Laser Impact Welding and High Strain Rate Embossing.

摘要

Small scale dissimilar materials welding is needed in micro-electronics, medical devices batteries, etc. Laser impact welding is a solid state welding method. The defects of dissimilar materials welding, which appear in fusion welding due to the liquid phase, can be avoided. Until now, only little research has been done on the topic of laser impact welding. The previous research is mainly about the description of welding phenomenon observed with this method. One goal of this work is to develop the technique to a level suitable for robust industrial application.;In this study, laser system and diagnostics were developed to investigate this laser impact welding process. The laser system has a maximum energy of 3J with pulse duration of 8ns. The maximum power is 3.75x108 W. The diagnostics monitors the laser energy and laser beam profile effectively. The laser beam profile not only shows the laser intensity distribution, but also reflects the degradation of the optics. An easy accessible and operation experimental setup was developed for laser impact welding. In the selection of components of the laser impact welding process, the effect of each component on the energy efficiency was studied.;Impact velocity is one of the critical parameters in impact welding. In this study, impact velocity was studied with varied laser beam energy, laser spot size and physical parameters of materials. With the current experimental setup, the impact velocity reached up to 1000m/s and the energy efficiency reached up to 30%. It was also found that the energy efficiency decreases with laser beam energy. Joining of aluminum to copper, aluminum to titanium were studied. Methods for weld strength and weld area measurement were proposed. Microstructure and hardness across the weld interface was examined. It was found that the weld strength increased with laser spot size and flyer thickness. The weld area also increased with laser spot size. Severe plastic deformation and twinning were observed along the weld interface at titanium side, which resulted in the hardness increase at that region. Spall phenomenon in laser impact welding was also studied. Spall strength was measured for several materials and the fracture surface microstructure was studied.;In the last chapter, a high strain rate embossing method was proposed. With this method, the die surface can be fully replicated while it is impossible with quasi-static forming method. Hardness was measured and compared for samples with other three forming methods. The corresponding microstructure was studied.