Multi-steps Deformation Process And Mechanical Behavior Of As-cast AZ80 Alloy

Results have shown that plastic deformation can significantly improve the mechanicalproperties of magnesium alloy by grain refinement. But considering the weak formability andthe difficulty for forming the large-scale complex workpieces by one step, the workpieces areoften completed by multi-steps forming and the final microstructure develops fromprevious microstructure. Therefore, in order to improve the mechanical properties, it cancontrol the microstructure evolution by optimizing the process and parameters, whichhas great significance for engineering applications.The invariable temperature multi-steps forming and the decreasing temperaturemulti-steps forming were applied with the as-cast AZ80 alloy. To get the best mechanicalproperties, the effect of initial state, deformation temperature, the number ofintermediate heating and heating treatment on strength-toughness behavior was studiedmainly by means of tensile test, hardness testing, optical microscope, SEM and so on.Finally, the trial-manufacture of magnesium alloy AZ80 wheel was carried out by theoptimized process and parameters to verify the reliability of the result.The compression was carried out with the as-cast and homogenized AZ80, and then theanalysis and comparison of the microstructure and mechanical properties of different initialalloy state were completed. Homogenization treatment made the coarse second phasedissolve into the matrix. The solute could effectively promote the occurrence of dynamicrecrystallization, which increased the proportion of fine grains and improved the mechanicalproperties. These results showed that the homogenization treatment should be carriedout prior to the deformation.At 390℃, the results of the invariable multi-steps forming showed that the intermediateheating contributed to the uniform distribution of grain size and the improvement of thetensile properties owing to the lack of coarse grains. But the hardness was low. In the case ofthe drop of elongation being not obvious, the strengthening and hardening effect couldincrease further by optimizing the aging time of T5 heating treatment. For magnesium alloyAZ80, the optimal number of intermediate heating was three and optimal aging time of T5 heating treatment at 170℃was 12～14h. At last, the obtained mechanical properties is Rm≥340MPa, A≥9% and hardness≥HRB40. Meanwhile, the uniform distribution of grain size,the grain refinement mechanism under different compressed strain, the characteristics of theprecipitated phase were observed and explained in this paper.When the deformation temperature was lower thanβ-phase transition temperature in thedecreasing temperature multi-steps forming process,the coarse grains were difficult todeform owing to the pinning of precipitated phase, which led to stress concentration.Moreover, the coarse grains still existed in the matrix after deformation. The obtained tensileproperties were low due to the above reasons. The T6 heating treatment not only couldincrease the elongation by forming the uniform distribution of grain size, but also couldimprove its strength and hardness by precipitation. For AZ80, the mechanical properties afterT6 heating treatment (415℃+1.5h,170℃+11.2h) is Rm=320MPa, HRB31, A=9%, and it waslower than the optimal value of the invariable multi-steps forming.For the as-cast magnesium alloy, homogenization treatment should be carried out beforedeformation and deformation temperature should be higher thanβ-phase transitiontemperature. The uniform distribution of grain size was obtained by appropriately adding theintermediate heating process,and the high comprehensive mechanical properties were gainedthrough the parameters optimization of T5 heating treatment. For magnesium alloy AZ80, theoptimal process route obtained in the experiment was as follows: 415℃+16h forhomogenization treatment→deformation at 390℃and the number of intermediate heatingwas three→170℃+12～14h for T5 heating treatment.At last, the trial-manufacture of magnesium alloy AZ80 wheel was carried out by theabove optimal process route and parameters, and the high comprehensive mechanicalproperties were obtained. The result verified the reliability of the optimal process route andparameters.