Hot Forging Feasibility And Microstructure Evolution Mechanism Of AZ80-Ce Magnesium Alloy

Magnesium alloys are widely used in automotive,aerospace and other fields due to their excellent properties such as low density and high specific strength.However,their poor formability and mechanical properties at room temperature limit their further application in the field of high performance.The hot forging process takes advantage of the characteristics that magnesium alloy can improve its formability at high temperature,and can produce parts whose size and shape reach or close to the finished product,and can improve the mechanical properties of magnesium alloy.However,the magnesium alloy in the hot forging process has the characteristics of poor fluidity,uneven deformation structure,small forging temperature range and sensitivity to strain rate,which seriously affects the forming and mechanical properties of complex forgings.In order to solve this problem,the uniaxial compression,isothermal forging,extrusion+forging and holding time before forging of AZ80 and AZ80-Ce magnesium alloys were studied in this paper.The purpose of uniform deformation and improvement of mechanical properties of magnesium alloy forging at high strain rate was realized,and the evolution law and mechanism of magnesium alloy microstructure under different forging process conditions were revealed.The effects of Ce addition on the true stress-true strain curve,constitutive equation and hot processing map of AZ80 magnesium alloy were studied by thermal simulation experiments,and the hot deformation behavior and microstructure evolution mechanism of magnesium alloy AZ80 and AZ80-Ce were revealed.The effects of stress-strain state,forging temperature and Ce element addition on the microstructure and mechanical properties of forgings were studied by isothermal die forging process,and the influence mechanism of stress-strain state on the mechanical properties of magnesium alloy was clarified.The free forging experiments of AZ80-Ce magnesium alloy with different grain sizes and textures obtained by different extrusion processes were carried out.The effects of different initial microstructure states on the microstructure and mechanical properties of AZ80-Ce magnesium alloy forgings were studied.It is clear that pre-extrusion treatment can improve the formability of forgings and reveal the mechanism of pre-extrusion treatment to improve the mechanical properties of forgings.Finally,by comparing the effects of different holding time on the microstructure and mechanical properties of AZ80-Ce magnesium alloy die forgings,the influence of different holding time on the temperature field of billet was studied,and the mathematical model of’holding time before forging’was constructed,which laid a theoretical foundation for improving the forging formability of magnesium alloy and improving the comprehensive mechanical properties of forgings.The main conclusions are as follows:（1）The addition of Ce formed Al₄Ce phase in the microstructure of AZ80 magnesium alloy,which reduced the concentration of Al atoms in theα-Mg matrix and the degree of lattice distortion.The hot deformation activation energy of AZ80 magnesium alloy decreased from136.068 k J/mol to 127.096 k J mol,and the area of instability zone in the hot processing map decreased,thus improving the hot deformation ability.In addition,due to the existence of Al₄Ce phase,the movement of dislocation is hindered,and the critical stressσ_c of dynamic recrystallization of AZ80 magnesium alloy is increased,which promotes the dynamic recrystallization of magnesium alloy through particle stimulated nucleation（PSN）mechanism.（2）In the process of magnesium alloy die forging,in the low stress and high strain parts of die forgings,it is more inclined to refine grains by continuous dynamic recrystallization,which is beneficial to the improvement of yield strength and compressive strength of forgings.In the high stress and low strain parts,the twinning mechanism is more inclined to coordinate the deformation,which is beneficial to the improvement of the elongation of the forging.Reducing the die forging temperature can reduce the grain size and improve the yield strength.The addition of Ce can reduce the precipitation ofβ-Mg₁₇Al₁₂ phase and hinder the dislocation movement during the deformation process to promote uniform forming,thereby improving the comprehensive mechanical properties of the die forgings.（3）The initial microstructure state has heredity to the forging microstructure:the grain size of the initial microstructure is coarse,and the grain size of the forging microstructure is also coarse;the non-fiber basal texture of the initial extruded state will evolve to the fiber texture after forging,but the characteristics of the extruded non-fiber basal texture are generally retained.The pre-extrusion treatment can obtain fine initial grain size,thereby improving the formability of the forgings,and also improving the yield strength and compressive strength of the magnesium alloy forgings.（4）By establishing the heat conduction temperature field model of the blank:（?）,the’holding time before forging’can be quickly determined.When the holding time is short,the forging is easy to form sub-grain structure,with the highest yield strength and tensile strength,but its elongation is low,cracks appear after die forging,and can not be completely filled.Under the appropriate holding time,the billet is heated to the forging temperature.After forging,the grains are fine and the comprehensive mechanical properties are high.However,with the extension of the holding time,the yield strength,tensile strength and elongation of the sample gradually decrease.