| Arc welding is the most widely used sets of joining technologies in the industry today.The automotive tier supplier network and light manufacturing are significant users of arc welding,particularly gas metal arc welding(GMAW)and pulsed GMAW(GMAW-P).For sheet metal welding the majority of welds are single pass fillet welds on lap joint or T-butt joint edges.Increase in productivity can be realized in high travel speed pulse GMAW welding,however,in practice,the constraints are the threshold limit of the traveling speed beyond which generate humping defect,a weld profile defect with a wavelike profile to the weld bead that has peaks and troughs in the longitudinal direction.The weld bead quality is severely undermined due to the humping defect formation,which significantly limits the efficiency gain in high travel speed welding process.In this study,a lap joint welding of steel sheets was considered.The effects of parameters such as wire feed speed(WFS),travel speed(TS)and the lead travel angle on the shape and size of weld hump defects are systematically studied.The optimum process parameters for the GMAW and the hybrid laser-GMAW welding were obtained.The main research contents and results of this study are as follows:1)A visualized hybrid laser-GMAW welding platform was built to monitor the welding process in real time through a high-speed camera.With the filter of a specific band,the image of the molten pool during the welding process was obtained.Analysis of the weld pool image under different welding parameters was used to determine the relationship between weld hump formation and main welding parameters.2)Based on the SWAT model,the relationship between the travel speed(TS),the sheet thickness,and the lead travel angle was established.The maximum travel speed of the 1.5 mm steel sheet and the 3 mm steel sheet at 15° and 30° lead travel angles was obtained by experimental analysis.The experimental values were consistent with the prediction results given by the SWAT model.The relationship between the wire feed speed(WFS)and the travel speed(TS)was obtained through experiments.The SWAT model and experimental analysis show that as the thickness of the welded sheet increases,the travel speed decreases,and increasing the lead travel angle was beneficial in realizing higher travel speed.The travel speed(TS)is also related to the position of the hybrid heat source(HHS).When the HHS is position at the top sheet at positive(+?y),the observed welds produced were better in weld appearance and produced a bit more convex in shape.However,when the HHS is position at the bottom sheet at negative(-?y),shows both ends of the bead appearance was irregular with lack of fusion,undercutting at the top sheet edge at high travel speed and evidence of humping at low travel speed.In both cases,the best welding results were obtained when the HHS was position at +0.5 mm.3)A comparative experiment of the hybrid laser-GMAW welding and GMAW welding processes were conducted.The results show that when the laser welding head and the GMAW welding head are in the same position,the welding width of the hybrid laserGMAW welding is narrowed,the lead angle becomes smaller,and the travel speed increased by about 40% compared with the GMAW.However,the ratio of WFS to TS remains unchanged during the two welding processes,indicating that the WFS is only related to TS.4)The weld humping formation process was simulated by welding pool image analysis.In the hybrid GMAW welding,when the droplet comes in contact with the molten pool at TS lower than 3.5 m/min,the droplet breaks into tiny flows,with the dominant flows moving toward the top and bottom sheet edge of the weld.As a result of surface tension,the weld pool subsequently moves backward.This flow pattern results in a stable arc and flow of molten metal,resulting in a good appearance of the weld bead without a hump.When the TS is higher than 3.5 m/min,the rate of heat input reduces resulting in less metal vapor and elongated molten pool.The two dominant flows conveyed bulk of the molten metal along towards the weld boundary exposing the impact area.This modifies the conductive channel resulting in intermittent arc instability.As the velocity of the molten pool increases,backflow end and the new rear-flow collision caused swollen and thin valley formation.Subsequently,the thin valley solidifies truncating the flow channel.The persistence of these complex phenomena periodically and slowly induced hump formation.The scenario of the next hump formation shows that the swollen buildup height shortens the distance between the back-flow and the impact area thereby causing the collision closely at the impact area,with the metal surface exposed after conveying of bulk molten metal away by the two dominant flows.This modifies the conditions at the impact area resulting in to,arc instability,copious spatter and metal loss.This truncates the flow channel as eventually a thin valley is created and subsequently the flow channel solidifies leading to the next hump formation.Higher laser power is required at higher TS to maintain weld pool conductivity and inhibit hump formation.The research results of the thesis have practical guiding significance for rational selection of hybrid laser-GMAW welding process parameters,suppression of hump defect formation,improvement of welding quality and welding efficiency. |
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