The shield gases used in M.I.G.-M.A.G. welding procedures can be divided into two basic categories: inert and active. Argon, helium and argon-helium mixtures belong to the first type, while carbon dioxide and argon/oxygen or argon/carbon dioxide mixtures are considered active gases.
Argon (Ar) is an inert gas, produced by fractional distillation of the atmosphere. The gas is extracted from the air and may, therefore, contain traces of impurities such as oxygen, nitrogen or water vapour, but it is nevertheless considered suitable for most welding applications.
The use of this gas in M.A.G. applications gives good arc stability and an easy strike. Moreover, given its low thermal conductivity, the central part of the arc column remains at a high temperature giving more fluidity to the drops of material passing through the arc zone.
Helium (He) is an inert, rather rare gas that is scarcely present in the atmosphere and is extracted from underground: it is therefore much more costly than argon.
Compared to argon, helium has a less stable arc but greater penetration; it is mostly used for thick welds and for materials with high thermal conductivity, such as, for example, copper and aluminium.
Since helium, unlike argon, is lighter than air and hence more volatile, a greater quantity of gas is needed to ensure a sufficient shield for the welding zone.
Carbon dioxide (CO2) is an active gas, present in the air and underground. The commonest problem caused by this type of shield is that it can cause the formation of excessive spray and establish an unstable arc; if the arc is kept rather short and of constant length, however, it is possible to keep it under control. With a CO2 shield good penetration is generally obtained.
Active mixtures. It is often possible to take advantage of the qualities of individual gases, by using a mixture for the gas shield e.g. argon-oxygen, argon-oxygen-CO2, argon-CO2.
Even if the inert gases in their pure state are able to perform their shielding effect at any temperature, the addition of active gases improves arc stability and the transfer of the wire electrode metal to the weld pool. This occurs without impairing the shielding effect.
