Study On The Hot Extrusion Process Of ZK60 Magnesium Alloy Bar

As the lightest light metal structure material,Magnesium alloy has extensive application prospect in the business areas and is considered as eco-structural material in the 21st century.ZK60 magnesium alloy attracts many researchers because of highest deformation strength.However its poor plasticity capability hampers the deformation at room temperature which may hinder the further application.From the perspective of hot forming,this article studied on extrusion process parameters of the micro structure and mechanical properties of ZK60 magnesium alloy bars.Specific contents are as follows:1)Research on the flow stress change law of ZK60 magnesium alloy under different temperatures and strain rates,observed the micro-structure of thermocompression specimen so that we can obtain organization transformation at different temperatures and strain rates of ZK60 magnesium alloy thermocompression process.2)Based on the phenomenological constitutive model,sorted Arrhenius equation in the form of hyperbolic sine,which contained deformation activation energy,strain rate,deformation temperature and temperature compensation parameter Z.3)Determine the dynamic recrystallization critical strain of ZK60 magnesium alloy.Establish dynamic recrystallization kinetics model of ZK60 magnesium alloy and the model of the average grain size.4)The finite element software was employed to simulate the thermocompression process and verify the reliability of both the established constitutive model and recrystallization model.Nine groups of orthogonal technological parameters with extrusion ratio being 5,10,15 and extrusion temperature being 523K,573K and 623K were formulated to perform finite element modeling.The variation of both stress field and strain field during thermocompression process was simulated,accompanied with a comparison of the variation of temperature field and strain rate field,both of which were utilized to elaborately analyze the effects of deformation degree,strain rate and deformation temperature on the average grain size.The extrusion process with an extrusion ratio of 10 and extrusion temperature of 523K was finally determined to obtain the minimum average grain size.5)The extrusion mould was designed and processed,followed by the experiment on the combined nine groups of as-simulated technological parameters.The micro-structure difference between bar extrusion direction and vertical extrusion direction was observed.Room-temperature compression experiment was performed for the bar after extrusion,the strength of magnesium alloy after extrusion was thereafter compared and it was found that the variation of average grain size presented a similar tendency with that of finite element modeling.6)The micro-structures of bars under different extrusion processes were compared,the sample with an extrusion ratio of 10 and extrusion temperature of 573K demonstrated high recrystallization degree,additionally,and the growth rate of recrystallized grain under high temperature was almost the same with that of the grain after deformation and refinement.Uniform and fine isometric grains accounted for the majority of the micro-structure,thereby indicating that highest compression strength and rate can be yielded with the extrusion ratio and temperature being 10 and 523K,respectively.However,the micro-structure was relatively nonuniform despite of the minimum average grain size when extrusion ratio was 10 and extrusion temperature was 523K,probably due in large part to the high recrystallization degree and fine recrystallized grain.Therefore,best performance can be achieved for the group with an extrusion ratio of 10 and extrusion temperature of 573K,the compression strength and rate of which were determined to be 532MPa and 14.7%,respectively.