Investigation On Microstructures And Mechanical Properties Of Mg-Sn-Zn-Cu Alloys And Numerical Simulation Of Hot Deformation

With the increasing applications of magnesium(Mg)and its alloys in the field of transportation and aerospace,the development of low-cost high-performance wrought Mg alloy is particularly important.In addition to well-studied Mg-Zn,Mg-Al and Mg-RE alloy systems,Mg-Sn system has great potential to expand the application range of Mg alloy and reduce the cost.In this work,the influence of Cu content on the microstructures and properties of Mg-8.0Sn-2.0Zn alloy was studied,as weel as the corresponding strengthening mechanism;Based on the thermal compression test,the microstructures and flow stresses of Mg-8.0Sn-2.0Zn-0.5Cu alloy were analyzed and strain compensated hyperbolic sine constitutive equation,dynamic recrystallization(DRX)kinetics model and dynamic material model(DMM)were presented.Meanwhile,in order to study the effects of second phase particles,hot deformation parameters and initial grain size on the DRX evolution of Mg-8.0Sn-2.0Zn-0.5Cu alloy,the grain growth CA model and the DRX CA model for single phase material and multiphase material were constructed.The finite element method(FEM)subroutine was developed with the purpose of combination the consititutive equation and DRX kinetics model,the microstructure evolution and its difference during hot compression deformation process of AZ31 and 5052 alloys were predicted.The main results can be summarized as follows:? With the increase of Cu content,the refinement of Mg-8.0Sn-2.0Zn-xCu alloy is not significant.In addition to Mg2Sn phase,MgZnCu phase is formed by adding Cu element in Mg-8.0Sn-2.0Zn alloy.When the conten of Cu is low,MgZnCu phase exists in the form of lamellae.When the Cu addition exceeds 1.0 wt%,the block-like MgZnCu phase appears.MgZnCu phase with different morphology are face-centered cubic structure,and lattice constant a=0.7169 nm.? With the increase of Cu content from 0 to 0.5 wt%,the strength of as-extruded Mg-8.0Sn-2.OZn-xCu alloy increases,the plasticity decreases and the maximum ultimate tensile strength(UTS)is 388.4 MPa,the yield strength(YS)is 365.9 MPa and the corresponding elongation(EL)is 5.8%.The continuous increasing Cu content leads to increasing in plasticity and decreasing in strength:1.0 wt%Cu alloy has an EL of 8.1%,a UTS of 360.2 MPa and a YS of 337.7 MPa.The EL of 1.5 wt%Cu alloy is 10.4%,the UTS is 345.5 MPa and the YS is 313.0 MPa.? The addition of an appropriate amount of Cu is favorable for the grain refinement of the as-extruded Mg-8.0Sn-2.0Zn-xCu alloy:when the Cu content is 1.0 wt%,the average grain size is 3.3 ?m,and when the Cu content is 1.5 wt%,the grains be coarsened.When the Cu content exceeds 1.0 wt%,with the increase of Cu content,the size of Mg2Sn and MgZnCu phase particles in the as-extruded alloy is gradually increased,and the spacing between the particles,as well as the particle size,increase gradually,that is to say,the number of small particles decreased and the second phase strengthening effect weakened,which result in decreasing in strength and increasing in plasticity.Furthermore,with the increase of Cu content,the texture of the as-extruded alloys decreases first and then increases.? Based on the hot compression test of Mg-8.0Sn-2.0Zn-0.5Cu alloy,the effect of different thermal deformation conditions on the flow stresses of the alloy was studied.And the numerical simulation of the strain compensated hyperbolic sine constitutive equation was carried out.Results show that the constitutive equation parameters a,n,Q and InA of the alloy vary with the strain.With the increase of the degree of deformation,the recrystallization activation energy of the alloy decreases first and then increases:the activation energy is 164.68 KJ/mol,144.31 KJ/mol and 148.62 KJ/mol while strain is 0.2,0.6 and 0.9,respectively.The DRX critical stress and steady state stress for Mg-8.0Sn-2.0Zn-0.5Cu alloy can be simplified as ?c=0.9?p,?sat=1.07ap.And the DRX kinetics model of Mg-8.0Sn-2.0Zn-0.5Cu alloy can be expressed as(?).? The main thermal deformation mechanism of Mg-8.0Sn-2.0Zn-0.5Cu alloy is discontinuous dynamic recrystallization when Z parameter is small.In contrast,the main thermal deformation mechanism of Mg-8.0Sn-2.0Zn-0.5Cu alloy is continuous dynamic recrystallization while Z parameter is large.The DMM of Mg-8.0Sn-2.0Zn-0.5Cu alloy suggested that the preferred hot processing parameters of the alloy are 300-320?,0.001-0.01S-1 and 390-440?,0.001-1s-1.? The grain growth CA model for single phase and multiphase material were established.The simulation results show that the grain boundary migration CA model driven by grain boundary energy and curvature can accurately reect the law of grain uniform growth during the heating process.The average grain size,grain size distribution and grain geometry are all agree with the experimental results.The numerical simulation results show that the second phase particles can suppress the grain boundary migration and refine the structure,so that the grain size distribution within the alloy has a wide range of physical phenomena.? The DRX CA model,which is driven by dislocation,was established to simulate the recrystallization nucleation and growth process of AZ31 and Mg-8.0Sn-2.0Zn-0.5Cu malloys,respectively,and the simulated flow stresses consistent with the hot compression test results.The CA simulation results of Mg-8.0Sn-2.0Zn-0.5Cu alloy reappeared that the nucleation of the recrystallized grains distribute along the original grain boundary,and the second phase particles act as the recrystallization nucleation sites,as well as inhibiting grain boundary migration and thus play an important role in the renement of alloys.The simulation results show that the larger the size of the second phase,the weaker the effect of nucleation sites and pinning grain boundary migration,and the DRX grains become bigger,which agrees with the extrusion test results of Mg-8.0Sn-2.0Zn-0.5Cu alloy.The microstructure characteristics of the studied alloys are consistent with the simulated results,indicating the DRX CA model is credible to predict the typical DRX behavior.? Based on the strain compensated consititutive model and DRX kinetics model for AZ31 and 5052 alloys,the corresponding subroutine UHARD of ABAQUS were developed,respectively.The changes of stress field,strain field and temperature field in dierent regions of cylinder sample during hot deformation process were calculated.Based on the FEM results and DRX CA model,the DRX evolution and grain characteristics of dierent regions during the hot compression deformation of AZ31 and 5052 alloys were predicted,and the plastic deformation and microstructure evolution of dierent regions were studied.The results show that the simulation results are in agreement with the distributions of dierent characteristics of hot deformed AZ31 and 5052 samples.