Microstructure evolution of a SIMA processed AZ91D magnesium alloy based on repetitive upsetting-extrusion (RUE) process

Repetitive upsetting-extrusion (RUE) was introduced into the strain induced melt activation (SIMA) process. The semi-solid feedstock of AZ91D magnesium alloy with superior thixotropic features was successfully prepared by the modified SIMA process. Prior to partial remelting, the process involving the application of RUE pre-deformation to as-cast alloy was conducted. Consequently, a severely strained structure consisting of fine α-Mg grains and highly fragmented β-Mg17Al12 particles was obtained. During the subsequent isothermal treatment between the solidus and the liquidus, the microstructure evolution of the RUE processed alloys subjected to various deformation cycles and temperatures was investigated at the temperature range of 550-580 °C for various durations. The results indicated that the RUE-based SIMA process was a valid route for preparing high-quality thixotropic materials, and the isothermally treated microstructure consisted of fine solid globules surrounded by uniform liquid films. The microstructure evolution model for the modified SIMA process was proposed with reference to the micrograph examination. And the mechanisms responsible for the spheroidization and coarsening of the globular structure were discussed in detail. Further study on effective parameters clarified that the thixotropic features of semi-solid slurry were affected by the parameters of RUE process as well as the isothermal treatment process. And the desirable globular structure, featured by fine, homogeneous and well globularized solid grains and continuous liquid films, was obtained by three-cycle RUE process at 285 °C followed by isothermal treatment at 560 °C for 15 min.