The Influence Of Trace Ca And Annealing On Microstructure And Forming Performance Of Mg-Al-Mn Alloys

Magnesium alloys have been widely favored by the automobile industry due to their low density,high specific strength,good shock absorption and other advantages,especially the wrought magnesium alloy sheets which exhibit the wider application prospects,since they can be formed by the secondary stamping to meet the production requirements of different shapes and sizes body structures.Mg-Al-Mn alloys have become one of the hot spots in the field of wrought magnesium alloys owing to the higher strength,better ductility and corrosion resistance,etc.However,similar to the other wrought magnesium alloys,the strong basal texture can be produced during the plastic deformation,which seriously restricts the improvement of the forming performance,thereby limiting their applications and developments in the automotive field.Therefore,how to weaken the basal texture of the Mg-Al-Mn alloys becomes the key to enhance their forming performance,which is of great significance for expanding the applications of magnesium alloys in the automobile industry.This paper was based on the Mg-5Al-0.3Mn alloy,the influence of trace Ca and annealing on the microstructure and forming performance of the alloy were studied by means of the metallographic observation,SEM and EDS analysis,XRD analysis,EBSD analysis,tensile and Erichsen tests.The alloy composition and annealing process were optimized to improve the formability of the sheet at room temperature.The main research results are as follows:?1?Trace Ca addition did not result in the formation of new phases in the as-cast Mg-5Al-0.3Mn alloy,but with the increase of Ca content,the microstructure was gradually refined,the reticulate and coarse bone-like?-Mg₁₇Al₁₂ phases distributed at the grain boundary gradually changed to the fine bone-like and granular shape,and their distributions in the Mg matrix became more discrete and uniform.The same rolling process was carried out on the Mg-5Al-0.3Mn-xCa alloys.It was found that the addition of Ca has promoted the formation of {1011} compression twins and shear bands during the hot rolling and also caused the basal pole deviated from theND,which resulted in the obvious decrease of the basal texture intensity.The weakening of the basal texture was mainly ascribed to the {1011} compression twins and shear bands promoting the nucleation of the recrystallization grains.In addition,the decrease of c/a due to the addition of Ca can also contribute to the basal texture weakening.?2?The Erichsen value of the as???olled Mg-5Al-0.3Mn alloy sheet was increased obviously with the addition of trace Ca,but when the additive amount was higher than 0.1wt.%,the rising trend became smooth.The same annealing treatment was carried out on the Mg-5Al-0.3Mn-xCa alloys.It was found that the basal textures were further weakened and the forming performance were significantly improved after the annealing.With the increase of Ca content,the yield strength???_s,the elongation S and the plastic strain ratio f were decreased gradually,but the strain hardening index n and the Erichsen value IE were increased gradually.The comprehensive properties of the alloy reached the best when the Ca content was 0.3wt.%,the yield strength???S,the elongation???,the???,the??? and the Erichsen value IE were 187 MPa,22.3%,0.25,2.46 and 4.18 mm,respectively.?3?The annealing process has a significant effect on the microstructure and forming performance of the Mg-5Al-0.3Mn-0.3Ca alloy sheet.With the increase of the annealing temperature and the annealing time,the proportion of the recrystallized grains and the average grain size were increased gradually,the Erichsen value was increased firstly and then decreased.When the annealing temperature was 400? and the annealing time was 1.5h,the Mg-5Al-0.3Mn-0.3Ca alloy sheet has the highest Erichsen value as well as the best comprehensive properties.Under this annealing process,the yield strength???S,the elongation???,,the???,the f and the Erichsen value IE of the Mg-5Al-0.3Mn-0.3Ca alloy sheet were 187 MPa,22.3%,0.25,2.46 and 4.18 mm,respectively.