Microstructure Evolution during Roller Hemming of AZ31B Magnesium Sheet

The evolution of microstructure and texture during multipass roller hemming (flanging, pre-hemming and flat hemming) of commercial grade AZ31B-O sheet has been studied using electron backscatter diffraction. The pre-hemming operations were performed with and without local heating using a laser source. It was observed that samples pre-hemmed at room temperature could not be flat hemmed even after applying heat; whereas flat hemming was possible in the sample pre-hemmed with laser heating. The major difference between these samples was the formation of contraction/double twins on the outer radii of the bend in the sample pre-hemmed at room temperature. It is believed that such twins contributed directly or indirectly to the fracture of this sample upon the third pass by leading to the formation of shear bands and/or by significantly hardening the material and not allowing for further deformation.