Mitigation of root defect in laser and hybrid laser-arc welding: A model is developed that predicts the range of processing conditions that produce defect-free, complete-joint-penetration welds

Even though laser and hybrid laser-arc welding processes can produce single-pass, complete-joint-penetration welds in excess of 12 mm, root defects, such as root humping, have been observed at these greater plate thicknesses. The competition between the surface tension and the weight of the liquid metal in the weld pool is expected to govern root-defect formation. A series of laser and hybrid laser-gas metal arc welds has been completed in which each force is independently varied. The internal morphologies of the resulting root defects are characterized by X-ray computed tomography and found to vary significantly when welding with either the laser or hybrid laser-arc process. In order to compute the surface tension and liquid metal weight, a model based on the approximate geometry of the weld pool is developed and successfully predicts the range of processing conditions where root defects form. Process maps are then constructed for low-carbon steel and 304 stainless steel alloy systems. These maps can then be used to select welding parameters that produce defect-free complete-jointpenetration welds over a wide range of plate thicknesses.