15th World Conference on Titanium Chapter 1: Additive Manufacturing &amp; Repair Technology
Creators: name: IOM3
Publisher: name: F1000 Research Limited
Year: 2024

description: Titanium powders used in additives manufacturing equipment are generally free flowing; yet flow problems such as ratholing in storage vessels and preferential flow can occur, especially if the powder is allowed to remain at rest.  This paper will present the fundamental flow properties of titanium powders and discuss design methods for designing reliable handling equipment.<br>
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This study investigates the complex Ti-Al-V-Cu alloy system using samples produced by in-situ alloying during the additive manufacturing process laser powder bed fusion. A composition-modulated alloy was produced by processing a mixture of Ti-6Al-4V and Cu powders, which exhibited various microstructures. High-resolution STEM investigations confirmed the occurrence of spinodal decomposition with β → β1 + β2 → β1 + Ti2Cu. β1 + β2 were arranged in a tweed-like microstructure with an average composition of Ti69.9Al4.6V2.5Cu23. β1 and Ti2Cu were present as separate phases next to each other.<br>
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In recent years, additive manufacturing (AM) has emerged as a topic of considerable interest due to its ability to fabricate components that possess distinct advantages over traditional manufacturing methods, such as cost efficiency and design adaptability. However, a critical limitation of AM for titanium alloys is the presence of microstructural heterogeneity, primarily resulting from epitaxial grain growth and martensitic transformations, which are induced by the directional and rapid cooling processes. The present investigation endeavors to manipulate the microstructure of additively manufactured titanium alloys by integrating alloying elements and applying heat treatment procedures. Direct Energy Deposited (DED) cost-effective titanium alloys facilitate significant grain refinement, characterized by fine equiaxed grains, discontinuous grain boundary α, and ultrafine α' martensite. The subsequent heat treatment promotes the formation of near-equiaxed α, and fully α' phases. The tensile properties of both the as-fabricated and heat-treated alloys exhibit remarkable improvements when compared with conventionally processed titanium alloys. As a result, the proposed methodology unveils new opportunities and potentialities for the fabrication of titanium alloy systems with enhanced manufacturing efficiency, thereby overcoming limitations inherent in alternative manufacturing processes.<br>
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The paper will present a unique martensite-decomposed α+β microstructure in ultra-hard Ti-6Al-2Sn-4Zr-6Mo alloy produced by laser powder bed fusion. Specifically, nano-sized α and β laths appear as the alternant structure and two variants of β plates present inside the α laths. This unique microstructure can be achieved by single-step ageing. And it provides ultra-high hardness and excellent wear resistance.<br>
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Surface defects pose a significant challenge for utilizing electron beam powder bed fusion (PBF-EB) as-built Ti-6Al-4V in aerospace applications for fatigue critical applications. Several parameters determine the surface properties of the PBF-EB built Ti-6Al-4V, particularly the melting strategy adapted for the contour section. This study evaluated different contour melting strategies (number of contours, order of contours, and extra contour overlapping hatch contour interface) and their influence on the surface roughness using surface imaging tools. In addition, the effect of these contour melting strategies on the sub-surface microstructure has been investigated. The build has been designed to generate variations in surface and sub-surface properties by building samples with different contour melting strategies, and post-heat cycle variations. Furthermore, the effect of surface roughness, resulting from different melting strategies, on fatigue life has been studied through four-point bend fatigue testing. Post-test fractography on the failed specimens revealed that cracks initiated from the surface valleys. Results from this study indicate that the contour overlapping hatch theme has low valley depth on the as-built surface enabling higher fatigue performance.<br>
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Titanium alloys, exhibiting excellent specific strength, have been widely used in the additive manufacturing of aerospace components. While large material property data sets for Ti-6Al-4V (ATI 6-4 TM) have been useful in the adoption of this alloy for additive manufacturing, the manufacturability of complex components in laser powder bed fusion applications has proven difficult. Component suitability is greatly limited by the residual stresses that are inherent to additive manufacturing of ATI 6-4 TM. Additionally, the problem of extensive development of the additive manufacturing processes is compounded by the high scrap rates during development. Other β-rich Ti alloys such ATI Titan 23TM exhibit superior printability, lower residual stress, and higher mechanical properties than ATI 6-4 TM in laser powder bed fusion applications. This combination of properties in ATI Titan 23TM can lead to improvements in the manufacturing process, reduce printing costs, and enable wider application of titanium additive manufacturing for complex components. The ability to produce ATI Titan 23TM with excellent mechanical properties by laser powder bed fusion additive manufacturing has been evaluated, and as such, the microstructure, mechanical properties, and residual stress of the printed material are presented herein.<br>
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Superelastic TiNbTaZrO β-Ti alloys possess interesting properties, combining low modulus, high strength, and high recoverable strains, making them an ideal choice for medical implants and shock absorption applications. In this study, the processability of the Ti-34Nb13Ta-5Zr-0.3O alloy using Laser Powder Bed Fusion (L-PBF) was studied, to assess the impact of the process on the microstructural and mechanical properties development. Pre-alloyed powders were used to study the influence of L-PBF parameters on the build density and texture. Following process optimisation to achieve maximum density and the least possible anisotropy, the as-fabricated optimum condition was found to possess a low modulus of 56.5 GPa, a high tensile strength of 756 MPa, and a high recoverable tensile strain of 1.3%. Nonetheless, the alloy did not demonstrate the typical β↔α’’ deformation induced transformation, possibly due to the high Ocontent. Heat treatment optimisation (via controlling the solution treatment and ageing temperatures) was performed to further improve the strength through the precipitation of  resulting in an increase in both the strength and elastic modulus, while maintaining the recoverable strain levels. Microscopic investigations were also performed to probe the mechanism for superelasticity, which indicated that the deformation is slip-dominated, with no evidence of either twinning or a deformation-induced phase transformation, echoing similar observations of O-containing β-Ti alloys in the literature. A structure-property model has been developed to rationalise the mechanical behaviour of the different conditions. The study will also highlight other potential applications that may benefit of the combination of the exotic mechanical behaviour and the use of additive manufacturing.<br>
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Additively manufactured structures are inherently anisotropic due to the layered nature of the process that result in the formation of interlayer defects and a preferred crystallographic orientation due to the epitaxial growth along the build direction. Lattice and metamaterial structures suffer from a further anisotropy associated with the arrangement of the struts or features in 3D. In this work, the utility of microstructural control in Laser Powder Bed Fusion (L-PBF) of Ti-6Al-4V Triply Periodic Minimal Surface (TPMS) lattice structures was assessed to control the levels of anisotropy. Microstructural control was achieved through controlling the heat input parameters in LPBF (laser power and scan speed), as well as the scanning strategies in an attempt to control the heat distribution. The combined influence of the heat input and heat distribution parameters on the grain structure and α lath thickness was studied. Estimates of the cooling rates were obtained through correlating the final microstructure with the literature, in particular in relation to the columnar-to-equiaxed transition and the α lath size development with hardness using Jominy end quench data. Compression testing was finally used to assess the degree of anisotropy and to deconvolute the effect of the defects and the microstructure on the development of anisotropy. The study will highlight the potential for achieving near isotropic properties in additively manufactured bulk or lattice structures through microstructural control and design improvements.<br>
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Additive manufacturing (AM), and more specifically powder bed fusion (PBF), offers an unrivalled opportunity to improve part complexity, and thus functionality and performance, through its unique design freedoms. This creates strong interest in PBF technologies in industries where high performance and lightweight are of high importance (e.g., aerospace, motorsport). Ti-6Al-4V offers a good combination of mechanical properties to these industries and as such has been heavily investigated. Thermal post-processing of (laser and electron beam) PBF components is an often-necessary manufacturing step in achieving the desired material properties. Although the understanding of PBF manufactured Ti-6Al-4V is well-developed, this knowledge typically relates to standard heat treatment routes (ASTM F2924-14 &amp; AMS 2801B). Due to the wealth of knowledge in utilising Ti-6Al-4V in both wrought and cast forms there is an inherent adoption of the thermal post processing routes associated with these processes to AM. However, the starting microstructure of PBF manufactured Ti-6Al-4V differs from that of wrought and cast product and thus this practice of adopting thermal post processes is not optimal. In conjunction with this there is a large amount of focus on the mechanical integrity of AM components due to inherent defects (such as porosity and lack of fusion) in the process. Thermal post processing operations like hot isostatic pressing (HIP) are wellknown to reduce the influence of defects in AM materials and thus are an area of interest. This has motivated the MTC to investigate the effect of altering thermal post processing in both laser and electron beam based PBF of Ti-6Al-4V to improve material properties. Performance was characterised through metallography, residual stress measurement, tensile and strain-controlled fatigue testing.<br>
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Complex structural components of Ti6Al4V are often fabricated by Powder Bed Fusion – Laser Beam (PBF – LB) technology. Although Ti6Al4V has a generally excellent combination of properties suitable for aerospace and biomedical applications, it is well-documented that its poor wear resistance limits its use. The advancement of PBF – LB technology, however, allows for an increased ability to tailor the alloy microstructure, which could reduce the shortcomings of Ti6Al4V. Nevertheless, Ti6Al4V produced by PBF – LB technology is normally comprised of columnar martensitic α’ microstructure. While this microstructure provides fair hardness (380 HV), the alloy tends to be anisotropic and incapable of work hardening. We hypothesized that by combining the addition of a β-stabilizer with tailored PBF – LB processing, we could achieve a graded alloy structure with increasingly hard surface, potentially enhancing the wear properties of the material. To achieve this aim, 4.5 wt.% 316L stainless steel alloy was added to Ti6Al4V powder and used to produce printed parts by PBF – LB. Given the β-stabilizing elements in 316L, the as-built alloy was found to predominantly consist of β phase. By varying the printing strategy, it was possible to locally affect the solidification of the alloy during the printing process. We thereby showed that the as-built microstructure can be controlled from dominantly β to α’ + β, depending on the printing strategy employed. This microstructural change was reflected in Vickers hardness values varying from about 330 HV (β) to above 500 HV (α’ + β). The results of this work are especially important to applications that require varying properties, such as the femoral head of a hip implant, where the bulk can be comprised of a tougher β-titanium and the surface could benefit from the hardness of α’ + β microstructure.<br>
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Understanding of the composite powder properties is essential in the design of high-performance metal matrix composites (MMCs) by laser powder bed fusion (L-PBF). Conventional particle or fibrous fillers are known to result in poor powder flowability and uncontrolled morphology of composite powders. In this study, we demonstrate a first example of a metal powder decorated with a twodimensional (2D) nanofiller that improves powder properties and L-PBF manufacturability. 0.5 wt% graphene oxide (GO) nanosheets were uniformly attached to the surface of Ti-6Al-4V (Ti64) powder by electrostatic self-assembly while maintaining spherical morphology and particle size. Centrifugation and recoating experiments showed that the quality of Ti64 (GO@Ti64) powder coated with GO was significantly improved and was attributed to significantly reduced powder/powder and powder/substrate adhesion. Furthermore, the thermal conductivity of the GO@Ti64 powder was found to decrease, while the laser absorption rate increased as a result of multiple local absorptions of wrinkled GO. Therefore, the optimum processing window of GO@Ti64 powder was extended and broadened, indicating improved printability of L-PBF. The builds fabricated using GO/Ti64 powder showed an ultrafine α' martensitic structure with a lath width of 91 nm. This fine structure was attributed to the high-temperature nonequilibrium condition of L-PBF, in which 2D GO nanosheets completely dissolved into the Ti64 matrix, resulting in a carbon-supersaturated, precipitation-free Ti64 alloy. Due to grain refinement and solid solution strengthening, the yield strength of the GO/Ti64 composite increased from 1375 MPa to 1793 MPa, and the compressive strength increased from 1796 MPa to 2032 MPa. These results contribute to a better understanding of the properties of composite powders and will be an important step toward the fabrication of high-performance MMCs using L-PBF.<br>
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Additive Manufacturing is a key technology in the Fourth Industrial Revolution. Since its conceptualisation and initial development, this technology has been based on processes performed in the digital environment, such as computer-aided design and slicing of threedimensional solid models into two-dimensional layers. For acceptance in industry, components produced through additive manufacturing require qualification. The conventional route to qualification entails expensive and time-consuming testing and experimentation, which could significantly delay market entry. However, dramatic improvement in computing power has made modelling and simulation across the additive manufacturing process chain, to speed up the qualification process, an exciting reality. In this paper, examples of how digital techniques are being applied in South Africa towards rapid qualification of laser powder bed fusion (LPBF) Ti6Al4V(ELI) processes and parts are discussed. In the design for additive manufacturing stage, the roles of topology optimisation, finite element analysis and optimisation of part orientation on the LPBF build platform, are discussed. The benefits derived from combining finite element-based simulation of the fatigue performance of a scaled-down aircraft nose wheel fork with physical performance testing are discussed. An assessment of the potential to accelerate the determination of LPBF process parameter sets through modelling of melt pool characteristics is presented. Successes in predicting the effect of different heat treatment cycles on the microstructure and high strain rate properties of Ti6Al4V(ELI) parts through constitutive numerical modelling are shared. It is concluded that the drive towards complementing the physical LPBF process chain with more extensive operation in the digital environment, promises to be highly beneficial for the broader acceptance of additive manufacturing in industry.<br>
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Additive manufacturing (or 3D-printing) has evolved rapidly to become a disruptive technology capable of revolutionizing the way that complex parts from a variety of industrial (such as aerospace, automotive, biomedical, marine, and offshore) sectors are produced. It provides design freedom and environmental/ecological advantages. However, additive manufacturing processes induce a high orientation in the microstructure of the printed parts due to the strong thermal gradients developed during the process stemming from the highly concentrated heat source that is applied in one direction to melt the metal powders layer-by-layer. The resulting microstructural anisotropy and heterogeneity have a direct effect on the mechanical properties, which in turn influence the load-bearing structural applications. It is thus necessary to understand the effects of anisotropic and heterogeneous microstructures and potential defects on the deformation behavior of additivelymanufactured alloys. This paper presents our recent work on the tensile properties of a Ti-6Al-4V alloy additivelymanufactured via powder bed fusion using an electron beam (PBF-EB) in relation to the microstructure and sample orientation. The anisotropy in the tensile properties was mainly associated with the orientation of prior-beta grain boundaries toward the building direction. With increasing temperature, the strength decreased moderately below 500C and more rapidly from 500C to 800C, while the ductility increased in the samples oriented in all three directions.<br>
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A study of creep behaviour of additively manufactured (AM) Ti64 alloy through Small Punch Creep Testing (SPCT) is presented. The AM technique used here is Laser-Powder Bed Fusion (L-PBF) and led to a fully martensitic microstructure. SPCT are performed on two AM Ti64 batches. Comparisons between SPCT and uniaxial tests performed on additively manufactured material allow us to validate the used SPCT models. The validity of the used model has been confirmed and deformation mechanisms appear to be the same for both batches. Thus, SPCT method seems to be quite reliable for studying the creep behaviour of AM Ti64.<br>
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Additive manufacturing offers potential for engineering applications as a means to produce functional parts in low volumes through the unique capability to make components directly from CAD files. What is absent from the technology, however, is a comprehensive understanding of how these components are determined to be fit for purpose. Metal castings, for example, are considered to be “materials with uncertainties” and for aerospace applications have “casting factors” specified for their utilization in demanding applications. This requirement typically relates to recommended procedures for their non-destructive testing which are specified on engineering drawings (see for example standard AMS 2175). Following from the precedent of aerospace castings and definitions of classes, it is possible to consider, additive manufacturing design factors that may be applicable for various classes or types of product. In the current work, comparisons are made between equivalent materials in either wrought or hot isostatically pressed (HIP) additively manufactured Ti-6Al-4V (Ti64) forms, manufactured from a commercial laser powder bed (SLM) system. It will be shown that, similar to castings, additive manufactured titanium materials also require definition as ‘materials with uncertainties” due to inherent manufacturing defects. It is therefore recommended that designs should be modified to account for the presence of such discontinuities and an “AM factor” should preferably be applied in demanding or structural applications. This suggestion impacts the parameters for topological optimisation, but its incorporation inevitably improves reliability and usability of the manufactured products. What may be required for manufacture without this AM factor is considered in context of the outcomes of part manufacture and likelihood of failure under given loading conditions.<br>
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Additive manufacturing (AM) is a process that involves directional heat flux and repeated melting and cooling, resulting in microstructures that exhibit directional heterogeneity along the building direction. These process-induced microstructural, such as layer band, columnar grain boundary, and melt pool boundary, can contribute to the anisotropic mechanical properties of AM parts. In this study, we investigate the role of these process-induced microstructure on anisotropic compressive behaviour of Ti-6Al-4V ELI alloy fabricated by laser-based powder bed fusion (L-PBF). The engineering stress-strain results show that the as-built sample exhibits higher strain hardening rate and fracture strain, and lower yield strength when compressed along the transverse direction (TD) compared to the building direction (BD). The dislocation density estimated by X-ray diffraction (XRD) evolves more sharply in the TD sample than in the BD sample as the strain increases. The kernel average misorientation (KAM) analysis reveals that local dislocation distribution is localized at β grain boundaries in the TD sample, while it is uniform in the BD sample. Therefore, the anisotropic properties of the material are attributed to the directional response of the process-induced microstructure, and their interaction with respect to the loading direction.<br>
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Additive manufacturing offers the ability to produce multi-material parts, generally through the use of multiple feedstock materials. Typically, the composition differences are designed as a gradient and restricted to the build direction. The high temperatures involved with additive manufacturing of alloys can lead towards the preferential vaporization of select elements with higher vapour pressures than the other alloying elements (e.g., chromium in stainless steels, magnesium in aluminium alloys, aluminium in titanium alloys), especially when processed under vacuum. Samples of Ti-6Al-4V from a single feedstock material were produced through electron beam melting wherein the process parameters were spatially controlled to tailor preferential vaporization and thus chemistry in specific locations within a single layer. Subsequent microstructural evolution was controlled through heat treatments. The resulting chemical and microstructural variations were analysed with optical and electron microscopy, energy dispersive spectroscopy, cyclic hardness and microhardness measurements, and spatially resolved acoustic spectroscopy, demonstrating that the chemistry control through the modification of processing parameters was significant enough to enable the tailoring of microstructural and mechanical properties, both elastic and plastic.<br>
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Titanium alloy is widely used in aerospace because of its high specific strength, good corrosion resistance and high thermal strength. However, the Ti6Al4V alloy is prone to react with oxygen, nitrogen, hydrogen and other elements in the air at high temperature, which affects its mechanical properties. The low temperature solid deposition characteristics of cold spraying technology are conducive to the remanufacturing/ repair of oxidation sensitive titanium alloy in the atmosphere. For cold-spraying technology, the titanium alloys with high hardness are difficult to deposit, and the porosity of the deposits is high. Ti6Al4V alloy deposits prepared by cold spray (CS) usually show porous microstructure due to insufficient plastic deformation of Ti6Al4V alloy particles. The interfacial bonding and the microstructural evolution mechanisms of Ti6Al4V particles during cold spray are still unclear. In this study, the Ti6Al4V deposits were prepared by cold spray with nitrogen as process gas at a temperature of 800°C and a pressure of 4 MPa. The micron CT results show that the porosity is 8.25%. The deformation ultrafine grain zone and un-deformation martensitic laths zone at the bonding interface between Ti6Al4V particles were observed. The EBSD results show that the grain orientation signal was only detected at the edge of the particles deformation area and the 23% of grain sizes detected by EBSD are less than 500 nm, which indicates that the nanocrystallization induced by dynamic recrystallization occurs at the inter-particle boundaries between cold-sprayed Ti6Al4V particles. The TEM and SADPs demonstrate a gradient structure consisting of equiaxed nanograins zone, crushing dendrites transition zone and original martensitic laths zone from the interface to the core of particles.<br>
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The fracture surfaces of six fatigue tested specimens were examined using SEM in order to determine the predictability of the variation in fatigue cycles based on the morphology, size or location of pores. The deviation from the average fatigue cycles, for a given applied fatigue stress and material condition, were computed for each examined specimen. Several samples that exhibited notably inferior and superior performance compared to the average were analyzed, along with a few that demonstrated typical performance.<br>
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In recent years, the proportion of titanium alloys for aerospace applications has increased. The ratio of raw material required to the finished part is on average a ratio of 6:1. This means that in many cases more than 80% of the material is wasted. For this reason, the aviation industry's trend is moving towards the use of additive manufacturing technologies. Laser or electron beam-based processes for the standard alloy Ti6Al4V are established for components up to 500 mm in development or introduction. Nevertheless, such components are costly, because specific powders with high demands and thus high costs have to be used, low deposition rates are achieved, and therefore production times are slow. Additive manufacturing processes for components with a high buy-to-fly ratio and dimensions larger than 500 mm have not yet been widely used on the market. This study aims to produce by Plasma Metal Deposition (PMD) process the deposition of Ti6Al4V components using a wire-fed process that meets aerospace requirements and allows the production of components larger than 500 mm with high deposition rates. A key challenge in the production of titanium materials is the sensitivity of titanium to oxygen. The oxygen content in the built-up component has a serious influence on the properties. This affects not only the microstructure but also the mechanical properties (tensile strength, elongation at break, hardness, etc.) and even the machinability of the material. Within the study, an appropriate inert gas cell was developed that allows the monitoring of the oxygen content during the manufacturing process. In addition, the influence of the starting material (wire) was also investigated and the process for the high-performance Ti6Al4V alloy was developed.<br>
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The design freedom of Additive manufacturing (AM) allows production of complex parts with light weight and additional functionally. As AM does not require specific tools or moulds, it is an attractive option for small lot sizes, and also for mass customization, i.e. the production of customer specific variants. Another benefit comes from the ability to produce integral designs, reducing the number of parts of an assembly and thus the respective costs and efforts. Due to these advantages, today’s applications include the production of patientspecific implants, as well as aerospace and sports car parts. Titanium is often used as the material of choice for these applications due to its biocompatibility and high specific strength, and laser powder bed fusion is today typically the favoured AM process due to the resulting superior part properties. The trends towards AM of integral parts of larger dimensions, as well as for higher lot sizes in the mass customization market, both require a lean, more robust and highly productive AM production. This article investigates different approaches for optimization of the laser powder bed fusion printing process itself as well as in combination with other steps along the process chain. It is shown, that the productivity may be increased significantly by increasing the layer size to 60 µm and 90 µm. Furthermore, the use of beam shaping to guide the energy input into the material in a preferable way can lead to a significant increase in the printing speed of around 40 %. Additional improvements to productivity as well as microstructure and resulting properties may be realized by looking at the printing process in combination with subsequent heat treatment steps, such as hot isostatic pressing (HIP). A productivity increase of around 25 % is reported for combining high-speed printing with HIP in order to close remnant porosities.<br>
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Wire and Arc Additive Manufacturing (WAAM) has been successfully applied to build plain geometries such as multi-layer walls. In this work, more complex parts in the form of T-joints have been investigated on the correlation between process parameters, microstructure and mechanical properties as the intrinsic heat budget totally differs from that of plain geometries. The built samples are analysed in terms of their microstructure and mechanical properties using Scanning Electron Microscopy, hardness measurements and compression tests. Effects of the intrinsic heat-treatment on the microstructure and mechanical properties were evaluated and a comparison between the simple and complex geometries was drawn. An additional stress-relief annealing further improved the mechanical properties. The results of the geometrically complex parts reveal a great potential of WAAM for future manufacturing of Ti‑6Al‑4V parts.<br>
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In this study, we employed laser deposition manufacturing to manufacture Ti-35421 (Ti-3Al-5Mo-4Cr-2Zr-1Fe wt.%) alloy, containing key elements that can refine grains, with the aim of transforming columnar to equiaxed grains. The results demonstrated that the microstructure was predominantly composed of equiaxed grains, including grain boundary α, lath α, and acicular α', which accounted for its high tensile strength (1446±46 MPa) but low elongation after fracture (1.7±0.3 %). Upon undergoing solution and aging treatment, the grain boundary α became coarser, and the matrix β transformed into a large number of fine lamellar α, resulting in decreased strength with increasing aging temperature and gradual enhancement of plasticity. The optimal heat treatment process for the LDM Ti-35421 alloy was found to be 780℃/2h/AC + 575℃/8h/AC, exhibiting comprehensive mechanical properties such as tensile strength of 1196±7 MPa, yield strength of 1162±8 MPa, elongation of 6.8±1.6 %, and reduction of area of 16.7±1.9 %.<br>
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Laser powder bed fusion (PBF-LB) is one of the most widespread additive manufacturing (AM) methods, spanning multiple industrial sectors such as medical, automotive and more recently aerospace. Current limitations to its large-scale adoption include low build rates, machining often required and consolidation via hot isostatic pressing (HIP). Productivity enhancement of PBF-LB has been investigated extensively and among the strategies adopted, that of combining high-speed parameters with HIP to achieve full density, has proven to be viable. This study puts forward a new approach for Ti-6Al-4V material, investigating if employing a wide particle size distribution (PSD) of the powder, with a range between 15 to 90μm, with a high layer thickness achieves comparable levels of bulk density, whilst improving the sustainability of the overall process and decreasing build times. If packing density can be improved by ensuring a more varied spread of particle sizes, the thermal conductivity of the powder bed increases. Combination of small and large diameter particles would result in a reduction of the number of interparticle cavities upon powder spreading, therefore enhancing the contact and the efficiency of melting between neighbouring particles, upon laser heating. An EOS M290 machine was used to establish a processing window for the extended PSD and increased layer thickness. Laser power, scanning velocity and hatch distance were varied to identify and exclude parameter values that render extremes such as lack of fusion or keyholing defects. Single, multiple tracks and cubes were produced as part of a study that aims to characterise the material’s response in terms of microstructure, defect density and hardness. It was possible to establish correspondence between tracks and cubes behaviour and isolate a design region that yielded minimal porosity.<br>
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Research on metastable β Ti alloys has recently shown the possibility to achieve outstanding ductility and work hardening behaviours by engineering the materials’ unique strain transformable attributes and responses to heat treatment. As quenching is central to the development of the metastable phase in these alloys, it is of interest to understand how these materials could be designed and developed for use in laser additive manufacturing where feedstocks experience numerous rapid thermal cycles. In this context, a material design framework is proposed to identify printable metastable Ti alloys for laser powder bed fusion (L-PBF). The ternary Ti-Cr-Sn system is investigated for both its affordable character and interest in the biomedical field. The design framework combines criteria based on wellestablished empirical parameters to compare stability of β phase and resistance to cracking. The design work is supported by series of microstructural investigations on arc-melted buttons of selected compositions. Using such methodology, Ti-7.5Cr-4.5Sn has been identified as strain transformable alloy of good printability, and this alloy is then characterised in depth. Printability is demonstrated via printing experiments using a variety of processing conditions, while the deformation character is studied using compressive testing. It was found that the identified Ti-7.5Cr-4.5Sn alloy can achieve a high compressive strain exceeding 70% thanks to deformation mechanisms that combine dislocation slip and {3 3 2} twinning. This research paves the way towards the identification of novel Ti alloys for L-PBF beyond those used in the aerospace sector, opening the way for a broader field of applications.<br>
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Metal additive manufactured parts have strict metallurgical requirements for aerospace. Quality control during the laser powder bed fusion process involves monitoring the processing route from feedstock, the building process and postprocessing. Currently quality assurance procedures such as in-situ monitoring, non-destructive testing and metallurgical witness specimens are implemented. These methods are expensive and time consuming as they require the manufacturer to either send the parts to accredited laboratories for testing or investing in testing equipment and training. This work shows the first developments which investigates the possibility of designing an alternative metallurgical witness specimen that is inexpensive and quick to test. It is required that the specimen has a clear relationship to the actual metallurgical properties such as tensile properties which is often used to verify the material. This work shows the specimen design which is based on a unique custom bending specimen and rig. The specimen design and testing procedure is reported. Ti6Al4V specimens are manufactured using an EOS M290. The procedure is tested to verify thermal postprocessing in as-built, stress relieved and hightemperature annealed conditions. Comparison between specimen deformation and ASTM E8/E8M tensile properties for the abovementioned conditions are reported. This method can be specifically developed and tailored to be approved by the quality assurance organization.<br>
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The additive manufacturing of titanium alloys has been getting a lot of attention in the past two decades since this technic is a perfect manufacturing solution for small quantity and complex structure. However, a lot of researchers found that titanium alloys manufactured via additive manufacturing are suffering from coarse columnar grains and significant anisotropic mechanical property due to the distinct columnar grains along the building direction. Therefore, to promote the additive manufacturing on a wider application, the columnar to equiaxed grain transition (CET) control need to be addressed first. In this study, several approaches of CET control in additive manufacturing have been discussed in detail. Meanwhile, based on the author’s previous works, a sufficient method of elements addition control was proposed. The result shows that a fully equiaxed grain morphology could be obtained after several elements addition (i.e., Fe, B etc.). The mechanical test results presents that the additive manufactured parts with equiaxed grain could obtain a higher ultimate tensile strength and a better elongation rate than forged samples. The mechanism of CET analysis indicates that the idea and controlling method could been applied on other titanium alloys as well which could promote the development of titanium additive manufacturing application in future.<br>
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This study aims to obtain understanding of the microstructural evolution (recovery and decomposition of martensite α’ during post heat treatment in the Ti-6Al-4V alloy manufactured by laser powder bed fusion (L-PBF). The microstructural evolution was tracked using synchrotron radiation facilities. In addition, interrupted heating at different temperatures was conducted to observe the microstructural evolution during heating by Scanning Electron Microscopy and Transmission Electron Microscopy (TEM). The phase fractions, the mean lattice parameters and the Full width at half maximum (FWHM) variations were determined by Rietveld refinement from XRD patterns during continuous heating from room temperature up to the single β-phase domain. The complementarity of characterization tools clearly evidenced that the decomposition of the ’martensite resulted in a noticeable modification of its mechanical state and a progressive change of its chemical composition, leading to the characteristics of the  phase. In addition, the change in the chemical composition of ’ martensite during heating leads to the formation of the  phase at the interface ’/’ and on the internal twinning by a diffusional mechanism.<br>
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Due to the interplay of fast cooling rates and part geometry, additive manufacturing (AM) of titanium alloys exhibits microstructure heterogeneities that span multiple length scales with high dependencies on process parameters and variabilities from process to process. Today, large titanium components (measured in feet) with complicated geometries can be manufactured by wire laser AM (WLAM), wire arc AM (WAAM), electron beam AM (EBAM) and hence add further heterogeneities to microstructure and the resulting properties. For example, Ti6Al4V may show basketweave microstructure with thin alpha laths (less than 1 micrometer) within large columnar prior-beta grains (many millimeters long) that span multiple layers. Not only does the morphology of the alpha and the parent beta-phase differ from one process to another, but also the crystallography of the beta-phase and the transformed alpha-phase varies. Further complications arise from the fact that for each parent beta, there are 12 possible alpha-variants during the solid-state transformation that could be affected by the cooling rates. Hence, microstructure quantification using large-area electron backscatter diffraction (EBSD) is a challenging task due to stringent sample preparation, sample size limitation (to fit within the SEM chamber), and limited data collection rates. An efficient method for microstructure quantification of titanium alloys made by AM using orientation imaging by polarized-light microscopy (PLM) is presented. Quantitative microstructure comparison of various large titanium samples made by WLAM, WAAM, EBAM will be discussed using data collected by a table-top polarized-light instrument developed by MRL (TiPolarTM). At a current effective data collection rate of about 1,000,000 pixel per second at ~ 0.6 micrometer resolution (many orders of magnitudes faster than EBSD) without the size or atmosphere limitations typical of EBSD, production implementation for quality control is a possibility. Further comparisons with laser powder bed fusion (LPBF) will also be presented. This data can be utilized to affordably evaluate the effect of processing on microstructure and the associated properties of large AM components. It also demonstrates the capabilities and robustness of the method across various AM processes.<br>
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Additive manufacturing (AM), and in particular laser powder bed fusion (PBF‑LB), is a growing technology for many critical, high‑performance industries, including aviation, space and defence. For these applications, understanding and controlling the output of a manufacturing process is critical to ensuring safety and enabling widespread adoption. Furthermore, a full understanding of the process and its outputs is necessary prior to qualifying any full‑scale production of AM components. Repeatability and stability of PBF‑LB is a known issue, in particular due to the interaction between a large number of process variables (e.g. gas flow, humidity, and build location). The Manufacturing Technology Centre (MTC) presents a study on the repeatability and stability of processing Ti‑6Al‑4V across three builds on a state‑of‑the‑art PBF‑LB system. Performance was measured using tensile testing, X‑ray computed tomography (XCT), metallographic density testing, metrology, and powder characterisation. Data was statistically assessed, allowing for correlations to be drawn between the measured properties and process variables. The study showed that reduction of area (RoA) remained statistically stable across the three builds. However, yield strength, ultimate tensile strength (UTS) and elongation differed between runs. There were also statistically significant differences between build locations, with the left side of the machine typically performing poorer than the front or centre in terms of yield strength, UTS, elongation and RoA. However, although statistically significant, any identified variations in properties are small, at ~3‑4%.<br>
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Additive manufacturing (AM) by laser powder-bed fusion (LPBF) generates very high cooling rates within a very small length-scale that has a significant impact on the resulting microstructure and the associated properties of many metallic alloys. In particular, LPBF of Ti-6Al-4V parts could have highly preferred alpha crystallographic orientations with columnar prior β phase that is often aligned with the build direction. Sometimes, A martensitic α’ phase forms that often transforms into α-laths with post processing heat treatment. Grain boundary alpha could decorate prior beta grains under slower cooling rates or adequate reheating conditions. This study leverages the TiPolarTM technique that is based on polarized light optical microscopy for orientation imaging of optically anisotropic materials (e.g. HCP titanium). The underlying c-axis orientation is calculated from generated polarized images as inclination angles relative to the sample coordinate system. TiPolar generates c-axis orientation maps (C-maps) as a spatial representation of the c-axis crystallographic orientation using two Euler angles. While yielding similar information as electron backscatter detection (EBSD) regarding c-axis orientation, it does not generate the third Euler angle. With TiPolar data collection rate characterization (~ 1 million pixels per second) with large area scans (~ 50 mm x 75 mm for TiPolar-S) at ~0.7 μm, valuable orientation information can be generated from AM titanium alloys. To explore effects of varying temperature during the LPBF process, two “inverted pyramid” parts were printed from Ti-6Al-4V on an EOS M290 machine. The geometry was intentionally designed to accumulate heat in the higher layers, except that one of the two builds was given more time between layers to cool down. Initial TiPolar scans of sections from this experiment revealed marked differences between the two samples both in terms of spatial distribution of alpha phase orientation and morphology. To explore results of varying process parameters, TiPolar has been performed on fatigue samples that were printed and tested as part of the NASA University Leadership Initiative (ULI) at CMU.<br>
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In this work, high-strength alloys Alloy1 and Alloy2 were studied and compared with high-strength alloys VT22, VT16, VST3553, as well as with titanium alloy Ti6Al4V. The investigated high-strength alloys showed a low sensitivity to β-grain size, which makes it possible to provide advantages in mechanical properties in a wide range of grain sizes (up to 4000 μm).