Study On Numerical Simulation And Process Optimization Of Laser Bending Of Tubes

Laser forming is a new type of metal forming process. Sheets and tubes can be formed conveniently depending upon the non-uniform thermal stresses induced by laser heating. It has special advantage in tube bending. Compared with the traditional tube mechanical bending methods, laser bending of tubes is wrinkless and springback-free. Neither a hard bending tool nor external forces are required, and thus the cost of tube bending is greatly reduced. The laser bending of tubes can prevent the tube thickness reduction and less ovalization of the cross-section. The proper temperature distribution can be obtained only by matching laser parameters reasonably. It is easy to automate the process and obtain the desired deformation..This dissertation uses the FEM software MSC.Marc to simulate the laser bending process of tubes and analyzes its the deformation characteristics. The process parameters optimization in laser tube bending is realized based on the optimization platform-iSIGHT. The main contents are as follows:The moving laser spot is simplified as a heating source. A user subroutine program developed by the author is used to define the flux density, geometry and scanning velocity. Therefore, the heating load is exerted automatically to the relative element faces. The curving moving model of laser beam is established in the FEM simulation. The relativity between material properties and temperatures is considered fully to guarantee the analytical precision.A thermo-mechanical FEM model of the Laser bending of tubes is established, and the laser bending process of tubes is analyzed numerically. Being similar to laser bending of sheets, different deformation stages also appear with the laser spot scanning. The first stage is reverse bending that is away from the laser beam during the heating stage, the second stage is forward bending that is towards the laser beam during cooling stage. At last, the positive bending is caused by a combination of the shortening along the axial direction of the tube and the thickening of the scanned region. The main mechanism of laser bending of tubes is upsetting mechanism. The stress-strain field shows that the higher residual stresses occur near the start and end of the scanning line. The heated area generates plastic compressive strain. Meanwhile, depending on different process parameters, unheated area appears no strain basically or small tensile stain The axial compressive strain at the outer surface of heated area is smaller than that at the inner surface. This situation is similar to the inverse bending of sheets and opposite to the positive bending of sheets. The different compressive strain between heated and unheated areas cause the whole tube bending towards the laser beam finally.The wall thickness at the unheated area is basically changeless during laser tube bending process. The fracture problem that occurs in traditional mechanical bending methods can be avoided. The bent tube slightly tilts towards the end of scanning because the temperature at the scanning end is higher than that at the beginning.Based on FEM simulation, influence of process parameters on laser tube bending is studied. When the other parameters keep constant, the laser bending angle augments with the increase of the laser power. The relationship between them is about linear. The laser bending angle decreases with the increase of the scanning velocity. Meanwhile, the bending angle diminishes with the increase of the diameter of the laser spot. The bigger energy density leads to bigger bending angle. The bending angle enlarges if the scanning wrap angle augments. The bending angle is largest when the scanning wrap angle is 180°. When the scanning wrap angle is over 180°, the bending angle decreases with the increase of the scanning wrap angle.Integrating numerical simulation with the optimization technology, process optimization of laser bending of tubes is realized. Forming a certain angle and forming with high efficiency are regarded as objective functions respectively, and design variables are laser power, spot diameter, scan speed, scan wrap angle and so on. According to different forming demands, the optimum process parameters are obtained. The optimization effects are good.Aiming to the actual laser forming, a process design method of multi scanning is put forward. By means of process parameters optimization based on different forming objects, the desired deformation can be obtained rapidly and accurately.