Effects of Alloying elements and Rolling Conditions on Stretch Formability of Mg Alloys

Abstract

The development of formable Mg sheets is essential to meet the needs of the automotive industry for lightweight material. However, it is difficult to archive high formability due to their crystal structure. Basal slip mode and {10-12} tensile twinning are main deformation modes during deformation at room temperature. An activation of these 2 deformation modes is intricately connected with microstructures, textures, and alloying elements. Various alloys have been developed by using various rolling conditions in order to investigate effects of texture and grain size. Microstructures and textures of by Mg-Al-Zn, Mg-Zn and Mg-Sn-Al alloys were examined, and then change of IE values through microstructures and textures were investigated. Basal pole intensity significantly decreased in Mg-Al-Zn and Mg-Sn-Al alloys, but there was the slight change in Mg-Zn alloys. In Addition, tensile tests were also performed to investigate the relationship with IE value and corresponding parameters from tensile test such as uniform elongation, the work hardening exponent (n) and the Lankford value (r-value). To identify effects of grain size and texture on stretch formability more detailed, AZ31 alloys have been developed using various rolling conditions. Increasing rolling temperature enhances the formability of AZ31 alloys because of a weakening basal texture. Increasing annealing temperature and time increase grain size and larger grain size promotes the activation of {10-12} tensile twins. There are two roles of the tensile twinning in deformation. One is to contribute to the deformation through twin strain and another is to support to strain compatibility in grain boundaries. The tensile twins also play a critical role in Erichsen test (bi-axial tension condition).On the other hands, tensile twinning occurring in coarse grain size can decrease the IE value by acting as an origin of cracks because twin growth is so fast that the twin boundary acts as the obstacle interrupting the movement of basal dislocation. Not only do grain size and texture affect to form {101 ̅2} tensile twins at room temperature, but alloy elements determine twinning, 5 Mg-Zn alloys were fabricated and their grain size varied under different annealing conditions. The large-grain Ca-containing alloy showed the highest amount of {101 ̅2} tensile twin. Tensile twinning is attributed to weak basal texture and large grain size as assessed from measurements of microstructures and VPSC calculations. The results of EBSD analysis and the small Erichsen test reveal that twinning contributes to stretch formability but the interaction between twin and slip causes cracks as grain size increases.