The role of shielding gas
What shielding gas can do
The primary function of the shielding gas in shielded arc welding has been to protect molten and heated metal from the damaging effects of the surrounding air to provide suitable conditions for the arc. If air comes in contact with the molten or heated metal, the oxygen in the air will oxidize the metal, the nitrogen might cause porosity or brittleness in the weld metal and moisture from the air might also cause porosity.
The shielding gas composition affects the material transport from the stolen electrode to the weld pool, which in turn influences the amount and size of the spatter created. It also affects the appearance of the weld bead, the weld geometry, the possible welding speed and plays a key role in the possible burn off of alloying elements (which affects material strength) or oxide formation on the bead surface.
These functions are affected by such factors as:
The emission of fume and gases is influenced by the shielding gas.
Molten or heated metal is shielded from the air in a controlled shielding gas atmosphere.
The type of metal transfer is strongly influenced by the shielding gas. The shielding gas also influences the size and forces acting on the droplets.
Arc stability and arc ignition are influenced by the shielding gas.
The amount of spatter and surface slag is also influenced by the shielding gas.
Metallurgy and mechanical properties
The loss of alloying elements and pick-up of oxygen, nitrogen, and carbon is influenced by the shielding gas. This loss and pickup will influence the mechanical properties of the weld metal.
The profiles of the weld bead and penetration are influenced by the shielding gas.
Also the choice of shielding gas will affect the welding speed and the total welding cost.