Aluminothemic welding or thermit welding principles

Principles And Working Of Aluminothemic Welding Aluminothemic welding is also known as Thermit welding.

This is a process which uses the heat from exothermic chemical reactions of solid, particulate materials to yield melting and joining, which is known as coalescence, between metals.

Often, the reactants used are oxides with lower heats of formation and metal reducing agents, which on oxidization gains high heats of formation, but a mix of two metals or a metal and a non-metal which will react exothermically to yield a compound with high heat of formation can be used as well.

The excess heat of formation of the reacting products, in either case, gives the energy to weld. For instance, if finely divided aluminium and metal oxides of, say, (Fe) iron or (Cu) copper are blended and ignited by external heat source, the aluminothermic reaction will proceed according to a given general reaction.

General Reactions and Principles Metal oxide + aluminum = aluminum oxide + metal + heat The reaction is exothermic and the heat yielded results in the metal which is formed as a reaction product being liquid. The most common Thermit reactions used to produce welds are – Thermit welding reactions By allowing the reaction to occur such that the molten metal product can fill a joint, a weld can be created. In Thermit welding the reaction is made to occur in a vessel located above the mould around the abutted joint elements. Once the reaction occurs, the molten metal product, which is denser than the solid AI, O, product, pours down into the mould under the influence of gravity and casts into the joint to create a weld. To allow the reaction proceed, mainly for large volumes of reactants and large welds to be created, the mould is preheated. The joining of steel railroad rails and copper electric

cables or buss bars to terminal connectors are widely practiced applications of the process.

Aluminotherrnlc Welding or Thermit Welding Principle Though the theoretical maximum temperature which results from such reactions can be calculated from reaction thermodynamics, the actual maximum temperature attained is less accurate because the reaction never occurs adiabatically.

In case of most common reactions maximum theoretical temperature is about 3200°C (5800°F). Though the maximum temperature in reality ranges from 2200°C (4000°F) to 2400°C (4350°F) owing to various losses, there is enough superheat in the molten metal yield to melt the surfaces of the joint elements, hence yielding a real weld.

Recently due to the result of Merzhanov et al. work in Russia, several exothermic reactions are studied and used to achieve surface welding by allowing reaction to occur in a reactant packed on the surface, and covering, and allowing the reaction to occur in a reactant pressed between layers. Reactions to yield refractory carbides, oxides,nitrides, carbonitrides, silicides, and other non-oxide ceramics and intermetallics also were studied and offered potential to join ceramics with each another and to metals. The former processes are classified as exothermic welding processes, while the latter as exothermic brazing processes, the differences being if any melting of the substrate(s) takes place, it must be considered as welding.

They have given alternative names for these processes, because of the propagation and simultaneous ways in which the processes occur: self-propagation via high-temperature synthesis and combustion synthesis.

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