On the Development of Mg Sheets with Improved Formability by Applying Additional Shear Strain during Processing

Abstract

Reducing vehicle weight and emissions by lightweight design is a major goal of the automotive industry. Magnesium as the lightest structural metal offers a significant weight saving potential compared to steel and aluminum. However, the poor formability of magnesium semi-finished products (e.g. sheets) has hindered the massive application of this metal. This poor formability arises due to the formation of strong textures. Typically the basal planes of the HCP-structure align parallel to the sheet plane. Such preferred orientation of the basal planes limits the ability of basal <a> slip to accommodate plastic strain in the sheet plane and is thus unfavourable for ductility and formability of the sheets, especially at room temperature. A combination of new technologies could help to alleviate the strong textures formed in Mg sheets. In this regard the utilization of twin-roll casting (TRC) in combination with processing techniques which apply additional shear strain such as differential speed rolling (DSR) and/or equal channel angular pressing (ECAP) could refine the microstructure and modify the resulting deformation texture. In this study an AZ31 strip (3Al-1Zn-Mg Bal. wt.%) produced by TRC was hot rolled at 400 °C using a rolling speed of 10 m/min. A speed ratio of 10% between rolls was used. Rolled samples with dimensions of 200 x 200 x 1.8 mm receive a single pass of ECAP at 200 °C. After processing the microstructure of the samples were analyzed by means of optical microscopy, electron backscatter diffraction and X-ray diffraction. The results showed that the microstructure can be refined and the texture altered in comparison to conventionally rolled AZ31 sheets. The samples processed by ECAP show an increase in formability up to 50% higher than conventionally rolled sheets. This demonstrates the potential of using shear deformation for processing Mg sheets.