A Study On The Effects Of Laser Shock Processing On Stress Corrosion Cracking Susceptibility Of AZ31B Magnesium Alloy

Mg alloys have been widely used in automotive body structures, aerospace, computer or mobile phones components due to its excellent properties such as low densities, high specific strength and good shock absorption ability. However, Mg is a kind of very active metal and Mg alloys are easy to be corroded in most of environments, especially as structural materials, Mg alloys suffer from the combination actions of mechanical stress and corrosion medium, and stress corrosion cracking (SCC) is easy to take place. SCC is a common local corrosion and its destructive effect is very great.The AZ31B magnesium alloy was taken as the research object in the present work. The samples were treated by laser shock processing (LSP) to induce compressive residual stress to improve corrosion cracking resistance. The analysis methods, such as OM (Optical Microscopy),XRD (X-Ray Diffraction),SEM (Scanning Electron Microscopy) and SCC test were carried out during the research process. The main contents of this paper are listed as follow:A refined microstructure with an average grain size of 5.8μm after four laser impacts can be obtained. The average grain size is further homogeneous after four laser impacts. The grain size refinement can be attributed to the high strain rate plastic deformations which are induced by the LSP. From the XRD patterns of AZ31B Mg alloy before and after LSP treatments with different laser impacts, it can be concluded that the diffraction peaks strength of (002)α-Mg phase for the LSP sample is weaker than those of the untreated sample.Residual stress distribution as a function of depth was charactized by X-ray diffraction technology. It was observed that with increasing the number of laser impacts, the compressive residual stress near the surface increased, whereas the increasing rate of the residual stress decreases with increasing the number of the impacts during LSP. The depth of the compressive residual stress induced by LSP exceeded 0.8 mm from the treated surface.SCC test in 1 wt. % NaOH solution showed that LSP retarded the SCC initiation and growth in AZ31B Mg alloy. From the SEM micrograph of the SCC fracture surface, it can be concluded that the morphologies exhibits obvious brittle fracture from the cleavage facets. The SCC initiation in AZ31B Mg alloy involves highly localized dissolution. Hydrogen lowering the decohesion energy of the slip plane, the crack opens along this plane under the normal stressThe results show that LSP introducing a compressive residual stress filed on the sample surface is selected to prove that LSP prevents the SCC initiation and retards the SCC growth.