Gas Metal Arc Welding (GMAW), particularly Metal Active Gas (MAG) welding, is a pivotal process in Kenya’s welding industry, especially in fabrication workshops and manufacturing plants. This chapter focuses on carrying out MAG welding effectively, emphasizing the equipment and gases involved. Mastery of these elements enables welding professionals to produce strong, defect-free welds on carbon steel and other ferrous metals commonly used in construction, automotive repair, and industrial maintenance.

2.1 MAG welding equipment and accessories

Metal active gas (MAG) welding uses the same type of equipment as MIG welding: a constant-voltage power source, a wire feeder, a torch and a gas supply. The difference is in the shielding gas. In MAG welding the gas is active, which means it takes part in the arc and the weld pool and does not only keep air away. The gas improves the penetration of the weld, the stability of the arc and the shape of the bead on carbon and low-alloy steels, which are the metals that MAG welding is mainly used for.

Because the gas is the feature of the process, the cylinder that holds it is the main item of equipment studied in this topic. The power source, the wire feeder and the torch are those described under MIG welding equipment and accessories, and need only the changes that come from the active gas: a regulator that suits the gas, and the care of the cylinders.

2.1.1 Active gas cylinders

An active gas cylinder is a pressure vessel that stores the gas for MAG welding. The gas is either carbon dioxide on its own, or a mixture in which argon is the main part, with carbon dioxide or a little oxygen added. The cylinder is fitted with a valve and a regulator that brings the pressure down to a working level, and a flowmeter and hose carry the gas to the machine.

The content of the cylinder is identified by the label from the supplier, and the label is read each time before the cylinder is connected, because the wrong gas on a job can ruin the weld. The two kinds of cylinder behave differently:

• Mixed gas cylinders: the gas is stored as a compressed gas, at a pressure of about 200 bar when full, and the pressure on the content gauge falls steadily as the gas is used, so the gauge shows how much is left.
• Carbon dioxide cylinders: the gas is stored partly as a liquid, with the gas above it. The pressure stays steady until the liquid is nearly all used, and then it drops quickly, so the gauge does not show the contents. The cylinder is weighed to find what is left, and the empty and full weights are marked on it.

2.1.1.1 Applications

The gas is chosen from the metal, its thickness, the finish required and the cost. The table sets out the main choices.

Notes on the use of the cylinders:

• Mixtures: the proportion of carbon dioxide in an argon mixture may be 20% or 25%, and in some work a smaller amount is used. The supplier states the mixture on the label, and the welding procedure names the mixture to be used.
• Thin sheet: a mixture is preferred because a lower heat and a stable arc reduce the risk of burn-through and spatter.
• Thick sections: carbon dioxide or a rich carbon dioxide mixture gives the deep penetration that is needed.
• Oxygen: it is never supplied to the welding set from a plain oxygen cylinder. Where a little oxygen is needed, the supplier mixes it with argon in a single cylinder. A plain oxygen cylinder is for gas cutting and welding and must never be connected to a MAG set, because it can cause a violent fire with oil and grease.
• Cost and supply: the cost of the gas is a part of the cost of the weld. Carbon dioxide is cheaper than argon mixtures, so it is chosen where the finish of the weld is less important.

Example: a workshop that makes steel frames for lorry bodies welds plate of 8 mm in carbon dioxide for its penetration and low cost. The same workshop uses an argon and carbon dioxide mixture on the 2 mm sheet panels, where a smoother arc reduces spatter and burn-through.

A cylinder has a limited amount of gas, and the time that it lasts is worked out from its contents and the flow rate. One kilogram of liquid carbon dioxide gives about 0.5 m³ of gas.

Gas in a 10 kg cylinder = 10 kg × 0.5 m³/kg = 5 m³ = 5 000 L

Time at a flow of 15 L/min = 5 000 L ÷ 15 L/min = 333 min

333 min ÷ 60 = about 5.5 hours of welding with the gas flowing

The gas flows only when the trigger is held, so the cylinder lasts for more working hours than this when welding is done in short runs, and for fewer if the valve is left open or the hose leaks.

2.1.1.2 Care

Cylinders hold gas at high pressure, so their care is a matter of safety as well as of weld quality. A cylinder that is dropped, heated or damaged can release its gas suddenly or burst. The rules below apply to every active gas cylinder.

Storing and moving:

• Upright and secured: cylinders are stored standing, chained or strapped to a wall or a rack, with the valve caps on.
• Away from heat: they are kept out of the sun, away from furnaces, and well clear of the welding area and sparks.
• Ventilated place: stored in a place with good air flow, because a leak of carbon dioxide or argon in a closed room displaces air and can cause suffocation without warning.
• Moving: they are moved on a cylinder trolley with the valve closed and the cap on, and never rolled, dragged or carried by the valve.
• Full and empty apart: full cylinders are separated from empty ones, and empty cylinders are marked as such.

Fitting and using:

1. Check the label to confirm that the gas is the one required for the job.
2. Fix the cylinder upright on the trolley or against the wall, with a chain.
3. Remove the cap, and open the valve for an instant to blow out dust, standing to one side and pointing the outlet away from people.
4. Fit the regulator with a clean nut and washer and tighten it with the correct spanner, without oil or grease.
5. Open the valve slowly, and read the content gauge. For carbon dioxide, also check the weight of the cylinder.
6. Set the flow with the flowmeter while the trigger is held, and test the joints for leaks with soapy water.
7. At the end of the work close the valve, release the gas from the line, and fit the cap when the cylinder is empty or is to be moved.

Special points for carbon dioxide are that the gas cools strongly as it expands at the regulator, and the regulator may freeze up at high flows, so a heater is fitted between the cylinder and regulator for heavy use. A cylinder is not drawn below a small residual pressure, so that air and moisture do not enter it, and it is returned to the supplier with the valve closed.

Regulators and hoses

The regulator and flowmeter must be made for the gas in use, and they are kept clean and dry. The hose is checked for cracks, cuts and loose clamps before each job, and a damaged hose is replaced. Joints are tested with soapy water and never with a flame. Equipment that has been used with a different gas is not swapped over without the supplier's advice. The regulator reduces the cylinder pressure to a steady working pressure, and the flowmeter sets the flow in litres per minute. A contents gauge on the regulator shows how much mixed gas is left. A check valve in the line stops gas flowing backwards. Hoses are made for the gas in use, and their colour or marking helps to avoid connecting the wrong gas.

Power source and wire feeder

The power source of a MAG set is a constant voltage (CV) type that supplies direct current. The welder sets the voltage and the wire feed speed sets the current, and the arc length corrects itself: if the arc becomes longer the current falls, the wire melts more slowly and the feeder shortens the arc again, and if the arc becomes shorter the current rises, the wire melts faster and the arc lengthens again. This gives a steady arc and an even bead. Inverter sets are light, efficient and easy to carry, and transformer-rectifier sets are heavier and robust for fixed workshops. A single-phase set suits light work, and a three-phase set carries the higher currents of heavy work. The set is earthed, kept dry, clear of dust and well ventilated. The wire feeder pushes the wire at a steady speed through drive rollers: rollers that match the wire diameter, the correct tension and a clean liner prevent slipping and jams.

Torch

A MAG torch is air-cooled or water-cooled. An air-cooled torch is lighter and simpler and is used at lower currents. A water-cooled torch has water circulating through the head and cable, so it carries a higher current for longer and suits heavy or continuous welding, but the water level, the flow and the hoses must be checked for leaks. The contact tip and nozzle must match the wire and the current.

Chapter Summary

This chapter focused on the components and operation of Metal Active Gas (MAG) welding, starting with a detailed overview of MAG welding equipment and the essential active gas cylinders used in the process. It highlighted the specific applications of these gas cylinders and the necessary care measures to ensure safe and effective usage. The discussion then moved to shielding gases, emphasizing the role of Carbon IV Oxide in protecting the weld area from contamination. The MAG welding process was explained step-by-step, outlining the correct procedure to achieve quality welds. Practical applications were covered with an emphasis on welding metallic materials such as plates and pipes ranging from 4 to 16 millimeters in thickness. The chapter also stressed the importance of executing MAG welding in the flat position to optimize weld quality and consistency. Overall, the chapter provided a comprehensive understanding of MAG welding equipment, shielding gases, and operational techniques essential for successful welding outcomes.