Laser bending of thin metal sheets

Abstract

Laser forming is a relatively new technique which forms metal sheets by thermal residual stresses instead of external forces. Any three dimensional geometrical shapes in principle could be obtained by controlling the path of the laser beam. Laser forming is shown to be particularly suitable for one-off or small batch production. Since the introduction of laser forming in 1985, there have been considerable amount of investigations on the process. However, less research work on studying bending of thin metal sheets has been reported. In this project, a low power Nd:YAG laser was used to study the bending behaviour of three types of thin metal sheets, namely AISI 304 stainless steel, A42 lead frame alloy and 6061 aluminium alloy. It was observed that the bending angle decreased significantly with increasing material thickness, and that there existed a threshold heat input below which no laser bending occurred. On the other hand, when the heat input was greater than a critical value, bending angle would no longer increase with increasing heat input. It was also found that bending efficiency increased as the holding time between each irradiation decreased. For the 6061 aluminium alloy, the samples were heat treated by full annealing and age hardening before bending, and their effect on laser bending angle was examined. It was found that there was no significant difference when the heat input was high. In this thesis, a 2-dimensional model originally developed for flame bending was modified and extended to laser bending. Instead of making assumption on stress paths, a total of 36 stress states were considered. These covered most of the combinations that would possibly occur. The success of the current work was the realisation of transient stress paths without solution divergence. By such a transient analysis, the changes of stress and plastic strain were visualised. The predicted results were compared with the experimental findings and the trends of them are in reasonable agreement. The outcomes of the project are significant in better understanding the deformation behaviour of thin metal sheets in laser forming, and these provide a basis for further investigation of laser forming of micro-devices.