# Laser Welding under Vacuum: A Review
## 4 Fundamental Phenomena during Laser Welding under Vacuum  
### The Influence of Ambient Pressure on Penetration Depth and Weld Geometry  
The increase in penetration depth and change in weld geometry are the most obvious achievements brought by the reduction of ambient pressure. According to the present experiment results [31; 33; 36; 38; 44], the penetration depth increases with the decrease of the ambient pressure. Based on the effect of ambient pressure on penetration depth, there exists a critical threshold of ambient pressure. That means the penetration no longer increases, and sometimes probably decreases when the ambient pressure is lower than the critical threshold. Because of the different machines and different experimental conditions, the critical pressures are not consistent. The previous results [31; 33; 36; 38; 44] showed that the critical pressure was in the range of 0.1 kPa to 10 kPa. [PERSON] et al. [44] carried out  
Figure 3: The systematic equipment of laser welding under vacuum (**a**) LAVA-L95. Reproduced with permission from [45], FOCUS electronics GmbH, 2017. (**b**) LASVAC PL 01. Reproduced with permission from [47], PTR Strahltechnik GmbH, 2017.  
the bead-on-plate welding of type 304 stainless steel plates and A5052 aluminum alloy with a 16 kW laser power at a welding speed of 1 m/min under reduced pressure. The results showed that the critical pressure was about 10 kPa and the penetration depth of both type 304 stainless steel plates and A5052 aluminum alloy at 10 kPa is nearly twice as deep as that at 101 kPa. The results of [PERSON] et al. [44] also revealed that the weld width decreased with the decreasing ambient pressure. The increase in penetration depth and the decrease in weld width led to a large difference in aspect ratio (penetration depth/weld width). That meant that the weld geometry between under vacuum and atmosphere were totally different. In comparison with conventional laser welding at atmospheric pressure, a deep and parallel weld, which is similar to electron beam welding, is achieved during laser welding under vacuum. The results of ISF at RWTH Aachen university also proved the strong penetrating ability of laser welding under vacuum [31; 48]. The cross section profiles of laser welding at atmosphere, laser welding under vacuum and electron beam welding are shown in Figure 4. With the same heat input, the penetration depth of both laser welding under vacuum and electron beam welding is about 50 mm. However, the pressure used in electron beam welding is two orders of magnitude lower than in laser welding under vacuum. The strong penetrating ability under low vacuum is the unique advantage of laser welding under vacuum. The ambient pressure, welding speed and defocused distance are the main parameters that control the weld appearance and penetration depth during laser welding under vacuum. It was found that, under vacuum condition, the increase of penetration depth was quite different at various welding speed [33; 49]. As shown in Figure 5 (Figure 6 of \"Influence of ambient pressure on spattering and weld seam quality in laser beam welding with the solid-state laser\" reproduced with permission from Proceedings of the International Congress on Applications of Lasers and Electro-optics, October 2011), a remarkable increase  
Figure 4: Comparison of cross section profiles using different welding processes (**a**) Laser welding (laser power is 16 kW, welding speed is 0.3 m/min, ambient pressure is 1000 mbar), (**b**) Laser welding under vacuum (laser power is 16 kW, welding speed is 0.3 m/min, ambient pressure is 10\({}^{-1}\) mbar), (**c**) Electron beam welding (electron beam power is 16 kW, welding speed is 0.3 m/min, ambient pressure is 10\({}^{-3}\) mbar). Reproduced with permission from [48], John Willey and Sons, 2015.  
of penetration depth occurs at low to medium speeds (up to approx. 3.0 m/min), it is true that there is a limit in the ambient pressure for improving penetration depth.