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Basic Technique of MIG Welding

MIG welding or also known as Gas Metal Arc Welding (GMAW) was developed in the 1940's and is a welding process that uses an arc of electricity to create a short circuit between a continuously fed anode (positive the wire-fed welding gun) and a cathode (negative the metal being welded).

The heat produced by the short circuit, along with a non-reactive inert gas locally melts the metal and allows them to mix together. Once the heat is removed, the metal begins to cool and solidify, and forms a new piece of fused metal.

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Advantages of MIG welding:

  • The ability to join many different types of metals: carbon steel, stainless steel, aluminum, magnesium, copper, nickel, silicon bronze and other alloys.
  • All-position welding capability
  • A good weld bead
  • A minimum of weld splatter
  • Easy to learn

 

Disadvantages of MIG welding:

  • MIG welding can only be used on thin to medium thick metals
  • The use of an inert gas makes this type of welding less portable than arc welding which requires no external source of shielding gas
  • Produces a somewhat sloppier and less controlled weld as compared to TIG (Tungsten Inert Gas Welding)

 

MIG welding process

An arc is struck between the end of a wire electrode and the workpiece, melting both of them to form a weld pool. The wire serves as both heat source (via the arc at the wire tip) and filler metal for the joint. The wire is fed through a copper contact tube (contact tip) which conducts welding current into the wire. The weld pool is protected from the surrounding atmosphere by a shielding gas fed through a nozzle surrounding the wire. Shielding gas selection depends on the material being welded and the application. The wire is fed from a reel by a motor drive, and the welder moves the welding torch along the joint line. Wires may be solid (simple drawn wires), or cored (composites formed from a metal sheath with a powdered flux or metal filling). Consumables are generally competitively priced compared with those for other processes. The process offers high productivity, as the wire is continuously fed.

Manual MIG/MAG welding is often referred as a semi-automatic process, as the wire feed rate and arc length are controlled by the power source, but the travel speed and wire position are under manual control. The process can also be mechanised when all the process parameters are not directly controlled by a welder, but might  still require manual adjustment during welding. When no manual intervention is needed during welding, the process can be referred to as automatic.

The process usually operates with the wire positively charged and connected to a power source delivering a constant voltage. Selection of wire diameter (usually between 0.6 and 1.6mm) and wire feed speed determine the welding current, as the burn-off rate of the wire will form an equilibrium with the feed speed.

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Things to take note in MIG welding

It can take a good amount of practice to start welding reliably every time, so don't worry if you have some problems when you first stop. Some common problems are:

  • No or not enough shielding gas from the torch is surrounding the weld. You can tell when this happens because the weld will start splattering little balls of metal, and will turn nasty colours of brown and green you can also see air bubbles on top of the weld (porosity). Turn up the pressure on the gas and see if that helps.
  • Weld is not penetrating. This is easy to tell as your weld will be weak and won't be fully joining your two piece of metal. Try more power and wire speed.
  • Weld burns a while right through your material. This is caused by welding with too much power. Simply turn down your welding power and it should go away.
  • Too much metal in your weld pool or the weld is sloppy like porridge. This is caused by too much wire coming out of the torch and can be fixed by slowing down your wire speed.
  • Poor weld penetration, erratic weld. Check condition of earth clamp and ensure where it fits to bench is clean and free from rust/paint. If earth clamp shows signs of overheating fit new clamp or complete lead assembly.
  • Welding torch spits and does not maintain a constant weld. This could be caused because the torch is too far from the weld.
  • You want to hold the tip of the torch about 1/4" to 1/2" away from the weld.