Development of Mg alloy sheets with enhanced formability and corrosion resistance

Abstract

In recent years, the interest in lightweight structural materials has been rapidly increasing due to the strong demand for mass reduction in the automotive industry and portable devices. In response to this trend, the use of Mg and its alloys, which are lightest structural metallic material, has been actively pursued. However, as compared to rival lightweight materials such as Al alloys, commercial Mg alloys have some disadvantages, including frightening ignition at high temperatures, poor corrosion resistance, and low formability at room temperature. Until now, considerable effort has been invested into designing alloys to provide solutions to these drawbacks, which have increased the industrial application capacity of Mg alloys. For example, it has been found that alloying with Ca and a rare earth (RE) element are effective in terms of reducing the ignitability and increasing room temperature formability of Mg and its alloys. In addition, a new Mg alloy system containing RE elements has been proposed to meet the industrial requirements with a better corrosion resistance than that of conventional Mg alloys. Notwithstanding the efforts outlined above, systematic research on the effect of alloying elements on both formability and corrosion resistance has been scarce. In the present study, we investigate the role of Ca element on the stretch formability of Mg–1Al–0.2Zn–0.03Mn (at%) alloy and Mg–0.7Zn (at%) alloy sheets. The sheets are found to exhibit a significant increase in limiting the dome height (LDH) value between alloyed Ca contents of 0.05% and 0.1% of Ca in Mg–1Al–0.2Zn–0.03Mn alloy and between 0.07% and 0.09% of Ca in Mg–0.7Zn alloy. Accordingly, significant changes are observed in both texture and Ca segregation at grain boundaries. Microstructural factors affecting the formability of the alloy sheets are discussed through the simple Mg–0.7Zn-based alloy regarding the Ca contents. Furthermore, combined additions of Ca and RE in Mg–1Al–0.2Zn–0.03Mn (at%) alloy are found to result in a better combination of stretch formability and corrosion resistance. We also discuss the microstructural factors affecting the formability and corrosion property of the alloy sheets.