A Fundamental Study On Laser Forming Of Sheet Metal Without Mould Based On Thermal Stress

In the light of analyzing existent problems of flexible and die-less sheet metal forming technology by laser induced thermal stress as a high spot, the research on die-less sheet metal forming technology at home and abroad was studied. And what is in need of research in applying sheet metal forming technology at present has been put forward.In order to make a systematic study the mechanism of laser forming with thermal stress and its key technology, the phenomenon of forming for sheet steel by laser thermal stress was studied and analyzed. In the experiment, sheets of AISI304 stainless steel, TA2 and QT450 were selected as test samples, the laser beam was produced by a 2.5kW CO₂ laser. Test samples were cooled by water or air. With water-cooling, the surface of the samples must be exposed out of the water. When the pattern of cooling was selected, the experiment adopts the single-factor method, which means that only one of the parameters such as laser power, laser spot, scan speed, number of passes is changed to investigate the mechanism of laser induced thermal stress forming. The scanning type is reciprocating and continuous, and the scan path and the time interval are consistent. It is carried out by numeral control. In order to ensure validity of experimental data, it is necessary to analyze according to known theory before designing experiment plan, and estimate combination process paramerters of the forming to bring bending, then design the experiment, The thickness of the test samples was a constant in the research, and the process parameters were multipacket, air-cooling and water-cooling were used in investigating the relationship of AISI304 stainless steel sheet laser bending. The results show that there is no bending for 0.6 mm thickness of AISI 304 sheet steel under the condition of water cooling when linear energy density is less than 80J/mm; the best value of the linear energy density is between 150 and 180J/mm for it; the technological parameters such as the thickness of sheet metal, laser spot and scanning times have great effect on the bending angle. And the ablation and stability of bending for AISI304-sheet metal in the experiment is evaluated as well. The new idea of sheet forming by laser thermal stress based on water-cooling is put forward, which provides the experimental foundation to control accurately the sheet forming by laser induced thermal stress. The above experimental result shows that the power density increases with the increase of laser power when other parameters keep constant, which eventually results in the distinct increase of the bending angle. The above-mentioned mechanisms are non-linear and there is an optimal process parameter. Bending angle induced by laser with thermal stress increases approximately in the linear proportion to the feed number. Increasing scan speed may decrease laser energy density, and then lessens bending angle. The effect of scan speed on the bending angle is approximately linear. Bending angle decreases when laser spot increases while other parameters keeps constant. The relationship mentioned above is approximately linear. It is obvious that the reasonable size of laser spot is one of the prerequisites in laser forming with thermal stress; the value (P/v) of the linear power density is the key process parameter. The best value of the linear power density is 30-40J/mm for 0.6 mm thickness AISI 304 sheet steel under the condition of air-cooling. Otherwise, bending angle is very small and ablation occurs on the surface. When the linear power density is less than 80 J/mm, there is no bending angle under the condition of water-cooling. And the best linear power density is 150-180J/mm; there is slight ablation on the surface of the samples, and the stability of forming is superior, the acquired angle of curve bending drops sharply, and decreases with the increase of the curvature.In order to optimize process parameters of bending with thermal stress induced by laser, an orthotropic experimental method was adopted here. In this experiment, laser bending of the 0.6mm thick AISI 304 sheet was studied by changing the laser power density, laser spot, scan speed, number of scan passes and thickness of sheet, and it can make a comprehensive evaluation on coupling of process parameters. According to the principle of orthotropic experimental method, the relationships of the interacted parameters can be distinguished. The results show that the five parameters develop different effects under the condition of water-cooling. Number of passes ranks first, followed by power density, thickness of sheet, laser spot and scan speed in terms of their effects on the bending angle, and the two most potential important parameters are the number of passes and the power density, They have strong potential to increase bending angle in laser bending.In the interest of further optimization of process parameters, Genetic algorithms was studied here, and the applying plan to optimize process parameters of bending with thermal stress induced by laser using Genetic algorithms was set forth. It is thought that the scope of population should be large, and the population should come from grope test. The condition of forming should be considered firstly when choosing optimization model, choose the mechanism of laser forming, and then apply mechanically the computational model.For the sake of studying mechanical performance and metallographic phase of forming area, a 2.5 kW CO₂ laser, a HVS-1000 Micro-hardness measuring instrument and an X-350A X-ray stress-measuring-instrument were adopted in the experiment where 0.6mm thickness sheet steel of TA2 was studied to evaluate the process parameters of bending with thermal stress induced by laser. Bending angle was measured as the function of main processing parameters. These processing parameters were arranged according to the theory of orthotropic experiment. Distribution of residual stress on the surface and variation of micro-hardness on the cross section of samples were studied after forming. The results show that these parameters bring different effects. The number of passes ranks first, followed by laser spot in terms of their effect on the bending angle, with power of laser beam and scan speed being the last two factors. Parameters of laser forming also have some influence on the distribution of residual stress on the surface of samples. Certain regularity appears in the variation of micro-hardness on the cross section of samples. The structure in the deforming area changed.To control forming quality effectively, the problem of how to evaluate forming quality was raised; the evaluating index of the surface quality on the samples after laser bending were studied here; surface roughness and its measurement, surface flaw and its measurement, surface association mechanical property and its measurement were introduced. The method of how to establish quality-evaluating system was discussed, which provides reference to a comprehensive evaluation on the surface quality for test sample after laser bending.By way of widening the forming technology, a RS2000SM CO2 laser of 2.5kW was used here to irradiate the nodular cast iron plate and hard alloy plate, the forming rule of brittle material QT-450 and YG10 was studied by changing the parameters of the laser beam and the machine tool; and the change of the surface properties of the QT-450 plate after laser forming was investigated. In order to study the oxidation in laser forming, Nitrogen-protecting and no protecting are adopted in the test; the structure and the content of elements of forming area was investigated. The analyses showed that the brittle material, such as nodular cast iron, can also be bent by thermal effect of laser beam; the structure, appearance, hardness and the mechanical performance of the plate in the deforming area also changed; and the case with Nitrogen-protecting and water-cooling is more excellent than that without air-protecting and water-cooling. The content of elements of forming area and other area is sameness.