Fine Grain Aluminum Superplasticity

Abstract
Fine grained 7475 aluminum alloy sheet was evaluated for its superplastic properties and potential for superplastic forming. The sheet alloy was specially processed by the ALCOA Technical Laboratory utilizing procedures prescribed by the contractor to achieve a fine grain size. The grain size measurements on this sheet alloy after complete recrystallization revealed a short transverse dimension of 7.8 microm and longitudinal and long transverse dimension of 14 microm. Subsequent grain growth studies showed that this fine grain size developed is quite stable at temperatures as high as 516 C . High temperature testing was conducted over the temperature range of 427 C to 516 C in order to establish the strain rate sensitivity of flow stress, strain hardening characteristics, and total elongation capabilities. Superplastic forming tests were also conducted utilizing commonly used gas pressure forming methods. Small demonstration parts were formed over a range of strain rates and thinning characteristics evaluated. Metallographic evaluation of tensile test specimens and formed parts revealed the tendency of the alloy to form internal voids, or cavities, at large strains. The cavitation appears to be sensitive to the conditions of deformation, being minimized at the higher temperature and lower strain rates.

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