Study On Mechanism Of AZ31B Magnesium Alloy Thin Sheets Edge Cracks During Rolling

Flat rolling is most potential method to produce massive magnesium alloy sheet and strips with low cost. Edge fracture and cracks are one of the most obvious and serious defects which have great impact on the continuity and production yield among different types of defects during rolling process of magnesium alloys sheets. Edge cracking will not only decrese the finished product rate, but also further development of side crack is likely to lead to break of the strip, even more destroy the stability of the production process. To study the generation and evolution of the edge cracks of magnesium alloy sheets during rolling, the effect of rolling process on edge fracture of AZ31 B magnesium alloy thin sheets and evolution of the pre-cracks during rolling were studied systematically. AZ31 B magnesium alloy thin sheets were produced by different rolling temperature, reduction(total reduction and each pass reduction) as well as the coil tension. Microstructure observation, SEM, edge fracture statistics, and room temperature tensile test were carried out to investigate the effect of process parameters on the microstructure of magnesium alloy thin sheets.The cracks with different types, depth, angle were prefabricated on edge zone of AZ31 B magnesium alloy sheets and then rolled. The situation of edge crack extention and maximum tension of sheets were measured. The mechanical properties and edge cracking extendion during rolling of AZ31 B magnesium alloy sheets with pre-cracks were discussed.The results show that:(1) Rolling temperature have a significant impact on edge cracks of magnesium alloy. With the rolling temperature increase, the degree of magnesium alloy edge crack significantly reduced. Since rolling temperature increase, twins was reduced in AZ31 B magnesium alloy microstructure, and significantly enhanced mechanical properties. When rolling temperature arise above 200℃, slip system was activated, and recrystallization occured. Rolling temperature should be controlled above 200 ℃ of AZ31 B magnesium alloy sheet rolling.(2) Effects of reduction on edge cracks of magnesium alloy include pass reduction and total reduction. With the decrease of pass reduction, the shear deformation decreases, and the degree of cracks reduced, because magnesium alloy edge cracks is caused by shear deformation. With the increase of the total reduction, the extent of the cracks increased, which is the result of crack propagation. But the microstructure is fine and uniform, and the degree of crack propagation will be reduced.(3) Coiling equipment is necessary for magnesium alloy strip rolling consecutively. And coiling tension also has a significant impact on the edge cracks of sheets. The degree of the edge cracks and recrystallized grains has increased.(4) AZ31 B magnesium alloy sheets with different depths and angles of edge cracks were rolled. With the crack angle increases, the decrease of the crack angle drops down. The greater depth of the cracks, the less trend to expand of the crack. The cracks with small angle and depth are most likely to extend.(5) AZ31 B magnesium alloy sheets with different depths and angles of edge cracks were tensile tested at room temperature. Tensile strength of the cracks increases with increasing angle, but the rate of increase has been reduced. The greater the depth of cracks, the lower tensile strength of the sheets, and the drop rate has increased. With increasing depth of the crack, the effect of crack angle on tensile strength will be reduced.