Study Of Mechanism And Numerical Analysis On Forming 3-D Shape Of Plate By Laser Peening

Laser peen forming(LPF) is an emerging modern technique to bend metal plate. It employes the short-duration high-power laser to induce residual compressive stress distributed unevenly along the thickness of plate, and then the moment exerted by the residual compressive stress impetus the plate to develop plastic curvature. In this article, based on the analysis of the applications of the short-duration and high-power laser in material processing, a novel technique on LPF of plate was presented. A systematical study on the mechanism of LPF and the related technology were carried out from the theory and experiment, and the main contents as following:On the basis of analyzing propagation and attenuation of the stress wave in material, the mechanism of residual compressive stress was analyzed and the mathematic model on LPF was established. According to the theory of pure bending, the formulation of curvature radius after laser peening was established to correlate the curvature of deformation with laser pulse parameters. The shape of deformation of plate peened by laser along the centerline was the bulge shape of V, which is the basic shape and the intricate shape can be composed of it. The main advantage of LPF under the condition of pre-bend state was to reduce the spherical surface double phenomenon curvatures induced by LPF under the condition of free state.The charateristics of bending and the relationship among the plate deformation and the material laser pulse prameters, covering ratio of laser spot, geometrical parameters of plate as well as laser peening paths were investigated in detail from the experiments. The experimental results show that the deformation of plate increases nonlinearly with the increase of laser power density, with the increase of covering ratio of laser spot and the numbers of laser shot, while decreases with the increaseing thickness of plate. When the size of plate changed, the curvatures in the different directions changed correspondingly, The location of plate and the intensity of laser were selected purposefully to laser peening, the complex 3-D shape of plate can be gained.The experiments on LPF under the condition of pre-bend state were also carried out. The results show that the pre-bending can decrease the curvature parrallel to the direction of moment exerted and increase the curvature radius in the vertical diretion of pre-bending, that the larger the strain exerted by pre-bend is, the distincter the curvature radius in two direction is. The precision of LPF was investigated further and the precision of LPF can attain to 0.4mm in the radius direction, which goes beyond other forming methods and indicates that the LPF can fulfill the exact forming.Residual compressive stresses along the thickness and on both surfaces of workpiece were measured with X-rays diffraction after laser peening. The results show that the magnitude of residual compressive stress increases with the increasing laser power fluence and numbers of laser shot. The residual compressive stress lies on both surfaces. Comparing the magnitude of residual compressive stress induced by traditional shot peening and by laser peenig, the magnitude of residual stress induced by laser shot peening is two to three times as large as that induced by traditional shot peening, which indicates that thickness of plate formed by laser peening is greater than that by shot peening.The surface quality of workpiece after laser peening and its effect on mechanical properties were investigated. The surface quality includes the surface finish, hardness, the magnitude of residual compressive stress and so on. The experimental dada show that the surface finish decreases because of the dents in surface impacted by high pressure plasma, and the surface finish mainly depended on the curvature radius of workpiece and the material mechanical properties. The hardness after laser peening enhanced and the grain of material was refined, which reduces the rate of fatigue cracking and stress corrosion cracking in structural metals needed for aerospace and extends the fatiguelife of workiece. It indicates that the LPF combines the laser shock processing with plastic deformation.Based on the above analysises and experiments, the model of finite element method (FEM) was established and the processes of stress wave propagating, dynamic stress on surface, residual compressive stress and deformation of plate were simulated with ABAQUS software. The simulation results were agreed well with the analytic results and the experimental results. So the developed finite element method is able to predict the deformation and residual stress field in the processing of LPF. It can help to optimize the laser parameters and select the peening paths to implement the precise forming.