GRAIN-REFINEMENT AND SUPERPLASTICITY IN 5083-AL

A preliminary investigation of thermomechanical processing of 5083 aluminum plate (Al-4.7%Mg-0.7%Mn) was undertaken to develop a fine-grain sheet for superplastic forming applications. Significant differences in grain size and the extent of superplasticity are seen in hot-rolled vs. cold-rolled sheets, with tensile elongations exceeding 600% for the cold-rolled alloy. Additionally, a separate fine-grain sheet of the same alloy, produced by Alusuisse Co., was studied in greater detail. Superplastic deformation behavior of this sheet was investigated under uniaxial tension over the temperature range of 500-565 degrees C. Strain rate sensitivity values greater than 0.3 were observed over a strain rate range of (3 x 10(-5))-(1 x 10(-2)) s(-1) with a maximum value of 0.65 obtained for strain rate of 5 x 10(-4) s(-1) at 565 degrees C. Constant-velocity tension tests consistently show larger strains to failure and lower strain hardening rate than the corresponding ''constant-strain-rate'' tests for the range investigated. A short but rapid prestraining step, prior to the normal superplastic straining, produced enhanced tensile elongation at all temperatures. Under the two-step schedule, a maximum tensile elongation of 600% was obtained at 550 degrees C, which was regarded as the optimum superplastic temperature under this condition. This paper and a companion paper are used to provide the details of results obtained to date from this study.