Macroscopic superplastic behaviour of metallic or non metallic materials is usually represented by the strain rate sensitivity, and it can be determined by tensile tests at elevated temperature. However, a uniaixal stress state occurs in the test whilst a multiaixal stress state is induced in actual superplastic forming processes. The difference of the stress state might affect the reliability of the strain rate sensitivity obtained from the tension test. In the current study, the material characterization of duplex stainless steel, UNS S31803, has been carried out to determine the strain rate sensitivity throughout superplastic free bulging test under constant pressure. The measured height of bulged plate during the test was transformed into strain rate and stress corresponding to height at each time by theoretical considerations. The strain rate sensitivities were calculated for each pressure level. After calculating the strain rate sensitivity and strength coefficient for each test case, the flow stress equations have been employed in the finite element simulation to verify the experiments. The bulged height obtained from the simulation showed a reasonable agreement with the experimental findings. In the future, if a study on the effect of fillet radius of test tool and microstructure evolution is carried out, it is thought that the reliability of the material characterization by superplastic free bulging test can be enhanced
展开▼
