DISLOCATION MECHANISM OF SURFACE MODIFICATION FOR COMMERCIAL PURITY ALUMINUM AND ALUMINUM ALLOY BY LASER SHOCK PROCESSING

Surface modification experiment of the commercial purity aluminum (alpha-Al) and Al-Cu-Mg alloyed aviation aluminum alloy 2A02 by laser shock processing (LSP) was implemented. The surface strengthening effect of both the target materials was investigated from dislocation mechanisms of microstructural response by means of TEM method. The results show that the strengthening effect of the two kinds of materials by laser shock processed is significantly different. The strengthening mechanism of alpha-Al by laser shock can be attributed to the multiplication of a large number of dislocations. With the increase of the impact number of laser shock and the degree of deformation, the new-generated dislocations will pile up and interact with the forest dislocations, and the dislocation lines will gradually evolve into waved-like, or wind into dislocation tangles and dislocation networks. But the hardness curve of the laser shocked (alpha-Al) will fast and linearly decline due to Bauschinger effect (BE) and stress wave damping. The laser shock strengthening mechanisms of the aging hardened aluminum alloy 2A02 can be summarized to the enhancement of the matching between the elastic energy of dislocations with the ultra-high energy of laser shock processing due to the higher matrix strength and the dislocation-pinning effect of large number of dispersed precipitates, as well as the complex dislocation networks in between the precipitates constructed by the dislocations induced by laser shock. The matrix strengthened by laser shock processing and the precipitates keep the extra semi coherent relationship to coordinate the total deformation, with the number of laser shock increase, dislocation multiplication and the vacancy motion constitutes geometrically necessary boundaries (GNBs), which consists of the sub grain boundaries to refine the matrix into the nanometer grains. The strengthening mechanism of surface modification of aluminum alloy by laser shock processing is formed of the internal stress state caused by the combination of the complex dislocation configurations and the Hall-Petch effect of the nanocrystalline grains.