Key Engineering Materials Vol. 1009

Abstract: This presentation provides an overview of the recent collaboration between CSIRO and The Boeing Company focused on developing preforms of high-temperature titanium alloys. This collaboration devised a new method for manufacturing preforms, shaped intermediates and mill products directly from titanium powder. These preforms can then undergo thermomechanical processing to produce parts requiring minimal surface finishing with the desired microstructure and mechanical properties.

Abstract: Excellent properties of titanium alloys as the high strength-to-weight ratio and exceptional corrosion resistance result very attractive for the industry. However, the high manufacturing costs of conventional processing methods, as well as the material waste remain challenging making these alloys only accessible to sectors such as aerospace.Additive manufacturing (AM) becomes an attractive solution to manufacture titanium based high-added value components due to the rapid prototyping, complex geometries manufacturing and waste reduction. Specifically, Laser Beam Directed Energy Deposition based on Metal Wire (DED-LB/Mw) process is becoming a key manufacturing process, mainly due to higher deposition rate, ability to build larger structures and high material efficiency compared with other usual AM technologies, as powder bed fusion (PBF), or powder-based DED processes (DED-LB/Mp). However, there are some challenges to industrialise the AM printed titanium alloys due to their high reactivity with oxygen at high temperature and mechanical properties of the deposited material because of extremely high residual stresses (RRSS).This paper reports the work carried out to demonstrate the feasibility of coaxial DED-LB/Mw process to manufacture a semi-spherical part, with potential application in the aerospace sector as a fuel tank. The work has been focused in three key issues:The effect of the argon shielding environments has been evaluated on three different configurations (local, inert chamber, local + inert chamber) to bring a deep understanding on the influence of protective conditions on process stability, surface quality, metallurgy and microhardness.The DED-LB/Mw processing of Ti-6Al-4V alloy has been parametrized to achieve the optimum process parameters attending to deposition rate and process stability. Mechanical properties have been also assessed on samples manufactured under the acceptable atmosphere condition.The optimal manufacturing strategy, with the established process parameters and protective atmosphere conditions, has been selected to manufacture the semi-spherical part also considering the trajectory limitations imposed by the required working movement conditions within the chamber

Abstract: Additive manufacturing has significantly advanced in the last two decades and can now produce various mechanical components. However, some limitations exist, such as the size and surface finish of the part. This work reports preliminary results of joining dissimilar metals or alloys by additive manufacturing inspired by welding dissimilar joints. Single laser tracks were produced by laser surface alloying, simulating laser powder bed fusion, using homemade additive manufacturing equipment composed of a computer numerically controlled table and a ytterbium-doped fiber laser operating inside an Argon chamber. Mo, Co, and Cr powders were used to obtain additively manufactured layers on a Ti substrate, aiming to produce a gradient from the Ti to the Co-Cr-Mo alloy. The results showed that increasing the heat input increased the dilution of Ti in the Co-Cr-Mo modified layer and decreased the hardness and the formation of cracks. Obtaining an intermediate layer of Mo reduced the dilution in the upper layers. It increased hardness by 629 ± 20 HV, indicating the feasibility of manufacturing multi-material pieces made of commercially pure Ti with an intermediate layer in Mo and a Co-Cr-Mo alloy. Such pieces aim for future application in hip implants, in which, in addition to the more flexible Ti stem, a femoral head with properties close to the Co-Cr-Mo alloy for excellent wear resistance.

Abstract: The technological advancements in laser powder bed fusion (PBF-LB/ L-PBF) processing has led to the potential in utilizing larger powder bed layer thicknesses aimed at increasing the productivity. Moreover, by increasing the layer thickness, coarser powder particle size distribution (PSD) may be employed, further improving cost-effectiveness of the process. This drives the shift towards a more sustainable process chain, while reinforcing the business cases in additive manufacturing (AM). In this study, the effect of larger layer thickness (i.e., 90 µm) using recommended PSD of 15 to 45 µm, as well as feedstock powder with a coarser PSD (i.e., 45 to 90 µm) on the surface characteristics, heat treated microstructure, and mechanical properties of Ti64 components is evaluated. The results were then compared with that of 30 and 60 µm layer thicknesses, using a standard PSD of 15 to 45 µm.

Abstract: Field-assisted sintering technology also known as spark plasma sintering is starting to be recognised as a potential route for metals processing and near net shaping for a range of sectors. FAST/SPS is an effective way of rapidly consolidating powder and particulate feedstocks, including waste streams such as machining swarf into shaped billets with as-forged properties. FAST/SPS can also be used as an intermediate step prior to conventional closed die forging or hot rolling (FAST-forge and FAST-roll, respectively). The solid-state technique has also been demonstrated to be an effective way to functionally grade and diffusion bond different alloys in the same FAST billet (FAST-DB). In this paper, we summarize some of the developments at The University of Sheffield around FAST/SPS over the last few years, with examples from different particulate types for a range of different sectors.

Abstract: This work is part of a project that aims to carry out industrial research on recovery of metallic Ti6Al4V waste material through the generation of recycled powders for their incorporation into the feedstock (in granular form) of extrusion based additive manufacturing processes. Preliminary printing tests show that commercially available 3D printing extrusion-based systems are ineffective at printing granules with particle sizes below 100 μm. Thus, a novel material extrusion screw-based 3D printing extrusion head is designed and tested with granular feedstock of different particle size distributions (1-100, 1-400, 100-475 and 475-550 μm). By means of this novel system, granular feedstock with sizes between 1 and 475 μm was successfully employed for the printing of green Ti6Al4V highly complex geometries. Critical printing parameters were investigated and optimized, demonstrating the feasibility of the incorporation of recycled Ti6Al4V powders into thermoplastic feedstocks for material extrusion additive manufacturing processes.

Abstract: The main interest in Additive Manufacturing, specifically Powder Bed Fusion - Laser/Metal (PBF-L/M) technology relates to its ability to produce complex components with significant weight reduction using the minimum material required by the application. Being the aerospace sector one of the sectors where this technology has more interest and applications, particularly the Ti-6Al-4V alloy, ensuring the quality of parts thus the processed material becomes more critical since the criticality of the using powder affects directly.This study aims to analyze the effect of powder reuse on titanium alloy Ti‑6Al‑4V manufactured by PBF-L as well as the material processability when a significant processing parameters optimization is conducted by modifying the laser scan speed for the process efficiency by keeping the energy density without compromising the material performance. The chemical composition and the physical properties of specimens manufactured with virgin powder and after several build jobs are analyzed and compared to assess the influence of virgin and reused powder material on the consolidated material.Furthermore, the manuscript also provides perspectives and recommendations to enable AM users to develop a well-defined and standard powder reuse process to maintain the desired characteristics' consistency. By optimizing the laser parameters, both manufacturing efficiency and material behavior can be improved. Fatigue and tensile testing should be done too to prove this after several heat treatments in different conditions, to improve alloy properties and minimize residual stress at the same time.Finally, it is worth mentioning that the development carried out around this dual-phase Ti alloy has contributed to some parts manufacturing of structural components for aerospace applications. Hence, the material performance has a long journey ahead of that industry.

Abstract: Due to its low density, high strength to weight ratio, and been unreactive to the human body, titanium is commonly used in human bone implants. Titanium in bone implants can be used in its porous form because the porosity reduces the elastic modulus of the implant, near to that of human cortical or trabecular bone, which prevents the effects of stress-shielding. To date, majority of the published studies using the space holder (SH) method to produce porous titanium, utilized-45 μm titanium hydride dehydride (Ti-HDH) powder, or similar titanium powder. However, there is limited research conducted on the use of coarse titanium powder particles, such as-150 μm Ti-HDH powder to produce porous titanium. Fine Ti-HDH powders are known to have higher oxygen content than coarse Ti-HDH powders, thus the specimens produced from fine powders are harder, require higher compaction pressures and are expected to have lower impact resistance. The following study thus investigated the use of-150 μm Ti-HDH powder to produce porous titanium specimens, by the SH method. The porous specimens of 45 mm diameter were produced by uniaxially compacting mixtures of sodium chloride (NaCl) powder and Ti-HDH powder at 500 MPa. The NaCl powder utilized was hand sieved to a range of-500 μm. The specimens were sintered at 1150 for 4 hours in a high-vacuum tube furnace. Three porosity levels were investigated i.e. 40%, 50% and 60%. The sintered compacts were assessed for density, porosity and elastic moduli. It was found that the sintered porosity of the specimens ranged from 42.7-59.1%, and the sintered density ranged from 1.84-2.58 g/cm³. The elastic moduli of the specimens were found to reduce as the porosity increased, and ranged from 0.59-1.3 GPa, which is similar to the elastic moduli of human trabecular bone. The use of-150 μm Ti-HDH powder is thus potentially a lower cost alternative, than the use of-45 μm Ti-HDH powder, to produce porous titanium for human bone implants.

Abstract: The exceptional corrosion resistance, low weight, and high strength of titanium (Ti) make it an excellent choice for components in proton exchange membrane fuel cells (PEMFC). However, during PEMFC operation, Ti undergoes passivation, which diminishes the bipolar plates' (BP) ability to transport electrons between cells. Applying titanium nitride (TiN) coatings, known for their good conductive properties, can resolve this issue and enhance corrosion resistance. Additionally, using additive manufacturing (AM) to produce BP offers numerous benefits in terms of structural control for more intricate designs. This study examines the impact of TiN coating via gas nitriding on Ti-6Al-4V open structures created by powder bed fusion-electron beam/metal (PBF-EB/M) or PM routes, focusing on the surface characteristics such as composition and interfacial contact resistance (ICR).