Experiment-3: Study of Metal Inert Gas (MIG) Welding.
AIM: To Study about MIG Welding Process
APPARATUS: Power Source & Cables, Welding Torch & Wire Electrode Coiled on a Spool,
Wire Feed Mechanism & Controls, Shielding Gas Cylinder, Pressure Regulator, Flow Meter.
OBJECTIVES:
➢ To study the principle of arc welding process, welding cycle & different time phases
involved.
➢ To understand various welding parameters of MIG welding process.
➢ To study the importance of pulsed DC in an arc welding process.
WHAT IS MIG WELDING?
MIG stands for metal inert gas welding or sometimes it is known as Gas Metal Arc welding.
This welding is same as TIG welding except a non-consumable electrode replaced by
consumable electrode wire. This process is automated or semi-automated arc welding process
in which inert gases are used as shielding gas and a consumable wire electrode is used. This
welding was developed to welding aluminum or other non-ferrous metals in 1940. Now days,
this is found in every industrial or manufacturing process due to its high speed, easy operation
and easily automated qualities. It is mostly used in sheet metal industries or
in automobile industries.
PRINCIPLE:
Mig works on same principle of TIG or arc welding. It works on basic principle of heat
generation due to electric arc. This heat is further used to melt consumable electrode and base
plates metal which solidify together and makes a strong joint. The shielded gases are also
supplied through nozzle which protect the weld zone from other reactive gases. This gives good
surface finish and a stronger joint.
EQUIPMENTS:
POWER SOURCE:
In this type of welding process, a DC power supply is used with reverse polarity. Reverse
polarity means the electrode or in case of MIG welding electrode wire is connected positive
terminal and work piece to negetive terminal. It is due to principle of electric circuit which
state that 70% of heat is always on positive side. So reverse polarity ensures that the maximum
amount of heat liberate at tool side which melt the filler metal in proper way. Straight polarity
can cause unstable arc that result into large spatter. The power source consist of a power supply,
a transformer, a rectifier which change AC into DC and some electronic controls which control
the current supply according to weld requirement.
WIRE FEEDER SYSTEM:
We know that MIG welding needs continuous consumable electrode supply for welding two
plates. This consumable electrode used in form of wire. These wire is continuously supplied
by wire feed mechanism or system. It controls the speed of the wire and also pushes the wire
form welding torch to welding area. These are available in different shapes and sizes. It consist
a wire pool holder, a driving motor, a set of driving rollers and wire feed controls. The wire
feed speed is directly control the current supply through power supply. If the wire feeding speed
is high, it required more current in welding zone to produce proper heat for melting of it.
WELDING TORCH:
This torch is slightly different as used in TIG welding. In this torch there is a mechanism which
hold the wire and supply it continuously with the help of wire feed. The front end of the torch
is fitted with a nozzle. The nozzle is used to supply inert gases. These gases form a shielding
area around the weld zone and protect it from oxidization. The welding torch is air cooled or
water cooled according to the requirement. For high current supplied, the torch is water cooled
and for low supply it is air cooled.
SHIELDING GASES:
The primary function of shielding gases is to protect weld area from other reactive gases like
oxygen etc. which can affect the strength of welding joint. These shielding gases are also form
plasma which helps in welding. The choice of gas is depend on the welding material. Mostly
argon, helium and other inert gases are used as shielding gases.
REGULATORS:
As the name implies, they are used to regulate the flow of inert gases from the cylinder. The
inert gases are filled into cylinder at high pressure. These gases cannot be used at this pressure
so a regulator is used between the gases supply which lower down the gases pressure according
to welding requirements.
WORKING:
Its working can be summarized as follow.
Figure 1. Schematic of Gas metal Arc Welding (GMAW)
▪ First, a high voltage current is change into DC current supply with high current at low
voltage. This current pass though welding electrode.
▪ A consumable wire is used as electrode. The electrode is connected to the negative
terminal and work piece from positive terminal.
▪ A fine intense arc will generate between electrode and work piece due to power supply.
This arc used to produce heat which melts the electrode and the base metal. Mostly
electrode is made by the base metal for making uniform joint.
▪ This arc is well shielded by shielding gases. These gases protect the weld form other
reactive gases which can damage the strength of welding joint.
▪ This electrode travels continuously on welding area for making proper weld joint. The
angle of the direction of travel should be kept between 10-15 degree. For fillet joints
the angle should be 45 degrees.
PULSED DC IN ARC WELDING
In normal mode the machine is operated at constant voltage and current. In Pulse mode the heat
input is supplied in the form of short pulses at the period in milli seconds. In the pulse mode
the machine is operated at peak current to melt the electrode wire for a very short period of
time.
After the melting period the voltage and current are lowered to a very low value which can
sustain the arc but not used in melting the electrode. This voltage and current termed as back
ground voltage and background current. During this background period cooling of the weld
bead takes place. The process of applying peak and background pulse is repeated during the
welding process.
• Pulsed GMAW result in a very good bead shape with less amount of residual stress due to
pulsing of current and voltage.
• The higher pulsing rates increase puddle agitation → a better grain molecular structure within
the weld.
• High speed pulsing constricts and focuses the arc; Increases arc stability, penetration and
travel speeds.
• Reduces arc blow (created by influence of magnetic field).
• A smaller heat-affected zone.
• 4 Variables: peak amperage, background amperage, peak time and pulse rate.
APPLICATIONS:
• MIG is best suited for fabrication of sheet metal.
• Generally, all available metals can be weld through this process.
• It can be used for deep groove welding.
ADVANTAGES AND DISADVANTAGES
ADVANTAGES:
▪ It provide higher deposition rate.
▪ It is faster comparing to arc welding because it supply filler material continuously.
▪ It produce clean weld with better quality.
▪ There is no slag formation.
▪ Minimize weld defects.
▪ This welding produces very little slag.
▪ It can be used to make deep groove weld.
▪ It can be easily automated.
DISADVANTAGES:
▪ It cannot be used for welding in difficult to reach portions.
▪ Higher initial or setup cost.
▪ It cannot be used for outdoor work because wind can cause damage of gas shield.
▪ It required high skilled labor.
