In order to understand how exothermic welding works, it is important to first understand what an exothermic reaction is…
What is an Exothermic Reaction?
The term, exothermic, comes from exo, which is Greek for outside, and thermic, which means heat, and relates to a chemical reaction that produces heat. All combustion reactions are exothermic, including fire and explosions. However, there are less obvious exothermic reactions, such as that between iron and oxygen that creates rust (iron oxide). This reaction releases heat, making it exothermic, but the process is so slow that the temperature difference is barely noticeable.
By contrast, aluminium and iron (III) oxide can be used to produce molten iron in a highly exothermic reaction that reaches temperatures of around 2000°C. This high heat is over the melting point of iron (1535 °C), creating practical uses such as for joining railway lines together. This exothermic reaction (iron(III) oxide + aluminium → aluminium oxide + iron) shows that iron is below aluminium in the reactivity series.
Some exothermic reactions require an amount of energy to be applied, called the energy of activation, to get them started, such as using a flame to make wood burn. CAD welding uses a heating torch to deliver the energy of activation to begin the welding process, while exothermic connections between railway lines can be started with a flint spark and a booster powder to increase the heat generated.
How Exothermic Welding / CAD Welding Works
CAD welding uses a clampable crucible mould to hold the weld pieces in place. Once clamped together a sealing compound is placed at the entry point of the metal pieces to prevent any weld metal from escaping during the welding process.
The thermite, which is a mixture of filler metal and the oxidisation agent, is then inserted into the mould. Heat is applied to start the chemical reaction, such as by a heating torch, and the welding begins. The exothermic reaction causes the molten metal to flow into the weld cavity and form a joint around the two pieces of metal. Once the weld is finished the clamp is released and, unless it is discarded, the mould will need to be cleaned to remove leftover slag before using it again.
The process is different when using exothermic welding for railway rails, which has been in operation since at least 1935 in the United States, where it was used on the Delaware and Hudson Railroad.
Firstly, the end of the rails to be joined are preheated until they are orange to make sure the molten steel is not chilled as it flows between the rail edges. The thermite mix will include pellets or rods of a high carbon alloying metal, like steel, which melts into the weld metal during the thermite reaction. The reactants are usually supplied as a powder that are triggered with a spark from a flint lighter. Because rail welding requires a high activation energy, a booster powder such as magnesium or a very hot flame source are also used.
The exothermic process can create a lot of smoke and even explosions, so protective equipment is required when CAD welding.
Exothermically welded connections are more effective than mechanical connections, particularly when joining conductors, as they create a strong, permanent connection between the conductors.
The advantages of exothermic welding include:
- Creating a long lasting connection that will not deform or weaken
- An easy to learn process that can be visually evaluated
- Creates a molecular weld unaffected by high current surges, allowing conductors to endure recurring fault currents without losing operation
- Welds can offer holding power and current carrying capacities at least as high as the conductors themselves
- Able to weld materials such as copper, brass, bronze, stainless steel and steel rail to produce a connection with an alloy without dielectric influence at the weld point that is also impervious to corrosion
Despite the advantages, there are still some difficulties associated with exothermic welding:
- Can be hazardous, depending on the method and application
- Unsuited to certain environmental conditions
Because of the advantages associated with CAD welding and electrical connectors, this CAD welding is used for a wide range of electrical protection systems in order to help keep the connections stable and grounded. Exothermic connections can avoid increased electrical resistance, minimising oxidation and removing the problem of vibration-related weakening in the connections, leading them to be used in a variety of industries.
Exothermic welding systems are widely used for electronics and electrical wiring, including everything from earthing systems for sub-stations and power plants to telecommunications devices. Exothermic welding is also used for cathodic protection systems and lightning protection.
Of course, outside of these applications, exothermic welding is also widely used to join railroad rails. These can be difficult to weld using other methods due to the thickness of the material to be joined and the remote locations where these welds often need to be performed.
Different materials are exothermically joined for different applications, including:
Copper wire is found in power distribution equipment as well as other applications. CAD welding is ideal for joining copper as it neither damages the copper or lowers conductivity, while also producing copper that is unmatched for purity by other processes. This purity prevents the conductivity from weakening. As a result, exothermic welding is widely used to make high quality electrical connections between copper and copper, as well as welding copper alloys, such as brass and bronze, or copper to steel.
Wires for electronic applications can also be joined using exothermic brazing, which is similar to exothermic welding. Both processes use filler metals and a metal oxide to cause a reaction, but brazing only applies the heat input for a short time so as to not damage or distort the sensitive electronics. Exothermic brazing for electronics uses various alloyed metals and solder filler materials.
The United States National Electrical Code determined that exothermic welding was the only acceptable method for joining copper to galvanised cable. The zinc coating on galvanised materials means that they do not have as good conductivity as bare metals. This galvanisation is usually ground away before welding takes place and the coating can also introduce porosity and other defects to the weld. These problems are avoided by using exothermic welding.
What is the Purpose of Exothermic Welding?
Exothermic welding is frequently used to join together conductors for earthing (grounding) or lightning protection. The technique can also be used for joining dissimilar metals and is commonly used to join rails for railways. The thick rails on railroad tracks would take a long time to weld with other processes and they would also require more joint preparation to ensure that the weld penetrated through the rail thickness properly. Exothermic welding does not require as much joint preparation and the welding process is also much faster than with other processes. Flash butt welding can be used as an alternative method for joining rails but, due to the requirement for electricity, coupled with the remote locations where the welding is often required, exothermic welding is generally preferred.
What does Exothermic Welding Mean?
Exothermic welding, also known as thermite welding or CAD welding, is a fusion welding process where the source of the energy is from a chemical reaction. The heat generated by the chemical reaction, which may need a flame or spark to begin, melts the metal powders so that the molten metal flows to the weld area within a crucible to join the materials together. Exothermic comes from the Greek for outside (‘Exo’), and heat (‘Thermic’).
Who Invented Exothermic Welding?
Hans Goldschmidt, who was working on using the thermite reaction to produce high purity chromium and manganese, first developed modern thermite welding for rails in the mid-1890s. The process was first used on a rail line in Essen, Germany I 1899, with exothermically welded rails quickly gaining popularity due to the greater reliability of the welds. As the process was picked up for railways from Dresden to Leeds and Singapore, Goldschmidt established the New York-based Goldschmidt Thermit Company in 1904, which went on to extend the practice of thermite welding to the railways of North America.
What is the temperature of thermite welding?
Thermite welding reaches temperatures of around 2,200°C (about 4,000°F). This is less than many arc welding processes, which are typically performed at around 6,100°C (about 11,000°F). However, this heat is still sufficient to form a strong bond.
CAD welding and exothermic welding are the same thing. CAD welding is simply a brand name that is commonly used for the process (like ‘Hoover’ and vacuum cleaning).
Exothermic welding is a fusion process that is used to create a molecular connection between two metal parts that are held in a mould without the use of an external heat source (other than to start the exothermic reaction).
Once cooled, an exothermically welded joint should be homogenous, with the two materials fully fused together.