Yongxiang Hu

Laser peen forming induced two way bending of thin sheet metals and its mechanisms

Laser peen forming, is a purely mechanical forming method achieved through the use of laser energy to form complex shapes or modify curvatures. It is flexible and independent of tool inaccuracies that result from wear and deflection. Its nonthermal process makes it possible to form sheet metal without material degradation or even improve them by inducing compressive stress over the target surface. Experimental investigation has been carried out to understand the effect of process parameters such as laser intensity, scanning velocity on the bendingdeformation of thin sheet metal with different thicknesses. It is found that the sheet metal can be made to bend not only toward but also away from the laser beam depending on the process condition. The bendingdeformation is found to be varied continuously and smoothly from the concave form to the convex by increasing the sheet thickness or decreasing the laser intensity. There is also a specific thickness for sheet metal to remain flat after laser peen forming. Two mechanisms are proposed to understand all experimental results, which are shock bending mechanism and stress gradient mechanism for bending toward and away from the laser beam, respectively. Due to the coupling effect of two mechanisms, the smoothly switching from one mechanism to the other depending on process conditions makes the laser peen forming to be an easily controlled process for forming complex shapes and high precision curvature modification.

Three-Dimensional Numerical Simulation and Experimental Study of Sheet Metal Bending by Laser Peen Forming

Laser peen forming (LPF) is a purely mechanical forming method achieved through the use of laser energy to form complex shapes or to modify curvatures. It is flexible and independent of tool inaccuracies that result from wear and deflection. Its nonthermal process makes it possible to form without material degradation or even improve them by inducing compressive stress over the target surface. In the present study, a fully three-dimensiorcal numerical model is developed to simulate the forming process of laser peen forming. The simulation procedure is composed of several steps mainly including the shock pressure prediction, the modal analysis, and the forming process calculation. System critical damping is introduced to prevent unnecessary long post-shock residual oscillations and to greatly decrease the solution time for simulation. The bending profiles and angles with different thicknesses are experimentally measured at different scanning lines and scanning velocities to understand the process and validate the numerical model. The calculated bending profiles and angles agree well with the trend of the measured results. But it is found that simulations with the Johnson―Cook model are more consistent, matching the experimental results for the thick sheet metal with a convex bending, while the elastic-perfectly-plastic model produces a better agreement even though with underestimated values for the thinner sheet metal with a concave bending. The reason for this phenomenon is discussed, combining the effects of strain rate and feature size. Both the simulation and the experiments show that a continuous decrease in bending angle from concave to convex is observed with increasing specimen thickness in general. Large bending distortion is easier to induce by generating a concave curvature with LPF, and the angle of bending distortion depends on the number of laser shocks.

Study on residual stress of laser shock processing based on numerical simulation and orthogonal experimental design

Abstract Laser shock processing (LSP) is a competitive alternative surface enhancement process. Shock pressure with high magnitude and short duration induced by LSP is one key parameter to induce a residual stress field in the material. Finite element method (FEM) simulation based on the orthogonal experimental design is adopted to analyse the interaction of different shock pressure profiles with the material. It can give a relatively accurate description of the effect of shock pressure profile on the residual stress field induced in the material with a representative coverage of all parameters by less calculation cost. The analysis results show that laser shock conditions should be selected carefully to ensure a favourable shock pressure to obtain an optimised residual stress field. The simulation on the overlapping laser shock is also presented and indicates that it is an effective method to settle the residual stress drop on the top surface and can obtain a much more favourable residual compressive stress field to enhance the fatigue resistance of the material.

An Analytical Model to Predict Residual Stress Field Induced by Laser Shock Peening

Laser shock peening (LSP) is an innovative surface treatment technique similar to shot peening. An analytical model to predict the residual stress field can obtain the impact effect much quickly, and will be invaluable in enabling a close-loop process control in production, saving time and cost of processing. A complete analytical model of LSP with some reasonable simplification is proposed to predict residual stresses in depth by a sequential application of a confined plasma development model and a residual stress model. The spatial distribution of the shock pressure and the high strain rate effect are considered in the model. Good agreements have been shown with several experimental measured results for various laser conditions and target materials, thus proving the validity of the proposed model.