Theoretical And Experimental Research On Laser Shock Forming

Laser Shock Forming (LSF) of metal sheet is a new and competitive laser based manufacturing technology. The dynamic response of the metal sheet and the deformation field are two key problems in the research of LSF rescently. LSF is a very complicated dynamic process. Lots of science and technology such as the diathermaneity,dynamics, material science, laser technology and computer science are all involved in LSF.. The laser irradiates on the metal sheet. And in a very short time, the absorption of the laser energy, the production and the combustion of the plasma, the expansion and the production of the laser shock waves, the confinement of the waves, the procession of the dynamical response of the sheet to the laser shock loads, the effect of the laser shock waves on the material microstructure, the change of the mechanical capability of the material and etc, all these factors have effect on each other. Although some theories, experiments and simulation of the LSF have been performed by some researchers and some important accomplishments are achieved, lots of supposion of the influencing factors and some simplified analysis are made because of too many interactional factors and the extremely short time about nanoseconds. The deforming mechanism of the LSF treatment, the mathematical computation and simulation of the displacement field and the discipline of the LSF are all need further research. The research of LSP with single laser pulse can simplify the model and display the mechanism directly. The relationship among the displacement of the sheet, the laser parameters, the confined conditions, the material properties and the loading time can be established and offers strong proof for the analysis of process of LSP. In this paper, through the analysis of the deforming procession of the metal sheet, reasonable supposition of velocity field is made. Applying equivalent constant load to simplify the computation, the mathematical model with single laser pulse of the displacement field is established and its precision is validated by experiments; applying finite elements, the numerical simulation is used to analyze the procession and results of the LSF. Combining the mathematical model and the numerical simulation, the main parameters of process are discussed and the dynamic process of the LSF treatment is analyzed. In the analysis, the deforming time, the displacement history, the strain and the strain rate are intensively discussed. In the end of the paper, the effect of the LSF on the material is studied. The main research work is listed as the following:1. The mathematical model of the LSF displacement field was established. Selecting the LSF with single laser pulse as the research object, the dynamic response of the metal sheet to the LSF load and the mechanism of the plastic deforming with high strain rate are studied. When the pressure generated by the laser shock waves is bigger than the dynamic yield strength of the structure, the plastic deformation is occurred. In the LSF treatment, the load time is extremely short and the reasonable muzzy velocity model is established. The process of the LSF can be divided into two phases. And then the displacement of the two phases is discussed and computed. In the process of the computation, the equivalent constant load is used to simpfy the work. In the end of this part, select the metal sheet of Brass and TA2 as the object, the precision of the computation is validated by experiments.2. The numerical simulation of the LSF treatment. According to the features of the LSF, the numerical simulation research is done to study the LSF. How to choose the parameter of the FEM model is analyzed. Comparing the simulated results with experiments, the precision and reliability are validated. The displacement history and the energy history of the structure changed with time are analyzed, which will be beneficial to the research of dynamic response of LSF. 3. The effect of the main LSF parameters is studied. In the LSF treatment, given the main parameters such as the material properties and the size of metal sheet, the displacement field is mainly determined by the laser energy and the size of the laser pulse. In order to study the effect of different parameters, the dynamic deforming process is analyzed with the numerical simulation and mathematical model. And then the displacement field can be predicted with the LSF treatment parameters. The parameters can also be specified with the objective displacement field. The relation of the parameters and the displacement field is studied qualitatively.4. The effect on the mechanic properties is studied. The surface quality of the LSF is examined and analyzed. The confining layer and the coating, which are two important factors in the treatment, are discussed. Three conditions, without coating, insufficient and reasonable coating are compared according to the experimental results. The investigation shows that the surface quality and the microstructure have no remarkable change; LSF is a mechanical process, not a thermal process and no visible ablation is observed. The micro-hardness of brass specimen increases by about 10~20% after the LSP treatment and the TA2 specimen has no obvious change.