Effect Of Multiple Direction Forging And Annealing On Microstructure And Mechanical Properties Of AZ80A Magnesium Alloy

Abstract:Effect of multiple direction forging and annealing on microstructure and mechanical properties of AZ80A magnesium alloy was systematically investigated using OM, SEM, EDS, XRD and TEM. The deformation characteristics of multidirectional forging and the mechanism of grain refinement in the deformation process was studied. The microstructure evolution of magnesium alloy AZ80A under different temperatures and times has been observed. The process of annealing static recrystallization kinetics equation was analyzed. The main conclusions are presented as follows:(1) Effect of forging process and deformation passes on microstructure and properties of AZ80A magnesium alloy was studied. The results show that after multidirectional forging, equiaxed grains was relatively uniformly distributed in all directions because of the orientation of deformation zone changing along with the load axis and staggering mutually during deformation process. While the microstructure and properties were severely anisotropic and high density streamlined microstructure was obtained after unidirectional compression deformation. Therefore, multidirectional forging is more favorable to grain refinement of alloy. In multidirectional forging, the microstructure became finer and more homogeneous and the strength was increased with increasing passes and strain. The microstructure was the finest and the strength was much higher than undeformed alloy after7passes of forging. However, when increasing deformation to9passes, repeated heating lead to recovery and softening of alloy during the deformation, giving rise to larger grain size and better ductility with slight change of strength(2) Grain refinement mechanism of AZ80A magnesium alloy in the multidirectional forging process:First, in the early stage of deformation, deformation band was easy to from in the coarse structure of as cast alloy, at the same time, few recrystallized grains appeared in the deformed zone. It’s mainly caused by discontinuous dynamic recrystallization; then, as the change of loading axis, deformation band staggered mutually and the coarse grains were broken into tiny several tiny grains by deformation band gradually, which provided preferential area for the dynamic recrystallization nucleation, in favor of the further grain refinement; Finally, with the increase of strain, continuous dynamic recrystallization occurred, leading to a fully equiaxial and refined grains.(3) With the extension of annealing time, the recrystallized grain increases gradually, at the time of3600s, the volume function of recrystallization is about88%at150℃annealing heat treatment; when the annealing temperature is175℃, the fully static recrystallization has occurred and some grains have grew up; at the annealing temperature of200℃, the volume fraction of recrystallized grains were about70%with the annealing time of360s, some of the recrystallied grains have grew up with the time of900s; at the temperature of250℃, the recrytallized grains have grew up after360s. Based on the kinetics analysis of static recrystallization process, the full recrystallization time at150℃,175℃,200℃and250℃are4160s,1387s,485s and85.6s, respectively. The recrystallization energy for multiple forged AZ80A magnesium alloy is71.48KJ/mol, and calculated results are consistent with the experimental results.