MIG-MAG WELDING

The purpose of the power source is to feed the welding 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 M.M.A. and T.I.G power sources, where there is only one regulating parameter (welding current), on M.I.G.-M.A.G. 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:

2. TORCH
The 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 handgrip 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 sheath.There is a variety of different types of welding torches and pistols available on the market.

Water-cooled torches are used when the current intensity used is such that it generates a considerable amount of heat energy; they are used for working currents of over 300 A or for pulsed currents. Self-cooled torches are cooled by the gas shield and are used when the working current is below 300 A; these are very commonly used. Swan neck torches are also cooled by the gas shield and are used for low intensity current applications (immersion transfer – short arc).

3. 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 section and length of the cable are determined by the maximum current from the power source.

4. WIREFEEDER

The wirefeeder device is powered by a motor whose job is to push the wire electrode, initially wound round a reel, towards the torch and hence to the welding zone. The choice of wire feed rate value 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 wire puller is the number of wire-feeder rollers; devices with 4 rollers feed the wire more uniformly with respect to those with 2 rollers.

5. WATER COOLING SYSTEM

The water cooling unit is a device used to cool the torch, if it is water-cooled, 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, controls overheating.

6. GAS CYLINDER WITH REGULATION SYSTEM

The cylinder contains the shield gas/gases such as argon, helium, carbon dioxide or 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. It is also fitted with a solenoid valve, which are controlled by a button on the torch, to open and close the gas flow as welding is started or ended.

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The wires can be identified by their chemical composition and also their section morphology, which may either consist of a single metal (solid wires) or have an internal core containing granules (tubular wire).

Particular attention should be paid to the presence of grease or moisture on the surface of the wire electrode, because they could cause cracks, porosity or blowholes; in addition, if the wire electrode reel is not wound uniformly this could cause uneven wire feed resulting in unstable welding.

Solid wires usually have the same composition as the base material, with added elements able to help clean the base material. The most widely used diameters are 0.6 – 0.8 – 0.9 - 1 – 1.2 – 1.6 mm.

Tubular wires, with gas shield, do not consist of solid metal but have an internal core filled with granular powder (flux); this has the same functions as the coating on coated electrodes. The granular powder or flux can be of rutile, basic or special type. Compared to solid wires, tubular wires have better arc stability and deeper penetration, ensure a better-looking seam, often eliminating the need for further finishing (e.g. spray grinding) and reducing the risk of defect formation, such as porosity; of course the use of tubular electrodes requires slag removal, as for welding with M.M.A. electrodes.
The most widely used diameters are 0.6 – 0.8 – 0.9 – 1.2 – 1.6 mm.
 

With M.I.G.-M.A.G welding procedures, the method of transferring the welding metal from the wire electrode (either solid or tubular) to the weld pool depends, as well as on the electrical welding parameters, on the wire diameter, the type of power source and the gas used. Depending on these parameters, drop transfer may be by:

1. immersion transfer (short-arc, dip-transfer or short circuit)
The welding metal is transferred to the weld pool in the form of drops which are immersed in the pool itself, creating continuous short circuits.
This "short arc" transfer is characterised by the presence of current intensities of up to 200 A, by the use of thin solid wires, from 0.6 m to 1.2 mm, enabling thin welds and welding in all positions possible. This is obtained using direct current power sources.

2. spray-arc transfer
This method is designed so that the drops of material are not transferred to the weld pool by contact but rather, due to the effect of the high current, are sprayed into the bath creating a continuous flow of material.
This feature is obtained using direct current power sources when the current used is high (greater than 200 A) and the wires have a diameter of more than 1 mm. A very fluid weld pool is generated, with sizeable penetration, which is suitable for horizontal welding of mainly medium and large thicknesses.

3. pulsed-arc transfer
This procedure can only be obtained with pulsed current power sources. The pulsations actually cause small-sized drops to detach themselves and therefore the typical spray arc is obtained, even with low currents. Heat generation/contribution, pool size and penetration are similar to those achieved with the spray arc method. This procedure is most often used for materials such as aluminium or stainless steel, for which the short arc welding procedure cannot guarantee sufficiently high quality results.

1. Mild, carbon steels
Carbon steels are welded with direct current and electrode positive polarity (the wire electrode is connected to the positive pole) using exclusively the M.A.G. welding procedure. Applications may range from the use of CO2 only, to argon - CO2 mixtures in differing proportions (the most common is 80% argon, 20% CO2 ).
The greater the percentage of argon in the mixture the better the properties and arc stability.
The weld has very good properties, especially with short arc where dense/thick weld pools are obtained, allowing welding in all positions.
The use of wires with silicon and manganese content will lead to the elimination of impurities present in the base material and obtain good quality welds.
Bevelled seams should be prepared with thicknesses greater than 3 mm.

2. Stainless steels
Stainless steels are welded with direct current and electrode positive polarity (the wire electrode is connected to the positive pole) using exclusively the M.A.G. welding procedure. The gas shield used should consist of Ar + CO2 or Ar + Oxygen mixtures. The proportion of argon in the mixture should never fall below 98% so as to avoid strong oxidation of the chrome present in the base material.
Seams thicker than 2.5 mm should be bevelled. Care should be taken to choose the most suitable welding material for the quality of the stainless steel being welded.
To achieve a good weld, we advise grinding the spots.

3. Aluminium and its alloys
Aluminium and its alloys are welded in direct current or pulsed current with electrode positive polarity (the wire electrode is connected to the positive pole) using the M.A.G. welding procedure.
The gas shield used is usually pure argon. Pure helium or an argon + helium mixture can also be used.
For horizontal welding, irrespective of the thickness, the spray-arc and/or pulsed-arc technique is used; the short-arc technique is used, instead, on thin vertical or corner welds. Given the properties of aluminium, milling the spots instead of grinding is advisable.

4. Other materials
The M.I.G.-M.A.G. welding procedure is also used on materials such as nickel and its alloys, copper and its alloys; for all these materials direct current with electrode positive polarity is used.
For welding copper more than 5 mm thick we advise adoption of the M.I.G. procedure but in any case the current intensity should be adjusted according to the welding position and the seam thickness.