GAS METAL ARC WELDING (GMAW)

GAS METAL ARC WELDING (GMAW)

This is also known as Metal inert-gas arc welding (MIG) because it utilizes a consumable electrode. There are other gas-shielded arc- welding processes utilizing consumable electrodes such as flux-cored arc welding, all of which coming under MIG. It’s quite suitable for thicker sheets because filler metal requirement in welding process makes gas-tungsten arc welding (GTAW or TIG ) difficult to use, which is more suitable for thin sheets.

The consumable electrode is in the form of wire reel fed at a constant rate through the feed rollers. The welding torch is connected to the gas supply cylinder, which provides the necessary inert gas. The electrode and the work piece are connected to the welding power supply. The power supply is always of the constant voltage type. The current from the welding machine is changed by the rate of feeding of electrode wire. Normally, DC arc-welding machine are used for GMAW with electrode positive (DCEP). The DCEP increases the metal- deposition rate and also provides a stable arc and smooth electrode metal transfer. With DCEN the arc become highly unstable and also results in spatter. But special electrodes having calcium and titanium oxide mixtures as coatings are found to be good for steel with DCEN.

Metal Transfer in GMAW process

Metal transfer takes place from the electrode to the joint in GMAW process. The metal transfer is done in different ways depending on the current and voltage used for a given electrode.

Short circuit or dip transfer,

Spray transfer,

Pulsed spray transfer, and

Rotating spray transfer.

The short-circuiting metal transfer occurs with relatively low current settings of the order of 75 to 175 A for an electrode diameter of 0.9 mm. The number of times that the pinching takes place depends on the inductance of welding machine used and the parameters set. The rate at which the short-circuiting current increases is controlled by inductance of the welding machines. Too low an inductance gives rise to very high short-circuiting current and consequently high pinching rate. With high inductance, the short-circuiting becomes low and results in somewhat lower pinching force. The effect of inductance is measured in terms of response rate (raising rate of current due to short-circuiting, kA/s). It can be observed that the optimum response rate depends on electrode wire size. Hence, he welding machines to be used for short-circuiting transfer make use of a variable choke system which can be tuned for different wire sizes. The frequency of metal transfer may be of the order of 50 to 200per second. This rate also depends on the open-circuit voltage and wire-feed rate employed.

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