How MIG Welding Works

MIG welding is an abbreviation of Metal Inert Gas welding, although you may also hear it referred to as Gas Metal Arc Welding (GMAW) or Metal Active Gas (MAG) welding. It was developed during World War II as an easy to use, portable means of creating strong and reliable metal joints. Its ease of use has led it to become by far and away the most popular technique for welding amongst the keen amateur, and for most beginners, this is where they start. 

Based upon the same principles as arc welding, MIG welding uses the power of an electrical charge to melt the metal being worked with. The main differences between the two are that in arc welding the flux is contained within metal rods, and in MIG a spool of flux core wire is used plus MIG welding uses a shielding gas, making it easier to create and maintain the arc during the process.

In essence, MIG welding is a type of arc welding itself in that it uses an electrical arc to form the joint. However, unlike arc welding, it is much more accessible to the keen amateur, being easier to learn and less hazardous. This type of welding is primarily performed inside shops and factories

as the shielding gas used can be dissipated by the movement of outside air, making it difficult to weld out of doors or in breezy locations. It is worth noting that MIG welders require far more consumables such as nozzles and contact tips, so for bigger projects they can end up being more costly.

The Equipment

1. Welding gun and wire feed unit

This part is made up of a number of components that it is useful to be familiar with. These include the control switch, power cable, contact tip, gas nozzle, gas hose and electrode conduit and liner. Some guns also have water hoses built into the gun for cooling.

2. Power supply

MIG welding requires a constant voltage power supply. Unlike arc welding where changes to the arc length can result in loss of the arc or other problems, the constant voltage used in MIG welding means changes in arc length dramatically affect the current and heat input, thereby making the electrode melt quicker and restoring the original arc length. This makes it easy to weld accurately even with hand held guns. 

3. Electrode

As with arc welding, the selection of an electrode will largely depend on the materials you are working with and the size of the weld to be performed. Plenty of information is available to help you with your choice, or professional welding equipment suppliers will be able to advise you further.

4. Shielding gas

In any type of arc welding, a gaseous shield is required to protect the arc, the electrode and the metal from atmospheric gasses such as oxygen and nitrogen that can cause brittle welds and porosity. In arc welding, the flux coating on the electrode produces a protective cloud of carbon dioxide, but as MIG welding does not use such a coating it is necessary to protect the weld with a gas. A number of gasses are used as shielding gas including argon, helium and carbon dioxide and the choice of which to use will depend on the metal you are working with. Again, there is plenty of further information on this, or ask your supplier for guidance.

The process

For the majority of applications MIG welding is very easy to learn, requiring only a week or two of practice to learn the basics. Unlike arc welding there is no need to keep stopping to clean the slag off your work, and the only real requirement is that the operator is able to keep the gun in a proper position along the area being welded.

MIG welding – Step by Step

1. Gather your MIG welder, your PPE items and select the correct wire and shielding gas for your project.

– When selecting a wire, choose a high quality wire with a similar thickness to that of the metal you are planning to weld.

– For gas, carbon dioxide is by far the cheapest and most versatile, but may

not be suitable for aluminium or thin pieces of steel. 

2. Mount the wire onto the spool and feed it through the welder as per manufacturers instructions.

– This can be the trickiest part of setting up a MIG welder. Removing the tip of the MIG gun will help to make it easier to pull the wire through

– Another tip is, once the wire is in place, turn on the machine and pull the

trigger to feed the wire into the nozzle with the tip off. Once the wire is 

through, replace the tip and trim the wire to around 30 – 60mm.

3. Set the power supply polarity to DCEP.  

4. Set up the pieces of metal you will be welding and secure them in place with clamps

• G clamps or locking pliers are adequate for this 

5. Begin to weld the joint using either a pushing or dragging technique

– pushing the electrode through the weld will create a thicker bead

• Dragging the electrode will give a deeper penetration but a narrower bead 

6. Inspect the weld for imperfections  

Note that, unlike arc welding, MIG welding does not require repeated passing of the weld joint upon first attempt. MIG welds need to be allowed to cool slightly and then if imperfections are detected, be ground down and rewelded until a perfect weld joint is achieved.

Tips for better operation

With a process this straightforward, it can be tempting to think this is an easy art to master. However, as many amateur welders have found out, it is not as easy as it looks and despite the straightforward nature of the equipment, welding perfectly with MIG takes time and practice. Here are some tips to get you started: 

• The orientation of the gun is important: it should be around 90 degrees if you are welding a flat surface and 45 degrees if you are performing a fillet weld. 
• Electrodes and workpieces should be properly cleaned with chemicals or brushed with a wire brush to remove oxides from their surfaces. If oxides remain and come into contact with your welding bead, the weld may become porous and therefore unstable. 
• Practice makes perfect, so before taking your MIG welder anywhere near your car or bike gather some scrap metal and test variations in wire speeds, power settings and hand movements to perfect your technique.