Analysis of laser arc hybrid welding experiments

Abstract: 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 edge mismatch 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 arc mode, the effects of increasing the standoff between the LAHW tool and the workpiece are described and a mechanism is identified to explain the flow behaviour and its consequences on the resulting weld pool shape. In Paper II, again using pulsed arc mode in LAHW, the effects of the vertical edge mismatch and of the gap width on the weld pool shape are described, again followed by a theoretical description. Paper III compares the pulsed arc mode with the CMT mode in LAHW, again for the effect of the gap width on the weld surface shape, in particular the top bridging phenomenon that was identified. Also these results are theoretically described, explaining the mechanisms via HSI. Altogether, the three papers comprise a theoretical description of the LAHW weld shape behaviour and weld quality depending on the fit-up tolerances, based on experimental evidence and analysis. From this chain of evidence and associated understanding, conclusions were drawn and practical guidelines were derived.