Spot welding is a type of electric resistance weld that uses resistance to weld two or more metal sheets together using pressure and heat to the weld part.
The spot welding process uses two copper alloy electrodes to focus the welding current into a small area and hold the sheets together. This process involves pressure and an electric current which creates heat through the channel of current through resistive materials like low carbon steels.
How Does Spot Welding Work?
Spot welding is a type of resistance weld and is one of the oldest welding processes that weld two or more sheets of metal together without any filler materials.
Spot welding works by applying pressure and heat to the weld area using copper alloy electrodes. These electrodes transport an electrical current through the weld segments. As the material melts the parts are fused. At this point, the current is turned off and pressure from the electrodes is upheld. This enables the molten “nugget” to solidify and create the joint.
The electric current uses the copper alloy electrodes to transfer heat to the work piece creating the weld. The reason copper is used for the electrodes is that it can maintain a high thermal conductivity and has a low electrical resistance compared to most other metals. This guarantees that the heat is created in the work pieces rather than the electrodes.
The degree of heat varies based on three factors:
- thermal conductivity
- the electrical resistance of the metal
- amount of time the current is applied
The formula used to express heat:
Q=I2Rt
Q = heat energy
I = current
R = electrical resistance
t = time for which the current is applied
Materials Appropriate for Spot Welding
Spot welding involves lower thermal conductivity and higher electrical resistance. Due to this, steel is an easy material to spot weld and low carbon steel is the best material for spot welding. On the other hand, high carbon steels are not ideal materials to spot weld because they often fracture or crack in the welds and hard or brittle microstructures form.
Galvanized steel which is zinc coated, requires higher welding currents to weld than uncoated steels. Unfortunately, with zinc alloys, the copper electrodes quickly damage the surface and result in poor weld quality. Spot welding zinc-coated steels require the electrodes to be exchanged often or the electrode surface needs to be ‘dressed.’ The term dressed in this context means that the cutter gets rid of the contaminated material to uncover a clean copper surface and reshapes the electrode.
Other material commonly used in spot welding:
- Stainless steel (austenitic and ferritic grades)
- Nickel alloys
- Titanium
The melting point for aluminum is low even though aluminum has thermal conductivity and electrical resistance that is similar to copper. This low welding point makes the welding possible. Since aluminum has a low resistance it takes extremely high levels of current to weld the material. For example, aluminum takes two to three times higher current than steel that has the same thickness.
After only a couple of welds, aluminum damages the surface of copper electrodes. This means that stable high-quality welding is extremely difficult to achieve. As a result of this difficulty, only specialist applications of aluminum are found in the trade. Fortunately, new technological developments are evolving to help permit stable high-quality spot welding in aluminum.
Another material that is able to be joined by resistance spot welding is copper and its alloys. However, spot welding copper cannot be simply accomplished with the traditional copper-alloy spot welding electrode because the heat created in the electrodes and the work piece are way too alike.
The best way to weld copper is to use an electrode made of an alloy with excessive electrical resistance and a melting temperature that is way above the melting point of copper. For example, much higher than 1,976 degrees Fahrenheit.
Electrode materials commonly used to spot weld copper:
- Molybdenum
- Tungsten
Where is Spot Welding Used?
Industries that spot welding is used:
- Automotive
- Aerospace
- Building and construction
- Medical building and construction
- Metal furniture
- Rail
- White goods
- Electronics
Spot welding can be easily automated when used with robots and manipulations systems. This makes it ideal for the joining process in high volume manufacturing lines. Over the past 100 years, spot welding has been the foremost joining process in the construction of steel cars.