A Study on Laser Forming Processes with Finite Element Analysis

Laser forming is an innovative technique that uses a defocused laser beam to form sheet metal by thermal stresses rather than external forces. Promising potential applications of laser forming include rapid prototyping, straightening, aligning and adjusting of macro/micro-metallic components. Research to-date on laser forming has been largely focused, theoretically and experimentally, on the problem of characterization of process parameters on the forming results, and computational simulations of laser forming remain limited only providing the insight into the process. This study investigates the laser forming processes using the finite element analysis with respect to material responses during the processes, including complex processes, process optimization, process reliability and the effects of thermal and mechanical material properties. The first part of this thesis describes a nonlinear transient three-dimensional heat transfer finite element model and a rate dependent three-dimensional deformation model, which are developed for the laser forming simulations. Simulations are performed using an indirect coupled thermal-structural method for the processes of a straight-line heating, a circle-line heating, and a laser micro-adjustment. The thermo-mechanical behaviours during the straight-line heating process are presented in terms of temperature, stress and strain, and displacement distributions. The emphasis in the circle-line heating simulations is placed on the characterization of the quality of the deformed geometry by obtaining the radial and circumferential waviness. The micron size movements induced by laser point heating are focused the simulations of the micro-adjustment process. Simulation results are validated by comparison with published data or correlation to engineering point of view. The second part of this thesis presents the development of an effective method to determine optimum process parameters in laser forming. For the process optimization, design optimisation techniques are introduced into the finite element analysis of the laser forming process. The optimum parameter values to produce a predefined bend angle of 3° in the straight-line heating process are sought by two optimization procedures - one is the procedure involving the non-gradient method and the other is the gradient-based method. Optimum values of laser power, feed rate, beam diameter and number of passes are determined to produce a predefined bend angle in a multiple straight-line heating process using the two optimization procedures. A more suitable optimisation method for laser forming is chosen, which is used for a new optimisation problem to generate a maximum bend angle in a single pass of laser forming. In the third part of this thesis, a strategy to assess the reliability of the laser forming process is established by employing a well-known reliability analysis method, the Monte Carlo simulation. Robustness of the straight-line heating process of producing 3° with the optimum parameters determined by process optimization is evaluated with regard to the uncertain input variables of laser power, feed rate, plate thickness and coefficient of thermal expansion via the Monte Carlo simulations based on the finite element simulations of the process. The final part of this thesis identifies the effects of material properties on the bend angle resulting from laser forming. Process sensitivity to the properties of coefficient of thermal expansion, thermal conductivity, specific heat capacity and elastic modulus is investigated by measuring the Pearson product-moment correlation coefficient between the properties and the bend angle, which are based on the Monte Carlo simulations of laser forming. The conclusion is that the developed finite element models contribute to a better understanding of the laser forming process, and the optimization procedure is able to be used for straightening, aligning and adjusting of components