Numerical Simulation And Experimental Study On Laser Micro-bending Processes

Laser bending or laser forming (LF) is a newly developed, flexible technique for forming sheet metal by means of thermal stress instead of external force or hard tools, which has many advantages compared with cold and mechanical processing. Especially, the laser can be focused into very small beam and is very suitable for precision micro-scale processing. The study works of laser micro-bending mechanisms analyzing and process controlling have an important significance to micro manufacture both in theory and practice.This thesis focuses on bending direction and deformation controlling of micro-scale stainless steel specimens. The study contents includes: investigation of temperature gradient mechanism(TGM) and buckling mechanism(BM) of laser micro-bending, laser micro-bending process with pre-stress, laser polarization affection and laser micro-bending process optimization .Based on the analysis of the characteristics of laser micro-bending, the numerical model was developed by using the FEM software MSC.Marc. The simulation results show that the bending in TGM also could be obtained on micro-scale specimens. The heat affection zone was almost constrained in the area of the laser beam during heating process. Large temperature gradient and stress gradient were generated along thickness of the specimen. Both compress stress and plastic strain induced on the top surfaces were larger than that on the bottom surface, thus positive bending towards to laser beam was produced. The main reason of the edge effect during laser bending process is that the peak temperature distribution along the scanning path is non-uniform.Laser micro-bending experimental system was built for studying the forming law of the micro-parts in TGM. The FEM model was verified under different powers and scanning speeds. The experimental results indicated: the peak temperature on the top surface is a critical point, when it is lower than the material's melting point, the bending angle increased with increasing power and decreasing scanning speed; when it is higher than the melting point, the bending angle decreased with increasing laser and decreasing scanning speed. Based on the affection of the distance between multi-passes and length of the specimen on the bending angle, a process design method for forming single-curvature surface was proposed and verified through experiments.The FEM model was developed for laser micro-bending process simulation in BM, and experiments also were carried out. The results showed that the bending direction in BM was strongly affected by the initial stress of the specimen. For controlling the bending direction, pre determined pre-stress was introduced in the specimen's heated zone and the effect of the pre-stress was studied by numerical simulation and lots of experiments. The results showed that the pre-stress could enlarge the stress gradient and the bending direction could be controlled freely by controlling the value and direction of the pre-stress.The influence of the beam polarization in laser bending process was experimental studied. The results showed that the laser absorption of the metallic specimen could be enhanced by increasing the incident angle when using linearly polarized laser, while at the same time, the laser beam area increased with increasing incident angle, thus the laser energy and bending angle are influenced by those two factors during multi-scanning process. When the deformation part is parallel to the laser incident direction, the worked laser energy decreased drastically and the deformation couldn't be produced any more, thus a strategy of bending angle controlling by changing the incident angle was proposed. The experimental results showed that the precision of bending angle can reach±0.1°.Considering the computation efficiency, the improved strategy for parameters optimization of laser bending process was developed. The FEM software and optimization design software were integrated through secondary development. Based on the analysis of the deformation law and temperature distribution during laser forming process, the temperature field computation was used instead of the previous coupled thermal-mechanical simulation in the optimization process. The optimization efficiency was considerably improved by the premise of guaranteeing the precision of the results. A process design route was developed based on optimization results and bending angle controlling strategy, experiments were also carried out to verify this process design method.