Fusion welding: major arc welding methods

TIG welding

TIG (Tungsten Inert Gas) welding uses an inert gas for welding. This type of arc welding does not throw sparks and can be used to weld various metals, including stainless steel, aluminum, and iron.
Non-consumable tungsten is used for the discharging electrode and an inert gas such as argon or helium is used as the shielding gas. The process strikes an arc in an inert gas and uses the arc heat to melt and weld the base material. Although a filler material is used, spatter is rare because the weld area is covered with the inert gas and the arc is stable.

TIG welding
  1. Shielding gas
  2. Tungsten electrode
  3. Ar gas
  4. Arc
  5. Weld metal
  6. Weld pool
  7. Filler rod
A semi-automatic TIG welding machine mainly consists of the following:
  • Welding power supply
  • Welding torch
  • Gas cylinder and gas flow controller
Some other instruments are added when the torch is a water-cooled type or the filler material is a wire. The polarity of the electric current (positive or negative) should be selected depending on the base material. Consequently, the welding power supply requires a controller for selecting the polarity according to the base material.
TIG welding
  1. Gas cylinder
  2. Welding power supply
  3. Remote controller box
  4. Torch

There are various types of TIG welding that can be classified according to the use of AC or DC power, the use of pulse or non-pulse current, and whether a filler wire is used or not.

AC or DC is selected depending on the base material. A pulse or non-pulse current can be selected. The method using a pulse current is called pulse TIG welding. In pulse TIG welding, the welding current is changed alternately at a constant frequency between a pulse current and a base current. The base material melts while the pulse current flows, and cools while the base current flows. This creates weld spots periodically, resulting in a bead looks like a string of beads.

There are two types of TIG welding that use a filler wire: cold wire welding and hot wire welding. Cold wire welding uses a normal filler wire. Hot wire welding heats up the wire beforehand by passing a current through it. This can increase the amount of deposition per unit time. Since about three times the amount of filler material can be melted compared with cold wire welding, welding can be completed more quickly. Hot wire welding makes up for the weakness of TIG welding where it can provide high-quality welding but takes time to melt the necessary amount of filler material.

Classification by output current waveform
Output current Pulse Frequency
Direct current (DC) Yes Low frequency (0.5 Hz to 20 Hz)
Medium frequency (20 Hz to 500 Hz)
High frequency (20 kHz or higher)
No -
Alternate current (AC) Yes Low frequency (0.5 Hz to 20 Hz)
Medium frequency (20 Hz to 500 Hz)
No -
Classification by the use of a filler wire
Wire Method
No
Yes Cold wire
Hot wire

The classifications above are just an example. There are various ways to classify the types, and some may differ from the table above.

MAG welding

MAG (Metal Active Gas) welding is a type of arc welding that uses an active gas (carbon dioxide [CO2] or a gas mix of argon and CO2). The process is also called as CO2 arc welding or CO2 welding. This process is generally used for automatic or semi-automatic welding of ferrous metals. It is not suited for nonferrous metals such as aluminum because of the chemical reaction of CO2.

Automatic or semi-automatic MAG welding uses a coiled welding wire as an electrode instead of the welding rod used in shielded metal arc welding (manual arc welding).
The coiled wire is attached to the wire feed unit and is sent automatically to the torch tip by a feed roller that is driven by an electric motor. The wire is energized when it passes through the contact tip that holds the wire.
An arc is struck between the wire and base material, which melts the wire and base material simultaneously to weld them. During the process, the shielding gas is supplied through a nozzle into the weld area and its surroundings to shield the arc and weld pool from the atmosphere. For the shielding gas, CO2 gas, a gas mix of argon and CO2, or a gas mix of argon with a few percent of oxygen is used.
Compared with shielded metal arc welding, the deposition rate at which the electrode becomes the weld metal is faster, which brings the advantage of high work efficiency due to the deep penetration of the base material. There are other important advantages, such as the weld metal being of high quality and that installing the welding torch on a robot enables automatic welding.

MAG welding
  1. Ar + CO2 gas mix
    or CO2 gas
  2. Solid wire electrode
A semi-automatic MAG welding machine mainly consists of the following:
  • Welding power supply
  • Wire feed unit
  • Welding torch
  • Gas cylinder
The wire has to be fed at a constant speed from the feed unit. Consequently, a constant-voltage characteristic power supply is generally used for the welding power supply. The wire feed unit is a constant speed feeding type.
MAG welding
  1. Gas cylinder
  2. Gas flow controller
  3. Welding power supply
  4. Wire feed unit
  5. Remote controller box
  6. Welding torch

MAG welding can be classified by the shielding gas or the type of the welding wire.

MAG welding

Speaking of the welding wires, solid wires have a cross section that is made up entirely of the same material. The wires for carbon steel have copper plating on their surfaces in order to improve rust resistance and electrical conductivity. Plating-free solid wires without copper plating offer advantages such as stable arcs and easy maintenance of the inside of the welding torch.
Flux-cored wires contain a core of flux inside the wire. They provide advantages such as stable arcs, less spatter and good appearance of the weld bead.
In addition to above, there are slag-cored wires and metal-cored wires. The former is characterized by fast deposition rate and the latter is characterized by less slag formation.

MIG welding

MIG (Metal Inert Gas) welding is another method of arc welding. As with TIG welding, an inert gas is used for the shielding gas, but MIG welding is a consumable electrode type of welding that uses a discharge electrode that melts during welding.
The process is generally used for joining stainless steel or aluminum alloy workpieces. An appropriate type of shielding gas must be used depending on the metal to be welded.

A coiled welding wire is used for the electrode. The coiled wire is attached to the wire feed unit and is sent automatically to the torch tip by a feed roller that is driven by an electric motor. The wire is energized when it passes through the contact tip. An arc is struck between the wire and base material, which melts the wire and base material simultaneously to weld them. During the process, the shielding gas is supplied through a nozzle into the weld area and its surroundings to shield the arc and weld pool from the atmosphere.

MIG welding
  1. Ar gas or
    Ar + 2% O2 gas
  2. Solid wire electrode
A semi-automatic MIG welding machine mainly consists of the following:
  • Welding power supply
  • Wire feed unit
  • Welding torch
  • Gas cylinder
The configuration is almost the same as that of a MAG welding machine, except for some improvements added to the wire feed unit. Since MIG welding is often used for welding aluminum, the wire feed unit has to be improved to allow stable feeding of soft aluminum wire (four-roll system).
MIG welding
  1. Gas cylinder
  2. Gas flow controller
  3. Welding power supply
  4. Wire feed unit
  5. Remote controller box
  6. Welding torch

MIG welding can be classified according to its use of AC or DC, or a pulse or non-pulse current.

Classification of MIG welding
Pulse Welding method
Direct current (DC) No Short-arc MIG welding
Spray MIG welding
Large-current MIG welding
Yes Pulse MIG welding
Low-frequency superimposed pulse MIG welding
Alternate current (AC) Yes AC pulse MIG welding
Low-frequency superimposed AC pulse MIG welding
DC + AC Yes AC/DC composite pulse MIG welding

Short-arc MIG welding is a welding method that uses the short-circuit transfer (short arc) phenomenon. It is often used in semi-automatic systems intended for thin plates due to the low heat input to the base material. MAG welding using short arc is commonly used to weld plates of medium thickness in forced positions. In case of MIG welding, such workpieces are often welded with pulse MIG welding.

Spray MIG welding is a process that sets the welding current higher than the critical current to set a higher arc voltage. It uses the spray transfer phenomenon where the molten filler material vaporizes. When an aluminum workpiece is welded such that no spatter is generated, lack of fusion or other welding defects may result. To prevent this problem, the arc voltage must be decreased a little to allow welding in small spray transfer mode. Spray MIG welding is no longer commonly used because pulse MIG welding that can handle workpieces with low to medium thickness has become common.

Large-current MIG welding uses welding wires with thick diameters (approx. 3.2 to 5.6 mm). The welding system includes a welding torch with double-shielded gas nozzle and a constant-current characteristic power supply with a rated output current of about 1,000 A.

MIG welding using DC and pulse current is also called conventional pulse MIG welding.
The basic principle is the same as that of pulse MAG welding.
This welding method passes a small base current to maintain an arc and a pulse current exceeding the critical current alternately to allow spray droplets to transfer from the wire even when the average current drops below the critical current. They ensure effective and high-quality welding of thin to thick plates.

Low-frequency superimposed pulse MIG welding is a method developed based on pulse MIG welding to achieve high-value added welding of aluminum workpieces. Since the process can create beautiful scale-like beads, it is used to weld thin aluminum plates for automobiles or motorbikes.

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