Bed Fusion Additive Manufacturing Process Research Articles

3D Modeling of the Solidification Structure Evolution of Superalloys in Powder Bed Fusion Additive Manufacturing Processes

Recently, a few computational methodologies and algorithms have been developed to simulate the microstructure evolution in powder bed fusion (PBF) additive manufacturing (AM) processes. However, none of these have attempted to simulate the grain structure evolution in multitrack, multilayer AM components in a fully 3D transient mode and for the entire AM geometry. In this work, a multiscale model, which consists of coupling a transient, discrete-source 3D AM process model with a 3D stochastic solidification structure model, was applied to quickly, efficiently, and accurately predict the grain structure evolution of IN625 alloys during Laser Powder Bed Fusion (LPBF). The capabilities of this model include studying the effects of process parameters and part geometry on solidification conditions and their impact on the grain structure formation within multicomponent alloy parts processed via AM. Validation was accomplished based on single-layer LPBF IN625 benchmark experiments, previously performed and analyzed at the National Institute of Standards and Technology (NIST), USA. This modeling approach can also be used to quantitatively predict the solidification structure of Ti-6Al-4V alloys in electron beam AM processes.

An analytical model for prediction of denudation zone width in laser powder bed fusion additive manufacturing

Accurately modeling the physics of laser-powder interaction is needed for predicting the probabilities of performance-limiting defects in parts additively manufactured using laser powder bed fusion. In this paper, we present an analytical model for rapid prediction of denudation width in laser powder bed fusion additive manufacturing process. The model is based on the force balance analysis for a powder particle sitting on top of the powder bed. The model relates the laser power and laser scan speed to the vapor jet speed and consequently to the inward entrainment gas velocity. The effects of laser power and laser speed on denudation width are investigated and compared with experimental data. In addition, a criterion for the onset of denudation is presented which accounts for chamber gas, powder particle size and density. The results show that for a given laser power the denudation width increases as the laser speed decreases.

Microstructure and mechanical properties of multilayer (TiB+TiB2+TiC)/Ti-6Al-4V composite in laser powder bed fusion additive manufacturing process

The paper presents the analysis of the physical and mechanical properties of the heterogeneous material based on the ceramics TiB, TiB2, TiC, B4C and metal alloy Ti-6Al-4V formed by the SLM method. The effect of ceramic particles TiB, TiB2, TiC, B4C resulting from in situ synthesis under the laser action on the microstructure and hardness of the formed metal-matrix composite has been studied. Under discussion are the main mechanisms of change of the microstructure with secondary ceramic insertions, the hardness is measured at the micro-level.

Cost-Aware Design and Fabrication of New Support Structures in Laser Powder Bed Fusion: Microstructure and Metallurgical Properties

This study investigates the effect of support structures on the properties of Inconel 718 (i.e., IN718) parts produced by the laser powder bed fusion (LPBF) additive manufacturing process. Specifically, the effects of support structure shape (i.e., pin-type, angled-type, cone-type) and geometry (i.e., support wall thickness, and gap) on their composition, hardness, microstructure, and material/time consumption are investigated and compared to the conventionally fabricated Inconel 718. From the microstructural analysis, the deepest melt pools appeared to be formed in the sample fabricated on top of the pin-type support structure having a relatively low wall thickness. The XRD results conveyed that a proper selection of geometrical variables for designing support structure results in elevated levels of the strengthening phases of IN718. The sample fabricated on top of the pin-type support structure showed the highest Vickers hardness value of 460.5 HV, which was even higher than what was reported for the heat-treated wrought Inconel 718 (355–385 HV). Moreover, for the thinner support wall thickness, an improvement in the hardness value of the fabricated samples was observed. This study urges a reconsideration of the common approach of selecting supports for additive manufacturing of samples when a higher quality of the as-fabricated parts is desired.

A Simulation Study on the Effect of Particle Size Distribution on the Printed Geometry in Selective Laser Melting

Abstract Selective laser melting (SLM) process is a powder bed fusion additive manufacturing process that finds applications in aerospace and medical industries for its ability to produce complex geometry parts. As the raw material used is in the powder form, particle size distribution (PSD) is a significant characteristic that influences the build quality in turn affecting the functionality and esthetic aspects of the product. This article investigates the effect of PSD on the printed geometry for 316L stainless steel pow der, where three coupled in-house simulation tools based on discrete element method (DEM), computational fluid dynamics (CFD), and structural mechanics are employed. DEM is used for simulating the powder bed distribution based on the different powder PSD. The CFD is used as a virtual testbed to determine thermal parameters such as heat capacity and thermal conductivity of the powder bed viewed as a continuum. The values found as a stochastic function of the powder distribution are used to analyze the effect on the melted zone and deformation using structural mechanics. Results showed that mean particle size and PSD had a significant effect on the packing density, melt pool layer thickness, and the final layer thickness after deformation. Specifically, a narrow particle size distribution with smaller mean particle size and standard deviation produced solidified final layer thickness closest to nominal layer thickness. The proposed simulation approach and the results will catalyze the development of geometry assurance strategies to minimize the effect of particle size distribution on the geometric quality of the printed part.

Additively manufactured Haynes 282: effect of unimodal vs. bimodal γʹ- microstructure on mechanical properties

This study examines the effects of lower-than-nominal heat treatment (HT) temperatures and cooling rates —possible during the treatment of larger components— on microstructure and tensile properties Haynes 282 manufactured via laser powder bed fusion (L-PBF) additive manufacturing process. Two main HT schedules, i.e., conventional treatment and its lower temperature variants were performed. The microstructural evolution at each step of the HTs was tracked and compared. It was shown that solution temperature of a wide range (1062–1146 °C) could moderately remove the as-solidified dendritic microstructure resulting in similar distribution of grain boundary (GB) carbides. In addition, the lower temperature double aging process resulted in a bimodal γ′-precipitates consisting of both nano-sized secondary γʹ- and large primary (>∼100 nm) γ′-precipitates, while the conventional aging gave rise to a unimodal γ′-precipitates with precipitates of diameter ∼60 nm. Owing to the bimodal γ′-precipitates and a similar GB carbide distribution, the low temperature HTs resulted in comparable tensile properties to the conventional HT despite having much larger γ′-precipitates. The results imply that moderate HT temperature deviations due to excessive component size, such as lower temperatures and slower heating/cooling rates, does not negatively impact the mechanical properties of L-PBF Haynes 282.

Effects of build orientation and heat treatments on the tensile and fracture toughness properties of additively manufactured AlSi10Mg

The purpose of this study aimed to examine how build orientation and heat treatments affect microstructure, and consequently, the mechanical properties obtained from tensile and fracture toughness tests of additively manufactured AlSi10Mg samples. Samples were manufactured in several orientations using a laser-based powder bed fusion (L-PBF) additive manufacturing (AM) process, following which they were subjected to three separate heat treatments: (i) stress relief at 300 °C for 2 h (SR); (ii) homogenization at 540 °C for 2 h followed by artificial aging at 170 °C for 8 h (HA-1); (iii) homogenization at 540 °C for 6 h followed by artificial aging at 170 °C for 14 h (HA-2). Tensile tests showed that the AB samples presented the highest mechanical strength; however, low ductility was also observed. Therefore, a model for crack propagation during tensile testing was proposed for L-PBF typical microstructure. Samples subjected to fracture toughness tests showed sensitivity to porosity and microstructure. The crack-tip opening displacement (CTOD) of the AB and HA-2 samples showed similar average values of approximately 10 µm in all three orientations. The SR samples showed the best fracture toughness behavior with average values ranging from 19 µm to 32 µm. The HA-1 samples presented average values between 13 µm and 22 µm. The fracture toughness values reported in the J-integral ranged from 6.0 to 8.4 kJ.m−2, 10.4 to 15.5 kJ.m−2, 5.5 to 7.8 kJ.m−2, and 4.6 to 5.7 kJ.m−2, respectively, for the AB, SR, HA-1, and HA-2 heat treatments.

Digitally twinned additive manufacturing: Detecting flaws in laser powder bed fusion by combining thermal simulations with in-situ meltpool sensor data

The goal of this research is the in-situ detection of flaw formation in metal parts made using the laser powder bed fusion (LPBF) additive manufacturing process. This is an important area of research, because, despite the considerable cost and time savings achieved, precision-driven industries, such as aerospace and biomedical, are reticent in using LPBF to make safety–critical parts due to tendency of the process to create flaws. Another emerging concern in LPBF, and additive manufacturing in general, is related to cyber security – malicious actors may tamper with the process or plant flaws inside a part to compromise its performance. Accordingly, the objective of this work is to develop and apply a physics and data integrated strategy for online monitoring and detection of flaw formation in LPBF parts. The approach used to realize this objective is based on combining (twinning) in-situ meltpool temperature measurements with a graph theory-based thermal simulation model that rapidly predicts the temperature distribution in the part (thermal history). The novelty of the approach is that the temperature distribution predictions provided by the computational thermal model were updated layer-by-layer with in-situ meltpool temperature measurements. This digital twin approach is applied to detect flaw formation in stainless steel (316L) impeller-shaped parts made using a commercial LPBF system. Four such impellers are produced emulating three pathways of flaw formation in LPBF parts, these are: changes in the processing parameters (process drifts); machine-related malfunctions (lens delamination), and deliberate tampering with the process to plant flaws inside the part (cyber intrusions). The severity and nature of the resulting flaws, such as porosity and microstructure heterogeneity, are characterized ex-situ using X-ray computed tomography, optical and scanning electron microscopy, and electron backscatter diffraction. The digital twin approach is shown to be effective for detection of the three types of flaw formation causes studied in this work.

Effects of processing parameters on densification behavior, microstructure evolution and mechanical properties of W–Ti alloy fabricated by laser powder bed fusion

Tungsten and its alloys have been widely used in various industries such as nuclear energy, medical shielding, rocket nozzle. However, the intrinsic high melting point and high ductile-to-brittle transition temperature (DBTT) normally limit their further applications. In this work, the transition element Ti modified W alloy was processed by laser powder bed fusion (LPBF) additive manufacturing process with different laser processing parameters. The densification behavior, microstructure evolution, and mechanical performance of LPBF-processed W–Ti samples were systematically investigated through the optimization of laser processing parameters. It indicated that the highest relative density of 99.1% without obvious pores and balling phenomenon were obtained at the applied energy density of 848 J/mm3 with laser power of 350 W and scan speed of 275 mm/s. The typical microstructure of LPBF-processed W–Ti alloy could be divided into two main regions including W bulk region and W dendrite region with the chemical compositions of W–Ti solid solution, which was caused by the significant difference of melting point between W and Ti. The high microhardness of 731 HV0.2 and compressive performance (σbc = 1719 MPa, δ = 24.7%) were achieved. Compared with pure tungsten samples of our previous work (902 MPa), the compressive strength of W–Ti alloy by LPBF displayed significant improvement of 90%, which could be attributed to the increased densification level and solid solution strengthening mechanism. These findings provided the knowledge of the relationship between laser processing parameters and properties of LPBF-processed W–Ti alloy, further providing reference values applicable for development of W-based alloys by laser additive manufacturing.

Giant 5.8% magnetic-field-induced strain in additive manufactured Ni-Mn-Ga magnetic shape memory alloy

Laser powder bed fusion (L-PBF) additive manufacturing process was employed to manufacture polycrystalline Ni-Mn-Ga samples. The samples were heat-treated for chemical homogenization and grain growth. It is demonstrated that the chemical composition, resulting martensitic crystal structures, and phase transformation temperatures of the L-PBF-built Ni-Mn-Ga can be precisely changed in-situ by controlling the selective evaporation of Mn through adjusting the process parameters. Subsequently, repeatable and fully reversible magnetic-field-induced strain of 5.8% was measured in a single crystalline grain of an additive manufactured polycrystalline Ni-Mn-Ga sample exhibiting a 10M martensitic structure at ambient temperature. The results indicate that L-PBF can be used to manufacture Ni-Mn-Ga devices containing active parts that can be strained by an external magnetic field.

Time Dependent Scanning Path Optimization for the Powder Bed Fusion Additive Manufacturing Process

In this paper, scanning paths optimization for the powder bed fusion additive manufacturing process is investigated. The path design is a key factor of the manufacturing time and for the control of residual stresses arising during the building, since it directly impacts the temperature distribution. In the literature, the scanning paths proposed are mainly based on existing patterns, the relevance of which is not related to the part to build. In this work, we propose an optimization algorithm to determine the scanning path without a priori restrictions. Taking into account the time dependence of the source, the manufacturing time is minimized under two constraints: melting the required structure and avoiding any over-heating causing thermally induced residual stresses. The results illustrate how crucial the part’s shape and topology is in the path quality and point out promising leads to define path and part design constraints.

The effects of powder reuse on the mechanical response of electron beam additively manufactured Ti6Al4V parts

High cost of metal powders has increased the demand for recycling of unmelted powder in electron beam powder bed fusion additive manufacturing process. However, powder characteristics are likely to change during manufacturing, recovery and reuse. It is important to track the evolution of powder characteristics at different stages of recycling to produce components with consistent properties. The present work evaluates the changes in Ti6Al4V powder properties during manufacturing by characterising powder particles at different locations in the powder bed; recovery and reuse, through evaluating the effects of the powder recovery system and sieving for 10 build cycles. Heterogeneous powder degradation occurred during manufacturing with the particles closer to the melt zone showing higher oxygen content and thicker α laths with β phase boundaries. Most of them had a hard-sintered and agglomerated powder morphology in contrast to particles at the edges of the powder bed. Recovery and reuse resulted in a refined particle size distribution, but only marginal change in powder morphology. The increased oxygen caused a slight increase in the yield and tensile strengths of the build. The effect of powder reuse on material elongation, hardness and Charpy impact energy was negligible. The high cycle fatigue performance deteriorated with reuse due to the increased lack-of-fusion defects. This might be attributed to the voids formed in the powder bed due to decrease in the number of fine particles coupled with an increase in the number of high-aspect ratio particles.

Relationships between porosity and mechanical properties of polyamide 12 parts produced using the laser sintering and multi-jet fusion powder bed fusion processes

Laser powder bed fusion (L-PBF, or Laser Sintering) and Multi-Jet Fusion (MJF) are two commercial powder bed fusion (PBF) additive manufacturing (AM) processes for polymers. While the strength of parts produced with polymer PBF processes is typically good, the toughness of AM parts is significantly reduced compared to traditional processes, such as Injection Molding (IM). This study evaluates the impact of residual porosity on the strength and ductility of polyamide-12 (PA-12) parts produced by powder bed fusion. Hot isostatic pressure (HIP) treatment is used to eliminate virtually all porosity from treated L-PBF and MJF parts. The relationship between porosity and mechanical properties (ultimate tensile strength, elongation at break, toughness) is evaluated for groups of parts with and without HIP treatment. For untreated parts, the thickness of the part was found to have a significant impact on mechanical properties, with 5 mm parts having more than 25% higher UTS, 39% higher maximum strains, and 21% higher Young's Moduli than 1 mm parts produced using the same process. In L-PBF parts, the HIP treatment greatly reduced porosity and increased ultimate tensile strengths by an average of 12% compared to parts that were not subjected to HIP, but they also exhibited reduced max strains. While HIP eliminated porosity in MJF parts, MJF parts did not exhibit any statistically significant improvements in mechanical properties from HIP. Though it was not possible to correlate the strength of an untreated part with density, a correlation does exist between density and some mechanical properties (UTS, Young's Modulus) as a result of a density change in L-PBF parts due to a post-processing HIP treatment. • No direct correlation between density and mechanical properties for L-PBF or MJF. • Hot isostatic pressure (HIP) treatment eliminated porosity in PA12 parts. • HIP increased tensile strength but reduced strain at failure. • Pore elimination did not improve ductility in PA12 AM parts.

A framework for effective and clean conversion of machining waste into metal powder feedstock for additive manufacturing

The dependence on conventional metal cutting technologies to satisfy consumer demands for more customised products and services is contributing to the generation of ever-increasing metallic waste in the form of machining chips (MCs). Metallic MCs are of high value if recycled by sustainable techniques. Conventional recycling by melting the MCs for industrious use is neither economical nor environment friendly. Thus, the advanced technologies for clean recycling of MCs for industrious use are the need of the hour. In this paper, consideration is given to the role of one such advanced recycling technology: ball milling (BM). The efficacy of BM for recycling MCs by converting them into chip powder (CP) is evaluated. The produced chip powder (CP) can be utilized as feedstock powder in various powder bed fusion (PBF) additive manufacturing processes like laser engineered net shaping (LENS) and electron beam melting (EBM) for the manufacturing of near-net shaped products. The consequences of adopting this novel recycling technology on industrial sustainability are not well understood and this exploratory study draws on publicly available data to provide insights into the impacts of BM on sustainability. Benefits of BM are found to exist which include promising results in obtaining 70–90% sphericity with the utilization of 20–30% less energy in comparison to conventional powder production techniques. As an immature technology, there are substantial challenges to these benefits being realised. This paper summarises these advantages, challenges, and discusses the implications of BM as sustainable recycling technology in terms of process parameters on the mechanical, physical, and morphological properties of the CP produced. In addition, a framework is presented which suggests the plausible methodology for recycling MCs into CP from the generation stage to the final utilization stage. • Sustainable pathway for the conversion of machining chips to powder feedstock. • Discretization of machining chips can save averagely 10% energy during recycling. • Multi-stage ball milling shows promising results in obtaining 70–90% sphericity. • Sustainability of the method is attributed to the utilization of 20–30% less energy. • Metal loss is only 5–7% compared to conventional recycling techniques.

Effect of post-processing on microstructure and mechanical properties of Alloy 718 fabricated using powder bed fusion additive manufacturing processes

PurposeThis study aims to investigate additive manufacturing of nickel-based superalloy IN718 made by powder bed fusion processes: powder bed fusion laser beam (PBF-LB) and powder bed fusion electron beam (PBF-EB).Design/methodology/approachThis work has focused on the influence of building methods and post-fabrication processes on the final part properties, including microstructure, surface quality, residual stresses and mechanical properties.FindingsPBF-LB produced a much smoother surface. Blasting and shot peening (SP) reduced the roughness even more but did not affect the PBF-EB surface finish as much. As-printed PBF-EB parts have low residual stresses in all directions, whereas it was much higher for PBF-LB. However, heat treatment removed the stresses and SP created compressive stresses for samples from both PBF processes. The standard Arcam process parameter for PBF-EB for IN718 is not fully optimized, which leads to porosity and inferior mechanical properties. However, impact toughness after hot isostatic pressing was surprisingly high.Originality/valueThe two processes gave different results and also responses to post-treatments, which could be of advantage or disadvantage for different applications. Suggestions for improving the properties of parts produced by each method are presented.

Simulation Study of Hatch Spacing and Layer Thickness Effects on Microstructure in Laser Powder Bed Fusion Additive Manufacturing using a Texture-Aware Solidification Potts Model

Microstructure control in the laser powder bed fusion additive manufacturing processes is a topic of major interest because of the submillimeter length scale at which the manufacturing process occurs. The ability to control the process at the melt pool scale allows for microstructure control that few other manufacturing techniques can match. The majority of work on microstructure control has focused on altering laser parameters to control solidification conditions (Ref (R.R. Dehoff, M.M. Kirka, W.J. Sames, H. Bilheux, A.S. Tremsin, L.E. Lowe, and S.S. Babu, Site Specific Control of Crystallographic Grain Orientation through Electron Beam Additive Manufacturing, Mater. Sci. Technol., 2014, 31(8), p 931–938. R. Shi, S.A. Khairallah, T.T. Roehling, T.W. Heo, J.T. McKeown, and M.J. Matthews, Microstructural Control in Metal Laser Powder Bed Fusion Additive Manufacturing Using Laser Beam Shaping Strategy, Acta Mater., 2020, 184, p 284–305, https://doi.org/10.1016/j.actamat.2019.11.053.)). Other machine parameters, besides the laser parameters, have also been shown to affect the microstructure of AM parts (Ref (N. Nadammal, S. Cabeza, T. Mishurova, T. Thiede, A. Kromm, C. Seyfert, L. Farahbod, C. Haberland, J.A. Schneider, P.D. Portella, and G. Bruno, Effect of Hatch Length on the Development of Microstructure, Texture and Residual Stresses in Selective Laser Melted Superalloy Inconel 718, Mater. Des., 2017, 134, p 139–150, https://doi.org/10.1016/j.matdes.2017.08.049. F. Geiger, K. Kunze, and T. Etter, Tailoring the Texture of IN738LC Processed by Selective Laser Melting (SLM) by Specific Scanning Strategies, Mater. Sci. Eng. A, 2016, 661, p 240–246, https://doi.org/10.1016/j.msea.2016.03.036.)). We propose an investigation of the effects of hatch spacing and layer thickness on microstructure development in laser powder bed fusion additive manufacturing. A Monte Carlo Potts model with textured solidification capabilities is used to study the effects of these parameters on a unidirectional scan strategy. The simulation results reveal substantial changes in grain morphology as well as texture. Additionally, EVP-FFT crystal plasticity simulations were performed to evaluate the effect of the microstructural shifts on mechanical response. The conclusions from this work elucidate the effects of these parameters on part microstructure as predicted by the texture-aware solidification Potts model and inform understanding of how bulk texture is predicted by the simulation approach.

Measurement Uncertainty of Surface Temperature Distributions for Laser Powder Bed Fusion Processes.

This paper describes advances in measuring the characteristic spatial distribution of surface temperature and emissivity during laser-metal interaction under conditions relevant for laser powder bed fusion (LPBF) additive manufacturing processes. Detailed descriptions of the measurement process, results, and approaches to determining uncertainties are provided. Measurement uncertainties have complex dependencies on multiple process parameters, so the methodology is demonstrated on one set of process parameters and one material. Well-established literature values for high-purity nickel solidification temperature and emissivity at the solidification temperature were used to evaluate the predicted uncertainty of the measurements. The standard temperature measurement uncertainty is found to be approximately 0.9% of the absolute temperature (16 AC), and the standard relative emissivity measurement uncertainty is found to be approximately 8% at the solidification point of high-purity nickel, both of which are satisfactory. This paper also outlines several potential sources of test uncertainties, which may require additional experimental evaluation. The largest of these are the metal vapor and ejecta that are produced as process by-products, which can potentially affect the imaging quality, reflectometry results, and thermal signature of the process, while also affecting the process of laser power delivery. Furthermore, the current paper focuses strictly on the uncertainties of the emissivity and temperature measurement approach and therefore does not detail a variety of uncertainties associated with experimental controls that must be evaluated for future generation of reference data.

Application of Deep Transfer Learning and Uncertainty Quantification for Process Identification in Powder Bed Fusion

Abstract Accurate identification and modeling of process maps in additive manufacturing remains a pertinent challenge. To ensure high quality and reliability of the finished product researchers, rely on models that entail the physics of the process as a computer code or conduct laboratory experiments, which are expensive and oftentimes demand significant logistic and overheads. Physics-based computational modeling has shown promise in alleviating the aforementioned challenge, albeit with limitations like physical approximations, model-form uncertainty, and limited experimental data. This calls for modeling methods that can combine limited experimental and simulation data in a computationally efficient manner, in order to achieve the desired properties in the manufactured parts. In this paper, we focus on demonstrating the impact of probabilistic modeling and uncertainty quantification on powder-bed fusion (PBF) additive manufacturing by focusing on the following three milieu: (a) accelerating the parameter development processes associated with laser powder bed fusion additive manufacturing process of metals, (b) quantifying uncertainty and identifying missing physical correlations in the computational model, and (c) transferring learned process maps from a source to a target process. These tasks demonstrate the application of multifidelity modeling, global sensitivity analysis, intelligent design of experiments, and deep transfer learning for a meso-scale meltpool model of the additive manufacturing process.

Algorithm Development for Generation of Powder Particles for Use in Numerical Simulations of Powder Bed Fusion Additive Manufacturing Processes

This article presents an algorithm development methodology for incorporating the effects of powder quality into numerical simulations of powder bed fusion technologies. The framework leverages x-ray microtomography measurements of samples representative of changes in powder quality with powder reuse. Individual particles are labelled and analysed by postprocessing the three-dimensional data. Included in this data is the particle surface normal and point cloud information which can be used to reconstruct the particle using Poisson Surface Reconstruction in Open3D. This reconstruction can then be exported as a Standard Tessellation Language (STL) file that can be incorporated into discrete element method solvers such as YADE or finite element frameworks such as MOOSE. In this sense, libraries of particles can be generated and used in simulations. To demonstrate the applicability of this method, a publicly available NIST dataset is leveraged. In lieu of experimental data, a second method using Computer-Generated Imagery (CGI) software, Blender, is also presented. The methods are able to replicate powder features, such as satellites and surface irregularity, that are functions of both powder manufacturing methods as well as powder reuse cycles. These particle irregularities have important implications on part quality and manufacturing cost.

A Coupled DEM/SPH Computational Model to Simulate Microstructure Evolution in Ti-6Al-4V Laser Powder Bed Fusion Processes

A new multi-stage three-dimensional transient computational model to simulate powder bed fusion (L-PBF) additive manufacturing (AM) processes is presented. The model uses the discrete element method (DEM) for powder flow simulation, an extended smoothed particle hydrodynamics (SPH) for melt pool dynamics and a semi-empirical microstructure evolution strategy to simulate the evolving temperature and microstructure of non-spherical Ti-6Al-4V powder grains undergoing L-PBF. The highly novel use of both DEM and SPH means that varied physics such as collisions between non-spherical powder grains during the coating process and heat transfer, melting, solidification and microstructure evolution during the laser fusion process can be simulated. The new capability is demonstrated by applying a complex representative laser scan pattern to a single-layer Ti-6Al-4V powder bed. It is found that the fast cooling rate primarily leads to a transition between the β and α martensitic phases. A minimal production of the α Widmanstatten phase at the outer edge of the laser is also noted due to an in situ heat treatment effect of the martensitic grains near the laser. This work demonstrates the potential of the coupled DEM/SPH computational model as a realistic tool to investigate the effect of process parameters such as powder morphology, laser scan speed and power characteristics on the Ti-6Al-4V powder bed microstructure.