# Laser Welding under Vacuum: A Review
## 4 Fundamental Phenomena during Laser Welding under Vacuum
### The Influence of Ambient Pressure on Molten Pool and Keyhole Behaviors  
During laser welding, the molten pool and keyhole behaviors have a close relationship with weld quality and defect formation. Due to the specialty of molten pool and keyhole behaviors, it is difficult to understand and investigate the molten pool and keyhole behaviors. Therefore, the advanced imaging technology and numerical simulation are the main methods to study the molten pool and keyhole behaviors. Both methods are used to explore the effect of ambient pressure on the molten pool and keyhole behaviors. The surface molten pool and keyhole inlet of SUS 304 stainless steel under different ambient pressures were observed by [PERSON] et al. [44]. As shown in Figure 9, the surface molten pool and the keyhole inlet changed following the ambient pressure decreasing. The average diameter of keyholes decreased, the surface molten pool became narrower and more stable under vacuum conditions. Moreover, there were quite different liquid flows between atmospheric pressure and vacuum. [PERSON] et al. [20] presented the liquid flow in the molten pool at various ambient pressures using the micro focused X-ray real-time observation. Based on the results of [PERSON] et al. [20], there were two main differences in liquid flow between atmosphere and vacuum. On the one hand, the liquid flowed downward along the rear keyhole at atmospheric pressure. While the liquid flowed upwards along the rear keyhole wall under vacuum. That means the liquid flow along the rear keyhole is opposite. On the other hand, there was a molten flow along the bottom molten pool at atmospheric pressure, while there was no such strong flow near the bottom under vacuum. The differences in molten pool flow had a direct relationship with the porosity formation. In addition, the keyhole  
Figure 8: The spot behaviors of probe laser beam passing through the plasma plume under various ambient pressures (laser power is 5 kW, welding speed is 0.25 m/min, defocus distance is 0 mm). Reproduced with permission from [36], Elsevier, 2017.  
shapes at different pressures were observed directly by [PERSON] et al. [40]. Figure 10 (Figure 5 of \"Time-resolved X-ray Analysis of the Keyhole Behavior during Laser Welding of Steel and Aluminum at Reduced Ambient Pressure\" reproduced with permission from Proceedings of the International Congress on Applications of Lasers and Electro-optics, October 2015) shows the averaged keyhole shapes at ambient pressures from 0.5 kPa to 101 kPa. The keyhole depth increased with the decreasing of ambient pressure. This phenomenon corresponded to the increase in penetration depth under vacuum. In addition, the reduction of ambient pressure resulted in an increased inclination angle of the keyhole front and an increased bending of the keyhole tip. Figure 10: X-ray images of averaged keyhole shapes at ambient pressures from 0.5 kPa–101 kPa (laser power is 2 kW, welding speed is 1.2 m/min, defocus distance is 0 mm). ICALEO® 2015 Proceedings. Reproduced with permission from [40], Laser Institute of America, 2015. Figure 9: High speed video picture of keyhole inlet and molten pool in type 304 stainless steel weld observed under each reduced pressure(laser power is 16 kW, welding speed is 1 m/min, defocus distance is 0 mm). Reproduced with permission from [44], Taylor & Francis, 2014. [PERSON] et al. [23; 57; 58] investigated the physical phenomenon of laser welding under vacuum by numerical simulation. Based on the 3D transient multiphase model of laser welding, an improved model of recoil pressure for laser welding under any ambient pressure was proposed. The comparison of the predicted keyhole wall temperature and the predicted velocity distribution of metallic vapor under atmosphere and vacuum are presented. The results show that the average keyhole wall temperature became lower and the predicted speed of metallic vapor increased under vacuum. Based on the previous results of numerical simulation, [PERSON] et al.