that make the process very complex. In the present research, a comprehensive heat transfer and solidification model of AlSi10Mg in direct metal laser sintering process has been developed on ANSYS 17.1.0 platform. The model helps to understand the flow phenomena, temperature distribution and densification mechanism on the powder bed. The numerical model takes into account the flow, heat transfer and solidification phenomena. Simulations were carried out for sintering of AlSi10Mg powders in the powder bed having dimension 3 mm × 1 mm × 0.08 mm. The solidification phenomena are incorporated by using enthalpy-porosity approach. The simulation results give the fundamental understanding of the densification of powder particles in DMLS process.
Additive manufacturing (AM) enables the fabrication of parts of unprecedented complexity. Dedicated topology optimization approaches, that account for specific AM restrictions, are instrumental in fully exploiting this capability. In popular powder-bed-based AM processes, the critical overhang angle of downward facing surfaces limits printability of parts. This can be addressed by changing build orientation, part adaptation, or addition of sacrificial support structures. Thus far, each of these measures have been studied separately and applied sequentially, which leads to suboptimal solutions or excessive computation cost. This paper presents and studies, based on 2D test problems, an approach enabling simultaneous optimization of part geometry, support layout and build orientation. This allows designers to find a rational tradeoff between manufacturing cost and part performance. The relative computational cost of the approach is modest, and in numerical tests it consistently obtains high quality solutions.
This study investigates the feasibility of a novel concept, contact-free support structures, for part overhangs in powder-bed metal additive manufacturing. The intent is to develop alternative support designs that require no or little post-processing, and yet, maintain effectiveness in minimizing overhang distortions. The idea is to build, simultaneously during part fabrications, a heat sink (called “heat support”), underneath an overhang to alter adverse thermal behaviors. Thermomechanical modeling and simulations using finite element analysis were applied to numerically research the heat support effect on overhang distortions. Experimentally, a powder-bed electron beam additive manufacturing system was utilized to fabricate heat support designs and examine their functions. The results prove the concept and demonstrate the effectiveness of contact-free heat supports. Moreover, the method was tested with different heat support parameters and applied to various overhang geometries. It is concluded that the heat support proposed has the potential to be implemented in industrial applications.
The melt pool dynamics consideration in the numeric modeling of the Laser Material Deposition (LMD) process can be enormously difficult and expensive, especially if this calculation is not strictly necessary. The increased cost comes mainly from the necessity of considering a higher number of input parameters into the model in addition to the computational cost. Therefore, an analysis of the influence of the melt pool dynamics in a LMD model and its impact on the accuracy is presented. For this purpose, a numeric model that simulates the melt pool fluid-dynamics has been developed and experimentally validated for different situations. After a detailed analysis of the results, an exponential formula based on the response surface methodology (RSM) that quantifies the influence of the fluid-dynamic phenomena inside the melt pool has been obtained. The main conclusion of the present work is that the LMD process can be addresses as a thermal problem without considering the melt pool dynamics and without losing accuracy for a certain window of process parameters, what reduces the computational cost and will allow an easier integration of the model in CAE tools for process simulation.
Direct Metal Laser Sintering (DMLS) is a new technology in the field of additive manufacturing, which builds metal parts in a layer by layer fashion directly from the powder bed. The process occurs within a very short time period with rapid solidification rate. Slight variations in the process parameters may cause enormous change in the final build parts. The physical and mechanical properties of the final build parts are dependent on the solidification rate which directly affects the microstructure of the material. Thus, the evolving of microstructure plays a vital role in the process parameters optimization. Nowadays, the increase in computational power allows for direct simulations of microstructures during materials processing for specific manufacturing conditions. In this study, modeling of microstructure evolution of Al-Si-10Mg powder in DMLS process was carried out by using a phase field approach. A MATLAB code was developed to solve the set of phase field equations, where simulation parameters include temperature gradient, laser scan speed and laser power. The effects of temperature gradient on microstructure evolution were studied and found that with increase in temperature gradient, the dendritic tip grows at a faster rate.
The high thermal gradients experienced during manufacture via selective laser melting commonly result in cracking of high γ/γ′ Nickel based superalloys. Such defects cannot be tolerated in applications where component integrity is of paramount importance. To overcome this, many industrial practitioners make use of hot isostatic pressing to ‘heal’ these defects. The possibility of such defects re-opening during the component life necessitates optimisation of SLM processing parameters in order to produce the highest bulk density and integrity in the as-built state.
In this paper, novel fractal scanning strategies based upon mathematical fill curves, namely the Hilbert and Peano-Gosper curve, are explored in which the use of short vector length scans, in the order of 100 μm, is used as a method of reducing residual stresses. The effect on cracking observed in CM247LC superalloy samples was analysed using image processing, comparing the novel fractal scan strategies to more conventional ‘island’ scans. Scanning electron microscopy and energy dispersive X-ray spectroscopy was utilised to determine the cracking mechanisms.
Results show that cracking occurs via two mechanisms, solidification and liquation, with a strong dependence on the laser scan vectors. Through the use of fractal scan strategies, bulk density can be increased by 2 ± 0.7% when compared to the ‘island’ scanning, demonstrating the potential of fractal scan strategies in the manufacture of typically ‘unweldable’ nickel superalloys.
Selective laser melting (SLM) offers significant potential for the manufacture of the advanced complex-shaped aluminium matrix composites (AMCs) used in the aerospace and automotive domains. Previous studies have indicated that advanced composite powders suitable for SLM include spherical powders with homogeneous reinforcement distribution, a particle size of b100 μm and good flowability (Carr index b 15%); however, the production of such composite powders continues to be a challenge. Due to the intensive impacts of grinding balls, the high-energy ball-milling (HEBM) process has been employed to refine Al particles and disperse the nano Al 2 O 3 reinforcements in the Al matrix to improve their mechanical properties. Notwithstanding, the specific characteristics of ball-milled powders for SLM and the effect of milling and pause duration on the fabrication of composite powders have not previously been investigated. The aim of this study was to synthesise Al-4 vol.% Al 2 O 3 nano-composite powders using HEBM with two different types of milling and pause combinations. The characteristics of the powders subjected to up to 20 h of milling were investigated. The short milling (10 min) and long pause (15 min) combination provided a higher yield (66%) and narrower particle size distribution range than long milling (15 min) and a short pause (5 min). The nano Al 2 O 3 reinforcements were observed to be dispersed uniformly after 20 h of milling, and the measured Carr index of 13.2% indicated that the ball-milled powder offered good flowability. Vickers micro-hardness tests indicated that HEBM significantly improved the mechanical properties of the ball-milled powders.
The aim of this study is to investigate the properties of high-energy ball-milled nanocrystalline aluminium powders and to determine the optimum milling time required to produce an advanced aluminium powder for selective laser melting (SLM). Previous research has indicated that powders suitable for SLM include milled nanocrystalline aluminium powders with an average grain size of 60 nm and good flowability (Carr index less than 15 %). This study employs advanced nanometrology methods and analytical techniques to investigate the powder morphology, phase identification, average grain size and flowability of ball-milled powders. Stearic acid is used to prevent excessive cold welding of the ball-milled powder and to reduce abrasion of the grinding bowl and balls. The results indicate that, whilst the average particle size achieves a steady state after 14 h of milling, the grain size continues to decrease as the milling time progressed (e.g. the transmission electron microscopy measured average grain size is 56 nm after 20 h of milling compared to 75 nm for 14 h of milling). The aluminium powders milled for 16 and 20 h exhibit good flow behaviour, achieving a Carr index of 13.5 and 15.8 %, respectively. This study shows that advanced nanocrystalline aluminium powders suitable for SLM require ball milling for between 16 and 20 h, with 18 h being the optimum milling time.
Selective laser melting (SLM) is being widely utilised to fabricate intricate structures used in various industries. Widening the range of applications that can benefit from such promising technology requires validating SLM parts in load bearing applications. Recent studies have mainly focussed on static loading, with minor attention to cyclic loading despite its vital importance in many applications. In this work, the fatigue performance of SLM AlSi10Mg was investigated considering the effects of surface quality and heat treatment. Compared to heat treatment, machining the samples played a minor role in improving the fatigue behaviour. This is potentially attractive to industries interested in latticed structures and topology-optimised parts where post-processing machining is not feasible. The characteristically fine microstructure in the as-built samples provided good fatigue crack propagation resistance but none of them survived nominal fatigue life of 3x107 cycles within the maximum stress range of 63-220 MPa. A specially-tailored heat treatment increased the material’s ductility, significantly improving its fatigue performance. At 94 MPa, the heat-treated samples survived beyond the nominal fatigue life, outperforming the reference cast material. The combined effect of machining and heat treatment yielded parts with far superior fatigue properties, promoting the material for a wider range of applications.
Additively manufactured (AM) conformal cooling channels are currently the state of the art for high performing tooling with reduced cycle times. This paper introduces the concept of conformal cooling layers which challenges the status quo in providing higher heat transfer rates that also provide less variation in tooling temperatures.The cooling layers are filled with self-supporting repeatable unit cells that form a lattice throughout the cooling layers. The lattices increase fluid vorticity which improves convective heat transfer. Mechanical testing of the lattices shows that the design of the unit cell significantly varies the compression characteristics.A virtual case study of the injection moulding of a plastic enclosure is used to compare the performance of conformal cooling layers with that of conventional (drilled) cooling channels and conformal (AM) cooling channels. The results show the conformal layers reduce cooling time by 26.34% over conventional cooling channels.
There is a growing interest in using additive manufacturing to produce smart structures, which have the capability to respond to thermal and mechanical stimuli. In this report, Selective Laser Melting (SLM) is used to build a Negative Poisson's Ratio (NPR) TiNi-based Shape Memory Alloy (SMA) structure, creating a multi-functional structure that could be used as reusable armour. The study assesses the influence of SLM process parameters (laser power, scan speed, and track spacing) on the microstructural and structural integrity development in a Ti-rich TiNi alloy, as well as the impact of the post-process homogenisation treatment on the microstructure and phase transformations. The builds generally shows stress-induced cracks and residual porosity, which could be minimised through process optimisation. Nonetheless, the homogenisation treatment is essential to reduce the fraction of Ti2Ni intermetallics, which are known to disturb the B19′-chemistry, and hence the required phase transformation temperatures. The optimum process parameters are finally used to fabricate NPR structures, which were mechanically tested to validate the Poisson's ratio predictions. A higher ductility was observed in the structures that have undergone the homogenisation treatment.
The efficiency of fabricating an overhanging structure by selective laser melting (SLM) is an important indicator of the performance of metallic parts. This is due to the fact that defects such as warpage and adherent dross may occur during fabrication of the curved surfaces of overhanging structures. In order to investigate the optimum conditions for fabrication of the curved surfaces of the overhanging structures, experiments were carried out using 316-L stainless steel powder. Initially, the almost 100 % dense parts were fabricated. Then, a model that has a circular curved surface along the Z axis was designed. For a given fabrication depth of 25 μm, several overhanging structures were produced when the laser scanning energy input ranges from 0.15 to 0.6 J/mm. Results show that the upper surface of the almost 100 % dense cube fluctuates like ripples and that the fabrication quality of the curved surface of the overhanging structure varies greatly depending on the energy input and the obliquity angle. For a given energy input of 0.2 J/mm, the obliquity angle for fabricating a totally overhanging surface is as low as 30°. The warpage and adherent dross grow with an increase in the energy input and a decrease in the obliquity angle. Warpage may accumulate, and the accumulated warpage of many layers significantly exceeds the predetermined thickness of the layer. All the four overhanging structures fabricated using varying energy inputs have the following four zones: no dross surface, dense-sinking transition surface, totally sinking surface, and forming failure surface. In the overhanging structures, fabricated with varying laser energy parameters, the angle corresponding to each region was different. The quality of the overhanging surface can be improved by reducing the laser energy. Additionally, a better overhanging surface can be obtained by increasing the obliquity angle. The variation trend of the roughness Rz was almost the same as that of Ra, but the variation range of Rz was much larger than that of Ra. Finally, a foldable abacus with several curved-surface overhanging structures was fabricated to verify the research results. Fundamental methods for controlling and optimizing the SLM-based direct fabrication of curved surfaces of overhanging structures are proposed in this paper, from the perspectives of crafting and design.
Direct metal laser sintering (DMLS) is an additive manufacturing technique for the fabrication of near net-shaped parts directly from computer-aided design data by melting together different layers with the help of a laser source. This paper presents an investigation of the surface roughness of aluminum samples produced by DMLS. A model based on an L18 orthogonal array of Taguchi design was created to perform experimental planning. Some input parameters, namely laser power, scan speed, and hatching distance were selected for the investigation. The upper surfaces of the samples were analyzed before and after shot peening. The morphology was analyzed by means of field emission scanning electron microscope. Scan speed was found to have the greatest influence on the surface roughness. Further, shot peening can effectively reduce the surface roughness.
Part distortion is a critical issue during Additive Manufacturing (AM) of metallic parts since it prevents this technology from being implemented at industrial level. To this regard, distortion prediction even from design stage has become crucial. Actually, numerical modelling methodologies play an important role here. Different modelling approaches have been developed but one of the most computationally efficient methodology to predict distortion is the so called inherent strain method. In this work an empirical methodology to determine inherent strains is presented. This is the input data in simplified Finite Element (FE) models in order to predict distortion and residual stress fields. These inherent strains are calculated considering layer lumping strategies that might be adopted in the numerical model as well. The procedure has been developed and validated using the well-known twin-cantilever beam structure. Ti-6Al-4V beams have been manufactured by LPBF technology following different scanning strategies. Distortion after support removal has been measured in order to be compared against numerical results. The methodology has been applied at coupon level giving accurate results and providing a preliminary validation.
Developing patient-specific biomedical implants for clinical application requires the integration of material science, manufacturing engineering, and biology. As selective laser melted (SLM) metallic additive manufactured implants become common, a key, but overlooked design parameter is its inclination angle. In this study, we have fabricated Ti6Al4V implants at three different inclination angles (0, 45 and 90 degrees) reporting the relationship between cell attachment, surface topography and surface chemistry at each angle. During the SLM process, we show that as the inclination angles increase, there is a corresponding increase in the number of partially melted particles adhering to the surface, greatly affecting the surface topography, morphology, roughness, chemistry, and wettability of the implant. In order to validate the approach, the effect of surface properties on cell fate was determined. In each case, the overall viability of Chinese hamster ovarian cells (CHO) was found to be statistically indistinguishable; however, the number of spindle cells and their dimension were found to increase significantly at higher inclination angles. This work demonstrates a novel approach for combining SLM technology in manufacturing metallic biomedical implants and provides a novel insight in case of switching cell‑titanium interface by modifying one process parameter, inclination angle, during rapid prototyping process.
In this paper, we formulate the generation of support structures for additive manufacturing as a topology optimization problem. Compared with usual geometric considerations based support structure design, this formulation affords mechanistic meaning to the computed support structures. Moreover, our study reveals that the topology optimization formulation generally leads to self-supporting designs without extraneous self-supporting constraints. We show the generality of the procedure by computing supports for a variety of parts in both two and three dimensions, including a complex model of the mascot of the University of Wisconsin-Madison. The resulting support structures have been 3D printed, demonstrating that the computed designs can successfully be used as supports.
Laser powder bed fusion (L-PBF) process has had a rapid growth in the industrial fields because of the capability to manufacture metallic complex shapes. The purpose of this study is to investigate the accuracy and surface roughness of parts manufactured by L-PBF in the AlSi10Mg alloy. The results showed that the choice of parameters of conversion from the CAD model to STL file and the setting of process parameters can affect the accuracy. In the L-PBF process, the staircase effect, inherent in additive manufacturing technologies due to the layered nature of the process, is not visible due to the melting of thin layers of metal powder. The surface roughness is mainly caused by the process parameters, orientation and position of the part with respect to the recoating blade and by the presence of partially fused particles that adhere to the molten part.
Additive manufacturing (AM) enables fabrication of multiscale cellular structures as a whole part, whose features can span several dimensional scales. Both the configurations and layout pattern of the cellular lattices have great impact on the overall performance of the lattice structure. In this paper, we propose a novel design method to optimize cellular lattice structures to be fabricated by AM. The method enables an optimized load-bearing solution through optimization of geometries of global structures and downscale mesostructures, as well as global distributions of spatially-varying graded mesostructures. A shape metamorphosis technology is incorporated to construct the graded mesostructures with essential interconnections. Experimental testing is undertaken to verify the superior stiffness properties of the optimized graded lattice structure compared to the baseline design with uniform mesostructures.
Selective laser melting (SLM) technology was recently introduced to fabricate dental prostheses. However, the fatigue strength of clasps in removable partial dentures prepared by SLM still requires improvement. In this study, we attempted to improve the fatigue strength of clasps by adding support structures for overhanging parts, which can generally be manufactured at an angle to be self-supporting. The results show that the fatigue strength of the supported specimens was more than twice that of unsupported specimens. Electron back-scattered diffraction analysis revealed that the supported specimens exhibited lower kernel average misorientation values than the unsupported specimens, which suggested that the support structure reduced the residual strain during the SLM process and helped to prevent micro-cracks led by thermal distortion. In addition, the supported specimens cooled more rapidly, thereby forming a finer grain size compared to that of the unsupported specimens, which contributed to improving the fatigue strength. The results of this study suggest that the fatigue strength of overhanging parts can be improved by intentionally adding support structures.
Commercially pure titanium was produced using laser engineered net shaping (LENS) and selective laser melting (SLM) processes. The SLM and LENS processing parameters as well as critical aspects including densification and balling effect were investigated. The resulting properties were studied and compared with those from traditional casting. Investigation of the processing parameters showed that significantly higher laser power and energy density is required in LENS compared to SLM in order to obtain near full density (99.5%). The microstructural investigations revealed an α microstructure with mixed morphologies including plate-like and widmanstätten for LENS somewhat similar to the serrated and fine acicular α obtained from casting. In contrast, the SLM samples showed only martensitic α′ phase mainly with a lath-type morphology. The difference between SLM and LENS microstructures was discussed based on interrelated aspects including energy density, solidification rate and specific point energy. Differences in their microstructures are mainly associated with differing rates of cooling and differing energy densities during SLM and LENS processing. Compression and hardness tests indicated that SLM titanium possesses better mechanical properties due to a fine grain size and martensitic phase composition, whereas LENS and cast titanium with α microstructures show similar mechanical properties.
This paper investigates the effect of processing parameters on the dimensional accuracy and mechanical properties of cellular lattice structures fabricated by additive manufacturing, also known as 3D printing. The samples are fabricated by selective laser melting (SLM) using novel titanium-tantalum alloy. The titanium-tantalum alloy has the potential to replace commercially pure titanium and Ti6Al4V as biomedical material. In this study, the unit cell used is specially designed to carry out the analysis using regression method and analysis of variance (ANOVA). Due to the effect of the SLM process parameters, the elastic constant of the cellular lattice structures ranged from 1.36 ± 0.11 to 6.82 ± 0.15 GPa using the same unit cell design. The elastic constant range, while showing the versatility of titanium-tantalum as biomedical material, is rather wide despite using the same lattice structure designed. This shows that there is a need to carefully control the processing parameters during the lattice structures fabrication so as to obtain the desired mechanical properties. Based on the statistical analysis, it is found that the dimensional accuracy and mechanical properties such as elastic constant and yield strength of the cellular lattice structures are most sensitive to laser power as compared to other parameters such as laser scanning speed and powder layer thickness.
In this paper, a selective laser melting (SLM) physical model describing the melt pool dynamics and the response of downward-facing surface morphology evolution of overhanging structure under different laser processing conditions was proposed, in which an enormous difference in thermal conductivity and laser absorption capacity between the as-fabricated part and powder material was taken into consideration. The underlying thermal physical mechanism of the dross formation phenomenon during SLM preparing overhanging surface was revealed by numerical simulation analysis and experimental studies. It was found that both high and low laser volume energy density (ω) resulted in an inferior downward-facing surface quality. As an optimal processing parameter (60–80 J/mm³) was settled, the overhanging structure obtained a relatively smooth downward-facing surface due to the sound melt pool dimension and steady melt flow behavior. The experimental studies were compared with the simulated results, showing a good agreement with the predictions obtained in the simulations. It was interesting to find that the variation rules of surface quality and densification level of overhanging structure with different ω were exactly converse. As the ω decreased from 80 J/mm³ to 60 J/mm³, the surface roughness could be reduced from 59 μm to 33 μm while, contrarily, the porosity was elevated from 3.2% to 8.4%. In order to fabricate complicated metal parts with lower risk, four solutions for improving the processability of hard-to-process overhanging structure were provided.
Aluminium-based composites are increasingly applied within the aerospace and automotive industries. Tribological phenomena such as friction and wear, however, negatively affect the reliability of devices that include moving parts; the mechanisms of friction and wear are particularly unclear at the nanoscale. In the present work, pin-on-disc wear testing and atomic force microscopy nanoscratching were performed to investigate the macro and nanoscale wear behaviour of an Al-Al2O3 nanocomposite fabricated using selective laser melting. The experimental results indicate that the Al2O3 reinforcement contributed to the macroscale wear-behaviour enhancement for composites with smaller wear rates compared to pure Al. Irregular pore surfaces were found to result in dramatic fluctuations in the frictional coefficient at the pore position within the nanoscratching. Both the size effect and the working-principle difference contributed to the difference in frictional coefficients at both the macroscale and the nanoscale.
Aluminium-based metal matrix composites are widely used in the aerospace and automotive industries, but their manufacturability and mechanical properties are not well understood when these new materials are employed in additive manufacturing. This is an important consideration because, compared with traditional manufacturing technologies, additive manufacturing technologies such as selective laser melting (SLM) offer the ability to manufacture engineering parts with very complex geometries. This paper systematically studied the SLM of an advanced Al-Al 2 O 3 nanocomposite that was synthesised using high-energy ball-milling. A finite element model was developed in the study to predict the thermal behaviour of the composite in order to narrow down the process parameters to be explored in the experiments; in particular, the SLM hatch spacing and scanning speed were found to be worth further experimental investigation for the composites. The experimental results demonstrated that the optimum laser-energy density and scanning speed in fabricating nearly fully dense composite parts were 317.5 J/mm 3 and 300 mm/s, respectively. Furthermore, the as-fabricated composite parts were observed to exhibit a very fine granular-dendrite microstructure due to the rapid cooling, while the thermal gradient at the molten pool region along the building direction was found to facilitate the formation of columnar grains. Compared to pure Al, the addition of 4 vol% Al 2 O 3 nanoparticulates was found to contribute to a 36.3% and 17.5% increase in the yield strength and microhardness of the composite samples, respectively, because the reinforcement particulates improved the dislocation density by offering more grain boundaries. The paper also examines the influence of cold working on microstructural and mechanical properties. In comparison with the as-fabricated composite sample, the cold-worked composite sample was found to offer a 39% increase in microhardness; this is thought to have been caused by plastic deformation, which in turn resulted in grain deformation and elongation.
Additive manufacture (AM) enables innovative structural design, including the fabrication of complex lattice structures with unique engineering characteristics. In particular, selective laser melting (SLM) is an AM process that enables the manufacture of space filling lattice structures with exceptional load bearing efficiency and customisable stiffness. However, to commercialise SLM lattice structures it is necessary to formally define the manufacturability of candidate lattice geometries, and characterise the associated mechanical response including compressive strength and stiffness. This work provides an experimental investigation of the mechanical properties of SLM AlSi12Mg lattice structures for optimised process parameters as well as the manufacturability of lattice strut elements for a series of build inclinations and strut diameters. Based on identified manufacturability limits, lattice topologies of engineering relevance were fabricated, including both stretch-dominated and bending-dominated structures. Manufactured lattice morphology and surface roughness was quantified, as were the associated mechanical properties and deformation and failure behaviour.
This study investigated the effect of the processing parameters on the quality and mechanical properties of a biomedical titanium alloy (Ti–24Nb–4Zr–8Sn) scaffolds fabricated