Electron Beam Powder Bed Fusion Research Articles

Globular surface layer in electron beam powder-bed fusion Ti-6Al-4V after standard powder removal blasting and hot isostatic pressing

A globular α recrystallization has been discovered at the surface of net shape electron beam powder-bed fusion (PBF-EB) Ti-6Al-4V alloy parts. This recrystallization is the result of powder recovery blasting followed by hot isostatic pressing treatment (HIP), which mimics the thermomechanical treatment necessary to achieve globularization in wrought Ti-6Al-4V. The thickness of the globular surface α is variable depending on the blasting intensity as well as the presence of surface protrusions that block the line-of-sight of the blasting media. The globular layer contains a 0001 orientation relationship parallel to the blasting direction, plus a low intragranular orientation deviation, when compared to the bulk. The results indicate that this globular α at the surface may be present for all net-shape Ti-6Al-4V components that have been manufactured via PBF-EB and subjected to a HIP treatment. Tunable fatigue and wear performance may be possible through controlled blasting parameters and globular surface layer thickness.

Thermophysical properties of Ti-6Al-4V fabricated by powder bed fusion methods

Three types of Ti-6Al-4V alloys were prepared by powder bed fusion methods using a laser powder bed fusion ( L -PBF) and two different electron-beam powder bed fusion (E-PBF) systems. The specific heat capacity, linear thermal expansion, and thermal diffusivity were measured from room temperature to ~ 1000 °C. The thermal conductivity values were calculated using the above values. The thermal diffusivity values of all samples along the build direction are similar and a few percent higher than those perpendicular to the build direction. The effect of measurement direction on the linear thermal expansion is evident and related to the processing conditions. A correction for the effect of low porosity (<1%) on thermal diffusivity and thermal conductivity shows that regardless of production method, the porosity corrected values are close for all the PBF samples. Attempts were made to correlate the subtle differences between the samples to the oxygen content, microstructure processing route, and preferred orientation. In the E-PBF samples with a lower cooling rate during production, thermal expansion and thermal diffusivity were nearly isotropic. The subtle changes between the E-PBF samples in both directions directly correlate to a preferred orientation parameter, the peak intensity ratio I (002) α /I (101)α . These results were compared to both cast and additive manufacturing literature, and differences were discussed. • Slight differences exist in L\E-PBF samples’ thermal expansions in both directions. • Anisotropy of the thermal diffusivity exists for both L-\E-PBF samples. • Apparent dependence on grain size exists in thermal diffusivity for various samples. • A dependency on O content was found in E-PBF’s thermal expansion and diffusivity. • A relationship was found between the thermal diffusivity and I (002)α /I (101)α ratio.

Comprehensive and Comparative Heat Treatment of Additively Manufactured Inconel 625 Alloy and Corresponding Microstructures and Mechanical Properties

This study examines and compares the microstructures, Vickers microindentation hardness, and mechanical properties for additively manufactured (AM) samples built by a variety of AM processes: wire arc AM (WAAM), electron beam powder bed fusion (EB-PBF), laser wire direct energy deposition (LW-DED), electron beam direct energy deposition (EB-DED), laser-powered direct energy deposition (LP-DED), and laser powder bed fusion (L-PBF). These AM process samples were post-processed and heat-treated by stress relief annealing at 1066 °C, HIP at 1163 °C, and solution annealing treatment at 1177 °C. The resulting microstructures and corresponding microindentation hardnesses were examined and compared with the as-built AM process microstructures and hardnesses. Fully heat-treated AM process samples were mechanically tested to obtain tensile properties and were also evaluated and compared. Principal findings in this study were that high-temperature heat treatment &gt;1100 °C of AM process-built samples was dominant and exhibited recrystallized, equiaxed grains containing fcc {111} annealing twins and second phase particles independent of the AM process, in contrast to as-built columnar/dendritic structures. The corresponding yield stress values ranged from 285 MPa to 371 MPa, and elongations ranged from 52% to 70%, respectively. Vickers microindentation hardnesses (HV) over this range of heat-treated samples varied from HV 190 to HV 220, in contrast to the as-built samples, which varied from HV 191 to HV 304.

Effect of porosity on mechanical properties of porous tantalum scaffolds produced by electron beam powder bed fusion

The effect of porosity on compressive, bending, and tensile properties of the porous tantalum scaffolds fabricated by electron beam powder bed fusion (EB-PBF) was investigated. The porous tantalum scaffolds with porosity from 69% to 77.8% were obtained by varying the designed porosity and adjusting the processing parameters. It is found that the pores and unfused powder decrease with the increase of deposited energy density. The decrease of porosity leads to an improvement in mechanical properties. The relevancy between compressive/bending/tensile yield strength and relative density can be described appropriately by exponential model, while the relationship between elastic modulus and relative density is in good agreement with the Gibson−Ashby model. All the porous tantalum scaffolds exhibit good ductility in compressive, bending and tensile tests. No fragmentation of struts is observed during the compression process, but cracks are formed on the strut surface after 90° bending, mainly due to the high sensibility to defects caused by the oxide.

Revealing the microstructural evolution of electron beam powder bed fusion and hot isostatic pressing Ti-6Al-4V in-situ shelling samples using X-ray computed tomography

Electron beam powder bed fusion/hot isostatic pressing (E-PBF/HIP), also known as in-situ shelling, is an emerging technology that produces components by only forming their shells whilst retaining sintered powder at the core, and then using HIP to consolidate the entire structure. E-PBF/HIP can boost additive manufacturing productivity, however, the fundamental understanding of the process-microstructure-property correlations remains unclear. Here, we systematically investigate the microstructural evolution of E-PBF/HIP Ti-6Al-4V parts as a function of hatch melting parameters. All HIPped samples achieve full densification, however, their microstructures are significantly different from one another. Using X-ray computed tomography (XCT) and optical microscopy, our results show that the HIPped Ti-6Al-4V microstructure can be controlled by varying the porosity, P (%), pore surface areas and morphology in the as-built parts with a single set of HIP parameters. The HIPped microstructures still exhibit the as-built columnar grains when the as-built porosity, P < 3 % with mainly spherical micro-pores; a mixture of columnar and equiaxed grains when the 3 % < P ≤ 5 % with a tortuous and interconnected pore network; and equiaxed grains when P > 5 % with a highly dense pore network. This work suggests two main drivers for the grain morphology transitions during HIP: (1) a dramatic increase in pore volume increases the localised applied pressure up to 4 times at the core region of the sample and (2) minimise lack-of-fusion pores with high surface energies, promoting dynamic recrystallisation. This study provides a fundamental insight into the E-PBF/HIP technology, showing the feasibility to tailor microstructural properties of E-PBF built parts whilst boosting additive manufacturing productivity.

Effects of local processing parameters on microstructure, texture, and mechanical properties of electron beam powder bed fusion manufactured Ti–6Al–4V

Electron beam powder bed fusion scan strategies for parts or part groupings of various sizes and scan line lengths have been found to inadvertently lead to significant variations in crystallographic texture and mechanical properties for Ti–6Al–4V. This occurs over a range of scan line length that is well below the maximum scan line length recommended by the electron-beam powder bed fusion machine manufacturer. The microstructural attributes such as α lath thickness, prior β grain size, and β phase fraction were similar for materials fabricated under short and long scan line length conditions, but the crystallographic textures varied considerably. Electron backscatter diffraction measurements indicate the basal pole [0001]α was strongly aligned with the build direction for long scan line length conditions, whereas 112‾0α were aligned with the build direction for short scan line length conditions. Subsequent prior β reconstructions indicate a preference for alignment of 110β and 100β with build direction for long and short scan line length conditions, respectively. Both yield and ultimate tensile strengths along the build direction were approximately 8% higher for long scan line lengths, consistent with the observed differences in texture. The impact of a sub-solvus hot isostatic pressing was also investigated, however, this did not significantly change the textures of as-built materials or the relative strengths for the two processing conditions.

Process-aware optimisation of lattice structure by electron beam powder bed fusion

Lattice structures are 3D open topologically ordered geometries that repeat an elementary cell in a predefined 3D space. Struts connected in specific nodes define the cell. Lattice structures are typical geometries that represent the design freedom unlocked by additive manufacturing (AM) and are unachievable with traditional processes. By tuning the morphometric parameters of the cell, its mechanical response can be significantly altered. Because of that, an accurate understanding of the process capabilities is crucial for achieving the nominally designed properties. Considering an electron beam powder bed fusion process, in this work, the same nominal lattice structure is produced under different processing conditions to determine the relationship between the process parameters, the actual cell morphometric parameters, and its mechanical response. Strut dimension, relative density and cross-section are measured using advanced X-ray computed tomography scanning analyses. Uniaxial compressive tests describe the mechanical performance. Inferential and descriptive statistical analyses are applied to investigate the effect of process parameters on the actual strut dimension and infer regression models. The results show that even slight variations of the process parameters significantly affect the morphometric structure parameters that result deviated from the nominal ones. The work demonstrates a strong correlation between all morphometric structure parameters and corresponding mechanical properties. The obtained regression model can predict the strut dimension from the process parameters, which can be then used to estimate the actual relative density and strut size. With this control and without any complex design procedure, a fine-tuning of process parameters allows a precise 3D spatial and localised control of structure properties to produce functionalised structures directly.

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.

Importance of Build Design Parameters to the Fatigue Strength of Ti6Al4V in Electron Beam Melting Additive Manufacturing.

The fatigue properties of metals resulting from Powder Bed Fusion (PBF) is critically important for safety-critical applications. Here, the fatigue life of Grade 5 Ti6Al4V from Electron Beam PBF was investigated with respect to several build and component design parameters using a design of experiments (DOE). Part size (i.e., diameter), part proximity, and part location within the build envelope were considered. Overall, metal in the as-built condition (i.e., no post-process machining) exhibited a significantly lower fatigue life than the machined surface condition. In both conditions, the fatigue life decreased significantly with the decreasing part diameter and increasing radial distance; height was not a significant effect in the machined condition. Whereas the surface topography served as the origin of failure for the as-built condition, the internal lack of fusion (LOF) defects, exposed surface LOF defects, and rogue defects served as the origins for the machined condition. Porosity parameters including size, location, and morphology were determined by X-ray micro-computed tomography (XCT) and introduced within regression models for fatigue life prediction. The greatest resistance to fatigue failure is obtained when parts are placed near the center of the build plane to minimize the detrimental porosity. Machining can improve the fatigue life, but only if performed to a depth that minimizes the underlying porosity.

Saturation pressure of nonequilibrium titanium evaporation during additive manufacturing by electron powder bed fusion

Electron beam powder bed fusion (E-PBF) is an attractive technology for the additive manufacturing of metal parts. However, process improvements require precise control of the energy transferred to the powder by the electron beam. Here, we used tunable diode laser absorption spectroscopy (TD-LAS) to measure the velocity distribution functions of titanium atoms evaporated during E-PBF. The narrow spectral ranges emitted by laser diodes allow for high-resolution absorption profiles of the evaporated atoms and thus accurate determinations of their Doppler broadening, density, and temperature during melting. The obtained vapor temperature reveals overheating at the surface of the melt pool relative to the low-pressure (0.1 Pa) boiling point of titanium, indicating that evaporation occurs under nonequilibrium conditions. We characterized the influence of the linear energy density on titanium evaporation and found it to be consistent with the saturation vapor pressure. Our characterization of the vapor properties provides reliable inputs for melt pool simulations. Furthermore, TD-LAS may be further exploited to prevent the evaporation of low-concentration alloy elements, which can induce defects in the printed part.

The effects of post-processing on the local fracture toughness properties of electron beam powder bed fusion Ti-6Al-4V alloy

Rapidly developing additive technologies for metallic parts production have led to the development of a wide range of methods supporting this field, including mechanical properties characterization. Components produced by AM processes are built spot to spot and layer by layer and that leads to varying local heat absorption and distribution, resulting in varying local properties depending on the shape complexity and build parameters. Different properties in different directions and at various component locations can be expected.Since AM parts are often sub-scale and/or with topological complexity, mechanical characterization with the use of standard specimens is not typically possible and small-sized specimen techniques have to be developed and applied. In the current paper, three AM produced parts made of Ti-6Al-4V by Electron Beam Powder Bed Fusion (EB-PBF) technology have been investigated. Three material conditions are reported here: as-deposited, stress relieved and HIP-ed. Local mechanical properties are assessed with the use of miniaturized compact tension (MCT) fracture toughness specimens and miniaturized tensile tests (MTT). The results are complemented by microstructural and fractographic analysis and are discussed in the light of literature values.

In-situ alloying of a fine grained fully equiaxed Ti-based alloy via electron beam powder bed fusion additive manufacturing process

Alloy design using Additive Manufacturing (AM) methods is an interesting research area attracting the attention of researchers across the world. Formation of strong texture, columnar grains, and chemical inhomogeneity in AM-designed alloys are among the serious challenges dealt with in various research works. This paper addresses these challenges for the first time for a Ti6Al4V–7Cu alloy produced via Electron Beam Powder Bed Fusion (EB-PBF) method following an in-situ alloying approach. In this work, a mixture of Ti6Al4V and elemental Cu powders was used as the feedstock. The results showed a relatively homogenous Cu distribution in the product due to the large melt pool created during the EB-PBF process, a development indicating the method's applicability for in-situ alloying and alloy design. The prior β columnar grains were effectively converted into equiaxed grains due to a constitutionally supercooled zone created by the rejection of Cu solutes in front of the solidification front and a relatively lower thermal gradient during EB-PBF compared with other AM processes. The addition of Cu enhanced the microhardness of Ti6Al4V, which could be considered an outcome of equiaxed grain formation, solid solution strengthening mechanism, and Ti2Cu intermetallic precipitation.

Surface chemistry in Ti-6Al-4V feedstock as influenced by powder reuse in electron beam additive manufacturing

X-ray photoelectron spectroscopy (XPS) as well as scanning and transmission electron microscopy (SEM/TEM) analysis was carried out on four Ti-6Al-4V powders used in electron beam powder-bed fusion (PBF-EB) production environments: virgin low oxygen (0.080 wt% O), reused medium oxygen (0.140 wt% O), reused high oxygen (0.186 wt% O), and virgin high oxygen (0.180 wt% O). The two objectives of this comparative analyses were to (1) investigate high oxygen containing Grade 23 Ti-6Al-4V powders which were further oxidized as a function of reuse and (2) comparing the two virgin Grade 23 and Grade 5 powders of similar oxygen content. The microstructure of the low oxygen virgin Grade 23 powder was consistent with martensitic α′ microstructure, whereas the reused powder displayed tempered α/β Widmänstatten microstructure. XPS revealed a decrease in TiO2 at the surface of the reused powders with an increase in Al2O3. This trend is energetically favorable at the temperatures and pressures in PBF-EB machines, and it is consistent with the thermodynamics of Al2O3 vs. TiO2 reactions. An unexpected amount of nitrogen was measured on the titanium powder, with a general increase in nitride on the surface of the particles as a function of reuse in the Grade 23 powder.11This work is an official contribution of the National Institute of Standards and Technology and is not subject to copyright in the United States.

Utilization of L-PBF process for manufacturing an in-situ Fe-TiC metal matrix composite

In-situ Fe-TiC metal matrix composite (MMC) was successfully fabricated via Laser Powder Bed Fusion (L-PBF) using an in-house gas atomized Fe-TiC composite powder. The optimization campaign of printing parameters targeted the maximum density of printed cubic samples. The highest Archimedes density of 7.615 g cm−3 was achieved by laser power of 210 W, scan speed of 763 mm s−1, and hatch spacing of 80 µm corresponding to a volume energy density (VED) of 68.78 J mm−3 given the constant layer thickness of 50 µm. The relative density of the optimized sample was also measured by Computer Tomography (CT) scanning yielding a relative density of 99.85%. According to measured chemical composition and subsequent thermodynamic calculation, the TiC phase can be up to 7.2 vol%, and X-ray diffraction analysis (XRD) confirmed the presence of TiC phase in bcc-iron. Furthermore, Scanning Electron Microscopy (SEM) of sample cross-sections revealed well-dispersed submicron and nano TiC precipitates of two morphologies and size ranges – primary cubic particles (100–400 nm in size) and secondary plate-like particles (up to 300 nm length and below 50 nm width). No micro-cracks and contaminations were detected between the matrix and reinforcement. Electron Back Scattered Diffraction (EBSD) analysis of a cross-section parallel to build direction revealed extremely fine equiaxed grains with a mean size of 1.3 µm which suggests Columnar to Equiaxed Transition (CET). CET and grain refinement were most likely induced by the presence of TiC particles and rapid solidification within L-PBF. Finally, the micro-hardness tests were conducted to evaluate the reinforcement potential of TiC particles. The mean value of 357.0 ± 9.3 HV1 was obtained for the optimized sample which is superior to pure Fe fabricated via L-PBF and a similar MMC fabricated by Electron Beam Powder Bed Fusion (EB-PBF). Furthermore, the microstructure and hardness of the optimized sample was compared to the sample from optimization campaign with the highest coincident VED and relative density. Some micro-defects were observed in the optimized build and their morphologies and origins were discussed.

Analysis and Fowability of β Type Ti-15Mo-5Zr-3Al Alloy Powders for Laser and Electron Beam Powder Bed Fusion Methods

Analysis and Fowability of β Type Ti-15Mo-5Zr-3Al Alloy Powders for Laser and Electron Beam Powder Bed Fusion Methods

Hierarchical effects of multi-layer powder spreading in the electron beam powder bed fusion additive manufacturing of pure tungsten material

In multi-layer powder spreading process of electron beam powder bed fusion (PBF-EB) additive manufacturing, the quantitative correlation between characteristics (actual height of the deposited layer, packing density, average surface roughness) of each spread powder layer and corresponding process parameters (set thickness value of the layer, spreading velocity) has not been well established. And the underlying mechanism is still not clear. For this objective, PBF-EB additive manufacturing of pure tungsten component in physical experiment is numerically reproduced by a validated coupled discrete element method-computational fluid dynamics (DEM-CFD) model, where the cumulative effects of process parameters are comprehensively analyzed through systematic parametric studies. The results show that with the increase of set thickness value of the layer, the number of layers that reaching steady state first decreases and then increases. After stabilization, the packing density and the average surface roughness of the powder bed are mainly determined by spreading velocity, while the actual height of the deposited layer is determined by both the packing density and the set thickness value of the layer. The mechanism can be ascribed to different motion behaviors of particles located in the top and bottom regions of a new powder layer induced by the impediment of the melted region in the powder bed. The bottom particles can be squeezed by strong forces to form a dense structure, while the top particles escape from the force arches to the zone consisted of weak forces before they are spread and their motion behaviors are dependent on their residual velocity. The obtained highlighted results are not only of theoretical significance, but also of practical value for parameter setting and optimization in the actual electron beam powder bed fusion additive manufacturing of pure W and other materials.

Sintering during Electron Beam - Powder Bed Fusion (EB-PBF) of Ti6Al4V Alloy

The partial sinter between the powder particles during the electron beam – powder bed fusion (EB-PBF) process is fundamental to guarantee adequate thermal and electrical conductivities and conduct the process safely. The sintering degree is tuned by using the process parameters that, at the present day, are mainly optimised with an experimental trial and error approach. Simulation has proven the capability to reduce costs and time related to parameter optimisation. In the current work, a phase field model was developed to simulate sintering during the EB-PBF process. The novelty lies in simulating the sintering process under non isothermal conditions which emulate the heating of the powder due to the preheating and subsequent temperature decrease due to the layer additions. The results show a strong influence of the thermal history on the neck growth and dimension, which differ significantly with respect to the traditional approach to the sintering simulation which considers only constant temperature.

Microstructure, phase composition and hardness of Ti–Au cladding deposited on Ti–6Al–4V substrate by electron beam powder bed fusion method

In this study, a Ti–Au cladding was deposited on a substrate from the Ti–6Al–4V alloy by the electron beam powder bed fusion method in a vacuum. The main goal was to assess the possibility of using titanium powders and gold foils as a feedstock for additive manufacturing of such dental products. The microstructure, chemical element distributions, phase composition and hardness of the formed Ti–Au alloy were studied using optical microscopy, energy dispersive X-ray analysis, as well as X-ray diffraction and nanoindentation tests. Gold-containing intermetallic compounds were observed through the entire cladding thickness. The α-Ti, α-Au, AuTi and AuTi3 phases were found, in addition to the Ti3Au one, which provided hardness values greater than those of conventional titanium alloys. It was shown by results of the crystal-geometric and X-ray phase analysis that the AuTi3 phase possessed the most densely packed A15 structure. This fact correlated with the obtained data on the deviation of the atomic volume per ion from Zen's law and the high hardness levels. Metallurgical patterns of the microstructure formation that affected the functional properties of such claddings were discussed and a further research direction was proposed.

The role of pore structures on the fatigue properties of additively manufactured porous tantalum scaffolds produced by electron beam powder bed fusion

In this work, porous tantalum scaffolds with structures of cubic, G7 and rhombic dodecahedron with different strut angles were fabricated by electron beam powder bed fusion (EB-PBF). Uniaxial compressive testing and compression–compression fatigue testing were conducted on the EB-PBF built tantalum scaffolds. The role of pore structures on the deformation behavior and fatigue properties of the tantalum scaffolds were analyzed. The effect of buckling component in the rhombic dodecahedron structure on fatigue performance of the porous tantalum scaffolds was investigated. It was demonstrated that the G7 and cubic porous tantalum scaffolds were dominated by buckling deformation under the axial force. Due to the presence of vertical struts, the G7 scaffold showed higher fatigue strength and fatigue ratio than the dodecahedron which deformed mainly via bending, the cubic scaffold showed the highest fatigue ratio. As the strut angel increased from 35.3° to 43°, the fatigue strength and fatigue ratio of the rhombic dodecahedron tantalum scaffolds improved by 59% and 20% at 2 × 106 cycles, respectively. The ratcheting strain and fatigue damage strain during the fatigue test were both decreased with the increase of buckling component, leading to the boosting of compressive fatigue properties of tantalum scaffolds accordingly.

Assessment of intra-build variations in tensile strength in electron beam powder-bed fusion Ti–6AL–4V part 1: Effects of build height

In this work, rectangular blocks of electron beam powder-bed fusion (PBF-EB) additively manufactured (AM) Ti–6Al–4V were built, such that a total of 68 mini-tensile test coupons could be extracted for mechanical testing over a range of build height and build plate locations. These as-built tensile test coupons were uniaxially tested to fracture and subsequently studied via scanning electron microscopy, micro X-ray computed tomography, and inert gas fusion chemical analysis to systematically study the intra-build variations in tensile properties, microstructure, porosity, and chemistry. A strong correlation was observed between YS and build height, where YS was observed to decrease with increasing build height, up to 35 mm. There were no significant contributions to this variation in yield strength (YS) from build location, porosity, grain size, or crystallographic texture. Inert gas fusion oxygen measurements reveal the same trend, suggesting that oxygen content variation as a function of build height is contributing to the observed YS variation. The source of the oxygen content variation is thought to be a transient source of oxygen, such as moisture in the powder and build chamber. The establishment of this oxygen variation and its correlation to tensile properties motivates and facilitates improvements to PBF-EB Ti–6Al–4V processing, including powder mixing and handling.