Spray transfer, Mechanical Engineering

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Spray Transfer

In a gas shield of argon or argon rich mixture [i.e., Ar+CO2 gas mixture containing more than 80 % argon] the metal transfer changes from globular to spray type as welding current is increased beyond the transition current. The current value at which the metal transfer mode changes from globular to spray is known as transition current.This type of transfer has a typical fine arc column and pointed wire tip associated with it. A very fine stream of droplets is projected axially through the arc. This mode of transfer is specifically called as streaming spray transfer. It occurs more readily with high resistivity, small diameter wires (eg. Austenitic stainless steel). The reduction in droplet size also accompanied by an increase in the frequency of droplet transfer. One important characteristic of spray mode of transfer is the "finger" penetration which it produces. The spray transfer mode can be used to weld any metal. However, applying the process to thin sheets may be difficult because of the high currents needed to produce the spray arc. Also the large weld pool produced due to the high deposition rate makes it difficult to use it vertical or overhead positions.

When CO2 is used for shielding, a globular to spray transition does not occur. Drops become smaller as the increases, but they are not at all axially directed. The spatter is more than that with argon or argon rich mixed gas shield.The spatter is minimised if "buried arc" technique is used. Buried arc technique involves using a low voltage for any given current, forcing the arc to locate within the cavity created by the arc force. Under this condition, most of the non axially directed droplets are trapped in the weld pool.


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