Study On Shape Forming In Laser Forming

Laser forming which requires no mechanical contact has its origins in sheet bending for ship construction. Laser forming can utilize localized heating for the forming of high strength alloys and use combinations of straight and curve lines to produce three dimensional formed components. Laser forming is of significant value to industries of aerospace, shipbuilding and microelectronics.This thesis focuses on improving shape forming in laser forming. Study includes six aspects: analytical formula of bending angle, analytical model of bending angle, finite element model of laser forming, experiment study, varying velocity scan and the post-forming mechanical property. Based on the theory of heat transfer, elasto-plastic mechanics and metallography, two key problems including bending angle mathematic model and"Edge Effects"are solved using methods of theoretical calculation, numerical simulation and experiment.Based on the conventional equilibrium and compatibility conditions used in solid mechanics, and the temperature distribution model along the thickness of the plate, the formula for TGM (temperature gradient mechanism) is derived based on the assumptions that the plastic deformation is generated only during heating, while during cooling the plate undergoes only elastic deformation. The formula includes two new parameters, the yield strength reduction factor and the characteristic depth of plastic zone. The comparison of the present model with other analytical models and published experimental data shows that the present formula is better than those proposed models and is in good agreement with the published experiment results.Based on the heat conduction equation for a semi-infinite solid, an analytical model to describe the three dimensional temperature field was derived for a finite plate with a Gaussian heat source moving at a constant velocity. A new analytical model for estimating the bending angle by the laser forming is proposed. The formula is derived based on a history-dependent incremental stress–strain relationship. The model accounts for the laser parameters, the temperature dependency of the material properties and sheet thickness. Numerical results agree well with published experimental results. The bending angles not only for TGM and BM (buckling mechanism) but also for co-existed BM and TGM can be well predicted.Based on the principle of FEMs (finite element model) of temperature field and deformation field of laser forming, the FEM of laser forming based on ANSYS is set up by analysis of geometry model, mesh model, material properties, loads and boundary conditions. The transverse strain study reveals a residual compressive transverse strain within the irradiated track. Analysis of the longitudinal strains reveals little or no residual strain in this direction. Analysis of the transverse stress reveals that near the centre of the plate there is no residual stress, however near the edges there is a large compressive transverse residual stress. Analysis of the longitudinal stress reveals that there is a large tensile residual stress on cooling along the scan line surrounded by a compressive zone. However, a considerable difference in strain and stress cycle is observed between the first edge, the centre and the second edge, demonstrating the asymmetry of the process.Based on carbon dioxide laser processing system and treatment of the irradiated surface, the absorption coefficient of the coating is calculated by comparison of thermocouples data and the results of FEA of temperature field. Reliability of FEA of temperature field is verified under different laser power. Using laser displacement sensor, reliability of FEM of deformation field is also verified under different parameters of laser power, scan velocity, scan line position and scan interval between two scans. The scan interval study shows that the final deformation of the plate is dependent on the time interval between the two sequent scans. For short intervals, the final deformation of the plate after the process of using two sequent scans is almost the same. For large intervals, the final deformation of the plate is identical but the value is much smaller than that corresponding to the short interval.By analysis of"Edge Effects"in laser forming under different scan velocities and scan line position, the varying velocity scan is proposed based on the reasons for"Edge Effects"that the temperature distribution and geometry constraints along the scan line. Based on FEM of constant velocity scan, the variation of the scan velocity thus can be implemented in the program by specifying the dwell times on corresponding elements. Using numerical simulation and experiment and considering the relation between temperature distribution and deformation, the staircase velocity scan is optimized by comparison of the bending angle under acceleration scan, deceleration scan, acceleration then deceleration scans and staircase velocity scans, which can largely reduce the"Edge Effect"in laser forming.By tension test, the tension properties of tension samples are analyzed to establish Ramaberg-Osgood constitutive equations under different laser processing parameters. Experimental data show that the yield strength and tensile strength are improved after laser forming, while the elongation percentage is lowered. Because the distribution of residual stresses as well as strains changes during laser forming process, the fatigue life of different laser processing parameters is studied by low cycle fatigue tests. Fatigue fracture mechanism is shown through the analysis of macro-fracture and micro-fracture using scanning electronic microscope.This thesis aims to improve shape forming. Based on elasto-plastic bending theory, the analytical formula of bending angle for TGM is established. Based on the incremental plastic theory, the analytical model of bending angle is derived for both TGM and BM. Based on the verified FEM of constant velocity scan and the phenomenon of"Edge Effects", the varying velocity scan is proposed and FEM of varying velocity scan also is established and verified. The scan is optimized to reduce"Edge Effects". By the study of analytical models of bending angle and varying velocity scan, the forming accuracy in laser forming can be improved.