Thermite flux (also called exothermic welding) is a method of combining metals by heating the superheated molten metal produced by the exothermic reaction between metal oxides and metal aluminum.

Exothermic welding was started in the late 19th century, when H. Goldschmidt found that the exothermic reaction between aluminum powder and metal oxide could be initiated by an external heat source, and once the reaction could be self-sustainable. The general formula of this reaction is: metal oxide + aluminum (powder) → alumina + metal + heat

Exothermic fusion welding can mainly weld pure copper, brass, bronze, copper, copper clad steel, pure iron, stainless steel, wrought iron, galvanized steel, cast iron, etc.