Analysis of laser arc hybrid welding experiments

The thesis is about the analysis of experimental results on the laser arc hybrid welding (LAHW) process as well as about the methodology and tools behind. Due to the high potential on weld penetration and welding speed, the industrial importance of LAHW techniques has been increasing in the recent years. However the massive use of them is constrained because of high investment costs, complexity and novelty. These obstacles provoke a lack of experienced operators and they desire a base of knowledge related to optimal parameters to obtain good quality welds. In this scenario, understanding of the physical LAHW phenomena has been proven useful for better control of the process,particularly to predict and avoid groups of parameters that can originate defects. For this reason, it is valuable to carry out experiments and to systematize the analysis methods.The presented work is focused on the impact of geometrical joint fit-up properties on the weld surface quality. The papers included here are organized as a comprehensive study of the effects and impact of various geometrical aspects of the laser-arc-workpiece arrangement on the surface quality of the welds, i.e. tracking from joint fit-up tolerances like gap width to critical weld shape aspects like undercuts via observation of fluid flow at the weld pool surface. Three frequent and critical geometrical aspects were identified from industrial edge preparation, namely gap width variations, vertical edgemismatch and vertical plate position to the laser-arc tool. These aspects can cause defects when the surface is bended or when the LAHW tool is automatically moved. Although in production they arise all simultaneously, to understand the respective contribution, systematic experiments were designed. These experiments were carried out to measure and surpass the stability threshold related to each aspect, to observe the flow behaviour and to evaluate the physical phenomena related to weld bead formation.The three Papers I, II, III describe a systematic methodology based on High Speed Imaging, HSI, on quantitative weld surface measurement obtained from a laser triangulation scanner and on statistical analysis of different experimental results. This methodology is based on the observation, measurement, automatic location and calculation of fundamental bead variables (top and root undercuts, reinforcement, melt pool length, pool width and flow speed, or mass balance) related to the melt flow.In Paper I, using a pulsed