Equal Channel Angular Pressing Of Magnesium Alloys And Their Microstructural Evolution Of During Partial Remelting

As the lightest structural metal materials in the industry,magnesium alloy will be used more and more widely in the future.Semi-solid forming which has many advantages will become a potential technology for metal forming in 21 century. However,the grain size of semisolid formed magnesium alloy is still relatively larger. Among the grain refining technologies of magnesium alloy,equal-channel angular pressing(ECAP)is a promising method to be used in industry.The production of semisolid nondendrtic microstructure ingot is the key techque for semisolid forming. If a magnesium alloy is ECAPed and then isothermal heat treatment at a semisolid temperature,a semisolid microstructure with small and spherical primary particles that is suitable for thixoforming can be obtained.This can overcome the problem of the large grians in essential.Therefore,in this thesis,the effects of ECAP parameters, such as extrusion pass and processing route on the microstructure of the thixoformed (TF)AZ91D alloy were investigated.Simultaneously,the microstructural evolutions of the ECAPed traditional permanent mould casting(PMC)AZ91D alloy and a new magnesium alloy ZW21 developed by ourselves were mainly studied during partial remelting.The results indicated:After being ECAPed,the microstructure of the thixoforming AZ91D alloy was obviously refined.As the extrusion pass increased,the effect of grain refinement was further improved.A large numbers of short dendrites and equiax grains formed after being precessed for four passes.It was also found that the sequence of the refining effect from good to bad is in turn of the route B_c,B_A,C and A.In addition,the pinning effect of eutectic structure to the primaryαphase could mechanically break up and refine them.The distribution of eutectic structure in the PMC alloy was more uniform than that in the TF alloy,so the grain refining effect of the PMC alloy was better than that of TF alloy due to the improved mechanical saeparation and uniform deformation.During partial melting,the microstructure evolution of both the ECAPed AZ91D and ZW21 magnesium alloys could be divided into four steps,the initial coarsening due to the dissolution of interdendritic eutectic,structure separation resulted from the melting of the residual eutectic and the penetration of the firstly formed liquid into the recrystallized boundaries,spheroidization due to the partial melting of primary particles and final coarsening attributed to the merging and Ostwald ripening. Correspondingly,a series of phase transformations occurred in turnβ→α,α+β→L andα→L for the AZ91D alloy.But for the ZW21 alloy,the phase transformations of Mg₃RE₂Zn₃+Mg₁₂REZn+Mg₂Sn→αandα→L occurred.The variation of the primary particles with reheating time obeyed the LSW law,D_(t)³-D₍₀₎³=Kt,after the semisolid system was up to solid-liquid equilibrium state.With the increase of stir pass,the size of the primary particles decreased and their morphology tended to be more spherical. Simultaneously,the amount of liquid phase slightly increased because more amount of structure melted due to the increased energy stored in alloy.Based on the same reason,the microstructural evolution speed also increased with increasing stir pass. At a given stir pass,the semisolid microstructure of the alloy processed by route Bc was quite ideal for thixoforming while that of the alloy processed by route A was not completely suitable for thixoforming.Also because of the difference in the stored energy,the liquid amount of the former alloy was obviously larger than that of the later alloy.Proper increasing the reheating temperature was beneficial for obtaining an ideal semisolid microstructure because of the decreased merging coarsening tendency.The difference was that the microstructural evolution speed of the ZW21 magnesium alloy in the partial remelting was slower than that of the AZ91D alloy because the ZW21 contained some rare elements with large atom radius,the diffusion rate of the large radius atom was quite slower,which resulted in that the phase transformation rate decreased,and thus the microstructural evolution speed slowered.