In laser welding, the shielding gas, sometimes referred to as ‘cover gas’, has three main roles:
- Protect the weld metal from reacting with the ambient environment, (e.g. oxygen, nitrogen, hydrogen),
- Prevent or minimize formation of a plasma, or cloud of ionized gas, that can form above the weld. The plasma is undesirable since it can partially block and/or distort the focused laser beam.
- Maintain a stable process and stable weld pool.
In general, the type of shielding gas used during high power laser welding process can play an important role in the process and can affect the resulting weld through influences on welding speed, microstructure, and shape.
Commonly used shield gases
The most frequently used shield gases for laser welding are helium, argon and nitrogen. The table below provides a comparison of these and other shield gases used for high power laser welding.
Table 1: Comparison of gases commonly used for shielding in laser welding
Shield gas type and laser wavelength
The interaction of the shield gas and laser melted metal can create a plasma above the weld. As the focused laser beam is absorbed by the plasma, laser power is absorbed (reduced) and the beam shape changed. This interaction between the laser beam and material generally reduces penetration and changes the shape of the weld.
Formation of plasma is more critical when welding with a CO2 laser (10.6-µm wavelength) than when using Nd:YAG or Yb-fiber based solid-state lasers (1 µm wavelength). This is because absorption of the 10.6 µm wavelength CO2 laser beam by the plasma is greater than absorption of the near infrared beam of solid-state lasers.
Welding with Nd:YAG and Yb-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 and shape of the weld.
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 (1 µm wavelength) welding, helium 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 other, lower cost gases (argon and nitrogen) more attractive and economical.
Shield gas reaction with weld metal
Certain metals and alloys react with nitrogen in a way that changes the microstructure of the weld. For example, nitrogen reacts strongly with titanium to form titanium nitride compounds that can make the laser weld brittle. For this reason, argon is the preferred shield gas for welding titanium-based alloys.
This is also the case for certain types of stainless steels. Nitrogen should not be used for welding austenitic stainless steels alloyed with titanium and niobium. Nitrogen forms nitrides with these elements, reducing the amount of free titanium and niobium available for preventing chromium carbide formation and sensitivity to intergranular corrosion.
For ferritic stainless steel, nitrogen shield gas has the same effect as carbon. Introduction of nitrogen into the material during welding of ferritic steels leads to an increased quantity of martensite in the weld metal. This, in turn, can make the weld more brittle and more susceptible to hydrogen embrittlement.
Shield gas type and porosity
The type of shield gas can also influence the presence and character of weld defects, such as porosity.
The type of shield gas affects porosity primarily through:
- Its influence on the stability of the molten pool.
- The solubility of the shield gas within the molten metal.
Metals and alloys containing constituents with high vapor pressure are less prone to porosity formation.
Why? High vapor pressure leads to greater stability of the weld pool in keyhole welding. For example, alloys containing significant amounts of Mn tend to exhibit less porosity since the keyhole is more stable. For alloys for which the weld pool is inherently stable, the type of shield gas has a negligible effect on porosity.
On the other hand, a metal with a low vapor pressure tends to have a less stable keyhole making it more prone to entrapment of gas. For these, our experience, at least related to aerospace alloys, and that of other researchers suggest that nitrogen favors lower porosity.
High solubility and high reactivity of the shield gas with the weld pool also tends to minimize porosity formation. For this reason, nitrogen also tends to yield lower porosity than argon in welding steels, stainless steels, and nickel-based alloys.
The following case study provides evidence to support these principles.
Case Study: Inconel 625
An aerospace manufacturer with a long history of CO2 laser welding indicated interest in fiber laser welding 3 mm thick Inconel 625 parts. From their experience, they believed that the correct shield gas for the application was argon.
To identify the best shield gas, we produced a series of Yb-fiber laser welds using nitrogen, argon, and a mixture of helium and argon shield gases.
Figure 1 shows the ‘bead-on-plate’ welds produced using the following common laser and process parameters:
- Laser power: 1.8 kW
- Laser output: CW
- Focused spot size: 214 µm
- Weld speed: 1.25 m/min
- Shield gas flow rate: 30 l/min
- Shield gas delivery: coaxial
Results, summarized in Figure 1, show the corresponding shapes and porosity, the two main parameters of interest for this application.
The micrographs in Figure 1 show that all welds have somewhat of an hour-glass shape. However, the weld produced using nitrogen is less tapered and more columnar than those produced using argon-based shield gas.
Welds with nitrogen did not exhibit porosity in x-ray inspection. However, those produced using argon and an argon-helium mixture both included significant porosity.
Delivering the shield gas
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 2), tubing, and the so-called ‘shoe’ (Figure 3) may be used. The ‘shoe’ is particularly useful for metals, such as titanium, which must be shielded over a wider range of temperature as the weld cools.
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. 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.
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.
|Figure 2: Coaxial nozzle for laser welding. A larger diameter opening delivers the required volume of gas at a relatively low velocity.||Figure 3: SmartShield™ cross jet 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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