Electron Beam Melting Process Research Articles

Electron Beam Based Additive Manufacturing of Alloy 247 for Turbine Engine Application: From Research towards Industrialization

In this contribution it will be shown how the metal powder bed fusion technology electron beam melting (EBM) enables a successful crack- and defect-free processing of the non-weldable Ni-based superalloy Alloy 247. Besides a defect-free processing, EBM process capabilities offer unique opportunities to tailor the microstructure according to material requirements for high-temperature applications. It could be demonstrated that EBM material properties are comparable or even better than traditionally casted Alloy 247 material. Tailored polycrystalline and columnar microstructures were achieved by process parameter adoption for solidification control. The EBM as-built microstructure was characterized by conventional metallography techniques and the results were used to define an adopted HIP + heat-treatment cycle which was applied prior to mechanical testing. Thus, corresponding mechanical and microstructural properties in different conditions will be presented and discussed to show EBM material capabilities. As a result, directionally solidified (DS) EBM material can achieve similar creep performance as DS cast material, despite an observed fine-grained “micro-DS” structure in EBM.Graphical

Effect of mechanical ball milling on the electrical and powder bed properties of gas-atomized Ti–48Al–2Cr–2Nb and elucidation of the smoke mechanism in the powder bed fusion electron beam melting process

Smoke is unexpected powder-splashing caused by electrostatic force and is one of the main problems hindering the process stability and applicability of the powder bed fusion electron beam (PBF-EB) technology. In this study, mechanical stimulation was suggested to suppress smoke of gas-atomized (GA) Ti–48Al–2Cr–2Nb powder using Al2O3 and WC ball milling. The deformation mechanism of the GA powder depending on the ball milling media was discussed based on the developed particle morphology distribution map and contact mechanics simulation. It was revealed that the rapid decrement of flowability and packing density after WC ball milling owing to the formation of angular fragments by the brittle fracture. The variation of surface and electrical properties by mechanical stimulation was investigated via XPS, TEM, Impedance analysis. The electrical resistivity of the ball-milled powders gradually decreased with increasing milling duration, despite the increased oxide film thickness, and the capacitive response disappeared in Al-60 and WC-30 via metal–insulator transition. This could be due to the accumulation of strain and defects on the oxide film via mechanical stimulation. The smoke mechanism of ball-milled powders was discussed based on the percolation theory. In the smoke experiment, smoke was more suppressed for WC-10 and WC-20 than that for Al-40 and Al-50, respectively, despite the longer charge dissipation time. This could be due to the high probability of contact with conductive particles. For the Al-60 and WC-30 powders, smoke was further restricted by the formation of a percolation cluster with metal-like electrical conductivity. We believe that this study will contribute to a better understanding of the smoke mechanism and process optimization of the PBF-EB.

Effect of Powder Characteristics on Relative Density and Porosity Formation During Electron Beam Selective Melting of Al2024 Aluminum Alloy

Abstract Defects, such as pores and cracks, can be found in parts fabricated by powder-bed additive manufacturing techniques. The origin of certain defects, such as some voids, can be linked to initial powder quality, which makes it an important factor in the process. Powders used in additive manufacturing processes are produced by different methods such as gas atomization (GA), plasma atomization (PA), and plasma rotating electrode process (PREP); each gives different powder quality. In this study, two different Al2024 powders, produced by electrode induction GA and PREP techniques, were used to investigate the effect of powder characteristics on defect formation during electron beam melting process (EBM). Powders were first characterized by using Hall flowmeter funnel and scanning electron microscope (SEM); then, the EBM process was carried out, and finally, samples were examined by density measurement using Archimedes method, SEM analysis, and tensile test. PREP powder showed higher levels of sphericity and surface smoothness without attached satellites. Consequently, a higher apparent density and decreased flowing time were achieved in PREP powder. Moreover, gas-induced internal pores were observed in GA particles. The results also revealed the average relative density of 96.7% and 99.4% for the parts built by GA and PREP powders, respectively. SEM micrographs confirmed the results of density measurement of the fabricated parts and showed higher degrees of both spherical and irregular-shaped pores in samples built by GA powder. Additionally, they showed deprived mechanical properties due to the higher porosity contents which can form stress-concentrated areas.

Effects of recycling on defects and microstructure in Ti-6Al-4V powder particles and samples fabricated by Electron Beam Melting process

Effects of recycling on defects and microstructure in Ti-6Al-4V powder particles and samples fabricated by Electron Beam Melting process

Powder Bed Fusion of Multimaterials

Powder bed fusion (PBF) process has been used successfully to produce 3D structures using single material properties. The current industrial demand is to use the technology to produce 3D structures of multimaterial properties. An electron beam melting (EBM) process has been used to produce 3D structures of multimaterial properties. However, due to the large beam size, the EBM process lacks geometrical accuracy, requiring a large machining allowance. A laser powder bed fusion (LPBF) process could be used to produce multimaterials with geometrical precision. However, the thermal gradient within the molten pool and the thermophysical difference between the multimaterials leads to defects (weak interfacial bonds, interlayer and intralayer cracks). Other challenges such as poor powder delivery system, powder cross-contamination, and lack of appropriate data processing software for producing 3D multimaterial structures are not yet fully resolved. Nonetheless, there have been encouraging results for producing the next generational multimaterial 3D components of intricate geometrical characteristics.

Improvement in the high temperature mechanical properties of additively manufactured Ti–48Al–2Cr–2Nb alloy using heat treatment

Gamma Ti–48Al–2Cr–2Nb (Ti4822) alloy, which is manufactured by an electron beam melting (EBM) process, has excellent ductility but poor high-temperature strength, because it has a near gamma structure. The objective of this study was to control the microstructure of Ti4822 by considering various forms of heat treatment to improve the mechanical properties at room and high temperatures. The heat treatment conditions were varied to create nearly lamellar (NL) and fully-lamellar (FL) structures. First, the EBM-built NL-Ti4822 exhibited a lamellar structure with an average layer thickness of about 92 nm and equiaxed γ-phase particles with a size of several tens of micrometers. On the other hand, the EBM-built FL-Ti4822 had an average lamellar thickness of about 347.2 nm, but in this case the equiaxed γ phase was not observed. The results of the room- and high-temperature compression test showed that the mechanical properties of the two heat-treated materials were significantly superior to those of the as-built Ti4822 across all temperature ranges. Moreover, both materials were highly ductile at room and high temperature despite having a lamellar structure, and exhibited intensified yield stress anomaly and dynamic recrystallization as the fraction of lamellar structure increased. Based on the above results obtained by subjecting the materials to different heat treatment methods, the effects of the microstructures of the two materials on the mechanical properties and deformation mechanisms at room and high temperatures were investigated.

Excellent compression strength and energy absorption of additively manufactured porous titanium with trapezo-rhombic dodecahedron cell

Porous metals with excellent properties for lightweight structural applications and energy absorption are manufactured via additive manufacturing (AM). The 3D-Voronoi division method generates trapezo-rhombic dodecahedron cells as a new cell structure. Porous titanium specimens with trapezo-rhombic dodecahedron cells were prepared using the electron beam melting process. The normalized first maximum compressive strength of the [0001] direction was determined to be the highest, while the normalized first maximum compressive strength of the [ 2 ̅ 110 ] direction was the lowest. The bending moment of cell struts could be used to explain the anisotropic compressive strength. In addition, AM porous titanium with trapezo-rhombic dodecahedron cells had higher normalized first maximum compressive strength and energy absorption than AM porous titanium with truncated octahedron and rhombic dodecahedron cells. Porous titanium with trapezo-rhombic dodecahedron cells exhibits significantly larger anisotropy than truncated octahedron and rhombic dodecahedron cells. • Trapezo-rhombic dodecahedron cell is a new cell structure similar to rhombic dodecahedron. • Porous titanium specimens with trapezo-rhombic dodecahedron cells have excellent first maximum compressive strength and energy absorption. • Porous titanium consisting of trapezo-rhombic dodecahedron cells exhibited large anisotropy.

An electron beam melting system for in-situ synchrotron X-ray monitoring

Electron beam melting (EBM) additive manufacturing (AM) has grown rapidly in recent years due to its capability to manufacture high-performance metals and conductive ceramics. Whereas in-situ synchrotron X-ray imaging and diffraction have proven invaluable tools in understanding of laser powder bed fusion (L-PBF), the characterization of the EBM process lagged what has been accomplished in laser-based AM. This is principally due to the lack of an open architecture system. Here we report an open architecture EBM system for in-situ synchrotron X-ray monitoring with complementary high-speed visible light imaging and high-speed infrared (IR) imaging. We demonstrate that the developed system enables the characterization of sub-surface transient phenomena. The open architecture system reported here provides a powerful tool for studying the dynamics and mechanisms of EBM AM process. The promising results reported here may ignite more future research activities on using in-situ synchrotron x-ray techniques to study EBM AM processes.

Process optimization and mechanical property investigation of Inconel 718 manufactured by selective electron beam melting

To accelerate the optimization of selective electron-beam melting (SEBM) processing parameters, two machine learning models, Gaussian process regression, and support vector regression were applied in this work to predict the relative density of Inconel 718 from experimental data. The experimental validation indicated that the trained algorithms can precisely predict the relative density of SEBM samples. Moreover, the effects of different parameters on surface integrity, internal defects, and mechanical properties are discussed in this paper. The Inconel 718 samples with high density (>99.5%) prepared by the same SEBM energy density exhibit different mechanical properties, which are related to the existence of the unmelted powder, Laves phase, and grain structure. Finally, Inconel 718 sample with superior strength and plasticity was fabricated using the optimized processing parameters.

Review of quality issues and mitigation strategies for metal powder bed fusion

PurposeSelective laser melting and electron beam melting processes are well-known for the additive manufacturing of metal parts. Metal powder bed fusion (MPBF) is a common term for them. The MPBF process can empower the manufacturing of intricate shapes by reducing the use of special tools, shortening the supply chain and allowing small batches. However, the MPBF process suffers from many quality issues. In literature, several works are recorded for qualification of the MPBF part. The purpose of this study is to recollect those works done for quality control and report their helpful findings for further research and development.Design/methodology/approachA systematic literature review was conducted to highlight the major quality issues in the MPBF process and its root causes. Further, the works reported in the literature for mitigation of these issues are classified and discussed in five categories: experimental investigation, finite element method-based numerical models, physics-based analytical models, in-situ control using artificial intelligence (AI) and machine learning (ML) methods and statistical approaches. A comparison is also prepared among these strategies based on their suitability and limitations. Additionally, improvements in MPBF printers are pointed out to enhance the part quality.FindingsAnalytical models require less computational time to simulate the MPBF process and need a smaller number of experiments to confirm the results. They can be used as an efficient process parameter planning tool to print metal parts for noncritical applications. The AI-ML based quality control is also suitable for MPBF processes as it can control many processing parameters that may affect the quality of the MPBF part. Moreover, capabilities of MPBF printers like thinner layer thickness, smaller beam diameter, multiple lasers and high build temperature range can help in quality control.Research limitations/implicationsThis study converts the piecemeal data on MPBF part qualification methods into interesting information and presents it in tabular form under each strategy. This tabular information provides the basis for further quality improvement efforts in the MPBF process.Originality/valueThis study references researchers and practitioners on recent quality control efforts and their significant findings for a better quality of MPBF part.

Influence of dimension, building position, and orientation on mechanical properties of EBM lattice Ti6Al4V trusses

The advent of additive manufacturing technologies significantly encouraged the development and usage of lattice structures. This paper aims to experimentally investigate the influence of dimension, building position, and orientation on the mechanical properties of Ti6Al4V trusses, manufactured by electron beam melting process, to be used in lattice cells. Specimens were manufactured considering the following parameters: truss diameter (1, 1.5, 2 mm), growth orientation (0°, 45°, 90°), and specimen position inside the building chamber. Trusses with diameter of 1 mm showed inconsistent mechanical properties caused by the poor manufacturing quality. Specimen position was found to influence the analyzed mechanical properties. Unmelted powders were observed to affect the outer surfaces of all specimens and the whole cross-sections of specimens manufactured at 0°. Specimens manufactured at 45° with diameter of 2 mm demonstrated the best performances, whereas specimens manufactured at 90° with diameter of 2 mm displayed the highest elongation at fracture.

A Design of Experiment Approach for Development of Electron Beam Powder Bed Fusion Process Parameters and Improvement of Ti-6Al-4V As-Built Properties

Additive manufacturing is a novel and breakthrough technology by which parts can be manufactured for various purposes and services. As in any production process, the desired properties of additively manufactured components, particularly in electron beam melting processes, ultimately depend on the manufacturing process parameters. Process parameters should be designed accordingly to manufacture parts with specific and desired characteristics. This study focuses on examining the effect of process parameters, such as beam current and velocity, focus offset, and line offset, at three different values each, on the properties of Ti-6Al-4V alloy. The study on the effect of the process parameters on the as-built material’s performance was performed using the Taguchi approach using an L9 (34) orthogonal array. The properties of printed parts (density, surface roughness, elastic moduli, hardness, tensile characteristics, fractography, and microstructure) were tested. A wide range of properties was obtained and analyzed; namely, porosity varied from 8% to almost fully dense materials with density higher than 99.9% and a range of yield and ultimate tensile strength values and brittle samples with less than 1% elongation to ductile samples with an elongation greater than 16%. The overall performance of printed parts was determined based on an evaluation criterion. Several parameter combinations were found and yielded the fabrication of parts with high density and relatively fine microstructure. The comparison of the best parameter combinations determined in this study and the parameters recommended by the machine manufacturer showed that improved results were obtained, and even when using the optimal parameters, they can be improved even more. This result highlights the ability of the proposed DOE method to further develop existing results and even for development of manufacturing parameters for new materials.

Influence of Powder Characteristics on the Microstructure and Mechanical Behaviour of GH4099 Superalloy Fabricated by Electron Beam Melting

A Chinese superalloy, GH4099 (~20 vol.% γ′ phase), which can operate for long periods of time at temperatures of 1173–1273 K, was fabricated by electron beam melting (EBM). Argon gas atomized (GA) and plasma rotation electrode process (PREP) powders with similar composition and size distribution were used as raw materials for comparison. The microstructure and mechanical properties of both the as-EBMed and post-treated alloy samples were investigated. The results show that the different powder characteristics result in different build temperatures for GA and PREP samples, which are 1253 K and 1373 K, respectively. By increasing the building temperature, the EBM processing window shifts towards a higher scanning speed direction. Microstructure analysis reveals that both as-EBM samples show a similar grain width (measured to be ~200 μm), while the size of γ′ precipitated in the PREP sample (~90 nm) is larger than that of the GA sample (~130 nm) due to the higher build temperature. Fine spherical γ′ phase precipitates uniformly after heat treatment (HT). Furthermore, intergranular cracking was observed for the as-fabricated PREP sample as a result of local enrichment of Si at grain boundaries. The cracks were completely eliminated by hot isostatic pressing (HIP) and did not re-open during subsequent heat treatment (HT) of solution treatment and aging. The tensile strength of the PREP sample after HIP and HT is ~920 MPa in the building direction and ~850 MPa in the horizontal direction, comparable with that of the wrought alloy.

Enhancement of grain refinement and heat resistance in TiB2-reinforced Al-Cu-Mg-Fe-Ni matrix composite additive manufactured by electron beam melting

Additive manufacturing (AM) by electron beam melting (EBM) has many advantages over the laser-based AM techniques, but is subjected to a lower cooling rate and a long time thermal exposure with a much higher temperature. Therefore, the EBMed aluminum alloys commonly have coarser solidification structures such that lower mechanical properties compared to their SLMed counterparts. In this work, we proposed a strategy of introducing TiB2 particles into heat-resistant Al-Cu-Mg-Fe-Ni alloy to enhance the grain refinement and heat resistance for EBM fabrication. Applying a “presintering-densification” scanning strategy, near fully dense samples were build up with precise contours and good surface quality. The as-built EBMed TiB2/Al-Cu-Mg-Fe-Ni composite possessed a highly homogeneous microstructure with uniformly distributed TiB2 and Fe/Ni rich intermetallic particles. A fully equiaxed grain structure was achieved in the composite with a profoundly refined grain size of ~9 µm. The improved grain refinement effect was attributed to the sufficient activation of TiB2 particles as heterogeneous nuclei under a high cooling rate. Additionally, the solidified microstructure has shown an excellent thermal stability under ~500 °C during EBM process. The coexistence of thermal stable TiB2 and Fe, Ni-rich intermetallics at grain boundaries restricted the grain growth effectively. The as-built composite exhibited a high tensile strength of 253.4 MPa with an outstanding elongation of 13.5 %. After a T6-like heat treatment, the tensile strength was increased to 322 MPa with an elongation of 9.8 %. This study might shed a new light on designing high performance aluminum alloys/composites suitable for EBM technique.

An analytical method to calculate effective elastic properties of mapped rhombic dodecahedron structures fabricated with electron beam melting

Cellular rhombic dodecahedron (RD) structures fabricated in Ti6Al4V using the electron beam melting (EBM) process have been proposed for tissue scaffolds and medical implants that approximate the properties of bone. The mechanical properties of these structures have often been determined through compressive tests. The introduction of a uniaxial mapping ratio allows structures with personalized parameters and desired mechanical properties to be manufactured with respect to different border constraints without changing the porosity of the structure. In this study, a non-linear relationship between the effective modulus of elasticity and the number of cells was found for periodically arranged RD porous structures, and there were discrepancies in the modulus for different mapping ratios of specimens having the same size. An analytical model based on beam theory is therefore proposed to estimate the effective compressive elastic modulus and Poisson's ratio for such phenomenon. The analytical model predicts cases including 1 × 1 to ∞ ×∞ cells without boundary constraints and arbitrary m× n with boundary constraints, among which the 1 × 1 and ∞ ×∞ cases without boundary constraints give the lower and upper limits of the effective compressive modulus, respectively. The prediction accuracy of m× n cells with analytical solutions was verified by beam elements neglecting shearing effects in the FE method, and both gave identical results under the same constraints. However, the analytical method has advantages that are difficult to replace when dealing with models where the number and dimension of cells far exceed the computational capacity of numerical simulations. Influenced by surface quality, the effective modulus of the actual test is generally between the values predicted by the input-defined inner and outer diameters of the strut as an input.

Study of the Electron Beam Melting Process Parameters' Influence on the Tensile Behavior of 3D Printed Ti6Al4V ELI Alloy in Static and Dynamic Conditions.

The Ti6Al4V alloy is widely adopted in many high-end applications in different fields, including the aerospace, biomechanics, and automotive sectors. Additive manufacturing extends its range of possible applications but also introduces variations in its mechanical performance, depending on the whole manufacturing process and the related control parameters. This work focuses on the detailed tensile stress–strain characterization at low and high strain rates of a Grade 23 Ti alloy manufactured by electron beam melting (EBM). In particular, the main aim is to study the effect of the variation of the EBM process parameters on the performance of the material and their consequent optimization in order to obtain the best printed material in terms of ductility and strength. The adopted optical experimental setups allow the semi-local scale analysis of the neck section which makes possible the accurate estimation of stress, strain, and strain rate, all over the post-necking range and up to the very incipient specimen failure. Among the EBM printing process parameters, the speed function was previously identified as the one mainly affecting the material performance at static rates. Therefore, two different parameter sets, corresponding to the standard value and to an optimized value of the speed function parameter, respectively, are tested here at dynamic rates of 1, 15, and 700 s−1, for assessing the effect of the speed function on the dynamic material response. The results show that the optimized parameter set has a better performance compared to the standard one in terms of strength and ductility. In particular, in both static and dynamic conditions, it presents an increase of the true stress–strain curve (about 5% on average) and an increase of the failure strain (about 11% on average). Moreover, in respect to the standard parameter set, the optimized one is also characterized by a huge increase of the amplification due to the strain rate (about 49% on average for the considered strain rates).

Damage-induced failure analysis of additively manufactured lattice materials under uniaxial and multiaxial tension

Mechanical behavior of additively manufactured lattice materials has been mainly investigated under uniaxial compression, while their performance under uniaxial and multiaxial tension are yet to be understood. To address this gap, a generic elastoplastic homogenization scheme with continuum damage model is developed, and three different lattice materials, namely cubic, modified face-center cubic and body-center cubic, are analyzed under uniaxial, biaxial and triaxial tension. The influence of micro-architecture on the material’s failure behavior as well as its macroscopic mechanical performance is thoroughly discussed. For validation, a set of uniaxial tensile experiments are conducted on functionally graded cubic lattice samples that are additively manufactured using Electron Beam Melting (EBM) process. Digital image correlation technique is employed to obtain the macroscopic stress–strain curves, and manufacturing imperfections are inspected using light omitting microscopy. It turns out that the behavior of as-built samples could substantially differ from numerical predictions. Thus, a defect-informed numerical model is employed to accommodate the effect of imperfections. The outcome is in a very good agreement with experimental data, indicating that with proper input data, the developed scheme can accurately predict the mechanical and failure behavior of a given lattice material.

Effect of Aging and Cooling Path on the Super β-Transus Heat-Treated Ti-6Al-4V Alloy Produced via Electron Beam Melting (EBM).

This work focuses on the effect of different heat treatments on the Ti-6Al-4V alloy processed by means of electron beam melting (EBM). Super β-transus annealing was conducted at 1050 °C for 1 h on Ti-6Al-4V samples, considering two different cooling paths (furnace cooling and water quenching). This heat treatment induces microstructural recrystallization, thus reducing the anisotropy generated by the EBM process (columnar prior-β grains). Subsequently, the annealed furnace-cooled and water-quenched samples were aged at 540 °C for 4 h. The results showed the influence of the aging treatment on the microstructure and the mechanical properties of the annealed EBM-produced Ti-6Al-4V. A comparison with the traditional processed heat-treated material was also conducted. In the furnace-cooled specimens consisting of lamellar α+β, the aging treatment improved ductility and strength by inducing microstructural thickening of the α laths and reducing the β fraction. The effect of the aging treatment was also more marked in the water-quenched samples, characterized by high tensile strengths but limited ductility due to the presence of martensite. In fact, the aging treatment was effective in the recovery of the ductility loss, maintaining high tensile strength properties due to the variation in the relative number of α/α’ interfaces resulting from α’ decomposition. This study, therefore, offers an in-depth investigation of the potential beneficial effects of the aging treatment on the microstructure and mechanical properties of the EBM-processed super β-transus heat-treated Ti-6Al-4V alloy under different cooling conditions.

Effect of Temperature on Sliding Wear Behavior of Ti-6Al-4V Alloy Processed by Powder Bed Fusion Additive Manufacturing Techniques

Ti-6Al-4V is suitable for powder bed fusion additive manufacturing processes; however, until now, limited studies are conducted to investigate the high temperature tribological performance of Ti-6Al-4V samples made by selective laser melting (SLM) and electron beam melting (EBM) techniques. This paper investigated dry sliding wear resistance of Ti-6Al-4V alloy manufactured by SLM, EBM and conventional processes at elevated temperatures up to 600 °C in contact with WC-Co. Linear reciprocating sliding wear tests were carried out under different applied loads and temperatures, and different wear mechanisms were identified and related to the manufacturing technique. Deviations of wear track width measurements indicated shape irregularly, which has been presented and discussed by SEM images of the wear tracks. Energy-dispersive x-ray analysis of surface layer showed how increasing temperature affects the surface oxide layer and debris. The results showed that for all three types of samples, the oxygen richer oxide debris layer at higher temperatures provided a protective layer with higher wear resistance, although strength and hardness of Ti-6Al-4V are lower at a higher temperatures. So, the combination of these two effects results in no significant effect of temperature on wear rate.

Microstructure and Mechanical Properties of Electron Beam Melted Ti–5Al–5Mo–5V–1Cr–1Fe Alloy

Electron beam melting (EBM) has been prevalently employed in fabricating titanium alloys, and it is regarded as an important additive manufacturing technique that attracts significant attention. This study aims to investigate the microstructure and mechanical properties of EBM‐fabricated near β Ti‐5Al‐5Mo‐5V‐1Cr‐1Fe (Ti55511) alloy. Optical microscope (OM) and electron backscatter diffraction (EBSD) results show that the columnar prior β grains with a < 100> texture are formed on a vertical plane. Furthermore, a lamellar structure comprised of α phase (56% vol.) and retained β phase (44% vol.) is observed within the prior β grains, indicating the occurrence of a β → α transformation during the EBM process. To evaluate the anisotropy and establish the relationship between microstructures and mechanical properties, tensile and fracture toughness tests are carried out with a loading direction either parallel or perpendicular to the building direction. Tensile results exhibit high ductility (elongation > 11%) and high strength (ultimate tensile strength > 1080 MPa) in both orientations of the EBM‐fabricated specimen. The anisotropy in mechanical properties is attributed to the morphological difference between the prior β grains along the vertical and horizontal directions.