Titanium Powder Metallurgy and Additive Manufacturing

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Authors: Lerato Criselda Tshabalala, Ntombizodwa Mathe, Hilda Chikwanda
Abstract: In this paper, titanium powders from various sources were characterized to compare powder intergrity for additive manufacturing by selective laser melting process. Selective laser melting by powder-bed based Additive Manufacturing (AM) is an advanced manufacturing process that bonds successive layers of powder by laser melting to facilitate the creation of engineering components. This manufacturing approach facilitates the production of components with high geometrical complexity that would otherwise be impossible to create through conventional manufacturing processes. Although the use of powder in AM is quite common, powder production and optimization of powder properties to yield desired performance characteristics has posed a serious challenge to researchers. It is therefore critical that powder properties be studied and controlled to ensure reliability and repeatability of the components that are produced. Typically, the desired feature of high quality titanium metal powders for AM are a combination of high sphericity, density and flowability. Scanning electron microscopy, EDS, particle size distribution and powder rheology were extensively performed to investigate the properties of gas-atomized Ti-6Al-4V powders.
Authors: Eugene Ivanov, Eduardo del Rio, Igor Kapchemnko, Maija Nyström, Juha Kotila
Abstract: The use and application of alloys as biomedical alloys have increased over the past few years owing to their excellent biocompatibility, corrosion resistance, high mechanical and fatigue resistance, low density, adequate wear resistance, and low elastic modulus. Orthopedic implant materials are exposed to high mechanical loading. Conventional materials based on Ti-6Al-4V, stainless steel or cobalt-chromium alloys demonstrate good mechanical strength, but also some toxicological concerns due to release of toxic elements which may result in inflammatory reactions. Metal alloys based on titanium, zirconium, tantalum and niobium represent higher biocompatibility with appropriate mechanical properties for avoiding stress-shielding and consecutive implant loosening. Application of specifically designed spherical β-titanium alloy powders in additive manufacturing, such as selective laser melting (SLM) or electron beam melting (EBM); enable the production of components with a high degree in freedom of design. Accordingly, SLM or EBM of Ti/Nb (/Ta) alloys offer the possibility to fabricate patient-specific orthopedic implants. The present paper describes development of β-titanium alloys powders designed for application in additive manufacturing technologies. TiNbZrTa (TNZT)-based 3D structures were successfully manufactured and mechanically tested.
Authors: Jin Gou Yin, Gang Chen, Shao Yang Zhao, Ping Tan, Zheng Feng Li, Jian Wang, Hui Ping Tang
Abstract: Microstructure of Ti-28Ta powders produced by plasma rotating electrode process (PREP) was investigated by using scanning electron microscopy (SEM), optical microscopy (OM), and transmission electronic microscopy (TEM). Phase constituent of the PREP Ti-28Ta powders was analyzed by X-ray diffraction (XRD). It was found that microstructure of the PREP Ti-28Ta powders was dependent on the powder particle sizes. Predominant martensitic α”, some metastable β and trace athermal ω phases were observed in the powders with the small diameter. While, phase constituent of the PREP Ti-28Ta powders with the large particle size was predominant metastable β, some martensitic α” and trace athermal ω. With the reduction of the powder particle size, the amount of martensitic α” increased and the metastable β decreased. The martensitic α” was formed preferentially on the metastable β grain boundaries of the PREP Ti-28Ta powders. The increase of α” phase and decrease of β with reduction of the powder particle size is attributed to the increase of the volume of the grain boundaries due to the grain refinement.
Authors: Harshpreet Singh, Muhammad Dilawer Hayat, Raj Das, Xin Gang Wang, Peng Cao
Abstract: Metal matrix composites (MMCs) are the new generation materials that combine both the metallic properties (ductility and toughness) and ceramic characteristics (high strength and modulus), leading to higher strength in shear and compression, at higher service temperatures. Titanium matrix composites possess light weight, high strength and good corrosion resistance and are used as structural materials in automobiles and aerospace industries. In the present study, in situ Ti-TiB composites were fabricated by reinforcing (2, 5, 10 and 20 wt. %) TiB2 powder (mean size <10 microns) into titanium powder (mean particle size ~26.58 μm) and subsequently consolidated by vacuum sintering at 1300 °C for 3 h. X-ray diffraction, scanning electron microscopy (SEM) and density measurements were carried out to characterize the prepared composites. The results showed that all compositions led to high density composites, and the hardness of the composites increased with an increase in the amount of reinforcement. The mechanism of vacuum sintering is yet to be understood in the consolidation of composites and the detailed evolution of microstructure needs to be analysed.
Authors: Fei Yang, Brian Gabbitas, Stiliana Raynova, Ajit Pal Singh, Leandro Bolzoni
Abstract: Ti-5553 (Ti-5Al-5V-5Mo-3Cr, wt. %) alloy is a recently developed near β titanium alloy and it has a very good hardenability, good ductility and high strength. In this study, we discussed the feasibility of preparing Ti-5553 alloy by different processes from powder mixtures of hydride-dehydride titanium powder, elemental powders and master alloy powders, including (1) direct extrusion of powder compact in argon, (2) extrusion of the vacuum-sintered billet in air and (3) extrusion of the hot-pressed billet in air. XRD, OM and SEM were used to determine the phase constitutions and microstructures of the prepared Ti-5553 alloys, and mechanical test was performed to examine their mechanical properties. The results showed the microstructures and phase constitutions of Ti-5553 alloys were significantly affected by different processes, which resulted in the relevant mechanical properties. The effect of the selected heat treatment on the microstructures and properties of Ti-5553 alloy were investigated as well.
Authors: Ming Tu Jia, Clément Blanchard, Leandro Bolzoni
Abstract: Blended elemental powder metallurgy is a cost effective approach to produce near net shape titanium alloy parts; however, the residual pores remaining in sintered parts are detrimental to the mechanical properties. In this study, elemental powders (Ti, Al and Fe) were used to produce the Ti-5Al-2Fe alloy by a powder forging process, involving cold compaction, vacuum sintering, forging and heat treatment. The residual pores of the sintered parts were removed completely by forging at the temperature of 1250oC. The effect of solution and aging and mill annealing heat treatments on the mechanical properties of the forged Ti-5Al-2Fe parts were studied. It is found that the ductility of the forged Ti-5Al-2Fe parts is improved significantly by both solution and aging treatment and mill annealing, without decreasing their ultimate tensile strength, which sits around 1000 MPa. The enhancement of the mechanical behaviour is justified via understanding the evolution of the residual porosity and of the microstructural features of the materials.
Authors: Carlos Romero, Fei Yang, Stiliana Raynova, Leandro Bolzoni
Abstract: In this study, Ti-6Al-4V bars were first prepared by extrusion of powder compacts from blended powder mixtures in the beta phase region, then the as-extruded bars were heat-treated following four different conditions: beta quenching and aging (βQA), broken up structure (BUS) treatment, solution treatment and aging (STA) and recrystallization annealing (RA). The effect of the heat treatments on microstructure and mechanical properties was studied using optical microscopy, scanning electron microscopy, and mechanical test to determine which heat-treatment condition has the greatest impact on the mechanical properties of the as-extruded Ti-6Al-4V alloy. The results show that the as-extruded condition has the best balance of strength (1120 MPa of UTS) and ductility (11% of elongation to failure). βQA and STA lead to a slight increase in strength but ductility decreases considerably. After BUS and RA treatments, both strength and ductility are reduced. The relationship between processing, microstructure and properties was studied, and their implications towards fatigue behaviour and fracture toughness were discussed.
Authors: Stiliana Raynova, M. Ashraf Imam, Hunter Taylor, Fei Yang, Leandro Bolzoni
Abstract: Microwave sintering (MWS) was used to consolidate hydride de-hydride Ti powder and blended elemental Ti6Al4V, Ti5Fe and Ti5Al5Mo5V3Cr (Ti5553) powder mixtures. The amount of powders used to prepare the powder compacts was scaled up to 500g.The effect of the MWS conditions on the relative density, porosity distribution, microstructure and tensile properties were studied. Furthermore, uniformity in distribution of the alloying elements was checked. For most of the materials considered, a combinations of sintering temperature of 1200oC and 1300oC and holding time of 5 to 30 min resulted in significantly improved density. Nevertheless sintering temperature of at least 1300oC was required for pore coalescence and high tensile properties.
Authors: Ajit Pal Singh, Fei Yang, Rob Torrens, Brian Gabbitas, Barry Robinson, Leandro Bolzoni
Abstract: In this paper, a Ti-6Al-4V rectangular bar was successfully produced from a 5kg blended powder mixture using an industrial scale extrusion facility. The elemental hydride-dehydride (HDH) titanium and 60Al-40V master alloy mixture was warm pressed and vacuum sintered prior to β extrusion in air. The as-processed material was characterised for compositional homogeneity, oxygen pickup, microstructure, tensile properties and fracture behavior. Variation in microstructure and properties along the length of the extruded bar were also studied. It was found that oxygen pickup mainly occurred during vacuum sintering of the green billet and consequently the as-extruded material had an oxygen content of 0.55 wt.%. The processed material had a typical lamellar morphology with some evidence of micro-cracks at high magnification. A significant deviation in prior β grain and α colony sizes was observed along the length of the bar, due to variations in extrusion temperature and cooling rate. Both grains and colonies became finer as the location changed from the tip of the extruded bar to the back end. The as-processed material had ultimate tensile strength in the range of 1068-1268 MPa and elongation to fracture of 1.2-4.5%, mainly due to the high oxygen content and non-optimised microstructure. Fractographic analysis was consistent with the variation in mechanical performances obtained.

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