Effect of homogenization on the microstructure and mechanical properties of the repetitive-upsetting processed AZ91D alloy

The current research investigates the effect of homogenization on the microstructure and mechanical properties of the AZ91D alloy processed by repetitive upsetting (RU). Results show that during RU processing, the initial large Mg17Al12 particles in the as-cast specimen accelerate the dynamic recrystallization (DRX) due to the particle stimulating nucleation (PSN) mechanism. With the progress of RU, the inherent large strain breaks the large second phases into small fragments, which indicates the PSN gradually disappears, while the pinning effect becomes obvious. As for the homogenized specimen, a pre-heat treatment leads to the absence of Mg17Al12 particles but a uniform distribution of Al atoms in the Mg alloy. Though the subsequent RU promotes the precipitation of Mg17Al12 particles, the relatively small particle size and the uniform distribution are more favorable to act as obstacles for grain growth than contributors to PSN. Finally, a more homogeneous and refined microstructure is obtained in the specimen with prior homogenization than the as-cast one.