# Laser Welding under Vacuum: A Review
## 3 The Equipment of Laser Welding under Vacuum  
In order to carry out laser welding under vacuum, the first issue is to build up a vacuum chamber for laser welding. The simplest way is to refer to the basic design of electron beam welding. Instead of an electron beam generator, a laser processing head is installed in vacuum chamber. The design of a built-in laser head has the advantage of a more flexible laser welding under vacuum machine. However, a special seal should be designed to couple optical fiber and water-cooled pipelines into vacuum chamber. With the cooperation of ISF at RWTH Aachen University and II-VI HIGHYAG, a fiber-fiber-coupler was design to develop an easy and affordable method to transfer laser power into the vacuum. The fiber-fiber-coupler open the possibility of high power laser processing with built-in laser head under vacuum [42]. Unlike electron beams, laser radiation is a matter of light. Laser beams can pass through the glass into a vacuum. Therefore, there is another structural design for laser welding under vacuum. Laser processing head is fixed outside the vacuum chamber. Only the laser beam is coupled into the vacuum chamber through glass. As shown in Figure 2, most of the research institutions used the second design according to the current reports. JWRI at Osaka University created a transparent vacuum chamber using acrylic [43; 44]. The rotary pump was used to achieve vacuum conditions. The design of transparent vacuum chamber was beneficial to take photos with high speed camera and other optical tests. AWJ of Harbin Institute of Technology set up a 680 \(\times\) 400 \(\times\) 400 mm vacuum chamber. The theoretical limit vacuum level was 6.6 \(\times\) 10\({}^{-4}\) Pa by using a rotary pump and a molecular pump simultaneously. The ifs at TU Braunschweig reformed a 1.5 m\({}^{3}\) vacuum chamber of former electron beam welding plant [33]. A blank flange was manufactured in which a quartz glass was located as a coupling-in window to couple laser into chamber. ISF at RWTH Aachen University established a cylinder vacuum chamber of 0.6 m\({}^{3}\)[31]. The vacuum chamber was equipped with two rotary vacuum pumps. This vacuum chamber had a minimum vacuum pressure of 10\({}^{-3}\) hPa and the ability to reach 0.1 hPa in 5 min. Besides, the vacuum chamber could realize different welding positions by turning around the longitudinal axis. Figure 2: The equipment of laser welding under vacuum. Reproduced with permission from [31], Springer, 2016; Reproduced with permission from [33], Laser Institute of America, 2011; Reproduced with permission from [44], Taylor & Francis, 2014. According to the above designs of vacuum chamber, the equipment of laser welding under vacuum basically consists of three parts: laser processing head, vacuum chamber and coupling-in window. The laser processing head should be set above the vacuum chamber. A conventional system for laser welding at atmosphere can easily fulfill the requirement. Compared with electron beam welding, laser welding under vacuum has a relatively low requirement of vacuum level. Therefore, the vacuum chamber structure of laser welding is relatively simple. Hence, the key part of the vacuum chamber is the coupling-in window for coupling the laser beam into the vacuum. A ZnSn glass were used as coupling-in window in CO\({}_{2}\) laser welding under vacuum. A quartz glass was used for solid laser welding under vacuum. In laser welding under vacuum, intense metal vapor ejected from keyhole and deposited on the surface of glass. That means that the stability of laser propagation and the lifetime of glass could be seriously affected. A laser head with long focusing length [31; 43; 44] (JWRI at Osaka University and ISF at RWTH Aachen University) or a special coupling-in window [32; 34] (ISF at RWTH Aachen University and its at TU Braunschweig) with a rather small gas flow under the glass are used to protect the coupling-in window from process emissions.