Study On Surface Plastic Deformation And Constitutive Parameters Inversion Of AZ31B Magnesium Alloy By Ultrasonic Rolling

Magnesium alloy is widely used in aerospace,automobile,communication electronics and military industry,owing to its low density,high specific strength and good mechanical processing properties.However,magnesium alloy is easily damaged during use,due to its low strength,poor wear and corrosion resistance,so it cannot provides adequate mechanical support.Ultrasonic rolling processing can effectively improve the comprehensive performance of materials and extend their service life.The effect of ultrasonic rolling can be reflected by the mechanical properties of the plastic deformation layer.Therefore,in this paper,ultrasonic rolling strengthening treatment of AZ31 B magnesium alloy is carried out to study the effects of different rolling passes on the microscopic mechanical behavior and parameters of the plastic deformation layer.The aim of this study is to provide basis for process optimization of ultrasonic rolling and establishment of constitutive equation of the surface strengthening layer of magnesium alloy.Firstly,the surface morphology and microstructure of AZ31 B magnesium alloy after turning and ultrasonic rolling are studied.The effects of different rolling passes on the surface morphology and thickness of plastic deformation layer are analyzed.The results show that the surface of AZ31 B processed by ultrasonic rolling is smoother than that processed by turning.The surface roughness Sa and Sq are reduced by a maximum of 75.14% and 74.62%,respectively.As the rolling passes increases from 1 to 3,the depth of plastic deformation layer increases from approximately 186?m to 335?m.The depth of the outer grain refinement zone increases from 56 ?m to 135 ?m,and the average grain size decreases from 6 ?m to 3 ?m.The depth of the transition twin region increases from 130 ?m to 200 ?m,and the average grain size decreases from 41 ?m to 36 ?m.The average grain size maximum decreases 44.61%,compared with the matrix grain size(about 65?m).Secondly,nanoindentation tests are performed on the surface layer of AZ31 B magnesium alloy using nanoindentation technique.Analysis of the experimental results reveal that the elastic modulus and nano-hardness along the diameter direction decrease from the material surface to the interior.The surface layers of the specimen after 1,2 and 3 passes of ultrasonic rolling are compared with the substrate.The nano-hardness at 45?m from the surface is increased by 62.24%,74.49% and 82.65%,respectively,and the modulus of elasticity is increased by 11.79%,16.09% and 17.29%,respectively.The nano-hardness at 145?m from the surface is increased by 24.49%,37.76% and 56.12%,respectively,and the modulus of elasticity is increased by 8.57%,6.8% and 11.74%,respectively.The nano-hardness at 245?m from the surface is increased by 8.16%,30.61% and 44.9%,respectively,and the modulus of elasticity is increased by 1.16%,4.56% and 9.73%,respectively.The nano-hardness at 345?m from the surface is increased by 4.08%,21.43% and 37.76%,respectively,and the elastic modulus is increased by 0.72%,3.78% and 6.55%,respectively.It indicates that the values of elastic modulus and nano-hardness at the same depth increase with the increasing rolling passes,and the influence layer of ultrasonic rolling strengthening becomes deeper.The load-displacement curves at the different depths of the surface layer reflect the degree of the plastic deformation at different depths under the same load,which provide experimental data for the inversion of the elastic-plastic constitutive equation of the gradient surface layer of AZ31 B magnesium alloy by ultrasonic rolling.Finally,the yield strength and strain hardening index at each depth of the plastic deformation layer of AZ31 B magnesium alloy are determined by the inverse analysis method combining the dimensional analysis with finite element simulation.Based on the power function constitutive equation,an expression for its stress-strain relationship is determined and input to the material properties for finite element simulation of nanoindentation.By comparing the simulation curves with experimental curves,the relative errors of maximum press-in load and maximum press-in depth are all within 1%,and the curves are in good agreement with each other,which verify the rationality of this inversion method.