## 2 Materials and Methods  
### VLBW System Design and Welding Material  
Figure 1 shows the laser welding system for VLBW and a magnified schematic of the cylindrical protective system used to address contamination of the coupling glass. The VLBW system is a superordinate concept that includes a cylindrical protective system. The VLBW system is a term used collectively for laser optic, vacuum chamber, and interior construction equipment (coupling glass, positioning stage, and protective system), and a cylindrical protective system that can protect the  
coupling glass inside the vacuum chamber. A 1070 nm wavelength laser beam was delivered using a 200 \(\upmu\)m diameter fiber and a 300 mm focal length optic. The laser beam was irradiated vertically through a coupling glass into a 74 L capacity vacuum chamber, where the vacuum level was controlled using a rotary pump with a maximum pumping rate of 1500 L/min. During welding, the pressure inside the vacuum chamber was maintained at 10 mbar. The distance between the focal lens of the laser optic and the coupling glass was 145 mm. The laser transmission region between the laser optic and the coupling glass was protected by a bellows, and the beam diameter at the focal point was 356 \(\upmu\)m. The diameter of the effective area of the coupling glass was 39 mm, while the laser was transmitted through a 10 mm-diameter area.  
A cylindrical protective system was installed at the bottom of the top surface of the vacuum chamber, as shown in Figure 1. The diameter of the aperture on the bottom of the protective system was chosen to avoid disturbing the laser beam propagation. In this study, N\({}_{2}\) shielding gas was supplied by a gas nozzle mounted on the side wall of the protective system.  
Figure 2 describes two hypothetical mechanisms of the protective system. The first mechanism is the pressure difference across the aperture. As the shielding gas is continuously supplied from the upper part of the protective system, the internal pressure increases and a local pressure difference is generated across the aperture. If the pressure difference \(\Delta\mathrm{P}=(\mathrm{P}_{1}-\mathrm{P}_{2})\) is positive, the shielding gas flows downward, as shown in Figure 2a, which disturbs the upward motion of the fume particles from the outside to the inside of the vacuum chamber [11]. Where \(D_{A}\) is diameter of the aperture. The second mechanism is the horizontal deflection of the fume particles due to the horizontal shielding gas flow, which is guided by the shielding gas nozzle, as shown in Figure 2b. The shielding gas flow V\({}_{\mathrm{N}}\) modifies the trajectory of the fume particles out of the coupling glass by changing the velocity of the fume particle V\({}_{\mathrm{F}}\) into V\({}_{\mathrm{F}}\)' [12]. Where \(D_{N}\) is diameter of the shielding gas nozzle.  
Using the two hypothetical approaches discussed above, contamination of the coupling glass by inflowing fume particles was primarily prevented by the local pressure difference across the aperture. Secondarily, the shielding gas flow was designed to change the trajectory of fume particles toward the outside of the coupling glass, to address fume particles that managed to get into the protective system through the aperture.  
The materials used in this experiment were 1.4 mm-thick dual phase (DP) 980 steels with galvanized (GI) coatings. The steel had a tensile strength of 1027 MPa and Zn coating layers on both sides with a weight of 79 g/m\({}^{2}\). The specimen size used in this experiment was 70 mm \(\times\) 70 mm. The vaporization of zinc during welding accelerates fume generation and was used to verify the effect of the process parameters on the contamination of coupling glass.  
Figure 1: Schematic diagram of the vacuum laser beam welding (VLBW) system.