| Based on laser shock processing, short pulsed laser can be used to form sheetmetal. With the outstanding advantages of non-contact, no heat-affected zone and high precision, it has wide range of application prospects. In recent years, domestic and foreign scholars have conducted extensive research, the mechanism of laser and laser micro-shot peening forming a large number of results achieved in the field. Based on analyzing of peen forming and sizing technology of sheetmetal techniques based on the laser induced shock wave, the peening forming mechanism of laser shock wave was researched. And this technology was applied to sizing the sheetmetal firstly. Therefore, this work not only has important scientific significance and theoretical value, but also has significant economic value.This paper studies the basic theory of laser peen forming. Three major steps of laser peen forming process were analysis: (1) Generation of laser shock wave. (2) Distribution of residual compressive stress was induced by laser shock waves. (3) With the releasing of residual stress, there is the deformation on the sheet meal. As the different residual stress distribution, there are two kinds of deformation mechanisms: convex and concave mechanisms. Residual stress determined with the peak value of pressure wave: When the laser shock wave peak pressure P is less than the material Ph (HEL) , the material plastic strain does not occur, there is no residual stress; When the shock wave peak pressure P in the 1HEL and 2HEL between, the materials began plastic strain and plastic deformation increased linearly; when the shock wave peak pressure P is greater than the material 2HEL, while a plastic unloading plastic strain to reach saturation.The experiments show that there are different deformations shocking with same laser parameters because of the thickness of the different samples. Compare the thickest materials 1.75mm with 0.25mm thinnest material, we can clearly see that aluminum sheet of 0.25mm forming concave state with the energy various in 0.2-0.5J, while the aluminum sheer of 1.75mm forming convex bending under the same condition. There should be a thickness threshold with a certain laser energy density, and the process parameters. The thickness is greater than the threshold deformation mechanism is convex; the thickness is less than the threshold deformation mechanism is concave. In this paper, the thickness threshold of the research process parameters is between 0.7-0.88mm. And as the energy increases, the threshold of deformation mechanism changes increased slightly. Under the mechanism of the concave deformation of the material more sensitive to the energy, while in the convex mechanism is not too sensitive.The papers study on the deformation of the concave deformation mechanism. The factors of deformation: The material thickness, laser power, scanning frequency and scanning speed of were studied. In the concave deformation mechanism, the sheetmetal bending angle decreased rapidly with increasing of the thickness. With a cubic curve can be well fitted, due to the bending stiffness of the material was cubic with the increase in thickness. With the increasing of laser energy, the energy density subsequently increased. Bending angle increases rapidly at the beginning and it stop growing after reached peak bending angle. Bending angle of the material increased with the overlaps rapidly, bending angle are not increased after three times overlaps. Bending angle decreases as the scanning speed increases rapidly. Residual stress test result of samples also shows that there are positive residual stresses on the material surface. With orthogonal test method, the laser shot peening bending process parameters (laser power, scanning speed and repetition times) were conducted a comprehensive assessment. The affecter of the bending angel that followed by scan speed, scan frequency and laser energy. Scanning frequency, scanning speed are two important factors, obvious impact on the bending deformation. They have great potential to increase the bending deformation.With ANSYS and LS-DYNA, deformation mechanisms of pulsed laser shock forming were simulated with the order of the transient steady-state method. The simulation results show that with the increasing of material thickness concave deformation mechanism convert to convex. The threshold is about 0.25mm. With certain material thickness, with the increasing of energy convex deformation changes to concave deformation, the threshold value is about 0.5J. This trend is consistent with test results, but the threshold is smaller than the experimental results because of estimating of press. Based on the transient-steady solution, equivalent initial stress load static analysis methods were proposed. The laser shock processing parameters was equal to the depth of initial stress load. It can be used to a large-scale analysis of the problem. Equivalent stress method was established with equivalent relationship of testing and simulation, and avoiding the pressure wave estimation error. The deformation of the material can be simulated accurately and fast. First, relationship between stress and deformation should be established through the simulation and the relationship of test parameters and the deformation should be established, then relationship of test parameters and equivalent stress were established. All these work form the basis for the bands distribution problems of laser shock forming.Two-dimensional deformation was studied with the regional distribution of laser shock. Test was conduct with the conditions that 0.52mm thickness of aluminum constrained with 10mm water layer with 1.4mm laser spot diameter, and the speed of the table is 5mm/s. with the interval of 2mm, 3mm, 4mm, and the 5mm, test results show that with the increase of the band spacing the total deformation of the material decreased. With the smaller spacing, the total distortion will be close to single band deformation×number. At the same time, there will be trace on the surface of the specimen when the interval increases. So the reasonable distance of the strip is 2-3mm. the finite model were built with the equivalent stress method, simulation results are in good agreement with experiment. It cans simulation distribution of the strip effectively.The optimized model was built with ANSYS/optim. With the FEA model, the equivalent stress of sizing was solved. Then the technological parameter can obtain with the diagram. The process parameters used to size a sheet with the arc high of 2.1mm and obtained a satisfactory result. |
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