| The purpose of this paper is to examine the uniaxial tensile behavior, bulk forming and the microstructure evolution in the bulk forming of casting AZ31 Mg alloy under different deformation conditions, and to determine the optimum process conditions for the best deformation properties of this alloy, thus supplying a reliable basis for the development and application for the bulk forming technology of Mg alloy.Uniaxial tensile tests were carried out at the temperature range 300-450°C and the strain rate range 4.25×10-4 s-11.0×10-2 s-1 to evaluate the plastic and superlasticity of casting AZ31 Mg alloy. The threshold stress characterizing the difficulty for grain boundary sliding and the deformation activation energy at various temperatures were calculated. The topography and fracture surfaces of superplastically deformed specimens were observed and analyzed by using optical microscope and scanning electronic microscope. It is shown that the alloy exhibited superplastic deformation behavior when the stain rate less than 1.0×10-3 s-1 in a range of the deformation temperature from 300 to 450°C. It was found that the elongation percentage of the alloy at 400°C withε = 4.25×10-4 s-1 is almost 200% and the strain rate sensitivity coefficient m=0.41 The microstructures undeformed and deformed specimens were observed by optical microscope(OM). The initial grain size of tensile specimens is about 25μm. The grain size growth near the necking zone is not obvious. Along the direction of deformation, it is found that grains become a little elongate, but the grain shape almostly maintains equiaxed.Based on the two-stage forming technology, the casting AZ31 Magnesium alloy bar was forged into cylindrical straight inner gear between the temperature 250°C to 400°C. At 250°C, the gear form of the inner gear is almost formed. Butthere are some cyclicity cracks on the surface of the sample. When improving the temperature above 300°C, the surface quality of the sample has greatly improvement. According to the result of this experiment, the best temperature range for forging AZ31 magnesium alloy gear is 300°C to 400°C .The forming load gradually reduced with the temperature improved. The microstructure evolution during the warm deformation was observed by optical microscopy (OM). It is demonstrated that the grain refinement happened during the deformation process.With Yada model which is amended, a finite element soft ware called SuperForm was used to analyze microstructure evolution in the precision forging process of AZ31 magnesium inner spur-gear. The results show that the microstructure evolution can be realized by computer simulation, which has the illustrative and high efficient advantages.
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