Electron Beam Melting Process Research Articles

Influences of Post-Heat Treatment on the Microstructure Evolution and Creep Properties of Ni-Based Superalloy IN718 Fabricated by Electron Beam Melting

In this study, the Ni-based superalloy IN718, fabricated using an electron beam melting process, was investigated in as-built and various heat-treated conditions. The relationships between the microstructure characteristics and creep properties were elucidated. Under testing conditions of 650 °C and 650 MPa, the direct-aged specimen exhibited the lowest steady-state creep rate, at 0.15 × 10−8 s−1. The superior creep resistance can be attributed to the higher volume fraction of γ’/γ”-strengthening precipitates within the grain and fine δ precipitates along the grain boundaries. Being coherent to the γ matrix, the nano-sized γ’/γ” precipitates effectively hindered the dislocation motion in the grain interior. In addition, controlled grain boundary δ precipitates inhibited grain boundary sliding and decelerated the steady-state creep strain rate during creep deformation.

Compression Behavior of EBM Printed Auxetic Chiral Structures.

In this study, the cyclic compression and crush behavior of chiral auxetic lattice structures produced from titanium alloy (Ti6Al4V) metallic powder using electron beam melting (EBM) additive manufacturing technology is investigated numerically and experimentally. For material characterization and understanding the material behavior of EBM printed parts, tensile and three-point flexural tests were conducted. Log signals produced during the EBM process were investigated to confirm the stability of process and the health of the produced parts. Furthermore, a compressive cyclic load profile was applied to the EBM printed chiral units having two different thicknesses to track their Poisson’s ratios and displacement limits under large displacements in the absence of degradation, permanent deformations and failures. Chiral units were also crushed to investigate the effect of failure and deformation mechanisms on the energy absorption characteristics. Moreover, a surface roughness study was conducted due to high surface roughness of EBM printed parts, and an equation is offered to define load-carrying effective areas to prevent misleading cross-section measurements. In compliance with the equation and tensile test results, a constitutive equation was formed and used after a selection and calibration process to verify the numerical model for optimum topology design and mechanical performance forecasting using a non-linear computational model with failure analysis. As a result, the cyclic compression and crush numerical analyses of EBM printed Ti6Al4V chiral cells were validated with the experimental results. It was shown that the constitutive equation of EBM printed as-built parts was extracted accurately considering the build orientation and surface roughness profile. Besides, the cyclic compressive and crush behavior of chiral units were investigated. The regions of the chiral units prone to prematurely fail under crush loads were determined, and deformation modes were investigated to increase the energy absorption abilities.

Improved quality and functional properties of Ti-6Al-4V ELI alloy for personalized orthopedic implants fabrication with EBM process

Understanding the manufacturing process parameters and their correlation with final product functional properties is needed to ensure performance in critical applications in the medical industry. This article presents the holistic procedure for verifying the quality of manufactured products, including the determination of basic material characteristics and strength parameters based on EBM (Electron Beam Melting) technology for the titanium alloy Ti-6Al-4V ELI. The aim of this study was to investigate the effects of the manufacturing and post-processing methods used, on the geometric accuracy, microstructure, mechanical and cytotoxic properties of additively manufactured parts, especially in the form of personalized jaw implants. The effect of Hot Isostatic Pressing (HIP) treatment on microstructure and properties is also taken into account. The EBM manufacturing process was developed and samples were built using three different orientations: 0° (horizontally), 45° (diagonally), and 90° (vertically). The novel results are systematically presented and discussed beginning from the selection of process parameters, sample manufacturing, wide microstructural characterization, determination of static mechanical properties as well as geometric accuracy, and quality control verification depending on the manufacturing process and the post-processing methods used. The conducted research allowed for the presentation of the entire process chain of the EBM manufactured medical implant made of high-purity titanium alloy (Ti-6Al-4V ELI), thus filling the scientific gaps related to the selective presentation and discussion of research results published in the literature on the subject.

Alloy design of Ni-based superalloy with high γ′ volume fraction suitable for additive manufacturing and its deformation behavior

The alloy design of Ni-based superalloy with high γ′ volume fraction has been studied to be suitable for additive manufacturing (AM) by using thermodynamic calculations. The design approach intended to increase AM processability while maintaining the high-temperature strength of the reference superalloy René 80. In this study, electron beam powder bed fusion also known as a selective electron beam melting (SEBM) process was utilized for alloy fabrication. All René 80 products manufactured by SEBM were cracked despite their small sizes. Using the conventional René 80 composition as a baseline, Ti reduction (5→3 wt%), 2.5 wt% Ta addition, and 1.0 wt% Hf addition successfully prevented crack formation after SEBM fabrication and considerably improved the characteristics of alloy powders, which exhibited more spherical shapes with fewer satellites. The as-SEBM modified René 80 alloy possessed a unidirectional columnar grain microstructure with a primary γ′ area fraction of 40% and size of 382 nm, which were higher than those of the conventional René 80 alloy. Atom probe tomography results indicated that the partitioning of Ti, Al, and Ta atoms into the γ′ phase was more pronounced in AM80 alloy than in René 80 alloy. This facilitated to increase the lattice misfit at the γ/γ′ interface, made it more difficult for dislocations to cut through γ′ particles, which was confirmed by the results of first-principles calculations. The SEBM products, which were subjected to a standard heat treatment demonstrated superior tensile properties at 870 °C as compared with those of the conventional cast René 80 alloy. The primary γ′ phase was sheared by a/3 < 112 > partial dislocation leaving stacking fault behind, and a high density of tangled dislocations was observed as a result of the strong interaction with fine secondary γ′ with sizes smaller than 50 nm.

Hybrid structures in Titanium-Lattice/FRP: effect of skins material on bending characteristics

Lattice structures represent a valuable solution for applications demanding both high mechanical properties and lightweight. However, the skin is required to obtain uniformity of the load distribution and avoid stress concentration. The characteristics of the skin materials can influence the mechanical properties of the structure; therefore, in the present work the bending properties of various kinds of lattice-cored structures, with different skin materials, were investigated by means of experimental tests. In particular, flat lattice panels were produced by the Electron Beam Melting process. Then, composite material skins were added to these cores through the autoclave-vacuum bagging process. Three types of reinforcement were adopted for the skin: carbon, aramid, or glass, and the produced sandwich specimens were subjected to a three-point bending test in order to evaluate the flexural characteristics. The experimental tests showed that the specimens with the carbon skin were able to reach the maximum load, but presented the lowest ultimate transversal displacement. On the contrary, the aramid skins were characterized by the highest applied displacement and the lowest failure load.

Simulating the sintering of powder particles during the preheating step of Electron Beam Melting process: review, challenges and a proposal

The powder bed preheating before melting is a distinctive manufacturing step of the Electron Beam Melting (EBM) process. During preheating slight sintering occurs and small necks are formed between neighbouring particles. The necks improve the heat transfer and the powder bed strength allowing a reduction of supports structures and the neutralisation of the so-called smoke. However, preheating may represent over 50% of the total production time. This work investigates the major strategies in literature for preheating phase optimisation and proposes a numerical simulation approach to evaluate the neck growth and the corresponding sintering level.

Distortion discrepancy of large-scale components in electron beam melting processes

Amongst many factors that hinder the industry from adopting additive manufacturing technology, dimensional printing error and thermal distortion are crucial, especially for large-scale components (length > 100 mm). Although residual stress in an electron-beam melting (EBM) printed part is relatively low, part distortion cannot be neglected in quality control and post-processing. To investigate distortion rules of EBM parts along the X and Y direction of the build plate, a series of L shape beams, with a length of 130 mm in both X and Y, were designed, printed, and characterized. Both 3D scanning and multi-functional coordinate-measuring machine (CMM) were used to measure designed hexagonal marks with a 10 mm interval along part gauge length. The dimensional error between designed and printed parts was calculated and analyzed. Results showed that the beams shrank orthotopically in terms of X and Y directions in discrepant rules. In the X direction, total shrinkage was around 0.7 %, accumulating with a stable increment. While in the Y direction, the shrinkage was indistinctive without apparent accumulation. We observed a slight wall thickness effect on shrinkage non-uniform accumulation in the X direction. Thin wall structures (T = 1 mm and 2.5 mm) distorted more at the beginning 40 mm of the part length from the origin point than the rest to the rear end. On the other hand, thick wall (T = 10 mm) distorted more at the rear end.

Investigating the Effect of Electron Beam Melting Parameters on the Ti6Al4V Alloy: A Simulation Study

Electron beam melting is one of few techniques that can melt powder directly; is a powder bed technique, classified as an additive manufacturing method. The electron beam melting method is based on melting the material to build a high-density solid part by applying a high-energy electron beam to a powder layer in a vacuum chamber. The parameters used during the electron beam melting process significantly affect the properties of the final part being printed. These parameters are beam power, spot size, scanning speed, and layer thickness. It is necessary to choose the best values for these parameters to increase the surface integrity of the part built. This study developed a computational fluid dynamics model with a moving Gaussian distribution electron beam source applied to one layer consisting of Ti6Al4V alloy powders. The model was solved with the ANSYS software to investigate the effects of the selected parameters. The results indicated that the temperature gradients, molten pool sizes, and melting-evaporation temperature ranges were determined by applying the parameter combinations used on the beam-powder interaction throughout the process. The simulation results have emphasised the outcomes determined by the power and scanning speed parameters that directly affect the molten pool dimensions. This way, the status of powder-beam interaction zones has been identified as either within or outside the melting-evaporation range.

Effect of recycling on internal and external defects of Ti-6Al-4V powder particles for electron beam melting process

Electron Beam Melting is an additive manufacturing process that allows the creation of high-resolution parts with complex shapes, starting from a powder feedstock and using an electron beam as the energy input. Due to the high costs of the powder, the excess one must be reused otherwise a high percentage of unused powder would be lost after the printing process. The virgin and reused powders may have not the same characteristics, and therefore the printed parts may not have the same final quality which is detrimental to mechanical performances. However, this topic still needs research, as the influence of recycling on powder characteristics is not fully understood. In this study, the internal defects, and their distribution in virgin and recycled Ti-6Al-4V powders particles, are investigated by means of an optical microscope. For the analysis of the surface morphology and the external characteristics, it was used the Scanning Electron Microscope (SEM). At the end, a commercial software ImageJ was used in order to provide some quantitative results on the internal porosity percentage. The results demonstrate the dependence on the powder atomization process, which is primarily responsible for the micro-porosities, due to the trapped gas in virgin powders, and the rough surface morphology, including the presence of various satellites. As the number of reuses increases, the quantity of satellites decreases, probably due to the partial melting on the surface, which consequently returns in rougher surfaces of the recycled powder particles. In addition, most of the internal porosities disappeared making the cross-sectional faces of the particles near fully dense.

Electron beam sintering (EBS) process for Ultra-High Temperature Ceramics (UHTCs) and the comparison with traditional UHTC sintering and metal Electron Beam Melting (EBM) processes

Ultra-high temperature ceramics (UHTCs) are refractory materials with unusual properties making them strong contenders for applications involving adverse and chemically aggressive environments. This paper presents an effort to process UHTCs using an additive manufacturing method, specifically Electron Beam Sintering (EBS) - adoption of the Electron Beam powder-bed fusion (PBF) process. Such a process shows that the processing and consolidation phenomenon of UHTCs are different from what is observed in the traditional sintering process of UHTCs and Electron Beam Melting (EBM) of typical metals or alloys. In this article, the EBS process of UHTCs is studied. The differences between EBS and the two aforementioned traditional processing methods are analyzed. The scientific hypotheses have been backed by experimental results on a ZrB2 – 30 vol% ZrSi2 UHTC mixture using an electron beam via Liquid Phase Sintering (LPS) theory. Efforts to develop processing conditions to fabricate dense and defect-free UHTC components are explored with the help of finite element (FE) simulations. Characterization of the EBS processed samples using Scanning Electron Microscopy (SEM) coupled with Energy Dispersive X-ray Spectroscopy (EDS) and X-ray Diffraction (XRD) revealed unique needle-like patterned grains of ZrB2 in a ZrSi2 matrix. Our findings demonstrate the feasibility of the EBS process to produce dense layers of UHTCs for coatings, potential three-dimensional, as well as complex-shaped applications.

Titanium lattice structures manufactured by EBM process: Effect of skin material on bending characteristics

Lattice structures are very interesting for applications in the aerospace field since they combine high stiffness with lightweight. Moreover, they can constitute the core material for sandwich structures. In this work, the bending properties of two kinds of sandwich structures were compared through experimental activity: both of them had a lattice core of titanium alloy manufactured through Electron Beam Melting process, but one presented composite material skin and the other titanium skin. For the same geometrical parameter, the bending stiffness of the all-titanium structure was found higher, but the hybrid one was deemed more performant if the weight is equal.

Anisotropic Compression Behavior of Additively Manufactured Porous Titanium with Ordered Open-Cell Structures at Different Temperatures

Anisotropic compression behavior of open-cell porous titanium is evaluated at different temperatures. Porous titanium specimens with truncated octahedron unit cells are designed by 3D-Voronoi division. Cubic specimens with the porosities of 85 and 92% are manufactured from commercially pure titanium powder using an electron beam melting process. Compression tests are carried out at different temperatures of 300, 473 and 673 K for three different compression directions of [001], [011] and [111]. In all specimens, the flow stress of [001] direction is highest and the flow stress of [111] direction is lowest. Anisotropic compressive strength can be explained by comparing the bending moment of cell struts. Activation energy obtained by Arrhenius plot is the same as that of base titanium and is independent of the compression direction.

In Situ Monitoring of Powder Bed Fusion Homogeneity in Electron Beam Melting.

Increasing attention has been devoted in recent years to in situ sensing and monitoring of the electron beam melting process, ranging from seminal methods based on infrared imaging to novel methods based on backscattered electron detection. However, the range of available in situ monitoring capabilities and solutions is still quite limited compared to the wide number of studies and industrial toolkits in laser-based additive manufacturing processes. Some methods that are already industrially available in laser powder bed fusion systems, such as in situ detection of recoating errors, have not yet been investigated and tested in electron beam melting. Motivated by the attempt to fill this gap, we present a novel in situ monitoring methodology that can be easily implemented in industrial electron beam melting machines. The method is aimed at identifying local inhomogeneity and irregularities in the powder bed by means of layerwise image acquisition and processing, with no external illumination source apart from the light emitted by the hot material underneath the currently recoated layer. The results show that the proposed approach is suitable to detect powder bed anomalies, while also highlighting the link between the severity of in situ detected errors and the severity of resulting defects in the additively manufactured part.

Failure energy and stiffness of titanium lattice specimens produced by electron beam melting process

Lattice structures allow achieving high stiffness and strength, maintaining the part weight low. There exist different technologies for the manufacturing of such structures, but the one having high flexibility and offering the possibility of producing parts with complex geometries is the additive manufacturing process. In this paper, titanium specimens with different lengths, presenting a lattice structure as a core, were manufactured by electron beam melting (EBM) process. Then, the bending properties, like stiffness and failure energy, were experimentally determined by subjecting the specimens to the three-point bending test. The analysis of the fracture surface was carried out too. The three-point bending test evidenced that the longer the span was, the higher the elastic contribution over the plastic one was; moreover, the fracture morphology evidenced a ductile behaviour of the material.

A multi-grid Cellular Automaton model for simulating dendrite growth and its application in additive manufacturing

The dendrite growth in casting and additive manufacturing is rather important and related to the formation of some defects. However, quantitatively simulating the growth of dendrites with arbitrary crystallographic orientations in 3-dimension(3D) is still very challenging. In the present work, we develop a multi-grid Cellular Automaton (CA) model for the dendrite growth. In this model, the interfacial area is further discretized into a child grid, on which the decentered octahedron growth algorithm is performed. The model is comprehensively and quantitatively verified by comparing with the prediction of analytical models and a published x-ray imaging observation result, proving that the model is quantitatively and morphologically accurate. After that, with the temperature gradient and cooling rate extracted from a finite-volume-method(FVM)-based thermal-fluid model, the model was applied in reproducing the dendrite growth process of nickel-based superalloy during a single-track electron beam melting process. The simulation results agree fairly well with the experimental observation, demonstrating the feasibility and effectiveness of using the model in additive manufacturing.

Investigation the effect of electron beam melting parameters on overhang structure deformation

ABSTRACT Electron beam melting (EBM) is a new and rapidly developing metal additive manufacturing (AM) technology. The unique capabilities of EBM have opened the door for this technology in high-performance applications such as medical implants, aerospace, automotive, etc. However, the building of overhang surfaces without support structure leads to dross formation, distortions and warping. Using the support structures is the solution to avoid the mentioned problems, but increased support structure leads to increased use of materials, and post-processing time, which eventually increases the costs of the final process. This study aims to investigate the effect build parameters EBM process on the warping deformation. The main process parameters are considered and evaluated. The results indicate that the process parameter play a role in overhang structure warping deformations. The minimum deformation (0.28 mm) is resulted with 15 mA current, 4530 mm/s scan speed, 3 mm focus offset and 0.3 mm line offset.

Peculiar microstructural evolution and tensile properties of β-containing γ-TiAl alloys fabricated by electron beam melting

The microstructure and tensile properties of β-containing Ti–44Al–4Cr alloy rods additively manufactured by electron beam melting (EBM) process were examined as a function of input energy density determined by the processing parameters. To the best of our knowledge, this is the first report to demonstrate that two types of fine microstructures have been obtained in the β-containing γ-TiAl alloys by varying the energy density during the EBM process. A uniform α 2 /β/γ mixed structure containing an α 2 /γ lamellar region and a β/γ dual-phase region is formed at high energy density conditions. On the other hand, a lower energy density leads to the formation of a peculiar layered microstructure perpendicular to the building direction, consisting of a ultrafine α 2 /γ lamellar grain layer and a α 2 /β/γ mixed structure layer. The difference in the microstructures originates from the difference in the solidification microstructure and the temperature distribution from the melt pool, which are dependent on the energy density. Furthermore, it was found that the strength of the alloys is closely related to the volume fractions of the β phase and the ultrafine α 2 /γ lamellar grains which originates from the massive α grains formed by rapid cooling under low energy density conditions. The alloys with high amounts of these peculiar microstructures exhibit high strength comparable to and higher than the conventional β-containing γ-TiAl at room temperature and 1023 K, respectively.

Investigation of elimination of powder spreading in manufacture of thin and wide preheating beads from Co–Cr alloy powders using a P-ebeam

A preheating step consisting of the heating of substrates and powder particles is necessary to prevent spreading phenomenon of powders in an electron beam melting (EBM) process. The preheating step, however, increases the overall manufacturing time as well as the energy consumption of the EBM process. In addition, the preheating causes thermal damages of surrounding parts in the vicinity of the powder bed. These drawbacks are particularly notable when both thin and wide parts are fabricated from the EBM process. The present paper newly investigates the manufacture of thin and wide preheated beads from a plasma electron beam (P-ebeam) and a Co–Cr alloy with a low temperature heating of the substrate. The effects of process parameters, including the working distance of P-ebeam, the probe current of the P-ebeam and the travel speed of the table, on the formation of preheated beads are examined to obtain feasible preheating conditions. The influence of the probe current and the travel speed on heat transfer characteristics in the vicinity of the irradiated region by P-ebeam are investigated via finite element analyses to analyze the cause of the powder spreading and defects. Finally, suitable preheating methods are proposed to fabricate thin and wide preheated beads through the elimination of the powder spreading.

Microstructure evolution and globularization mechanism of lamellar phases in Ti6.5Al2Zr1Mo1V produced by electron beam melting

Electron beam melting (EBM) was adopted to produce Ti6.5Al2Zr1Mo1V titanium alloy, and the microstructure of as-built samples was characterized to illustrate the effect of complex thermal history on microstructure evolution. Under the combined action of cyclic heating and powder bed preheating, lamellar phases were globularized simultaneously during the EBM process. Boundary splitting, lamellar termination migration and epitaxial growth were involved in the globularization process of lamellar phases. Sub-grain boundaries were formed according to the climbing or recombination of dislocations, and then lamellas were segmented through boundary splitting and the edge globularization. As the cooling rate decreased, some globular phases were formed by epitaxial growth on the split segments, and then the microstructure was coarsened through terminating migration and Ostwald ripening.

Smoke Suppression in Electron Beam Melting of Inconel 718 Alloy Powder Based on Insulator-Metal Transition of Surface Oxide Film by Mechanical Stimulation.

In powder bed fusion–electron beam melting, the alloy powder can scatter under electron beam irradiation. When this phenomenon—known as smoking—occurs, it makes the PBF-EBM process almost impossible. Therefore, avoiding smoking in EBM is an important research issue. In this study, we aimed to clarify the effects of powder bed preheating and mechanical stimulation on the suppression of smoking in the powder bed fusion–electron beam melting process. Direct current electrical resistivity and alternating current impedance spectroscopy measurements were conducted on Inconel 718 alloy powder at room temperature and elevated temperatures before and after mechanical stimulation (ball milling for 10–60 min) to investigate changes in the electrical properties of the surface oxide film, alongside X-ray photoelectron spectroscopy to identify the surface chemical composition. Smoking tests confirmed that preheating and ball milling both suppressed smoking. Furthermore, smoking did not occur after ball milling, even when the powder bed was not preheated. This is because the oxide film undergoes a dielectric–metallic transition due to the lattice strain introduced by ball milling. Our results are expected to benefit the development of the powder bed fusion–electron beam melting processes from the perspective of materials technology and optimization of the process conditions and powder properties to suppress smoking.