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Shield Gas in Laser Welding

Selecting and Delivering Shield Gas in Laser Welding

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

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

  2. Protective gas can protect the laser head lens from metal vapor pollution and liquid droplet sputtering.

  3. 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.  The metal vapor absorbs the laser beam and ionizes into a plasma cloud. If there is too much plasma, the laser beam is consumed by the plasma to some extent. The shielding gas can disperse metal vapor plumes or plasma clouds, reduce the protective effect of the laser, and increase the effective utilization rate of the laser.

  4. Maintain a stable process and stable weld pool.

  5. Cool down the welding torch and keep the torch on a stable temperature

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.

What protective gas should I use for laser welding?

Does a laser welding 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.

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.

There are currently two main ways to blow in protective gas:

  • It is the side-blow protection of the side shaft
  • It is coaxial protection (standard in our PhoyonWeld laser Welding Machines

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.

The shielding gas blown in not only needs to protect the weld pool in a timely manner, but also needs to protect the just solidified area that has been welded. Therefore, side-shaft side blowing protection is generally used, because this method of protection is relatively The protection range of the coaxial protection method is wider, especially for the area where the weld has just solidified.

Sideshaft side blowing For engineering applications, not all products can be protected by sideshaft side blowing. For some specific products, only coaxial protection can be used. It needs to be targeted from the product structure and joint form. 

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.

The table below provides a comparison of these and other shield gases used for laser welding

Shield Gas Plasma suppression Prevention against oxidation Relative cost Typical flow rates Weld profile Limitations
He Excellent Good High 20-40l/min Deepest penetration None
Ar Lower Excellent Medium 12-25l/min Wide Plasma cloud reduces power density
N2 (O2 free) Lower Good Low 15-25l/min Deepest penetration Embrittlement of certain alloys (ex Ti)
CO2 Lower Poor Lowest 20-45l/min Nominal No usefull for reactive materials
He+Ar (20/80%) Good Very Good Medium 20-35l/min Nominal None

​​​​​​​The role of shielding gas in laser welding

What shielding gas should I use for laser welding?

In laser welding, shielding gas will affect not only the weld formation, the weld quality, the weld depth, and the weld width, etc. In most cases, the blowing of the shielding gas will have a beneficial effect on the weld, but bad use may also bring adverse effects.

The positive effect of shielding gas on laser welding

  • Proper blowing of the shielding gas will effectively protect the weld pool from oxidation and even avoid oxidation;
  • Proper blowing of the shielding gas can effectively reduce the splash generated during the welding process and protect the focusing mirror;
  • Proper blowing of the shielding gas can promote the uniform spreading of the weld pool during solidification so that the weld is uniformly formed and beautiful;
  • Proper blowing of the shielding gas can effectively reduce metal vapor or the shielding effect of the plasma cloud on the laser so that the laser energy reaching the surface of the workpiece, and thereby increase the effective utilization rate of the laser;
  • Proper blowing of the shielding gas can effectively reduce weld porosity.

As long as the gas type, gas flow rate, and blowing method are correctly selected, the ideal effect can be obtained. However, improper use of shielding gas can also have adverse effects on welding.

Adverse effects of improper use of shielding gas on laser welding

  • Improper blowing of shielding gas may cause the weld to deteriorate;
  • Choosing the wrong type of gas may cause cracks in the weld, and may also lead to the reduction of weld mechanical properties;
  • Choosing the wrong type of gas may cause the weld to be more oxidized (whether the flow is too large or too small), or it may cause the weld pool metal to be seriously disturbed by the external force to cause the weld to collapse or form unevenly;
  • Choosing the wrong gas blowing method will result in the weld not achieving the protective effect or negatively affecting the weld formation;
  • Insufflation of the shielding gas will have a certain effect on the weld penetration, especially when the thin plate is welded, the weld penetration will be reduced.

Shield gas reaction with weld metal

Which gas does react with metals during laser welding?

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.