Research On Lowered Temperature Rolling Of ZK60 Plates And The Effect Of Texture On Formability

Wrought magnesium alloy sheets owing to their low density are widely used in many fields,especially in aerospace,automotive and electronics industries.However,it is hard to fabricate magnesium alloy sheets e fficiently because of high rolling temperature,small thickness reduction per pass and intermediate heat treatments in current rolling technology.Meanwhile,coarse grain and strong basal texture emerging in rolling sheets generally led to their poor forma bility.Due to the above reasons,the applications of wrought magnesium sheets had been severely restricted.Therefore,this article compared the microstructure and mechanical properties evolution during multi-pass lowered temperature rolling and different speed lowered temperature rolling of extruded ZK60 plates and focused on the impact of final rolling temperature and deformation on sheet microstructure and mechanical properties.According to the above study,the technique of effective microstructure refining and basal texture control formed,which was further applied to fabricate fine-grained ZK60 sheets with various texture states.Based on these sheets,the bending and stretch formability influenced by texture states were deeply investigated,which pro vided theoretical guidance for sheets fabrication and microstructure design in press forming field.By 3~5 passes lowered temperature rolling,fine-grained magnesium sheets of 7~9?m grain size were obtained even without intermediate annealing.Thickness reduction per pass of 30~40% and rolling temperature of 270~300 ? were conductive to achieve fine uniform microstructure.As the lowered temperature rolling proceeded,initial extruded(0002)//ED fiber texture gradually turned into(0002)//RD-TD basal texture,and this transition could be delayed with the increasing temperature and the decreasing pass number.Besides,different speed lowered temperature rolling could weaken the basal texture in rolling direction in certain level.Subsequently,the influence of final rolling temperature and deformation on microstructure and mechanical properties were investigated,which had shown that final rolling temperature at 200 ? and thickness reduction per pass of 31% could refine the grains to 5.2?m based on dynamic recrystallization.According to EBSD results,the deformed grains was <10-10>//RD texture component,while new dynamic recrystallized grains was <11-20>//RD texture component.With the rolling proceeded,dynamic recrystallized grains would turn to <10-10>//RD and <10-10>~<11-20>//RD texture component was formed.Correspondingly,the yield stress was the combined effect of grain refinement strengthening and weak basal texture weakening and the elongation was positively correlated to grain refinement and basal texture weakening.Samples taken along rolling direction(RD)and transverse direction(TD)were subjected to the bending process.Results revealed that,compared to RD sample with strong basal texture,the bending property in TD sample with weak basal texture could be improved by 32.2% and the minimal bending radius reached 1.51.The difference of bending property mainly stemmed from the different deformaiton mechanism: tension twin for inner region of RD sample;mainly basal slip along with prismatic slip for outer region of RD sample;mainly tension twin along with basal slip for inner region of TD sample;mainly basal slip along with tension twin for outer region of TD sample.Hence,weak basal texture magnesium sheets with basal plane deviated from bending direction were suitable for bending forming,and the more it deviated,the better bending property was.Besides,the difference in the texture forming induced by tension twin in inner and outer regions was investigated.Three ZK60 resulting sheets with same grain size but various texture states were successfully fabricated by a combination process of hot rolling,cold rolling and annealing treatment,and the basal intensity in the sheet plane was of anisotropy.The stretch forming tests suggested that weak ening basal texture could remarkably improve the stretch formability,and the forming height was increased by 80% from 5.2mm to 9.7mm.However,basal texture weakened only in single direction could not improve the stretch formability.The strain hardening and crack propagation analysis indicated,along basal texture weakening direction,the deformation ability and strain hardening ability were greatly improved,which led to excellent formability.However,stress concentration tended to occur along strong basal texture direction and then caused the early crack of sheets.Hence,weak basal texture magnesium alloy sheets with uniformly weakened basal intensity on the sheet plane were suitable for stretch forming.In addition,forming limiting diagram was furthe r used to investigate the forming mechanism of magnesium alloy sheet influence by texture.