PRINCIPLES OF MIG WELDING TECHNOLOGY

PRINCIPLE OF MIG WELDING

Gas Metal Arc Welding (GMAW) is commonly referred to as MIG welding (Metal Inert Gas

welding). It is also referred to as MAG welding (Manual Metal Arc Welding). The basic principle of

MIG Welding is, an arc is maintained between the end of the bare wire electrode and the work piece

where the heat source required to melt the parent metal is obtained. The arc melts the end of the

electrode wire, which is transferred to the molten weld pool. For a given wire material and diameter,

the arc current is determined by the wire feed rate. The arc and the weld pool is shielded from the

atmospheric contamination by an externally supplied shield gas. Metal Inert Gas (MIG) welding is a

'flat' arc process (constant) voltage. The required voltage is selected by adjusting the voltage control

knobs provided at the power source. The process itself can be manual, partly mechanized, fully

mechanized or automatic. An example for MIG welding is show below

MIG welding Setup MIG welding component

MIG Hard Faced component

Schematic representation of MIG welding process:

MIG WELDING EQUIPMENT

The basic MIG welding equipment consists of the following components:

• Power source

• Welding Gun

• Gas cylinders with pressure-reducing valve and flow meter

• Wire feeder unit

The basic circuit and the equipment of MIG welding is shown below,

Basic MIG Welding Circuit: Fully Automated MIG Welding Equipment:

WELDING VARIABLES AND PARAMETERS

1. Electrode extension - affects the amperage. Stick out length should be 10 - 15 mm.

2. Inductance - smoothes the arc characteristic. Also called the choke. If it is Set low it gives excess penetration and if set high, no penetration.

3. Wire feed speed - amperage. Controls fusion and penetration.

4. Travel speed - controls depth of penetration.

5. Gas flow rate - protects weld from atmosphere.

6. Voltage - set on the welding machine and controls the arc length.

7. Tilt angle - back or fore hand should be not greater than 15° from the perpendicular.

8. The welding position and type of weld are further variables to be considered.

METAL TRANSFER ACROSS THE ARC

The weld metal transfer from the electrode to the work is classified into four different types 

Short circuiting transfer Globular transfer Spray transfer Pulsed spray transfer

 

The mode of weld metal transfer is determined by the following: 

Welding current Electrode size Electrode composition Electrode stick out Shielding gas

Short circuiting transfer: Short circuiting transfer uses the lowest welding currents and voltages, which consequently produces very low heat input. In this mode of welding, the metal is not transferred across the arc gap, but from the electrode to the work only during a short period when the welding wire is in contact with the weld pool. When the electrode wire tip touches the weld pool, the arc extinguishes, the voltage goes down and amperage rises. At this moment, metal is transferred from the melted electrode tip to the weld pool with the help of surface tension of the melted weld metal. Globular metal transfer: Globular metal transfer occurs at relatively low operating currents and voltages but these are still higher than those used in short circuiting transfer. This metal transfer mode is characterized by a drop, two or three times larger in diameter than the wire, formed at the tip of the electrode. This droplet is detached from the tip of the electrode by the effect of a pinch force and the transfer of the droplets in irregular form across the arc is aided by the effect of the weak electromagnetic and strong gravity forces. As the droplets grow on the tip of the wire electrode they wobble around and disturb the arc plasma stability. Consequently, the heat-affected zone in the work becomes narrow, penetration of the weld becomes small, and the weld deposit is irregular and large amounts of spatter takes place. 

Spray Transfer: Under an argon-rich shielding gas, increasing the current and voltage causes a new mode of metal transfer to appear, the tip of the wire electrode is tapped, the sizes of the droplets become smaller and they are directed axially in a straight line from the wire to the weld pool. The current level above by which this mode of metal transfer begins is called transition current. The droplets are much smaller than the diameter of the wire and they detach with pinch force much more rapidly than with the globular transfer mode, there is very little spatter and the surface of the weld bead is smooth. Pulsed spray transfer: The Pulsed mode of metal transfer in MIG is used for applications where a good penetration and reduced heat input are required. A pulsed current transfer is a spray type of transfer that occurs at regularly spaced intervals instead of constantly. This mode of metal transfer can only be produced if the power source is able to supply a pulsed current. The level of a welding current supplied by a pulsing type of power source varies between high and low levels. Whereas high level is above the transition current and produces the droplets, low level or background current has only sufficient energy to sustain the arc.

WELDING TORCH

The Welding torch feeds the wire and directs inert gas to the weld area with the help of gas nozzle. They are highly insulated as electric current is flowing in order to ensure operator safety.

The selection of the proper MIG torch, commonly called a MIG gun, depends upon the following factors:

 o Type of welding: semiautomatic, hard automation or robotic automation.

o Level of current (amps) required by the welding application and capacity of the torch.

o Shielding gas selected.

o Duty cycle of the torch.

o Preference of an air-cooled or water-cooled torch. 

Both air cooled (natural) and water cooled (when the current is above 200 amp) torches are available. The basic welding torch consist of  

o Gas Nozzle

o Copper contact tube

o Water hose (if water cooled)

o Gas hose

o Welding cable

o Wire conduit

o Trigger switch  The duty cycle of the MIG torch selected relates to the shielding gas and the maximum current that is

specific to the welding application.

SCHEMATIC REPRESENTATION OF WELD TORCH:

WELDING TORCH

POWER SOURCE

Only Direct Current DC (Reverse Polarity - DCRP) source is used for MIG welding process. A power source with a flat characteristic is almost used for Metal Inert Gas Welding process as it offers several advantages e.g., latitude in setting the welding condition and self-regulated arc besides it also meets the special requirements of dip transfer welding. Power sources incorporate output characteristics designed to optimize the arc performance for a given welding process. For MIG, the output characteristics fall into two main categories:

1. constant current

2. constant voltage

The important advantage of flat characteristic power source is its ability to produce self-regulated arc. Self-regulated arc means maintaining the arc length constant. With a constant potential power source, change in arc voltage will have marked effects on current. Thus if the arc length is reduced from the set value, there will be an increase in current resulting in faster burn off rate and the arc length will be adjusted back to its original value.

In other hand increase in arc length from a set value would increase the arc voltage resulting in less current

and a low burn off rate. The arc length would be restored to the original length. Self-adjustment will operate

successfully only when the change in current produced by voltage fluctuations is sufficiently large to

produce a large alteration in burn off, and rapid response rate and correction that the disturbance cannot

beat. Current density is defined as the current employed with a particular electrode diameter divided by its

current carrying cross-sectional area. If the wire feed speed is low, then the current density will be low, and

vice versa. Lower current density applied to a given electrode is associated with the short-circuit mode of

metal transfer. Higher current density is associated with the higher energy modes of metal transfer: globular,

axial spray transfer or the more advanced pulsed spray metal transfer.

SHIELDING GASES

The primary purpose of shielding gas is to protect the molten weld metal from contamination by the oxygen and nitrogen in air. The type of shielding gas used has a major influence on the weld quality. Only inert gases and their mixtures are utilized for welding. The required purity of the gases must be guaranteed.  

• MAG welding + Reactive shielding gasses (oxygen, nitrogen, carbon dioxide &

hydrogen)

• MIG welding + Inert shielding gasses (Argon & Helium)

Although pure inert gas protects metal at any temperature from reaction with constituents of air, they are not suitable for all welding applications. Controlled quantity of reactive gas mixes with inert gas improves the arc action and metal transfer characteristics when welding the steel, but such mixtures are not used for the reactive metals. The heavier a gas, the more effective it is for gas shielding. Helium is very light, argon is about 10 times heavier than helium and about 30% heavier than air.

The reactive gas are not generally used alone for arc shielding, carbon dioxide can be used alone or mixed with an inert gas for welding many carbon and low alloy steels. Oxygen is used in small quantity with one of the inert gasses – usually Argon. Nitrogen is used occasionally but is mixed with argon, as a shielding gas to weld copper. The most extensive use of nitrogen is in Europe, where helium is relatively unavailable.

 

PENETRATION PATTERN FOR VARIOUS SHIELDING GASSES:

ADVANTAGES

• Ability to provide high quality welds, for a wide range of ferrous and non-ferrous alloys, at a low cost.

• The ability to join a wide range of material types and thicknesses.

• Simple equipment components are readily available and affordable.

• MIG welding has higher electrode efficiencies (93% - 98%).

• All-position welding’s are possible.

• Excellent weld bead appearance.

• Low hydrogen weld deposit

• Low heat input when compared to other welding processes.

• Less weld spatter and slag makes weld clean up fast and easy.

• Less welding fumes when compared to SMAW (Shielded Metal Arc Welding) and FCAW (Flux-Cored Arc Welding) processes.

DEFECTS AND CAUSES

• Lack of fusion.

• Excessive penetration.

• Silica inclusions (with steel only).

• Solidification (center line) cracking: Spray transfer current too high, deep narrow prep.

• Porosity: Gas flow too high or too low, blocked nozzle, leaking gas line, draughty conditions, nozzle to work distance too long, Painted, primed, wet or oily work surface, damp or rusty wire.

• Lack of penetration: Current too low, prep to narrow, root face too thick, root gap too small, worn tip causing irregular arcing, irregular wire feed, poor technique, mismatched joint.

• Undercut: Speed too high, current too high, irregular surface, wrong torch angle.

• Crater cracking: Poor finishing technique.

• Spatter: Inadequate choke, voltage too low, rusty or primed plate.

LIMITATIONS

The lower heat input characteristic of the short-circuiting mode of metal transfer restricts its use to thin

materials.

The higher heat input axial spray transfer generally restricts its use to thicker base materials.

The higher heat input mode of axial spray is restricted to flat or horizontal welding positions.

The use of argon based shielding gas for axial spray and pulsed spray transfer modes is more expensive

than 100% carbon dioxide (CO2).

APPLICATIONS

It is used in:

• Structural steel.

• Aluminum sections.

• Stainless steel and nickel alloys.

• Some offshore applications (flux core only).

Arcraft Plasma Equipments (I) Pvt Ltd., 124, Diamond Industrial Estate, Ketkipada, Dahisar (East),

Mumbai- 400068.

INDIA.Ph : 00-91-22-28965890, 28965745, 28963247 Fax: 00-91-22-28966418