867Wrought Product Fabrication

Wrought products (mill products) are fabricated to desired configurations with the same concerns regarding microstructural control as discussed in the previous section. Examples of forming of wrought product include isothermal/hot forging, sheet metal forming, foil production, rod and wire, and superplastic forming/diffusion bonding.

Isothermal and hot forging are special forging operations in which the die temperatures are close to the metal temperature, that is, much higher than in conventional forging. This reduces chill effects and allows close to net-shape production. Strain rates are much lower than normal, contributing to the near-net-shape capability. The metastable P alloys, with a low P-transus temperature, are particularly amenable to the isothermal forging process.

Sheet metal forming is conducted either hot, which generally allows larger, more precise amounts of deformation, or cold, which is lower cost. Hot forming of titanium alloys is conducted in the range 595-815°C (1105-1500°F) with increased formability and reduced spring back. Formability increases with increasing temperature, but at the higher temperatures contamination can become a problem, sometimes necessitating an inert atmosphere or a coating. Beta alloys are easier to cold form than a and a-P alloys. The high degree of spring back exhibited by titanium alloys sometimes requires hot sizing after cold forming. This reduces internal stresses and restores a compressive yield strength.

Superplastic forming/diffusion bonding makes use of the fact that fine-grained material can deform extremely large amounts, especially at very low strain rates (0.0001-0.01 s-1). Superplastic forming (SPF) is the propensity of sheet material to sustain very large amounts of deformation, without unstable deformation (tensile necking); for example-fine-grained (<10 ^m) Ti-6Al-4V can be deformed >1000% in tension at 927°C (1700°F). Diffusion bonding (DB) is a solid-state bonding process in which a combination of pressure and temperature allow production of a metallurgically sound bond. Superplastic forming is now used routinely as a commercial sheet metal fabrication process for reduced cost and production of complex shapes, generally using gas pressure. The combined SPF/DB process has seen less commercial use than initially anticipated, predominantly because of problems in inspecting the integrity of the bond region.

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