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Introduction Metal additive manufacturing (AM) is no longer solely a prototyping technology; it is now being used for production of industrial parts ranging from orthopedic implants to aircraft components.1 The last two decades have seen the development of a diversity of metal AM processes. Figure 1 summarizes these processes categorized in terms of energy source, feedstock form, and additive method with corresponding key equipment manufacturers. In principle, all weldable metals can be fusion-printed today. In a broader sense, all powdered metals can be printed at room temperature using the ExOne binder jetting process (see Figure 1)2 to achieve a preliminary shape, with the final shape and required properties developed through a subsequent infiltration or sintering process. Titanium alloys are advanced structural materials that possess an array of unique properties. These include corrosion immunity to seawater, high strength-to-weight ratios, excellent fracture toughness, high-fatigue performance, significant compatibility with composites, long durability with little or no maintenance, and superb biocompatibility. However, titaniumbased materials are not easy to machine due to their low thermal

conductivity and acute reactivity.3 In addition, they are difficult to cast since molten titanium is extremely reactive, which is problematic for both melt handling and casting.3 As a result, historically, titanium components have been mostly machined from forged titanium blanks at a slow speed, entailing prolonged manufacturing cycles and up to 95% of the raw material being lost as scrap (recycling titanium chips from machining is not straightforward).4,5 The concept of metal AM was therefore especially attractive for the manufacture of titanium components. From a research perspective, Ti-6Al-4V, which accounts for more than 50% of total titanium usage,6 has been the single most extensively studied alloy for AM. Apart from the liquid metal printing process, Ti-6Al-4V has been additively manufactured using every other AM process (shown in Figure 1) to gauge the capability of each process. This article offers an overview of AM and postprocessing of AM Ti-6Al-4V by focusing on microstructures and mechanical properties. The discussion is limited to selective laser melting (SLM), selective electron-beam melting (SEBM), and laser metal (powder and wire) deposition (LMD).

M. Qian, Centre for Additive Manufacturing, School of Engineering, RMIT University, Australia; [email protected] W. Xu, Department of Engineering, Macquarie University, Australia; [email protected] M. Brandt, Centre for Additive Manufacturing, School of Engineering, RMIT University, Australia; [email protected] H.P. Tang, State Key Laboratory of Porous Metal Materials, Northwest Institute for Non-ferrous Metal Research, China; [email protected] doi:10.1557/mrs.2016.215

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