Selective Electron Beam Melting Process Research Articles

Effect of substrate orientation on crystal growth in selective electron beam melting of nickel-based superalloy

Selective electron beam melting (SEBM) has proven to be a possible method for fabricating single-crystal structures owing to its high preheating temperature and directional temperature gradient, which can promote the directional growth of columnar grains. Researchers have conducted experiments to obtain a CMSX-4 single-crystal structure; however, when a polycrystalline substrate was adopted, a 9–15 mm competitive growth stage could not be avoided. There are still two key questions that need to be addressed: 1) Whether the use of a single-crystal substrate can avoid the competitive growth stage and a single-crystal structure grown directly from the substrate, and 2) the effect of single-crystal substrate orientation (001, 001, or 111) on grain growth behavior.Polycrystalline substrates and single-crystal substrates with different crystallographic orientations were used in this study. It was found that the (001) substrate promoted single-crystal structure growth in the SEBM process. When SEBM was carried out with (011) and (111) substrates, because of the complicated temperature field and crystallographic orientation in the first few layers, stray grains were found and epitaxial growth was difficult to maintain. These findings provide an important foundation for the direct fabrication of single-crystal blades.

The microstructure and mechanical properties of selective electron beam melting manufactured 9–12Cr ferritic/martensitic steel using N- and Ar-atomized powder

Low-carbon 9-12Cr heat-resistant ferritic/martensitic steels were fabricated by SEBM using two types of powders (Ar gas- and N gas-atomized powders). Both the as-built Ar- and N-components showed a tempered and fine lath-like ferritic/martensitic microstructure, high tensile strength (approximately 1300 and 1167 MPa for the as-built N-component and the as-built Ar-component, respectively), and tensile elongation of approximately 8%. The high N content (0.44 wt%) of the as-built N-component resulted in a refined lath martensite phase and promoted the precipitation of a fine M 2 X phase which give rise to higher strength. SEBM is a suitable and rapid method for fabricating low-C martensite steels with a fine lath martensitic microstructure and enhanced mechanical properties for use in power plants without employing welding. • 12Cr heat-resistant steels fabricated by selective electron beam melting (SEBM). • The formation of tempered ferritic/martensitic structure after SEBM process. • High tensile strength caused by refined microstructure during SEBM process.

Grain growth prediction in selective electron beam melting of Ti-6Al-4V with a cellular automaton method

An integrated modeling framework coupling the discrete element method for powder spreading, finite volume method for powder bed melting, and an extended cellular automaton method for grain structure evolution during solidification is proposed. In this framework, the initial grain structure of both the substrate and metal powders can be taken into account and used to capture epitaxial and competitive grain growth. The framework is used to provide an in-depth understanding of microstructure development in Ti-6Al-4V during the selective electron beam melting process. The complex process of grain growth during deposition of multiple tracks and multiple layers is modeled through an analysis restarting scheme. The epitaxial growth of grains from pre-existing grains, in particular the grains of partially melted powders, is reproduced. The mechanism of microstructure development within the overlap region of consecutive tracks and layers for various scan strategies is revealed. The simulation results are in qualitative agreement with experimental observation in the literature. The proposed modeling framework is a powerful tool to guide optimal process parameters that lead to designed, site-specific microstructure control and therefore to tailored mechanical properties of parts fabricated by the powder bed fusion additive manufacturing process.

Process optimization and mechanical property investigation of non-weldable superalloy Alloy713ELC manufactured with selective electron beam melting

An efficient optimization method based on a support vector machine (SVM) is used to optimize multiple process parameters of selective electron beam melting (SEBM) for a non-weldable nickel-base superalloy Alloy713ELC. The global optimum condition and the near optimum conditions are extracted to fabricate SEBM samples. All the SVM optimized conditions lead to near net shaped samples with even top surfaces. The sample fabricated under the global optimum condition for sample dimension of 10 mm exhibits pore-less cross-sections, columnar grains with fine γ′ precipitates and fine substructure, a small amount of grain boundary crack and excellent room temperature tensile properties. The samples fabricated under the global optimum condition and a near optimum condition with increased beam current for sample dimension of 15 mm exhibit excellent creep properties under 980 °C. In both the two situations for sample dimensions of 10 mm and 15 mm, SEBM samples with mechanical properties superior to conventional cast alloys can be achieved by testing only 1–3 SVM optimized conditions. We demonstrate the current method is effective for optimizing SEBM process, especially when multiple parameters need to be considered simultaneously. Besides, this method can rapidly provide not only a batch of conditions leading to samples with good top surfaces but also the optimum conditions leading to good building quality and superior mechanical properties.

Processing of High‐Carbon Steel by Selective Electron Beam Melting

In addition to the production of lost moulds, additive manufacturing (AM) is increasingly used for the direct manufacture of tools, inserts, or parts thereof. Depending on the material and tool geometry, the combination of additive and conventional technologies (hybrid production) are advantageous. Commercially available AM tool inserts have their limits on the kinds of the materials that can be used. High‐carbon, particle‐reinforced, or crack‐prone materials are indispensable for many areas of tool making but so far can hardly be processed using the common laser‐based AM methods, as rapid solidification in these brittle materials results in high residual stresses, which may lead to crack formation. In contrast, selective electron beam melting (SEBM) is working under elevated temperatures of up to 1100 °C and, thus, minimizes thermal stresses. This study shows how such materials can be processed by SEBM. Results are presented for the first‐time production of high‐carbon iron–chromium alloy. Herein, powder properties and their reusability are focused upon, as well as process parameters and their influence on part quality. Investigations on density, microstructure, and hardness are shown to illustrate the potential of the SEBM process. Final heat treatments reveal that a further increase in hardness is possible in this alloy.

Multi-scale modeling of selective electron beam melting of Ti6Al4V titanium alloy

Selective electron beam melting (SEBM) is one of the most promising powder-based metal additive manufacturing (AM) technologies. However, controlling the SEBM process parameters in such a way as to maximize the quality of the fabricated components represents a significant challenge. Accordingly, taking Ti6Al4V titanium alloy for illustration purposes, the present study constructs a novel multi-scale modeling framework to examine the effects of three SEBM process parameters (namely the beam current, the beam diameter, and the scanning speed) on the dimensions and temperature distribution of the melt pool. In microscale, an effective model considering the interactions between the electron beam and the Ti6Al4V atoms was constructed based on the Monte Carlo ray-tracing simulations, and the electron beam total absorption of Ti6Al4V material as a function of incidence angle was obtained. In mesoscopic level, in order to take the absorption property of the porous structure of the powder layer into account, absorptivity profile along the depth of the powder bed was calculated by utilizing the absorptivity of Ti6Al4V for various incidence angles and the 3-D powder bed model. It is found that the total laser absorptivity of the Ti6Al4V powder bed is around 12% higher than that of the bulk material of Ti6Al4V due to the multiple reflection of electron beam inside the powder layer. Finally, three-dimensional finite element heat transfer simulations are performed using the proposed volumetric heat source to predict the melt pool geometry and thermal distribution during the SEBM process. It is shown that the simulation results are in good agreements with the experimental results reported in the literature.

Effect of processing parameters on the density, microstructure and strength of pure tungsten fabricated by selective electron beam melting

Pure tungsten samples were prepared by the selective electron beam melting (SEBM) process. The effect of the SEBM process parameters on the density, microstructure and compression strength of pure tungsten was studied. In addition, the influence of substrate preheating temperature during SEBM was studied. A processing window for additive manufacturing of pure tungsten by SEBM was preliminarily determined. Pure tungsten samples with relative density of 99.5% and without obvious pores and microcracks were successfully fabricated. The as-built pure tungsten samples showed strong columnar grain structures. Compression strength along the columnar grains in the build direction was measured to be 1560 MPa. Fracture occurred predominantly along the columnar grain boundaries by decohesion, in addition to brittle transgranular fracture. Refinement and strengthening of the columnar grain boundaries are expected to improve the compression strength of the SEBM-fabricated pure tungsten.

Additive manufacturing of WC-13%Co by selective electron beam melting: Achievements and challenges

Additive manufacturing is a powerful tool for rapid prototyping and fabricating metal articles having a complicated geometry. This method is known to be used almost solely for the manufacture of articles consisting of pure metals and alloys. In the present work the possibility of obtaining dense carbide articles by a single-step process of additive manufacturing based on selective electron beam melting was evaluated. A new technology for fabricating cemented carbide granules suitable for selective electron beam melting was developed. It includes conventional granulating WC-Co powders followed by solid-state pre-sintering and preliminary screening of the granules. After that their liquid-phase sintering and final screening are carried out to obtain a desired fraction needed for the additive manufacturing process. Results of experiments on selective electron beam melting at different scan rates and current values indicated that it was possible to obtain non-porous carbide articles of complex geometry from WC-Co granules initially containing 13 wt% Co. The selective electron beam melting process led to the evaporation of some liquid Co and very intense local WC grain growth resulting in peculiar microstructures of the cemented carbide articles comprising layers with medium-coarse and abnormally large WC grains. A near-surface layer of the cemented carbide articles obtained by additive manufacturing is characterized by a high roughness comparable with the mean size of the original WC-Co granules.

Immediate development of processing windows for selective electron beam melting using layerwise monitoring via backscattered electron detection

The availability of a reliable processing window is the basic requirement for processing new materials via selective electron beam melting (SEBM). Typically, these processing windows are derived by a time-consuming procedure comprising fabrication, metallographic preparation and analysis of standardized specimens for every parameter set. This study demonstrates the immediate development of a processing window during one single SEBM process. An electron optical image acquisition system provides the necessary information for evaluation and subsequent adaption of process parameters. It is shown that this immediate approach delivers processing windows with sufficient accuracy, while the required time is substantially reduced from weeks or even months to several hours.

Selective electron beam melting of NiTi: Microstructure, phase transformation and mechanical properties

Recent years have witnessed extensive investigations of additively manufactured NiTi alloys using laser-source techniques. This study reports a selective electron beam melting process to make Ni-rich NiTi parts using the plasma rotating electrode processed pre-alloyed NiTi powder. Microstructures of the as-printed NiTi samples were investigated, including micro-morphology, phase constituent and texture. Chemical compositions (primarily oxygen and nickel) and martensitic transformation temperatures were compared between the in house made NiTi powders and as-printed NiTi samples. X-ray diffraction results show that the as-printed samples exhibit a dominant austenitic B2 phase and a minor in situ formed Ni4Ti3 phase. The as-printed samples demonstrate a strong B2 (001) orientation texture along the building direction. Differential scanning calorimetry results show asymmetric phase transformations: a two-step of B2 → R → B19′ phase transformation during cooling but a one-step B19′ → B2 transformation during heating. Excellent superelasticity and large reversible strain have been observed. The as-printed NiTi samples achieve a fracture tensile stress of 1411.0 ± 59.3 MPa and an elongation of 11.8 ± 0.9%. The observed tension-compression asymmetry has been also discussed.

A sensitivity analysis of the columnar-to-equiaxed transition for Ni-based superalloys in electron beam additive manufacturing

Solidification conditions experienced during Selective-Electron Beam Melting (S-EBM) of Nickel-Base superalloys generally lead to epitaxial grain growth from previous layers, resulting in highly textured columnar grain structures, and consequently, highly anisotropic mechanical properties. However, in recent work, researchers have been able to produce equiaxed grain structures by changing the scan strategy to influence the solidification conditions, thereby obtaining isotropic properties. The competition between columnar and equiaxed grain formation is governed by both the local thermal conditions and alloy thermodynamics. The above examples demonstrate successful manipulation of the thermal conditions through changes in process parameters. However, there is no guarantee that process conditions that encourage the formation of equiaxed grains for a given alloy will coincide with the production of defect free material. The goal of this work is therefore to understand the influence of alloy composition on the columnar-to-equiaxed transition (CET) so that alloys may be designed such that grain structure control is accessible to a wider range of S-EBM process conditions. A CET model originally developed by Hunt and extended by Gäumann was used to relate processing conditions to resulting microstructure. A sensitivity analysis of the CET model was performed using the Element Effects Methods. The results lead to a comparison of the relative effects of various alloying additions on the CET, as well as an assessment of other model parameters, including, most significantly, the nucleation density of equiaxed grains.

Microstructure, texture and tensile properties of Ti-47Al-2Cr-2Nb alloy produced by selective electron beam melting

A Ti-47Al-2Cr-2Nb alloy was fabricated by selective electron beam melting, and the influence of energy density on porosity, microstructure, texture, Schmid factor and tensile property was investigated systematically. As the energy density increased continuously, the microstructure in the vertical cross-section transformed from fine-equiaxed grains with a low aspect ratio and a weakly crystallographic texture to elongated grains with a strong texture. The angles between the lamellar planes and the building direction accumulated 0 ± 5° and 45 ± 5° preferentially with the increase of energy density. Additionally, the average value of Schmid factor increased gradually from 0.39 to 0.45 with the enhancing energy density. The tensile properties of as-built samples produced at different energy densities were tested and the reason for the difference in tensile properties at different energy densities was explained. The formation mechanism of the preferred lamellar orientation that was fabricated at a high energy density was discussed in detail. The findings provide a valuable reference to fabricate TiAl alloy parts with a desired lamellar orientation, texture and tensile properties by tuning energy density during the selective electron beam melting process, and will be of significant interest for future applications.

Mechanical and corrosion properties of AlCoCrFeNi high-entropy alloy fabricated with selective electron beam melting

Additive manufacturing is expected to be the manufacturing method for components made with high-entropy alloys (HEAs). In this study, the mechanical and electrochemical behaviors were investigated for equi-molar HEA (AlCoCrFeNi) obtained with selective electron beam melting (SEBM). The mechanical properties of SEBM products were improved compared with those of a cast specimen. Electrochemical measurements in artificial seawater revealed the corrosion behaviors of HEA (AlCoCrFeNi). The pitting potential of SEBM specimens (0.112 V vs. Ag/AgCl) was lower than that of a cast specimen (0.178 V vs. Ag/AgCl). The mechanical and electrochemical properties of SEBM products were influenced by the phase morphologies formed during the SEBM process.

Selective electron beam melting of TiAl alloy: Microstructure evolution, phase transformation and microhardness

In this research, a Ti-47Al-2Cr-2Nb alloy was fabricated by selective electron beam melting (SEBM). The microstructures at different heights of SEBM-produced TiAl alloy sample and their contribution to microhardness were investigated. The results revealed that the last deposited layers displayed near lamellar structure with fine α2 lamellae. Lamellar colonies degraded at the height of 19 mm of the as-built sample, accompanied with the α2 lamellae decomposition and B2 precipitation along primary α2 lamellae. Meanwhile, equiaxed γ and blocky B2 phase dispersed on the colony boundaries. Lamellar colonies further degraded and colony boundaries further coarsen at the heights of 15 mm and 5 mm due to the in-situ heat treatment during SEBM process. Three kinds of phase transformations could be observed during SEBM process, which were α2 → γ, α2 → B2 and α2 + γ → B2. The microhardness exhibited slight fluctuation as the building height increased from 3 mm to 19 mm, and the microhardness rapidly increased with the building height increasing from 19 mm to 20 mm. Eventually, the mechanism of microstructure evolution during SEBM process was discussed in detail.

Physical and Tensile Properties of NiTi Alloy by Selective Electron Beam Melting

NiTi is characterized as a shape memory alloy that has found interesting applications from aerospace to biomedical engineering. The use of NiTi in biomedical applications is due to its excellent biocompatibility, shape memory and pseudoelastic properties. These properties make NiTi an excellent candidate for many functional designs in biomedical fields. However, difficulties in manufacturing and processing of this alloy are significant hindrance to widespread applications. Advances in additive manufacturing (AM) such as selective laser and electron beam techniques have provided opportunities in manufacturing complex shaped NiTi parts. In this research paper, we demonstrate manufacturing of NiTi parts using a selective electron beam melting (SEBM) technique. Complete evaluation of physical, chemical and mechanical properties was carried out to determine the suitability of SEBM process. Differential scanning calorimeter (DSC), X-ray diffraction (XRD), and metallographic analyses were employed for the thermal and structural characterizations. The obtained results suggest that it is imperative to, and challenging to control the additive manufacturing process in order to obtain the desired microstructures and avoid unwanted texture. An exhaustive heat treatment of the samples after SEBM process might also be necessary.

Process parameter optimization and mechanical properties for additively manufactured stainless steel 316L parts by selective electron beam melting

This work presents an experimental study of process optimization of the pair of critical parameters (speed function (SF) and focus offset (FO)) for stainless steel 316L (SS316L) parts additively manufactured by selective electron beam melting (SEBM). Here, there are two sets of optimized SF-FO parameters that could build SS316L parts with high relative densities (>99%) and well-melted top build surfaces. Tensile test results show that most of the SEBM-built SS316L samples exhibit higher tensile strengths than the conventional cast and wrought counterparts, whereas their ductility is lower. In addition, strong anisotropic tensile properties are observed for the SEBM-built SS316L samples, e.g. they generally have better tensile properties when loaded parallel to the build direction as compared to the horizontal direction. However, a large number of σ phase was found to precipitate at grain boundaries in the SS316L samples fabricated under lower SF and larger FO with a higher build temperature, which evidently deteriorates their tensile properties particularly for the horizontal direction. It is suggested that SEBM process parameters for SS316L must be optimized to avoid σ phase precipitation at elevated temperatures apart from a well-melted top build surface and a high relative density.

Powder layer deposition algorithm for additive manufacturing simulations

Powder bed fusion processes are additive manufacturing technologies, where parts are built up layer by layer in a powder bed. The highly complex geometries manufactured with these technologies are the main advantage and the difference to conventional technologies, where they are hardly machinable. Numerical models can replace the intensive and expensive trial-and-error principle to find suitable process parameters for the production of accurate parts.This work presents a stochastic algorithm to generate a random powder bed for numerical simulations of powder bed fusion processes. The main focus is the efficient coupling of a classical discrete element approach to a grid-based solver used to simulate the melting process. The algorithm is implemented and validated in the range of common relative powder layer densities of powder bed fusion processes. The computational efficiency is demonstrated and finally, the complete coupling to a process simulation of the selective electron beam melting process is presented.

Meso-scale modeling of multiple-layer fabrication process in Selective Electron Beam Melting: Inter-layer/track voids formation

Selective Electron Beam Melting (SEBM) is a promising powder-based metallic Additive Manufacturing (AM) technology. However, most powder-scale modeling efforts are limited to single track process, while it is also difficult to experimentally observe the interaction between tracks and layers. In this study, we develop an integrated modeling framework to investigate the SEBM process of multiple tracks and multiple layers. This approach consists of a Discrete Element model of powder spreading and a Computational Fluid Dynamics (CFD) model of powder melting. These two models exchange 3D geometrical data as a cycle to reproduce the manufacturing process of multiple tracks along various scan paths in multiple powder layers. This integrated modeling approach enables further understanding of how current tracks and layers interact with previous ones leading to inter-track/layer voids. It also incorporates more influential factors, particularly the layer-wise scan strategy. The inter-layer/track voids due to the lack of fusion are systematically discussed in light of our simulation results which qualitatively agree with experimental observations in literature.

Hot cracking mechanism affecting a non-weldable Ni-based superalloy produced by selective electron Beam Melting

A non weldable nickel-based superalloy was fabricated by powder bed-based selective electron beam melting (S-EBM). The as-built samples exhibit a heterogeneous microstructure along the build direction. A gradient of columnar grain size as well as a significant gradient in the γ′ precipitate size were found along the build direction. Microstructural defects such as gas porosity inherited from the powders, shrinkage pores and cracks inherited from the S-EBM process were identified. The origins of those defects are discussed with a particular emphasis on crack formation. Cracks were consistently found to propagate intergranular and the effect of crystallographic misorientation on the cracking behavior was investigated. A clear correlation was identified between cracks and high angle grain boundaries (HAGB). The cracks were classified as hot cracks based on the observation of the fracture surface of micro-tensile specimens machined from as-built S-EBM samples. The conditions required to trigger hot cracking, namely, presence of a liquid film during the last stage of solidification and thermal stresses are discussed within the framework of additive manufacturing. Understanding the cracking mechanism enables to provide guidelines to obtain crack-free specimens of non-weldable Ni-based superalloys produced by S-EBM.

Microstructures of as-Fabricated and Post Heat Treated Ti-47Al-2Nb-2Cr Alloy Produced by Selective Electron Beam Melting (SEBM)

The microstructures of as-fabricated and post heat treated Ti-47Al-2Nb-2Cr (at%) alloy produced by selective electron beam melting (SEBM) were examined. The solidification path and the phase transformation of Ti-47Al-2Nb-2Cr alloy during SEBM process were analyzed. Results show that the microstructure is fully lamellar (FL), consisting of fine γ -TiAl and α 2 -Ti 3 Al, which is formed in the as-fabricated TiAl specimen. Due to the rapid cooling and the cyclic heat treatment during the SEBM process, the lamellar colonies and the lamellae width become coarser and wider, respectively from the top to the bottom of the specimen, solidification takes place solely through the β phase, and the phase transformation path is L → L + β → β → β + α → α → α + γ . Different post heat treatments were carried out to get a homogeneous microstructure. Results indicate that a fine and homogeneous microstructure can be achieved after oil quenching from 1250 °C and holding at 1200 °C for 2 h.