Effectiveness of pre-scanning on zinc evaporation in laser spot welding of zinc-coated steels

Abstract

To prevent zinc vapor–induced spatter, pre-scanning was proposed as a means to evaporate the zinc coating prior to the welding process. This paper studied the effectiveness of various pre-scanning parameters such as beam defocusing distance, beam power, scanning velocity, and geometric scanning path. The gap conditions during pre-scanning were also studied in regard to outgassing of zinc vapor during pre-scanning. To complement the experimental testing, a thermomechanical numerical model was developed and validated by comparing the size of the predicted versus the physical fusion zone. The effectiveness of the pre-scanning process was evaluated by examining the temperature distribution at both the top and faying surfaces of the stack-up under the condition that the temperature at the faying interface is greater than the zinc boiling point through the entire pre-scanning process to prevent condensation of the zinc vapor back onto the faying interface. The study found that no gap at the faying interface should be allowed with the use of pre-scanning to ensure an effective heat conduction across the faying interface. When the pre-scanning path is spiral or circular, the beam translation should initiate at the center and work outward towards the periphery. The line energy is suggested to decrease as the radius increases to avoid excessive accumulation of heat in the path center.