Electron Beam Powder Bed Research Articles

Characterization of surface asperities to understand its effect on fatigue life of electron beam powder bed fusion manufactured Ti-6Al-4 V

Surface asperities play a leading role in determining the fatigue life of as-built Ti-6Al-4 V components manufactured by electron beam powder bed fusion (PBF-EB). Several roughness parameters are available to characterize the surface asperities This study focuses on identifying the surface roughness parameter that correlates best with fatigue life. To this end, several fatigue test specimens were manufactured using the PBF-EB process and utilizing different contour melting strategies, thus producing as-built surfaces with varying roughness. The focus variation microscopy technique was employed to obtain surface roughness parameters for the as-built surfaces. Selected specimens were characterized using x-ray computed tomography (XCT). Tomography can detect surface-connected features obscured by other parts of the surface that are not visible through optical microscopy. The fatigue life of all specimens was determined using four-point bend testing. Through regression model analysis, maximum pit height (Sv) was identified as the statistically significant roughness parameter with the best fit affecting fatigue life. The fracture zone was closely inspected based on the data collected through XCT prior to fatigue tests. This led to another estimate of the worst-case value for the statistically significant roughness parameter Sv. The Sv parameter values obtained from optical microscopy and XCT were used as the initial crack size in a crack growth model to predict fatigue life. It is observed that life estimates based solely on optical measurements of Sv can be overly optimistic, a situation that must be avoided in predictive design calculations.

Influence of powder particle size distribution on the creep performance of Ti-48Al-2Cr-2Nb alloy fabricated via electron beam-powder bed fusion

The effects of particle size distributions (PSDs) on the microstructure and creep behavior of Ti-48Al-2Cr-2 Nb fabricated via electron beam-powder bed fusion (EB-PBF) were investigated. Three PSDs of 45–150 (PSD-A), 38–150 (PSD-B), and 38–180 µm (PSD-C) were used as powder feedstock. Widening the PSD range decreased the relative density of fabricated parts and promoted the formation of banding microstructure. Moreover, samples fabricated with wider PSDs had a higher volume of lack-of-fusion (LoF) defects; however, the content of gas pores decreased significantly in PSD-C, which was related to the presence of coarse powder particles. The volume of the LoF defects in PSD-C was approximately 2.8 times of the same defects in PSD-A, which influenced the creep performance (at 750℃) and failure mechanism of Ti-48Al-2Cr-2 Nb. A longer steady-state creep period at different stress levels was identified for the samples fabricated with the narrowest PSD (45–150 µm). Widening the PSD span also caused weakness in the creep properties and data scattering at higher stress levels of 300 and 350 MPa. The LoF defects and coarse grains accelerated the crack initiation and propagation in Ti-48Al-2Cr-2 Nb during creep. This study represents the first report on the impact of PSD on the creep properties of TiAl alloys fabricated via EB-PBF, which offers new insights and enhances understanding of the powder-process-microstructure-property relationship.

Dynamic Response of Ti-6Al-2Zr-1Mo-1V Alloy Manufactured by Laser Powder-Bed Fusion.

Titanium parts fabricated by additive manufacturing, i.e., laser or electron beam-powder bed fusion (L- or EB-PBF), usually exhibit columnar grain structures along the build direction, resulting in both microstructural and mechanical anisotropy. Post-heat treatments are usually used to reduce or eliminate such anisotropy. In this work, Ti-6Al-2Zr-1Mo-1V (TA15) alloy samples were fabricated by L-PBF to investigate the effect of post-heat treatment and load direction on the dynamic response of the samples. Post-heat treatments included single-step annealing at 800 °C (HT) and a hot isotropic press (HIP). The as-built and heat-treated samples were dynamically compressed using a split Hopkinson pressure bar at a strain rate of 3000 s-1 along the horizontal and vertical directions paralleled to the load direction. The microstructural observation revealed that the as-built TA15 sample exhibited columnar grains with fine martensite inside. The HT sample exhibited a fine lamellar structure, whereas the HIP sample exhibited a coarse lamellar structure. The dynamic compression results showed that post-heat treatment at 800 °C led to reduced flow stress but enhanced uniform plastic strain and damage absorption work. However, the HIP samples exhibited both higher stress, uniform plastic strain, and damage absorption work owing to the microstructure coarsening. Additionally, the load direction had a subtle influence on the flow stress, indicating the negligible anisotropy of flow stress in the samples. However, there was more significant anisotropy of the uniform plastic strain and damage absorption. The samples had a higher load-bearing capacity when dynamically compressed perpendicular to the build direction.

Achieving back-stress strengthening at high temperature via heterogeneous distribution of nano TiBw in titanium alloy by electron beam powder bed fusion

Back-stress strengthening has been proven to be an effective strategy in breaking the room temperature strength–ductility dilemma in hetero-structural metals matrix composite materials (MMC). However, back-stress is difficult to achieve at high temperature because of grain boundary softening, grain boundary sliding, dislocation annihilation and grain rotation. Here, back-stress strengthening is exploited to improve the high temperature tensile properties of Ti-6Al-4 V (TC4) alloy by precipitating a heterogeneous distribution of nano TiB whiskers (TiBw). In this work, the rapid solidification associated with electron beam powder bed fusion (EB-PBF) generates heterogeneous in-situ nano TiBw at both grain boundaries and grain interiors. The nano-TiBw on the grain boundaries (GB nano-TiBw) not only refines the grains but can also pin the grain boundaries. The intragranular nano TiBw (IG nano-TiBw) significantly reduces the dislocation mean free path, inhibit dislocation movement, and dramatically enhances the dislocation storage density. Meanwhile, specimens are strengthened by high-density < a + c > dislocations induced by the precipitated nano TiBw. The heterogeneous distribution of nano TiBw achieved a 50 °C increase in the service temperature of TC4 alloy while also increasing stability. This strategy provides a new perspective for further improving the high-temperature mechanical properties of titanium alloys.

Insights into Nb-silicide-based in-situ composite processed by electron beam powder bed fusion

This is the first report introducing the idea of processing Nb-silicide-based in-situ composite through electron beam powder bed fusion (PBF-EB). A high energy input together with a line order strategy resulted in cracking, whilst a lower energy input without the line order led to microstructural inhomogeneity characterised by alternating dark- and bright-contrast bands. In samples printed using the line order strategy, long cracks initiated close to the melt pool boundary and propagated into the previous layer. These cracks were due to the weak interface between the primary Nb3Si columnar grains and the equiaxed (Nb,Ti) solid solution phase or Nb3Si-(Nb,Ti) solid solution eutectic structure. The porosity level increased from 0.122 % to 0.547 % with the increase of area average energy input from 2.22 J/mm2 to 6.67 J/mm2. The density decreased to 6.80 g/cm³ in the line order strategy samples due to the presence of cracks, compared to 6.84 g/cm³ in samples without employing the line order strategy. The dark-contrast band consisted of (Nb,Ti) solid solution, Nb3Si and Nb5Si3, while the bright-contrast band was composed of (Nb,Ti) solid solution and Nb3Si. Within the bright-contrast band, a recurring layered microstructural inhomogeneity appeared, with columnar grains at the bottom, whilst fine- and coarse-grained eutectic networks in the middle and upper regions. The Nb3Si phase exhibited a pronounced {001} texture, whereas the (Nb,Ti) solid solution phase displayed a weaker texture. The Nb-silicide-based in-situ composite fabricated through PBF-EB had a microhardness of ∼700 HV0.5. This value is higher than that achieved through conventional means but falls within the mid-range for laser-based additive manufacturing counterparts.

Electron beam powder bed fusion of Ti-30Ta high-temperature shape memory alloy: microstructure and phase transformation behaviour

ABSTRACT The present study reports on additive manufacturing of a Ti-30Ta (at.%) high-temperature shape memory alloy (HT-SMA) using electron beam powder bed fusion (PBF-EB/M) technique. Detailed microstructure analysis was conducted to reveal the microstructural evolution along the entire process chain, i.e. from gas-atomised powder to post-processed material. PBF-EB/M processed structures with near full density and an isotropic, β-phase stabilised microstructure, i.e. equiaxed β-grains of around 20 µm in diameter with no preferred crystallographic orientation, are reported. As revealed by differential scanning calorimetry, post-process heat-treated Ti-Ta demonstrates a reversible martensitic phase transformation well above 100°C. Although partly unmolten Ta-particles after both gas atomisation and PBF-EB/M remain a challenge towards robust processing, PBF-EB/M appears to show significant potential for fabrication of Ti-Ta HT-SMAs, especially when functional metal parts and components with complex shapes are required, which are difficult to fabricate conventionally.

A novel titanium alloy for load-bearing biomedical implants: Evaluating the antibacterial and biocompatibility of Ti536 produced via electron beam powder bed fusion additive manufacturing process

Additive manufacturing (AM) of Ti-based biomedical implants is a pivotal research topic because of its ability to produce implants with complicated geometries. Despite desirable mechanical properties and biocompatibility of Ti alloys, one major drawback is their lack of inherent antibacterial properties, increasing the risk of postoperative infections. Hence, this research focuses on the Ti536 (Ti5Al3V6Cu) alloy, developed through Electron Beam Powder Bed Fusion (EB-PBF), exploring bio-corrosion, antibacterial features, and cell biocompatibility. The microstructural characterization revealed grain refinement and the formation of Ti2Cu precipitates with different morphologies and sizes in the Ti matrix. Electrochemical tests showed that Cu content minimally influenced the corrosion current density, while it slightly affected the stability, defect density, and chemical composition of the passive film. According to the findings, the Ti536 alloy demonstrated enhanced antibacterial properties without compromising its cell biocompatibility and corrosion behavior, thanks to Ti2Cu precipitates. This can be attributed to both the release of Cu ions and the Ti2Cu precipitates. The current study suggests that the EB-PBF fabricated Ti536 sample is well-suited for use in load-bearing applications within the medical industry. This research also offers an alloy design roadmap for novel biomedical Ti-based alloys with superior biological performance using AM methods.

A new approach of preheating and powder sintering in electron beam powder bed fusion

Preheating is an essential process step in electron beam powder bed fusion. It has the purpose of establishing a sintered powder bed and maintaining an elevated temperature. The sintered powder bed reduces the risk of smoke and in combination with the elevated temperature improves the processability. Today, the line-ordering preheating scheme is widely used. This scheme does not take the previously built layers into account and results in an inhomogeneous elevated temperature and consequently in a variety of sinter degrees, which is disadvantageous for the process. The main challenge is now to modify this scheme to establish a homogeneous temperature distribution. This study addresses this challenge and analyses as well as optimises this scheme. A GPU-parallelised thermal model reveals a heterogeneous temperature distribution during preheating because of varying thermal conditions within a build job. In addition, a work-of-sintering model predicts that the sinter degree of the current powder layer on top of previously consolidated material is smaller than on top of the surrounding powder bed. This work aims to invert this trend to improve powder re-usage and material consolidation. Consequently, this work proposes an extension of the current scheme, compensating for the specific energy loss with local adjustments to the energy input. This adaption results in a uniform temperature distribution and advantageous sintering. Applying the proposed numerical model proves to be an effective method to analyse the evolving process conditions and tailor the local energy input, thereby improving the efficiency of the preheating step.

Expanding manufacturability of sheet-based triply periodic minimal surfaces by electron beam powder bed fusion in Wafer theme

Porous biomaterials based on triply periodic minimal surfaces (TPMS) are intensely studied and discussed in the literature. Manufacturing of these structures with a complex geometry became possible using additive manufacturing methods, having their own specifics and challenges. Previously, we demonstrated that sheet-based gyroids, one of TPMS structures, manufactured by electron beam powder bed fusion in Melt and Wafer themes could possess identical mechanical properties (quasi-elastic gradient). In the current research sheet-based gyroids were produced using Wafer Theme with five different settings, including electron beam current and scan speed variation with MultiBeam turned on and off, in attempt to obtain thin walls without losing mechanical performance. Mechanical tests revealed that the specimens manufactured with higher beam line energy on average show better mechanical properties. The specimens produced with MultiBeam showed slightly lower mechanical properties, and do not demonstrate significant improvement in the TPMS surface roughness. According to SEM studies, the average wall thickness was about 0.4 mm for specimens with high line energy, and 0.3 mm for low line energy. Through-hole defects on the horizontal surfaces, noticed and discussed by us previously, have larger average area of the hole and more irregular shape for increased beam line energy. TPMS surface morphology of the specimens produced with higher beam energy and no MultiBeam was on average more even. According to EBSD, α/α` martensite with laths oriented in a Widmanstätten basket-weave pattern was observed with no β phase in the resulting material. No significant differences in microstructure between specimens produced with different beam energy was detected. FEA modeling revealed the impact of the wall thickness, through holes size and orientation on quasi-elastic gradient of a gyroid. It was demonstrated that, through holes with small size located on the horizontal surfaces (layer plane) have only a minor impact on the mechanical properties of the samples if they are not extremely abundant.

Innovative design of heterogeneous structures in Cu-containing titanium alloys to enhance mechanical properties, abrasion resistance, and antibacterial performance

How to precisely modulate the morphology and distribution of precipitated phases to have long-term antibacterial activity and outstanding strength-ductility has slowed the overall development and engineering applications of Cu-bearing biomedical titanium alloys. For the first time, the electron beam powder bed fusion (EBPBF) was employed to design titanium alloys with a completely solid solution, and containing fine Ti2Cu precipitates in both uniform and layered structures. The mechanical properties of the designed alloy with the layered (α-Ti and Ti2Cu) structure are superior to the other structures, especially with an outstanding compressive yield strength of 1221.9 MPa. Simultaneously, the wear resistance of the heterogeneous structures containing Ti2Cu precipitates was significantly improved, with a specific wear rate only half of that of the EBPBF-fabricated Ti6Al4V alloy. The compact arrangement of Ti2Cu phases created a large number of interfaces conducive to the formation of corrosion channels, which provided the capacity of continuous Cu2+ release. This work comprehensively analyzes the effects of heterogeneous structures on enhancing the sustained antibacterial capacity and optimizing the mechanical properties of a Cu-containing titanium alloy, laying a good foundation for their application in clinical and implantable devices.

Multiple interaction electron beam powder bed fusion for controlling melt pool dynamics and improving surface quality

Low surface quality of fabricated parts and long processing times are two commonly stated reasons against a more wide-spread adoption of electron beam powder bed fusion (PBF-EB) as a commercially viable manufacturing technique. Aside from the large particle size distribution (45–150 µm) of processed powders, the surface roughness, limited structural resolution and process instabilities are attributed to unwanted dynamics of the melt pool, like spattering, balling, evaporation and melt pool displacement. As a result, a contouring step using slow but stable melting parameters is usually added to the process to improve surface quality, leading to increased processing times.In this study, we demonstrate how the surface quality can be improved by supplying the energy for melt pool formation over multiple, timely separated interactions. To investigate the effect of this multiple interaction processing (MIP), series of single line melt structures with increasing number of interactions were fabricated, characterized and corresponding thermal diffusion simulations performed. The experimental results show how MIP allows to lower surface roughness of melted structures through a reduction of spattering, balling and melt pool displacement during processing, while our simulation model identifies these benefits to be caused by a significant reduction in surface temperature and thus evaporation, with increasing number of interactions for melt pool formation. Our model further demonstrates, how a relatively simple, computationally inexpensive and thus scalable thermal diffusion model is capable of providing valuable information about thermal conditions during melt pool formation. Lastly, we introduce a few simple equations to calculate appropriate MIP parameters for any kind of scan strategy.

Electron beam powder bed fusion enables crack-free, high-strength and sufficiently ductile chemically complex intermetallic alloys

ABSTRACT In contrast to traditional intermetallics that are difficult to process and prone to cracking during additive manufacturing, our work demonstrates the successful fabrication of crack-free, high-performance CCIMAs via electron beam powder bed fusion (EBPBF). The microstructures of CCIMAs (NiCoFeAlTiB) fabricated by EBPBF are remarkably complex, exhibiting a multiphase composition where the disordered FCC phase forms the matrix interleaved with L12-ordered intermetallic phase. The unique combination of ordered lattices and high-entropy disordered phases, tuned by optimised EBPBF processing, imparts exceptional mechanical properties with a high tensile strength (∼1 GPa) and a sufficient ductility (∼11%). It is found that additional minor HCP and L21 precipitates can also effectively slow down and block the crack extension. This work demonstrates a pathway for crack-free fabrication of CCIMAs with tailored microstructures using EBPBF which may provide new insights and manufacturing strategies to unlock the potential of these advanced intermetallic alloys.

Electron Beam Powder Bed Fusion of ATI C103TM Refractory Alloy

The study investigated the use of electron beam powder bed fusion (EB-PBF) to fabricate niobium ATI C103™ alloy articles for microstructural characterization and mechanical testing. The feedstock powder was consolidated into low-porosity articles, and both powder and sample chemistry were monitored. Oxygen uptake in the powder was limited to less than the ASTM B655/B655M (2018) specification limits for 7 uses. Manipulating vacuum chamber pressure showed stable hafnium content but decreasing titanium content with decreasing chamber pressure attributed to evaporation. AM samples were evaluated in the post-processed, as-fabricated, annealed, and hot isostatic pressing (HIP) condition with a maximum yield strength of 287 MPa, UTS of 375 MPa for the HIP, and maximum elongation of 32 pct for the annealed specimens, respectively. Mechanical properties are similar to typical wrought products, with a notable increase in yield strength after post-processing by HIP. The fracture behavior was driven by porosity in the as-fabricated specimens and grain boundary fracture after HIP.

Creep anisotropy of additively manufactured Inconel-738LC: Combined experiments and microstructure-based modeling

The current lack of quantitative knowledge on processing-microstructure–property relationships is one of the major bottlenecks in today’s rapidly expanding field of additive manufacturing. This is centrally rooted in the nature of the processing, leading to complex microstructural features. Experimentally-guided modeling can offer reliable solutions for the safe application of additively manufactured materials. In this work, we combine a set of systematic experiments and modeling to address creep anisotropy and its correlation with microstructural characteristics in laser-based powder bed fusion (PBF-LB/M) additively manufactured Inconel-738LC (IN738LC). Three sample orientations (with the tensile axis parallel, perpendicular, and 45° tilted, relative to the building direction) are crept at 850 °C, accompanied by electron backscatter secondary diffraction (EBSD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigations. A crystal plasticity (CP) model for Ni-base superalloys, capable of modeling different types of slip systems, is developed and combined with various polycrystalline representative volume elements (RVEs) built on the experimental measurements. Besides our experiments, we verify our modeling framework on electron beam powder bed fusion (PBF-EB/M) additively manufactured Inconel-738LC. The results of our simulations show that while the crystallographic texture alone cannot explain the observed creep anisotropy, the superlattice extrinsic stacking faults (SESF) and related microtwinning slip systems play major roles as active deformation mechanisms. We confirm this using TEM investigations, revealing evidence of SESFs in crept specimens. We also show that the elongated grain morphology can result in higher creep rates, especially in the specimens with a tilted tensile axis.

Revealing the Mechanisms of Smoke during Electron Beam–Powder Bed Fusion by High-Speed Synchrotron Radiography

Electron beam–powder bed fusion (PBF-EB) is an additive manufacturing process that utilizes an electron beam as the heat source to enable material fusion. However, the use of a charge-carrying heat source can sometimes result in sudden powder explosions, usually referred to as “Smoke”, which can lead to process instability or termination. This experimental study investigated the initiation and propagation of Smoke using in situ high-speed synchrotron radiography. The results reveal two key mechanisms for Smoke evolution. In the first step, the beam–powder bed interaction creates electrically isolated particles in the atmosphere. Subsequently, these isolated particles get charged either by direct irradiation by the beam or indirectly by back-scattered electrons. These particles are accelerated by electric repulsion, and new particles in the atmosphere are produced when they impinge on the powder bed. This is the onset of the avalanche process known as Smoke. Based on this understanding, the dependence of Smoke on process parameters such as beam returning time, beam diameter, etc., can be rationalized.

A surface quality optimization strategy based on a dedicated melt-pool control via the dashed-scan contouring technique in electron beam powder bed fusion

Electron beam powder bed fusion (EB-PBF) is a key metal additive manufacturing (AM) technology known for its high energy absorption and low heat stress. However, the high surface roughness of EB-PBF parts has limited its broader application. To address this issue, this paper proposes a dashed-scan contouring strategy to obtain lower surface roughness in EB-PBF. This strategy involves melting the contour in the form of segmented staggered scans and employing high-frequency jumping of the electron beam (EB) to melt multiple positions synchronously. A comprehensive analysis, combining experimental investigations and multi-physical simulations, elucidates the intrinsic connection between control processes and melt pool stability. Results show that by controlling the length-to-width ratio of the melt pool, regulating overlap distances between melt pools, and fine-tuning cooling times, balling phenomena and Plateau-Rayleigh instabilities can be suppressed. Additionally, these measures effectively mitigate irregularities in track formation when compared to the traditional process. Experimental validation demonstrates the efficacy of the approach, achieving reduced surface roughness (Ra) of thin-walled Ti-6Al-4 V parts from over 25 μm to below 12.6 μm, while enhancing dimensional accuracy and reducing melt anomalies at corners. The proposed dashed-scan contouring strategy opens new avenues for AM of highly reliable and intricate structures, such as lattice sandwich structures, thin walls, and internal flow passages.

Corrosion fatigue behavior of nanoparticle modified iron processed by electron powder bed fusion

Due to its excellent biocompatibility, pure iron is a very promising implant material, but often features corrosion rates that are too low. Using additive manufacturing and modified powders the microstructure and, thus, the material properties, e.g., the corrosion properties, can be tailored for specific applications. Within the scope of this study, pure iron powder was modified with different amounts of CeO2 or Fe2O3 nanoparticles and subsequently processed by Electron Beam Powder Bed Fusion (PBF-EB/M). The corrosion-fatigue behavior of CeO2 and Fe2O3 modified iron was investigated using rotation bending tests under the influence of simulated body fluid (m-SBF). While the modification using Fe2O3 showed reduced fatigue and corrosion-fatigue strengths, it could be demonstrated that the modification with CeO2 is characterized by improved fatigue properties. The superior fatigue properties in air are attributed to the positive impact of dispersion strengthening. Additionally, an increased degradation rate compared to pure iron could be observed, eventually promoting an earlier failure of the specimens in the corrosion fatigue tests.

In situ build surface topography determination in electron beam powder bed fusion

Electron optical imaging is the most promising process monitoring method in electron beam powder bed fusion. State of the art in modern machines is the installation of a single detector in the top center of the build chamber. Exemplary applications are the reconstruction of digital twins of manufactured parts to compare their dimensional accuracy or analysing the top surface of each layer to identify surface features like pores or material transport. Multi-detector systems are currently under research and have shown great potential in reconstructing the surface topography in situ. A recently developed ray tracing model, describing the image formation process, allows to formulate design guide lines for multi-detector systems and provides a method for the computation of the normal vector field of the build surface. This work utilizes the recent progress and presents a newly developed four-detector system and an updated computation chain, which enable build surface topography reconstruction in situ in every layer of a build process. The computation chain contains a normal integration algorithm, which employs Tikhonov regularization to cope with measurement irregularities. The integration method is validated with ex situ measured as-built surfaces. Additionally, first applications are demonstrated and connections to process parameter changes illustrated.

Extracting powder bed features via electron optical images during electron beam powder bed fusion

Electron beam powder bed fusion offers the unique opportunity to observe the process by measuring scattered electrons on a metal detector. This technique is the state of the art in generating electron optical images of the build area after melting using single- or multi-detector setups. The images enable the detection of surface defects like porosity or material transport by reconstructing the surface topography. Internal defects such as layer-bonding defects cannot be identified. Many of these defects, particularly layer-bonding defects, often originate from an irregular distribution of the powder bed.This work introduces an additional process step by recording an electron optical image after the distribution of the powder bed. Combining this with an electron optical image after melting the previous layer enables extraction of powder bed features such as the current powder bed height. The underlying method bases on the correlation of experimental measurements and numerical simulations of the intensity of the electron optical signal for different powder bed heights. With this approach, it is possible to identify irregular powder distributions, such as uncovered areas of previously molten material or locally varying powder bed heights. This information is crucial for online monitoring and real time process control. Exemplary, this opens the opportunity of healing the powder bed by an additional raking step.

Vapor chemical composition in electron beam powder bed fusion using Ti–6Al–4V powder

Vapor chemical composition in electron beam powder bed fusion using Ti–6Al–4V powder