Research On Microstructure Evolution And Mechanical Properties Of Cold Rolled ZK60 Magnesium Alloy Sheets By Electropulsing Treatment

The energy crisis is becoming one of the major challenge for the world. To cope with this issue, Magnesium alloys, the lightest structural material, is attracting more and more attention in recent years. Magnesium alloys behave with great advantages, such as high specific strength, high specific stiffness, and high electromagnetic shielding. The utilization of Mg alloys is increasingly expanding, especially in the automobile industry and aerospace industry. However, the poor formability at room temperature severely hinders their further wide applications. Fortunately, grain refinement can effectively enhance both the ductility and the strength of Mg alloys.In this study, the effect of electropulsing treatment(EPT) on the microstructure evolution and mechanical properties of cold rolled ZK60 Mg alloys was investigated. Different experiment parameters, such as rolling reduction, duration and conduction time, were applied. The thermal and athermal effect, nucleation and grain growth, were studied experimentally and theoretically.It was found that EPT induced recrystallization could produce ZK60 Mg alloys with desirable microstructure, contributing to enhancing the mechanical properties. The average grain sizes decreased from 110 μm to around 13 μm. Due to the grain refining strengthening, the tensile strength was improved from 182 MPa to 265 MPa, and meanwhile the elongation was increased from 7.6% to 18.1%. Besides, the basal-type texture of the cold-rolled sample was weakened and the dislocation density was reduced.The extra energy brought by EPT could low the nuclear barrier, promoting the recrystallization process. EPT was accompanied with thermal and athermal effect. In our work, the athermal effect actually predominated over the thermal effect, thus inducing recrystallization at 448.8 K, far below the recrystallization temperature of ZK60 Mg alloy. The athermal effect was proportional to the difference of electrical resistivity between the deformed state and the recrystallized state. With the rolling increasing reduction, this difference became greater, as well as the athermal effect, leading to the enhanced nucleation rate and the restrained grain growth. As a result, a modified microstructure with numerous small recrystallized grains was obtained. In addition, the dominated nucleation mechanism transferred form strain induced grain migration in 10% reduction samples to twinning induced recrystallization in 20% reduction samples.