Study On Compression Superplasticity Of AZ Series Commercial Extruded Magnesium Alloy

Magnesium alloy has such characteristics as low density, high specificstrength and specific stiffness and good dimensional stability, its products werefavored in the field of battery, transportation, aerospace and weapons.With ahexagonal closepacked structure and relatively coarse grains, commercialextruded magnesium alloy has poor plastically at room temperature. At thehigher temperatures, the plasticity of magnesium alloy is improved, andsuperplasticity can even be achieved, it creates the conditions for magnesiumalloy’s application in compression superplastic forming process such assuperplastic extrusion and superplastic forging, and superplastic weldingtechnology.This paper mainly studied the compression superplasticity of commercialextruded AZ31, AZ61and AZ91magnesium alloy. Before the superplasticcompression test, the material was grain refinement pretreated by recrystallization annealing to improve its plastic deformability firstly. Afterannealing for30min at300℃, the grain size of AZ31, AZ61and AZ91decreased to12.58μm,12.49μm and12.14μm from22.35μm,20.67μm and21.24μm respectively, three kinds of magnesium alloys both achieved therelatively better refinement results.Then, magnesium alloy after the grain refinement pretreatment at relativelybetter annealing parameters was superplastic compression tested. Superplasticcompression performance of magnesium alloy in different deformationtemperature and initial strain rate was explored; superplastic compressionperformance of magnesium alloy of different compositions was compared;different relatively better parameters were recommended according to differentprinciples; the variation of the flow stress, the microstructure and hardness wasstudied; finally, superplastic compression mechanism of AZ series commercialextruded magnesium alloy was analyzed preliminarily.The experiment results show that, when compressed above250℃, in theinitial strain rate between1×10-4s-1and1×10-2s-1, the periphery extensibilityratio of AZ61all exceeds121.4%, AZ61magnesium alloys exhibit goodcompression superplasticity, for AZ31and AZ91, they both crack at250℃, inthe initial strain rate of1×10-2s-1, in the other parameters, the peripheryextensibility ratio of both them exceeds138.7%, they also have compressionsuperplasticity. Composition of AZ series commercial extruded magnesiumalloy has almost no affect on the realization of compression superplasticity. For the superplastic forming, the recommended relatively better parameteris AZ31(300℃,1×10-2s-1), AZ61(250℃,1×10-2s-1), AZ91(300℃,1×10-2s-1);for the larger deformation forming, it is AZ31(250℃,1×10-4s-1), AZ61(400℃,1×10-2s-1), AZ91(250℃,5×10-4s-1); for the superplastic welding, it is400℃,1×10-4s-1~1×10-2s-1.Deformation temperature and initial strain rate both affect the flow stress.Dynamic recrystallization happens in the superplastic compression process, itmakes grains refined. Dynamic recrystallization is affected by both deformationtemperature and initial strain rate. After superplastic compression deformation,the hardness of the magnesium alloy increases to some extent, and as thedeformation temperature increases, the hardness gradually decreases. The maincompression superplasticity deformation mechanism of AZ series commercialextruded magnesium alloy is probably grain boundary sliding mechanism underthe coordination of dynamic recrystallization.