There are two main reasons to use shielding gas when laser welding;
1) Keep air (oxygen and nitrogen) away from the molten metal while it solidifies and cools
2) Reduce the size of the plume called “suppression”
Inert gases such as argon are frequently used for shielding. Helium is also inert and is also used for plume suppression.
The video shows the immediate influence of turning the argon shielding gas on and off during a pulsed Nd:YAG laser weld. This video shows how quicky defects can be created if shielding gas is disrupted.