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June 12, 2011

Spot welding

A resistance-welding process in which coalescence is produced by the flow of electric current through maintained at all times during the heating cycle to the resistance of metals held together under pressure. A low-voltage, high-current energy source is required. Usually the upper electrode moves and applies the clamping force. Pressure must be prevent flashing at the electrode faces. Electrodes are water-cooled and are made of copper alloys because pure copper is soft and deforms under pressure. The electric current flows through at least seven resistances connected in series for any one weld.
(1) upper electrode, (2) contact between upper electrode and upper sheet, (3) body of upper sheet, (4) contact between interfaces of sheets,(5) body of lower sheet, (6) contact between lower sheet and electrode, and (7) lower electrode.
Heat generated in each of the seven sections will be in proportion to the resistance of each. The greatest resistance is at the interfaces in 4, and heat is most rapidly developed there. The liquid-cooled electrodes 1 and 7 rapidly dissipate the heat generated at the contact between electrodes and sheets 2 and 6, and thus contain the metal that is heated to fusion temperature at the interfaces. After themetals have been fused together, the electrodes usually remain in place sufficiently long to cool the weld. An exception is in welding quench sensitive metals, where it is desirable to remove the electrodes as soon as possible to allow the heat to be conducted in the surrounding metal, preventing steep quench gradients
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