Why is argon used in arc welding?

$\begin{array}{1 1} \text{High calorific value } \\ \text{Lowers the melting point } \\ \text{Flammability} \\ \text{Acts as a shielding gas} \end{array}$

Answer: Acts as a shielding gas
Shielding gases are inert or semi-inert gases that are commonly used in several welding processes. heir purpose is to protect the weld area from oxygen, and water vapour. Improper choice of a welding gas can lead to a porous and weak weld, or to excessive spatter.
Argon being an inert gas is a cost-effective shielding gas because of its low relativity with metal to provide an inert atmosphere.