Powder Bed Research Articles

A Scan Strategy Based Compensation of Cumulative Heating Effects in Electron Beam Powder Bed Fusion

AbstractThe fabrication of complex geometries with uniform material properties in electron beam powder bed fusion (PBF-EB) remains a major challenge. Local material properties in PBF-EB are determined by the local thermal conditions and the spatio-temporal melt pool evolution. The local thermal conditions are governed by the cumulative heating effect on the hatch scale, which results from the superposition of temperature fields from adjacent hatch lines. The build-up of the cumulative heating effect at the beginning of a new hatch segment, without prior hatch lines, which results in regions with underdeveloped thermal conditions, is so far only rarely considered in the design of process strategies. This study introduces a numerical optimization scheme with the objective to minimize the extent of regions with underdeveloped thermal conditions at the beginning of line-based hatches, by means of scan strategy modifications. For this purpose, a simplified thermal solution is combined with an optimization approach to determine an optimal process strategy for line-based PBF-EB of a cuboid model geometry through the adaptation of individual hatch line spacing. Based on the approach determined for the model geometry, a generalized process strategy is derived for complex geometries and is numerically validated for different process parameter and geometry combinations.

Microstructure Evolution and High Strength-Ductility Synergy of Ti-13Nb-13Zr-2Ta Alloy Fabricated by Laser Powder Bed Fusion

Microstructure Evolution and High Strength-Ductility Synergy of Ti-13Nb-13Zr-2Ta Alloy Fabricated by Laser Powder Bed Fusion

A finite volume–based thermo-fluid-mechanical model of the LPBF process

The multi-physics coupling feature of the laser powder bed fusion (LPBF) process poses great challenges to numerical models regarding computational fidelity and efficiency. This paper proposed a finite volume–based model for predicting integrated thermo-fluid-mechanical behaviors of the LPBF process. The model directly unifies the heat transfer, fluid flow and solid mechanics simulations within a predefined mesh, enabling simultaneous solutions for the fluid domain under Eulerian description and the solid domain under Lagrangian description. Three benchmark tests accounting for individual problems were conducted to validate the model's accuracy and effectiveness. Track-scale LPBF simulations were performed to unravel the intricate interplay between thermal, fluid and mechanical behaviors. The numerical predictions of surface morphologies, molten pool dynamics and melt track dimensions aligned well with the experimental observations. The spatiotemporal evolution of transient thermal stress was accurately captured and the predicted residual stress field showed consistency with nanoindentation measurements. The proposed model was found robust in simultaneously predicting the temperature distribution, melt flow and residual stress evolutions of the LPBF process, and showed strong potential for addressing other similar multi-physics coupling problems.

Powder coarsening mechanisms of Ti-6Al-4 V with multiple build cycles in laser powder bed fusion

Laser powder bed fusion (LPBF) is an additive manufacturing process which can produce complex 3D parts from digital models. The performance of parts fabricated by LPBF is largely dependent on the characteristics of the powder feedstock, in particular, the particle size distribution. The coarsening of powder particles may limit the potential for reusing powder in further builds, as consistency in powder quality is crucial for ensuring consistent parts performance when using reused powder, especially in aerospace applications. However, there is a lack of systematic understanding regarding the causes and nature of powder coarsening in LPBF. In this work, the characteristics of powder samples from different locations in the build chamber during LPBF were studied to understand the particle size evolution and determine the origin of coarsening, which has not been previously reported. Meanwhile, powder coarsening was found to have a detrimental effect on the relative density and surface quality of as-built parts, highlighting the importance of exploring the mechanisms of powder coarsening and finding ways to control it in LPBF. The relationship between powder in key locations in the build chamber and its effect on powder coarsening has been established. Layer thickness is identified as a critical factor in causing powder coarsening due to the fine powder size characteristic in the powder bed. Spatter, in its various forms, plays a direct or indirect role in powder coarsening. Sintered powders resulting from spatter and the laser scanning borders of as-built parts were observed to contribute to the powder coarsening. Therefore, three main mechanisms (layer thickness, spatter, sintered powder) associated with the powder coarsening are therefore proposed. This work provides insight and potential solutions to control powder coarsening and maintain consistent parts performance.

Improvement of strength in single-lap adhesive joints of AlSi10Mg alloys fabricated by laser powder bed fusion

This study deals with the mechanical strength of metallic single-lap joints bonded with adhesive and fabricated by Additive Manufacturing (AM). To this aim, AlSi10Mg alloy has been used to manufacture the specimens using Laser Powder Bed Fusion (LPBF) process. Based on a series of tensile tests on AMed joints, the mechanical strength of the joints was determined. In this paper, a procedure is recommended to improve the strength of AMed single-lap joints. Particularly, a notch is introduced in the adherends, which plays a crucial role in the mechanical strength and failure load of the joints. Therefore, a series of single-lap joints with notch have been printed and examined. Based on the conducted tests, failure load and fracture behavior of the joints with and without notches have been compared. Here, we used the digital image correlation technique to determine strain fields on the surface of the joints. In addition, a series of finite element models has been developed to investigate the stress distributions in the adhesive and simulate the load-carrying behavior of the joints. The obtained results confirm the effective role of notches in adhesive joints for strength improvement of AMed single-lap joints. The outcome of this study can be utilized for fabrication of AMed metallic joints with a higher mechanical strength.

Elastocaloric effect of NiTi shape memory alloys manufactured by laser powder bed fusion

To address the challenges posed by the complex processing of NiTi alloys, the additive manufacturing technology has been employed for the preparation of NiTi alloys samples. Up to now, there is limited research on the elastocaloric effects of NiTi alloys manufactured by LPBF. This study focused on the utilization of laser powder bed fusion (LPBF) to fabricate NiTi samples, with a particular emphasis on the influence of laser power, scanning speed and hatch spacing on the microstructure and properties of NiTi alloys. The results indicated that the process parameters affect the stress hysteresis of superelasticity by influencing the Ni content in the NiTi alloys. The higher Ni content, the lower the stress hysteresis. In addition, the critical stress for plastic deformation of the texture in the <001> direction in NiTi alloys is the smallest. When only the P was changed, the proportion of the texture in the <001> direction in the sample increased with the increase of P, the proportion of plastic deformation increased, and the degree of recovery also gradually decreased. Furthermore, at a specific temperature, 10 K higher than the end temperature of austenite transformation, each sample exhibited good superelasticity. In particular, the 1st cooling capacity of the sample with the best elastocaloric effect reached 11.0 K, which was superior to other results obtained by LPBF under similar compressive strain levels.

Case study of the environmental impacts and benefits of a PBF-LB additively manufactured optimized filtrate nozzle

In this paper, we present a comprehensive case study of a laser-based powder bed fusion-manufactured optimized filtrate nozzle, focusing on environmental impacts and benefits. The scope of the study covers the raw material production, part manufacturing and use phases. We compare the results for similar components manufactured by conventional, mainly material-removing manufacturing technologies. Primary data, measured from processes, are utilized for life cycle assessment calculations, which are completed with database data when necessary. The results show that by exploiting the design freedom of additive manufacturing, it is possible to achieve positive environmental benefits, namely, a 40% reduction in the carbon footprint during the use phase, although the manufacturing phase per se requires a similar amount or even more resources than the conventional manufacturing route. In addition to environmental benefits, we present here some other potential benefits for using optimized, additively manufactured parts.

Numerical simulation and analytical modelling of temperature and morphology of melt pool in electron beam powder bed fusion of copper

Numerical simulation and analytical modelling of temperature and morphology of melt pool in electron beam powder bed fusion of copper

In-line process monitoring in laser welding and powder bed fusion of metals using directional reflection measurements

In-line process monitoring in laser welding and powder bed fusion of metals using directional reflection measurements

Process Developments in Electron-Beam Powder Bed Fusion Enabled by Near-Infrared Radiation

The use of an electron beam (EB) as a heating source in EB-based powder bed fusion (PBF-EB) has several limitations, such as reduced powder recyclability, short machine service intervals, difficulties with heating large areas and the limited processability of charge-sensitive powders. Near-infrared (NIR) heating was recently introduced as a feasible replacement and/or complement to EB heating in PBF-EB. This work further investigates the feasibility of using NIR to eliminate the need for a build platform as well as to enable easier repairing of parts in PBF-EB. NIR-assisted Ti-6Al-4V builds were successfully carried out by starting from a loose powder bed without using a build platform. The results do not only confirm that it is possible to eliminate the build platform by the aid of NIR, but also that it can be beneficial for the process cleanliness and improve the surface quality of built parts. Furthermore, a 430 stainless-steel (SS) component could be repaired by positioning it in a loose 316L SS powder bed using a fully NIR-heated PBF-EB process.

Impact of polymer molecular weight and dispersity on the coalescence of polypropylene powders suitable for laser powder bed fusion

Understanding and controlling the fundamental processes governing the coalescence of polymers is vital to enable the design of polymeric materials for improved mechanical performance and quality of manufactured structures, including those fabricated via additive manufacturing techniques. Our group has utilized thermally induced phase separation (TIPS) to produce spherical and size controlled polypropylene (PP) powders from 12,000 (12k), 250,000 (250k), and 340,000 (340k) molecular weight (Mw) PP, including 50-50 wt% blends of 12k/250k,12k/340k, and 250k/340k, and a 33-33-33 wt% blend of 12k/250k/340k to investigate the impact of polymer Mw, and thus zero-shear viscosity, on particle coalescence and its implication in laser powder bed fusion. The particles exhibit similar size distributions with an average particle size, Dx(50), in the range of 58 μm–86 μm. The coalescence behavior of the powders, evaluated via hot-stage microscopy, show that adding 12k PP in the blend significantly alters the coalescence dynamics of the 250k and 340k PP, dramatically increasing their coalescence rate. The substantial drop in zero-shear viscosity with addition of 12k PP provides the driving force for the pronounced enhancement in coalescence. Strong agreement between experimental results and the Hopper model of coalescence is observed only if corrected for extensional flow, exemplifying the importance of extensional flow on the coalescence process. The results also indicate that the 12k PP in the blend does not surface segregate in the TIPS process, but is homogeneously distributed in the blend. More broadly, these results provide molecular-level insight into how control of powder molecular weight characteristics and viscosity can offer pathways to optimize the consolidation of particles in manufacturing processes, including laser powder bed fusion.

Laser energy-dependent processability of non-equiatomic TiNbMoTaW high-entropy alloy through in-situ alloying of elemental feedstock powders by laser powder bed fusion

Pre-alloyed powder, which is primarily used in laser powder bed fusion (LPBF), has the disadvantages of requiring time and high manufacturing costs. To overcome these limitations, in-situ alloying, which mixes pure elemental powders and alloys them in real-time during the LPBF process, has attracted attention. In particular, manufacturing high entropy alloys (HEA) containing high-melting-point refractory elements through in-situ alloying presents considerable challenges. In this study, a non-equiatomic single body-centered cubic (BCC) solid-solution HEA was fabricated via in-situ alloying with Ti, Nb, Mo, Ta, and W powders through the LPBF process. Specifically, by applying a high volumetric energy density (VED), we successfully mitigated the segregation of constituent elements, leading to an enhanced crystallographic texture. Consequently, the reduction in the residual stress and high-angle grain boundary (HAGB) density progressed, contributing to an increased relative density. Thus, this study marks a pioneering endeavor for in-situ alloyed HEA fabrication via LPBF, illustrating the efficacy of in-situ alloying utilizing mixed powders.

Fatigue Damage Evolution in SS316L Produced by Powder Bed Fusion in Different Orientations with Reused Powder Feedstock

Abstract Background Metal Laser Powder Bed Fusion Melting (LPBF-M) is considered economically viable and environmentally sustainable because of the possibility of reusing the residual powder feedstock leftover in the build chamber after a part build is completed. There is however limited information on the fatigue damage development of LPBF-M samples made from reused feedstock. Objective In this paper, the stainless steel 316 L (SS316L) powder feedstock was examined and characterised after 25 reuses, following which the fatigue damage development of material samples made from the reused powder was assessed. Methods The suitability of the powder to LPBF-M technology was evaluated by microstructural observations and measurements of Hall flow, apparent and tapped density as well as Carr’s Index and Hausner ratio. LPBF-M bar samples in three build orientations (Z – vertical, XY – horizontal, ZX – 45° from the build plate) were built for fatigue testing. They were then subjected to fatigue testing under load control using full tension and compression cyclic loading and stress asymmetry coefficient equal to -1 in the range of stress amplitude from ± 300 MPa to ± 500 MPa. Results Samples made from reused powder (25 times) in the LPBF-M process exhibited similar fatigue performance to fresh unused powder although a lower ductility for vertical samples was observed during tensile testing. Printing in horizontal (XY) and diagonal (ZX) directions, with reused powder, improved the service life of the SS316L alloy in comparison to the vertical (Z). Conclusions Over the 25 reuses of the powder feedstock there was no measurable difference in the flowability between the fresh (Hall Flow: 21.4 s/50 g) and reused powder (Hall Flow: 20.6 s/50 g). This confirms a uniform and stable powder feeding process during LPBF-M for both fresh and reused powder. The analysis of fatigue damage parameter, D, concluded cyclic plasticity and ratcheting to be the main mechanism of damage.

In-melt electron analysis to accelerate process exploration of ceramics: Electron beam melting of TiB2

To enhance the versatility of electron beam powder bed fusion (EB-PBF), a widely utilized additive manufacturing (AM) technique for metallic materials, we propose a novel paradigm aimed at facilitating the exploration of the process parameters for less-studied materials, such as ceramics. The high melting points and poorly understood thermal properties of ceramics have constrained the comprehension of their melting behavior. In this study, titanium diboride (TiB2) sintered bodies were subjected to spot melting under four distinct electron beam currents and four different exposure times. By introducing a novel in-melt electron analysis (IMEA) approach, the various stages of melting were clearly identified. The analysis and interpretation of IMEA signals were found to be consistent with experimental observations on the spot-melted surface of TiB2. IMEA demonstrates significant potential for real-time process window optimization and quality assurance for challenging and novel materials.

Integrating Phase Field Modeling and Machine Learning to Develop Process-Microstructure Relationships in Laser Powder Bed Fusion of IN718

Integrating Phase Field Modeling and Machine Learning to Develop Process-Microstructure Relationships in Laser Powder Bed Fusion of IN718

High cycle fatigue behavior and strengthening mechanisms of laser powder bed fusion pure tantalum

The fatigue behavior and strengthening mechanisms of the laser powder bed fusion pure tantalum (LPBF-Ta), coupled with a fatigue life prediction model, were originally explored. The fatigue strength of LPBF-Ta fabricated at the optimal laser energy density (154.16 J/mm3) is 348 MPa (106 cycles), showing a remarkable improvement of about 80 % and 50 % compared with that of the annealed and cold worked commercially pure tantalum, respectively. The microstructural analysis identifies that cellular structures averaging 300 nm in size contribute 438.2 MPa to strengthening, serving as the main contributor to fatigue strength. The breaching of cell walls by dislocations, along with the activation of cross-slip, ensures plasticity and ultimately enhances fatigue strength. Besides, the analysis of the defects reveals that the internal lack of fusion pore can induce the secondary fatigue crack, resulting in a region of brittle crack propagation. Furthermore, a finite life prediction model is established by originally incorporating the size of the crack source defect, and it significantly improves the prediction accuracy of the fatigue life. These results provide guidance for the development of fatigue strengthening strategies for LPBF-Ta.

Alpha-case promotes fatigue cracks initiation from the surface in heat treated Ti-6Al-4V fabricated by Laser Powder Bed Fusion

This research investigates the effect of the formation of an oxygen-stabilised titanium alpha layer – called alpha-case at the surface – on the fatigue properties of Ti-6Al-4V (Ti64) alloy components produced by Laser Powder Bed Fusion (L-PBF). Three post processing heat treatments with different controlled atmospheres were carried out on samples with as-built surfaces to evaluate how differences in alpha-case layer thickness and hardness affect the material’s susceptibility to surface embrittlement and its overall fatigue performance. The investigation includes bulk and subsurface microstructural analysis, surface characterisation by X-ray computed tomography (XCT), and fatigue testing. Key findings show that alpha-case layers can reduce the fatigue resistance of L-PBF fabricated Ti64. The presence of a 70±3 µm thick alpha-case layer was found to promote crack initiation. This is emphasised by a higher density of initiated cracks, thus leading to a reduction in fatigue life. Conversely, thinner alpha-case layers were found to have a reduced impact on the fatigue performance, highlighting the critical role of post processing heat treatments in modulating the fatigue resistance of the material. The use of XCT to characterise the surfaces of the specimens in 3D confirms that fatigue cracks primarily initiate at surface notches, highlighting the predominance of as-built surfaces over microstructure in determining the fatigue resistance of L-PBF Ti64 components.

Microstructure evolution in laser powder bed fusion melted 2205 duplex stainless steel using in-situ EBSD during uniaxial tensile testing

Compared to duplex stainless steels (DSSs) prepared by traditional methods, specimens produced through laser powder bed fusion (LPBF) exhibit excellent yield strength but lower elongation. To improve elongation, understanding microstructure evolution during uniaxial tensile testing is crucial. This study investigates the slip behavior, grain boundary evolution, and phase transformation of LPBF 2205 duplex stainless steel during tensile deformation using in-situ electron backscatter diffraction (EBSD). Results show the formation of slip bands, which increase in number as loading stress rises. The primary slip systems activated are {110}<111> in ferrite and {111}<110> in austenite. In the ferrite phase, slip dislocations accumulate and generate low-angle grain boundaries (LAGBs), which evolve into high-angle grain boundaries (HAGBs), refining the grain structure. Numerous Σ3 annealing twins form in the austenite phase, but detwinning and twinning reduce Σ3 content as deformation processes. Ferrite and austenite exhibit good stability during the initial stages of tensile deformation. However, some austenite transforms into martensite through the transformation-induced plasticity (TRIP) effect at the final stages of deformation, which helps relieve stress concentration and delays material fracture. This study provides valuable insights for optimizing microstructure to improve the mechanical properties of LPBF materials.

Oxides improve the strength of Zn-0.5Mn-0.5Mg alloys proceeded by laser powder bed fusion

Laser powder bed fusion has recently been adopted to fabricate biodegradable Zn alloys. But the mechanical properties need to be further improved to meet the requirements for natural bone implants. In this study, a ternary Zn-0.5Mn-0.5Mg biodegradable alloy is prepared by laser powder bed fusion technique. The as-printed sample shows a large compressive yield strength of 267 MPa and considerable ductility that the sample maintains its integrity after 50 % compression. The improved strength is mainly attributed to the formation of brittle Zn oxides, Mn oxides and Mg oxides which induce lattice distortion. These oxides fragment easily during the compression and reduce the stress concentration, resulting in significant ductility. More importantly, the as-printed sample shows a moderate degradation rate of 0.08 mm/year after immersing in the simulated body fluid solution for 7 days. In addition, the sample has MC3T3-E1 cell viabilities higher than 75 %, showing no-toxicity characteristic. This study demonstrates that laser powder bed fusion Zn-Mn-Mg alloy is a promising candidate for biodegradable materials and proposes a novel route to tailor the mechanical properties of additive-manufactured Zn alloys by oxides compositing.

A Review of Non-Powder-Bed Metal Additive Manufacturing: Techniques and Challenges.

Metal additive manufacturing has significantly evolved since the 1990s, achieving a market valuation of USD 6.36 billion in 2022, with an anticipated compound annual growth rate of 24.2% from 2023 to 2030. While powder-bed-based methods like powder bed fusion and binder jetting dominate the market due to their high accuracy and resolution, they face challenges such as lengthy build times, excessive costs, and safety concerns. Non-powder-bed-based techniques, including direct energy deposition, material extrusion, and sheet lamination, offer advantages such as larger build sizes and lower energy consumption but also encounter issues like residual stress and poor surface finish. The existing reviews of non-powder-bed-based metal additive manufacturing are restricted to one technical branch or one specific material. This survey investigates and analyzes each non-powder-bed-based technique in terms of its manufacturing method, materials, product quality, and summary for easy understanding and comparison. Innovative designs and research status are included.