Study On High Temperature Plastic Deformation Mechanism And Extrusion Forming Process Of ZK60 Magnesium Alloy

ZK60 magnesium alloy has poor fluidity and difficulties in controlling extrusion process and profile quality.It is necessary to have an in-depth understanding of its high-temperature rheological properties and the evolution of microstructures.In addition,with the increase of extrusion velocity and extrusion stroke,severe plastic deformation will generate a lot of heat,resulting in a great increse in the extrusion temperature field,leading to a coarse structure,thereby reducing the comprehensive properties of the profile by extrusion.In view of the above problems,ZK60 magnesium alloy was used to conduct experiment and numerical simulation,and systematically researches its high-temperature thermal rheological behavior and the method to control extrusion temperature field.The development of this subject is helpful to provide a theoretical basis for the extrusion of high-strength magnesium alloys,and to further enrich the theoretical system of magnesium alloy extrusion.The main research and conclusions of this paper are as follows:(1)Based on the isothermal thermal compression test data,an Arrhenius constitutive model based on strain compensation was established to realize the accurate prediction of the flow stress of ZK60 magnesium alloy.By analyzing the microstructure under different thermal compression deformation conditions,it is found that temperature,strain,and strain rate have a great effect on the microstructure evolution of ZK60 magnesium alloy.High temperature and low strain rate promote the increase of recrystallized grain size and volume fraction;low temperature and high strain rate are conducive to the formation of smaller and larger number of recrystallized grains,but the recrystallized volume fraction is at a lower level,and its conducive to the formation of twins.Non-basal slips will lead grains to rotate,causing their orientation to deviate from the compression direction,resulting in the {0001} basal texture splitting.When twins are formed,non-basal slips are active in the twin region,and the climbing and crossing of prismatic slip and pyramidal slip can promote the nucleation of recrystallized grains.(2)A transient analysis model of the ZK60 magnesium alloy flat die extrusion was established,and combined with the extrusion experiment,the influence of heating billet with a temperature gradient and extrusion velocity on the flat die extrusion temperature field and the structure of the profile were studied.The study found that reducing the extrusion velocity and introducing heating billet with a temperature gradient can effectively control the flat die extrusion temperature field,which is beneficial to reduce the temperature of the profile at the die exit,improve the uniformity of the profile cross-sectional temperature distribution,and thus help to obtain a refined grain structure.When the extrusion temperature is higher than 400℃,continuous dynamic recrystallization and discontinuous dynamic recrystallization are the main mechanisms.Continuous dynamic recrystallization tends to occur at lower temperatures,and the new formed dynamic recrystallization grains are randomly distributed and deviate from the matrix.With more active basal slip at low temperatures,it will cause the reducuction of<10-10>//ED texture strength.In addition,reducing the extrusion velocity and introducing heating billet with a temperature gradient,the small grain size and larger internal residual strain were brought,which will help to improve the tensile strength of the profile.(3)The transient analysis model of porthole die extrusion of ZK60 magnesium alloy unequal-thickness square tube was established,and the temperature field change law of split die extrusion and its control method were studied.The study found that reducing the extrusion velocity,introducing heating billet with a temperature gradient,and reducing the mold temperature all help to improve the extrusion temperature field.Heating billet with a temperature gradient can reduce the exit temperature of the profile and improve the uniformity of the profile cross-section temperature,but it will cause the breakthrough of the extrusion force to increase.The above trend is more obvious when the extrusion velocity is higher.Reducing the mold temperature helps to reduce the extrusion temperature field,and the increase in the extrusion force is small,but it will cause the extremely non-uniformity of the cross-sectional temperature distribution of the profile.The stepped temperature heating of the casting rod and the decrease of the mold temperature can effectively control the extrusion temperature field,reduce the temperature unevenness of the profile section caused by the decrease of the mold temperature,and ensure that the increase in the extrusion force is within a small range,which is beneficial to obtain Ideal profile structure performance.