Periodic root humps in thick-plate laser welding using steady electromagnetic force

The transient and dynamic forming process of periodic root humps was observed using high-speed camera (CCD) and the 'sandwich' method. The formation mechanism of periodic root humps was analyzed. The magnitude and orientation of the flowing molten material under the keyhole were the primary factors to affect welding stability of back surface. The additional steady electromagnetic force (SEMF) could be used to prevent periodic root humps. The Laser-SEMF welding method not only could minimize the input laser power in welding process of thick plates, but also made the welding process insensitive to the laser power. The SEMF was little influenced by hysteresis effect and existed continuously in transient welding process.