Spot Welding

Specifications

What is the spot welding? 

• A form of resistance welding, spot welding is one of the oldest welding processes whereby two or more sheets of metal are welded together without the use of any filler material.
• The process involves applying pressure and heat to the weld area using shaped alloy copper electrodes which convey an electrical current through the weld pieces. The material melts, fusing the parts together at which point the current is turned off, pressure from the electrodes is maintained and the molten “nugget” solidifies to form the joint.
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• The welding heat is generated by the electric current, which is transferred to the workpiece through copper alloy electrodes. Copper is used for the electrodes as it has a high thermal conductivity and low electrical resistance compared to most other metals, ensuring that the heat is generated preferentially in the work pieces rather than the electrodes the amount of heat depends on the thermal conductivity and electrical resistance of the metal as well as the amount of time the current is applied.

Materials are Appropriate for Spot Welding, 

• Due to its lower thermal conductivity and higher electrical resistance, steel is comparatively easy to spot weld, with low-carbon steel being most suited to spot welding. However, high carbon content steels (Carbon equivalence > 0.4wt%) are prone to poor fracture toughness or cracking in the welds as they tend to form hard and brittle microstructures.
• Galvanized steel (zinc coated) requires slightly higher welding currents to weld than uncoated steel.  Also, with zinc alloys, the copper electrodes rapidly degrade the surface and lead to a loss of weld quality. When spot welding zinc coated steels, electrodes must either be frequently exchanged or the electrode tip surface should be ‘dressed’, where a cutter removes contaminated material to expose a clean copper surface and reshapes the electrode.
• Other materials commonly spot welded include stainless steels (in particular austenitic and ferritic grades), nickel alloys and titanium.
• Although aluminum has a thermal conductivity and electrical resistance close to that of copper, the melting point for aluminum is lower, which means welding is possible. However, due to its low resistance, very high levels of the current need to be used when welding aluminum (in the order of two to three times higher than for steel of equivalent thickness).
• In addition, aluminum degrades the surface of copper electrodes within a very small number of welds, meaning that stable high-quality welding is very hard to achieve. For this reason, only specialist applications of aluminum spot welding are currently found in the industry. Various new technology developments are emerging to help enable stable high-quality spot welding in aluminum.
• Copper and its alloys can also be joined by resistance spot welding, although spot welding copper cannot be easily achieved with conventional copper alloy spot welding electrodes, as heat generation in the electrodes and workpiece are very similar.
• The solution to welding copper is to use an electrode made of an alloy with high electrical resistance and a melting temperature far in excess of the melting point of copper (much greater than 1080°C). Electrode materials typically used for spot welding copper include molybdenum and tungsten.