Optimization of the microstructure for improving superplastic forming in magnesium alloys

The deformation mechanisms and the thermal stability of AZ31 alloy, processed by various routes, have been investigated. Superplastic deformations could be achieved at flow stresses lower than 10 MPa at 375 °C and 10^-4 s^-1 in the AZ31 alloy with a grain size of 17 (mu)m. The thermal stability of several magnesium alloys have been compared. It has been found that the forming temperature must be below but close to a limit temperature, which is a function of the grain size to obtain microstructural stability. Moreover, in the case of ultrafine grained magnesium alloys the stresses required for superplastic forming increase continuously reaching prohibitive levels because a fast decrease in the limit temperature occurs as the grain size is refined. To perform superplastic blow forming in magnesium alloys it is proposed the use of fine, but not ultrafine, grain sizes with the condition of deformation being controlled by grain boundary sliding.