This process differs from MIG in that it uses CO2 instead of inert gases (argon or helium) both the normal and fine-wire machines could be used. The differences are: metal transfer mode, power source, cost and field of application. The process is schematically shown in Fig. 2.8.

• In CO2 welding there is no threshold current to change transfer mode from gravita­tional to projected type. At low currents the free flight transfer is of repelled type and there is excessive scatter loss. This situation is quite common in fine wire welding but can be overcome by adjusting welding parameters to obtain short-circuiting mode of transfer (the drop comes in contact with the weld pool and is detached from the wire by surface tension and electromagnetic forces before it can be projected laterally). If the current is excessive during short-circuiting, detachement will be violent and will cause spatter.

• To get rid of this problem the power source is modified either by adjusting the slope of a drooping characteristic machine or by inserting a reactance in the circuit of a flat
characteristic machine. Thus the short circuit current is limited to a suitable level. At currents in excess of 200 A using 1.5 mm or thicker wires the process is sufficiently regular permitting free flight transfer but welding is to be done in flat position only.

• At arc temperature carbon di-oxide dissociates to carbon monoxide and oxygen. To save metal from oxidation, deoxidized wire for welding carbon steel is essential, otherwise 40% of the silicon and manganese content may be lost.

• This process finds its main application in the welding of carbon and low alloy steels.