Processing map and kinetic analysis for hot deformation of an as-cast AZ91D magnesium alloy

The hot deformation behavior of as-cast AZ91D magnesium alloy has been investigated by using the processing map and the standard kinetic analysis based on hot compression tests. The compression tests were conducted in the temperature range of 220-380 °C and at a strain rate from 0.001 to 1 s−1. The processing map for a true strain of 0.6 was developed on the basis of a dynamic material model by applying the flow stress data obtained from the compression tests. It was found that the processing map exhibited one single domain of good workability associated with dynamic recrystallization (DRX) in the temperature range of 280-380 °C and at strain rates lower than 0.01 s−1, with a peak power dissipation efficiency of 32% observed at 340 °C and 0.001 s−1. At temperature range of 260-370 °C, a wide region of flow instability was observed for the strain rates ranging from 0.01 to 1 s−1. The hot deformation behavior represented by the DRX domain and the unstable region was discussed by referring to microstructure evolution during compression. Kinetic analysis of the flow stress data at a true strain of 0.6 yielded a stress exponent of 6.99 and an apparent activation energy of 156 kJ/mol for deforming at 260 °C with the strain rates ranging from 0.001 to 0.01 s−1. A stress exponent of 6.49 and an apparent activation energy of 145 kJ/mol were determined for deforming at 300-380 °C with a relatively lower strain rates ranging from 0.001 to 0.1 s−1. The results suggest that the thermally activated climb process associated with basal glide is considered to be the rate controlling factor or deformation mechanism for the two regions.