An Introduction To MIG Welding

The Welding Circuit

The welding circuit consists essentially of the Power source, the MIG Torch, a Wirefeeder, optional Water cooling unit, gas cylinder with regulator and an earth clamp.



1. Power Source
The purpose of the power source is to feed the weld zone with the welding material, by means of a special torch, and to maintain the electric arc that is struck between the piece to be welded and the consumable wire electrode. Unlike Stick (MMA) and TIG power sources, where there is only one regulating parameter (welding current), on MIG power sources there are two regulating devices, one which regulates electric arc intensity (welding voltage), and another which regulates the rate of welding wire feed (welding current). Power sources can be divided into two categories:

a) direct current (DC) power sources
Direct current power sources are the most common type of power source and are characterised by their high stability; this is because they are based on the fact that an electric arc will tend to stabilise naturally if it is powered at constant voltage and generated on a wire fed at a constant rate. Given the flexibility of the process, there is some elasticity in the choice of voltage and wire feed rate parameters. In this way it is possible to obtain drop transfer, from the welding material to the material to be welded, using either the "short arc" immersion procedure or the "spray arc" procedure, which are explained later.

b) pulsed current power sources
In this case it is not the voltage size that is regulated in the power source, but the current, which is not held constant but modulated with a train of impulses (hence the name "pulsed"). The purpose of the impulses is to force the drop to detach itself from the welding material; in this case the arc is not stabilised naturally, so that the impulses and wire feed rate must be perfectly synchronised to achieve an acceptable weld.

Both the in first and second case, at least two knobs are used for regulation; recent research in the industry has led to the development and marketing of "synergic" type welding machines in which the operator uses only one control knob. The manufacturer memorises the optimal welding parameters in the power source and these can be recalled and/or corrected by the operator, depending on the particular job requirements.

The different polarities when connecting the poles of the power source to the material to be welded identify two operating modes:

i) direct current with straight polarity connection
With electrode negative connection, the torch is connected to the negative pole and the material to be welded to the positive pole of the power source; this type of connection is only used in welding with tubular wire (FLUX).

ii) direct current with reverse polarity connection
When welding with this operating mode, the torch is connected to the positive pole and the piece to be welded to the negative pole of the power source; this is the most frequently used type of connection.

2. MIG Torch
The MIG torch, which is used to transfer the welding metal to the welding zone, has an externally insulated body and allows the passage of the wire electrode, the gas and the welding current . Its handle contains a control button for switching on the current, gas output and wire electrode feed. The cable bundle consists of a current conductor, control cables, the gas pipe, cooling water circulation pipes (if present) as well as the wire-guide liner. There are a variety of different types of welding torches available on the market. Water-cooled torches are used when the current intensity used is such that it generates a considerable amount of heat: they are used for working currents of over 300A or for pulsed currents. Self-cooled torches are cooled by the gas shield and are used when the working current is below 300A; these are very common.

3. Wirefeeder
The wirefeeder device is powered by a motor whose job is to push the MIG wire, initially wound round a reel, up through the torch and in to the weld zone. The choice of wire feed speed is made by adjusting the motor regulator; a given wire feed rate implies a certain melting rate and hence a defined value for the welding current. A distinguishing property of a wirefeeder is the number of wirefeed rollers; devices with 4 rollers feed the wire more uniformly than those with 2 rollers and are often more reliable on softer wires (e.g aluminium).

4. Water Cooling System (Optional)
The water cooler unit is a device used to cool a water-cooled MIG torch, should the high welding currents used cause excessive overheating. A pump ensures continuous circulation of water in the torch and, by means of a cooling system, keeps the water at a low enough temperature to be effective.

5. Gas Cylinder With Regulator
The cylinder contains the shielding gas(es) such as argon, helium, carbon dioxide or a mixture of them, and is fitted with a pressure gauge with related pressure reducer, which is used to indicate the quantity of gas in the cylinder. The Power Source is also normally fitted with a solenoid valve, controlled by the button on the torch, which opens and closes the gas flow as welding starts or stops.

6. Earth Clamp
The clamp with earth cable is used to make the electrical connection between the power source and the base material to be welded. The diameter and length of the cable are determined by the maximum current from the power source.

An Introduction To MIG Welding

Types Of MIG Welding Shielding Gases



The shield gases used in MIG welding procedures can be divided into two basic categories: inert and active. Argon, helium and argon-helium mixtures belong to the first type, while carbon dioxide and argon/oxygen or argon/carbon dioxide mixtures are considered active gases.

Argon (Ar) is an inert gas, produced by fractional distillation of the atmosphere. The gas is extracted from the air and may, therefore, contain traces of impurities such as oxygen, nitrogen or water vapour, but it is nevertheless considered suitable for most welding applications. The use of this gas in MIG applications gives good arc stability and an easy strike. Moreover, given its low thermal conductivity, the central part of the arc column remains at a high temperature giving more fluidity to the drops of material passing through the arc zone.

Helium (He) is an inert, rather rare gas that is scarcely present in the atmosphere and is extracted from underground: it is therefore much more costly than argon. Compared to argon, helium has a less stable arc but greater penetration; it is mostly used for thick welds and for materials with high thermal conductivity, such as, for example, copper and aluminium.
Since helium, unlike argon, is lighter than air and hence more volatile, a greater quantity of gas is needed to ensure a sufficient shield for the welding zone.

Carbon dioxide (CO2) is an active gas, present in the air and underground. The commonest problem caused by this type of shield is that it can cause the formation of excessive spray and establish an unstable arc; if the arc is kept rather short and of constant length, however, it is possible to keep it under control. With a CO2 shield good penetration is generally obtained.
Active mixtures. It is often possible to take advantage of the qualities of individual gases, by using a mixture for the gas shield e.g. argon-oxygen, argon-oxygen-CO2, argon-CO2.

Even if the inert gases in their pure state are able to perform their shielding effect at any temperature, the addition of active gases improves arc stability and the transfer of the wire electrode metal to the weld pool. This occurs without impairing the shielding effect.

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