Study On Microstructure And Properties Of Magnesium Alloy Shot Peening Enhanced Composite Plasma Electrolytic Oxidation Film

Mg-Gd-Y alloy has excellent properties,such as light weight,high specific strength and good vibration damping,which has a wide application prospect in industrial fields.However,the corrosion resistance and mechanical properties,especially fatigue properties of Mg-Gd-Y alloy are still insufficient,which would restricts the development prospect of Mg-Gg-Y alloy,and surface modification technology is helpful to improve the comprehensive properties of Mg-Gg-Y alloy.This paper passes that shot peening introduces residual compressive stress to refine the surface prganization and increase mechanical properties of Mg-Gd-Y alloy.On this basis,plasma electrolytic oxidation(PEO)ceramic layer was prepared to increase the corrosion resistance of Mg-Gd-Y alloy,the comprehensive properties of Mg-Gd-Y alloy were improved.In this paper,the mechanism of surface nanocrystallization by shot peening was investigated,and the mechanism of fatigue crack initiation and fatigue crack propagation by shot peening on Mg-Gd-Y alloy were revealed.The effects of shot peening on the microstructure of Mg-Gd-Y alloy and PEO ceramic layer were investigated by metallographic microscope(OM),scanning electron microscope(SEM)and transmission electron microscope(TEM).Image Pro Plus software was used to maket statistical analysis on the area change of Mg₂₄Y₅phase on the surface and porosity change of PEO composite film before and after shot peening.X-ray residual stress analyzer,microvickers hardness tester,room temperature rotary bending fatigue testing machine and electrochemical workstation were used to analyze the residual stress,hardness,fatigue performance and corrosion resistance of the Mg-Gd-Y alloy.The conclusions are as follows:After shot peening,the average area of larger Mg₂₄Y₅phase decreases,and the area of?-Mg phase decreases with the decrease of shot diameter and the increase of shot peening pressure,the needle-like phase with parallel distribution presents different degrees of small Angle bending.With the decrease of pellet diameter,the smaller the surface nanocrystals are,the crystallographic orientation difference increased.After shot peening,the TEM morphology of mg-Gd-Y alloy at different depths showed that the gradient transition layer was formed,which was composed of four regions,including matrix region,deformation region,sub-crystal region and nanocrystalline region from outside to inside.In the end,nanocrystals with large grain orientation difference,large quantity and small size were formed on the surface of Mg-Gd-Y alloy,and there were deformation defects in the nanocrystals.The formation of surface nanocrystals is related to twin delivery,dislocation slip and rotational recrystallization.After shot peening,the maximum residual compressive stress of Mg-Gd-Y alloy substrate was 152 MPa,and the surface hardness value increased by 50.5%-65.8%.The fatigue strength of Mg-Gd-Y alloy is increased by 13.6%.It can be seen from the fatigue fracture that the fatigue crack of Mg-Gd-Y alloy was germinated on the surface of the specimen due to the asymmetry of the grain binding around the surface.After shot peening,fatigue cracks germinate on the subsurface due to the transformation of the crack germination mechanism into a slip cleavage mechanism.Before blasting,small fatigue cracks of the Mg-Gd-Y alloy specimen expand along the dissociation surface at the fracture surface,the surface of the fracture in the transient fault area before the induction of Mg-Gd-Y alloy was not shot peened,and the brittle fracture characteristics such as large dissociation plane and long secondary crack can be seen.After shot peening,due to the increase in the dislocation of Mg-Gd-Y alloy,the crack tip encounters dislocation plugging,resulting in small fatigue cracks being expanded by shearing ligament merger.After shot peening,the instantaneous fault zone of Mg-Gd-Y alloy can be seen in a small area of dissociation planes,secondary cracks and tough sockets,showing brittle-tough fracture characteristics.As the shot peening pressure increased,the number of ligaments increased.Compared with single PEO ceramic layer,the micropore area and surface porosity of PEO ceramic layer were decreased obviously.With the decrease of shot diameter and shot peening intensity,the number of micropores in the composite film increases but the area of single micropores decreases.When the same pressure is applied to the sample,the fatigue cycles of the sample after shot peening are larger,the fatigue cycles of the composite film layer are smaller,and the fatigue cycles of the single PEO ceramic layer are smaller.The fatigue cycle increases with the increase of peening pressure.It is found that as the diameter of the shot decreases and the shot peening pressure increase,the corrosion resistance of the composite film also increased.