Superplastic forming of rapid
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Table I.
achieve a steady-state strain rate and increasing the gas pressure to obtain an incrementally larger strain rate. To minimize errors introduced by strain softening, and to assess the effects of strain-induced microstructural changes, several incrementally increasing pressure cycles were repeated at successively higher strains in experiments 1 and 2. All experiments were conducted under superimposed hydrostatic pressures of 2.1 to 2.8 MPa (300 to 400 psi) to suppress cavitation. Preliminary experiments showed that the formability was optimized at 590 ~ where the lithium was completely dissolved and the flow stress minimized. Results of the incremental-strain-rate experiments (1 and 2 in Table I) show that RSP A1-4Li-0.2Zr is formable to large superplastic strains, as determined from cone-height measurements, and that the strain-rate sensitivity exponent m ranges from 0.33 to 0.36 independent of strain. The relatively low value of m is typical of P/M Zr-containing A1-Li alloys, which have fully-recovered substructures prior to thermomechanical processing of superplastically formable sheet. 2'3'6 This behavior contrasts with that of 7475-A1, where high-angle boundaries predominate and m = 0.9 under optimal forming conditions. 4 In contrast to the incremental-strain-rate tests, the constant-strain-rate tests (experiments 3 to 5 in Table I) yielded modest strains, it was thought that the initial microstructure may not have had enough favorably-oriented high-angle boundaries to allow extensive deformation by grain-boundary sliding. Following a suggestion by A. K. Ghosh, 7 experiment 6 was conducted at a strain rate of 3.7 x 10 ~ s -~ up to e , = 0.5 to promote dynamic formation of favorably oriented grain boundaries; the remainder of the experiment was performed at a strain rate of 6.8 x 10 -4 s -a. This procedure resulted in an improvement in strain to failure relative to that of experiment 4. which was conducted entirely at the lower strain rate. Experiments 1 and 2 were similar to experiment 6 in that the strain rate was incrementally increased to 3 x 10 -3 s -~ early in the experiment, followed by relaxation to a lower value. It was observed in experiments 1 through 6 that failure occurred where the rod used for the continuous measurement of cone height rested on the cone tip. Experiment 7 was therefore conducted in a similar manner to experiment 6 but without the rod. A qualitative increase in cone height was observed, indicating that the practical
Biaxial Superplastic Forming Experiments on Rapid Solidification Processed AI-4Li-0.2Zr at 590 ~
Experiment Number
Strain Rate, k (s t)
2.3 x 1.0 x 3.1• 5.9 x 2.5 x 3.7 • 4 x
10 -5 to 3.1 x 10 ~ (incremental) to 3.5 x 10 3 (mcremental) 4
10 - 4
10 -.4
10 10 ~ to e, = 0.5, then 6.8 x 10 4 10 3to e, = 0.5, then7 x 10 4(notinstrumcnted)
P.J. MESCHTER, Senior Scientist. R.J. LEDERICH, Scientist. and S. M. L SASTRY, Principal Scienust. are with McDonnell Douglas Research Laboratories, P.O. Box 516, St. Louis. MO 63166 Manuscript submitted June 2, 1986. METAL
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