Optical monitoring and analysis of laser welding

Abstract: After the laser was invented in 1960, it was not long until someone started using this powerful source of light to weld parts together. Laser welding became an industrial application during the 1970's and the field has developed ever since. In 2008 a new 15kW fibre laser was installed at Luleå University of Technology, and at the same time a considerable investment was made in new digital high speed cameras. This combination of equipment enabled research on a new level, and a first step towards a more general understanding of laser welding. This thesis presents the results acquired by analyzing high speed videos of laser welding. Qualitative and quantitative results from these high speed videos have revealed a considerable amount of information about the physics which underlies the laser welding process, including direct measurements of fluid flow within the melt pool and the interpretation of electromagnetic signals which emanate from the welding process. The thesis comprises three papers which are thematically linked by their concentration on the analysis of high speed imaging of the laser welding process. Paper A concerns the quantitative evaluation of high speed imaging of the time-dependent metal vapour jet that streams out of the laser welding vapour capillary. This work has revealed an important correlation. The output of commercially available process monitoring photodiodes (used for detecting infrared radiation) correlates with the fluctuating vapour jet above the weld, instead of, as was previously assumed, with the radiation from the molten and solid surface. In paper B, for the first time, ultra-high speed images of the surface of the laser welding vapour capillary have been obtained, (at a rate of 180 000 frames per second) with good spatial resolution and contrast. In addition, a streak technique was developed that measures and the time-dependent melt flow velocity along a selected line. Wave-like patterns that flow down the capillary have been directly observed. These phenomena are of essential importance for a basic understanding of the laser welding process and are a very powerful support for future research, e.g. for modelling and simulation. Using the above method, the velocity of the flowing vapour capillary waves was quantitatively evaluated in Paper C for varying process parameters, like laser power, focus position or welding speed, revealing clear, important trends of the laser welding process.