Study On Deformation Behavior And Microstructure Evolution Of AZ80M Magnesium Alloy In Solid And Semi-solid State

Magnesium alloy has been widely used in railway transportation,aerospace,3C electronic and electrical products and automobile due to its excellent mechanical properties,electromagnetic shielding property and light weight.The solid forming process?forging,extrusion,etc.?and the liquid forming process?casting?are some main method to produce magnesium alloy components.The former process has high production cost due to a lot of machining for complex parts,while the latter process can form complex parts,but the mechanical properties are poor.Semi-solid process is an efficient near-net forming technology,which combines the advantages of solid forming and liquid forming,but the mechanical properties may be slightly lower than solid forming.At present,some researchers have applied the“semi-solid”and“solid”composite forming technology to aluminum alloy,that is,semi-solid thixoforming and solid-state plastic forming for complex and simple parts respectively.The aluminum alloy parts can obtain excellent mechanical properties on the basis of near-net forming.Therefore,the“Solid-Semisolid”composite forming technology for magnesium alloy is proposed in this paper.In this paper,some key problems in the application of“solid-semisolid”composite forming technology in magnesium alloy were studied.The AZ80M alloy extruded bar was taken as the raw material.Firstly,the microstructure evolution of AZ80M semi-solid billet prepared by resistance furnace isothermal heating and electromagnetic induction heating was studied.Then,the deformation behavior of AZ80M alloy at solid state and semisolid state was analyzed by isothermal compression experiment,and the constitutive models of peak stress were established.Finally,the deformation mechanism in different temperature ranges was discussed based on the deformed microstructure.The main results are as follows:The average grain size,shape factor and liquid fraction increase with the temperature and holding time increasing during semisolid isothermal treatment.Both of the Ostwald ripening and grain coalescence results in grain coarsening.The coarsening rate is 510.38um³s^-1 and 621.92 um³s^-1 at 580?and 590?,respectively.The average grain size of solid particles in semi-solid microstructure obtained by electromagnetic induction heating is fine,and the shape factor is small.The liquid pool is mainly curved strip distribution.The true stress-strain curves of AZ80M alloy compressed at solid state temperature can be divided into three stages:work hardening,dynamic recrystallization softening and stable state;the true stress-strain curves of AZ80M at semi-solid temperature also can be divided into three stages:increased load,liquid phase softening and steady flow.The flow stress decreases with the deformation temperature increasing.The Arrhenius equation is used to fit the peak stress constitutive equation of AZ80M alloy in the solid and semisolid.The calculated results show a good agreement with the experimental measurements.The activation energy?=289 kJ/mol?for semisolid deformation of AZ80M alloy is larger than that for solid deformation?=198 kJ/mol?.Some obvious cracks and peripheral collapse are found in the lateral surface of the sample compressed at semisolid temperature.Partial recrystallized grains appeared in the solid-state deformed microstructure.The lower the strain rate is,the more complete the recrystallization is.Generally,the volume fraction of liquid phase increases with the strain rate decreasing or the deformation temperature increasing.Then the deformation mechanism changes from plastic deformation of solid grains to the change of solid spherical shape and size with liquid flow.As a result,the number of spherical solid particles in the microstructure increases gradually.