¹⁾Exothermic welding, also known as exothermic bonding, is a welding process for joining two electrical conductors, that employs superheated copper alloy to permanently join the conductors. The process employs an exothermic reaction of a copper thermite composition to heat the copper, and requires no external source of heat or current. The chemical reaction that produces the heat is an aluminothermic reaction between aluminium powder and a mixture of copper oxides (copper(II) oxide and copper(I) oxide), with chemical formula:[1]

  3CuO + 2Al → 3Cu + Al2O3 + Heat.[1]

This chemical reaction reaches a temperature of 1,400 °C (1,670 K). The reactants are usually supplied in the form of powders, with the reaction triggered using a spark from a flint lighter. The activation energy for this reaction is very high however, and initiation requires either the use of a “booster” material such as powdered magnesium metal or a very hot flame source. The aluminium oxide slag that it produces is discarded.[2][3]

The process employs a semi-permanent graphite crucible mould, in which the molten copper, produced by the reaction, flows through the mould and over and around the conductors to be welded, forming an electrically conductive weld between them.[1] When the copper cools, the mould is either broken off or left in place.[2] Alternatively, hand-held graphite crucibles can be used. The advantages of these crucibles include portability, lower cost (because they can be reused), and flexibility, especially in field applications.

The weld formed has higher mechanical strength than other forms of weld, and excellent corrosion resistance. It is also highly stable when subject to repeated short-circuit pulses, and does not suffer from increased electrical resistance over the lifetime of the installation. However, the process is costly relative to other welding processes, requires a supply of replaceable moulds, suffers from a lack of repeatability, and can be impeded by wet conditions or bad weather (when performed outdoors).[1][2]

Exothermic welding is usually used for welding copper conductors but is suitable for welding a wide range of metals, including stainless steel, cast iron, common steel, brass, bronze, and Monel.[2] It is especially useful for joining dissimilar metals.[3] The process is marketed under a variety of names such as Cadweld, Techweld, and Thermoweld.[2]

The first non-ferrous application for exothermic welding was developed in 1938 by Dr. Charles Cadwell, a professor at the Case School of Applied Science (now Case Western Reserve University), in Cleveland, Ohio. The original use of the process was to weld signal bonds to railroad tracks.

[1] Milenko Braunović, Valeriĭ Vasilʹevich Konchits, and Nikolaĭ Konstantinovich Myshkin (2006). Electrical Contacts: Fundamentals, Applications and Technology. CRC Press. pp. 291. ISBN 1574447270.

[2] John Crisp (2002). Introduction to copper cabling. Newnes. pp. 88. ISBN 0750655550.

[3] Jerry C. Whitaker (2005). The electronics handbook (2nd ed.). CRC Press. pp. 1199. ISBN 0849318890.