Effect of powder reuse on CoCrMo alloy feedstock and printed parts in laser powder bed fusion

Selective laser sintering (SLS) and Selective Laser Melting (SLM) are parent layer manufacturing processes that allow generating complex 3D parts by consolidating layers of powder material on top of each other. Consolidation is obtained by processing the selected areas using the thermal energy supplied by a focused laser beam. In SLS partial fusion of powder particles takes place, followed by a solidification of the created liquid. SLM is essentially the same process as SLS, with the difference that the particles are completely molten under the laser beam. This development is driven by the need to produce near full dense objects, with mechanical properties comparable to those of bulk materials and by the desire to avoid lengthy post processing cycles. Identification of the optimal process conditions (so-called process window) is a crucial task for industrial application of SLS/SLM processes. Operating parameters of the process are adjusted in correspondence with optical and thermal properties of the processed material. Nowadays in SLS/SLM there is a tendency to increase the speed of the fabrication as a consequence of the available higher laser powers. It leads to increase of laser scanning speeds. In these circumstances, to rely only on experimental investigations in order to adjust process and material parameters is time-consuming and ineffective. Simulation tools are strongly needed for the visualization and analysis of SLS/SLM processes. In SLM the powder grains under the laser are completely molten and form a liquid domain called melt pool. Evolution of the melt pool during the process, its interaction with the laser, the substrate and the surrounding non-molten powder strongly affect the quality of the final part. The goal of this work is to study the melt pool dynamics by means of the finite-element simulation software, built specially for SLS/SLM. The numerical model is based on the homogeneous medium hypothesis. It considers the interaction between the laser and the powder material, the phase transformations and the evolution of the material properties during the process. We also study the influence of the phase change on the process efficiency. The macroscopic model is completed by the sub-models, which allow to study at microscopic level the processes taking place in the powder bed during its laser heating and melting. Melting of separate powder particles during laser irradiation is studied by means of the improved Single Grain Model. The capillary phenomena taking place in the powder bed during SLS/SLM are also studied. The interconnection of powder grains during their melting is approached by the mechanism of liquid drops coalescence. According to the obtained results, the depth-dependent sintering threshold for powder materials is proposed.

Additive manufacturing (AM) has shown advantageous aspects over conventional manufacturing methods, e.g., additional design freedom, ability to fabricate complex shapes without the need for post-processing, and enhanced mechanical performance of the fabricated parts. The development and application of AM techniques, such as laser powder bed fusion (LPBF), however, are hampered by the relatively limited types of alloys that are amenable to AM, e.g., 316L stainless steel, Inconel 718, and Ti64. Therefore, metal AM is still in its nascent stage, with the exploration of new alloy chemistries and/or fabrication of components with unique functional and/or mechanical properties being relatively unexplored. The combination of in-situ alloying and compositionally graded alloy is promising to accelerate the composition screening/alloy designing for AM, as it can rapidly examine the printability, micro-/meso-structure, and properties of the AM-produced parts and therefore shorten the time span between conceptualization of new alloys and their deployment in service. In this study, among the several available AM processes, the LPBF technique is selected to fabricate compositionally graded alloys, as it has higher spatial resolution and lower cost, compared to directed energy deposition (DED) and electron beam powder bed fusion (EBPBF). With three different LPBF setups and four alloy systems examined in this study, the following objectives are achieved: (i) screening out, from a large range of compositions, the alloys that are amenable to LPBF, (ii) examining the effects of the LPBF characteristics, e.g., rapid cooling rate, on the micro-/meso-structure and properties of the fabricated parts with varying chemical compositions, and (iii) evaluating the fabrication methods in terms of types of achieved gradation (stepwise or smooth), efficiency, and level of chemical segregation. Through pre-packing the mixed powders in the powder supply bin, compositionally graded Fe-Al (Al contents in range of 9.8–40.8 at.%), and AlxCrCoFeNi (x = 0.07–0.88 molar ratio) were fabricated and studied. The chemical compositions, microstructures, and mechanical properties were examined to investigate the effect of Al contents. In Fe-Al alloys, the critical Al content for crack-free printing was examined to be 35 at.% and a columnar to equiaxed grain structure transition with increasing Al content is seen along the gradient/building direction, due to the varying levels of constitutional undercooling (CUC). Regarding the AlxCrCoFeNi alloy, the maximum Al that can enable crack-free printing was observed to be x (molar ratio) = 0.5, and the phase variations in the graded alloys were found to be closely associated with x. A CoCrMo-Ni graded alloy (26.8–9.8 wt.% Ni) with smooth compositional gradient was successfully fabricated using a customized LPBF system, where the gradient was created along the transverse direction instead of building direction. Two types of grain morphologies and chemical segregation bands, which are either rich in Ni or in CoCrMo alloy, were observed. Incomplete mixing of the powders combined with the high aspect ratio of the melt pool and the 67° scan rotation between successive layers are the reasons behind the formation of these bands. Detailed mechanical property characterization shows that the microscale chemical segregation can not only strengthen the matrix but also improve the work hardening ability of the bulk material through kinematic hardening mechanism. Compositionally graded Cu-Ni alloys with 0–9.8 wt.% Ni were fabricated using a novel and simple powder supply system in LPBF. With no need to alter the mechanical setup of the LPBF machine or its powder feeding system, this strategy can efficiently create smooth compositional gradient without introducing significant chemical segregations. 7.6 wt.% Ni was proved to be the lower threshold for fabricating nearly full-density Cu-Ni binary alloy. Cu-7.6Ni-3Al (wt.%) was further designed and fabricated. After aging till the peak condition, the Cu-Ni-Al alloy shows significantly higher strength and electrical conductivity than in the as-printed (AP) condition, due to the formation of the Ni3Al precipitates during aging. In this study, three different in-situ alloying methods are utilized on four alloy systems to explore and potential of utilizing compositionally graded alloy as a high-throughput method to study the composition-microstructure-property relationship of the LPBF produced alloys. On the basis of the findings, it is believed that such method can contribute greatly to the alloy designing and further development of AM.

Depending on the application, establishing a strategy for selecting the type of powder bed fusion technology—from electron beam (EB-PBF) or laser powder bed fusion (L-PBF)—is important. In this study, we focused on the β-type Ti–15Mo–5Zr–3Al alloy (expected for hard-tissue implant applications) as a model material, and we examined the variations in the microstructure, crystallographic texture, and resultant mechanical properties of specimens fabricated by L-PBF and EB-PBF. Because the melting mode transforms from the conduction mode to the keyhole mode with an increase in the energy density in L-PBF, the relative density of the L-PBF-built specimen decreases at higher energy densities, unlike that of the EB-PBF-built specimen. Although both EB-PBF and L-PBF can obtain cubic crystallographic textures via bidirectional scanning with a 90° rotation in each layer, the formation mechanisms of the textures were found to be different. The <100> texture in the build direction is mainly derived from the vertically grown columnar cells in EB-PBF, whereas it is derived from the vertically and horizontally grown columnar cells in L-PBF. Consequently, different textures were developed via bidirectional scanning without rotation in each layer: the <110> and <100> aligned textures along the build direction in L-PBF and EB-PBF, respectively. The L-PBF-built specimen exhibited considerably better ductility, but slightly lower strength than the EB-PBF-built specimen, under the conditions of the same crystallographic texture and relative density. We attributed this to the variation in the microstructures of the specimens; the formation of the α-phase was completely absent in the L-PBF-built specimen. The results demonstrate the importance of properly selecting the two technologies according to the material and its application.

In this work, the microstructure and hot deformation behavior of laser powder bed fusion (L-PBF) and conventionally cast Fe-25Al-1.5Ta (at.%) alloys were compared. The L-PBF builds recrystallized comparably to the as-cast samples during hot deformation. Nevertheless, distinct differences were observed in the flow behavior characteristics between the as-cast and L-PBF samples. The L-PBF builds exhibited lower flow stress than the as-cast material over the entire deformation conditions tested. The average activation energy of hot deformation (Q) of 344 kJ mol−1 was calculated for the L-PBF build and 385 kJ mol−1 for the cast material. The lower Q indicates lower deformation resistance of the L-PBF sample. The peak work hardening rate (θ) in the L-PBF sample (1.72 × 103 MPa) was significantly smaller than that of the as-cast sample (3.02 × 103 MPa), suggesting that the dislocation glide in the L-PBF sample is less hindered during deformation. Possible sources of the observed differences in the deformation behavior between the L-PBF and cast materials will be discussed. Initial and post-deformation microstructures were characterized using an X-ray diffractometer (XRD) and ultra-high-resolution scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (EDX) detector. The C14-(Fe, Al)2Ta Laves phase (P63/mmc) was predominantly formed at the A2 α-(Fe, Al) matrix phase grain boundaries in both the as-cast and L-PBF materials. The XRD results suggest that the ordering transition from B2-FeAl to a D03-Fe3Al phase occurs during casting, but rarely during ultra-high-cooling L-PBF processing. In summary, the L-PBF creates samples that are subject to less work hardening and require less deformation resistance, and thus, can be formed by a lower deformation force. It, in turn, reduces the loads imposed on the tooling and dies during the deformation processing, contributing to less wear and the high durability of dies.

Electric discharge aided surface post-treatment of laser powder bed fused non-planar metallic components for enhanced form accuracy

Effect of process parameters on the formation of single track in pulsed laser powder bed fusion

Advances in computational modeling for laser powder bed fusion additive manufacturing: A comprehensive review of finite element techniques and strategies

Additive layer manufacturing is a blanket term for a wide range of processes operating on the same underlying principle. 3D geometry is created by depositing material, layer by layer to create a final 3D geometry. Selective laser melting (SLM) is a branch of additive layer manufacturing, using a laser to fuse a powder bed of metal into each layer. This thesis investigates the SLM process and its application to nickel based superalloy materials, IN625 and IN718. IN625 and IN718 are similar nickel-based superalloys developed for use in aerospace gas turbine engines. In their conventionally manufactured form, these materials have excellent high temperature mechanical properties which make them idea for use in the hot section of gas turbine engines. The aim of this thesis was to investigate how these materials interact with the SLM process and how the material produced can be optimised to improve the range of applications it can be used for. A gap in knowledge regarding a detailed understanding of how the powders morphological and rheological properties influence its ability to be processed by SLM was identified and investigated. A wide range of characterisation methods were implemented with certain important properties being identified to assess a powders processability, namely the particle size distribution and how a significant content of fine particles below 10 μm in size can be detrimental to processability. There is also a lack of a standard powder characterisation methodology specifically for SLM applications. This is addressed with certain methods and measurements being suggested as most promising for wider SLM application. Avalanche flow testing is found to be most applicable to the critical recoating process in SLM and most able to differentiate suitable and unsuitable SLM powders. Following characterisation of the raw material feedstock powder, this thesis also investigates the influence of processing parameters on the microstructure of the material produced by the SLM process. Significant microstructural changes were observed as a result of process parameter changes. This was identified to potentially enable for in-situ modification of material microstructure to suit a manufactured material to its end application. Of the process parameters investigated, laser scan speed was most interesting, suggesting that a faster laser scan speed was able to create a similar microstructure to a much slower one. This was attributed to the reheating effect of the laser beam returning quickly to the adjacent scan line. The validity of this explanation was investigated using a simple, computational thermal model. The result is a new understanding of laser scan speed SLM and its nonlinear relationship with material temperature and microstructure evolution. Finally, post process heat treatments of SLM manufactured IN718 material were investigated. This investigation was in response to a gap in current knowledge regarding heat treatments designed specifically for SLM material. SLM IN718 has been found to have reduced high temperature mechanical properties, specifically stress rupture, which limits its application in demanding environments. In this thesis a range of post process homogenisation heat treatments were investigated, with treatments between 1030 °C and 1060 °C being found to produce material with characteristics consistent with material with excellent stress rupture properties. This novel heat treatment route could provide a method for SLM IM718, and the increased design and geometric freedoms, to be applied in more demanding applications. An evolution of the grain structure in the material was also observed and measured during high temperature homogenisation treatments. This was investigated in the final chapter, and a novel mechanism is suggested for the process of grain coarsening observed. Previously published literature explains similar evolutions as recrystallisation however this did not fit the observations during this thesis. The evolution of grain structure was observed using a process of quasi in-situ electron back scatter diffraction, and a mechanism of grain boundary length reduction, followed by grain growth, is suggested to better fit the observations. It was determined that grains are preferentially selected for growth based on their proximity to a ‘path of least resistance’ of lower angle grain boundaries. The results of this work should benefit industrial users of SLM in the fabrication of Nickel-Based Superalloy material for aerospace applications. The conclusions on powder characterisation offer an insight into available methods to better control and characterise powder feedstock materials for consistent production. Aerospace users especially may find the work regarding post process heat treatments designed specifically for SLM material, to recover lost stress rupture performance, useful in enabling the use of SLM materials, and the design freedom that brings with it, in mor demanding environments than are currently possible.

In the Laser Powder Bed Fusion (L-PBF) process, 3D components with complex geometries are fabricated in a layer-by-layer fashion by using a controlled laser beam to selectively melt particular regions of the metal powder bed. However, due to the stochastic nature of the L-PBF process, the top surface roughness of each solidified layer tends to be different even when the optimal processing conditions for the different positions on the build plate are employed. As a result, the mechanical properties of the built components frequently vary from one component to the next. Accordingly, this study proposes an Intelligent Additive Manufacturing Architecture (IAMA) for controlling the surface roughness of each build layer through an appropriate adjustment of the laser re-melting parameters. The IAMA architecture comprises five modules, namely In-Situ Metrology (ISM), Ex-Situ Metrology (ESM), Automatic Virtual Metrology (AVM), Additive Manufacturing Simulation (AMS) and Intelligent Compensator (IC). The feasibility of the proposed architecture is demonstrated by comparing the top surface roughness of cubic and mechanical strengths of tensile test samples built using the proposed method with those built using a traditional L-PBF approach without surface roughness control. It is found that the samples fabricated using the IAMA approach have an average top surface roughness of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.6~\mu \text{m}$ </tex-math></inline-formula> and a standard deviation is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.7~\mu \text{m}$ </tex-math></inline-formula> . By contrast, the samples produced using the traditional L-PBF approach have an average surface roughness of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$13.45~\mu \text{m}$ </tex-math></inline-formula> and a standard deviation of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.5~\mu \text{m}$ </tex-math></inline-formula> . In addition, the specimens produced with the assistance of IAMA architecture have an average tensile strength of 1013 MPa with a standard deviation of 69.5 MPa, while those printed without surface roughness control have an average tensile strength of 903 MPa with a standard deviation of 101.4 MPa Note to Practitioners—As L-PBF produce part in a layer-by-layer manner, therefore, the roughness on the top surface of previous layer have a strong influence on the printing quality of current layer. The variations of surface roughness will lead to the fluctuation of mechanical properties of the fabricated components. Additionally, to the best of author’s knowledge, current commercial L-PBF machines can not actively control the surface roughness of samples during the process. The proposed IAMA in this work can predict and control the roughness on the top surface of each layer during L-PBF process. Therefore, the constructed architecture has strong potential for integrating into the commercial L-PBF machine for controlling the surface roughness on the top of the parts during the manufacturing process. As a result, the quality of the fabricated components is expected to be in consistent. Accordingly, L-PBF machine equipped with IAMA will have a strong potential in applying for mass production of the components in aerospace and automobile industries.

Metal additive manufacturing techniques have been recognized for their capability of controlling the crystallographic orientations of stainless steels. However, the inherent anisotropic corrosion behavior has not been extensively studied. In this study, the corrosion properties of 316L stainless steels prepared by Laser Powder Bed Fusion (LPBF) additive manufacturing were investigated. The effects of different crystallographic textures, namely {100}, {110} and {111} on both general and pitting corrosion were characterized by several electrochemical measurements, including Electrochemical Impedance Spectroscopy (EIS), potentiodynamic polarization and Mott-Schottky analysis. The results were also compared to the polycrystalline and wrought 316L counterparts. It was found that the LPBF-{111} sample offered the highest general corrosion resistance, followed by the LPBF-{100}, LPBF-polycrystalline and LPBF-{110} samples (Figure 1). The origin of this trend was related to the atomic surface density. The LPBF-{111} surface exhibited a stronger atomic bonding than that of LPBF-{100} and LPBF-{110} samples, resulting in a higher corrosion activation energy and thus a higher general corrosion resistance. All the LPBF samples also offered a significantly higher pitting corrosion resistance (Figure 2), which was attributed to the lower concentration of oxygen vacancies (donor levels) in the passive film that serve as pits nucleation sites, as observed by the Mott-Schottky analysis (Figure 3). Figure 1

Recently, Additive Manufacturing (AM) technology has been attracting interest because of its process to fabricate directly from CAD data. A laser powder bed fusion (L-PBF), one of the AM technologies, is able to form the metal three dimensional (3D) objects from metal powder by building it layer-by-layer. Generally, intensity distribution of the laser beam employed by L-PBF is Gaussian. Therefore, these beam shape has energy gradient. It is difficult to fabricate with high precision due to metal powder is aggregated at the lower energy area of beam spot. In this study, fabricated with L-PBF for the purpose to clarify the effects of energy gradient of beam spot on surface roughness Ra of 3D objects. As the evaluation method of 3D objects fabricated with L-PBF, it is employed to measuring surface roughness Ra and Vickers hardness.Recently, Additive Manufacturing (AM) technology has been attracting interest because of its process to fabricate directly from CAD data. A laser powder bed fusion (L-PBF), one of the AM technologies, is able to form the metal three dimensional (3D) objects from metal powder by building it layer-by-layer. Generally, intensity distribution of the laser beam employed by L-PBF is Gaussian. Therefore, these beam shape has energy gradient. It is difficult to fabricate with high precision due to metal powder is aggregated at the lower energy area of beam spot. In this study, fabricated with L-PBF for the purpose to clarify the effects of energy gradient of beam spot on surface roughness Ra of 3D objects. As the evaluation method of 3D objects fabricated with L-PBF, it is employed to measuring surface roughness Ra and Vickers hardness.

Process parameters and powder spreading quality are important factors for aluminum matrix composites (AMCs) prepared using laser powder bed fusion (LPBF). In this study, a Box-Behnken Design (BBD) was used to optimize the process parameters, and near-spherical β-SiC was selected to improve the quality of powder spreading. The rationality of parameter optimization was verified by testing the density of samples prepared using different laser power levels. Al4C3 diffraction peaks were found in XRD patterns, which indicated that interface reactions occurred to form good interface bonding between the Al matrix and the SiC particles. The tensile strength and plasticity of LPBF α-SiC/AlSi10Mg were lower than that of LPBF AlSi10Mg, which was mainly due to the poor fluidity of the powder mixtures and powder spreading quality. For LPBF β-SiC/AlSi10Mg, the tensile strength increased and elongation decreased slightly compared to LPBF α-SiC/AlSi10Mg. The data in this study were compared with the data in other studies. In this study, LPBF AlSi10Mg and LPBF β-SiC/AlSi10Mg not only showed the inherent high strength of their LPBF parts, but also had relatively high plasticity. Matching between strength and plasticity was mainly dependent on the scanning strategy. Most studies use uni-directional or bi-directional scanning strategies with a certain rotation angle between layers. A chessboard scanning strategy was used in this study to form a coarse remelted connected skeleton inside the material and significantly improve plasticity. This study lays a theoretical and experimental foundation for the controllable preparation of SiC-reinforced AMCs using LPBF.

Laser powder bed fusion (LPBF) provides a rapid and cost-effective solution for fabricating metallic parts with near full density and high precision, strength, and stiffness directly from metallic powders. In LPBF, process variables are widely recognised as fundamental factors that have important effect on the quality of the built parts. However, activity of designing process variables for LPBF, i.e., process planning for LPBF, still heavily depends on knowledge from domain experts. This necessitates a knowledge base that enables the capture, representation, inference, and reuse of existing knowledge. In this paper, a description logic (DL) based ontology for knowledge representation in process planning for LPBF is presented. Firstly, a set of top-level DL entities and specific DL entities and semantic web rule language (SWRL) rules for part orientation, support generation, model slicing, and path planning are created to construct the ontology. The application of the ontology is then illustrated via process planning on an LPBF part. Finally, the benefits of the ontology are demonstrated through a few examples. The demonstration results show that the ontology has rigorous computer-interpretable semantics, which provides a semantic enrichment model for LPBF process planning knowledge and enables automatic consistency checking of the ontology, knowledge reasoning on the ontology, and semantic query from the ontology. This would lay solid foundation for development of a process planning tool with autonomous decision-making capability.

Tailoring microstructure and mechanical properties by laser powder bed fusion of Ti powder recycled and treated via discharge plasma modification

Corrosion behavior of additively manufactured PH steels in simulated nuclear reactor environments