LASER WELDING OF ZINC-COATED AND UNCOATED STEEL SHEETS AT ATMOSPHERIC AND SUBATMOSPHERIC PRESSURES

Abstract

In the automotive industry, zinc-coated steel is widely used because of its high corrosion resistance. Many automotive industry companies have tried to employ laser welding because of its many benefits, such as low heat input, high-intensity heat source, minimal distortion in heat affected zones, and high productivity. In lap joint laser welding of zinc-coated steel sheets, a proper gap needs to be maintained to avoid weld defects in weldment because the zinc vaporization temperature (1180 K) is lower than the steel melting temperature (1809 K). However, in this case, additional processes are required for application to actual industrial production lines, and it is difficult to precisely control the gap. Furthermore, although many researchers have investigated ways to mitigate the influence of high zinc vaporization pressure, it remains an issue because of erratic and unstable keyhole motion and melt pool behavior. Therefore, the purpose of this dissertation is to investigate the keyhole behavior and weldability of zero-gap laser welding of zinc-coated and uncoated steel sheets at atmospheric and subatmospheric pressures according to process parameters to develop the gap insensitive lap joint laser welding of zinc-coated steel. In this dissertation, firstly, a scaling law for predicting penetration depth was proposed, because the determination of penetration depth is the first consideration before the welding process. Moreover, then precisely observation method and analysis method were developed to observe clearly keyhole behavior, and effect of relative configuration of the laser beam and keyhole geometry on weldability for zero-gap lap laser welding of zinc-coated steel sheets. Also, the influence of ambient pressure on keyhole behavior and weldability were investigated to find solutions and possibilities for obtaining good welds for zero-gap lap laser welding of zinc-coated steel sheets by adjusting processing parameters (i.e. laser intensity and welding speed and ambient pressures).These studies can be summarized as follows. Firstly, a scaling law for predicting penetration depth was proposed that can be applied to both conduction mode and keyhole mode laser welding. The proposed scaling law was formulated based on a simple one-dimensional heat conduction model, and the effect of multiple reflections was accounted for. Because the scaling law was obtained from a laser heating problem, its physical meaning and why it needs to be formulated that way can be clearly explained. Experiments were conducted, and the obtained results were found to be in good agreement with the proposed scaling law. Secondly, in order to observe the keyhole behavior and reconstruct the keyhole geometry, a coaxial observation method was developed using a high-speed camera. A coaxial observation is a more useful and precise method to observe keyhole behavior than other lateral observation methods, and it was possible to study how the keyhole shape changes as the process parameters are varied. This chapter investigated the overall differences in the keyhole geometry between the zinc-coated and uncoated steels over a large process parameter space. Thirdly, using the obtained keyhole geometry data, the effect of keyhole geometry and dynamics on weldability was investigated by defining several key factors. It was found that the relative configuration of the keyhole and the laser beam is the most influential factor for obtaining good welds. For the zinc-coated steel, good welds were obtained at low welding speeds even zero-gap lap joint laser welding of zinc-coated steel sheets. Finally, based on the observation and analysis method from previous chapters, we investigated the laser welding of zinc-coated steel at subatmospheric pressures in order to compare between laser welding at atmospheric pressure and subatmospheric pressure. The purpose of this work is because the pressures inside the keyhole play a major role in weldability during zero-gap lap joint laser welding of the zinc-coated steel sheets. In this chapter, the main focus was to reconstruct time-averaged 3-D keyhole shapes and studying the influence of ambient pressures on keyhole behavior and weldability.