Study On Technique And Microstructure Of Magnesium Alloy By Twin-roll Continuous Casting

Magnesium alloys have many excellent characteristics such as low density, excellent damping capacity and good electromagnetic shielding ability, and attractive potential applications in automobile, electrical communication, and aeronautics and astronautics. Magnesium alloy sheet and strip have wide prospective applications. The present processes of magnesium sheet and strip take multi-annealing, with low rate of finished products and high cost, thus their wide applications are limited at some degree. By twin roll casting, magnesium alloy thin strips with near net shape can be prepared, which can shorten the process and reduce the cost. In the paper, AZ31 magnesium alloy thin strips were prepared by vertical twin roll casting and horizontal twin roll casting, with the analysis of the relationship between the process and microstructure. The two casting technologies were compared. The strips were rolled by using the hot rolling mill.Distribution nozzles were designed and optimized, corresponding to the vertical twin roll casting of AZ31. AZ31 strips with 2-4mm thick and 150mm wide were obtained. The casting process and strips'surface quality were analyzed as well as the microstructure of the rolled strips. The effects of casting speed, casting temperature on the microstructure of the strips were investigated as well as the microstructure evolution of the rolled strips. As a result, with the decrease of the casting speed, the grain size of the strips increases and its uniformity reduces. With the decrease of the casting temperature, the microstructure changes from columnar dendrites to cystiform dendrites.A distribution and protection system for horizontal twin roll casting of AZ31 was developed. AZ31 strips with 800mm wide and 7mm thick were obtained which were rolled to the thin sheets with 0.5-1.2mm thick and 700-1000mm wide. By heat treatment,σb,σ0.2 andδof the sheets are 290MPa, 193MPa, 16.4% respectively, which are equal to those of the sheets by hot rolling. The defects of the strips were analyzed and some ways were points out.By the comparison of the two casting technologies, some problems were analyzed and the future development was pointed out. The factors influencing the casting speed of twin roll casting for magnesium alloys were discussed. As a result, to improve the thermal conduction of the caster is an effective way to increase the casting speed.