MIG / CO₂ WELDING

In MIG / CO₂ welding the feeding of filler wire is machanised. If the welder moves the MIG / CO₂ torch the process is called semi automatic MIG / CO₂ welding. If the torch is fixed on a carriage and if it is moved by the bead or if the relative motion between the torch and job is mechanised then the process is called fully automatic MIG / CO₂ welding.

In MIG welding the melting rate of the wire is determined by two forms of heating that occur during the process. The arc is not struck between the torch tip and job. Some length of filler protrudes from the torch, and the arc is struck between the tip of the wire and the work piece. The free length of the wire from the torch tip to arc is called stick - out length. The welding current, before entering the arc zone, passes through this length of  wire.  Since, the filler wire diameters are generally low (1.0 mm to 2.4 mm) there is a resistance heating that is taking place before the filler wire enters the arc zone. This heating is also called joules heating.

The melting rate of the wire is determined by two components namely joules heating and arc heating.

Fig. shows how the flat characteristic power source responds to changes in stand ­ off distance. If we assume that the normal working arc of length 12 as shown in Fig. 10 takes about 12 A from the power source, then if the welder increases the stand-off distance then the arc length increases to 11 and hence the current determined by the intercept of arc characteristic with power source characteristic\ reduces to 11 amps. This reduction in arc current reduces the Melting Rate (MR). But the Feed Rate (FR) of the wire is kept constant. Therefore, now MR is less than FR. This disturbance causes less melting at the arc zone, to balance that the stick - out length increases increasing the joule heating of the wire, when the stick - out
length increases the arc length is reduced and it is restored.

The reverse phenomenon happens when the stand-off distance reduces. The arc length reduces to 13 there by increasing the current to 13 amps. Since the feed rate is kept constant, FR is less than MR. This disturbance causes the arc to burn more wire and the arc length is increased and restored back to its original value. This property of controlling the length for changes in stand-off distance is called self regulation with flat characterist ic power source. This advantageous property of flat- characteristic is the reason for its wide application in MIG / CO₂ welding. In addition to this, the setting of welding the use of parameters is made simple by flat characteristic power source. The arc voltage is selected in the power source. The current is selected by the feed rate selection. Both are independent. Hence setting of arc voltage and current is easier.