Article

Production Strategy for Manufacturing Large-Scale AlSi10Mg Components by Laser Powder Bed Fusion

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Abstract

The long production time required for large-scale parts fabricated by laser powder bed fusion (LPBF) tends to induce cracks, distortions, and overheating problems. In this work, to address these challenges, we explored and established a suitable strategy for producing large AlSi10Mg components. The platform temperatures to prevent cracks and distortions were firstly determined. Then, the in situ aging behavior was investigated for samples under various platform temperatures and holding times. Our results revealed that platform temperatures of 150°C and 200°C can effectively prevent cracks and minimize distortions. Besides, using 150°C, samples can reach peak hardness with a holding time less than 13 h. In comparison, those samples produced with a holding time longer than 13 h at 150°C and 200°C show obvious over-aging responses and thus lower hardness. However, such a hardness impoverishment can be recovered by using a T6 post-process heat-treatment.

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... Meanwhile, peak hardness of in situ ageing can be achieved at this temperature and, thus, post-heat treatments may not be required. For a higher platform temperature that is more effective for distortion reduction, however, the in situ ageing effect tends to impair the mechanical properties of Al-Si alloys [19,20]. In addition, for the crack-sensitive 2xxx and 7xxx Al alloy series, a heated platform at the temperature range of 100-500 • C has been previously considered [21][22][23][24][25][26]. ...
... On this basis, heating the platform to 200 • C throughout printing can diminish the crack propagation from the surface during part removal [40,41]. Particularly, large-scale parts that normally experience buckling and distortion due to overheating at long production times can be safely fabricated and removed, given that most of the residual stresses are relieved [19]. ...
... This suggests that finer and steadier Al 3 Sc cluster sizes can be achieved during LPBF processing compared to DED. In addition, this temperature level of <230 • C experienced by the underlying consolidated layers (see T3 in Figure 9a) cannot lead to over-aging in the printed Al-Sc parts regardless of the printing time, unlike other Al alloy systems that can reach the maximum strength at 120-200 • C [19]. ...
Article
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Platform heating is one of the effective strategies used in laser powder bed fusion (LPBF) to avoid cracking during manufacturing, especially when building relatively large-size components, as it removes significant process-induced residual strains. In this work, we propose a novel and simple method to spare the elaborate post-processing heat treatment typically needed for LPBF Al-Sc alloys without compromising the mechanical properties. We systematically investigated the effects of LPBF platform heating at 200 °C on the residual stress relief, microstructure, and mechanical performance of a high-strength Al-Mn-Sc alloy. The results reveal that LPBF platform heating at 200 °C is sufficient to largely relieve the process-induced residual stresses compared to parts built on an unheated 35 °C platform. Meanwhile, the platform heating triggered the dynamic precipitation of uniformly dispersed (1.5–2 nm) Sc-rich nano-clusters. Their formation in a high number density (1.75 × 1024 m−3) resulted in a ~20% improvement in tensile yield strength (522 MPa) compared to the build on the unheated platform, without sacrificing the ductility (up to 18%). The improved mechanical properties imply that platform heating at 200 °C can strengthen the LPBF-synthesised Sc-containing Al alloys via in situ aging, which is further justified by an in situ measurement study revealing that the developing temperatures in the LPBF part are within the aging temperature range of Al-Sc alloys. Without any post-LPBF treatments, these mechanical properties have proven better than those of most Al-Sc alloys through long-time post-LPBF heat treatment.
... The Authors found that printing at 250 • C was optimal for preventing parts' deformations, despite the relatively low hardening exhibited by the material after printing. In this regard, Shen and co-workers [25] demonstrated that the build plate heating promotes a simultaneous in-situ aging heat-treatment, thus creating different levels of hardening in the same part based on its distance from the build plate and the printing time. ...
... In this work, differently from recent studies [25,26,31], the platform heating was not only utilised to induce in-situ aging at relatively low temperatures but also to carry out solutioning heat-treatments in a higher temperature range. Therefore, to streamline the AM post-processing workflow, we assessed the viability of conducting in-situ heat-treatments on AlSi10Mg parts using a heated build plate strategy during PBF-LB manufacturing. ...
... In addition, the contour maps of the as-built samples showed hardness gradient along the build direction ( Fig. 2(b,f)). This gradient corresponds to the intensity of the heat transfer from the build plate to the sample at each particular height [25,35]. The platform's temperature also influenced the typical fish-scale mesostructure characteristic of PBF-LB ( Fig. 2(c,g)). ...
... The reason behind this difference can be attributed to the fact that these samples remained on the heated platform for longer, i.e., until the manufacture of the full height of the vertical samples was completed. It is envisaged that the elevated temperature of the build plate led to an in-situ heat treatment that stimulated some decomposition of the inter-dendritic Si and its diffusion within the α-Al matrix, thereby disrupting the eutectic Si network [44,45]. ...
... With the higher effective build plate temperature for horizontally placed samples slight decomposition of the inter-dendritic Si occurs along with diffusion within the α-Al matrix. This in turn affects the hardness of the material [44,45]. Hardness is a property that is local to the indented area, and in the case of L-PBF materials, it is heavily influenced by the cooling Metals 2023, 13, 151 9 of 15 rate fluctuations throughout the build and hence by the microstructural variations that eventually follow [47,48]. ...
... matrix. This in turn affects the hardness of the material [44,45]. Hardness is a property that is local to the indented area, and in the case of L-PBF materials, it is heavily influenced by the cooling rate fluctuations throughout the build and hence by the microstructural variations that eventually follow [47,48]. ...
Article
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Laser powder bed fusion (L-PBF) additive manufacturing has reached wide-scale technology readiness for various sectors. However, some challenges posed by the complex nature of the process persist. Limited studies investigated the correlation between the micro- and macroscopic properties of L-PBF AlSi10Mg parts and the features’ sizes with the build orientation in mind. Therefore, this study presents a comprehensive view on the “size effect” for samples larger than those available in the literature (up to 12 mm) on the defects, microstructure evolution, and mechanical properties in two build orientations using a fixed set of process parameters. Microstructural differences were observed between the build orientations, but no considerable difference with size change was detected. The porosity content was inversely proportional to the feature size irrespective of the build orientation, leading to an increase in ductility that was more evident in the horizontal specimens (~44%). This was attributed to an in-situ heat treatment. Although specimens oriented parallel to the build direction showed no significant size-effect in terms of the mechanical properties (hardness and tensile), anisotropy was evident. Based on the findings presented in the study and the scientific explanations discussed corroborated by thermal imaging during processing, it is concluded that although any set of ‘optimised’ process parameters will only be valid for a specific size range, the severity of the size-effect changes dynamically based on the range.
... The microstructure and mechanical properties can be optimized for the function of the targeted application. Solution heat treatments are employed with the idea of finding known properties, while direct artificial ageing is used to exploit the hardening potential of the rapidly cooled alloy and maintain the fine cell structure or reduce thermal stresses [10,14,15]. ...
... Platform preheating is an important process parameter that can affect the properties of the printed material. It was originally used to reduce thermal stresses or to help the prevention of hot cracking [14,16]. However, it was highlighted that preheating has an important role in the microstructural and hardening potential of the alloy. ...
... For a 160 • C preheating, Casati et al. noted a loss of hardening potential in comparison with an un-preheated platform [19]. Nevertheless, they did not notice any hardness variation with respect to the building height as opposed to Bosio et al. and Mauduit et al. [14,20]. The various authors do not agree on the consequences of the influence of the plate temperature on the final properties of the parts. ...
Article
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AlSi10Mg alloy is mainly produced by laser fusion on a powder bed. It offers a good compromise between easy processing and good mechanical properties. These properties depend on the manufacturing parameters, including the preheating temperature, as this alloy hardens by precipitation. This study explored the effect of preheating to 170 °C on the mechanical properties and microstructure of this alloy as a function of the manufacturing time. The mechanical properties were characterized by tensile, hardness and impact strength tests as a function of the sample height. An anisotropic behavior was confirmed: the horizontal orientation showed higher deformation and fracture energies. In addition, a gradient of properties appeared as a function of the distance from the platform; the closer the sample was to the platform, the higher its fracture energy and the lower its hardness. The hardness values followed the same evolution as a function of the distance to the platform as that of the hardness curve after ageing post-treatment. It was therefore shown that the preheating of the platform generated in situ ageing with respect to the manufacturing height: a hardness peak was obtained at a certain distance from the plateau (40 mm—10 h of remaining manufacturing time) and over-aging near the plateau was induced by long exposure times at 170 °C.
... For this reason, to trigger the precipitation of strengthening phases containing Cu from the SSSS, in-situ alloyed materials can be strengthened through direct ageing heat-treatments without presolutioning heat-treatments [16,17]. Unlike the well-known direct aging behavior of Al-Si-Mg alloys [18][19][20][21], to the best of our knowl-edge, the state-of-art on the aging response of Al-Si-Mg-Cu alloys processed by LPBF is limited to the work of Roudnická et al. [22]. In particular, the Authors have recently investigated the agehardening response of an AlSi9Cu3 alloy processed by LPBF. ...
... By raising the aging temperature to 175°C (Fig. 7c,d), the eutectic network was still identifiable. However, in Fig. 7d, it was noticed the presence of tiny particles within the a-Al cells, which likely precipitated out of the SSSS during the aging heat-treatment [18,20]. This was clearly observed after aging at 190°C. ...
Article
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In this work, a high strength Al alloy was successfully processed via in-situ alloying of AlSi10Mg and Cu elemental powders during Laser Powder Bed Fusion. To get superior strength, 4 wt.% of Cu was added to AlSi10Mg composition to benefit from the additional solid solution and precipitation strengthening mechanisms induced by Cu atoms. Microstructure, chemical composition, hardness, and tensile properties of as-built samples were firstly determined. The microstructure showed a dual eutectic formed by Si precipitates intermixed with θ -Al2Cu phase. The average Cu concentration was 3.96 ± 0.26 wt.% in line with the theoretical one. Hardness and yield strength of AlSi10Mg+4Cu alloy showed an increase of respectively 8.8 % and 35.6 % compared to the as-built AlSi10Mg, owing to the highly super-saturated solid solution of Si and Cu atoms upon rapid solidification. After that, a direct aging heat-treatment strategy was pursued to fully exploit the potential alloy precipitation by heating as-built samples at temperatures between 160 and 190 °C. A maximum hardness response was achieved after 1 h at 175 °C. The high hardening was primarily attributed to a mix of θ''/θ'+θ -Al2Cu and Si phases coupled with a still high solid solution content. Nevertheless, direct aging slightly decreased the alloy ductility due to the prominent precipitation of brittle Si particles during aging and the presence of Cu inhomogeneities formed after in-situ alloying.
... Therefore, while temperature peaks above melting temperature are important for the soundness of the LPBFed part, lower temperature peaks still play an important role in causing successive "in-situ" thermal treatments, which may partially trigger precipitation or other thermally induced phenomena. Moreover, the use of base-plate preheating causes a proper in-situ heat treatment of the built volume [86]: this feature may cause serious inhomogeneities across the built part due to the different exposures of the volumes at different heights to high temperatures, possibly inducing different levels of local age-hardening or even softening due to over-ageing (Fig. 6 b). In this light, the post-building precipitation treatments may be rendered less effective or even unnecessary by these features [79,87]. ...
... Side-effects of laser scanning and base-plate preheating during LPBF processing of aluminium alloys: (a) Local variation of temperature during successive laser scans with the employment of different laser powers; (b) hardness trends induced by base-plate preheating along the height of massive ASi10Mg components produced by LPBF. Reproduced from[88] (a) and[86] (b). ...
Article
Full-text available
Laser powder bed fusion (LPBF) is the most widely used additive manufacturing technique and has received increasing attention owing to the high design freedom it offers. The production of aluminium alloys by LPBF has attracted considerable interest in several fields due to the low density of the produced alloys. The peculiar solidification conditions experienced by molten metal during the SLM process and its layer-by-layer nature causes a variety of microstructural peculiarities including the formation of metastable phases and supersaturated solid solutions, extreme microstructural refinement, and generation of residual stresses. Therefore, post-build heat treatments, which are commonly applied to conventionally produced aluminium alloys, may need to be modified in order to be adapted to the peculiar metallurgy of aluminium alloys manufactured using LPBF and address the specific issues resulting from the process itself. A number of studies have investigated this topic in recent years, proposing different approaches and dealing with various alloying systems. This paper reviews scientific research results in the field of heat treatment of selective laser melted aluminium alloys; it aims at providing a comprehensive understanding of the relationship between the induced microstructure and the resulting mechanical behaviour, as a function of the various treatment strategies.
... During structure build-up, elevated temperature may activate diffusion processes; however, local temperature and holding time are indirectly influenced, e.g. the first layers may be held at an elevated temperature for relatively longer than the final layers due to prolonged heat input from the AM process. Achieving homogeneous peak ageing requires strict control over temperature and holding time, which cannot currently be achieved during AM, thus leading to undesirable precipitation and hardness gradients in the AM structure [35,36]. Although hardness cannot be directly correlated with mechanical properties such as tensile strength and ductility, areas with high hardness generally exhibit higher strength than areas with low hardness [37,38]. ...
Article
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Additive friction stir deposition (AFSD) is an emerging solid-state non-fusion additive manufacturing (AM) technology, which produces parts with wrought-like material properties, high deposition rates, and low residual stresses. However, impact of process interruption on defect formation and mechanical properties has not yet been well addressed in the literature. In this study, Al6061 aluminium structure with two final heights and deposition interruption is successfully manufactured via AFSD and characterised. Defect analysis conducted via optical microscopy, electron microscopy, and X-ray computed tomography reveals > 99% relative density with minimal defects in centre of the parts. However, tunnel defects at interface between substrate and deposit as well as kissing bonds are present. Edge of deposit contains tunnel defects due to preference for greater material deposition on advancing side of rotating tool. Virtual machining highlights the ability to remove defects via post-processing, avoiding mechanical performance impact of stress concentrating pores. Electron backscatter diffraction revealed regions with localised shear bands that contain 1–5 µm equivalent circular diameter grains. Kissing bonds are exhibited in areas separated by large grain size difference. Meanwhile, Vickers hardness testing reveals hardness variation with deposit height. This work advances the understanding of complex microstructure development, material flow, and mechanical behaviour of AFSD Al6061 alloy.
... 3.2. In this case, the discrepancy could be caused by a chemical variation through the height (BD) of the sample leading to different d 0 at the nominal and notch planes [43,45,49]. ...
Article
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Laser Powder Bed Fusion (PBF-LB/M) additive manufacturing (AM) induces high magnitude residual stress (RS) in structures due to the extremely heterogeneous cooling and heating rates. As the RS can be deleterious to the fatigue resistance of engineering components, great efforts are focused on understanding their generation and evolution after post-process heat treatments. In this study, one of the few of its kind, the RS relaxation induced in an as-built PBF-LB/M AlSi10Mg material by a low-temperature heat treatment (265 °C for 1 h) is studied by means of X-ray and neutron diffraction. Since the specimens are manufactured using a baseplate heated up to 200 °C, low RS are found in the as-built condition. After heat treatment a redistribution of the RS is observed, while their magnitude remains constant. It is proposed that the redistribution is induced by a repartition of stresses between the α-aluminium matrix and the silicon phase, as the morphology of the silicon phase is affected by the heat treatment. A considerable scatter is observed in the neutron diffraction RS profiles, which is principally correlated to the presence (or absence) of pockets of porosity developed at the borders of the chessboard pattern.
... The microstructure in LPBF parts differs significantly from that in the bulk components manufactured via traditional manufacturing processes (Avateffazeli et al. 2022). A typical fish scale pattern shown in Figure 7(b) consists of dual-half elliptical melt pools aligned along the building direction (Bosio et al. 2021). Figure 7(c) displays the crystal morphology along with the crystallographic orientation in two perpendicular views for an AlSi10Mg LPBF part. ...
Article
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Laser powder bed fusion (LPBF) is a promising additive manufacturing technique that allows layer-by-layer fabrication of metallic powders. Distinguished thermal dynamics result in the microstructure of intrinsic features. This review aims to provide a thorough insight into the solidification fundamentals and the microstructural tailoring during LPBF. It begins with the introduction of the LPBF and the challenges of its applications with aluminium alloys. The thermal dynamics and its influence on the microstructure in LPBF were thereafter discussed. The attempts to tailor microstructures by refinement in LPBF fabrication of various aluminium alloys were summarised. Finally, the review provides a conclusive remark on the microstructural controlling and an outlook on the remaining challenges and potential research topics in LPBF with Al alloys.
... This combination leads as-built LPBF AlSi10Mg to exhibit higher yield strength than conventional cast materials. Not that baseplate preheating (at 150-200 • C) during manufacturing is effectively used to prevent cracks and minimize distortions [35]. Another strategy to avoid distortion is by performing a stress relief heat treatment (300 • C for 2 h) prior to the part removal from the baseplate. ...
Article
For the first time, synchrotron X-ray refraction radiography (SXRR) has been paired with in-situ heat treatment to monitor microstructure and porosity evolution as a function of temperature. The investigated material was a laser powder bed fusion (LPBF) manufactured AlSi10Mg, where the initial eutectic Si network is known to break down into larger particles with increasing temperature. Such alloy is also prone to thermally induced porosity (TIP). We show that SXRR allows detecting the changes in the Si-phase morphology upon heating, while this is currently possible only using scanning electron microscopy. SXRR also allows observing the growth of pores, usually studied via X-ray computed tomography, but on much smaller fields-of-view. Our results show the great potential of in-situ SXRR as a tool to gain in-depth knowledge of the susceptibility of any material to thermally induced damage and/or microstructure evolution over statistically relevant volumes.
... The specimens at the bottom level (close to the substrate plate) experience longer thermal load, both from the substrate plate at preheating temperature T Preheat = 200°C and the laser beam energy input of higher layers of the part. In the work of Bosio et al. 29 it could be shown that longer build time for AlSi10Mg alloy can result in overageing of the microstructure in areas close to the substrate plate. This, in turn, resulted in lower hardness for longer build jobs and therefore could explain the improved mechanical performance at the top level of samples within this study. ...
Article
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By using additive manufacturing techniques like the laser powder bed fusion (LPBF) process, parts can be manufactured with high material efficiency because unfused powder material can be reconditioned and reused in consecutive manufacturing jobs. Nevertheless, process by-products like spatters may influence the powder quality and hence alter the mechanical properties/performance of parts. In order to investigate these dependencies, a methodology and a standard build job for the recycling behavior of the lightweight aluminum alloy AlSi10Mg was developed and built with ageing powder in 10 consecutive jobs with no refreshing between the cycles. The powder properties and mechanical performance of parts at static load for two build directions (horizontally and vertically to substrate plate) was evaluated. The influence of build height effects on mechanical performance was investigated as well. The findings may indicate that the coarsening of the powder material during recycling could lead to improved mechanical properties for the AlSi10Mg alloy.
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Recent efforts and advances in additive manufacturing (AM) on different types of new materials are presented and reviewed. Special attention is paid to the material design of cladding layers, the choice of feedstock materials, the metallurgical behavior and synthesis principle during the AM process, and the resulted microstructures and properties, as well as the relationship between these factors. Thereafter, the trend of development in the future is forecasted, including: Effects of the particles size and size distribution of powders; Approaches for producing fine microstructures; Opportunities for creating new materials by AM; Wide applications in reconditioning of damaged components; Challenges for deep understanding and applications of the AMed new materials. The idea of “Develop Materials” or “Create Materials” by AM is highlighted, but a series of scientific, technological and engineering problems remain to be solved in future.
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Precipitation hardening of selective laser melted AlSi10Mg was investigated in terms of solution heat treatment and aging duration. The influence on the microstructure and hardness was established, as was the effect on the size and density of Si particles. Although the hardness changes according to the treatment duration, the maximum hardening effect falls short of the hardness of the as-built parts with their characteristic fine microstructure. This is due to the difference in strengthening mechanisms.
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A357 samples were realized by laser powder bed fusion (LPBF) on building platforms heated up to different temperatures. The effect of the preheating temperature and of the post processing heat treatment on the microstructure and the mechanical properties of the samples was studied. It was demonstrated that building platform heating can act as an in situ ageing heat treatment following the fast cooling that arises during laser scanning. A 17% higher ultimate tensile strength was achieved by the selection of the optimum building platform temperature. Moreover, the possibility to further increase the mechanical properties by means of a direct ageing heat treatment was investigated.
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In this work a new heat treatment specific for selective laser melted (SLM) AlSi10Mg products is studied. Samples were analyzed by differential scanning calorimetry (DSC) and scanning electron microscopy (SEM); two exothermic phenomena were recognized, kinetically analyzed and associated to the precipitation of Mg2Si and to the rupture and spheroidization of the silicon network, which is characteristic of SLM built parts. Thermal treatments, based on the results of calorimetric analyses, were defined and applied to SLM built samples. The evolution of the silicon network morphology was confirmed by SEM observations and corresponding hardness variations were correlated to the treatment conditions. Dedicated thermal treatment is therefore proposed as a lower temperature alternative to the usually applied post – building annealing.
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Additive Manufacturing (AM), the layer-by layer build-up of parts, has lately become an option for serial production. Today, several metallic materials including the important engineering materials steel, aluminium and titanium may be processed to full dense parts with outstanding properties. In this context, the present overview article describes the complex relationship between AM processes, microstructure and resulting properties for metals. It explains the fundamentals of Laser Beam Melting, Electron Beam Melting and Laser Metal Deposition, and introduces the commercially available materials for the different processes. Thereafter, typical microstructures for additively manufactured steel, aluminium and titanium are presented. Special attention is paid to AM specific grain structures, resulting from the complex thermal cycle and high cooling rates. The properties evolving as a consequence of the microstructure are elaborated under static and dynamic loading. According to these properties, typical applications are presented for the materials and methods for conclusion.
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The aim of this work is to investigate the effects of Selective Laser Melting (SLM) on the microstructure and mechanical properties of A357 aluminium alloys. The SLM processing parameters were optimised to achieve maximum density, corresponding to an extremely fine microstructure with very few pores. This translates to differences in mechanical properties compared to conventional cast alloy. Porosity in SLMed A357 Al samples was analysed based on relative density versus laser parameter and energy input curves. Substrate temperatures and the combination of laser parameters influence the mechanical properties via changes in melt pool morphology and eutectic Si cell characteristics. The anisotropy of SLMed Al samples is explained in relation to the directionality of the microstructure based on differences in the deformation response of horizontal and vertical tensile samples. Fractographic studies have been performed to understand tensile properties by comparing fracture surfaces of tensile samples with microstructural features in different planes. This has led to an explanation of why the tensile properties are better for the horizontal test samples than for the vertical ones in an as-SLMed material.
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The study looks into the impact of thermal post-processing using Hot Isostatic Pressing (HIPping) and/or T6-peak aging treatment, post-process machining, as well as the build orientation on the microstructural and mechanical properties development in AlSi10Mg alloy fabricated using Selective Laser Melting (SLM). The builds contained fine columnar grains, with a fine Si-enriched cellular dendritic network, resulting in tensile strengths exceeding the castings. To elucidate the as-fabricated microstructure and strength, thermal modelling was employed, predicting cooling rates of 105-106°C/s.
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Selective laser melting (SLM) of aluminium is of research interest because of its potential benefits to high value manufacturing applications in the aerospace and automotive industries. In order to demonstrate the credibility of SLM Al parts, their mechanical properties need to be studied. In this paper, the nano-, micro-, and macro-scale mechanical properties of SLM AlSi10Mg were examined. In addition, the effect of a conventional T6-like heat treatment was investigated and correlated to the generated microstructure. Nanoindentation showed uniform hardness within the SLM material. Significant spatial variation was observed after heat treatment due to phase transformation. It was found that the SLM material's micro-hardness exceeded its die-cast counterpart. Heat treatment softened the material, reducing micro-hardness from 125±1 HV to 100±1 HV. An ultimate tensile strength (333 MPa), surpassing that of the die cast counterpart was achieved, which was slightly reduced by heat treatment (12%) alongside a significant gain in strain-to-failure (∼threefold). Significantly high compressive yield strength was recorded for the as-built material with the ability to withstand high compressive strains. The SLM characteristic microstructure yielded enhanced strength under loading, outperforming cast material. The use of a T6-like heat treatment procedure also modified the properties of the material to yield a potentially attractive compromise between the material's strength and ductility making it more suitable for a wider range of applications and opening up further opportunities for the additive manufacturing process and alloy combination.
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Optimum laser irradiation conditions to achieve dense A356.0 (AlSi7Mg0.3) specimens fabricated by selective laser melting (SLM) were studied. The SLM specimens with relative density of 99.8% were obtained. The microstructures and mechanical properties of the dense SLM specimens fabricated under the optimum laser irradiation conditions were also investigated. The dense SLM specimens had an ultimate tensile strength, yield strength, and breaking elongation of 400. MPa, 200. MPa, and 12-17%, respectively. All of these values considerably exceeded those in cast-melting materials. The superior mechanical properties of the SLM specimens could be attributed to fine dendritic cell microstructures and relative density of almost 100%. An investigation of the heat treatment effects on the microstructures and mechanical properties revealed a clear difference in annealing behaviors between the SLM specimens and the cast-melting materials. After annealing (T5), the breaking elongation of the SLM specimen increased from 15% in the as-fabricated specimen to 30% in the specimen annealed at 350. °C, while the ultimate tensile strength and yield strength decreased from 400. MPa and 250. MPa to 200. MPa and 125. MPa, respectively. This confirmed that the mechanical properties of the SLM specimens could be controlled in accordance with required functions for design.
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During selective laser melting, the irradiated material experiences large temperature fluctuations in a short time which causes unwanted thermal stresses. In order to assess thermal stresses in a simple and fast way, a new pragmatic method is developed, namely the bridge curvature method. The bridge curvature method is used to assess and qualitatively compare the influence of different laser scan patterns, laser parameter settings and more fundamental process changes on residual stresses. The results from the experiments, as well as the findings from literature, lead to two general conclusions: changes that reduce the high temperature gradient, like using short scan vectors and preheating of the base plate, reduce the thermal stresses. And, thermal stresses in a particular direction can be reduced by optimal choice of the orientation of scan vectors. The experiments indicate the reliability of the bridge curvature method. Statistical analysis is used to check the repeatability of the method and to quantify the uncertainties during measurement.
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In order to produce serial parts via additive layer manufacturing, the fatigue performance can be a critical attribute. In this paper, the microstructure, high cycle fatigue (HCF), and fracture behavior of additive manufactured AlSi10Mg samples are investigated. The samples were manufactured by a particular powder-bed process called Selective Laser Melting (SLM) and machined afterwards. 91 samples were manufactured without (30 °C) and with heating (300 °C) of the building platform and in different directions (0°, 45°, 90°). Samples were tested in the peak-hardened (T6) and as-built condition. The Wöhler curves were interpolated by a Weibull distribution. The results were analysed statistically by design of experiments, correlation analysis, and marginal means plots. The investigations show that the post heat treatment has the most considerable effect and the building direction has the least considerable effect on the fatigue resistance. The fatigue resistance of the samples, however, is high in comparison to the standard DIN EN 1706. The combination of 300 °C platform heating and peak-hardening is a valuable approach to increase the fatigue resistance and neutralize the differences in fatigue life for the 0°, 45°, and 90° directions.
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Selective laser melting (SLM) process allows fabricating strong, lightweight and complex metallic structures. To successfully produce metallic parts by SLM, additional structures are needed to support overhanging surfaces in order to dissipate process heat and to minimize geometrical distortions induced by internal stresses. However, these structures are often massive and require additional post-processing for their removal. A minimization of support structures would therefore significantly reduce manufacturing and finishing efforts and costs. This study investigates the manufacturability of overhanging structures using optimized support parts. An experimental study was performed to identify the optimal self-supporting overhanging structures using Taguchi L-36 design. Experimental results revealed that with optimized supports it is possible to build non-assembly mechanism with overhang surfaces. However, it is necessary to correctly orientate the part in the SLM machine in order to build it with a minimal support structure so to obtain the best trade-off between production time, cost, and accuracy.
Article
Purpose This paper presents an investigation into residual stresses in selective laser sintering (SLS) and selective laser melting (SLM), aiming at a better understanding of this phenomenon. Design/methodology/approach First, the origin of residual stresses is explored and a simple theoretical model is developed to predict residual stress distributions. Next, experimental methods are used to measure the residual stress profiles in a set of test samples produced with different process parameters. Findings Residual stresses are found to be very large in SLM parts. In general, the residual stress profile consists of two zones of large tensile stresses at the top and bottom of the part, and a large zone of intermediate compressive stress in between. The most important parameters determining the magnitude and shape of the residual stress profiles are the material properties, the sample and substrate height, the laser scanning strategy and the heating conditions. Research limitations/implications All experiments were conducted on parts produced from stainless steel powder (316L) and quantitative results cannot be simply extrapolated to other materials. However, most qualitative results can still be generalized. Originality/value This paper can serve as an aid in understanding the importance of residual stresses in SLS/SLM and other additive manufacturing processes involving a localized heat input. Some of the conclusions can be used to avoid problems associated with residual stresses.
Article
The additive manufacturing process selective laser melting (SLM) can be used to directly produce functional components made out of metal. During the construction process, however, thermally induced residual stress occurs due to the layered build-up and the local input of energy by means of a focused laser beam, which can lead to distortion of the component or sections of the component itself. Normally, distortion is prevented due to supporting structures between the component and the substrate plate. It is not always possible, however, to provide all the areas of a component with supporting structures or to remove them later, depending on how complex the geometry or how accessible the structures are. When the substrate plate is heated during the construction process, the distortion can be reduced or eliminated entirely. Nonetheless, a systematic investigation of the extent to which preheating influences distortion of aluminum components has not yet been conducted. This works aims at systematically investigating the effects of preheating during SLM of aluminum components and determining an appropriate preheating temperature at which distortion practically no longer occurs. A significant reduction in distortion compared to the distortion without preheating can be seen beginning at a preheating temperature of 150 degrees C. At a preheating temperature of 250 degrees C, distortion can no longer be detected within the scope of the measuring accuracy independent of the twin cantilever test geometry investigated. In addition to reducing distortion, the preheating avoids the stress-related cracks in the component, which can lead to tearing of the parts of the test geometry. With 90 HV 0.1 at a preheating temperature of 250 degrees C, the hardness is greater than the required minimum hardness according to DIN EN 1706 of die-cast parts from the material AlSi10Mg. From these results, it can be concluded that a preheating temperature of 250 degrees C is suitable for reliably manufacturing components made out of the material AlSi10Mg using SLM free of defects and for preventing distortion completely. VC 2014 Laser Institute of America.
Article
This study shows that AlSi10Mg parts with an extremely fine microstructure and a controllable texture can be obtained through selective laser melting (SLM). Selective laser melting creates complex functional products by selectively melting powder particles of a powder bed layer after layer using a high-energy laser beam. The high-energy density applied to the material and the additive character of the process result in a unique material structure. To investigate this material structure, cube-shaped SLM parts were made using different scanning strategies and investigated by microscopy, X-ray diffraction and electron backscattered diffraction. The experimental results show that the high thermal gradients occurring during SLM lead to a very fine microstructure with submicron-sized cells. Consequently, the AlSi10Mg SLM products have a high hardness of 127 ± 3 Hv0.5 even without the application of a precipitation hardening treatment. Furthermore, due to the unique solidification conditions and the additive character of the process, a morphological and crystallographic texture is present in the SLM parts. Thanks to the knowledge gathered in this paper on how this texture is formed and how it depends on the process parameters, this texture can be controlled. A strong fibrous 〈1 0 0〉 texture can be altered into a weak cube texture along the building and scanning directions when a rotation of 90° of the scanning vectors within or between the layers is applied.
Article
For establishing Selective Laser Melting (SLM) in production technology, an extensive knowledge about the transient physical effects during the manufacturing process is mandatory. In this regard, a high process stability for various alloys, e.g. tool steel 1.2709 (X3NiCoMoTi 18-9-5), is realisable, if approaches for the virtual qualification of adequate process parameters by means of a numerical simulation based on the finite element analysis (FEA) are developed. Furthermore, specific methods to evaluate and quantify the resulting residual stresses and deformations due to the temperature gradient mechanism (TGM) are required. Hence, the presented work contains particular approaches using the FEA for the simulation of transient physical effects within the additive layer manufacturing (ALM) process. The investigations focus on coupled thermo-mechanical models incorporating specific boundary conditions and temperature dependant material properties to identify the heat impact on residual stresses and deformations. In order to evaluate the structural effects and simultaneously validate the simulation, analysis on residual stresses based on the neutron diffractometry as well as considerations concerning part deformations are presented.
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