In laser welding, the shielding gas (sometimes referred to as ‘cover gas’) fulfils two main roles:
- to protect the weld metal from oxidation and
- to reduce the formation of a plasma, or cloud of ionized gas, above the weld, that can partially block and/or distort the focused laser beam.
Formation of plasma is more critical when using a CO2 laser (10.6 µm wavelength) for laser welding than when using Nd:YAG or Yb-fiber based solid-state lasers (1 µm wavelength). This is because absorption of the far infrared CO2 laser beam by the plasma is greater than absorption of the near infrared beam of solid-state lasers.
As the focused laser beam is absorbed by the plasma, laser power is absorbed (reduced) and the beam shape changes. This interaction between the laser beam and material generally reduces penetration and changes 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 (>4 mm) 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, the 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 these and other shield gases 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 must be high, especially for open, three-dimensional components. This coupled with the high cost of helium makes use of other lower cost gases (argon and nitrogen) preferred.
For whichever shielding gas type and delivery method 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, underbead shielding is recommended to produce an acceptable appearance of the weld.
A second important consideration, after the choice of shield gas, is the means used to deliver the shield gas to the weld.
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) may be used.
Shield gas delivered using an auxiliary tube design is typically aimed at the trailing portion of the weld (hot material). For full penetration welds requiring protection of the bottom side of the weld, 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 delivers the required volume of gas at a relatively low velocity.||Figure 2: SmartShield™ crossjet welding nozzle with shield gas ‘shoe’ to protect the metal at the point of welding and behind the weld during cooling.|
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