Welding by exothermic chemical reaction was devised in the late 1890s but combining it with centrifugal force to clad cylindrical components, for example, is relatively new.
It should be stressed that the process demands stringent safety precautions because violent, explosive reactions can occur.
Exothermic reaction is achieved through reduction of metal oxides (e.g. iron oxide) by reaction with aluminium powder. When ignited, the powder reactants produce the necessary welding heat within a few seconds. The superheated molten metal produced by the reactions is so hot (2500-2960°C for steel) that the surface layers of the substrate melt. These layers, together with the liquid weld metal, coalesce to form a welded joint or clad layer.
When centrifugal cladding of the bore of a tubular component, the component is rotated at speed and the powdered exothermic charge held in contact with the bore by centrifugal force. The charge is then ignited to initiate exothermic reaction and produce a uniformly clad surface with predetermined properties.
Potentially, almost any composition of steel can be obtained by introducing the required alloying elements into the exothermic charge. Copper and nickel alloys can be produced through selection of the appropriate oxide for reduction during the process.
Copyright by TWI, 1999
