# The Pre-Heating Effect for Porosity Control during the Laser Welding of GAlvanized Steel Sheets
## 1 Introduction  
Zinc-coated steel sheets [1], widely utilized as automotive materials, have consistently seen an increasing adoption rate compared to non-coated steel sheets [2]. However, the application of zinc-coated steel sheets has been accompanied by challenges during welding due to their significantly lower melting and vaporization temperatures of the zinc layer compared to steel sheets, resulting in welding defects such as blow holes and pits [3; 4; 5; 6; 7]. This issue arises from the formation and growth of porosities within the molten pool caused by zinc vapor [8; 9; 10; 11], and various methods have been continuously devised to prevent such welding defects [12; 13; 14; 15; 16; 10; 17].  
[PERSON] et al. verified the reduction in welding defects by creating a fine gap for zinc vapor discharge between the overlapping joints of galvanized steel sheets [12]. [PERSON] et al. improved weld quality by combining a YAG laser with a TIG welding process, using additional energy from the TIG plasma to sustain the keylhole during welding [13]. [PERSON] et al. devised a method for inserting a thin aluminum foil for generating an Al-Zn alloy between the overlapping joints of a galvanized steel sheet using a relatively low melting point (about 660 \({}^{\circ}\)C) and a high vaporization point (about 2519 \({}^{\circ}\)C) [14]. [PERSON], [PERSON] et al. obtained a higher performance of pore-free welding in the GMA process by reducing the CTWD and using ER705-3 wire (Harris Products Group, Mason, OH, USA) [15]. Although these methods have suggested a solution to significantly reduce welding defects, they have a series of problems that additional processes or equipment are indispensable.  
Therefore, methods for improving the quality of galvanized steel sheet welds solely through laser welding equipment, without the need for an additional separate process, have been newly proposed. [PERSON], [PERSON] et al. reported that reducing the spiral spacing in laser spiral welding processes of galvanized steel sheet material had a greater effect on pore suppression than increasing keyhole velocity [10]. [PERSON], [PERSON] et al. improved the quality of the welds by devising a two-pass welding process to remove the galvanized layer by irradiating the galvanized steel sheet with a laser in the pre-heating mode before the main welding [16]. [PERSON], [PERSON] et al. reviewed and verified the applicability of two-pass welding during the laser welding of galvanized steel sheets [17].  
In this study, we tried to apply a pretreatment procedure that added pre-heating steps before the main welding to control the porosity of galvanized steel sheets by eliminating the galvanized layer. This welding process, named three-step laser stitch welding, was devised to secure sound welds during the overlap welding of SGARC 340 material for automobile body parts. Through the first and second steps, zinc vaporization was induced to remove the zinc plating layer, and in the third step, penetration depth was secured by increasing the power, and main laser welding was performed. These entire steps were carried out using a single laser heat source without the need for additional processes or equipment. Furthermore, to investigate the characteristics of weld quality according to the change in process parameters during laser welding, the reliability of the weld was assessed by considering the weld bead characteristics and the mechanical and metallic characteristics.