No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Influence of the particle size distribution on surface quality of Maraging 300 parts produced by Laser Powder Bed Fusion
Abstract
This work investigates the effect of the powder particle size distribution on the surface finish of Maraging 300 specimens, produced by the Laser Powder Bed Fusion (LPBF) process. Although it is recognized that the initial powder morphological characteristics play an important role on LPBF part density, mechanical properties and surface quality, there is a lack of empirical data that could help to link the powder properties to actual metrological established surface parameters, like Ra or Sa. For this reason, an extensive initial powder characterization is presented in this paper, for three Maraging 300 batches, and first insights on the different obtained LPBF surface quality are disclosed. The results demonstrate how small differences in particle size distribution can decrease the LPBF surface roughness consistently. Moreover, it is shown how the use of fine powder can unlock novel LPBF processing strategies to further improve surface finish, down to 1.5 µm measured Ra, and thus reduce eventual post-processing efforts.
... The conventional LPBF process is widely adopted in industry, however, it comes with a number of stringent requirements with respect to the feedstock powder. Sinico et al. [2] observed that depositing Maraging steel powder particles between 5-15 µm into thin and homogeneous layers was impossible due to inter-particle Van Der Waals forces leading to agglomeration. Balbaa et al. [3] investigated the importance of particle size of AlSi10Mg alloy powder and found that the finer powder fractions (d 50 of 15 µm) had a lower flowability and packing density than coarser fractions, leading to less dense final components. ...
... Powder layer movability, such as spattering and denudation, contributes to defect formation and quality uncertainty during the process, making the powder feedstock intrinsic to stochastic characteristics in L-PBF [46]. The powder feedstock particle size and its interaction with the layer thickness influence various quality aspects, including density, accuracy, internal defects, surface quality, and mechanical and fatigue properties [47][48][49][50][51][52][53][54][55][56][57]. Powder flowability/rheology significantly affects the formation of a homogenous powder layer, with improved flowability observed for coarser powder particles and reduced flowability with an increase in moisture content up to the saturation point [47][48][49][50]. ...
Thin features are integral components of most lightweight cellular lattice structures; however, limited studies are carried out to understand their property/quality aspects. This experimental study investigates the influence of powder feedstock size, feature geometry, and process parameters on the property/quality of thin lightweight features fabricated using a laser powder bed fusion additive manufacturing (L-PBF-AM) system. Three different Ti6Al4V powder feedstocks (fine, medium, and coarse) were utilized, and the dimensions of the features were varied in the range of 0.1 to 0.5 mm. The experimental data were analyzed to gain insights into the powder feedstock-geometry-process-property/quality (PGPP/PGPQ) characteristics, which had not been previously explored but are crucial for designing lightweight structures using L-PBF-AM. The results indicated that both powder feedstock size and feature dimension significantly influenced the properties and quality of the fabricated thin features. Additionally, feature type, volumetric energy density, and their interactions exhibited varying effects on geometrical accuracy, porosity, grain size, and flexural properties. Struts showed lower success rates, grain sizes, and dimensional errors but higher mechanical properties compared to walls. However, both features exhibited similar porosity characteristics. Regarding powder feedstock size, smaller powder sizes were found to be advantageous for fabricating lower-dimensional features and improving their mechanical properties. The feature geometry type also significantly influences the final material properties. A notable observation was that the 0.1 mm wall features exhibited the lowest mechanical properties, particularly in terms of yield strength, while the 0.5 mm strut features exhibited lower mechanical properties among the struts. The findings of the study underscored the importance of understanding the compound relationships between powder feedstock, feature geometry, process parameters, and the resulting properties/quality for lightweight features in L-PBF-AM. Further research is necessary to establish the knowledge and understanding of L-PBF-AM thin features and elucidate the PGPP/PGPQ characteristics in greater detail.
... Since the relative density depends on laser power, scan speed, thickness, and hatching space, this can be expressed by the volumetric energy density. This energy is the ratio of the laser power to the product of the speed, thickness, and hatching space [16], as described in equation 4. Figure 7 illustrates the evolution of the relative density in terms of volumetric energy density (VED). Results show that the energy density proportionally increases with the volumetric energy density. ...
Selective laser melting (SLM) presents significant assets for both industrial and academic fields. However, the process parameters selection is yet challenging. It presents tens of parameters to be carefully selected, including laser power and speed, bed thickness, hatching space, and other parameters, for the manufacturing of parts with high density. This paper provides a deeper understanding of the processing parameters’ effect on the evolution of the product’s density. A series of numerical simulations of porosity is achieved on Ansys Additive© software and it shows the evolution of the relative density at different laser powers and scan speeds. Numerical results show that low laser power and accelerated scan lead to the generation of a small melt pool, and consequently low density. In the opposite case, at high power and slow scan, the created melt pool is wide enough to avoid porosity and generate fully dense products. The product density is proportionally related to the melt pool size. Hence, it could be estimated through the correlation with the melt pool width, which enables the perfect selection of the hatching space for the selected set of parameters.
... Feedstock powder characteristics such as particle size and its interplay with layer thickness plays an important role in the PBF process characteristic and is related to various part quality aspects such as density, surface quality, accuracy, internal defects, static mechanical properties and fatigue properties [16][17][18][19][20][21][22][23][24][25][26]. During the process of PBF, powder flowability/rheology has significant effect on the formation of homogenous powder layer. ...
In this work, three different types of Ti6Al4V powder feedstock of different particle size ranges (fine, medium, and coarse) were utilized to fabricate thin strut lightweight features using laser powder bed fusion additive manufacturing (L-PBF-AM) using different process parameter settings. Thin strut features of varying dimensions from 0.1mm to 0.5mm were fabricated. The resulting sample sets allow for the analysis of the compound powder feedstock-process-geometry-material (PPG-M) characteristics for lightweight features fabricated by L-PBF-AM, which have not been previously explored. Various material characteristics were experimentally determined and analyzed, including success rate, geometry quality, porosity, pore size, grain size, and mechanical properties of the lightweight thin strut samples. The results clearly demonstrated the significance of the compound PPG-M relationships for lightweight structures, which calls for further studies to "re-establish" the knowledge base for L-PBF-AM materials at small dimension scales.
... Spierings et al. (2011) reported that surface roughness value and porosity of SLM 316L steel were reduced when using small powders. Sinico et al. (2019) found that the use of fine powders could result in the reduction of surface roughness (Ra) down to 1.5 µm when they fabricated maraging steel using SLM. On the other hand, due to the low thickness, the remelted part of the previous layer will increase, which is beneficial for the improvement of the wettability between the adjacent layers. ...
The performance of the selective laser melting (SLM) parts was critically affected by the surface quality and internal defects that are closely related to process parameters. An in-depth understanding of the relationship between the formation and evolution of surface and internal defects and process parameters is needed to achieve defect-free and high-performance SLMed parts. In this study, the influencing mechanism of laser power, scanning speed, hatch spacing and layer thickness on melt pool morphology, surface quality and internal hole defect of SLMed 18Ni300 maraging steel was investigated. The thermal and physical behaviour and instability of the molten pool, as well as the formation and distribution behaviour of internal hole defects, were also analyzed and discussed. Recoil pressure, the insufficient overlap between tracks and remelting between layers, Plateau-Rayleigh instability and material aggregation caused by the Marangoni effect were characterized as the main factors closely related to molten pool morphology and surface quality. Within the selected parameters in this study, the obtained surface roughness and tensile strength range from 9.08–26.40 μm and 544.14–1246.24 MPa, respectively. The internal defect changes from irregular lack-of-fusion at low energy density to the keyhole-included spherical hole at high energy density. In addition, the volumetric energy density (VED) has a certain limitation in predicting surface quality and mechanical properties due to the complex physical characteristics of the molten pool.
... The samples were built from maraging steel 300 with a nominal particle size distribution of 15-45 µm. A detailed study on the morphology of the powder used in this study was reported by Sinico et al. (2019). The building param- Table 1 were optimized for a maximal part density (98.9 ± 0.1%), assessed following the Archimedes principle with a theoretical density of 8.1 g/cm 3 . ...
Additive manufacturing techniques such as laser powder bed fusion (LPBF) enable production of components with highly complex geometries. However, their as-built surface quality and geometrical precision remain insufficient for high-end applications, requiring further post-processing. Unlike horizontal surfaces, inclined up-facing surfaces cannot be improved by in situ remelting, as they remain covered with powder. In this research a dual laser setup replaces the traditional single-laser layout. Surface enhancement is achieved in two steps: a nanosecond pulsed laser is used to remove powder from selected areas via laser-induced shock waves (LISW), so that these surfaces can be remelted with a continuous wave laser. This work presents an investigation of in situ remelting (laser polishing) of inclined surfaces in their as-built orientation, after powder removal by brushing or using LISW. Particular focus is given on the effect of process parameters (laser power, scanning speed, number of scans) on the surface and subsurface quality and dimensional accuracy. As a result, this in situ processing leads to a comparable surface texture on horizontal and inclined surfaces, without introducing any major subsurface defects.
... The material selected for this study is gas atomized maraging steel 300 with a nominal particle size distribution 15-45 µm. The powder particles morphology was investigated within a prior study [19] and the chemical composition is provided in Table 1. The samples were built either using a simple "fill" scanning strategy (Fig. 2b) or "contours and fill" strategy (Fig. 2c). ...
Considerable surface texture is one of the typical drawbacks of laser powder bed fusion (LPBF). Especially up-facing inclined surfaces suffer from insufficient quality, due to the combination of the staircase effect (also called stair-stepping effect), attached powder particles and elevated edges (edge effect). In this work, the contribution of the edge effect to the topography of up-facing inclined surfaces is investigated in detail for the first time. Moreover, the effect of contour scanning was evaluated on up-facing surfaces over the whole range of inclinations, from horizontal to vertical. The edge effect was found to play a dominant role for surfaces with a low inclination angle, especially when contour scanning is used. A suitable scanning strategy can thus be determined for each surface inclination. In the present study, an optimization of scanning strategy allowed a significant improvement of the inclined surface quality. The arithmetical mean height of the roughness profile Ra could be reduced up to 52% for low inclinations, 20% for high inclinations and 32% for vertical walls.
... To reduce the cost of the powder, research is being conducted on the spheroidization of such powders. When fusing powders of various particle size distribution, it may also be necessary to select fusion modes that is discussed in a number of papers [1][2][3][4][5][6][7][8]. For example, [9] and [10] analyzed the effect of particle size on the absorption of laser radiation by the surface of a powder layer: relationship between the absorption, distribution of absorbed irradiance within the powder layers, and surface morphology and geometric characteristics (e.g., contact angle, width and height of tracks, and remelted depth) of the laser scanning tracks. ...
The paper studies effects of particle size distribution on the structure and mechanical properties of monolithic samples obtained by L-PBF. A powder of 321 austenitic stainless steel of one batch was divided into three fractions 0–20, 20–40, and 0–40 μm. It was established that for narrow fractional powder composition, hardness anisotropy is observed that depends on the building direction, whereas for wide fractional powder composition, hardness anisotropy is practically absent. It was found that the particle size composition of AISI 321 steel powder does not fundamentally affect the morphology of the grain structure. Despite the general preferred orientation of the {101} planes, a weak effect of the powder composition on the crystallites orientation is observed.
... The voids between the particles lead to less material available for ablation, plasma formation and shock wave generation in the beam interaction zone. Sinico et al. (2019) reported an apparent density of 52.3% ± 0.3% compared to the bulk material density, analyzing the same material and particle size distribution. In addition, the argon gas present in the voids can reduce the heat transfer to the surrounding particles and the heat can be partially removed with the gas flow. ...
Insufficient surface quality is one of the common issues encountered in laser powder bed fusion (LPBF). In order to meet the industrial requirements, it is typically necessary to proceed to expensive and time-consuming post-processing. Improving surface quality during the building process would hence be very beneficial. However, in situ remelting (laser polishing) is possible only for horizontal up-facing surfaces. The quality of inclined surfaces remains limited to the as-built state, as after building they are covered with loose powder, excluding the possibility for in situ polishing. This work presents a novel strategy to improve the quality of up-facing inclined surfaces using a dual-laser setup with a continuous wave and a pulsed laser. This approach consists in two steps: after building, the powder is selectively removed from the inclined surfaces using shock waves generated by a nanosecond pulsed laser. These newly exposed surfaces can be subsequently remelted with a continuous wave laser or alternatively treated with a pulsed laser. This paper discusses various scanning strategies for selective powder removal, their efficiency and the surface quality of the treated parts. The initial results are promising and indicate that the surface texture of LPBF parts in maraging steel 300 can be significantly improved for surface inclinations up to 45°, provided a sufficient powder removal efficiency is reached. The latter was found to be dependent on the maximal depth and volume of the powder to be removed, but also on part geometry.
... It is possible to identify an optimal dimensional distribution for each material, for achieving minimum residual porosity, good flowability and better final part properties. This is usually a balance between smaller and larger dimensions, the smaller particles beign able to fill the voids left between the bigger particles that cover the platform relatively quickly and uniformly [51,52]. ...
In this PhD project at DTU MEK, Mandaná Moshiri investigated how to define an integrated process chain for first time production of mould components to be used for plastic injection moulding using laser powder bed fusion metal additive manufacturing (AM) technologies. This technology is already used for fabricating injection moulding components, but its maturity and readiness for full-scale industrial applications it is still far from reality. Mandaná articulated her project over five main topics, each addressing a specific aspect for the major development of AM and its full adoption in the manufacturing industry. The topics started from a clear assessment of the advantages of using AM over conventional manufacturing in terms of technologies required and production cost impact, followed by an AM machines benchmarking to understand what are the current capabilities and limitations of AM. The technology gap between what AM can deliver and what it is required by injection moulding industrial applications was defined, exploring also new way of exploiting AM products for mould inserts, beyond the well-known enhancement of thermal management. In the last two topics, the systems and research areas for creating an integrated first-time-right process chain were analysed in the context of the Industry 4.0 framework including key-enabling technologies such as monitoring and simulation.
... Multiple studies have been published regarding additively manufactured maraging steel: fully dense parts made from the laser processing of 18Ni-300 powders were accomplished by Stanford et al. on an EOS M250 extended platform [13] and the effects that powder size and printing parameters (e.g., scan speed and layer thickness) have on the mechanical properties and microstructure of 18Ni-300 studied by Yasa et al., Kempen et al. and others [5,6,[14][15][16]. Jägle et al. investigated the properties of heat-treated 18Ni-300 and found three Ni-based precipitates form and observed austenite reversion after aging [17]. ...
Changes in the mechanical properties of selective laser melted maraging steel 300 induced by exposure to a simulated marine environment were investigated. Maraging steel samples were printed in three orientations: vertical (V), 45° (45), and horizontal (H) relative to the print bed. These were tested as-printed or after heat-treatment (490 °C, 600 °C, or 900 °C). One set of specimens were exposed in a salt spray chamber for 500 h and then compared to unexposed samples. Environmental attack induced changes in the microstructural features and composition were analyzed by scanning electron microscopy and energy dispersive spectroscopy respectively. Samples printed in the H and 45° directions exhibited higher tensile strength than those printed in the V direction. Corrosion induced reduction in strength and hardness was more severe in specimens heat-treated between 480 °C and 600 °C versus as-printed samples. The greatest decrease in tensile strength was observed for the 45°-printed heat-treated samples after exposure. A comparison between additive and subtractive manufactured maraging steel is presented.
... Besides ensuring that you get what you want, PAM 2 also aims to push the limits in terms of precision. As a result, low surface roughness [8,9], reduced edge effects [10] and high-precision CT techniques [11] are obtained. ...
Numerous challenges of additive manufacturing (AM) are tackled in the European Horizon 2020 project PAM^2 by studying and linking every step of the AM process cycle. For example, PAM^2 researchers from the design, processing and application side have collaborated in this work to optimise the manufacturability of metal AM parts using an improved Topology Optimisation (TO) approach, including a thermal constraint. Additionally, the project is focusing on modelling, post-processing, in- and post-process quality control and industrial assessment of AM parts, with the aim of moving beyond the state-of-the-art of precision metal AM. || Professional oriented publication for Mikroniek Issue 5 - 2019 (https://www.dspe.nl/mikroniek/archive).
The use of ultrashort-pulsed (USP) lasers in Additive Manufacturing (AM) enables the processing of different materials and has the potential to reduce the sizes and shapes manufactured with this technology. This work confirms that USP lasers are a viable alternative for Laser Powder Bed Fusion (LPBF) when higher precision is required to manufacture certain critical parts. Promising results were obtained using tailored and own-produced stainless steel powder particles, manufacturing consistent square layers with a series of optimized processing parameters. The critical role of processing parameters is confirmed when using this type of lasers, as a slight deviation of any of them results in an absence of melting. For the first time, melting has been achieved at low pulse repetition (500 kHz) and using low average laser power values (0.5–1 W), by generating heat accumulation at reduced scanning speeds. This opens up the possibility of further reducing the minimum size of parts when using USP lasers for AM.
Electron beam powder bed fusion (EB-PBF) has been given much attention in recent years for its potential in the aerospace and medical industries, particularly with Ti-6Al-4V. However, these processes produce parts with inherent rough surface finishes, impacting the performance and generating additional challenges and costs with surface post-processing. This review examines the primary mechanisms responsible for surface roughness generation in EB-PBF and the process variables that have been verified to influence the surface quality of Ti-6Al-4V parts. The challenges in surface metrology of metallic PBF parts are also discussed, as are new perspectives and guidelines for future research.
In the field of metal additive manufacturing (AM), one of the most used methods is selective laser melting (SLM)—building components layer by layer in a powder bed via laser. The process of SLM is defined by several parameters like laser power, laser scanning speed, hatch spacing, or layer thickness. The manufacturing of small components via AM is very difficult as it sets high demands on the powder to be used and on the SLM process in general. Hence, SLM with subsequent micromilling is a suitable method for the production of microstructured, additively manufactured components. One application for this kind of components is microstructured implants which are typically unique and therefore well suited for additive manufacturing. In order to enable the micromachining of additively manufactured materials, the influence of the special properties of the additive manufactured material on micromilling processes needs to be investigated. In this research, a detailed characterization of additive manufactured workpieces made of AISI 316L is shown. Further, the impact of the process parameters and the build-up direction defined during SLM on the workpiece properties is investigated. The resulting impact of the workpiece properties on micromilling is analyzed and rated on the basis of process forces, burr formation, surface roughness, and tool wear. Significant differences in the results of micromilling were found depending on the geometry of the melt paths generated during SLM.
A nearly fully dense grade 300 maraging steel was fabricated by selective laser melting (SLM) additive manufacturing with optimum laser parameters. Different heat treatments were elaborately applied based on the detected phase transformation temperatures. Microstructures, precipitation characteristics, residual stress and properties of the as-fabricated and heat-treated SLM parts were systematically characterized and analyzed. The observed submicron grain size (0.31μm on average) suggests an extremely high cooling rate up to 107 K/s. Massive needle-shaped nanoprecipitates Ni3X (X=Ti, Al, Mo) were clearly present in the martensitic matrix, which accounts for the age hardening. The interfacial relations between the precipitate and matrix are revealed by electron microscopy and illustrated in detail. Strengthening mechanism is explained by Orowan bowing mechanism and coherency strain hardening. Building orientation based mechanical anisotropy, caused by “layer-wise effect”, is also investigated in as-fabricated and heat-treated specimens. The findings reveal that heat treatments not only induce strengthening, but also significantly relieve the residual stress and slightly eliminate the mechanical anisotropy. In addition, comprehensive performance in terms of Charpy impact test, tribological performance, as well as corrosion resistance of the as-fabricated and heat-treated parts are characterized and systematically investigated in comparison with traditionally produced maraging steels as guidance for industry applications.
The powder bed fusion additive manufacturing process enables fabrication of metal parts with complex geometry and elaborate internal features, the simplification of the assembly process, and the reduction of development time; however, its tremendous potential for widespread application in industry is hampered by the lack of consistent quality. This limits its ability as a viable manufacturing process particularly in the aerospace and medical industries where high quality and repeatability are critical. A variety of defects, which may be initiated during powder bed fusion additive manufacturing, compromise the repeatability, precision, and resulting mechanical properties of the final part. One approach that has been more recently proposed to try to control the process by detecting, avoiding, and/or eliminating defects is online monitoring. In order to support the design and implementation of effective monitoring and control strategies, this paper identifies, analyzes, and classifies the common defects and their contributing parameters reported in the literature, and defines the relationship between the two. Next, both defects and contributing parameters are categorized under an umbrella of manufacturing features for monitoring and control purposes. The quintuple set of manufacturing features presented here is meant to be employed for online monitoring and control in order to ultimately achieve a defect-free part. This categorization is established based on three criteria: (1) covering all the defects generated during the process, (2) including the essential contributing parameters for the majority of defects, and (3) the defects need to be detectable by existing monitoring approaches as well as controllable through standard process parameters. Finally, the monitoring of signatures instead of actual defects is presented as an alternative approach to controlling the process “indirectly.”
The surface texture of additively manufactured metallic surfaces made by powder bed methods is affected by a number of factors, including the powder’s particle size distribution, the effect of the heat source, the thickness of the printed layers, the angle of the surface relative to the horizontal build bed and the effect of any post processing/finishing. The aim of the research reported here is to understand the way these surfaces should be measured in order to characterise them. In published research to date, the surface texture is generally reported as an Ra value, measured across the lay. The appropriateness of this method for such surfaces is investigated here. A preliminary investigation was carried out on two additive manufacturing processes—selective laser melting (SLM) and electron beam melting (EBM)—focusing on the effect of build angle and post processing. The surfaces were measured using both tactile and optical methods and a range of profile and areal parameters were reported. Test coupons were manufactured at four angles relative to the horizontal plane of the powder bed using both SLM and EBM. The effect of lay—caused by the layered nature of the manufacturing process—was investigated, as was the required sample area for optical measurements. The surfaces were also measured before and after grit blasting.
Selective Laser Melting is an efficient process for producing metal parts with minimal subtractive post-processing required. Analysis of the parameters controlling the part quality has been performed focussing on the energy intensity during processing and the effect of the particle size distribution on factors such as ultimate tensile strength and surface finish. It is shown that the controlling the energy intensity is key to quality and can be affected by varying, for example, laser beam diameter or the scanning rate.
Selective Laser Melting (SLM) is an Additive Manufacturing process in which a part is built in a layer by layer manner. A laser source selectively scans the powder bed according to the CAD data of the part to be produced. The high intensity laser beam makes it possible to completely melt the metal powder particles to obtain almost fully dense parts. In this work, the influence of process parameters in SLM (e.g. scan speed and layer thickness) and various age hardening treatments on the microstructure and mechanical properties of 18Ni-300 steel is investigated. It is shown that almost fully dense parts with mechanical properties comparable to those of conventionally produced maraging steel 300 can be produced by SLM.
Metal Additive Manufacturing (AM) has begun its revolution in various high value industry sectors through enabling design freedom and alleviating laborious machining operations during the production of geometrically complex components. The use of powder bed fusion (PBF) techniques such as Selective Laser Melting (SLM) also promotes material efficiency where unfused granular particles are recyclable after each forming operation in contrast to conventional subtractive methods. However, powder characteristics tend to deviate from their pre-process state following different stages of the process which could affect feedstock behaviour and final part quality. In particular, primary feedstock characteristics including granulometry and morphology must be tightly controlled due to their influence on powder flow and packing behaviour as well as other corresponding attributes which altogether affect material deposition and subsequent laser consolidation. Despite ongoing research efforts which focused strongly on driving process refinement steps to optimise the SLM process, it is also critical to understand the level of material sensitivity towards part forming due to granulometry changes and tackle various reliability as well as quality issues related to powder variation in order to further expand the industrial adoption of the metal additive technique. In this review, the current progress of Metal AM feedstock and various powder characteristics related to the Selective Laser Melting process will be addressed, with a focus on the influence of powder granulometry on feedstock and final part properties.
Powder-bed fusion is a class of Additive Manufacturing (AM) processes that bond successive layers of powder to facilitate the creation of parts with complex geometries. As AM technology transitions from the fabrication of prototypes to end-use parts, the understanding of the powder properties needed to reliably produce parts of acceptable quality becomes critical. Consequently, this has led to the use of powder characterisation techniques such as scanning electron microscopy, laser light diffraction, X-ray photoelectron spectroscopy, and differential thermal analysis to study the effect of powder characteristics on part properties. Utilisation of these powder characterisation methods to study particle morphology, chemistry, and microstructure has resulted in significant strides being made towards the optimisation of powder properties. This paper reviews methods commonly used in characterising AM powders, and the effects of powder characteristics on the part properties in powder-bed fusion processes.
Selective Laser Melting (SLM) is an Additive Manufacturing process (AM) that built parts from powder using a layer-by-layer deposition technique. The control of the parameters that influence the melting and the amount of energy density involved in the process is paramount in order to get valuable parts. The objective of this paper is to perform an experimental investigation and a successive statistical optimization of the parameters of the selective laser melting process of the 18Ni300 maraging steel. The experimental investigation involved the study of the microstructure, the mechanical and surface properties of the laser maraging powder. The outcomes of experimental study demonstrated that the hardness, the mechanical strength and the surface roughness correlated positively to the part density. Parts with relative density higher than 99% had a very low porosity that presented closed and regular shaped pores. The statistical optimization determined that the best part properties were produced with the laser power bigger than 90 W and the velocity smaller than 220 mm/s.
Laser powder-bed fusion additive manufacturing of metals employs high-power focused laser beams. Typically, the depth of the molten pool is controlled by conduction of heat in the underlying solid material. But, under certain conditions, the mechanism of melting can change from conduction to so-called “keyhole-mode” laser melting. In this mode, the depth of the molten pool is controlled by evaporation of the metal. Keyhole-mode laser melting results in melt pool depths that can be much deeper than observed in conduction mode. In addition, the collapse of the vapor cavity that is formed by the evaporation of the metal can result in a trail of voids in the wake of the laser beam. In this paper, the experimental observation of keyhole-mode laser melting in a laser powder-bed fusion additive manufacturing setting for 316L stainless steel is presented. The conditions required to transition from conduction controlled melting to keyhole-mode melting are identified.
Selective laser sintering (SLS) is a layered manufacturing process that builds prototypes by selective sintering of materials in powder form, like thermoplastic polymer powder (Polyamide 2200), using a CO2 laser. Prototypes made by SLS are widely used in product development as they can be used for product testing. SLS prototypes, therefore, should have a very good surface finish for functional performance as well as aesthetics. However, prototypes made by the SLS process have comparatively high surface roughness due to the stair stepping effect. Surface roughness of the prototypes also depends on the various process parameters. This paper attempts to study the effect of process parameters, namely build orientation, laser power, layer thickness, beam speed, and hatch spacing, on surface roughness. Central rotatable composite design (CCD) of experiments was used to plan the experiments. Analysis of variance (ANOVA) was used to study the significance of process variables on surface roughness. In the case of upward-facing surfaces, build orientation and layer thickness have been found to be significant parameters. In downward-facing surfaces, other than build orientation and layer thickness, laser power has also been found to be significant. Empirical models have been developed for estimating the surface roughness of the parts. A trust-region-based optimization method (standard module of MATLAB) has been employed to obtain a set of process parameters for obtaining the best surface finish. A confirmation experiment has been carried out at an optimum set of parameters and predicted results were found to be in good agreement with experimental findings. A case study of a standard part 'Truncheon' is also presented.
Purpose
A recent study confirmed that the particle size distribution of a metallic powder material has a major influence on the density of a part produced by selective laser melting (SLM). Although it is possible to get high density values with different powder types, the processing parameters have to be adjusted accordingly, affecting the process productivity. However, the particle size distribution does not only affect the density but also the surface quality and the mechanical properties of the parts. The purpose of this paper is to investigate the effect of three different powder granulations on the resulting part density, surface quality and mechanical properties of the materials produced.
Design/methodology/approach
The scan surface quality and mechanical properties of three different particle size distributions and two layer thicknesses of 30 and 45 μm were compared. The scan velocities for the different powder types have been adjusted in order to guarantee a part density≥99.5 per cent.
Findings
By using an optimised powder material, a low surface roughness can be obtained. A subsequent blasting process can further improve the surface roughness for all powder materials used in this study, although this does not change the ranking of the powders with respect to the resulting surface quality. Furthermore, optimised powder granulations lead generally to improved mechanical properties.
Practical implications
The results of this study indicate that the particle size distribution influences the quality of AM metallic parts, produced by SLM. Therefore, it is recommended that any standardisation initiative like ASTM F42 should develop guidelines for powder materials for AM processes. Furthermore, during production, the granulation changes due to spatters. Appropriate quality systems have to be developed.
Originality/value
The paper clearly shows that the particle size distribution plays an important role regarding density, surface quality and resulting mechanical properties.
Purpose
– Selective laser melting (SLM) is a powder metallurgical (PM) additive manufacturing process whereby a three‐dimensional part is built in a layer‐wise manner. During the process, a high intensity laser beam selectively scans a powder bed according to the computer‐aided design data of the part to be produced and the powder metal particles are completely molten. The process is capable of producing near full density (∼98‐99 per cent relative density) and functional metallic parts with a high geometrical freedom. However, insufficient surface quality of produced parts is one of the important limitations of the process. The purpose of this study is to apply laser re‐melting using a continuous wave laser during SLM production of 316L stainless steel and Ti6Al4V parts to overcome this limitation.
Design/methodology/approach
– After each layer is fully molten, the same slice data are used to re‐expose the layer for laser re‐melting. In this manner, laser re‐melting does not only improve the surface quality on the top surfaces, but also has the potential to change the microstructure and to improve the obtained density. The influence of laser re‐melting on the surface quality, density and microstructure is studied varying the operating parameters for re‐melting such as scan speed, laser power and scan spacing.
Findings
– It is concluded that laser re‐melting is a promising method to enhance the density and surface quality of SLM parts at a cost of longer production times. Laser re‐melting improves the density to almost 100 per cent whereas 90 per cent enhancement is achieved in the surface quality of SLM parts after laser re‐melting. The microhardness is improved in the laser re‐molten zone if sufficiently high‐energy densities are provided, probably due to a fine‐cell size encountered in the microstructure.
Originality/value
– There has been extensive research in the field of laser surface modification techniques, e.g. laser polishing, laser hardening and laser surface melting, applied to bulk materials produced by conventional manufacturing processes. However, those studies only relate to laser enhancement of surface or sub‐surface properties of parts produced using bulk material. They do not aim at enhancement of core material properties, nor surface enhancement of (rough) surfaces produced in a PM way by SLM. This study is carried out to cover the gap and analyze the advantages of laser re‐melting in the field of additive manufacturing.
Effects of process parameters on surface quality of parts produced by selective laser melting -ANFIS modelling
- N A Derahman
- M S Karim
Derahman N A, Karim M S, and Amran N A M 2018 Effects of process
parameters on surface quality of parts produced by selective laser
melting -ANFIS modelling Proceedings of Mechanical Engineering
Research Day 2018 115-116