Research On Corrosion Resistance Of Magnesium Alloy Strengthened By Combination Of Laser Shock Peening And Surface Phosphate Conversion

Magnesium(Mg)alloy,as an excellent lightweight material,is an ideal industrial material in many application fields.Laser shock peening(LSP)and surface phosphate conversion(SPC)are two effective surface treatment methods to improve the corrosion resistance of Mg alloys.However,with the progress of industrialization,complex and extreme service environment has put forward higher requirements for structural materials,and the single surface treatment technology is gradually difficult to meet the requirements of corrosion resistance.In this paper,the surface of AZ31 Mg alloy was strengthened by LSP and SPC.The feasibility and superiority of the composite anticorrosion process were evaluated by surface morphology characterization and corrosion experiments.The main research contents and results of this paper are as follows:Theoretical basis of LSP and SPC.Based on the precipitation mechanism of phosphate conversion coating(PCC)on Mg alloy surface,the factors affecting the quality of PCC were analyzed.Combined with the principle of LSP and the deformation mechanism of material with ultra-high strain rate,the mechanism of the response of LSP induced microstructure and stress field on the regulation of coating forming process was studied.The effect of LSP and SPC composite strengthening on the corrosion of Mg alloy was analyzed,and the synergistic anti-corrosion mechanism of composite strengthening was revealed.Effect of LSP on the surface morphology and microstructure of AZ31 Mg alloy.Confocal microscope,scanning electron microscope and X-ray diffraction were used to study the effect of LSP on the surface morphology,phase composition and microstructure of Mg alloy.The experimental results show that the greater the singlepoint laser energy,the greater the strain on the surface of the material.After LSP,no new phase is produced,but the surface roughness of Mg alloy is increased and residual compressive stress is formed at a certain depth.Massive LSP can induce crystal defects such as dislocation entanglement and twins on the surface of Mg alloy to achieve surface grain refinement.Effect of LSP pretreatment on formation and quality of PCC deposited on Mg alloy.The effects of LSP pretreatment on process,thickness and bonding strength of PCC were systematically studied by morphology observation,element analysis and cross cut test.The results show that LSP pretreatment can promote the deposition of phosphide and inhibit the dissolution of phosphide.When the preparation time is 30min,the amount of phosphide deposition is the largest.Compared with that of the untreated sample,the PCC deposited on the LSPed surface has fewer defects,larger thickness and stronger adhesion.Corrosion resistance of Mg alloy strengthened by LSP and SPC.The corrosion resistance of AZ31 Mg alloy with different anticorrosion treatments in 3.5%NaCl solution was studied by electrochemical corrosion and immersion corrosion experiments,and the corrosion mechanism was analyzed by combining the corrosion morphology.The results show that the plastic deformation layer formed by LSP provides good surface characteristics for the subsequent SPC,forming better quality PCC on the surface of the LSPed sample.In addition,the deformed layer at the bottom of the PCC develops grain refinement and introduces a certain depth of residual compressive stress.When the PCC is eroded off,the deformed layer can continue to provide anti-corrosion protection for the Mg alloy.Compared with the single anticorrosion method,the composite strengthening technology has more obvious protective effect and further improved corrosion resistance,and can better resist the corrosion damage of corrosion solution on Mg alloy substrate.