The Influence Of Annealing On The Microstructure And Texture Of Cold Rolled AZ31Magnesium Alloy Sheet

Due to its close-packed hexagonal (HCP) structure, Magnesium and its alloysshow low capability of deformation at room temperature. Meanwhile, during theforming process the magnesium alloy sheet is inclined to produce a strong basal texture,and this basal texture is extremely unfavorable to the subsequent rolling process and thefollowing forming process. Therefore, it is crucial to develope new processing methodsand heat treatment process to improve the plasticity deformation capacity of magnesiumalloys for the cost-effective magnesium alloys and the expansion of their application.Magnesium alloy sheet is generally produced by rolling, within which the mostcommonly used methods in industrial production include three kinds: rolling directlyfrom the ingot rolling, namely hot rolling (HR); extrusion the ingot then followedrolling, namely extrusion rolling (ER); and molten metal’s direct injection into the roll,namely twin-roll casting (TRC). And the obtained sheets are usually sustain basaltexture. This study aimed to explore the adjustments and improvements of themicrostructure and basal texture of the AZ31magnesium alloy sheet by cold rolling andannealing heat treatment to find one processing method that can effectively weaken thebasal texture of AZ31magnesium alloy sheet, and the effect of texture weakening ofthree kinds of AZ31magnesium alloy sheet, i.e, twin-roll casting (TRC), hot rolling(HR) and extrusion rolling (ER) were compared to provide guidance on technicalimprovement of magnesium alloy sheet in industrial production.The experimental results show that the single pass deformation reduction ofmagnesium alloy sheet is about10%during cold rolling, and excessive deformationreduction will cause the edge crack or central crack, due to the poor deformationcapability at room temperature of magnesium alloy sheet. The microstructurecharacteristics of the rolled sheet is shear zone and twinning, and the volume of shearzone and twinning will increase with the deformation reduction. The shear zone andtwinning During annealing, the recrystallization nucleation is mainly concentrated in theshear zone, twinning boundaries and inside the twinning. The recrystallization time isgetting shorter with the increased annealing temperature. After completion of therecrystallization, the microstructure is stable, with little change in grain size and basaltexture.Pre-annealing can not change the type of basal texture of the sheet, only with the intensity of basal texture increased; post-annealing can not change the type of basaltexture, but with the intensity of basal texture decreased. The texture weakening effectof different kinds of sheets during post-annealing firstly emerges at differenttemperature, with the TRC sheet for300℃, HR sheet for350℃, ER sheet for450℃.Also the weakening effect of the three kinds of sheets shows difference, with TRC sheetthe best, HR sheet second, ER sheet third. And the three kinds of AZ31magnesiumalloy sheets show different thermal stability. TRC AZ31sheet shows good thermalstability with small equiaxed grains, while abnormal grain growth occurs in the HR andER sheets.