Basic Research On Prediction And Control Of Edge-cracking Behaviors Of AZ31 Mg Alloy Sheet During Rolling

Edge-cracking is a significant defect of rolled Mg alloy sheet,its vertical depth directly determines the sheet loss of side in the subsequent finishing process and the final sheet yield,which has become an important index for evaluating the formability of rolled sheet.In this paper,the AZ31 Mg alloy is taken as the research object.By combining isothermal and constant speed hot compression physical simulation,rolling test,finite element analysis and residual stress test,based on the deformation and damage behaviors of AZ31 Mg alloy,the prediction of edge-cracking behaviors under different rolling conditions was carried out,and the edge-cracking control method based on tensile stress rolling was proposed.Based on the thermal compression physical simulation results,the mathematical relationships between the stress and strain,strain rate and temperature of AZ31 Mg alloy were analyzed,and a phenomenological constitutive model was proposed.The modified Arrhenius model based on the strain effect and the Zerilli-Armstrong(ZA)model for HCP metals based on the dislocation mechanism of heat activation were compared.The predictive ability of the three models was assessed by calculating the goodness-of-fit as well as the error in the relative instability zone.The results show that only in the small strain zone(??0.4)can the modified Arrhenius model have certain predictive ability.Both the ZA model and the phenomenological constitutive model can well describe the hot compressive deformation behaviors of AZ31 Mg alloy over a wide range of the deformation conditions,and the phenomenological constitutive model has a reasonable number of material constants,as well as a higher predictive accuracy.The rolling test combined with finite element simulation were used to analyze the macroscopic morphology and transverse cracking depth of the rolled sheet under different deformation conditions.Results show that the fracture mode of edge cracks is dominated by 45° crossed shear cracking in thelongitudinal(RD–ND)section,and the cracking depth along the TD increases as the rolling temperature decreases and reduction rate increases.The transverse effective stress distribution,transverse temperature distribution and damage distribution characteristics based on Freudenthal criterion were further characterized by finite element simulation.Combined with the results of rolling test,the critical damage values of fracture damage were obtained.Due to the increase of the temperature and the decrease of the effective stress along the direction of TD of the AZ31 Mg alloy sheet,the transverse cracking damage values based on Freudenthal criterion decreases linearly from the edge to the inside,and there is a clear linear relationship with ln(Zener-Hollomon parameter,Z).By introducing the critical damage values of fracture and Zener-Hollomon parameters,the Freudenthal criterion was reconstructed,and a criterion for predicting edge-cracking behaviors in Mg alloy sheet rolling was presented.The transverse distribution of the residual stress and the relation between the stress direction and the edge-cracking trend were analyzed by the residual stress test results.It can be found that the distribution of residual stress varies greatly in low temperature rolling of AZ31 Mg alloy sheets due to their poor plasticity,and the edge metal is subjected to greater pull stress during rolling,which makes the boundary more prone to cracking.As the temperature increases,the transverse distribution of residual stress is more uniform,and the reduction rate has little effect on the change of residual stress.The edge stress distribution of Mg alloy sheet can be changed by applying tension in the rolling process,so as to reduce the difference of the transverse stress distribution and regulate the residual stress,effectively improve the edge-cracking defect.The simulation results show that the damage value and crack depth are the smallest when the Mg alloy sheet is rolled with a small tensile stress of about 3MPa at 250?.At 350 ?,it is better to use higher tensile stress of about 12 MPa,so as to effectively reduce the damage value and avoid the occurrence of edge crack defects to a certain extent.