Development Of Microstructure And Mechanical Properties During Deferential Speed Rolling Of Twin-roll Cast Magnesium Alloys Strips

Mechanical properties of cast magnesium (Mg) alloys can be improved by grain refinement during hot deformation. Mg has great potential in the use of light structural materials. However, two major barriers to the industrial applications of Mg alloys are the high manufacturing cost and limited formability at room temperature. Twin roll casting (TRC) technology combines casting and hot rolling into a single step and it could produce strips from molten Mg alloys, and it can refine grain and second-phrase size, reduce segregation, improve plastic deformation capability, and also shorten the fabrication process of Mg alloys. However, conventional warm rolling of TRC Mg strips preserves a strong basal 0002 texture, and this leads to a very limited ductility near the room temperature and high anisotropy. Differential speed rolling (DSR) can improve the formability of Mg alloys by grain refinement and weakening the basal texture. The objective of this study was to explore the possibility of further enhancement of plasticity of TRC Mg alloys by texture controlling and grain refinement through the DSR technique.Different values of velocity ratio (Rv) between the two rollers were considered to examine its effect on the microstructure, texture and mechanical properties of the processed sheets during DSR. At first, the DSR technique on ZK60 strip was investigated. The rolling force at the same degree of rolling reduction decreased with an increase of the Rv, which could be attributed to the friction forces produced in the opposite direction in the shear zone; the sheets processed by DSR were characterized by the unidirectional shear bands with an inclination at about 25~30°with respect to the rolling direction; tilted basal textures were developed both in equal speed rolling (ESR) and DSR-processed sheets. With the increase of Rv, the intensity of the titled basal texture decreased and the inclination of the -axis became more asymmetric about the normal direction; the increase of the Rv led to significant increase in the elongation of the sheet, at a less significant expense in the yield and tensile strengths. Excellent mechanical properties could be obtained after annealing, and the tensile strength and elongation was about 270 MPa and 23% respectively. The enhancement of the plasticity could be attributed to the weakening of basal texture.The DSR technique was also applied to TRC-processed AZ41M alloy. Compared with the inconspicuous shear bands in the ESR-processed sheet, the DSR-processed sheet was characterized by the unidirectional shear bands and more severe deformation. After annealing, fine grains were found at the locations of the previously developed shear bands. The DSR-processed sheet possessed a high fraction of fine-grained region. The DSR-processed sheet not only had the advantage to weaken the basal texture, but also had a basal fiber texture with splitting along the rolling direction. The annealed DSR specimens exhibited an obvious increase in elongation, with a little expense of the yield and tensile strengths. The enhancement of the elongation could be mainly attributed to the weakening of the basal texture and grain refinement. At last, an investigation was made to compare the DSR on ingot and TRC-processed strips, respectively, for the same AZ41M alloy. Compared to the DSR technique on TRC strips, more recrystallization was observed after ESR or DSR of the ingots. The sheets processed by DSR on TRC failed to show superior mechanical properties, which could be attributed to the inner cracks in the middle of the sheets.TRC could decrease the manufacturing cost and DSR is effective to produce thin sheets with high plasticity. The combined process of TRC and DSR on AZ41M alloy is effective in producing high strength and high formality Mg alloy thin sheets.