Helium (He) is an inert gas, rather rare, scarcely present in the atmosphere and extracted from the subsoil, consequently it is much more expensive than argon.
The properties of helium compared to argon, are lower arc stability but higher penetration depth, its use is predominant in welding of great thicknesses and of materials with high thermal conductivity such as copper and aluminum.
Since helium, unlike argon, is less heavy than air and therefore more volatile, a greater quantity of gas is required to ensure the correct protection for the welding area.