Study On Hot Deformation Behavior Of ZE21B Magnesium Alloy And Hot Extrusion Process Of Micro Tube Billet Based On DEFORM-3D

In recent years,the incidence of cardiovascular diseases has increased sharply,and the implantation of vascular stents is an effective method to treat cardiovascular diseases.Magnesium alloy has become a hot research spot for biodegradable vascular stents due to its degradability,good biocompatibility and suitable mechanical properties.Vascular stents are mainly obtained by hot extrusion with cold drawing to obtain micro-tubes,followed by laser cutting.Therefore,the quality of magnesium alloy tube billet will directly affect the subsequent processing and the quality of magnesium alloy vascular stents.In order to optimize the extrusion process of magnesium alloy microtube billets and reduce the experimental cost,the thermal deformation behavior of ZE21B magnesium alloy and the effect of billet hot extrusion process on the quality of microtube billets were systematically investigated based on DEFORM-3D simulation in this paper,In this study,Gleeble thermal simulation compression experiments were carried out at different temperatures of 598 K,623 K,673 K and 698 K with strain rates of0.005 s^-1,0.01 s^-1,0.1 s^-1and 1 s^-1using homogenized ZE21B magnesium alloy bar.Heating and holding experiments at different temperatures and times were performed.The constitutive equation of ZE21B magnesium alloy and dynamic recrystallization models were constructed by linear and multivariate nonlinear fitting methods.Using DEFORM-3D software,the hot extrusion process of micro-tube billets was investigated and experimentally verified.The optimal billet hot extrusion process parameters were determined by orthogonal simulation.The experimental results indicate that the true stress-true strain curve of ZE21B magnesium alloy exhibits a typical dynamic recrystallization（DRX）feature.The rheological stress increases rapidly with the increase of strain to the peak and then decreases,and finally stabilizes.The grain size of ZE21B did not change too much when the holding time is within 1h.When the holding temperature exceeds 500℃,the grain grows rapidly.The simulation results of the billet positive extrusion process with extrusion temperature 380℃,extrusion speed 6 mm/s and friction coefficient 0.1 show that the distribution of effective strain in the formed tube billet is uneven,which is consistent with the experimental phenomenon of bamboo cracking.The formed tube billet undergoes complete dynamic recrystallization,and the unformed part gradually increases from the outer part of the billet to the center.The volume percentage distribution is consistent with the experimental results.The average grain size of the tube billet is 4.08μm,and the experimental one is 4.66±0.26μm,which is in agreement with the simulated ones.The simulation results for different extrusion parameters show that the maximum temperature of the tube billet increases with higher extrusion temperature,the grain size increases,the dynamic recrystallization rate increases,the maximum die temperature increases,and the maximum principal stress decreases.The friction coefficient increases,the maximum temperature of the tube billet increases rapidly,the grains grow rapidly,the maximum temperature of the die increases,and the maximum principal stress increases.When the extrusion speed increases,the maximum temperature of the tube billet increases,the recrystallization speed decreases,the grain size does not change significantly,the maximum temperature of the die decreases,and the maximum principal stress increases.Therefore,lowering the extrusion temperature,reducing the friction and increasing the extrusion speed are beneficial to obtain a tube billet with finer grains and prolong the die life.