Research On The Plastic Forming Properties And Microstructure Evolution Of AZ31 And AZ91 Magnesium Alloies

The research on the plasticity property of Mg alloy is of great significance for the engineering application of the Mg alloy parts which are formed by plastic forming technology. In this paper with the AZ31and AZ91 Mg alloy as the research object which is typical of wrought Mg alloy and cast Mg alloy respectively. On the basis of the two fine-grained Mg alloys of AZ31 and AZ91 were prepared with the isothermal extrusion process. The uniaxial tensile deformation behavior of the two alloys was studied, and the mechanical properties of the two alloys were contrasted. The microstructure evolution during isothermal extrusion of AZ31 and AZ91 Mg alloy was simulated by the finite element method, and the microstructure evolution regularities were obtained.The AZ31 and AZ91 Mg alloy were extruded by isothermal extrusion technology at the temperature of 300℃, 350℃and 400℃with extruding ratios of 4:1 and 9:1. The deformed properties under different extrusion conditions were researched. The extrusion specimens of AZ31 Mg alloy is better forming quality than specimens formed by AZ91 Mg alloy. The specimen surface for AZ31 Mg alloy didn't appear cracks. But the specimen surface for AZ91 Mg alloy appeared cracks extruded at 4:1 extruding ratio, 300℃temperature; 9:1 extruding ratio, 350℃and 9:1 extruding ratio, 400℃temperature. The microstructure of extrusion specimens of AZ31 and AZ91 Mg alloy were observed by optical microscopy. The dynamic recrystallization occurred during hot extrusion process, the grain of extruded samples were obviously refined. The grain size of forming specimens is different for different zone, and the size in center portion is bigger than that in the brim portion. Grain size increases with the increasing of the extrusion temperature, and with the extrusion ratio increasing the grain size decreases. But in the same extrusion conditions, the grain size of AZ31 Mg alloy is apparently smaller than that of AZ91 Mg alloy.The uniaxial tensile behavior of AZ31 and AZ91 Mg alloy at room temperature was studied for undeformed samples and samples deformed under different extrusion conditions. The influence of extrusion conditions on mechanical property for the two alloys was analyzed. The mechanical property of AZ31 and AZ91 Mg alloy can be remarkably improved by extrusion process, and the tensile strength and elongation percent increase with the increasing of extrusion ratio, decrease with the increasing of temperature. But in the same extrusion conditions, the tensile strength of AZ31 Mg alloy is lower than that of AZ91 Mg alloy and the elongation percent of AZ31 Mg alloy is higher than that of AZ91 Mg alloy, which indicates that the plasticity of AZ31 Mg alloy is better than that of AZ91 Mg alloy.The microstructure evolution of AZ31 and AZ91 Mg alloy during the isothermal extrusion process is simulated with Yada model by MSC.Marc, which is basically consistent with the experiment results.