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What protective gas should I use for laser welding?

Does the machine need auxiliary gas?

Yes, Nitrogen (N2), Argon (Ar) and Helium (He) are all OK. But for materials easy to be oxidised, Argon is better.

 

The gas can isolate the air from the welding plate to prevent reaction with the air. So the welding surface of the metal plate will be white and beautiful. The gas also can protect the lens from welding dust.

Type of protective gas

Commonly used laser welding protective gases are mainly N2, Ar, He, and their physicochemical properties are different, so the effect on the weld is also different.

1. Nitrogen N2 - a welding protective gas of stainless steel

The ionization energy of N2 is moderate, higher than that of Ar, lower than that of He, and the degree of ionization is generally under the action of laser, which can better reduce the formation of plasma cloud, thereby increasing the effective utilization of laser.

Protective Aluminum and carbon steel welds: Nitrogen can react with aluminum alloy and carbon steelat a certain temperature to produce nitrides, which will improve the brittleness of welds and reduce the toughness, which will have a great adverse effect on the mechanical properties of welded joints. Therefore, it is not recommend to use nitrogen.

Protective Stainless steel welds: The nitride produced by the chemical reaction between nitrogen and stainless steel can improve the strength of the welded joint and improve the mechanical properties of the weld. Therefore, when welding stainless steel can use nitrogen as a shielding gas

2. Argon Ar - high cost performance, the most conventional protective gas

The ionization energy of Ar is relatively low, and the degree of ionization is high under the action of laser, which is not conducive to controlling the formation of plasma cloud, which will have certain influence on the effective utilization of laser, but the activity of Ar is very low, and it is difficult to chemicalize with common metals.  The reaction, and the cost of Ar is not high, in addition, the density of Ar is larger, which is favorable for sinking above the weld pool, and can better protect the weld pool.  So Argon is a VERY conventional shielding gas.

3. Helium He - the best but also the most expensive protective gas

He has the highest ionization energy and low ionization under the action of laser. It can control the formation of plasma cloud very well.  So The laser can work well on metal, and the activity of He is very low, and it does not chemically react with metal. VERY GOOD weld protection gas, but the cost of He is too high, generally not used in mass production products.  He is generally used for scientific research or products with very high added value.

Selecting and Delivering Shield Gas in Laser Welding

In laser welding, the shielding gas, sometimes referred to as ‘cover gas’, has three main roles:

  1. Protect the weld metal from reacting with the ambient environment, (e.g. oxygen, nitrogen, hydrogen),
  2. 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.
  3. 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

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.