Microstructure Evolution and Mechanical Properties of Large-Scale AZ80 Magnesium Alloy Billets Produced by Multitemperature Multidirectional Forging

Grain refinement is an effective method to improve the mechanical properties of magnesium (Mg) alloys. In the present research, the combination of multidirectional forging (MDF) and forward extrusion for sizing with decreasing temperature pass by pass was employed to deform as-cast AZ80 magnesium alloy billet with large size to obtain fine grains and homogeneous microstructure. It was found that with the increasing number of MDF passes, the average grain size decreases; the homogeneity of microstructures and the percentages of dynamic recrystallised grains increase simultaneously. After six-pass MDF, fine and homogeneous microstructure with an average grain size of 15 μm was achieved; however, grain refinement became very difficult with a further increase in the MDF pass number. After eight-pass MDF plus forward extrusion for sizing, excellent mechanical properties of large-scale AZ80 magnesium alloy billet were obtained. The tensile strength, the yield strength and the elongation of billet are 410 MPa, 292 MPa and 14%, respectively. Furthermore, during multitemperature MDF, the inverse of the square root of grain size and tensile yield strength fit well with the Hall–Petch relationship.