What is MIG Welding?
MIG Welding or also called Metal Inert Gas Welding (MIG) or metal active gas welding (MAG welding) is a gas shielded arc welding process. It is a semi-automatic welding process. In this process, a continuous solid wire filler rod is used in the form of wire as a welding consumable.
Externally supplied welding shielding gas (e.g., argon, CO2, 75-25 Welding gas) is required to protect the molten weld pool from oxidation. These gases blanket the solidifying weld puddle from contamination of atmospheric gases (e.g., Nitrogen & oxygen) to avoid weld oxidation and porosity.
MIG Welding Process Principle
MIG is welding one of the important welding processes in fusion welding processes. This process is very versatile and makes it suitable to weld both thin sheets and thick sections.
Welding can be performed in all positions with high productivity. As shown in figure 1, an arc is generated between the end of the solid wire protruding out of the nozzle and the workpiece. The heat generated melts both the workpiece and filler wire to form a weld pool.
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MIG Welding Equipment setup
The welding process works on the constant voltage principle. The workpiece is connected to the negative terminal and the welding torch to the positive terminal. The on/off switch is located on the welding torch. In on mode, the wire is continuously fed via the wire feeder. The welder moves the welding torch along the welding joint line or axis.
Two types of filler wire are available in the market for the GMAW process:
- Solid wire (AWS A 5.18 for carbon steel electrodes)
- Metal cored wire (AWS A 5.36 for carbon steel electrodes and low alloy steel)
Different size of filler wires is used ranging from 0.8 mm to 1.6 mm in the industries according to the requirements. A higher diameter offers more deposition although less suitable to weld thin sheet metals.
To perform the GMAW, a DCEP polarity welder with the following auxiliaries are required as shown in figure 2 are:
- DC Welder
- Wire feeder
- Welding torch
- Welding Gas
- Earthing cable, gas hose.
Figure 3 shows the construction of the welding torch along with the main components. Parts such as contact tip, nozzle, gas diffuser, and liner are replaceable thus offering a cost-saving in case of damage to any of these components instead of full torch replacements.
Gas metal arc welding torch is having two variants:
- Air-cooled welding torch
- Water-cooled welding torch.
An air-cooled welding torch is used for normal duty welding activities. Welding requires a high duty cycle and high welding current normally uses water-cooled torched.
For example in robotic welding MIG welding and thick plate joints welding. The air-cooled torches due to their simple construction are cheaper than water-cooled torches. They are also easy to repair compared to the latter ones.
Metal transfer mode in MIG Welding
Metal transfer mode means the form of metal transfers from filler wire to the weld pool is defined by the welding parameters, shielding gases type, and operating feature of the welding power source.
The four-principle mode of metal transfer in MIG Welding or gas metal arc welding are:
- Short circuit transfer mode: Short-circuit transfer mode happen when the welding current and voltage are low. Hence, this mode offers low heat input in the weld. This mode is beneficial for sheet metal welding, root runs welding in butt joints, out-of-position welding, and low heat input weld depositions. The normal current for short circuit mode is 90-180 ampere and voltage within the range of 16V to 22 volts.
- Spray transfer mode: This mode requires a high current and voltage to melt the welding wire faster to avoid contact with the base metal during welding. This mode offers advantages of high weld deposition and improved weld profile but this mode is not good to perform out-of-position welding. The metal transfer happens in the fine metal droplets deposited in the weld pool. For spray transfer mode to occur two conditions shall be met:
- Minimum 80% argon in the welding gas, and
- Minimum 220 amperes and 20 voltages.
3. Pulsed transfer mode: This mode uses a modified waveform of the output current. This offers a controlled metal deposition enabling controlled heat input, and enhanced weld profile with very low spatters. Pulsed mode is beneficial for welding thin sheets, exotic materials, and out-of-position welding.
4. Globular transfer mode: Globular transfer mode takes place with a high current and 100% CO2 gas in the shielding. The metal transfer takes place in the form of irregular metal transfer. This mode is beneficial for thick plate joints. The disadvantages are high spatters and high heat input. Because of this reason, it is not recommended for sheet metal welding.
Welding Gases for MIG Welding
Shielding gases protect the weld pool from the adverse influences of atmospheric contamination. Shielding gas for GMAW is classified into two groups:
- Inert gases such as argon & helium, GMAW using inert gas is called Metal inert gas (MIG) welding.
- Active gases such as CO2, Ar + CO2 mixture, GMAW using inert gas is called Metal active gas (MAG) welding.
CO2, argon + CO2, and CO2 + argon + oxygen mixtures are used for carbon steel and low alloy steel welding. Usually, inert gases such as argon and argon + helium mixtures are used for the welding of non-ferrous materials.
Advantages of MIG Welding
- GMAW process can be carried out using different metal transfer modes for various welding conditions.
- GMAW is a versatile process and is used for welding most engineering materials such as carbon steel, low alloy steel, stainless steel, aluminum, copper, and nickel alloy, etc.
- The process offers high productivity usually 8-10kg deposition is made per shift. Very high productivity is possible using the tandem MIG option.
- The process can be used for all positions welding.
- GMAW provides higher weld deposition due to semi-automatic welding mode with continuous wire supply.
- In this process, there is no continuous start/stop like in stick welding. This also minimizes the start-stop points cleaning or grinding.
- The process being flux less requires minimal cleaning after welding.
- Since the GMAW is a semi-automatic welding process, it is easy to get skilled.
- GMAW can be easily used with automated setup for robotic welding and for special purpose machines (SPM).
Limitations of MIG Welding
- The MIG-MAG welder are costlier compared to stick welding and require additional setup to make a weld.
- The weld metal solidifies faster as compared to SMAW and SAW and can result in degraded weld metal properties.
- The force relies on external welding gas to protect weld pool and this restricts the process for indoor welding only.
- The equipment setup is not suitable for outdoor work.
Applications of MIG Welding
GMAW welding is widely used in various industries and accounts for a major weld deposition made worldwide. It is being used in automobile, railway, structural, heavy engineering, and oil & gas areas.