What is GMAW or MIG welding ? - explained | The Mechanical post

What is GMAW or MIG welding ? - explained | The Mechanical post

Welding is one of the most widely used manufacturing process in the industrial world. Welcome back to The Mechanical Post ! Today let's see, what is GMAW or MIG welding starting with their full forms:

MIG -  Metal Inert Gas welding.
GMAW - Gas Metal Arc Welding.

Now, Let's define GMAW :

GMAW is a type of manufacturing process in which two workpieces are joined togther by the application of  electric arc. This electric arc causes the wire electrode to melt and form a part of the weld. Upon solidification of the weld, the two workpieces are joined together.

Unlike TIG welding which uses a non-consumable tungsten electrode, MIG uses a consumable metal electrode for welding purposes.

What is meant by consumable electrode?

Simply saying, in welding if the electrode melts due the heat produced by the electric arc and becomes the part of the weld, it is known as consumable electrode. In this type continuous feed of wire electrode is required. The speed of the wire electrode feed will depend upon the voltage of the arc.

In welding process, for joining two metal pieces together either the metal ends are melted to fuse together or a filler material is used. The filler material has a relatively low melting point and fills the gap between two pieces when melted and thus joins the metal pieces. 

In TIG the tungsten electrode gets hot enough to melt the metal directly thus no filler material is used. Whereas in MIG, the electrode which produces the arc gets melted to form a part of the melt. Thus, continuous feed is given from a reel which is controlled by a variable speed motor.

What is an electric arc?

An electric arc is a continuous stream of electrons flowing through a medium between two conductors of an electric circuit and accompanied by intense heat generation and radiation.

MIG or GMAW are one of the types of " electric arc welding ". The electric arc welding is the most extensively used method of joining components of metallic parts using the heat produced due to electric arc.

Construction of MIG.

To do MIG or GMAW, the basic eelment is a welding gun (also known as welding torch or handle), a welding power supply, a welding electrode wire, wire feed unit and a shielding gas supply. MIG welding is done with the help of a welding torch. It is a handle like structure within which the electrode wire along with shielding gas is present.

The GMAW welding torch consists of following components
  • control switch for controlling the gas flow and power supply,
  • contact tip, 
  • power  supply cable for producing the arc,
  • gas nozzle for shielding gas, 
  • electrode conduit and liner
  • gas hose.
By Mysid, TTLightningRod.-Vectorized by Mysid on a JPEG by TTLightningRod.,

(1) Torch handle which is held in hand, 
(2) Molded phenolic dielectric (shown in white) and threaded metal nut insert (yellow), 
(3) Shielding gas transmitter, 
(4) Contact tip, 
(5) Output of the Torch.

The control switch, or trigger, when pressed by the technician, starts the wire feed, electric power, and the shielding gas flow, producing an electric arc.

The wire electrode melts during welding, thus continuous supply is provided via a wire feed rolls which is controlled by a variable speed motor. The speed of feed depends upon the voltage of the arc produced.The greater the arc is the voltage, faster the wire electrode is feed. Most GMAW models supply the wire electrode at a constant feed , however advanced machines can vary the feed rate according to the arc length and voltage. Feed rates as high as 30 m/min (1200 in/min) can be achieved in advanced GMAW machines, while feed rates for semi-automatic MIG or GMAW range from 2 to 10 m/min (75 – 400 in/min).

Shielding gases are also passed on over the weld site with the help of pressurized gas cylinders via the welding torch. The shielding gases are usually inert gases like Helium, Argon, or mixture of argon (80%)  with carbon dioxide(20%).

Why are inert gases used in MIG or GMAW welding?

Use of inert gases cuts off the contact between the workpiece and the atmosphere, thus preventing chances of oxidation. It forms a protective layer over the weld site and thus the name 'shielding gases'.

Working of the MIG welding.

The workpiece is placed and clamped firmly. The shielding gas is fed to the weld site. Current supply is switched ON. Direct current is used and the electrode is positively charged. This causes it to flow from the electrode to the workpiece.
Since anode tends to have a greater heat concentration, which causes faster melting of the feed wire, helps in increasing weld penetration and welding speed. 

As the current flows through the electrode an arc is form between it and the workpiece. The high temperature of  the arc causes the electrode to melt and form a part of the weld. 
Nathaniel C. Sheetz (User:Spangineer) / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

(1) Direction of motion of the weld torch, 
(2) Contact tube, 
(3) MIG wire Electrode, 
(4) Shielding gas for better arc formation and weld, 
(5) Melted weld or filler metal, 
(6) Solidified weld metal upon cooling, 
(7) Workpiece upon which welding is performed.

The electrode is made up of metallic alloy wire, called a MIG wire, whose alloy and size, is depends on the composition of the metal being welded, Depending on the base material being welded the diameters of the electrodes used in MIG or GMAW range from 0.7 to 2.4 mm (0.028 – 0.095 in). For heavy duty operations, the electrode size could be upto 4mm in diameter.

In GMAW the technician doesn't need to maintain a precise arc length, as well as feeding filler metal at the weld site this eases the welding process. In GMAW the operator  is required to guide the torch properly over the area being welded, and also maintain the torch's gas nozzle to remove spatter buildup after regular time intervals.

 Any type of welding process which uses Electric Arc can be hazardous if proper precautions are not taken. Technicians or operators must wear suitable protective gear to prevent any harm.

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Advantages of MIG or GMAW welding :

Higher Productivity

In GMAW not having to constantly change electrodes as done in TIG or chip away slag, as well as not having to brush the weld repeatedly saves the operator  lot of time. Thus he can complete more work in less time.

Easy to Learn

It is so simple thanks to the semi-automated processes that one can learn GMAW in a very short time.

Good quality Welds

MIG provides better weld pool visibility. And with better control offered by the auto-feed wire, GMAW makes it simple to produce a great looking weld.

Less slag produced

GMAW uses a shielding gas to protect the arc, thus there is very little loss of alloying elements as the metal transfers across the arc. This reduces the amount of slag produces.

High flexibility

MIG welding is very flexible as it can weld a wide variety of metals and alloys, while operating at a variety of ways, such as semi and fully automatic. This gives a lot of option to choose from.

Faster Welding Speed

Due to semi and fully automatic processes possible time taken is reduced significantly.

Disadvantages of MIG Welding :

High initial as well as maintainance cost

The machines used in GMAW are complex and are expensive. Also the replacement cost of the shielding gases, feed wire electrode and electricity costs come along with the already expensive price tag.

Not suitable for Outdoor Welding

GMAW isn't portable and can't be used outdoors as the wind could scatter away the shielding gas.

Cannot be used for Thick Metals

While MIG welding is good for thin metals, it does not deliver proper penetration for thicker steel that requires a solid weld.

Radiation effects 

The ultraviolet radiation from the electric arc may cause sunburn-like damage if the skin is  exposed to the arc.

Applications of GMAW :

  • For welding pipe joints.
  • In ship building process.
  • For Welding railway tracks.
  • For Welding in automobiles etc.

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