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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 8 is a condition in which a solid crystal- line material is deformed well beyond its usual breaking point, usually more than about 200% during tensile defor- mation. This state is usually achieved at a temperature typically half the abso- lute melting point. Superplastically de- formed material gets thinner in a very uniform manner, rather than forming a neck (a local narrowing), which leads to fracture. Generally, for superplastic behavior to occur, a relatively stable, fine equiaxed grain size (to allow grain boundary sliding) is required with the optimum strain rate increasing as grain size becomes smaller. Superplastici- ty enables applying very large plastic strains to sheet metal parts during hot forming operations. Although superplastic behavior canproduce strains greater than1000%, superplastic forming is generally lim- ited to the 100-300% range. The ma- jor advantage is formation of large complex structures in one operation, which eliminates assembly and reduces weight. The final product features ex- cellent precision and fine surface finish with no springback or residual stresses. Versatility of the SPF process is enhanced by combining it with diffu- sion bonding (DB), which also requires a fine, equiaxed stable grain size and an elevated temperature, and can be carried out simultaneously with SPF. Combined techniques are referred to as SPF/DB. Figure 3 shows a schematic of the process. NEAR-NET SHAPE POWDER METALLURGY PROCESSING Using the metal can approach with prealloyed (PA) powder and HIP, advanced process modeling enables achieving net surfaces and minimal machining stock on near-net surfaces despite the 30-35% volume shrinkage typical for HIP of PA powders [2] . Also, near-net shape titanium parts can be built up to the size of existing HIP fur- naces (e.g., up to 2 m, or 6.5 ft). A com- parison of tensile properties of powder HIP and conventionally processed tita- nium is shown in Fig. 4. Fracture tough- ness ( K Ic ) of net shape components ranges from 92.5 to 96.5 MNm − 3/2 . INJECTION MOLDING Metal injection molding (MIM) of titanium components is particularly applicable to small complex parts gen- erally weighing 450 gm or less [2] . The process is based on injection molding plastics, a method developed for long production runs of small (typically less than 400 gm) complex shaped parts in a cost effective manner. By increas- ing the metal (or ceramic) particle con- tent, plastic injection molding evolved into a process for producing high-den- sity metal, intermetallic, and ceramic components. A major contributor to mechanical properties is interstitial ox- ygen. Thus, the aerospace oxygen spec- ification for commercially pure (CP) titanium (Grade 4) is 0.4 w/o for low- er yield strength Grade 4 (≤80 ksi), and 0.2 w/o for Ti-6A l-4V for yield strength between 130-140 ksi. Thus, cosmetic parts such as watch cases are made of CP titanium. Currently, titanium MIM parts up to one foot in length are produced, but 3 to 4-in. parts (about 50 gm) are less common due to dimensional Fig. 4 — Tensile properties of titaniumproducts made using different processing techniques. Fig. 3 — Examples of combined SPF/DB fabrication process. Titanium sheet (gray) is forced into the cavity by inert gas pressure. Diffusion bonding occurs when adjacent titanium sheets come into contact. The stop-off prevents contacting sheets frombonding together.