| Laser shock processing (LSP) is a new and promising surface treatment technique for improving the fatigue durability, corrosion, wear resistance and other mechanical properties of the metals and alloys. During LSP, the generated shock wave can introduce a deep compressive residual stresses into the materials, due to its high-pressure (GPa-TPa), ultra-fast (several tens nanoseconds), ultra-high strain-rate (107-108S-1) and high-energy. The overall properties and behavior of metal materials subjected to LSP were significantly improved because a refined surface layer has been successfully obtained. Nevertheless, up to now, a clear scenery between micro-structure and macro-property of the refined surface layer, especially formation of sub-micrometer grains from coarse grains during severe plastic deformation, is still pending. Therefore, a basic study of the underlying mechanism for grain refinement by ultra-high strain-rate becomes more and more crucial.The aim of this paper was to provide some foundational researches on the macro-property, micro-structure evolution and plastic deformation. These researches can provide some scientific insights into the industry application of LSP technology. Four different topics were involved, i.e. the surface integrity and fatigue life of LY2 aluminum alloy after LSP with different processing parameters, the tensile property and fractural morphology of LY2 aluminum alloy by laser shock processing at strain-rate from 0.00001 s-1 to 0.1 s-1, the grain refinement mechanism based on the micro-structure evolution after multiple LSP impacts, and the estimation equation of the maximum depth of the dimple in the surface of LY2 aluminum alloy. Some important conclusions and innovative achievements of this work were listed as follows:Firstly, the micro-structure evolution of LY2 aluminum alloy during LSP were investigated systematically, and different vacancy cluster defects (void, SFT:Stacking fault tetrahedra, and vacancy-type dislocation loops) were found in the minor plastic deformation (MPD) layer of LY2 aluminum alloy and the formation mechanism of SFT was analyzed. The relation between macro-mechanical properties and micro-structure was developed, and a grain refinement mechanism induced by plastic deformation during multiple LSP treatment in LY2 Al alloy is proposed based on the microstructure obser-vations. The grain in the surface layer was obviously refined due to the ultra-high plastic strain induced subjected to LSP. After a single LSP impact, the change of dislocation structure can be also clearly seen at different layers, i.e., it varies from dislocation lines (DLs) to dislocation tangles (DTs) and dense dislocation walls (DDWs), to subgrains or refined grains as functions of the distance from the top surface. After multiple LSP impacts, the minimum grain size in the top surface was about 100-200 nm. The grain refinement process after multiple LSP impacts, can be described as follows:(ⅰ) the formation and development of DLs in original grains; (ⅱ) DTs and the formation of DDWs; (ⅲ) transformation of DTs and DDWs into sub-grain boundaries; (ⅳ) evolution of the continuous dynamic recrystallization (DRX) in sub-grain boundaries to refined grain boundaries. The high strains with a high strain rate during LSP are necessary for the formation of refined grains during plastic deformation of LY2 Al alloy. After multiple LSP impacts, different vacancy cluster defects (void, SFT:Stacking fault tetrahedra, and vacancy-type dislocation loops) were found in the minor plastic deformation layer of LY2 aluminum alloy, and the formation mechanism of SFT under ultra-high strain-rate plastic deformation was analyzed.Secondly, the effects of material property and the LSP processing parameter on the depth of surface micro-indention for aluminium alloy subjected to LSP were investigated systematically, and the approximate analytical expression of the maximum depth of the micro-indention was deduced. The overall plastic deformation of the thick metal plate happens after the action of the shock waves, so the overall plastic deformation of the thick metal plate has no influence on the pressure of the shock waves. When the stress waves get to the back surface of the ultra-thick plate, the strength of the stress waves has attenuated to a value lower than the dynamic strength of the material, so the overall plastic deformation in the plate doesn't occur, and only a micro-indention formed in the front surface. Based on stress-wave governing equation, Maxwell constitutive equation and the relation the movement velocity between stress wave and metal plane, and the residual strain expression of elastic-viscoplastic material and the velocity expression of the surface micro-indention were obtained. the estimation equation of the maximum depth of the micro-indention in the surface of the plate was deduced. The approximate analytical expression of the maximum depth of the micro-indention, which took viscosity coefficient, strain rate, and the pressure of shock wave as the characteristic parameters, was deduced. The relations between the maximum depth of the micro-indention and dynamic yield strength, peak pressure of shock wave, viscosity coefficient, elastic modulus were also obtained.Thirdly, the surface integrity (nano-hardness, elastic modulus, residual stress, and surface profile) of LY2 aluminum alloy after LSP with different processing parameters was investigated systematically, and the appropriate processing parameter and processing rule were obtained in different situations. The influences of LSP on nano-hardness and elastic modulus of LY2 aluminum alloy were discussed, and its enhancement mechanism of LSP on elastic modulus was first also addressed. Additionally, the effects of LSP on the residual stresses of the LY2 aluminum alloy samples with elliptical spot were experimentally investigated, and the effects of the overlapping rate on the residual stresses were studied. Results showed that the longger effective shocked-length can be obtained by using the smaller overlapping rate during LSP, thus it can improve the efficiency of the LSP. With the increase of the impact time in multiple LSP impacts, the surface compressive residual stress increases obviously. However, the increasing rate of residual stresses decreases as the impact time increases, and when the residual stresses at the locations far away from the surface (i.e.,≥a few mm) are almost not influenced by the impact time. The increasing rate of the affected depth in plastic deformation decreases with the increment of impact time, and when the impact time is from 4 to 5, there are little changes in the depth of plastic deformation.Finally, the tensile property with different strain-rate and the LCF performance with different processing parameters of the LY2 aluminum alloy notched specimen were investigated systematically. The phenomenon of dynamic strain aging (DSA) was found in the treated sample by LSP at the strain rate ranging from 0.00001 s-1 to 0.0001 s-1, which may be explained by the dynamic interaction between diffusing solute atom and the mobile dislocation in the SPD layer of LY2 Al alloy at the given strain-rate range. The values of time, axial displacement and axial force during the tensile experiment were measured, and scanning electron microscopy (SEM) observations of fracture morphology were carried out. Results showed that LY2 aluminum alloy subjected to LSP exhibited only the modest strain-rate sensitivity. The ultimate tensile strength increased and the elongation decreases gradually with the increment of strain-rate. Fracture morphology suggested the possibility that LY2 aluminum alloy evolved towards a more ductile dimple fracture mode associated with micro-void coalescence at higher strain-rates. The relations underlying the ultimate tensile strength, fracture morphology and strain-rate sensitivity were also addressed. By comparing with the untreated specimen, the LCF performance of the specimen treated by LSP with different processing parameter was obviously increased.
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