In laser welding, the shielding gas (also sometimes referred to as ‘cover gas’) fulfils two main roles: to protect against oxidation and to reduce plasma formation. The formation of plasma is more critical to welding when using a CO2 laser, as there is an interaction between the laser beam and the cloud of ionized gas above the weld, which reduces the penetration and can change the shape of the weld. Welding with Nd:YAG and fiber lasers, with their 1 µm wavelength, does not commonly suffer from plasma formation. However when welding thick sections (>4mm) at slow welding speeds, there can be a cloud of gas above the weld which can affect the quality of the weld.
The type of shielding gas used during high power laser welding process can play an important role in the process and resulting weld since it can influence welding speed, weld shape, presence and character of weld defects, such as porosity, and mechanical properties of the joint. The most frequently used shield gases are helium, argon and nitrogen. The table below provides a comparison of the these and other shield gases that are used for high power laser welding.
Helium is technically the most suitable shielding gas for CO2 laser welding due its ability to suppress any plasma formation. For Nd:YAG and fiber laser welding, helium gas can also be used for welding stainless steels, aerospace alloys and a range of aluminum alloys. However, due to its low mass, flow rates to provide effective protection from the atmosphere is high, especially for open, three-dimensional components. This coupled with high cost of helium, makes use of other lower cost gases (argon and nitrogen) attractive.
For whichever shielding gas type and delivery method are used, too low gas flow will result in a heavy oxidized weld surface while too high gas flow causes excessive weld undercut and a disrupted weld bead.
In most cases underbead (bottom surface) shielding is not required for welding at speeds greater than 1m/min. However, for stainless steels, nickel alloys, titanium alloys and aluminum alloys, the use of underbead shielding is recommended in order to produce an acceptable underbead appearance.
A second important consideration, after the choice of shield gas, is the means in which the shield gas is delivered to the weld area. The shield gas is typically directed centrally at the laser/material interface. A variety of methods, including coaxial nozzles (Figure 1), tubing, and the so-called ‘shoe’ (Figure 2), are all used. If the gas is delivered using an auxiliary tube design, shield gas is also directed towards the trailing portion of the weld (hot material). For full penetration welds requiring protection of the bottom side of the weld, the fixturing is often designed to incorporate a means of delivering the shield gas to the bottom side.
|Figure 1: Coaxial nozzle for laser welding. A larger diameter opening is used to provide a sufficient volume of gas at relatively low velocity.||Figure 2: Crossjet welding nozzle with shield gas ‘shoe’ to protect the metal at the point of welding and behind the weld during cooling.|