In MIG welding the arc is maintained between a consumable electrode and the workpiece in inert gas medium. It is used as a heat source which melts the electrode and thus supplies the filler metal to the joint. The principle of operation is shown in Fig. 2.7. The apparatus consists of a coil of consumable electrode wire, a pair of feed rolls, a welding torch having a control switch and an inert gas supply. Consumable wire picks up current while it passes through a copper guide tube.

• Electrode wire diameter is between 1 .5 mm to 3.0 mm and current used is between 100 to 300 A for welding aluminium, copper, nickel and alloy steels (current density is of the order of 100A per mm square: thus projected transfer occurs). The arc projects in line with the wire axis and metal also transfers in the same line.

• Projected transfer occurs within a range of current. Below the lower limit the trans­fer is gravitational and above the upper limit, for aluminium, the metal flow is unsta­ble resulting in the formation of dross, porosity and irregular weld profile.

• Welding may be done below the threshold current and conditions could be adjusted to get short-circuit transfer. Wires of 0.75 mm diameter or less with wire reel directly mounted on the gun itself could be used with short circuit or dip transfer. Such a welding is called fine-wire welding and is suitable for joining sheet metals.

• Dcrp is commonly used and a power source with flat characteristics is preferred for both projected and short circuiting transfer, as it gives more consistent arc-length.

Welding of aluminium is only possible with dcsp. Drooping characteristic power sources may also be used with a choke incorporated in the circuit to limit the short circuit current and prevent spatter.

• Shielding gas is normally argon, but argon-oxygen mixtures (oxygen: 20%) are some­times used for welding austenitic stainless steels in order to impove weld profile. Similarly 80% Ar + 20% CO2 improves weld profile of carbon steel and sheet metal and is cheaper and better than pure argon. CO2 shielding can also be used.

• The process is suitable for welding high alloy steels, aluminium, copper, nickel and their alloys. it is complementary to TIG, being particularly suited to thicker sections and fillet welds.

• MIG spot welding gives deeper penetration and is specially suitable for thick materi­als and for the welding of carbon, low alloy and high alloy steels.