Numerical Simulation And Experimental Study On Microscale Laser Peen Forming

Microscale laser peen forming (?LPF) is a high-strain-rate microforming technique, which employs the shock wave pressure induced by a laser pulse to deform thin metals to desired configurations. The process combines the advantages of laser shock peening and plastic deformation, and holds promise for fabrication of components for Micro Electro Mechanical Systems (MEMS).?LPF is a complex process which involves the physics of interaction between laser and materials, impact dynamics, high-strain-rate plastic mechanics, material science, and so on. There are several special technological requirements in this process, including laser parameters, absorbent coating and confining overlay. Furthermore, the metal sheet takes place complicated changes both in geometry and mechanical properties during a short period of time. Also, many technological factors affect the forming quality. Therefore, it is difficult to fully understand the process mechanism and forming laws of?LPF.This thesis is devided into three parts. In the first part, the microscale laser bulge forming was systematically investigated combining the numerical simulation and experiments. The process features were analyzed, and the effects of critical process parameters on bulge forming were presented in detail. Secondly, the application of microscale laser shock bending on Fe78 Si9B13 metallic glass was studied. In the third part, the dynamic facture phenomenon occurring in?LPF was discussed by analysis of the fracture surface morphology.The main contents and conclusions are as follows.(1) Based on the analysis of the characteristics of microscale laser bulge forming, the mechanical model of shock wave pressure, the dynamic constitutive description of materials and the FEM analysis procedure were discussed. The numerical model was developed by using the FEM software MSC.MARC. The distributions of displacement. strain, residual stress and thickness changes of pure copper foils were presented, and the evolution of equivalent plastic strain rate was also analyzed. The simulation results showed that the equivalent plastic strain rate during bulge deformation could reach 106/s. indicating that microscale laser bulge forming was an ultra-high-strain-rate forming technique. The radial residual tensile stress was induced within the laser irradiation zone on the top surface of the sample, while the bottom side was compressive. The significant changes of plastic strain and stress gradient were appeared both in the regions around the laser spot edge and near the die entrance. Stress concentration also occurred in these regions.(2) The experimental system of microscale laser bulge forming was built. The realization methods of laser path setup, absorbent coating, confining overlay and the design of micro-bulging mold were proposed. A novel method for measurement of micro parts was introduced. The FEM model was verified through profiles and maximum bulging heights, and the simulation results agreed with those of experiments. The effects of absorbent coating and confining overlay on the deformation behavior in microscale laser bulge forming were studied. Based on plenty of experiments, the influence of laser energy, spot diameter, shock number, die diameter, material type and sheet thickness on the maximum bulging height was investigated.(3) The experimental system of microscale laser shock bending was set up. The measurement method of the bending angle was presented. This system was employed to study the bending of Fe78 Si9B13 metallic glass strip with 30?m in thickness. The experimental results indicated that metallic glass strips could obtain large bending angle without heating the material to supercooled liquid region. The influence of scanning number, spot distance, shock position, laser energy, spot diameter, distance of scanning lines, specimen length and width on the bending angle was investigated.(4) The dynamic facture phenomenon occurring in?LPF was experimentally studied by analysis of the fracture surface morphology in view of microscopic fracture theory. The results showed that the fracture mode of materials had a relationship with the laser shock conditions. Moreover, the dynamic fracture behaviors of materials were different from those under static or quasi-static loading conditions. The ductile metals such as pure titanium and pure copper both exhibited ductile fracture. However, though the Fe78 Si9 B13 metallic glass displayed brittle fracture under the macroscopic view, it indeed showed the feature of ductile fracture from a microscopic view.