Effects of Laser Shock Peening on Surface Roughness and Residual Stress of AA 7050-T7451

Surface treatment techniques such as laser shock peening (LSP) represent a consolidated strategy to induce the presence of compressive residual stresses beneath the surface of various metallic alloys. However, surface roughening caused by the process must be monitored, since it may negatively affect fatigue life, resulting in earlier crack initiation. For this reason, the interplay between the key factors affecting both the surface roughness and the development of residual stresses was experimentally examined. The surface roughness was assessed through the determination of roughness average and roughness total height values, whereas residual stresses were assessed using the x-ray diffraction technique and hole drilling method to obtain information about the stress status of the components. Higher values of laser power density resulted in higher values of maximum compressive stress both along scanning and stepping direction and residual stresses remained approximately constant up to a depth of about 0.7-0.8 mm beneath the treated surface. No substantial difference was observed between the residual stress components in the two main LSP directions. Moreover, it was found that the approach to use lower values of nominal power density together with a higher number of layers allows obtaining the same stress condition determined by the use of the highest value of nominal power density but a significantly lower impact on the surface roughness.