Superplastic gas pressure forming of fine-grained AZ61 magnesium alloy sheet

Superplastic deformation behavior of fine-grained AZ61 magnesium alloy sheet during equi-biaxial tensile deformation has been investigated. Thin circular diaphragms were successfully deformed into the hemispherical domes at 673 K in applied gas pressure range of 0.46–1.20 MPa. In this pressure range, average shell stress in range of 7–23 MPa and average deformation rate in range of 2×10−4 to 5×10−3 s−1 were imposed on the deforming hemisphere. The thickness profile of the resulting shape, which is sensitive to strain-rate sensitivity (m) and the extent of deformation, was examined and compared with the analytical model. For the low pressure condition (0.46 MPa) the forming process obeyed the model but not at high pressures (0.8 and 1.2 MPa). In the latter cases, according to the deformation mechanism map for Mg, the rate-controlling deformation mechanism changes from lattice diffusion (DL) controlled GBS to DL-controlled dislocation climb creep during the forming process due to dynamic grain growth. The extent of uniformity in thickness distribution is less than predicted by the theoretical model. The reduction of m-value resulted from this change of deformation mechanism.