| Welding speed is a key parameter of high speed welding when joining thin sheets. Undercut is one of the most important problems which restrict welding speed.Both computer simulation and experimental studies were conducted to find the relations between undercut and surface tension temperature coefficient, temperature gradient, the direction of surface flow of welding pool, and dynamic ratio of depth/width. The preventive measures were given through the experiments of moveless welding, welding with single arc and pressing arc with electric magnetic field and double arc welding.The research indicated that surface flow direction of welding pool near the periphery is the most relevant reason of undercut occurred. When the existence of surface active elements (e.g. the content of S is about 0.02-0.04% in steel) change the sign of surface tension temperature coefficient from negative to positive, undercut always occurs in TIG moveless welding, and not occurs when the content of S in iron is very low( For example, S% is less than 0.01%).The temperature gradient is the most important factor which influences the depth of undercut. The depth of undercut is bigger with the increasing of temperature gradient when the other parameters are the same. The tendency and depth of undercut are bigger in the welding conditions of big current with short time than in the small current with long time in TIG moveless welding, and it is bigger under high speed welding with large current than under low speed welding with small current. By increasing molten angle, the movement resistance of liquid metal of the pool and the cooling speed near the periphery could be decreased, which restrains undercut obviously. Filling metal into welding pool restrains the molten metal moveing from periphery to the pool center, which reduces the tendency of undercut, and eliminates undercut ultimately.It was indicated by simulation and experiments that the biggest depth cross...
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