Laser shock peening (LSP) is an advanced surface modification technology that uses laser-induced shock waves as a direct source of force for the production of plastic deformations on the surface of metals. It can be used to regulate the surface quality of the material and improve its properties. Laser shock peening has several advantages over other surface modification techniques, such as high loading pressure, large influence range, and controllable impact area and loading pressure parameters, and can be easily automated. It has a wide range of applications in aerospace, automotive manufacturing, and marine engineering industries. The plastic deformation is the fundamental process for laser shock peening in the reconstruction of surface stress, improvement of surface morphology, and hardening of surface materials. This deformation is influenced by a complex nonlinear dynamic process that is dependent on various factors such as laser shock wave pressure decay, the dynamic yield strength of materials, plastic strain rate, internal dislocations, and microcracking of materials. In this study, the flow law of plastic deformation of the TC4 titanium alloy under the action of shock waves is analyzed by combining numerical simulations and experimental studies. The reliability of the numerical model is verified through experimental studies, and the volume changes of internal plastic deformations under different laser shock parameters are calculated based on the numerical model. Additionally, the influence of plastic flows on volume distribution, stress reconstruction, and internal grain distribution of the material is analyzed.
A TC4 titanium alloy is used in this study. A combination of numerical simulations and experiments is used to study the flow law of plastic deformation of the TC4 titanium alloy by laser shock peening. First, a numerical simulation model of the laser shock peening of a TC4 titanium alloy is established. The surface deformation and residual stress distributions in the numerical simulation are extracted and compared with the experimental results, and the numerical simulation model is validated. After verifying the validity of the numerical simulation model, the internal deformation distribution of the material is extracted from the numerical simulation model, and the internal deformation distribution data is processed. Subsequently, the deformation distribution data is fitted, and the fitted equations are calculated to evaluate the flow of plastic deformation by obtaining the volume change of each part after the LSP. In addition, the microstructures of the surface layers at different depths before and after the laser impact treatment are observed using a field-emission high-resolution transmission electron microscope (TEM), and particle size measurements and distribution statistics are processed.
The clouds obtained from the numerical simulation show that other plastic deformations occur beyond surface microindentation and superficial convex deformations after laser shock peening. The effects of the shock wave cause the volume of the microindentation deformation produced in the center of the impact area to shift to the surrounding area. This results in the formation of the superficial convex deformation at the edge of the spot impact, and the remaining volume is squeezed into the material, leading to internal convex deformations (Fig. 6). The distributions of the residual stress and deformations on the surface at a power density of 3.02 GW/cm2 are consistent with the numerical simulation results (Fig. 10). As the volume in the residual compressive stress zone decreases, the volume in the residual tensile stress zone increases, and a part of the volume is transferred to the internal part of the material during the plastic deformation process (Fig. 13). After laser shock peening, a large number of dislocations, dislocation walls, dislocation entanglements, and other sub-structural defects appear inside the grains. Lamellar dislocation accumulation occurs in the lamellar organization in the microindentation deformation region, and high-density dislocations are formed in the convex deformation region (Fig. 14). Variations in the grain size are directly related to the intensity of the plastic deformation. The grain size is the smallest in the microindentation deformation region, which is directly loaded by the laser spot. The superficial convex deformation region exhibits a larger grain size compared to the microindentation deformation region, while only a slight refinement in grain size is observed in the internal convex deformation region (Fig. 15).
In this study, the microindentation deformation is formed in the center area of a spot, the convex deformation is formed in the edge area of the spot, and the residual compressive stress is generated in the surface layers of the microindentation and convex deformation areas. The 3D morphology test results of the TC4 titanium alloy surface show that the volumes of the microindentation and convex deformations are not equal, mainly because the laser shock wave induces the flow of plastic deformation in the surface metal towards the interior of the material, forming a convex deformation area under the microindentation deformation layer and a residual tensile stress layer in this area. Based on the results of numerical calculations, the volume of the overall plastic deformation is extracted and calculated. The results show that the sum of the internal convex deformation volume and superficial convex volume is approximately equal to the microindentation deformation volume. In the absence of phase changes, the overall plastic deformation adheres to the volume constancy law. The TEM images before and after laser shock and particle size distribution results show that flow of plastic deformation of the TC4 titanium alloy by laser shock peening directly affects the grain refinement mechanism in each region; the degree of grain refinement decreases successively in the microindentation deformation area, the superficial convex deformation area and the internal positive deformation area. When a multispot laser shock peening is performed, all spots work together to form microindentation deformations, and small convex deformations appear at the edge of the indentation surface. In the part without an overlap of spots and impact of shocks, convex deformations are formed.
