Powder Bed Fusion Additive Manufacturing Research Articles

A new variant of the inherent strain method for the prediction of distortion in powder bed fusion additive manufacturing processes

A new variant of the inherent strain (IS) method is proposed to predict component distortion in powder bed fusion additive manufacturing (AM) that addresses some of the shortcomings of the previous work by accounting for both the compressive plastic strain formed adjacent to the melt pool and the thermal strain associated with the changing macroscale thermal field in the component during fabrication. A 3D thermomechanical finite element (FE) model using the new approach is presented and applied to predict the distortion of a component fabricated in an electron beam powder bed fusion (EB-PBF) machine. To improve computational efficiency, each computational layer is comprised of six powder layers. A time-averaged volumetric heat input based on beam voltage and current data obtained from the EB-PBF system was calculated and applied to each computational layer, consistent with the process timing. The inherent strains were applied per computational layer as an initial anisotropic contribution to the thermal strain at the time of activation of each computational layer, resulting in the sequential establishment of static equilibrium during component fabrication, which accounts for the variation in the local macroscale thermal field. The thermal field and distortion predicted by the thermomechanical model were verified using experimentally derived data. The model predicts in-plane compressive strains in the order of 10−3. Differences in the inherent strain were found at different locations in the component, consistent with differences in the macroscale thermal field. The proposed method is general and may also be applied to the laser powder bed fusion (L-PBF) process.

Influence of the physicochemical properties of reusable powders on the mechanical properties of Ti6Al4V manufactured via laser powder bed fusion additive manufacturing

Laser powder bed fusion additive manufacturing (L-PBF AM) technology holds notable advantages regarding raw material reusability. Nonetheless, a notable gap exists in extensive research assessing how the reuse of powders influences their intrinsic physicochemical traits as well as the mechanical attributes of the fabricated components. This study focuses on introducing an innovative approach to Ti6Al4V powder recycling in L-PBF AM, involving the incorporation of virgin spherical powder to maintain powder characteristics prior to printing. This results in sustained mechanical properties throughout seven cycles of printing (P7), with the fifth print (P5) showcasing the optimal mechanical performance (strength of 1185 MPa and elongation of 6.3%). Transmission electron microscopy (TEM) analysis was performed before and after the P5 tensile experiments，revealing a high amount of primary twinning {101¯1} in LPBF. This study offers both theoretical insights and practical validation, thereby lending substantial support to the advancement of metal AM industrialization.

Additive manufacturing of defect-free TiZrNbTa refractory high-entropy alloy with enhanced elastic isotropy via in-situ alloying of elemental powders

Laser powder-bed fusion (L-PBF) additive manufacturing presents ample opportunities to produce net-shape parts. The complex laser-powder interactions result in high cooling rates that often lead to unique microstructures and excellent mechanical properties. Refractory high-entropy alloys show great potential for high-temperature applications but are notoriously difficult to process by additive processes due to their sensitivity to cracking and defects, such as un-melted powders and keyholes. Here, we present a method based on a normalized model-based processing diagram to achieve a nearly defect-free TiZrNbTa alloy via in-situ alloying of elemental powders during L-PBF. Compared to its as-cast counterpart, the as-printed TiZrNbTa exhibits comparable mechanical properties but with enhanced elastic isotropy. This method has good potential for other refractory alloy systems based on in-situ alloying of elemental powders, thereby creating new opportunities to rapidly expand the collection of processable refractory materials via L-PBF.

Manipulating ionic conductivity through chemical modifications in solid-state electrolytes prepared with binderless laser powder bed fusion processing

Additive manufacturing of solid-state batteries is advantageous for improving the power density by increasing the geometric complexity of battery components, such as electrodes and electrolytes. In the present study, bulk three-dimensional Li1+x Al x Ti2−x (PO4)3 (LATP) electrolyte samples were prepared using the laser powder bed fusion (L-PBF) additive manufacturing method. Li3PO4 (LPO) was added to LATP to compensate for lithium vaporization during processing. Chemical compositions included 0, 1, 3, and 5 wt. % LPO. Resulting ionic conductivity values ranged from 1.4 × 10−6–6.4 × 10−8 S cm−1, with the highest value for the sample with a chemical composition of 3 wt. % LPO. Microstructural features were carefully measured for each chemical composition and correlated with each other and with ionic conductivity. These features and their corresponding ranges include: porosity (ranging from 5% to 19%), crack density (0.09–0.15 mm mm−2), concentration of residual LPO (0%–16%), and concentration and Feret diameter of secondary phases, AlPO4 (11%–18%, 0.40–0.61 µm) and TiO2 (9%–11%, 0.50–0.78). Correlations between the microstructural features and ionic conductivity ranged from −0.88 to 0.99. The strongest negative correlation was between crack density and ionic conductivity (−0.88), confirming the important role that processing defects play in limiting the performance of bulk solid-state electrolytes. The strongest positive correlation was between the concentration of AlPO4 and ionic conductivity (0.99), which is attributed to AlPO4 acting as a sintering aid and the role it plays in reducing the crack density. Our results indicate that additions of LPO can be used to balance competing microstructural features to design bulk three-dimensional LATP samples with improved ionic conductivity. As such, refinement of the chemical composition offers a promising approach to improving the processability and performance of functional ceramics prepared using binderless, laser-based additive manufacturing for solid-state battery applications.

On the thermal emissive behavior of four common alloys processed via powder bed fusion additive manufacturing

Current thermal monitoring methods used in metal powder bed fusion (PBF) additive manufacturing (AM) rely on a priori knowledge of the emissivity, which is usually assumed temperature, wavelength, and time invariant. Given the true dynamic nature of emissivity for a given material, these assumptions result in the calculation of inaccurate process temperatures, or in the reporting of radiant intensity or radiance temperature as a proxy for the absolute temperature. In this work, we detail the use of a multi-wavelength (MW) pyrometry technique, operating in the spectral range from approximately 900–1700 nm, to capture radiant intensity emissions during processing of various materials using the electron-beam powder bed fusion (PBF-EB) process. The technique measured spectral intensity and analyzed the results of these data after processing with Planck’s distribution law in its two-color or ratio pyrometry form to establish the spectral and temporal emissive behavior in the active range from 1080–1637 nm. Four commonly used alloys (Ti6Al4V, TiAl, 316 L, and IN625) were examined here as each of these alloys underwent transitions (from powder to liquid to structured solid) induced during processing. For each material studied, analysis of the aggregated data for ten layers, superimposed in time, identified specific and repeatable trends in the emissivity, with plotted measurements forming clusters or regions in each of the processing stages (preheating, melt scanning, liquid, and cooldown). Also, the data recorded by the MW pyrometer was used to analyze the spectral variability of measurements as well as temporal changes in emissive behavior (from gray to non-gray) at similar temperatures and points in time in each processing stage but during different layers or scans. The results show that the measured emissive behaviors for the four materials were highly variable during processing, with typical differences during transitions ranging from 20% to 75%, and as high as 300% for the case of Ti6Al4V, indicating emissive behaviors that are highly dynamic rather than temporally and spectrally invariant. This dynamic emissive behavior is associated with changes in temperature, morphology, phase, and chemistry of the processed metal that happen during the highly transient and non-equilibrium conditions of PBF AM, and that can only be accounted for by performing in situ measurements during processing using techniques that do not rely on prior knowledge of the emissivity. These results are intended to (1) better inform the additive manufacturing community on the physical nature of emissivity of metallic materials during processing, and (2) provide foundational emissivity data that can be used to improve numerical modeling and the application of radiation thermometry techniques in PBF AM. Further, these results indicate that the emissive behavior during processing can result in significant variations temporally and spectrally, and although the results can be used as foundational emissive behavior measurements, the authors recommend the use of in situ techniques that operate without prior knowledge of emissivity to reduce uncertainties in measurements during PBF AM.

Porosity control of copper-based alloys via powder bed fusion additive manufacturing for spacecraft applications

Porosity control of copper-based alloys via powder bed fusion additive manufacturing for spacecraft applications

In-situ synthesis of spatial heterostructure Ti composites by laser powder bed fusion to overcome the strength and plasticity trade-off

Recent research has focused on laser in-situ additive manufacturing of metal matrix composites with spatially controllable microstructures (phases). This study, inspired by the process of inserting mesh fibers into reinforced concrete, synthesizes TiN in situ using laser powder bed fusion and N2 gas. The laser-melted track, embedded with TiN particles, formed a spatially heterostructured Ti composite (SHTC) with a three-dimensional, artificially controlled architecture in a pure Ti matrix. The influences of process parameters on the mechanical properties of the spatially heterostructured Ti composite and the microstructural evolution of TiN/Ti were investigated emphatically. The results showed that the growth direction of the microstructure was changed by laser powder bed fusion additive manufacturing with alternating N2–Ar gas under suitable N2 concentration and melting track spacing. Among all spatially heterostructured Ti composites, the TiN–Ti heterolayer net-like structure achieved a high ultimate tensile strength of ∼1.0 GPa and elongation of 27 %, demonstrating a superior strength-ductility combination than intrinsic pure Ti and uniform TiN composites, as well as traditional layered structure Ti-based composites. During the tensile test, the deformation behavior was monitored in situ using digital image correlation, and the fracture mechanism was investigated. Hetero-deformation induced strengthening and toughening potentially explains the mechanism behind the strength enhancement of spatially heterostructured Ti composites. Furthermore, this work may stimulate research and development in additive manufacturing of spatial heterostructures with configurable structures, targeting synergistic regulation of strength and ductility in the integration of structure-material-function.

Investigating the effect of temperature on powder spreading behaviour in powder bed fusion additive manufacturing process by Discrete Element Method

In this study, a Discrete Element Method (DEM)-based model was developed to simulate the powder spreading process in Powder bed Fusion (PBF) for Polyamide 6 (PA6) powder, which considered the spreading speed (3 mm/s and 30 mm/s) and temperature (25 °C and 110 °C) as parameters affecting the final spreading powder layer quality. The particle horizontal and vertical velocities were analysed in regions near the spreading blade, where at the lower spreading speed, particle velocities are far less compared to the higher spreading speed. The lower particle velocities lead to gently settling down and rearranging particles during spreading on the bed, allowing a uniform powder layer to form. Increasing the spreading speed led to an increase in shear stress and inertia number. At higher temperatures, shear stresses also rise while the inertia number is slightly reduced due to the greater cohesion between particles. The generated powder layer by the DEM model was analysed using the wavelet analysis technique and compared to experiments. The spreadability index of experiments can be estimated with less than 5% error using DEM simulations, though in an approximately consistent manner that captures the experimental trends of spreading speed and temperature. The packing fraction of simulated powder layers was investigated in the spreading direction. The DEM simulations show that packing fraction decreases as temperature or spreading speed are increased, with its variation across the bed increasing for higher spreading speeds. Increasing the spreading speed leads to greater motion and inertia number of particles and, consequently, it intensifies the particle ejection and results in many unfilled areas in the spread powder layer. Increasing temperature leads to an increase in cohesivity between particles, resulting in aggregates forming on the spread powder layer.

Performance Improvement for the CuCrZr Alloy Produced by Laser Powder Bed Fusion Using the Remelting Process.

Owing to the high optical reflectivity of copper powder, the high-performance fabrication of copper alloys in the laser additive manufacturing (AM) field is problematic. To tackle this issue, this study employs the remelting process during laser powder bed fusion AM to fabricate defect-free and high-performance CuCrZr alloy. Compared to the non-remelting process, the remelting process yields finer grains, smaller precipitates, denser dislocations, and smaller dislocation cells. It realizes not only the dense molding of high laser reflectivity powders but also excellent mechanical properties and electrical conductivity (with an ultimate tensile strength of 329 MPa and conductivity of 96% IACS) without post-heat treatment. Furthermore, this study elucidates the influence of complex thermal gradients and multiple thermal cycles on the manufacturing process under the remelting process, as well as the internal mechanisms of microstructure evolution and performance improvement.

Corrosion behavior of laser powder bed fusion additive manufacturing produced TiNi alloy by micro-arc oxidation

To improve the corrosion resistance of TiNi alloy fabricated by laser powder bed fusion (LPBF), a porous oxidation layer was synthesized by micro-arc oxidation in a sodium aluminate and sodium silicate electrolyte. The influences of the applied voltage and the processing time on the morphology of oxidation layer were investigated, and the corrosion behavior of the oxidation layer in artificial saliva was evaluated and compared with that of the as-fabricated LPBF alloy. The results indicate that, as increasing the applied voltage and the processing time, the oxidation layer becomes uniform and integrated. The optimum parameters are with an applied voltage of 450 V and processing time of 40 min. The oxidation layer primarily contains α-Al2O3 and consists of two layers, i.e., a thin, compact and uniform inner layer and a porous outer layer. The formation of stable α-Al2O3 phase in the coating and its almost non-porous dense structure reduce the channels for corrosion ions to penetrate into the substrate through coating, thereby improving the corrosion resistance of TiNi alloy.

Understanding Melt Pool Behavior of 316L Stainless Steel in Laser Powder Bed Fusion Additive Manufacturing.

In the laser powder bed fusion additive manufacturing process, the quality of fabrications is intricately tied to the laser-matter interaction, specifically the formation of the melt pool. This study experimentally examined the intricacies of melt pool characteristics and surface topography across diverse laser powers and speeds via single-track laser scanning on a bare plate and powder bed for 316L stainless steel. The results reveal that the presence of a powder layer amplifies melt pool instability and worsens irregularities due to increased laser absorption and the introduction of uneven mass from the powder. To provide a comprehensive understanding of melt pool dynamics, a high-fidelity computational model encompassing fluid dynamics, heat transfer, vaporization, and solidification was developed. It was validated against the measured melt pool dimensions and morphology, effectively predicting conduction and keyholing modes with irregular surface features. Particularly, the model explained the forming mechanisms of a defective morphology, termed swell-undercut, at high power and speed conditions, detailing the roles of recoil pressure and liquid refilling. As an application, multiple-track simulations replicate the surface features on cubic samples under two distinct process conditions, showcasing the potential of the laser-matter interaction model for process optimization.

Prediction of Compressive Behavior of Laser-Powder-Bed Fusion-Processed TPMS Lattices by Regression Analysis

Triply periodic minimal surface (TPMS) structures offer lightweight and high-stiffness solutions to different industrial applications. However, testing of these structures to calculate their mechanical properties is expensive. Therefore, it is important to predict the mechanical properties of these structures effectively. This study focuses on the effectiveness of using regression analysis and equations based on experimental results to predict the mechanical properties of diamond, gyroid, and primitive TPMS structures with different volume fractions and build orientations. Gyroid, diamond, and primitive specimens with three different volume fractions (0.2, 0.3, and 0.4) were manufactured using a laser powder bed fusion (LPBF) additive manufacturing process using three different build orientations (45°, 60°, and 90°) in the present study. Experimental and statistical results revealed that regression analysis and related equations can be used to predict the mass, yield stress, elastic modulus, specific energy absorption, and onset of densification values of TPMS structures with an intermediate volume fraction value and specified build orientation with an error range less than 1.4%, 7.1%, 19.04%, 21.6%, and 13.4%, respectively.

The Printability, Microstructure, and Mechanical Properties of Fe80-xMnxCo10Cr10 High-Entropy Alloys Fabricated by Laser Powder Bed Fusion Additive Manufacturing.

This work investigated the effect of Fe/Mn ratio on the microstructure and mechanical properties of non-equimolar Fe80-xMnxCo10Cr10 (x = 30% and 50%) high-entropy alloys (HEAs) fabricated by laser powder bed fusion (LPBF) additive manufacturing. Process optimization was conducted to achieve fully dense Fe30Mn50Co10Cr10 and Fe50Mn30Co10Cr10 HEAs using a volumetric energy density of 105.82 J·mm-3. The LPBF-printed Fe30Mn50Co10Cr10 HEA exhibited a single face-centered cubic (FCC) phase, while the Fe50Mn30Co10Cr10 HEA featured a hexagonal close-packed (HCP) phase within the FCC matrix. Notably, the fraction of HCP phase in the Fe50Mn30Co10Cr10 HEAs increased from 0.94 to 28.10%, with the deformation strain ranging from 0 to 20%. The single-phase Fe30Mn50Co10Cr10 HEA demonstrated a remarkable combination of high yield strength (580.65 MPa) and elongation (32.5%), which surpassed those achieved in the FeMnCoCr HEA system. Comparatively, the dual-phase Fe50Mn30Co10Cr10 HEA exhibited inferior yield strength (487.60 MPa) and elongation (22.3%). However, it displayed superior ultimate tensile strength (744.90 MPa) compared to that in the Fe30Mn50Co10Cr10 HEA (687.70 MPa). The presence of FCC/HCP interfaces obtained in the Fe50Mn30Co10Cr10 HEA resulted in stress concentration and crack expansion, thereby leading to reduced ductility but enhanced resistance against grain slip deformation. Consequently, these interfaces facilitated an earlier attainment of yield limit point and contributed to increased ultimate tensile strength in the Fe50Mn30Co10Cr10 HEA. These findings provide valuable insights into the microstructure evolution and mechanical behavior of LPBF-printed metastable FeMnCoCr HEAs.

Effect of parameters on quality of IN718 parts using laser additive manufacturing

The objective of this study is to determine the effect of process parameters such as laser power, scanning speed and layer thickness on the mechanical properties, microstructure and fracture mechanism of Inconel 718 using laser powder bed fusion additive manufacturing process. Full factorial design, regression modelling and response surface methodology are used to optimise the process parameters. The highest tensile strength occurs at 350 W laser power, 1300 mm/s scanning speed and 30 µm layer thickness. The optimal parameters for multi-objective optimisation are identical to those used for single-objective optimisation for tensile strength. The difference between the predicted and actual tensile strength is 0.33%. This work shows a successful methodology using IN718 superalloy for various applications in aerospace industry.

Selective Laser Sintering versus Multi Jet Fusion: A Comprehensive Comparison Study Based on the Properties of Glass Beads‐Reinforced Polyamide 12

Selective laser sintering (SLS) and multi jet fusion (MJF) are the most widespread powder bed fusion additive manufacturing techniques for fabricating polymeric parts since they offer great design flexibility, productivity, and geometrical accuracy. However, these technologies differ in the thermal energy source used to melt the powders as well as the innovative use of printing agents featured in the latter one to promote material consolidation and to avoid thermal bleeding at the part contours. The use of a single powder made of glass beads‐reinforced polyamide 12 (PA12/GB) for the fabrication of MJF and SLS samples makes possible a systematic comparison of the printed parts properties. A thoughtful analysis of the microstructure and mechanical properties of the samples reveals differences and peculiarities between the two technologies. SLS exhibits lower porosity and higher mechanical performances when the parts are printed along the build plane thanks to the powerful heating ensured by the laser. In contrast, MJF samples show higher mechanical isotropy with great flexural and tensile behavior for vertically oriented parts. The role of glass beads in the material behavior is defined by their mechanical properties, meaning higher rigidity and lower strength compared to neat PA12, and fracture mechanism.

Laser powder bed fusion additive manufacturing of molybdenum using a nitrogen build atmosphere

A method was developed to suppress the crack formation during laser powder bed fusion (LPBF) of molybdenum (Mo). The method uses a nitrogen (N2) atmosphere to fabricate crack-free samples without any additional processing. To compare with the effect of the N2 atmosphere, samples were prepared under an argon (Ar) atmosphere as well with the same process parameters. The microstructure analyses by optical and high-resolution electron microscopy revealed that crack-free samples were produced under this condition, whereas grain boundary cracks were observed throughout the sample fabricated in Ar atmosphere. To understand the mechanism of crack prevention by nitrogen (N), the energetics of solute components in the BCC-Mo structure were studied by Density Functional Theory (DFT). The theoretical calculations showed that the introduction of N into the BCC-Mo hinders oxygen (O) diffusion, causing O entrapment in the lattice at interstitial positions. As a result, reduced oxides within grains and at grain boundaries (GB) were observed. The results are consistent with the experimental observation made from the fracture surface investigation. The reduced embrittlement of GB from the reduced GB oxide formation is believed to suppress GB cracking.

In-process monitoring and direct simulation of Argon shielding gas and vapour dynamics to control laser-matter interaction in laser powder bed fusion additive manufacturing

Laser powder bed fusion (L-PBF) additive manufacturing (AM) enables the fabrication of parts with precise dimensional control, freedom of design and material properties similar to or better than those fabricated using traditional manufacturing approaches. AM quality control depends upon the fundamental of the laser-matter interaction during metal AM using L-PBF to exploit the potential use of the materials and process control. In this work, thermal-fluid dynamics in gas chamber experimentally and computationally is used to elucidate the interplay between vapour, liquid, and solid phases in L-PBF. It is revealed that the argon (Ar) shielding gas flow with varied inlet velocities by different nozzles has a pronounced effect to minimise the laser-fume interaction, resulting in the reduction in unstable metal vapour flow and enhancing laser absorptivity. In-process monitoring via high-speed visualisation has been used to understand the simultaneous gas plume dynamics as a result of vapourisation and subsequent laser-fume interaction, backed up by thermal-fluid flow simulation. Unfavourable process dynamics associated with unwanted defects such as lack of fusion can be avoided to improve process design and enhance process stability.

Probabilistic Data-Driven Modeling of a Melt Pool in Laser Powder Bed Fusion Additive Manufacturing

Probabilistic Data-Driven Modeling of a Melt Pool in Laser Powder Bed Fusion Additive Manufacturing

Correlation between surface and processing parameters of an Nb-48Ti alloy produced by laser powder bed fusion additive manufacturing

Correlation between surface and processing parameters of an Nb-48Ti alloy produced by laser powder bed fusion additive manufacturing

Influence of burnishing process on surface integrity of inconel 718 fabricated by laser powder bed fusion additive manufacturing

Influence of burnishing process on surface integrity of inconel 718 fabricated by laser powder bed fusion additive manufacturing