Highly conductive materials like aluminum require very high power to form quality spot welds. However aluminum alloys are routinely spot welded (see Table I for weldability). Here, cleanliness is much more of a concern than with low-carbon steels because of aluminum’s rapid surface oxidation characteristics. For optimum quality and weld performance, expensive cleaning procedures to remove surface oxide are required. For demanding applications, equipment to monitor surface resistivity from lot to lot is necessary to assure consistency of quality. This leads to a related consideration. If aluminum has been chosen for an important reason, such as lightweight or high strength-to-weight ratio, the added expense of ensuring a high-quality weld should be justified. If it has not, re-evaluation of the original material selection is in order or, perhaps, another assembling method should be considered.
Position the welding torch with the wire in the center of the hole contacting the back sheet of metal. It is important to arc against this back sheet rather than on the edge of the hole, otherwise the weld might not penetrate into the back sheet. The torch should ideally be pointing directly into the hole rather than at the angle in the photograph. Start welding in this position and don’t move the welder until the hole is almost full of weld. Then move the welder outwards in ever increasing circles until the weld is completed.
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. Other materials commonly spot welded include stainless steels (in particular austenitic and ferritic grades), nickel alloys and titanium. See more info on Auto Body 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.