Electron Beam Melting Ti6Al4V Research Articles

Rapid screening of creep resistance in additive manufactured Ti-6Al-4V alloy

In this study, rapid screening of creep resistance for additively manufactured (AM) materials is enabled using an accelerated creep test (ACT) called the dynamic negative stepped test (DNST). Additively manufactured components are on the rise in aerospace systems, particularly turbine engine parts. It can be challenging to identify the AM process parameters that correspond to high-temperature strength and creep resistance. Tests are needed that rapidly screen and qualify creep resistance. In DNST, the temperature is held constant, and the specimen is taken through a series of load holds from high to low. The DNST contains two distinct characteristics: (1) the hold duration for each step is “dynamically” controlled to ensure the minimum-creep-strain-rate is achieved at every step, and (2) the “descending” profile maximizes damage accumulation while remaining on the edge of instability. The DNST assesses a material's ability to resist creep damage accumulated across multiple deformation mechanisms. In this study, electron beam melted Ti–6Al–4V, built at six (6) directions: 0°, 30°, 45°, 60°, 90°, and V, is tested using a four-step DNST at 650 °C. Analysis of the minimum-creep-strain-rate (at each step), creep ductility, and rupture were performed. A strain energy density (SED) benchmark metric was established to rapidly screen and classify creep resistance using an “Ashby-like” plot that balanced energy dissipated and energy dissipation rate. The creep resistance was found to be ranked from V, 60, 45, 30, 90, and 0 in descending order. Microstructural and crystallographic characterization revealed grain refinement progressing from the strain-free to the high-strain regions, indicative of dynamic recrystallization (DRX). Further observations revealed that the material's energy capacity increased with the alignment of the grains' long axis and tensile direction.

Microstructure and microhardness of electron beam melted Ti–6Al–4V components with differential thickness in initial deposition layers

Electron beam powder bed fusion (EB-PBF) is a process to additively manufacture Ti–6Al–4V. Along with process induced defects, the unique microstructure of EB-PBF Ti–6Al–4V leads to anisotropic properties, and therefore an understanding of how the microstructure evolves as layers are added is required. This experimental study presents the microstructural evolution of Ti–6Al–4V during the deposition of initial layers in EB-PBF process. Two geometries of specimens, ramp and step, were fabricated from 0.1 mm thickness (2 layers) to 0.7 mm (14 layers), and 0.2 mm (4 layers) to 1.4 mm (28 layers). The evolution of the microstructure was investigated through optical and electron microscopy, spatial microhardness mapping, and detailed microstructural characterization. Up to the deposition of seven layers, the microstructure comprised of α+β phases with partially retained α′ phase from rapid solidification cooling that resulted in higher microhardness. A unique microstructural gradient composed of three distinct regions (Region I, II, III) appears along the build direction from the deposition of 10 layers until 28 layers. Region I showed a finer α+β microstructure in irregular prior β grain with partial martensitic α’ phase and displayed greater microhardness. Region II showed coarser α+β microstructure and lower microhardness than both Region I and III. The average α-lath thickness and α-volume fraction increased as new layers were deposited. Perpendicular to the build direction, the hardness distribution was observed to be uniform along the longitudinal x-y direction.

Adjustable Elasticity of Anatomically Shaped Lattice Bone Scaffold Built by Electron Beam Melting Ti6Al4V Powder

This study investigates the elasticity of specific lattice structures made from titanium alloy (Ti6Al4V), namely, anatomically shaped lattice scaffolds (ASLS) aimed for reinforcement of the bone tissue graft that substitute a missing piece of the previously injured bone during its recovery. ASLSs that were used for testing were fabricated using the Electron Beam Melting (EBM) method. The mechanical properties of the ASLS were examined through uniaxial compression tests. Compression testing revealed the complex non-linear behavior of the scaffold structure’s elasticity, with distinct compression stages and deformation dependencies. The ASLS structures exhibited quasi-elastic deformation followed by the rupture of individual struts. Results demonstrate that the ASLSs can be stiffened by applying appropriate compression load and accordingly achieve the target elasticity of the structure for the specific load range. The modulus of elasticity was determined for different compression stages of ASLS, allowing interpolation of the functional relation between the modulus of elasticity and compressive force that is used for stiffening the ASLS. This study enhances the understanding of the mechanical behavior of the specific lattice structures made of Ti6Al4V and provides insights for the development of mechanically optimized anatomically shaped lattice scaffolds.

Improved seawater corrosion resistance of electron beam melting Ti6Al4V titanium alloy by plasma nitriding

Electron beam melting (EBM) is an additive manufacturing technology that allows flexible processing of parts with complex shapes, but the product surface is usually rough and porous, which affects its corrosion resistance. In this study, the plasma nitriding has been conducted on electron beam melted Ti6Al4V titanium alloy. The samples were analyzed using X-ray diffraction (XRD), optical microscope (OM), scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscope (HRTEM). The results showed that the plasma nitriding-treated EBM sample (PN-EBM) exhibited better corrosion resistance in natural seawater than the EBM sample due to the generation of a ∼16 μm compound layer, which can effectively prevent the intrusion of harmful ions and enhance the corrosion resistance of the EBM titanium alloy.

Multi-Response Optimization of Additively Manufactured Ti6Al4V Component Using Grey Relational Analysis Coupled with Entropy Weights

Due to its near-net-shape manufacturing and ability to treat challenging-to-manufacture materials such as titanium alloys, Additive manufacturing (AM) is growing in popularity. However, due to the poor surface quality of AM components, finishing processes such as machining are required. One of the most difficult aspects of finishing AM components is the fact that even when using the same machining parameters, the surface roughness can vary significantly depending on the orientation of the part. In this study, electron beam melting (EBM) Ti6Al4V parts are subjected to the finishing (milling) process in three potential orientations relative to the direction of the tool feed. The impact of the feed rate, radial depth of cut, and cutting speed on the surface roughness and cutting force of the Ti6Al4V EBM part is studied while taking the orientations of the EBM components into consideration. It is found that the machined surface changes in noticeable ways with respect to orientation. A factorial design is used for the experiments, and analysis of variance (ANOVA) is used to evaluate the results. Furthermore, the grey relational analysis (GRA) method coupled with entropy weights is utilized to determine the optimal process variables for machining a Ti6Al4V EBM component. The results show that the feed rate has the greatest impact on the multi-response optimization, followed by the cutting speed, faces, and radial depth of cut.

The influence of gaseous hydrogen charging on the microstructural and mechanical behavior of electron beam melted and wrought Ti–6Al–4V alloys using the small punch test

The influence of gaseous hydrogen charging at 600 °C on the microstructure and mechanical behavior of wrought and Electron Beam Melted (EBM) Ti–6Al–4V alloys was investigated for hydrogen contents between 0.14 and 1.0 wt%. The small punch test (SPT) technique was used to characterize the mechanical behavior of all specimens. Both EBM and wrought alloys containing ∼6 wt% β and similar impurity levels showed similar phase content and mechanical property changes at all hydrogen contents, regardless of their original microstructural differences. This similarity can be explained by the high hydrogen diffusivity at the high temperature at which gaseous charging was carried out, and is in contrast to previous reports where EBM Ti–6Al–4V was found to be more sensitive to hydrogen embrittlement due to low-temperature electrochemical charging. After hydrogenation, αH and βH solid solutions were formed. The quantity of the αH phase reduced gradually with hydrogen content, while forming βH, α2, and hydrides. It was found that βH saturated at 0.27 wt% hydrogen content. Both alloys demonstrated relatively high strength and ductility up to hydrogen content of 0.2 wt%, i.e. below the βH saturation concentration. Above the βH saturation concentration, the mechanical properties of the maximum load (Pmax), deflection at maximum load (δmax), and absorbed energy (E), degraded significantly due to hydride formation.

Energy saving in milling of electron beam–melted Ti6Al4V parts: influence of process parameters

Additive manufacturing (AM) is claimed to be a green technology because of its potential in improving material use efficiency. Electron beam melting (EBM) is among the most popular AM techniques adopted to manufacture titanium parts for medical and aerospace applications, as the technology offers an effective way of producing lightweight and complex parts. Nevertheless, additively manufactured parts hardly ever meet industrial quality standards, so post-treatments are always required resulting in additional resources and energy consumption. Moreover, still few works exist on joint analysis of energy consumption and roughness in milling EBMed parts by means of a non-typical tool, and this study aims to fill this gap of knowledge. Three EBM Ti6Al4V cylindrical samples were manufactured into a single job of the ARCAM A2X machine in the same process conditions. Three lengths, 120° apart from each other, were defined along the direction parallel to the axis of each cylinder to perform the milling by varying spindle speed, depth of cut, and machining speed. A high-performance complex-shaped insert has been used to perform the milling process to improve the surface finishing of the Ti6Al4V EBMed samples. Total energy consumption has been calculated as the sum of the machining time and the non-machining time. A joint investigation of both the surface roughness and the energy consumption in machining led to understanding which the best cut strategies are to perform milling with a complex cutting tool from a sustainability perspective. Results showed that it is not sustainable to choose the minimum depth of cut to obtain a fixed total depth of material removed as non-machining time showed to play a crucial role in the total energy consumption of the milling process.

CNC Turning of an Additively Manufactured Complex Profile Ti6Al4V Component Considering the Effect of Layer Orientations

Electron beam melting (EBM) is one example of a 3D printing technology that has shown great promise and advantages in the fabrication of medical devices such as dental and orthopedic implants. However, these products require high surface quality control to meet the specifications; thus, post-processing, such as with machining processes, is required to improve surface quality. This paper investigates the influence of two-part orientations of Ti6Al4V EBM parts on the CNC machining (turning) process. The two possible EBM part orientations used in this work are across EBM layers (AL) and parallel to the EBM layer (PL). The effect of the EBM Ti6Al4V part orientations is examined on surface roughness, power consumption, chip morphology, tool flank wear, and surface morphology during the dry turning, while using uncoated carbide tools at different feed rates and cutting speeds. The results showed that the AL orientation had better surface quality control and integrity after machining than PL orientation. Using the same turning parameters, the difference between the roughness (Ra) value for AL (0.36 μm) and PL (0.79 μm) orientations is about 54%. Similarly, the power consumption in AL orientation differs by 19% from the power consumption in PL orientation. The chip thickness ratio has a difference of 23% between AL and PL orientations, and the flank wear shows a 40% difference between AL and PL orientations. It is found that, when EBM components are manufactured along across-layer (AL) orientations, the impact of part orientation during turning is minimized and machined surface integrity is improved.

The effects of surface finish on the fatigue performance of electron beam melted Ti–6Al–4V

Additive manufacturing (AM) is an advanced manufacturing technique whose uptake within the aerospace industry is being limited by the lack of understanding of the effects of surface finish on the fatigue properties of AM built components. It is known that the surface profile of as-built AM parts is significantly rougher than its traditionally manufactured equivalent, promoting stress raising features that have an adverse effect on fatigue life.This paper will focus on developing a deeper understanding of the fatigue properties of the widely-used alpha-beta titanium alloy, Ti–6Al–4V, as manufactured through electron beam melting (EBM) and the role that surface roughness plays. Results have been generated on as-built and machined & polished EBM test coupons, supported by microstructural and fractographic analysis, X-ray computed tomography (XCT), advanced surface profilometry and hardness testing.Results have shown that as-built EBM Ti–6Al–4V samples have a significantly inferior fatigue life than machined & polished samples, despite the material in each of the surface finish types being hot isostatically pressed (HIP). It has been revealed that while surface parameters, MR2 and Ra, provide no correlation to high cycle fatigue life, it is shown that as the Rp surface parameter decreases, the respective number of cycles to failure increases. It can be concluded that a samples’ surface roughness is a highly contributing factor in influencing fatigue performance, yet it should not be considered in isolation since other features, including porosity, also play an important role.

Microstructural and machinability aspects of electron beam melted Ti–6Al–4V with different building orientations

Microstructural and machinability aspects of electron beam melted Ti–6Al–4V with different building orientations

Mechanical surface treatment of EBM Ti6Al4V components: Effects of the resulting surface layer state on fatigue mechanisms and service life

Although additive manufacturing of the titanium alloy Ti6Al4V has been an established process for years, problems still exist with regard to the design of components subjected to cyclic loads. In this context, process-related defects play an important role, such as surface roughness and pores. Typically, AM Ti6Al4V components are machined and subjected to a hot isostatic pressure (HIP) treatment, which, however, strongly limits the cost advantage. Mechanical surface treatment processes can be used locally where high loads are expected and thus make a cost-effective contribution to the service life of additively manufactured components. In the present work, the influence of the mechanical surface treatment process of deep rolling (DR) on the surface state of electron beam melted (EBM) and EBM+HIP components is investigated. In addition, rotating bending tests are presented in SN curves and these are evaluated in the context of the surface layer condition and the failure mechanisms. The fatigue strength could be improved from RAB=157MPa in the as built state (AB) to RAB+DR=251MPa in the deep rolled state (AB+DR). Through the deep rolling treatment, the surface could be strengthened to the extent that the location of the failure shifts into the component volume. The comparison of the fracture surfaces analysis of deep rolled samples shows that pores below the surface are the cause of failure, in a depth that is not influenced by the mechanical surface treatment. In an additional series of tests, the fatigue strength of the HIP and deep-rolled (HIP+DR) state was determined. Since HIP EBM specimens are nearly pore-free, the failure mechanism switches back to the strengthened surface, resulting in an increased fatigue limit of RHIP+DR=413MPa.

The effect of heat treatment on microstructure, microhardness, and pitting corrosion of Ti6Al4V produced by electron beam melting additive manufacturing process

There has been limited studies on corrosion behaviour of post-processed electron beam melted (EBM) Ti6Al4V, given that the factors affecting corrosion resistance of AM Ti6Al4V remain unclear. This paper proposes using heat treatment method to improve the pitting corrosion resistance of EBM Ti6Al4V. Different treatment profiles alter the microstructure of EBM Ti6Al4V. A clear trend is observed between microhardness and α lath width. As-printed EBM Ti6Al4V exhibits an inferior pitting potential, while heat treatment provided a significant improvement in the corrosion resistance. This study finds that the β phase fraction is a better indicator than the α lath width for pitting corrosion resistance. Solution air-cooled and ageing heat-treated EBM Ti6Al4V exhibits good mechanical and corrosion properties, and even performs better than commercial cast Ti6Al4V.

Mechanical behavior of electrochemically hydrogenated electron beam melting (EBM) and wrought Ti–6Al–4V using small punch test

The influence of electrochemical charging of hydrogen at j = −5 mA/cm2 for 6, 12, 48 and 96 h on the structural and the mechanical behavior of wrought and electron beam melting (EBM) Ti–6Al–4V alloys containing 6 wt% β and similar impurities level was investigated. The length of the α/β interphase boundaries in the EBM alloy was larger by 34% compared to that in the wrought alloy. The small punch test (SPT) technique was used to characterize the mechanical behavior of the non-hydrogenated and hydrogenated specimens. It was found that the maximum load and the displacement at maximum load of the wrought alloy remained nearly stable after 6 h of charging, showing a maximum decrease of ∼32% and 11%, respectively. Similarly, hydrogenation of the EBM alloy resulted in a gradual degradation in mechanical properties with charging time, up to ∼81% and 86% in pop-in load and displacement at the “pop-in” load, respectively. The mode of fracture of the wrought alloy changed from ductile to semi-brittle with mud-cracking in all hydrogenated specimens. In contrast, the mode of fracture of the EBM alloy changed from a mixed mode ductile-brittle fracture to brittle fracture with star-like morphology. The degraded mechanical properties of the EBM alloy are attributed to its α/β lamellar microstructure which acted as a short-circuit path and enhanced hydrogen diffusion into the bulk as well as δa and δb hydride formation on the surface. In contrast, a surface layer with higher concentration of δa and δb hydrides in the wrought alloy served as a barrier to hydrogen uptake into the bulk and increased the alloy resistivity to hydrogen embrittlement (HE). This study shows that EBM Ti–6Al–4V alloy is more susceptible to mechanical degradation due to HE than wrought Ti–6Al–4V alloy.

Indentation strain rate sensitivity of laser-powder bed fused and electron beam melted Ti–6Al–4V

Indentation hardness (Hin) and strain rate sensitivity (SRS) of additive manufactured Ti–6Al–4V (Ti64) alloys fabricated through both laser powder bed fusion (L-PBF) and electron beam melting (EBM) are investigated. One major difference between these processes is the higher cooling rates associated with the L-PBF. The indentation tests were performed at loading rates of 1, 10, and 50 mN/s. The L-PBF alloy showed a higher hardness value (3.45 GPa at 1 mN/s, 3.57 GPa at 10 mN/s, and 3.66 GPa at 50 mN/s) being correlated with its finer grain structure as compared with the EBM counter material (3.12 GPa at 1 mN/s, 3.37 GPa at 10 mN/s, and 3.55 GPa at 50 mN/s). The indentation SRS was explained based on the print orientation and dislocation density. EBM-Ti64 shows a higher SRS (0.0335 compared with 0.0147 in the L-PBF alloy) due to the rotation of α-prismatic planes along the building direction.

Insights into the wear behaviour of electron beam melted Ti–6Al–4V alloy in the as-built and the heat-treated conditions

This paper assesses the dry sliding wear behaviour of Ti–6Al–4V alloy components produced by electron beam melting (EBM). The effect of different post-processing heat treatments on the wear behaviour of EBM parts was studied by comparing the heat-treated samples with the as-built sample. The results obtained from wear tests showed that the samples subjected to water-quenching after heat treatment had the lowest wear loss. The furnace-cooled samples after heat treatment exhibited the maximum wear loss; this was due to the presence of coarser α-phase in prior β grains in furnace cooled specimens when compared to the as-built sample; producing the lowest microhardness amongst all the heat-treated samples. Most of the samples showed material loss due to abrasive mechanism, whilst the sample that was furnace-cooled after heat treatment above the β transus temperature showed material loss due to adhesive wear and delamination.

On the influence of heat treatment in suppressing the layer orientation effect in finishing of electron beam melted Ti6Al4V

The main issue in the finishing of the electron beam melted (EBM) parts is their directional properties that lead to different surface roughness values for different part orientations for the same machining parameters. In this study, EBM Ti6Al4V parts were subjected to heat treatment to suppress the effect of the part/layers’ orientations during the finish milling process. Three different heat treatments were employed to the as-fabricated EBM Ti6Al4V parts. The results of the heat treatments showed that the EBM Ti6Al4V parts heated at 600°C for 3 h, and then air-cooled exhibited the same fine lamellar microstructure and microhardness as that of the as-fabricated EBM parts. The as-fabricated and heat-treated EBM Ti6Al4V parts were subjected to the milling operation by considering the three possible parts/layers orientation with respect to the tool feed directions. The milling results showed that the as-fabricated parts showed up to a 27% difference in surface roughness for different part orientations. In contrast, the heat-treated parts (600°C@3h) showed uniform surface roughness for the three part orientations with a variation of 8%. Similarly, considerable differences were observed in the machined surface microhardness of the as-fabricated EBM parts (18%) as compared to the heat-treated parts) 2%). The heat-treated parts showed more uniform and superior surface morphology across all the part orientations. This is because of the more uniform microstructure, less porosity, higher consolidation of the EBM layers, and elimination of the columnar grains in the case of the heat-treated parts as compared to the as-fabricated EBM parts.

Thermal decomposition of titanium hydrides in electrochemically hydrogenated electron beam melting (EBM) and wrought Ti–6Al–4V alloys using in situ high-temperature X-Ray diffraction

Thermal decomposition of titanium hydrides in electrochemically hydrogenated electron beam melting (EBM) and wrought Ti–6Al–4V alloys containing 6 wt% β is compared. Differential scanning calorimetry (DSC) is used to identify phase transitions. High-temperature X-ray diffraction (HTXRD) is used to identify phases and determine their contents and crystallographic parameters. Both alloys are found to contain αH (hcp) and βH (bcc) solid solutions, as well as δa (fcc) and δb (fcc) hydrides after hydrogenation. δa is found to decompose between room temperature and 350 °C to αH (in both alloys) plus either βH and δb (wrought alloy) or δb only (EBM alloy). δb fully decomposes at either 450 °C (wrought alloy) or 600 °C (EBM alloy) to αH plus H2 desorption (which starts at 300 and 350 °C in the wrought and EBM alloys, respectively). In the case of the wrought alloy, βH is also formed in this decomposition reaction due to faster diffusion of hydrogen. The non-continuous, finer needle-like morphology of the β-phase in the as-printed EBM alloy combined with its smaller lattice constants seem to inhibit hydrogen diffusion into the bulk alloy through the β-phase, thus triggering δa dissociation into δb (rather than to βH+δb) and δb decomposition into αH (rather than to αH + βH). Hydrogen incorporation in the αH phase results in its expansion in the c direction in both alloys. HTXRD allows to conclude that both δa and δb hydrides decompose up to 600 °C. Hydrogen peaks measured at higher temperatures are due to hydrogen desorption from the hydride that is decomposed from the sample's bulk and/or hydrogen desorption from βH and/or αH during heating. These findings indicate that the EBM Ti–6Al–4V alloy might be more prone to hydrogen damage at elevated temperatures than its wrought counterpart when both have a similar β-phase content.

Defect analysis and fatigue strength prediction of as-built Ti6Al4V parts, produced using electron beam melting (EBM) AM technology

Ti6Al4V alloy has been highly employed in numerous applications of aerospace, medical, and chemical industries because of having excellent corrosion resistance, fracture toughness and high specific strength. Various studies on fatigue have shown that under cyclic loading fatigue cracks preferentially initiate and propagate from both the external surface defect or internal defect depending on severity and size of the defect. Although, electron beam melting (EBM) provides excellent prospect to the additive manufacturing (AM) technology in terms of energy, cost, and time efficiency, it produces rougher surfaces as compared to the other AM processes. For this reason, fatigue performances of as-built EBM parts could be poor. In the current study, the surface roughness profile on different sides of the as-built EBM Ti6Al4V parts with different orientations has been analyzed to determine anomaly of surface roughness parameters resulting from sample orientations. The average surface roughness parameter (Ra) was observed to fluctuate from 6 µm to 27 µm among different sides and build orientations of the samples. Furthermore, internal porosity of these parts was analyzed using X-ray computed tomography (CT) and found to vary between 0.05% to 0.17% in different build orientations. Based on this defect analysis, the fatigue strength of these parts has been calculated nondestructively using a quantitative method, Murakami's square root of area parameter model and Basquin's model. Besides, the severity of internal and external defects on fatigue strength have been evaluated and the external surface defects were rendered to be more critical as potential crack initiation sites.

Experimental Investigation on the Mechanical Properties of Thin Beams Built by Electron Beam Melting Technology and Ti6Al4V Material

Electron beam melting (EBM) technology has been popularly used to fabricate flexible devices that performance is directly determined by the elastic deformation of thin beams/flexures. This paper presents the experimental investigation on the effective thickness which determines the mechanical properties of beam-based flexures built by EBM method and Ti6Al4V material. The findings show that the effective thickness of EBM-printed beams is different from the designed value regarding to the building direction. A coefficient factor is proposed to compensate this difference. The experimental results suggest that with EBM-printed flexures having large thickness of ≥ 0.7 mm, the coefficient factors become consistent.

Hydrogen embrittlement of electron beam melted Ti–6Al–4V

Environmental-assisted degradation in hydrogen environment, also known as hydrogen embrittlement (HE), has a very important role on physical and mechanical properties of structural materials in the presence of hydrogen. In this research the interactions of high and low fugacity hydrogen with electron beam melted (EBM) Ti–6Al–4V alloy were studied and compared to conventionally manufactured alloy. Parts produced by additive manufacturing (AM) experience relatively high cooling rates and possess unique microstructure features, which might alter the intrinsic properties of the material. Our research describes in details the impact of hydrogen on EBM Ti–6Al–4V alloy by means of hydrogen trapping. The interactions of hydrogen with the material were studied by thermal desorption spectroscopy (TDS), X-ray diffraction, and microstructural observations. Our results suggest that EBM Ti–6Al–4V alloy with relatively fine microstructure has a better resistance for hydrogen induced damage in comparison to a wrought alloy.