MIG (metal inert gas) welding weldingtheory
MIG (Metal Inert Gas) welding, also known as MAG (Metal Active Gas) and in the USA as GMAW (Gas Metal Arc Welding), is a welding process that is now widely used for welding a variety of materials, ferrous and non ferrous
MIG WELDING TORCH
The MIG torch can be air cooled or water cooled and most modern air cooled torches have a single cable in which the welding wire slides through a Liner. Gas flows around the outside of this Liner and around the tube the Liner sits in is the power braid and trigger wires. The outer insulation provides a flexible cover. Water cooled MIG torches are similar to the above, but gas hose, liner tube, power lead (including water return pipe), water flow pipe and trigger wires are all separate in an outer sleeve. Most industrial MIG equipment uses a standard European MIG torch connector for easy connection of torch, some low cost smaller units use individual manufacturers fittings. The important areas of maintenance are: Liners are in good condition and correct type and size; Contact tips are lightly fitted, of correct size and good condition.
Shielding Gas
This is a complicated area with many various mixtures available, but the primary purpose of the shielding gas in the MIG process is to protect the molten weld metal and heat affected zone from oxidation and other contamination by the atmosphere. The shielding gas should also have a pronounced effect on the following aspects of the welding operation and the resultant weld.
• Arc Characteristics A basic position or starting point would be
• Mode of Metal Transfer
• Penetration and Weld Head Profile Aluminum - Argon
• Speed of Welding Magnesium - Helium
• Undercutting Tendency Copper Alloys - Argon - Helium Mix
• Cleaning Action Steel - CO2 not commonly used
Weld Metal Mechanical Properties today, Ar-CO2 mix is preferred
metal transfer
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. When the droplet from the tip of the wire passes to the weld pool there is no more metal connection and the arc is reestablished. At the heat of the arc tip, the electrode is melted and as the wire is fed towards the weld pool the next short circuit occurs. The rate of current increase during the short circuit is controlled by the induction of the power source, whereas the re-ignition and the maintenance of the arc are provided by the energy stored in the inductor during the short circuiting period
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 characterised 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.