Resistance welding is a process used to join metallic parts with electric current. There are several forms of resistance welding, including spot welding, seam selding, projection welding, and butt welding.

In all forms of resistance welding, the parts are locally heated until a molten pool forms. The parts are then allowed to cool, and the pool freezes to form a weld nugget. On a typical machine, the operator has control over the current setting, electrode force and weld time.

To create heat, copper electrodes pass an electric current through the work pieces. The heat generated depends on the electrical resistance and thermal conductivity of the metal, and the time that the current is applied. The heat generated is expressed by the equation

E = I²·R·t

where E is the heat energy, I is the current, R is the electrical resistance and t is the time that the current is applied.

Copper is used for electrodes because it has a low resistance and high thermal conductivity compared to most metals. This ensures that the heat is generated in the work pieces instead of the electrodes. When the electrodes get too hot, heat marks on the surface of the work pieces can form. The electrodes also become susceptible to "mushrooming". Electrode mushrooming reduces their usable lifetime. To prevent these problems, the electrodes are cooled with water. The water flows inside a cavity in the electrodes, removing excess heat.

The electrodes are held under a controlled force during welding. The amount of force affects the resistance across the interfaces between the work pieces and the electrodes. The force is adjusted to immediately create heat at the interface between the work pieces. The force also refines the grain structure of the weld. If the force is too low expulsion, or weld splash, can occur.

The heat needed to produce a molten pool depends on the thermal conductivity and melting point of the metal being welded. A material with a high thermal conductivity will quickly conduct heat away from the weld pool, increasing the total heat needed to melt the pool. A low melting point will lower the heat needed. The table below shows material properties for low carbon steel, aluminum, zinc (present on galvanized steel), and copper.

Steel has a higher electrical resistivity and lower thermal conductivity than the copper electrodes, making welding relatively easy. The copper electrodes must be well cooled, because the melting point of steel is significantly higher than that of copper.

Aluminum has an electrical resistivity and thermal conductivity that is closer to that of copper. However, aluminum's melting point is much lower than that of copper, making welding possible. Higher levels of current must be used for welding aluminum because of its low resistivity.

Galvinized steel, steel coated with zinc to prevent corrosion, requires a different welding approach than uncoated steel. The zinc coating must first be melted off before the steel is joined. Zinc has a low melting point, so a pulse of current before welding will accomplish this. During the weld, the zinc can combine with the steel and lower its resistivity. Therefore higher levels of current are requred to weld galvanized steel.

Resistance welding can be used to join a wide range of similar and dissimilar metals. For information on a particular combination of materials see the PDF document Weldability and Electrode Selection (from Lors Machinery). You will need the Adobe Acrobat Reader to view this document.