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RESISTANCE WELDING AND ITS PROCESS

In Resistance welding the metal parts to be joined are heated to a plastic state over a limited area by their resistance to the flow of an electric current and mechanical pressure is used to complete the weld. Recently, with air and hydraulic systems for applying pressure at correct time and in right amount through electronic controls, resistance welding has been advanced. In this process, preferably two copper electrodes are incorporated in a circuit of low resistance and the metals to be welded are pressed between the electrodes. The circuit is thus completed and the electrical resistance at the joint of the metals to welded is so high, in comparison with the rest of the circuit, that if the current is heavy enough the highest temperature will be produced directly at the joint. The heat generated in the weld may be expressed by 

H= I^2 * RT 

Where H is heat , I is the current, R is the Resistance of the assembly and T the time or duration of current flow. That is, the heat developed by the current is in proportion to the electrical resistant of the joint. 

The electrical pressure or voltage from either 120 or 240 V is reduced down from 4 to 12 volts, depending on the composition, area, thickness, etc., of the metal being welded. The amount of power supplied to the weld usually ranges from about 6 to 18 kW for each cm2 of area. Alternating current has been found most convenient for this purpose as it is possible to obtain any desired combination of  current and voltage by using a suitable transformer. 

The machine used for making resistance welds contains a transformer, a clamping device for holding the pieces, and a mechanical means for forcing the pieces together to complete the weld. In machines which are operated continuously , the electrodes are cooled by water circulating through hallow electrodes. 

Resistance welding, which is used with sheet metal from 0.5 to 3.2 mm thick and with steel pipe and tubing, is employed mainly for mass production, because of  the type of equipment required for its application. Metals of medium and high resistance, such as steel, stainless steel, monel metal and silicon bronze, are easy to weld. 

High-Frequency resistance welding is done with 400 to 450 kc current commonly supplied by an oscillator. The high frequency current readily breaks through oxide film barriers and produces a thin heat affected zone because it travels on the surface of the material. 


Resistance welding


Resistance Welding Methods :

The field of  resistance welding can be sub divided into several processes, The most important being :

1) Butt Welding.

2) Spot Welding.

3.) Seam Welding.

4.) Projection Welding.

5.) Percussion Welding. 



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