Effects of severe shot peening on the surface state of AW 7075 Al alloy

X-ray diffraction methods were used to study the surface layer properties of AW 7075 aluminium alloy before and after application of two different air blast shot peening treatments (similar Almen intensity and different coverage). Results have revealed important information about the strengthened surface layers. An increase of coverage does not result to any increase of the compressive residual stress field in the surface layer. However it plays the determining role in the process of the grain refinement by shot peening. After application of shot peening with 8.3 N/100 % parameters, the surface crystallite size decreased from an initial value of 80 nm to 60 nm. When 9.6 N/650 % parameters were applied, the crystallite size further decreased to a value of 51 nm. The relaxation process of the compressive residual stresses during the cyclic loading was observed; almost half of the residual stresses were lost, but the FWHM parameter, representing the crystallite size and dislocation density, remained more or less unchanged.