Common shielding gases used in laser welding machines
The most commonly used shielding gases in laser welding machines are helium, argon, and nitrogen.
1. Argon is inexpensive and has high density, providing excellent shielding performance. However, it is easily affected by ionization from the hot metal plasma, which can cause partial beam shielding, thereby reducing the effective laser power for welding and decreasing both welding speed and penetration depth. The surface finish of argon-shielded welded components is smoother than that of helium-shielded ones.
2. Nitrogen is the cheapest shielding gas, but it is unsuitable for welding certain types of stainless steel, primarily due to metallurgical issues. It may also cause porosity in the weld pool.
3. Helium resists ionization, allowing the laser to pass smoothly and delivering beam energy unimpeded to the workpiece surface. It is the most effective shielding gas for laser welding but is relatively expensive.
How to Select Shielding Gas for Laser Welding Machines?
Shielding Gas Type and Laser Wavelength
The interaction between the shielding gas and the laser-melted metal generates plasma above the weld. When the focused laser beam is absorbed by the plasma, laser power is absorbed (reduced), and the beam shape undergoes changes. This interaction between the laser beam and the material typically reduces penetration depth and alters the weld shape. Plasma absorption of the 10.6µm wavelength CO₂ laser beam differs from the absorption of near-infrared beams from solid-state lasers.
Technically, helium has the ability to suppress any plasma formation, making it the most suitable shielding gas for CO₂ laser welding machines. For welding with Nd:YAG and fiber lasers (wavelength 1µm), helium can also be used for welding stainless steel, aerospace alloys, and a range of aluminum alloys.
However, due to its low density, a higher flow rate is required to provide effective gas shielding, particularly for open three-dimensional components. Combined with the cost of helium, this makes other cost-effective gases (argon and nitrogen) more attractive.
Shielding Gas and Weld Metal
Certain metals and alloys react with nitrogen, altering the weld microstructure. For instance, nitrogen reacts strongly with titanium, forming titanium nitride compounds that make laser welds brittle. Consequently, argon serves as the shielding gas for welding titanium-based alloys.
The same applies to certain stainless steels. Nitrogen cannot be used for welding austenitic stainless steels alloyed with titanium and niobium. Nitrogen forms nitrides with these elements, thereby reducing the amount of free titanium and niobium available to prevent chromium carbide formation and intergranular corrosion.
For ferritic stainless steels, nitrogen shielding gas behaves similarly to carbon. Introducing nitrogen into the material during ferritic steel welding increases the amount of martensite in the weld metal, causing the metal to become brittle.