Improvement of magnesium alloy edge cracks by multi-cross rolling

In order for the edge cracks during the AZ31 Mg alloy rolling to be reduced, the hot-rolling microstructure and texture were modified through multi-cross rolling (MCR) during the deformation. The AZ31B magnesium alloy sheets were hot rolled at a temperature ranging from 250 °C to 400 °C and at a rolling speed of 0.5 m/s. Four different multi-cross rolling (MCR) routes were selected in the test. The macroscopic morphology, microstructure and texture of the as-rolled AZ31B sheets were characterized to investigate the edge-crack behavior during the rolling with various rolling routes and temperatures. Through the various rolling routes comparison, the grain size, the texture and the twins were the main factors that affected the cracks. It was demonstrated that the grain refinement and weak basal textures obtained by RII where the rolling direction was changed by 90° between two adjacent passes, could significantly reduce the edge cracks during the rolling at an elevated temperature. When rolling with RII at 400 °C, no apparent edge cracks appear on the sheet rolled with four passes. The propagation mechanism of the crack tips was studied in details at the temperature of 350 °C, whereas the results demonstrated that a high-sized area of grain boundaries in-between the finer grains could increase the crack propagation resistance.