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 2.5μm to a depth of about 40μm 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.