Superficial Stress Reduction By Laser Annealing: Preliminary Study

Most of the manufacturing processes introduce a locked-in stress state in the material that can have strong effect on the lifetime of the component. The presence of residual stress must be conveniently taken into account during the design process and minimized when possible. Thermal processing of the component is also possible in order to relax residual stress. If a proper thermal cycle is designed for the specific material it can be possible to reduce and even eliminate the state of stress. In this paper we explored an alternative approach to obtain stress relaxation based upon the focusing on the surface of an Al 5068 aluminium alloy of a high power diode laser (HPDL) source. In this way it is possible to obtain a localized effect of stress relaxation limited to the neighbourhood of the area irradiated by the laser beam. Final results, however, depend upon several factors, such as the adopted wavelength, the reflectivity of the material at that specific wavelength, the power of the HPDL source, the mode distribution and the duration of the irradiation. In this work temperature field distributions are calculated for the case of interaction with a point source, a Gaussian source and a double-Gaussian source. For these two, in particular, the influence of the HPDL power was studied. Results obtained by the double-Gaussian distribution analysis were used to set the proper experimental parameter for the experimental study. Finally, repeated annealing cycles were performed and the sample and the obtained stress reduction was evaluated by X-Ray diffraction.