Powder Bed Fusion Additive Manufacturing Research Articles

Sawtooth scanning strategy for additive manufacturing

Purpose This study aims to improve the acceleration in the additive manufacturing (AM) process. AM tools, such as extrusion heads, jets, electric arcs, lasers and electron beams (EB), experience negligible forces. However, their speeds are limited by the positioning systems. In addition, a thin tool must travel several kilometers in tiny motions with several turns while realizing the AM part. Hence, acceleration is a more significant limiting factor than the velocity or precision for all except EB. Design/methodology/approach The sawtooth (ST) scanning strategy presented in this paper minimizes the time by combining three motion features: zigzag scan, 45º or 135º rotation for successive layers in G00 to avoid the CNC interpolation, and modifying these movements along 45º or 135º into sawtooth to halve the turns. Findings Sawtooth effectiveness is tested using an in-house developed Sand AM (SaAM) apparatus based on the laser–powder bed fusion AM technique. For a simple rectangle layer, the sawtooth achieved a path length reduction of 0.19%–1.49% and reduced the overall time by 3.508–4.889 times, proving that sawtooth uses increased acceleration more effectively than the other three scans. The complex layer study reduced calculated time by 69.80%–139.96% and manufacturing time by 47.35%–86.85%. Sawtooth samples also exhibited less dimensional variation (0.88%) than zigzag 45° (12.94%) along the build direction. Research limitations/implications Sawtooth is limited to flying optics AM process. Originality/value Development of scanning strategy for flying optics AM process to reduce the warpage by improving the acceleration.

Thermokinetics driven microstructure and phase evolution in laser-based additive manufacturing of Ti-25wt.%Nb and its performance in physiological solution

A bio-compatible Ti-25 wt% Nb alloy fabricated from a blend of pure elemental powders using laser powder bed fusion additive manufacturing technique. The present work investigated the effects of processing conditions on the evolution of microstructures and its consequential material attributes, such as mechanical properties and corrosion performance. Thermal management strategies comprising laser powers of 200 W and 300 W in complement with a shorter scan length (1 mm) and substrate preheating above β-transus temperature (1123 K) were considered to achieve complete dissolution of niobium particles. The microstructure in the 200 W sample showed thin α′′ martensite needles in β matrix while martensite laths in the 300 W condition appear coarse and were twice the area fraction compared to that in 200 W build. On the other hand, microstructures in the heated substrate sample exhibited the evolution of α and β phases. A multi-scale finite element method based thermo-kinetic model spanning from melt pool scale to the component scale was incorporated to understand the mechanism of the evolution of microstructures during liquid–solid and solid–solid state transformation. Electrochemical performance in the simulated body fluid of the printed alloys was found to be significantly affected by the presence of martensite fractions. Both mechanical and corrosion behaviors were favorably influenced by adoption of the substrate preheating during additive manufacturing due to promotion of diffusional transformation of β to α at the expense of martensitic transformation.

Catalytic properties of Fe-based amorphous alloys based on LPBF

Laser Powder Bed Fusion (LPBF) additive manufacturing technology has the advantages of high forming accuracy and rapid cooling. It is expected to overcome the limitations of forming difficulties in amorphous alloys and provide a new opportunity for the production of amorphous alloy structural parts. In this paper, FeSiBCrC amorphous alloy powder was used to prepare Fe-based amorphous alloy samples by LPBF technology. In view of the excellent properties of Fe-based amorphous alloy, the catalytic degradation performance of Fe-based amorphous alloy samples prepared by LPBF was systematically investigated. The catalytic degradation performance of the Fe-based amorphous alloy was optimised by changing the reaction conditions. In the methyl orange (MO) dye degradation experiment, the degradation rate reached 90.99% within 10 min. Increasing the sample dosage and reaction temperature can improve the degradation efficiency, but excessive addition of H2O2, raising the pH and the initial dye concentration will reduce the degradation efficiency. In addition, in the cyclic catalytic experiment, it was found that it has self-renewing performance, and the degradation efficiency was still higher than 95% within 35 minutes after repeated degradation 12 times. Fe-based amorphous alloy catalysts prepared on the basis of LPBF have good catalytic activity and will provide economical and effective technical support for wastewater treatment.

Measuring fatigue crack growth using microscale specimens: Si-modified Inconel 939 alloy processed by laser powder bed fusion additive manufacturing

Quantitative assessment of fatigue crack growth (FCG) in a Si-modified Inconel 939 alloy, processed by laser powder bed fusion additive manufacturing (L-PBF AM), was conducted through cyclic bending of pre-notched cantilevers with a characteristic cross-sectional dimension of 25 μm × 25 μm. To the best of our knowledge, this work is the first to show that cyclic bending of such pre-notched microscale cantilevers can sense both the threshold regime and the Paris-law regime of FCG quantitatively, despite the small specimen size. The deliberate fabrication of such microscale test specimens enables control of the direction of fatigue crack propagation with respect to a given microstructure, while avoiding the inclusion of volumetric defects within the test volume. Significant differences in FCG behavior were observed when the direction of crack propagation was switched from being parallel to the AM build direction to being perpendicular to it. The present work demonstrates the efficacy of FCG assessment using microscale specimens, which can enhance our understanding of the influence of a given microstructure on crack growth behavior.

Tensile Properties and Fracture Analysis of Duplex (2205) and Super Duplex (2507) Stainless Steels, Produced via Laser Powder Bed Fusion Additive Manufacturing

Additive manufacturing of duplex (DSS) and super duplex stainless steel (SDSS) has been successfully demonstrated using laser powder bed fusion (LPBF) in recent years. Owing to the high cooling rates, as-built LPBF-processed DSS and SDSS exhibit close to 100% ferritic microstructures and require heat treatment at 1000–1300 °C to obtain the desired duplex microstructure. In this work, the mechanical properties of DSS and SDSS processed via LPBF were investigated in three building directions (vertical, horizontal, diagonal) and three processing conditions (as-built, stress-relieved, annealed, and quenched) using uniaxial tensile testing. As-built samples exhibited tensile and yield strength greater than 1000 MPa accompanied by less than 20% elongation at break. In comparison, the water-quenched samples and samples annealed at 1100 °C exhibited elongation at break greater than 34% with yield and tensile strength values less than 950 MPa. Stress relief annealing at 300 °C had a negligible impact on the mechanical properties. Austenite formation upon high-temperature annealing restored the reduced ductility of the as-built samples. The as-built and stress-relieved SDSS showed the highest yield and tensile strength values in the horizontal build direction, reaching up to ≈1400 and ≈1500 MPa (for SDSS), respectively, as compared to the vertical and diagonal directions. Fractographic investigation after tensile testing revealed predominantly a quasi-ductile failure mechanism, showing fine size dimple formation and cleavage facets in the as-built state and a fully ductile fracture in the annealed and quenched conditions. The findings in this study demonstrate the mechanical anisotropy of DSS and SDSS along three different build orientations, 0°, 45°, 90°, and three post-processing conditions.

Relationship Between Process Parameters and Defects in Laser Powder Bed Fusion Additive Manufacturing of Crack-Sensitive Al–Cu–Mg Alloy

Relationship Between Process Parameters and Defects in Laser Powder Bed Fusion Additive Manufacturing of Crack-Sensitive Al–Cu–Mg Alloy

Laser powder bed fusion additive manufacturing of Ti6Al4V-matrix composite with outstanding hardness/strength: Microstructural evolution and performance enhancement mechanisms

Incorporation of transition-metal boride with excellent mechanical performance as a reinforcing phase, represents a promising approach to address the challenge of insufficient hardness/strength in Ti6Al4V alloys. Despite the common challenge of achieving homogeneous distribution and refinement of TMB, this study leveraged a novel strategy involving laser-induced in-situ synthesis and a greatly strengthened laser-molten-pool effect, to additively manufacture highly-dense Ti6Al4V-matrix composites reinforced with mono-dispersed TiB nanowires. This was achieved through selective laser melting route of laser powder bed fusion. Remarkably, the unique microstructural morphology of these composites characterized by the uniform dispersion of high-aspect-ratio reinforcing phases, boron dissolution into α-Ti grain boundaries of the Ti6Al4V matrices, and unified crystallographic coordination of gradient transitional layers, contributed to their superior microhardness and compressive strength compared to the counterparts even with the higher contents of reinforcing phases. This achievement was attributed to the synergistic effects of various morphological mechanisms including second-phase, grain refinement, and stress-transfer reinforcing/strengthening. This research provided new insights into the precise control of microstructural evolution in metal-matrix composites and the enhancement of their overall mechanical performance.

Computationally guided alloy design and microstructure-property relationships for non-equiatomic Ti–Zr–Nb–Ta–V–Cr alloys with tensile ductility made by laser powder bed fusion

Single-phase body-centered cubic refractory complex concentrated alloys (RCCAs), exhibit strength retention at temperatures above the melting points of conventional Ni-based superalloys. However, their lack of room temperature tensile ductility leads to cracks during processing and/or premature failure under mechanical loading when fabricated by laser-based metals additive manufacturing (AM). We present an alloy design framework for tensile ductility in non-equimolar RCCAs amenable to AM processing within the Ti–V–Cr–Nb–Zr–Ta system. First, density functional theory (DFT) and machine learning informed screening of alloy compositions were used to down-select ∼106 alloys with potential for tensile ductility. Next, Scheil solidification modeling was used to eliminate compositions most at risk for micro-segregation and solidification cracking based on the estimated freezing range of each alloy. This led to the design of two new, representative alloys: Ti0.4Zr0.4Nb0.1Ta0.1 and Ti0.486V0.375Ta0.028Cr0.111. These alloys were evaluated for rapid solidification defect formation using in-situ synchrotron melt pool imaging, fabricated via laser powder bed fusion (L-PBF) AM, and then characterized for processing defects, microstructure, and mechanical properties. Overall, both alloys achieved tensile yield strengths >800 MPa and failure strains >5 %. However, the occurrence of un-melted powders of high melting point elements likely resulted in premature failure during tensile straining. Considerations about mitigating this defect source are discussed and the role of local micro-segregation on ductility are elucidated. Overall, these results highlight the success of our framework, and show potential for laser-based AM-RCCAs with tensile ductility.

Effect of processing parameters and thermal history on microstructure evolution and functional properties in laser powder bed fusion of 316L

In this paper, we explain and quantify the causal effect of processing parameters and part-scale thermal history on the evolution of microstructure and mechanical properties in the laser powder bed fusion additive manufacturing of Stainless Steel 316L components. While previous works have correlated the processing parameters to flaw formation, microstructures evolved, and properties, a missing link is the understanding of the effect of thermal history. Accordingly, tensile test coupons were manufactured under varying processing conditions. Their microstructure-related attributes, e.g., grain morphology, size and texture; porosity; and microhardness were characterized. Additionally, the yield and tensile strengths of the samples were measured using digital image correlation. An experimentally validated computational model was used to predict the thermal history of each sample. The temperature gradients and sub-surface cooling rates ascertained from the model predictions were correlated with the experimentally characterized microstructure and mechanical properties. By elucidating the fundamental process-thermal-structure–property relationship, this work establishes the foundation for future physics-based prediction of microstructure and functional properties in laser powder bed fusion.

Powder movement rules of laser powder bed fusion additive manufacturing aluminum alloy based on discrete element method

Laser Powder Bed Fusion (LPBF) is a promising metal additive manufacturing technology based on layer by layer powder spreading, and powder bed uniformity has a great influence on the forming quality. By Discrete Element Method and powder spreading experiment, the interaction and movement between powder were studied during powder spreading, including powder jamming, rebound, splash, eddy, and empty powder area. Additionally, five kinds of powder spreading schemes were explored, and the new process of one-way reciprocating with tri-splint blade was designed to change the motion state of powder spreading from “blade pushing powder” to “blade holding powder.” By increasing the distance between the blade and the working platform form 0 to 20 µm with the distance between the upper surface of the substrate and the working platform 50 µm, defects such as powder splash and empty powder decreased. And the uniform powder bed of aluminum alloy powder was achieved with the new process of one-way reciprocating with tri-splint blade structure.

Influence of contouring on the microstructure and micromechanical properties of Laser Powder Bed Fusion (LPBF) processed Al6061 enhanced with reactive particles

Age-hardenable aluminum alloys are of interest for powder bed fusion (PBF) additive manufacturing to produce lightweight, high-performance custom components. The successful processing of these alloys depends on specific alloying and print techniques to overcome defects that significantly degrade mechanical properties of PBF aluminum. The Reactive Additive Manufacturing (RAM) process modifieds alloy feedstock with reactive precursors which inoculate equiaxed microstructures during solidification, resulting in a reduction of columnar grain growth and shrinkage cracks. Additionally, surface quality is addressed by contouring, which retraces the perimeters of printed layers to produce smooth, fully fused surfaces. However, the combination of RAM and contouring methods has not been previously described in the literature, raising questions about the extent of refinement which is achieved when RAM particles are remelted by contouring laser sweeps. In this research, X-ray microcomputed tomography (XCT) was used to provide a complete characterization of RAM particles and defects in an AA6061-RAM alloy in 3D. The non-destructive XCT technique was used to quantify pore and particle distributions in the contoured alloy volumes and compared directly to hatched regions. The resulting three-dimensional analysis was correlated to traditional 2D microstructure quantification and nanoindentation experiments, providing a multimodal quantitative description of sub-micron grains, reaction products, and micromechanical properties. The study reveals that RAM particles inhibit cracking while simultaneously reducing the variance in mechanical properties between hatched and contoured regions of the build volume.

Active interlocking metasurfaces enabled by shape memory alloys

Interlocking metasurfaces (ILMs) are a newly developed joining technology that relies on arrays of interlocking features that transmit force and constrain motion between adjoining bodies in one or more directions. This study explores harnessing the shape memory effect in nickel-titanium shape memory alloys (NiTi SMAs) in structures fabricated using additive manufacturing (AM) to advance the development of active ILMs by creating unit cells that open or close at specific temperatures. The study encompasses designing and fabricating two distinct interlocking array configurations using near-equiatomic NiTi powder and the laser powder bed fusion (L-PBF) AM technique, following a previously developed AM process optimization framework to manufacture defect-free parts. To guide the design process, finite element analysis (FEA) was employed to predict strain values during engage-disengage cycles. The martensitic transformation characteristics of the ILMs were characterized. Thermomechanical testing revealed that the ILMs demonstrate high locking force once engaged, coupled with complete shape recovery and good cyclic stability. Digital image correlation (DIC) was also employed to validate the FEA predictions during the engage-disengage cycles. The results indicate that NiTi SMA-based ILMs can be designed and fabricated into complex shapes using L-PBF. By leveraging the shape memory effect (SME), the functionality of an ILM can be improved upon. The combination of computational modeling, additive manufacturing, and thermomechanical and physical property characterization provides a framework for designing future ILMs out of active materials.

Effect of Electric Pulse Treatment on Microstructure and Mechanical Property of Laser Powder Bed Fused IN718

This study investigated the impact of electric pulse treatment (EPT) on the microstructure and mechanical properties of laser powder bed fused Inconel 718 (IN718). Through a comprehensive experimental characterization, we found that EPT induced significant improvements in the microstructure of IN718. In the YOZ plane of EPT-700, the molten pool diminished and replaced by a grain boundary with granular Ni3Nb precipitates, and the dislocations increased while the irregular porosity decreased. Concurrently, enhanced mechanical properties of EPT-700 were obtained, including a hardness of 354.7 HV, an ultimate tensile strength of 930.21 MPa, and an elongation of 34.35%. Fractographic analysis revealed a transition in fracture mechanisms, highlighting the intricate relationship between microstructural modifications induced by EPT and mechanical response under load. These findings underscore the potential of EPT as a promising post-processing technique for optimizing the microstructure and mechanical properties of IN718 components fabricated via laser powder bed fusion additive manufacturing. This study contributes to the advancement of knowledge in the field of additive manufacturing and provides valuable insights for the development of high-performance metallic components.

Effects of Low-Temperature Heat Treatment on Mechanical and Thermophysical Properties of Cu-10Sn Alloys Fabricated by Laser Powder Bed Fusion.

This study investigated the impact of low-temperature heat treatments on the mechanical and thermophysical properties of Cu-10Sn alloys fabricated by a laser powder bed fusion (LPBF) additive manufacturing (AM) process. The microstructure, phase structure, and mechanical and thermal properties of the LPBF Cu-10Sn samples were comparatively investigated under both the as-fabricated (AF) condition and after low-temperature heat treatments at 140, 180, 220, 260, and 300 °C. The results showed that the low-temperature heat treatments did not significantly affect the phase and grain structures of the Cu-10Sn alloys. Both pre- and post-treatment samples displayed consistent grain sizes, with no obvious X-ray diffraction angle shift for the α phase, indicating that atom diffusion of the Sn element is beyond the detection resolution of X-ray diffractometers (XRD). However, the 180 °C heat-treated sample exhibited the highest hardness, while the AF samples had the lowest hardness, which was most likely due to the generation of precipitates according to thermodynamics modeling. Heat-treated samples also displayed higher thermal diffusivity values than their AF counterpart. The AF sample had the longest lifetime of ~0.19 nanoseconds (ns) in the positron annihilation lifetime spectroscopy (PALS) test, indicating the presence of the most atomic-level defects.

Mechanical response and failure modes of three-dimensional auxetic re-entrant LPBF-manufactured steel truss lattice materials

Abstract Auxetic architected materials present a novel class of damage-tolerant materials with tunable mechanical characteristics and high energy absorption due to their unique ability to laterally contract and densify when subjected to axial compressive loading. The current state of research on negative Poisson's ratio materials mainly focuses on 2D geometries and a few families of 3D geometries with limited experimental comparisons between different architectures and various geometrical features. Furthermore, when manufactured via laser powder bed fusion, the influence of as-built deviations of geometrical and material properties inherently present due to the melt pool solidification process for thin features is relatively unexplored in the case of metal architected materials. The authors aim to study the elastic properties, peak characteristics, and failure modes of steel auxetic truss lattices subjected to axial compression while also addressing the uncertainties inherent to the metal laser powder bed fusion additive manufacturing of architected materials. This work presents an experimental and computational exploration and comparison of two promising three-dimensional auxetic truss lattice families of low relative densities. A comprehensive investigation of metal negative Poisson's ratio mechanical metamaterials is presented, including the selection of the architectures, modeling, laser powder bed fusion additive manufacturing, as-built part characterization, material testing, and mechanical testing under axial compression. The study of such architectures can unlock their potential in making them readily adaptable to a wide variety of engineering applications.

Effects of the unit-cell size and arrangement on the compressive behaviors of lattice structures in powder bed fusion additive manufacturing

Lattice structures demonstrate a unique compressive behavior wherein the compressive strength increases and converges to a specific value as the number of unit-cell arrangements and overall dimensions of the lattice structure increase. In this study, the effects of the unit-cell size and cell arrangement on the compressive behavior of lattice structures were investigated to determine the convergence of compression characteristics at specific unit-cell configurations. Simple cubic lattice structures were designed and manufactured using powder bed fusion techniques. The lattice structures were characterized using universal testing machines. Experimental results revealed that the convergence of compressive strength was influenced by the size of the unit cell: the number of arrangements at which a specific compressive strength was reached increased as the unit-cell sizes decreased. Furthermore, specific convergence points were determined for different unit-cell sizes, providing valuable insights into the compressive strength behavior of lattice structures. This study emphasizes the significance of considering both unit-cell arrangements and overall dimensions when designing standard specimens for compression testing. Additionally, the methodology developed in this study can be applied to obtain an optimal configuration for desired strength characteristics.

Understanding precipitation during in-situ and post-heat treatments of Al-Mg-Sc-Zr alloys processed by powder-bed fusion

This study investigates the evolution of Sc-rich precipitates in Scalmalloy® processed using Powder Bed Fusion (PBF) additive manufacturing. By combining microstructural analysis, thermodynamic calculations, and an adapted precipitation model, we examine how precipitates change during solidification, in-situ heat treatment (IHT), and post-heat treatment (PHT). We establish a comprehensive classification system for primary and secondary Sc-rich precipitates based on their origin, location, morphology, composition, interaction with other precipitates, and size. Notably, primary precipitates are observed within the fine-grained (FG) zone, and their characteristics suggest further modification during IHT and PHT. The developed precipitation model, coupled with multi-scale thermal simulations, accurately predicts the formation of small homogeneous secondary L12-Al3Sc precipitates during PBF and PHT, showcasing its potential as a powerful tool for optimizing PBF processes for components with complex geometries and different thermal conditions. Our findings reveal that while the employed IHT conditions did not induce secondary precipitation, subsequent PHT at 400°C for 1 hour led to the formation of secondary precipitates via both continuous and discontinuous mechanisms. Addressing limitations in understanding primary precipitate formation during PBF and incorporating both homogeneous and heterogeneous mechanisms in future models are crucial for a deeper understanding of Scalmalloy® precipitation behavior and PBF process optimization.

Research progress on powder bed fusion additive manufacturing of TiAl-based alloy

Research progress on powder bed fusion additive manufacturing of TiAl-based alloy

Research progress on powder bed fusion additive manufacturing of TiAl-based alloy

Research progress on powder bed fusion additive manufacturing of TiAl-based alloy

Influence of Pre- and Post-Contouring Strategies to Downskin Sloped Surfaces in Laser Powder-Bed Fusion (L-PBF) Additive Manufacturing.

Among various metal additive manufacturing (AM) technologies, L-PBF is known for fabricating intricate components. However, due to step edges and powder particle attachments, attaining a good surface finish is challenging, especially on downskin surfaces. Contour scanning has potential to improve surface quality because such scanning may dominate the surface formation of sloped features. This study evaluates the effects of pre- and post-contouring strategies on the sloped downskin surfaces fabricated using a commercial L-PBF system with Ti6Al4V powder. L-PBF parts printed at inclination angles 30°, 45° and 60° were investigated. A double-contouring approach with varying processing conditions was employed and surface characteristics were analyzed using data acquired by white light interferometry. The average surface roughness, Sa, surface skewness, Ssk, and percentage area of powder particles attached onto surfaces were statistically evaluated. The lowest Sa obtained for pre- and post-contoured samples is 14.08 µm and 18.88 µm, respectively. For both strategies, the combination of a low laser power and a high scan speed on the interface of downskin surface and underneath powder results in smoother surfaces. However, while comparing both strategies, pre-contouring gives better surface finish for samples built at similar processing conditions, with a difference of nearly 5 µm in Sa.