ORIGINAL ARTICLE Effectiveness of pre-scanning on zinc evaporation in laser spot welding of zinc-coated steels Shengjie Deng 1 & Qian Guo 1 & Hui-Ping Wang 2 & Fenggui Lu 1 & Joshua Solomon 3 & Blair E. Carlson 2 Received: 10 September 2019 /Accepted: 23 December 2019 # Springer-Verlag London Ltd., part of Springer Nature 2020 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 devel- oped 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. Keywords Laser spot welding . Zinc-coated steel . Pre-heating . Zinc outgassing 1 Introduction Remote laser welding (RLW) has become an important tech- nology in manufacturing due to its high power density, high processing speed, and narrow fusion zone [1, 2]. Klinger [3] pointed out that a remarkable weight reduction of 12.2 kg could be achieved in a vehicle body-in-white structure by changing the resistance spot welded structure into a laser welded counterpart. The RLW is capable of maximizing struc- ture integrity by performing welding with a customized shape, which can embody the original intentions of the assembly designer as well as improve productivity [4, 5]. Zinc-coated steels are used extensively in the modern automotive industry to make a variety of components because of zinc coating’ s excellent protection to steels when exposed to corrosive envi- ronments [6, 7]. However, successful laser welding of the overlapped zinc-coated steels in a zero-gap condition is yet a technical issue to be resolved. The major challenges lie in two aspects: (1) the boiling point of zinc is 907 °C, while the melting point of steel is about 1370 °C; (2) zinc at the faying interface vaporizes due to laser heat and is often trapped in the molten pool, causing defects such as spatter, blowout holes, and/or pores [8]. A number of research initiatives have been conducted to resolve the aforementioned issues with some solutions having been proposed, such as introducing an ap- propriate gap at the faying interface by applying a spacer or dimpling on the bottom sheet [9–12], pre-fabricating sheets with special crimping [13–15], creating vent holes in the bot- tom steel sheet [16, 17], using two types of heating sources for pre-heating and keyhole welding, respectively [18], welding under subatmospheric pressure [19], or mitigating chemical composition at the faying interface [20, 21]. The above methods put forward ideas for addressing issues related to zinc * Hui-Ping Wang hui-ping.wang@gm.com * Fenggui Lu Lfg119@sjtu.edu.cn 1 Shanghai Key Laboratory of Materials Laser Processing and Modification, School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’ s Republic of China 2 GM Global R&D, Warren, MI 48092, USA 3 GM Manufacturing Engineering, Warren, MI 48092, USA The International Journal of Advanced Manufacturing Technology https://doi.org/10.1007/s00170-019-04891-9