Microstructure And Properties Of Surface Modification-plasma Electrolytic Oxidation Composite Layer On Al Alloy

Aluminum silicon multi element alloy is widely used in automobile engines fields as the hot end component and moving component.Aluminum alloys have good thermal and mechanical properties under high temperature and high-speed working environment.The thermal protection performance of Al alloy can be effectively improved by preparing the thermal protection ceramic layer on the surface of Al alloy,while the ceramic layer has limitations in improving the mechanical properties of Al alloy,especially the fatigue performance.After the surface strengthening of alloy,the matrix microstructure of the alloy can be improved,the fatigue and mechanical properties of alloy has improved and the compressive stress of alloy has introduced after surface strengthening.On this basis,the strengthened alloy was reconstructed by plasma electrolysis oxidation to prepare the thermal protection ceramic layer.This method has important research significance for improving the mechanical properties and thermal protection properties of alloy.In this study,the surface of alloy was strengthened by surface mechanical shot peening technology and laser remelting.On this basis,PEO process is used to improve the microstructure and crystal structure of strengthening layer,which can be analyzed by OM,SEM,XRD and TEM.Ansys was used to simulate the effect of primary Si morphology on the thermal stress distribution in the alloy.The residual stress of the alloy surface after shot peening was measured by stress analyzer.The electronic universal testing machine was used to characterize the fracture toughness and bending strength of alloy which strengthened by surface strengthening,Fatigue properties under room temperature,thermal insulation properties as well as thermal impact properties are analyzed by fatigue test machine,thermal insulation device and thermal impact test device.Though shot peening on the surface of alloy,we found that the surface of the alloy forms a deformation layer with a depth of 50-150?m.The average size of the primary Si is about 20?m,which is 50%lower than the matrix,and the roundness of the primary silicon is improved as well as its distribution becomes more uniform,the dislocation cells and the dislocation walls are formed on the surface of the alloy due to the dislocation aggregation,Under both of these factors which leads to the splitting and the refinement of primary Si,and disorder the grain orientation.the surface hardness is increased by about 35%-69%compared with the matrix,the surface residual stress is up to 180 MPa,the fracture toughness and flexural strength are improved,which are 4.7 MPa·m^1/2 and 275N/mm² respectively.The room temperature bending fatigue results show that the fatigue strength of the matrix is about 115 MPa,and the shot peening strength is 130 MPa.The fatigue life of alloy after mechanical shot peening is much higher than that of the matrix.On these foundations,The peening strengthening layer has reconstructed by PEO treatment and we found that the surface microspore size of the shot peening-PEO composite layer was reduced and its distribution becomes more evenly compared with the PEO coating.The flexural strength and fracture toughness of the composite layer are higher than that of the base PEO coating,which are 250 N/mm² and 4.5 MPa·m^1/2 respectively,the thermal insulation properties of the composite layer are not significantly changed,After 1200 times thermal shock experiments under 400?,the composite layer showed no macroscopic shedding,and the number of microscopic cracks was lower than that of the PEO coating.The fatigue strength of the shot peening-plasma electrolytic oxidation composite layer is 130 MPa,while the PEO layer is about 115 MPa.The composite layer has a higher fatigue life in the same loading mode.Laser surface remelting can improve the matrix microstructure of the alloy,refine the size of the primary Si,and improve the mechanical properties of the alloy.We found that the grain size of the microstructure is about 2-7?m on the surface of the alloy which was treated by laser remelting.and the irregular,coarse primary Si and the second phase structure are completely dissolved in the alloy.The surface hardness of the alloy is about 183HV,which is 56.4%higher than that of the base 117HV.The remelted alloy has a thermal insulation temperature difference of about 25?compared with the matrix,the fracture toughness and the flexural strength was increased by about 7%and 4%respectively.The surface of the alloy after laser remelting which was reconstructed by plasma electrolytic oxidation and reconstitution.We found that the film layer of the composite layer was denser and the pore distribution become more evenly compared with the PEO film layer.The flexural strength and fracture toughness of the composite coating at the laser power of 700 W were 269.3 N/mm ² and 4.86 MPa·m ^1/2,which was 9.8%and 12.8%higher than PEO ceramic layer.In this paper,the surface strengthening-PEO composite treatment of Al alloy improves the thermal protection properties and the mechanical properties,which making the alloy has better comprehensive performance in the hot end moving parts.In addition,it provides a theoretical basis for studying the evolution law of microstructure and properties of Al alloy composite strengthening layer.