Microstructure And Properties Of Shot Peening-plasma Electrooxidation Composite Layer On Aluminum Alloy

Al-Si alloy is widely used in hot end parts of automobile and tank engines because of its high specific strength and thermal conductivity low coefficient of thermal expansion and easy processing.With the development of engine to high speed and high performance,the mechanical properties and thermal protection performance of materials are more stringent.However,it is difficult for Al Si alloy to meet the increasing demand of working conditions.Surface treatment is one of the effective methods without changing the properties of the matrix material.However,a single coating can improve its performance to a certain extent,but in the face of complex working conditions,the improvement of comprehensive performance is still extremely insufficient.Composite surface treatment is the future development trend.In this paper,Al-12Si alloy was shot peened firstly.On this basis,the plasma electrolytic oxidation of Al-12Si alloy was carried out to prepare shot peening strengthened composite plasma electrolytic oxidation modified layer.The thermal stress and mechanical stress of precipitates in Al-12Si alloy were analyzed by Ansys simulation.The microstructure and morphology of shot peened Al-12Si alloy and composite layer were characterized by metallographic microscope,scanning electron microscope and transmission electron microscope The residual stress,hardness,tensile property,fracture toughness,bending strength,fatigue property and heat insulation property were characterized by X-ray residual stress analyzer,Vickers hardness tester,electronic universal testing machine,fatigue testing machine and self-made heat insulation device.By simulating the thermal stress and mechanical stress sensitivity of precipitates in Al-12Si alloy,the results show that the thermal stress of rod-shaped silicon is larger than that of elliptical silicon,and the thermal stress is the smallest.With the increase of Si phase size,the thermal stress of Si phase increases gradually.For other phases,the thermal stress of Al7Cu4 Ni is the largest,followed by Al3 CuNi,and that of Mg2 Si is the smallest.Under the static load of50 MPa,the mechanical stress produced by Al7Cu4 Ni s the largest,followed by Mg2 Si,Al3CuNi and silicon phase.Compared with the thermal stress and mechanical stress,the mechanical stress of Mg2 Si is greater than the thermal stress,while the thermal stress of Al7Cu4 Ni,Al3CuNi and Si phases is greater than the mechanical stress.As a hot end component,al3 cuni is the phase that affects the high temperature properties of Al-12Si alloy at high temperature,while al7cu4 ni is the phase that affects the room temperature properties of Al~12Si alloy at static load.After shot peening,Si phase and AlCuNi phase of Al-12Si alloy are refined in different degrees.The average area of primary Si phase decrease to s from 853 ?m2 523 ?m2.After shot peening,the surface structure of the material is refined and nanocrystals appear.With the increase of depth,the dislocation density decreases.After shot peening,there are four kinds of microstructure layers along the depth direction:nanocrystalline area on the surface,high density dislocation wall area,low density dislocation wall area,dislocation proliferation and dislocation entanglement area,matrix area.After shot peening,the hardness of Al-12Si alloy is increased by 27%-59% compared with the matrix,and the maximum residual compressive stress is 189.9MPa.When the diameter of shot peening is 0.6MPa and the pressure of shot peening is 0.4MPa,the bending strength is48.6% higher than that of the matrix.When the diameter of shot peening is 0.4mm and the pressure of shot peening is 0.4MPa,the fracture toughness is 51% higher than that of matrix.After shot peening,when the fatigue cycle is 2 × 107,the stress amplitude of Al Si alloy increases from 115 MPa to 130 MPa.When the projectile diameter is 0.4mm and the shot peening pressure is 0.4MPa,the median fatigue strength is 136 MPa.It can be seen from the fatigue fracture that shot peening makes the fatigue source migrate from the surface to the subsurface,and prolongs the crack propagation period.Compared with the plasma electrolytic oxidation ceramic layer,the surface of the ceramic layer prepared by shot peening and plasma electrolytic oxidation is smoother,the number of large area micropores decreases,and the average area of surface micropores decreases from11.2 ?m2 to 2.61 ?m2;when the shot peening diameter is constant,the diameter of surface micropores decreases with the increase of shot peening pressure.When the peening pressure is constant,the number of micro holes decreases with the increase of peening diameter.The thermal insulation performance of the composite layer is increased from 25? to 40 ?.At 120 MPa,the room temperature rotary bending fatigue increased from 5.6 × 105 cycles to 9.58 × 106 cycles.The results show that shot peening can refine the surface grains and make the plasma discharge easier,while the residual compressive stress counteracts the tensile stress during the plasma discharge process,making the composite layer more compact and uniform.The fracture toughness and bending strength of the composite coating are 53% and 32% higher than those of the plasma electrolytic oxidation coating.