# Laser Welding under Vacuum: A Review
## 4 Fundamental Phenomena during Laser Welding under Vacuum
### The Influence of Ambient Pressure on Plasma Plume  
It was known that the welding vapor plasma exerted a negative \"shielding effect\" in high-power CO\({}_{2}\) laser welding. The strong inverse bremsstrahlung (I.B.) effect decreases the penetration depth and even stops the CO\({}_{2}\) laser welding process due to the optical breakdown [51; 52]. The vapor plasma  
Figure 5: Effect of ambient pressure on penetration depth at various welding speed ICALEO® 2011 Proceedings. Reproduced with permission from [33], Laser Institute of America, 2011.  
Figure 6: Typical video pictures of A5052 aluminum alloy laser induced plume 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.  
exerts a different state between CO\({}_{2}\) laser welding and solid laser welding. Due to the presence of a large number of non-ionized metal particles, the vapor plasma is called \"metal vapor plume\" in solid laser welding [53]. However, the plasma plume still causes some disturbances by Rayleigh or Mie scattering in solid laser welding [54; 55; 56]. The previous studies [19; 33; 44] showed that no matter whether CO\({}_{2}\) laser welding or solid laser welding was used, the reduction of ambient pressure had a significant effect on plasma plume behavior. As shown in Figure 6 an extremely intensive and very bright plasma plume was exerted above the workpiece in high power laser welding at atmospheric pressure (101 kPa). The lower part plasma plume adjoined directly the keyhole has much higher brightness than the other part. Besides, a large number of spatters were observed around the plasma plume. The reduction of ambient caused an obvious change in plasma plume. When the ambient pressure reduced to 50 kPa, the change of plasma plume was already visible. Both the intensive luminescence and the volume of plasma plume decreased. At the reduced ambient pressure of 10 kPa, the intense illuminant plasma plume and obvious spattering disappeared. There was only a small brightness and volume of plasma plume above the keyhole. When the ambient pressure decreased at 0.1 kPa, the plasma plume was completely suppressed and invisible. The different materials and the welding parameters led the difference in critical ambient pressure of plasma plume disappearance.  
The enormous difference in the luminescence and the volume of plasma plume guide the research of the interaction between laser radiation and plasma plume under different ambient pressures. As shown in Figure 7, a probe laser, which horizontally passed through the plasma plume, was usually used to study the interaction between laser radiation and plasma plume [54; 56]. As shown in Figure 8, the spot behaviors of a probe laser beam passing through the plasma plume under various ambient pressures are presented [36]. The results showed that the interaction between laser radiation and plasma plume was minimal at low ambient pressures. Therefore, the laser process was more stable under vacuum. In addition, the extinction of fiber laser radiation decreased with decreasing ambient pressure [38]. Although the interaction between laser radiation and plasma plume under different ambient pressures has been discussed, the essential influence of ambient pressure on plasma plume has not been presented in detail. On the one hand, the illuminant change of plasma plume means that the ambient pressure has a significant effect on the ionization state and formation condition of the plasma plume. On the other hand, the ambient pressure has a direct relation with the stress state of metal vapor behavior. This may lead to a different laser welding process.  
Figure 7: The schematic of studying the interaction between laser radiation and plasma plume using a probe laser. Reproduced with permission from [54], Taylor & Francis, 2008.