Material characterization of duplex stainless steel by superplastic free bulging test
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.
The forming of superplastic sheet metal
Abstract The superplastic deformation of thin circular diaphragms subjected to onesided hydrostatic pressure is examined both theoretically and experimentally. Special attention is given to the thickness variations in bulged shapes of simple geometry (domes) and also in more complex shapes related to those encountered in practical forming operations.
Influences of material parameters and microstructure on superplastic forming
The strain rate sensitivity of flow stress has long been recognized as an important factor in determining superplastic ductility, and its relationship to high tensile elongations is well understood from the mechanics point of view. However, the measurements of this parameter and other properties of superplastic materials are challenging, and quite varied results are observed from different test procedures used. In this paper a discussion of the various characterization methods is presented, and the relationship between the superplastic characteristics and the microstructure is brought forth. The applicability of the uniaxial superplastic properties on the biaxial deformation behavior as in the superplastic forming process is addressed, including a relationship of such properties to the selection of forming parameters.