Study On The Mechanical And Electrochemical Corrosion Resistance Of A291D Magnesium Alloy Strengthened By Laser Shock-ultrasonic Rolling

At the era of vigorously promoting resource conservation and lightweight manufacturing,magnesium alloys have been widely used in aerospace,automotive,defense and military fields due to their low density and high strength.The fatigue fracture and corrosion of the material mostly occur on the surface Therefore,in order to solve the problems of low absolute strength and poor corrosion resistance of magnesium alloy,it is significant to strengthen the surface of the magnesium alloy to improve its service life in complex service environments.Laser shock peening(LSP)is an emerging and efficient surface strengthening method,which can introduce deep residual stress and refine grains on the surface of the material.However,the uneven plastic deformation caused by it restricted the strengthening effect.Ultrasonic surface rolling processing(USRP)is a new type of ultrasonic surface strengthening method.As a post-treatment process of laser shock peening,it can effectively reduce the surface roughness after LSP and introduce greater residual stress on the surface.Combining the effects of the two processes can further enhance the performance of the magnesium alloy.In this dissertation,AZ91 D magnesium alloy was used as the research object,and it was treated by LSP with various parameters,and USRP was used as the post-treatment process.The effects of composited LSP and USRP on the surface integrity,tensile properties and electrochemical corrosion resistance of AZ91 D magnesium alloy were investigated.The internal mechanism of the surface properties of magnesium alloys on the tensile properties and corrosion properties of the materials were clarified.The main research work and conclusions are as follows:(1)The AZ91 D magnesium alloys were subjected to different energy LSP and LSP-USRP composite strengthening treatment.The surface morphology,microhardness,residual stress distribution and microstructure of the magnesium alloy after composite treatment were studied.After LSP with different energies,the surface roughness of magnesium alloy was greatly improved from 1.270 μm to 3.917 μm(3 J)and 5.440 μm(6 J).After LSP-USRP composite strengthening,the surface roughness was dropped sharply to about 0.9 μm.LSP introduced high residual compressive stress on the surface of AZ91 D magnesium alloy,which increased with the increase of LSP energy,reaching-74.4 MPa(3 J)and-109.8 MPa(6 J)respectively,After LSP-USRP composite strengthening,greater residual stress was introduced in the near surface layer of 0.4 mm,reaching-160.8 MPa and-192.7 MPa respectively.After LSP,the grain on the surface of the material was remarkably refined from 60-70 μm to 30-40 μm,and the grain refinement depth is 450 μm.After LSP-USRP composite strengthening,the grain in the range of 0-250 μm away from surface was further refined.The microhardness of the surface of the material treated by LSP increased with the increase of laser energy,increasing by 39.4%(3 J)and 53.6%(6 J)respectively.After LSP-USRP composite strengthening,the microhardness was further increased,increasing by 39.4% and 53.6% respectively.(2)The effects of different energy LSP and LSP-USRP composite strengthening on the tensile properties of AZ91 D magnesium alloy was investigated by room temperature tensile experiment.LSP improved the ultimate tensile strength of AZ91 D magnesium alloy,which was 8.0%(3 J)and 15.7%(6 J)higher than that of the untreated specimens.However the plasticity of the material was decreased and the fracture mechanism transformed from quasi-cleavage fracture to brittleness.After LSP-USRP composite strengthening,the ultimate tensile strength of the material was further increased by 18.9% and 27.4%.The plasticity of the material has been improved to a certain extent and the fracture mechanism also tended to be ductile compared to material that only treated by LSP.The effects of residual stress,grain refinement and lower surface roughness introduced by composite strengthening processes on crack initiation,propagation and tensile properties of AZ91 D magnesium alloy were analyzed.(3)The effects of different energy LSP and LSP-USRP composite strengthening on the corrosion properties of AZ91 D magnesium alloys were investigated by electrochemical corrosion experiments.After LSP,the self-corrosion potential of AZ91 D magnesium alloy was shifted positively,the current density was reduced,the impedance was increased,the corrosion degree of material surface was reduced,and the corrosion resistance was improved.After LSP-USRP composite strengthening,the self-corrosion potential is positively shifted and the current density is reduced continuously,the AC impedance is further increased,the material exhibits better corrosion resistance as well.The effect of surface strengthening layer on the corrosion resistance of AZ91 D magnesium alloy was revealed.