Effect of laser shock peening on microstructure and fatigue crack growth rate of AZ31B magnesium alloy

The fatigue crack growth tests were introduced for studying the effect of Laser Shock Peening (LSP) on fatigue crack growth rate of AZ31B Magnesium (Mg) alloy. The surface layer structures of laser treated samples were analyzed by using transmission electron microscopy (TEM) and X-ray diffraction (XRD) method. The fracture surface morphologies of samples were characterized by scanning electron microscopy (SEM). Moreover, the residual stress and surface roughness were also examined. A modified incremental polynomial method was used to obtain the fatigue crack propagation rates of six samples each group under the same crack length. The results show that the nanometer grains (with an average size of 17.5 nm) can be generated in the surface layer by using the optimized laser parameters. Surface roughness decreased from 1.177 μm to 0.713 μm after LSP. The depth of compressive residual stress induced by LSP reached to about 0.8 mm from the top surface. Comparing with the original samples, the obviously lower fatigue crack growth rates for the LSP treated samples was observed due to a combination of grain refinement, residual compressive stress and the barrier effect of surface nanocrystalline layer.