# Laser Welding under Vacuum: A Review
## 6 Prospects of Laser Welding under Vacuum  
With the reference to the previous researches of laser welding under vacuum, the main advantages of laser welding under vacuum or reduced pressure are as follows [45; 47; 61; 62]:  
* Significant increase (More than two times) of the welding depth;
* Parallel-sided seams with reduced nail head;
* Increased process stability due to greatly reduced and stable plasma plume;
* Reduction of workpiece contamination by sputter and vaporization;
* Higher-quality, pore-free weld seams;
* Low operating costs due to efficient solid-state laser;
* Welding process without inert gas;
* Lower vacuum level and simpler device compared with electron beam welding. Despite the advantages of laser welding under vacuum above, the vacuum chamber causes laser welding loose the most important advantage: flexibility. In addition, the dimensions of the welded part were restricted by the dimensions of the vacuum chamber. A larger vacuum chamber means higher cost and associated pumping time [65]. A lot of trails were carried out to make the electron  
Figure 12: The survey of weldable plate thicknesses/materials with laser welding under vacuum. Reproduced with permission from [31], [PERSON], 2016. (**a**) Laser welding under vacuum of unalloyed steel S690 QL, (**b**) Laser welding under vacuum of nickel-base alloy 617.  
beam welding release the limitation of vacuum chamber. Rolls-Royce and TWI worked on a local vacuum, or \"out of chamber\" to expand the application of electron beam welding in thick section components [66]. However, the electron beam interaction with low pressure gases is the difficult point in the application of electron beam welding under local vacuum. However, the interaction of the laser with ambient gases is negligible. In addition, the relatively low vacuum can achieve an obvious increase in penetration depth. Therefore, laser welding under local vacuum or mobile vacuum has a more practical application value. ISF RWTH-Aachen and BAM, Germany has conducted some attempts in laser welding under mobile vacuum [66; 67; 68]. As shown in Figure 13, a vacuum cap, which provides a local reduced ambient pressure of about 20 kPa above the welding area, was presented by BAM. The reduced pressure in the vacuum cap generate 50% higher penetration depth in comparison to welding under ambient pressure conditions. Due to the restriction of this movable seal, the vacuum level is very low and the penetration depth has no significant increase. The laser welding under vacuum can be combined with local vacuum electron beam welding to accumulate the disadvantages of those two processes. Hence, a local vacuum seal with a laser, instead of the electron beam generator may be a promising technique in the future. Laser welding under vacuum not only has promising application prospects, but also provides a special physical environment to achieve a fundamental understanding of the laser welding process. Laser keyhole welding is a complex physical process involving heat flow, melting, evaporation, solidification phase changes, melt flow, and vapor flow. Most physical phenomena in laser keyhole welding have not been fully understood due to the complexity of laser welding. The phenomena of laser welding are completely different at the same welding parameter except for ambient pressure. By comparing the differences between atmosphere and vacuum and analyzing the influences of ambient pressure, the essences of laser welding can be more deeply understood. For example, the interaction of laser radiation and plasma plume is a focused issue in high power laser welding. Under vacuum conditions, the plasma plume was suppressed completely. By studying the effect of ambient pressure on the plasma plume, we could get a better understanding of the interaction of laser radiation and plasma plume. There is no such thing as a perfect technology. The laser welding under vacuum is no exception. There are also some challenges in laser welding under vacuum.