Study On Process And Microstructure Of AZ31 Magnesium Strips By Twin Roll Casting With High-dam

By twin roll casting, magnesium alloy thin strips with a thickness close to the final thickness of the sheet product can be prepared directly from the melt, which can shorten the process and reduce the cost. In this paper, based on high-dam molten pool system, the processing characteristics of AZ31 thin strips, the effects of compositions of AZ31 alloy and processing parameters on solidification microstructure of AZ31 strips, and evolution of solidification microstructure during twin roll casting process were researched by means of design and optimization of the nozzles, microstructure analysis and XRD test, And also. in order to investigate microstructure evolution of AZ31 strip, physical simulation was made. The following results are obtained.A better flow field distribution was obtained by using"g"nozzle, which was made of Q235 steel plate. High-dam molten pool system increased the height of melt level, improved the shape of meniscus, stabilized initial solidifying point, and therefore made microstructures of AZ31 strips more uniform, improved surface quality of AZ31 strips.According to the effects of different casting temperatures, different casting speeds, alloy elements on the thickness of AZ31 strips, twin roll casting process could be divided into three stages such as initial stage, 0~1m long, transition stage, 1~3m long, and stabilization stage, over 3m long. Furthermore, based on the microstructures analysis of the strips, twin roll casting process could be divided into two stages such as non-stabilization stage and stabilization stage. When the casting time was over 25.5s, the microstructures of strips reached stabilization stage. Solidification coefficient of AZ31 strips was analyzed at different casting speeds. It was considered that when casting speed was above 12r/min, the solidification coefficient kept constant.Through analyzing the influence of casting speed and casting temperature to microstructures, the conclusion was made that when the casting speed was 12r/min and the casting temperature was 675℃, the twin roll casting process was stable, the surface of strips better, and microstructures more uniform. The combination of casting speed and casting temperature could influence the position of final solidification point.When Ce or Ca was added into AZ31 magnesium alloy, the microstructure of AZ31 strips was finer. When the content of Ce was 0.4%, the refinement effect was optimal and new phase Al4Ce appeared. When the content of Ca was 0.08%, microstructure of AZ31-xCa strips was fine and an optimal refinement was got, and new phase Al2Ca appeared.By analyzing solidification microstructures in molten pool system, it was known that the solidification microstructures evolved from coarse dendrites shaped at a low cooling rate to fine dendrites and finally to fine casting-rolling microstructures resulted from casting-rolling stress.Preliminary investigation of physical simulation of solidification microstructure has made it clear that the dendrites in melt pool grow along the vertical direction of roll surface, and during casting and rolling process, due to the combination of solidification and casting-rolling stress, the spherical dendrites were formed.