| Magnesium alloy is a lightweight green engineering material,widely used in transmission parts,shell manufacturing and other important fields.However,the poor hardness and corrosion resistance of magnesium alloy limit its application scope in industrial engineering field to some extent.In this paper,the surface modification treatment of magnesium alloy by scanning electron beam was used to study the microstructure and mechanical properties of the modified magnesium alloy.In this paper,a mathematical physical model of the scanning electron beam AZ31 B magnesium alloy is established.Based on the three-dimensional transient heat conduction equation,a finite element model of the temperature field of magnesium alloy is established,and the simulation of the temperature field of additional thermally conductive solids is carried out to achieve a better grain refinement effect.Based on the thermoelasticity theory,a finite element model of the scanning electron beam stress field was established based on the simulation results of the scanning electron beam temperature field coupled with thermal stress.Finally,the distribution laws of temperature field and stress field during the modification process were derived,and the modification results of scanning electron beam AZ31 B magnesium alloy under different process parameters were experimentally verified to study the change laws of cross-sectional microstructure,surface morphology and mechanical properties.The results show that the whole scanning electron beam heating process is divided into three processes: down-beam stage,mid-stabilization stage and closing stage.The temperature reaches 1010 K at 0.1s of downbeam,which exceeds the melting point of AZ31 B magnesium alloy,and the surface material is in molten state.Compare the simulation results of temperature field of additional heat-conducting solid in different ways,the surface temperature of magnesium alloy with pressed sheet is the lowest,and it rises to 1004 K in0.3s,which has a greater rate of temperature rise and fall,and it is easy to form micro-melt polishing effect.Comparing the stress field simulation analysis under different currents,it can be obtained that the scanning area will show the phenomenon of convex deformation,and the maximum deformation appears at the closing end of 0.141 mm,and the surface stress value reaches 320 Mpa when the current is 6m A,which exceeds the strength limit of magnesium alloy,and cracks appear in the modified layer.After the scanning electron beam modification,the cross section of AZ31 B magnesium alloy was divided into three regions:molten layer,heat affected zone and matrix.Under the non-equilibrium solidification,the distribution density of Al elements in the modified layer is larger than that in the matrix,and a large number of fine and uniformly distributed hard phase ?-Mg17Al12 precipitates on the grain boundaries,and the grain refinement phenomenon occurs in the modified layer,and the grains change from coarse dendrites of 200?300?m to columnar and cellular crystals of1?5?m.The cross-sectional microhardness showed a pattern of increasing first and then decreasing,and the maximum hardness of AZ31 B magnesium alloy reached 108.7 HV at 60?m from the surface in the ring scanning mode;the maximum hardness of AZ31 B magnesium alloy reached 109.3 HV at 200 ?m from the surface in the concentrated downbeam mode.The mechanical properties of AZ31 B magnesium alloy were significantly improved after the modification,and its surface microhardness,wear resistance,?-Mgg The surface microhardness,wear resistance and volume fraction of ?-Mg phase increased and then decreased with the increase of heat flow density,and the surface microhardness was maximum when the beam flow was 5m A.The surface microhardness increased from 67.08 HV to 95.2 HV at a scanning speed of 300 mm/s,and the best wear resistance was obtained. |
PDF Full Text Request