Microstructure and Texture of AZ80 Magnesium Alloy Sheet Rolled by High Speed Warm Rolling

Abstract

The present authors have succeeded in single pass large draught rolling of AZ31 and ZK60A magnesium alloy sheet below 200°C without heating rolls by raising the rolling speed above 1000m/min. Maximum reduction attained in single pass rolling was 60%. Among magnesium alloys, AZ31 is known as the most ductile alloy. It remains uncertain whether the high limiting reduction by high speed rolling can be attained in other magnesium alloys that are less ductile but stronger than AZ31. In this study, AZ80A (Mg-8.1%Al-0.63%Zn) sheets with the thickness of 2.7mm cut from the extruded sheets were used. Rolling temperature was varied from RT to 350°C. Rolling speed was 1000m/min. The limiting reduction in thickness increases with rolling temperature, and the maximum reduction of 52% is obtained at 250°C. The fracture surface of sheet rolled at 100°C shows ductile fractured surface, while it shows brittle fracture surface at 350°C. This difference in fracture mode is attributed to the precipitation of -particles at grain boundaries during holding at 350°C before rolling. From this result, high speed rolling can also be an effective tool for improving the rolling deformability of AZ80 sheet. The hardness of the rolled sheets measured on the transverse plane increases with increasing temperature and reduction. The variation of hardness with rolling temperature and reduction indicates the occurrence of dynamic recrystallization (DRX). The sheet rolled at 200°C with the reduction of 50% shows the tensile strength of 353MPa and the elongation of 29%, which is an excellent strength-ductility balance. By applying high-speed rolling process to AZ80 magnesium alloy, we can obtain a remarkable improvement in the material characteristics as well as rolling deformability.