Study On Fatigue Properties Of AZ31 Magnesium Alloy By Supersonic Shot Peening And Micro Arc Oxidation Composite Treatment

Micro arc oxidation(MAO)could effectively improve the corrosion resistance of magnesium(Mg)alloy and strengthen the surface of the sample,but the residual tensile stress was produced in the Mg matrix.The residual compressive stress could be developed on the surface of the material by deformation strengthening.Whereas the Mg is the close-packed hexagonal(HCP)structure with small deformation capacity,which is easy to cause surface damage,and even affect the discharge process during the coating growth.In this paper,the residual compressive stress was enhanced on the surface of Mg alloy by using the supersonic shot peening(SP)technique,followed by MAO surface treatment.Further,the bending fatigue tests were carried out on the composite treatment samples.Combined with the fracture analysis of the fatigue sample,micro morphology of the coating,and the coating/matrix interface observation,the influence rule and mechanism of shot peening intensities and the coating thickness on fatigue properties of the composite treatment of AZ31 Mg alloy were studied.The results showed:(1)The residual compressive stress was introduced on the surface of AZ31 Mg alloy by supersonic shot peening.With the increase of SP intensity,the residual compressive stress on the surface was increased first and then decreased,and the compressive stress reaches the maximum value 92.3 MPa when the intensity was 0.5 mmN.The reason for the improved compressive stress was attributed to the changes in alloy microstructure during the deformation process.On the contrary,the drop in residual stress was caused by the stress release of surface defects such as holes and micro cracks on the surface,resulting from the poor deformation capacity of Mg alloy.(2)The AZ31 Mg alloy produced a large number of net shape-dislocations in the crystal when deformed,which was different from the morphology of the dislocation cell in the normal metal deformation.In the early stage of plastic deformation of AZ31 Mg alloy,it was dominated by dislocation slip.And the middle stage,the number of twins increased gradually,and the {1012} twinning was the main deformation mode.Besides,in the late stage of plastic deformation,the dislocation and twinning competed with each other.A great number of dislocation piled up in the twin boundary,and the number of lamellar twins also increased.The interaction of the twins between different directions divided the grain interior into several large and small blocks.(3)The fatigue limits of AZ31 Mg alloy were reduced by composite treatment.Firstly,when the thickness of the ceramic coating was 20 ?m,as the SP intensity increased,the fatigue limits of the composite samples were decreased by 2.5%,6.9% and 12.9%,compared with the MAO sample,respectively.Secondly,when the SP intensity is 0.15 mmN,the fatigue limits of the composite sample were maintained at about 74.0 MPa with the increase of the ceramic coating thickness,which was still lower than the MAO sample with the same coating thickness.It can be seen the main factor affecting the fatigue performance of the composite sample was the difference in the SP intensity.Fatigue fracture analysis showed SP pre-treatment increased the number of fatigue crack sources of MAO samples,and with the increase of SP intensity,the crack source number increased gradually.Besides,when the stress changed from high to low level,the fatigue crack source would change from single source to multiple sources.(4)The residual stress state of the matrix was compressive stress,but after SP pre-treatment,due to the uneven deformation of the sample surface,the MAO ceramic coating was over-growth in the local area.A stress concentration zone was formed at the interface between the coating and matrix.It could be simulated as an elliptical-notched sample,which was considered as the potential site of fatigue crack source.When the thickness of the MAO coating was the same,the equivalent notch radiuses of the composite samples were lower than that of the MAO sample,which indicated the stress concentration effect of the sample was aggravated,resulting in a decreased fatigue strength.When the SP intensity was 0.15 mmN,the notch radius didn't change obviously with the thickness of the ceramic coating,so the fatigue limit was basically the same.