Fundamentals of resistance welding

Resistance welding is a fusion process that takes place between two metals under a certain load and heat without need for any additional material. The heat required for the welding is generated by the electrical resistance of the materials to the current versus time.

It is one of the oldest of the electric welding processes in use by industry today. The weld is made by a combination of heat, pressure, and time. As the name resistance welding implies, it is the resistance of the material to be welded to current flow that causes a localised heating in the part. The pressure exerted by the tongs and electrode tips, through which the current flows, holds the parts to be welded in intimate contact before, during, and after the welding current time cycle. The required amount of time current flows in the joint is determined by material thickness and type, the amount of current flowing, and the cross-sectional area of the welding tip contact surfaces.

Welding methods

Resistance welding machines can be divided into four welding methods and three output current as electrical solutions. Among the four types of welding methods only Flash Butt/ Butt Welding requires no overlapping of the joining metals. In terms of output current, only Capacitor Discharge DC is not applicable in Seam Welding and Flash Butt/ Butt Welding due to its nature of instantaneous current flow time.

Output current solutions

? Single Phase AC is the most simple and basic output current for all welding machines. With single phase AC output current, the current returns to zero every half cycle, the phase control creates a “no current flow” period, hence the heat is intermittent. An AC welding machine with Constant Current Controller has the function of primary voltage compensation and primary and secondary constant current feedback control which automatically adjusts power line fluctuations.

? Capacitor Discharge DC is one of the direct current series, which extracts energy from the power line over a period of time and stores it in welding capacitors. The stored energy is then rapidly discharged to the transformer producing a flow of electrical current for welding. Capacitor Discharged Welding Machines are best for applications with multiple-embossments, hot rolled sheet or thick steel sheet.

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