The microstructural evolution of AZ61 magnesium alloy predeformed by equal channel angular extrusion（ECAE） during semisolid isothermal treatment（SSIT） was investigated by means of optical metalloscopy and image analysis equipment.The process involved application of ECAE to as-cast alloy at 310 ℃ to induce strain prior to heating in the semisolid region for different time lengths.The results show that extrusion pass,isothermal temperature and processing route have an influence on microstructural evolution of predeformed AZ61 magnesium alloy during SSIT.With the increase of extrusion pass,the solid particle size is reduced gradually.When isothermal temperature increases from 530 ℃ to 560 ℃,the average particle size increases from 22 μm to 35 μm.When isothermal temperature is 575 ℃,the average particle size decreases.The particle size of microstructure of AZ61 magnesium alloy predeformed by ECAE at BC during SSIT is the finest.
Numerical simulation and experiment of thixoforming angle frame of AZ61 magnesium alloy were investigated.The results show that with the increase in punch displacement,cylinder billet firstly fills into die cavity of angle frame from feed inlet and plastic deformation occurs in touching region between billet and die cavity.After central thin wall of angle frame is created,semi-solid billet fills toward two edges.Lastly,complete plastic deformation occurs in billet,leading to complete filling of semis-olid billet.Effective strain,effective stress and billet temperature decrease with the increase in punch displacement.Effective stress decreases with the increase in billet temperature,die temperature and punch velocity.The optimal conditions decided by numerical simulation are as follows:die temperature of 450 ℃,billet temperature of 560 ℃ and punch velocity of 30 mm/s.Angle frame components with high mechanical properties such as yield strength of 225 MPa,tensile strength of 309 MPa and elongation of 21.8% and fine microstructure could be thixoformed successfully according to process parameters decided by numerical simulation.
Semisolid billet of AZ80 magnesium alloy was prepared by new strain induced melt activated (new SIMA) process and thixoforging experiment was performed.The results show that after as-cast AZ80 magnesium alloy is processed by equal channel angular extrusion, microstructure is refined well due to heavy dynamic recrystallization occurring in severe plastic deformation.Compared with semisolid isothermal treatment and conventional SIMA, semisolid billet with fine and spheroidal grains are achieved in new SIMA.Thixoforging process of semisolid billet prepared by new SIMA has many advantages such as good surface quality of final component, high ability to fill cavity and net-shape.The fine and spheroidal grains and high mechanical properties such as tensile strength of 298 MPa and elongation of 28% can be developed in final part thixoforged.
New strain induced melt activation（new SIMA） method was employed to prepare high-quality semisolid billet of AZ61 magnesium alloy.Optical microscopy and tensile test were used to study the microstructure and mechanical properties of the thixo-extruded component.The results showed that the optimal process parameters for achieving the complete filling status involved the applied pressure of 784 MPa,the pressure holding time of 90 s and the die temperature of 450 ℃.Compared to semisolid isothermal treatment,high mechanical properties such as the tensile strength of 300.5 MPa and elongation of 22% and fine microstructure were obtained in the thixo-extruded parts.With increasing the isothermal temperature and holding time,the tensile strength and elongation were increased firstly and then decreased.When the press pass was increased from 1 to 4,the tensile strength and elongation of the thixo-extruded parts were greatly enhanced and microstructure was refined obviously.
