MIG Welding Process (Explained with Definition & Diagram)

MIG welding (gas metal arc welding, GMAW process)

What is MIG welding?
Definition: MIG welding is defined as a type of arc welding process which uses a welding gun to continuously feed the heated solid wire electrode into the weld pool. The welding gun also supplies shielding gas along with the electrode for protecting the weld pool from air contamination.

(Note: Metal Inert Gas (MIG) welding is also known as Gas Metal Arc Welding (GMAW) process.)

Having discussed the definition of MIG welding, now further I have explained MIG welding process with its diagram and procedure (i.e How MIG welding works?)

So let’s begin with the procedure of MIG welding (or GMAW welding process (Gas Metal Arc Welding Process).

MIG welding process (or Gas Metal Arc Welding process)

MIG welding diagram

The MIG welding diagram is shown in the above image.

You can see all the major components of MIG welding (or GMAW welding process) labeled in the diagram itself.

MIG welding procedure (13 steps)

Follow this 13 step MIG welding procedure to get your welding done.

Step 1: Setup the MIG welding machine

Step 2: Setup the gas tanks

Step 3: Setup the welding gun and feed wire

Step 4: Attach the group clamp

Step 5: Clean the weld surface

Step 6: Wear safety equipment

Step 7: Turn on the gas cylinders

Step 8: Setup the welding parameters as per recommendation

Step 9: Turn on welder

Step 10: Set the speed of feed wire and check the wire in welding gun

Step 11: Start welding the workpiece

Step 12: Close the valve of gas cylinder

Step 13: Turn off welder after welding is completed

Detailed explanation of MIG welding procedure

If you have read the above 13 steps of MIG welding, then you might have got the overview of the entire process. But there are lot more things to consider while welding using a MIG welder.

  • In MIG welding, the filler rod (or filler wire or filler material) is generally connected to the positive terminal of the DC power source of welding.
  • And as you can see in the diagram above, the negative terminal of DC power source is attached with the workpiece.
  • The main purpose of using DC power source is that it gives a stable arc as well as the smooth metal deposition. Also the weld spatter is minimized by using DC current in MIG welding.
MIG welder
  • If an AC power source is used in the MIG welding process, then it may give an irregular arc.
  • Hence DC power source is generally prefered in MIG arc welding.
  • The power sources used in Gas Metal Arc Welding process are rated at 60% duty cycle for semi-automatic operations. While it is rated at 100% duty cycle for automatic continuous operations. 
  • During the MIG arc welding operation, the amperage for semi-automatic operations is set around 600 Amps, while for automatic continuous operations the amperage is set around 1000 to 2000 amps.
  • The AC power source having constant voltage is generally used in MIG welding.
  • When the electrode feeding rate increases, the current flow also increases. And similarly when the electrode feeding rate decreases, the current flow also decreases. This is because more current flow is required to melt more electrode material.
  • For MIG welding process, the shielding gases like argon, helium, or its mixture is used to prevent the contamination of the atmospheric gases with the weld.
Short circuit transfer Spray transfer Globular transfer welding
  • For gas metal arc welding of steel plates, CO2 gas is used as a shielding gas.
  • If thin sheets are to be welded using MIG welding process, then short circuit metal transfer mode is used with low voltage (around 16 V to 22 V) and low current density (60 amps to 180 amps).
  • By using lower arc voltage, the arc length is reduced and the molten metal is deposited in the weld pool by direct contact.
  • If helium is used as a shielding gas in metal inert gas arc welding process, then the flow rate of helium should be kept higher. This is because helium is much lighter than air, so it rises in a turbulent manner and tends to mix rapidly in the air.
  • If oxygen or carbon dioxide gas is added to the shielding gas like helium or argon, then there are chances that welded joints may oxidize and it may also lead to weld porosity defect in some ferrous metals.
  • Carbon dioxide gas is suitable for MIG welding of mild steel as it gives sound weld deposition. But care should be taken that the electrode must have some appropriate balance of deoxidizers like Ti, Si, Zr and Al.
  • In MIG welding process, the filler wire can be heated to increase the deposition rate of the metal. This process is known as hot wire MIG welding.

What are the functions of the current passing through the MIG welding wire?

The main functions of the current passing through the filler wire are mentioned below.

  • To heat the filler metal to its melting conditions.
  • To set-up electromagnetic force at the tip of electrode that forces the molten metal to deposit on the workpiece surface.
  • To maintain the plasma.
  • To wet-in the weld bead and workpiece so that proper weld penetration can occur.

What motion to use when MIG welding?

electrode motion in MIG welding

In order to get the perfect weld, neither wider nor narrow and dipper penetration is required. But it is necessary to give proper motion to the welding torch during the ongoing MIG welding process. 

The welding movements can be back and forth, or oval motion.

The picture given above represents the different welding movements for MIG welding process (or Gas Metal Arc Welding process (GMAW process)).

What is the purpose of shielding gas in MIG welding?

shielding gas MIG welding

In MIG welding, the shielding gas plays a very important role in producing good quality and defect free welds.

  • The shielding gas provides protection (or shield) to the weld from the atmospheric gases (mainly oxygen), moisture content of air, etc which may cause oxidation of the weld.
  • Because of the flow of shielding gas, the welding gun remains cooler.
  • The major component of shielding gas mainly includes helium, argon, carbon dioxide and oxygen.
  • Inert gas used in MIG welding does not react with the weld. While the other gases like oxygen and carbon dioxide are useful in stabilizing the arc and it also helps in secure transfer of molten metal to the weld part. 

Argon (Ar): Argon is an inert shielding gas. This gas is chemically inert, that means it does not react chemically and so it does not cause oxidation of metal.

Argon gas is commonly used as a shielding gas in TIG welding process.

Helium (He): Just like Argon, helium is also an inert gas which is chemically inert and does react with metal.

In MIG welding and TIG welding, the mixture of helium and argon is used.

Helium gas helps in getting better side penetration and greater welding speed as compared to argon gas.

Carbon dioxide (CO2) and Oxygen (O2): These gases are used in metal arc active gas welding (MAG welding), in which the gas helps in smooth transfer of molten metal to the weld part.

The CO2 and O2 gases are mixed in different proportions based on the type of steel to be welded.


External links:
GMAW welding: Image credits Cierrex derivative work: OSH FPaD, CC BY-SA 3.0, via Wikimedia Commons

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