Selecting and Delivering Shield Gas in Laser Welding
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.
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
| Shield Gas | Plasma suppression | Prevention against oxidation | Relative cost | Typical flow rates | Weld profile | Limitations |
|---|---|---|---|---|---|---|
| He | Excellent | Good | High | 30-40l/min | Deepest penetration | None |
| Ar | Lower | Excellent | Medium | 20-25l/min | Wide | Plasma cloud reduces power density |
| N2 (O2 free) | Lower | Good | Low | 20-25l/min | Deepest penetration | Embrittlement of certain alloys (ex Ti) |
| CO2 | Lower | Poor | Lowest | 30-45l/min | Nominal | No usefull for reactive materials |
| He+Ar (20/80%) | Good | Very Good | Medium | 30-35l/min | Nominal | None |
Fiber Laser welding with and without protective Gas
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, tubing, and the so-called ‘shoe’ 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).
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. 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.
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.