Investigation Of Laser Shock Processing On Corrosion Resistance Of AZ31Magnesium Alloy

In order to conserve energy and reduce emissions of pollutants, magnesium alloy has been applied widely in the field of automotive, aerospace, computer, electronic communication, national defense military and other fields as a kind of lightweight materials. But magnesium alloy product, as a mechanical component, is prone to pitting and stress corrosion cracking under damp environment. Stress corrosion and fatigue fracture for magnesium alloy under static load or alternating load occur easily, which lead to destructive accidents. Although magnesium alloy as medical materials has excellent biocompatibility and biodegradability, its application is limited by its fast corrosion rate.Laser shock processing (LSP) is a new way of surface strengthening treatment, which produces ultra high strain rate and induces the useful residual compressive stress at surface of material under a blast wave. The surface properties can be improved to delay crack nucleation, propagation. The fatigue life of materials, wear resistance and corrosion resistance of magnesium alloy can be enhanced by grain refinement.Aimed at the poor corrosion-resistance of magnesium alloy, the effects of laser shock processing on surface integrity of AZ31magnesium alloy, microstructure evolution, corrosion properties including electrochemical property, stress corrosion and corrosion fatigue, has been studied. Some important conclusions are listed as follows:(1) A model of grain refinement for AZ31magnesium alloy by LSP has been established. The effect of LSP on surface integrity of AZ31magnesium alloy and microstructural evolution has been investigated. The results show that grains of AZ31magnesium alloy are refined obviously by LSP. Mechanism of grain refinement includes three stages. At the early stage of LSP, three dislocation slip systems, including basal plane system, prismatic plane system and pyramidal plane system, are activated sequentially. The deformation mechanism is dominated mainly by dislocation. When dislocation slip is hindered at grain boundary and becomes more difficult to continue during LSP, parallel twins appear. Parallel twins divide the original coarse grains into finer twin platelets. Finally, high-density dislocation walls are formed because of tangled dislocation and twins each other and subdivided twins into sub-grains. Sub-grains gradually develop into large-angle grain boundaries, which lead to grains refinement at the surface layer of laser shock.(2) The effect of different laser impact times on corrosion resistance of AZ31magnesium alloy has been studied. The polarization curve and impedance spectrum curve of samples were determined by electrochemical workstation CHI660in3.5%NaCl solution at different laser impact time. The results indicate that corrosion resistance of AZ31magnesium alloy can be improved by LSP. But the effect of multiple impact times on corrosion resistance of AZ31magnesium alloy is similar to that of one impact time.(3) The stress corrosion behavior of AZ31magnesium alloy before and after LSP has been carried out in air and NaCl solution respectively by slow strain rate tension (SSRT) test. The stress corrosion sensitivity index decreased from0.803to0.69in3.5%NaCl solution before and after LSP respectively. The stress corrosion sensitivity of AZ31magnesium alloy has been improved significantly after LSP. The reason is that residual compressive stress from the surface of laser impact layer reduces tensile stress within the material, which leads to the decrease of the crack propagation rate. Meanwhile, grain refinement and high density dislocation tangled at the surface of AZ31alloy by LSP hinder dislocation slipping at the crack tip, which inhibit initiation and propagation of cracks. Therefore, the resistance of stress corrosion sensitivity for AZ31alloy by LSP increases significantly under the same external load.(4) The effect of LSP on corrosion fatigue of AZ31magnesium alloy has been analyzed. The fatigue life of AZ31magnesium alloy after LSP increased about38.25%in air and183.47%in solution respectively. The fatigue cycles in air are higher than that of the sample in solution. The Paris equations in different medium before and after LSP have been determined by curve fitting. Fitting curves show that the crack propagation rate of LSP samples is less than that of un-LSP samples in air and in3.5%NaCl solution. The residual compressive stress, grain refinement and higher corrosion resistance by LSP are main factors to hinder the crack propagation of AZ31magnesium alloy.(5) Based on the stress equilibrium differential equation and constitution equation, a relation between residual stress and impact load, poisson ratio and yield strength has been established. Finite element (FE) analyses during LSP of AZ31magnesium alloy h performed by using the ABAQUS software and residual stress field was obtained by the visual processing. The results from FE consist with the theoretical formula.