Resistance Welding Technology

Bulk & Contact Resistance Values

Resistance Welding is a thermode-electric process in which heat is generated at the interface of the parts to be joined by passing an electrical current through the parts for a precisely controlled time and under a controlled pressure (also called force). The name "resistance welding" derives from the fact that the resistance of the workpieces and electrodes are used in combination or contrast to generate the heat at their interface.

Key-advantages of the resistance welding process include:

• Very short processtime
• No consumables, such as brazing materials, solder or welding rods
• Operator safety because of low voltage
• Clean and environmentally friendly
• A reliable electro-mechanical joint is formed

Resistance Welding is a fairly simple heat generation process: the passage of current through a resistance generates heat. This is the same principle used in the operation of heating coils. In addition to the bulk resistances, the contact resistances also play a mayor role. The contact resistances are influenced by the surface condition (surface roughness, cleanliness, oxidation and platings).

The general heat generation formula for Resistance Welding is:

Heat = I² x R x t x K

Where "I" is the weld current through the workpieces, "R" is the electrical resistance (in ohms) of the workpieces, "t" is the weld time (in hertz, milliseconds or microseconds), and "K" is a thermal constant. The weld current (I) and duration of current (t) are controlled by the resistance welding power supply. The resistance of the workpieces (R) is a function of the weld force and the materials used. The thermal constant "K" can be affected by part geometry, fixturing and weld force.

The bulk and contact resistance values of the workpieces, electrodes and their interfaces both cause and affect the amount of heat generated. The diagram at the right demonstrates three contact and four bulk resistance values, which, combined, help determine the heat generated.