# Laser Welding under Vacuum: A Review
## 2 The Research Status of Laser Welding under Vacuum  
Laser welding under vacuum or reduced pressure has a history of more than 30 years. As shown in Figure 1, the first report about laser welding under vacuum or under reduced pressure dates back to the 1980s. [PERSON] et al. [19] (Joining and Welding Research Institute, JWRI for short, at Osaka University) made the first effort to carry out the experiment of laser welding under vacuum conditions. The original intent of this research was to suppress the plasma and achieve a deep penetration in CO\({}_{2}\) laser welding. The problem of laser induced plasma, which is the most severe problem in laser welding at atmospheric, was completely solved by laser welding under vacuum. In addition, the exceeding expected penetration depth, which was approximate 2 times the depth of laser welding at atmospheric pressure, was achieved by laser welding under vacuum. However, this new laser welding method did not attract much attention due to the inadequate development of laser welding in industrial fields at that time. JWRI at Osaka University insisted on the research about laser welding under vacuum and paid their attention to fundamental phenomena in laser welding under vacuum. In 2001, the effect of vacuum on weld penetration and porosity formation was investigated in high-power CO\({}_{2}\) and YAG laser welding [20]. The reason for no porosity in vacuum was explained by keyhole and molten pool behaviors. In 2011, [PERSON] et al. [21] performed a high power disc laser welding experiment on 304 stainless steel and A5052 aluminum using a new chamber for laser welding under vacuum achieved by using rotary pumps. Sound deep single pass weld of 73 mm in penetration depth on type 304 stainless steel was obtained at a laser power of 26 kW, a welding speed of 0.3 m/min, a defocused distance of \(-40\) mm and an ambient pressure of 0.1 kPa. The research of [PERSON] et al. [21] showed the penetrability of laser welding under vacuum which was similar to that of electron beam welding and the application feasibility of laser welding under vacuum on thick plate welding. [PERSON] et al. (PIMM Laboratoire, Paris, France) have also been studying laser welding under vacuum or reduced pressure for many years. The research of [PERSON] et al. mainly focused on the explanation of weld geometry changes and penetration improvement under vacuum conditions. In 1995, [PERSON] et al. [22] observed the plasma suppression at low pressure by measuring plasma size, electron temperature and electron density. The energy deposition process inside the keyhole was  
Figure 1: The research status of laser welding under vacuum. compared to explain the different weld seam profiles between vacuum and atmosphere. Recently, [PERSON] et al. [23] made a detailed explanation of fundamental phenomena in laser welding under vacuum by theoretical analysis and numerical simulation. Since 2000, lasers have experienced an extremely rapid development. Especially during the last 10 years, the high-power (>10 kW, up to 100 kW) and high-brightness (<15 mm mrad) solid state lasers (fiber lasers or disk lasers) emerged into market [24; 25]. Due to the availability of a new generation of high brightness multi-kilowatt solid state lasers, high-power laser beam welding became a new stimulus. However, some new problems, for example, the weld defects of sagging and spatter, the intense plasma plume and turbulent molten pool flow, are accompanied during high-power laser beam welding [26; 27; 28]. That means that the application of high power single pass laser welding encounters a bottleneck. Under such a background, the method of laser welding under vacuum or reduced pressure has been enjoying a renaissance. In 2010, the Welding and Joining Institute (ISF) of RWTH Aachen University started research work of laser welding under vacuum using a 600 W single-mode fiber laser and a 12 kW multimode fiber laser [29]. The weld formations of both thick plate and thin plate were compared between laser welding under vacuum and electron beam welding.