MIG (Metal Inert Gas) welding
intro.
MIG (Metal Inert Gas) welding, also known as MAG (Metal Active Gas) and in the USA as GMAW (Gas Metal Arc Welding), is a welding process that is now widely used for welding a variety of materials, ferrous and non ferrous. The essential feature of the process is the small diameter electrode wire, which is fed continuously into the arc from a coil. As a result this process can produce quick and neat welds over a wide range of joints.
Power Source
MIG welding is carried out on DC electrode (welding wire) positive polarity (DCEP). However DCEN is used (for higher burn off rate) with certain self shielding and gas shield cored wires. DC output power sources are of a transformer-rectifier design, with a flat characteristic (constant voltage power source). The most common type of power source used for this process is the switched primary transformer rectifier with constant voltage characteristics from both 3-phase 415V and 1-phase 240V input supplies. The output of direct current after full wave rectification from a 3-phase machine is very smooth. To obtain smooth output after full wave rectification with a 1- phase machine, a large capacitor bank across the output is required.
Because of the expense of this, many low cost 1-phase machines omit this component and therefore provide a poorer weld characteristic. The switches to the main transformer primary winding provide the output voltage steps at the power source output terminals.Another method of producing different voltages at the power source output terminals is to use a Thyristor or a Transistor rectifier instead of a simple diode rectifier. This system offers continuously variable output voltage, which can be particularly useful on robot installations and the cost of this type of rectifier can be partly offset with no need for primary voltage switch or switches and a single tapped main transformer primary winding. Most MIG power sources have a contactor or relay used to switch the output ON/OFF with operations of the trigger on the MIG torch. The switch off operation of this contactor is normally delayed to allow the welding wire to Burn back out of the molten weld pool.
The Wire-feed Unit
The wire-feed unit, or sub-assembly where this is mounted in the power source cabinet (known as a composite MIG), provides the controlled supply of welding wire to the point to be welded. According to the welding wire size and Arc voltage provided by the power source, a constant rate of wire speed is required, in MIG welding the power source provides Arc voltage control and the wire feed unit provides welding wire speed control, ( in MIG this equates to welding current ).
Most modern wire feed units control the wire feed speed via a DC motor and thyristor control PCB to provide continuos control of Armature volts and hence RPM of motor.
Welding Current
The value of welding current used in MIG has the greatest effect on the deposition rate, the weld bead size, shape and the penetration. In MIG welding, metals are generally welded with direct current polarity electrode positive (DCEP, opposite to TIG welding), because it provides the maximum heat input to the work and therefore a relatively deep penetration can be obtained. The oxide removal effect of the DCEP, which is very important in the welding of aluminum and magnesium alloys, contributes to clean the weld deposit.
When all the other welding parameters are held constant, increasing the current will increase the depth and the width of the weld penetration and the size of the weld bead. In a constant voltage system, the wire feed speed and welding current are controlled by the same knob. As the wire feed speed is increased the welding current also increases, resulting in increases in the wire melt-off rate and the rate of deposition.
Welding Voltage (Arc Length)
The arc length is one of the most important variables in MIG that must be held under control. When all the variables such as the electrode composition and sizes, the type of shielding gas and the welding technique are held constant, the arc length is directly related to the arc voltage. For example, normal arc voltage in carbon dioxide and helium is much higher than those obtained in argon. A long arc length disturbs the gas shield, the arc tends to wander and thus affects the bead surface of the bead and the penetration. In MIG the arc voltage has a decided effect upon the penetration, the bead reinforcement and bead width. By increasing the arc voltage the weld bead becomes flatter and wider, the penetration increases until an optimum value of the voltage is reached, at which time it begins to decrease. High and low voltages cause an unstable arc. Excessive voltage causes the formation of excessive spatter and porosity, in fillet welds it increases undercut and produces concave fillet welds subject to cracking. Low voltage produces narrower beads with greater convexity (high crown), but an excessive low voltage may cause porosity and overlapping at the edges of the weld bead.