It is an arc welding process wherein coalescence is produced by heating the job with an electric arc established between a continuously fed metal electrode and the job. No flux is used but the arc and molten metal are shielded by an inert gas, which may be argon, helium, carbon dioxide or a gas mixture.
Principle of Operation ( Semi-Automatic ) :
Before igniting the arc, gas and water flow is checked. Proper current and wire feed speed is set and the electrical connections are ensured. The arc is struck by any one of the two methods. In the first method current and shielding gas flow is switched on and the electrode is scratched against the job as usual practice for striking the arc.
In the second method, electrode is made to touch the job, is retracted and then moved forward to carry out welding; but before striking the arc, shielding gas, water and current is switched on. About 15 mm length of the electrode is projected from the torch before striking the arc. During welding, torch remains about 10-12 mm away from the job and arc length is kept between 1.5 to 4 mm.
Arc length is maintained constant by using the principles of self-adiuste arc, and self-controlled arc in semi-automatic (manually operated) automatic welding sets respectively.
Self-adjusted arc :
The electrode is fed from a coil through oved rollers run by a constant speed motor. Suppose under certain condtions, arc length decreases from L, to L. It can be seen from Fig. below that, with the decrease in arc length, voltage decreases, and arc current increases. The increased current melts electrode at a faster rate, arc length increases and becomes normal.
The reverse will occur if arc length in creases above the set value. For self-adjusting arc, a welding power source with flat characteristics is preferred over another having drooping characteristics. The reason is that with the same change in arc length there is a bigger change in arc current (Fig. below) with Nat characteristics which increases or decreases electrode burn off rate and brings arc length to normal.
Self-controlled arc :
The electrode is fed from a coil through the rollers run by a variable speed electric motor whose speed increases or decreases as the arc voltage increases or decreases. Under certain circumstances, if the arc length decreases, arc voltage will decrease (Fig. above), which in turn reduces the speed of electric motor and electrode feed rate. This will increase and bring the arc length to the normal set value.
Reverse will occur if the arc length increases. For self-controlled arcs, a welding power source with drooping characteristics is preferred over another having flat characteristics. The reason is with the same change in arc length, there is a greater change in arc voltage with drooping characteristics which increases or decreases electrode feed motor speed, thus electrode feed rate and brings arc length to normal.
For carrying out welding, torch is moved along the joint in a steady manner. The torch angle is about 70° with the horizontal. Forehand welding technique may be used. The metal transfer* in MIG welding may be of globular, spray or short-circuiting type. In table below gives MIG welding parameters for different materials (approx. values).
References : A Textbook of Welding Technology by O. P. Khanna