The evaporation behavior of zinc and its effect on spattering in laser overlap welding of galvanized steels

In this paper, a computational fluid dynamics model considering zinc evaporation was innovatively established to investigate the interaction between zinc vapor and laser keyhole in both zero-gap and small-gap conditions. Under the zero-gap condition, it was observed by a high-speed camera that the entrance of zinc vapor gas into the keyhole had the cyclic characteristics, and the occurrence of spattering was closely related to the entrance of zinc vapor. Following the cyclic characteristics obtained from experiment, the zinc vapor with properties of mass and momentum was set to periodically entering the keyhole in the numerical model. The numerically calculated results under the zero-gap condition showed that the zinc vapor entered the keyhole at an initial velocity of 81 m/s, resulting in large deformation of the keyhole channel. The vapor flowed upward along the rear wall with a velocity of 15–20 m/s, pushing large amounts of melt in the molten pool upward and generating spatter. After the spattering event, the velocity of the remaining zinc vapor in the keyhole dropped to lower than 5 m/s, leading to shrinkage of the keyhole channel, and both the keyhole and the molten pool restored to a normal and stable state. Under the small-gap condition, the zinc vapor entered the keyhole continuously with an initial velocity of 33 m/s, which had little impact on keyhole geometry. The velocity of the zinc vapor inside the keyhole was significantly reduced comparing with the zero-gap condition, and the velocity of the upward flowing melt was lower than 0.75 m/s, thus no spatter occurred. The numerical analysis sufficiently explained why spatter was depressed under the gap condition.