October AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 1 8 1 7 Fig. 1 — Mechanical properties of Ti-6Al-4V alloy parts made using various AM processes and conventional processing techniques. DMD = direct metal deposition; LENS = laser engineered net shaping; DMLS = direct metal laser sintering; EBM = electron beammelting; HIP = hot isostatic pressing; HT = heat treatment; UTS = ultimate tensile strength; YS = yield strength. Fig. 2 — Room temperature fatigue properties of AM processed and conventionally processed Ti-6Al-4V: ■ , ♦ , and ▲ represent properties in the three orthogonal directions ( x, y, and z ). Courtesy of Jim Sears/EADS. melting (EBM) is higher than laser-pro- cessed material due to reduced resid- ual stress. Generally, as-built Ti-6Al-4V product exhibits similar or better fa- tigue resistance than that for conven- tional cast and wrought products even without HIP (Fig. 2). Mechanical properties of DED pro- ducts with repaired and added feature regions are of considerable interest. For example, repaired V-groove regions, and slot-repaired Ti-6Al-4V samples ex- hibit tensile strength equivalent to that of conventional wrought Ti-6Al-4V. In addition, Ti-6Al-2Sn-4Zr-2Mo alloy re- paired using laser cladding features higher high-cycle fatigue strength than conventional wrought alloy [3] . SUPERPLASTIC FORMING AND DIFFUSION BONDING Superplastic forming (SPF) of tita- nium alloys is a mature technology. It is commonly used in aircraft parts fabri- cation and is an effective means of both weight and cost savings. Superplasticity T itaniumalloys are attractive for use in many aerospace and terrestrial applications based on their excel- lent mechanical properties. However, in many cases their use is limited due to the high cost of fabricated components, es- pecially in the cost-conscious automobile industry [1,2] . This article discusses innova- tive and potentially lower cost fabrication processes including additive manufac- turing (AM), superplastic forming/diffu- sion, hot isostatic pressing (HIP) near-net shapes fromprealloyed powder, injection molding, rapid solidification, mechanical alloying, and thermohydrogen process- ing (i.e., using hydrogen as a temporary alloying element). ADDITIVE MANUFACTURING Powder bed fusion (PBF) and di- rect energy deposition (DED), two broad classes of AM component build- ing, are based on the concept of slic- ing a solid model into multiple layers and building the component layer by layer in compliance with sliced-model computer data [3] . PBF technologies in- volve placing a layer of metal powder onto a build platform and subsequent- ly scanning the powder bed with a heat source (either laser or electron beam). The powder is partially or complete- ly melted followed by resolidification. DED involves injecting material into the meltpool. The same procedure is fol- lowed for successive layers and builds the component layer by layer until the part is complete. Figure 1 shows tensile properties of Ti-6Al-4V alloy parts produced us- ing various AM techniques and material produced using conventional methods including casting, forging, and wrought and annealed techniques. All AM pro- cesses exhibit strength levels above those of material produced using con- ventional methods. As-built product made using laser-based processes such as direct metal deposition (DMD), laser engineered net shaping (LENS), and di- rect metal laser sintering (DMLS) have lower ductility due to martensite for- mation. Ductility can be enhanced by subsequent treatment by HIP and/or heat treatment. Ductility of Ti-6Al-4V product produced by electron-beam