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The process involves fusing two pieces of sheet metal together by penetrating entirely through one piece into the other. No joint preparation is required except proper cleaning of the overlap areas. The main operation in arc spot welding is to strike and hold an arc without travel at a point where the two parts to be joined are held tightly together.
A vented metal nozzle of a shape to suit the application is fitted to the MIG gun and is pressed against the workpiece at the desired area. The operation is carried out for a period of 1-5 seconds and a slug is melted between the parts to be joined. Timing is usually controlled automatically with the help of a timer. Thus, the process time can be varied to achieve welds of different sizes depending upon the thickness of the sheets. Arc initiation is a critical part of the process and therefore must be reliable and consistent. This is easy to achieve by a flat characteristics power source and clean surface of the work.
GMAW spot welding is a highly adaptable process which requires very little manipulative skill; does not require the use of a welding helmet. It is an extremely fast process and can be fully automated. Due to addition of extra metal the weld slug is free from piping defects. A wire composition different from the base metal may be used to control cracking, porosity, or strength. Argon and CO2 are shielding gases commonly used for GMAW spot welding.
GMAW arc spot process can be used more efficiently for downhand welding position. It can be successfully employed for horizontal position but fails for overhead welding position.
This process does not require a hole to be made in either member, thus in differs from plug welding in that respect. As the upper member is required to be melted through and through, its thickness is normally restricted to 3 mm. The thickness of the second member is not important. Through lap joints are the most often used type of joint for arc spot welding but fillet joints can also be successfully made by this process.
In this process the metal from the electrode wire scours deeply into the weld crater. This breaks up the oxide films at the faying surfaces so that the process can be used as successfully on aluminum as on mild, low alloys, and stainless steels.