In applications where space is limited, specifying one weld can produce a stronger bond than two spots, which may be limited in size and integrity because of contraints in positioning, accessibility and shunting effects (current loss). Typically, diameters of spot welds range from about 1/8 in. (3 mm) to l/2 in. (13 mm) depending on the thicknesses of the workpieces and the material. When the size of spot welds is designated, the designer should specify only one size throughout an assembly in the interest of manufacturing economy and total part cost.
Plug welding is an alternative to spot welding used by vehicle manufacturers where there is insufficient access for a spot welder. For DIY car restoration it’s generally used instead of spot welding on panels flanges that would have originally been spot welded. Plug welds when done properly tend to be stronger than the original spot welds. Rally car builders often use the technique, and it is acceptable in a UK MOT test as an alternative to spot welds where repairing older cars (it would not be suitable for modern high tensile steels).
Materials 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. Galvanised steel (zinc coated) requires slightly higher welding currents to weld than uncoated steels. 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. See more details at Spot Welder.
Spot welding machines represent a significant long-term investment that can decline in value. Their high performance has led to widespread use in automotive manufacturing; their use can be automated by combining them with extremely fast, precise robots. Radiators can be welded by the same process except using rollers to obtain a continuous, watertight weld. Industrially made furniture and other functional metal objects such as lampshade frames are also spot-welded. The process gives an impressive quality of weld without deforming the components it is used on.
