Aluminothemic Welding or Thermit Welding Principles

Aluminotherrnic welding is commonly known as Thermit welding. As a subset, these processes use the heat from highly exothermic chemical reactions of solid, particulate materials (or, occasionally, solid particles and a gas) to produce melting and joining, also called coalescence, between metals. Most often, the reactants employed are oxides with low heats of formation and metallic reducing agents, which when oxidized have high heats of formation, but combinations of two metals or a metal and a non-metal (e.g., H, C, 0, N, B, Si, S, or Se) that will react exothermically to produce a compound with a high heat of formation can also be used. The excess heat of formation of the reaction products, in either case, provides the energy to produce the weld. As an example, if finely divided aluminum and metal oxides of, say, iron or copper are blended and ignited by means of an external heat source, the aluminothermic reaction (after which the entire group is named) will proceed according to the following general reaction:

Aluminotherrnlc Welding or Thermit Welding General Reactions and Principles

Metal oxide + aluminum = aluminum oxide + metal + heat

The reaction is so exothermic that the heat liberated results in the metal formed as a reaction product being liquid. The most common Thermit reactions used to produce welds are –

By causing the reaction to take place such that this molten metal product can reach and fill a joint, a weld can be made. In Thermit welding as it is usually practiced, the reaction is made to take place in a vessel located above a mold around the aligned and abutted joint elements. Once the reaction takes place, the molten metal product, being denser than the solid AI,O, product, pours down into the mold under the influence of gravity and casts into the joint to create a weld. To help the reaction proceed, especially for large volumes of reactant and large welds to be made, the mold is often preheated. A typical arrangement for Thermit welding is shown schematically in Figure where concrete reinforcing steel bar is being welded in either a horizontal or vertical orientation. This and the joining of steel railroad rails and heavy copper electric cables or buss bars to terminal connectors are common applications of this process.

Aluminotherrnlc Welding or Thermit Welding Principle

Maximum Temperature of Thermit Reactions

While the theoretical maximum temperature that results from such reactions can be calculated from the reaction thermodynamics, the actual maximum temperature achieved is less precise because the reaction does not take place adiabatically. In the case of most common reaction maximum theoretical temperature is approximately 3200°C (5800°F). Even though the actual maximum temperature probably ranges between 2200°C (4000°F) and 2400°C (4350°F) due to various losses, there is more than enough superheat in the molten metal product to cause melting of the surfaces of the abutting joint elements, thereby producing a real weld.

More recently, as the result of work by Merzhanov et al. (1972) in Russia, a host of exothermic reactions have been studied and used to accomplish surface welding or overlaying by causing the reaction to take place in reactant packed on the surface, and cladding by causing the reaction to take place in reactant sandwiched between layers. Reactions to produce refractory oxides, carbides, nitrides, carbonitrides, borides, silicides, and other non-oxide ceramics as well as intermetallics (e.g., aluminides) have been studied (Hlavacek, 1991) and offer potential to join ceramics to one another and to metals. The former processes are generically classified by the AWS as exothermic welding processes, while the latter are classified as exothermic brazing processes, the difference being whether any melting of the substrate(s) occurs, as it must to be considered welding. Alternative names for these processes, because of the propagating and simultaneous modes in which the process can take place are: self-propagating high-temperature synthesis (SHS) and combustion synthesis (CS)

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Mechanical Engineering