TABLE OF CONTENTS The surface morphology of metals has a great effect on fatigue behaviour. Many investigations related to the surface morphology of laser-peened materials have been performed with scanning electron microscopy (SEM) observations and roughness measurements. When no protective laser-absorbent coating was used on the material surface, LSP can cause severe surface melting and vaporisation, particularly in aluminium (Clauer et al., 1976; Gerland and Hallouin, 1994). This can result in resolidified droplets and craters leading to very rough surfaces. These problems can be solved with energy absorbent coatings as discussed in previous sections. However, there has been no systematic fundamental understanding in the LSP literature of the interaction of the microstructure with laser-induced shock waves and the resulting changes in the microstructure.
The LSP process is not a thermal process but a mechanical process for metallic materials and it is accompanied by significant changes in microstructures and phases. These changes have been investigated by means of transmission electron microscopy (TEM), SEM and X-ray diffraction analysis. Microstructural changes induced by LSP have been related to the laser parameters and the treatment conditions of the alloys. In laser peened aluminium alloys such as welded 5086-H32, 6061-T6 (Clauer et al., 1976) and 2024 T62 (Zhang and Yu, 1998), it was observed that the dislocation density increased significantly. High dislocation densities were also a prominent microstructural feature in low carbon steels after LSP (Peyre et al., 1998b; Atshulin et al., 1990). LSP of Hadfield manganese steel was found to induce...
