Microstructure modification and bendability improvement of AZ31 magnesium sheet by bending-unbending and annealing process

AZ31 magnesium sheet was processed by room temperature multi-step cyclic bending-unbending (CBU) and subsequent annealing (CBUA) process to modify the microstructure and improve its bendability. The CBU process resulted in a complex stress and strain distribution through the thickness and a multi-layered microstructure after annealing. The grain structure, micro-texture, and micro-hardness of each of the layers were studied by optical microscopy, electron back-scattered diffraction (EBSD) and indentation measurements, respectively. The CBUA process resulted in static recrystallization of grains leading to average grain size of about 2.5 μm to a depth of about 40 μm from the surface. The grain size, however, significantly coarsened in two intermediate layers through the thickness. The recrystallization was studied by microstructural characterization and micro-hardness measurement after annealing. The through-thickness texture from EBSD experiments indicated texture spreading and weakening in the surface layers. The results are consistent with the mechanics of cyclic bending-unbending (CBU) process, mechanisms of strain induced static recrystallization, texture evolution and its stability with long term annealing. The grain refinement and texture spreading of multi-layered microstructure from CBUA process led to bendability (rmin/t value) improvement in V-bending tests at room temperature.