Exothermic welding is also known as exothermic bonding, thermite welding, or thermit welding. It was developed around 1895 by Hans Goldschmidt. The process was refined by Dr. Charles Cadwell, who performed the first non-ferrous application. Exothermic welding was first used to weld signal bonds to railroad tracks.
Exothermic welding is often used to join copper conductors. The welding process joins two electrical conductors by use of superheated copper alloy. An exothermic reaction requires no external heat or current source. The reaction occurs between a copper thermite composition that heats the copper. The chemical reaction is aluminothermic, occurring between aluminum powder and copper oxides. The reaction reaches very high temperatures. Reactant powders and a spark is all that is needed for a high energy reaction.
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How it Works
Molten copper produced by an exothermic reaction flows through a semi-permanent graphite crucible mold. The mold goes over and around conductors to be joined and the weld is electrically conductive. The mold can be broken off or left in place when the copper cools. Handheld graphite crucibles are portable, reusable, and flexible for field applications.
A common formula for the reaction is five parts iron oxide powder to three parts aluminum powder, by weight. The combination is ignited at high temperatures. The heat-generating reaction produces a white hot mass of molten iron and a slag of refractory aluminum oxide (through reduction and oxidation). The molten iron is the welding material. The aluminum oxide floats to the top of the reaction. When working, you must remember that the actual molten metal is below, at the bottom of the crucible and covered by floating slag.
When Exothermic Welding is Used
Exothermic welding is most commonly used for permanently joining copper conductors. It is the only acceptable means of bonding copper to galvanized cable. It can also be used on stainless steel, cast iron, steel, brass, bronze, and more. If joining two dissimilar metals, exothermic welding is a good option. You'll find thermite welding under the names of Ultraweld, Cadweld, Techweld, and Thermoweld.
Thermite welding is a common method for joining railway rails, though it is not chemically pure thermite. For sound welds the ends of the rails are preheated with a torch. When they reach orange heat, the molten steel will not be chilled during the pour. Without preheating, the low heat penetration into the joining metals would not create the quality weld needed. Also, because exothermic bonding yields mostly iron (not stronger steel) pellets or rods of high-carbon alloying metal are added to the thermite mix. The added materials melt from the heat and mix into the weld metal.
Advantages and Disadvantages
Compared to other forms of welding, exothermic bonds have a higher mechanical strength. The weld is also resistant to corrosion. You will not see an increase in electrical resistance despite repeated short circuit pulses; the weld is highly stable. The process is more costly, however. The process requires replaceable molds, is not easily repeatable, and can be hindered by poor weather conditions when the work is done outdoors.
Exothermic welding can also be done remotely. This reduces risks to operators. An igniter is used with standard graphite molds or a consumable sealed drop in weld metal cartridge, semi-permanent graphite crucible mold, and an ignition source that connects the cartridge with a cable.