In laser welding, the shielding gas, sometimes referred to as ‘cover gas’, has five main roles:
Protect the weld metal from reacting with the ambient environment, (e.g. oxygen, nitrogen, hydrogen)
Protective gas can protect the laser head lens from metal vapor pollution and liquid droplet sputtering.
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.
Maintain a stable process and stable weld pool.
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.
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.
|Shield Gas||Plasma suppression||Prevention against oxidation||Relative cost||Typical flow rates||Weld profile||Limitations|
|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|
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.
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.