Spot welding is a process that uses electrical current to join two metal pieces together. It creates a strong resistance between the two pieces of metal, which causes heat to be generated and the metals to fuse into one piece. Spot welding is used in many industries to manufacture cars, trains, planes, appliances and other machinery.
What is Spot Welding?
Spot welding (also known as resistance spot welding) is a resistance welding method. This welding process combines two or more metal sheets by applying pressure and heat to the weld area via an electric current.
It operates by applying pressure and electric current to the sheet surfaces, and heat is generated by the passage of current through resistive materials such as low-carbon steels.
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How Does Spot Welding Work?
One of the earliest methods of joining two or more sheets of metal together is spot welding, a type of resistance welding in which no filler material is used.
Pressure and heat are applied to the weld area using shaped alloy copper electrodes that conduct an electrical current through the weld pieces. The material melts, fusing the parts, after which the current is turned off, the electrode pressure is maintained, and the molten “nugget” solidifies to form the joint.
The electric current generates the welding heat, which is then transferred to the workpiece via copper alloy electrodes. Copper is used for the electrodes because it has higher thermal conductivity and lower electrical resistance than most other metals, ensuring that heat is generated in the workpieces rather than the electrodes.
Materials Appropriate for Spot Welding
Steel is relatively easy to spot weld due to its lower thermal conductivity and higher electrical resistance, with low-carbon steel being best suited. However, high carbon content steels (Carbon equivalence > 0.4wt%) are prone to poor fracture toughness or cracking in welds due to the formation of hard and brittle microstructures.
Galvanized steel (zinc-coated steel) requires slightly higher welding currents than uncoated steel. In addition, with zinc alloys, the copper electrodes rapidly degrade the surface, resulting in a loss of weld quality. When spot welding zinc-coated steels, electrodes must be changed frequently, or the electrode tip surface must be ‘dressed,’ which involves removing contaminated material to expose a clean copper surface and reshaping the electrode.
Although aluminium has similar thermal conductivity and electrical resistance to copper, its melting point is lower, making welding possible.
However, due to its low resistance, very high current levels are required when welding aluminium (two to three times higher than for steel of equivalent thickness).
Furthermore, aluminium degrades the surface of copper electrodes in aery sfewwelds, making stable, high-quality welding difficult to achieve. As a result, only specialized applications of aluminium spot welding are currently available in the industry. Several new technological developments are emerging to aid in producing stable, high-quality spot welding in aluminium.
Copper and its alloys can also be joined by resistance spot welding. However, spot welding copper is difficult to achieve with conventional copper alloy spot welding electrodes due to similar heat generation in the electrodes and workpiece.
To weld copper, use an electrode made of an alloy with high electrical resistance and a melting temperature far above the melting point of copper (much higher than 1080°C). Molybdenum and tungsten are common electrode materials used for spot welding copper.
Where Is Spot Welding Used?
Spot welding is used in various industries, such as automotive, aerospace, rail, white goods, metal furniture, electronics, medical building, and construction.
Spot welding is the most common joining process in high-volume manufacturing lines. It has been the main joining process in the construction of steel cars for over 100 years due to the ease with which it can be automated when combined with robots and manipulation systems.