Research On Microstructure, Texture And Mechanical Properties Of ZK61Mg Thin Sheets Produced By Rolling

Wide application of magnesium alloy sheets is limited by the immaturemagnesium alloy plate manufacturing technology at present. Therefore, exploring arolling processing technology with high efficiency and high quality for magnesiumalloy sheets is of great significance to promote the development of magnesium alloy.In this study, as-cast and as-extruded billets of ZK61magnesium alloy were used asthe starting materials to study multi-pass rolling process with decreasingtemperature and microstructure evolution, for the purpose of reducing productioncost and improving mechanical properties at room temperature. Further study wasfocused on the effect of rolling temperature and thickness reduction on theperformance of hot-rolled sheets with2.0mm thickness to design a warm rollingprocess for fabricating thick ZK61magnesium alloy sheets. In addition,precipitation during rolling process as well as relationships between microstructureand mechanical properties was also studied.As-cast ZK61magnesium alloy billet was subjected to6-pass rollingdeformation with30%thickness reduction per pass. The final ZK61magnesiumalloy sheets with grain size7.2μm attained an improved elongation of24%. It wasfound that30%thickness reduction applied on the rolling of as-extruded billet stillhad a good applicability. The effects of rolling parameters including rollingtemperature and reduction, on the microstructure and mechanical properties ofZK61magnesium alloy sheets rolled at160-300℃with thickness reductionranging from20-40%were investigated. Microstructure evolution of the as-rolledsheets was controlled by the rolling reduction and rolling temperature. Grain size ofZK61magnesium alloy was refined effectively at rolling reduction of30%-40%and rolling temperature of230-250℃. Homogeneous dynamic recrystallizedmicrostructure with an average grain size of3.2μm was fabricated, and theelongation was enhanced to about24%by grain refinement. Then, through coldrolling process, fine-grained ZK61magnesium alloy sheet with a thickness of0.4-0.7mm, yield strength of more than220MPa, tensile strength of about280MPa,and enhanced elongation of25%was successfully fabricated. Further annealingtreatment of180℃for2h can be used to eliminate the strength anisotropy and toimprove the elongation to above30%. This is of great significance for theapplication of magnesium alloy sheets in the field of stamping.Precipitation during rolling process was promoted by hot rolling under300℃ due to the effect of strain-induced precipitation. Particles of needle-like I type androd-like II-type were primarily distributed in ZK61magnesium alloy, and graduallytransformed into with rod-like II-type and spherical III-type. Subsequent agingtreatments suggested that a peak aging state occurred at about175℃for0.5h.Thus, multi-pass hot rolling process is beneficial to shortening the production cycleas a whole.Mechanical properties of sheets are closely related to microstructure. Grainsize effect on strength can be described by Hall-Petch equation. Tensile testssignificantly reveal two different Hall-Petch relations with the variation of texture,corresponding to the mechanical response. One is multislip-yield Hall-Petch fittingwith a texture dependent slope ranging from120to246MPa·μm^1/2and a slightdecreasing friction stress of about142MPa with basal texture weakening, which isstrongly dependent on combination of activated slip systems rather than thedominated; the other is twinning-yield Hall-Petch fitting, showing a uniqueempirical equation ofσ25+3001/2y≈d, indicating an insensitivity of twinning-yield strength to texture. This could be ascribed to the lower values in {1012}twinning and basal slip, and an unchanged but lower friction stress in term of theprofuse twinning with the onset of yielding. Tensile tests along transverse directionat room temperature revealed that post-uniform strain may be controlled by theenhancement of increasing strain-rate sensitivity and the deterioration of grainboundary sliding in grain refining, resulting to a maximal fracture strain at the grainsize of about6μm. Although the uniform strain is not associated with grain size, itcan be related exclusively to the texture patterns and increased with basal intensityweakening. The texture dependence of uniform strain was characterized by anexponential relationship on estimated Taylor factor deduced from the Kocks-Mecking hardening model. Additional results and discussions presented on theeffect of twinning deformation suggest that uniform strains can also be enhanced bytwinning deformation via retarding strain hardening.