Machinery's Handbook, 31st Edition
1436 Superplastic Forming and Diffusion Bonding • titanium (Ti-6Al-N) • aluminum (2004, 2419, 7475) • aluminum-lithium (2090, 2091, 8090) • stainless steel (2205 series). In general, the alloys chosen for superplastic forming should have a grain size below 10 microns in diameter. The grain size must not increase if it is kept at temperatures 90 percent of melting for a few hours, and the alloys must have strain rate sensitivity parameters of 0.35 < m <= 0.85. High strain rate sensitivity is necessary for reducing the rate of flow localization, i.e., necking. A low rate of damage accumulation, e.g., cavitation, is necessary to allow large plastic strains to be reached. Superplastic Forming.— Superplastic forming (SPF) is a metal forming process that takes advantage of the high extendability of certain materials in order to form compo- nents whose shapes might be otherwise very difficult to obtain. Today, superplastic form - ing of titanium and aluminum alloys is a standard industrial practice accepted worldwide. Due to the high temperatures and simultaneously relatively low gas pressure (typically less than 200 psi), superplastic forming has found widespread application. Because of titanium’s (and some other alloys’) high affinity with oxygen and hydrogen, inert gases are exclusively used as the pressure medium. Fig. 29 schematically illustrates the SPF process in four steps. Typically, the closed die and the blank with created seal are heated to the same temperature in a special hydraulic hot-press (Step 1). Inert gas pressure is introduced at a controlled rate using a sophisticated gas management system until the sheet is fully formed against the die surface (Steps 2 and 3). Each workpiece’s geometry is unique and requires a unique pressure/time profile to maintain the appropriate strain rate. As the sheet thins, it requires less forming pressure. However, as the workpiece radius decreases, more pressure is required. Blank Gas pressure
Step 1
Set-up of SPF die
Vacuum
p = f ( t )
Step 2
Membrane forming
p = f ( t )
Step 3
Final workpiece forming
Vacuum
Step 4
Die
Finished part removed from die
Fig. 29. Schematic Illustration of Superplastic Forming Process
The advantages of the superplastic process are the following: a) Using one of the alloys with superplastic capabilities means getting to the finished product in fewer steps, which means lower costs.
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