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

Al-Si alloy is widely used in the 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,higher requirements are put forward for the mechanical properties and heat resistance of Al-Si alloy.Surface modification is the most effective way to improve its comprehensive properties.But for Al-Si alloy,the single surface treatment method is difficult to meet the requirements of comprehensive properties.Composite surface treatment is an effective way to improve the comprehensive properties.In this paper,laser shock peening(LSP)surface modification technology is used to improve the matrix structurr and mechanical properties of Al-Si alloy.On this basis,plasma electrolytic oxidation(PEO)technology is used to reconstruct the surface modification layer by in-situ growth of ceramic layer on the surface of aluminum alloy,so as to further improve the heat resistance of Al-Si alloy,enhance the film-based bonding strength,and establish the microstructure and property evolution of the composite modified layer.The microstructure and morphology of Al-Si alloy before and after LSP were studied by means of metallographic microscope,scanning electron microscope and transmission electron microscope,and image pro plus was used to make statistics on the changes of microstructure in the alloy.The results show that the coarse and irregular primary silicon particle size in the surface layer of Al-12Si alloy decreases from 40 ?m to 20 ?m after LSP,and the distribution is more uniform.Some primary silicon also occurs fragmentation and separation.The average particle size of Al7Cu4Ni phase and A13CuNi phase decreases by about 50%after laser shock,and some of the Al7Ci4Ni phase are aggregated.The phase of Mg2Si and Al2Cu changed little before and after laser shock.With the increase of laser energy and impact times,the change of phase size and morphology is more obvious.At the same time,laser shock strengthening can significantly reduce the crystal surface spacing and disrupt the crystallographic orientation of the alloy.The effect of laser energy and impact times on the surface structure of Al-12Si alloy during LSP was studied by HRTEM.The results show that the change of laser energy and impact times makes the structure of the alloy surface change obviously.With the increase of laser energy,the structure of the alloy surface is mainly dislocations,and the dislocations are clustered,With the number of times of impact increased,the more density of the dislocation occurs at the alloy surface,and its distribution pattern changes from parallel structure arrangement to a large number of random tangled dislocations.Along the direction of impact depth,TEM and HRTEM were used to study the effect of laser shock strengthening on the microstructure of Al-12Si alloy at different depths.The results show that there are four kinds of transition layers after LSP:the mosaic structure of nanocrystalline and amorphous,the high-density dislocation zone,the low-density dislocation ring zone and the matrix.With the increase of impact depth,dislocation density increases first and then decreases,and grain size increases gradually.The phase change of Al-12Si alloy before and after laser shock strengthening was studied by X-ray diffraction(XRD).The results show that compared with that of Al-Si alloy matrix,the Al2Cu peak and Mg2Si peak of Al-Si alloy after laser shock strengthening have no obvious change,and the position of Al3CuNi peak and Al7Cu4Ni peak have obvious Bragg broadening phenomenon,which shows that the grain size of the alloy has obvious change after laser shock peening.With the increase of impact energy and impact times,the change of grain size is more obvious.In the process of laser shock peening,a new phase of AICu3 appears,which is not coherent with the matrix,and the intensity of the diffraction peak of AICu3 increases with the increase of laser shock energy and times.The changes of residual stress and hardness of the alloy before and after LSP were studied by using X-ray residual stress analyzer and hardness tester.The results showed that 143 MPa residual compressive stress was introduced into the alloy after LSP.With the increase of impact energy and impact times,the residual compressive stress on the alloy surface gradually increased and finally stabilized.The hardness test results show that the hardness of Al-12Si alloy can reach 145 HV after LSP,and the surface hardness of the alloy increases with the increase of laser energy and impact times.LSP can introduce about 250 ?m deformation layer into the alloy.With the increase of the depth of the deformation layer,the hardness gradually decreases,and finally the hardness is the same as that of the matrix.Through the study of the structure and morphology of the composite layer,it is found that the diameter of the discharge micropore of the composite ceramic layer is decreased,the porosity and the number of microcracks on the surface of the coating are decreased,and the thickness of the coating is increased obviously.TEM observation of the PEO ceramic layer strengthened by LSP from the depth direction shows that there are three different structures,namely,the lamellar Al2O3 ceramic layer at the top of the film,the regular Al2O3 crystal structure in the middle of the film and the Al2O3 nano-particle close to the substrate.Compared with the pure PEO structure,the film-based interface of LSP-PEO composite ceramic layer has about 2 nm thick amorphous layer.Moreover,the distance between the crystal faces corresponding to the two ends of the interface tends to increase after LSP.According to the XRD analysis of the coating,the ceramic layer is mainly composed of ?-Al,?-Al2O3 and?-Al2O3.With the increase of the thickness of the ceramic layer,the content of ?-Al decreases gradually,the content of ?-Al2O3 increases slightly,and the content of ?-Al2O3 increases obviously.The thermal protection performance of LSP-PEO composite ceramic layer before and after LSP was studied by using self-made thermal insulation device and thermal shock device.It was found that LSP-PEO composite ceramic layer has more excellent thermal insulation performance and thermal shock performance than pure PEO.With the increase of laser energy,the thermal insulation performance of the ceramic layer increases gradually,and there is no obvious crack in the thermal shock test at 400?.