Exothermic Welding or CAD Welding
Exothermic Welding or CAD Welding is used to join together the two sides of the rails, between which a gap is provided. The ends are placed in a refractory mold that isolates the metal from the external environment and sets the width and height of the welding joint. A hopper (crucible) with thermite powder is connected to the form.
The sides of the rails are pre-heated. Propane-oxygen or kerosene-oxygen flame is usually used. After that, the thermite (mix of filler metal and an oxidization agent) is set on fire at the top with a flame or fuse and covered with a lid. At the same time, the feed from the hopper from below to the docking zone is opened.
Liquid metal fills the mold and melts the edges completely. Welding of the sides takes place. Then wait until the product cools, and remove the covering shape. On the surface, irregularities, weld metal overflows are possible, so mechanical treatment (gas cutting & grinding) may be required.
Thermal welding is used to connect:
- Grounding circuits,
- Metal surfacing,
- Filling cracks.
CAD Welding or Thermit Welding is suitable for welding carbon steels and cast iron with a thickness of 10-15 cm.
Most often, with the help of thermite welding, railway tracks are repaired. The joints are made according to AWS D15.2 and ISO 23300.
In a miniature version, cables and wires are fused by this method. The technology allows you to connect metals of large cross-section in hard-to-reach places, saves time. But the seams are very rough and need grinding, so they are not suitable for the facade of the products.
The production process is characterized by the fact that its course does not require connection to the power supply network or the supply of specific gas compositions.
What is Thermite in CAD Welding
Various powder mixtures are used to combine metals: aluminum, iron oxide, magnesium. They are called thermites, because in the process of their combustion a large amount of thermal energy is formed.
The most popular is iron-aluminum thermite welding. The composition of the termite is dominated by iron oxide. There is also aluminum in the mixture, but its part is low and is 25%.
In addition to the main ingredients, there are additional ones – iron seed, flux, alloying additives and others. Thermite burns in the magnesite crucible, and here the metal melts. In addition to aluminum, the mixture may contain other components. So with iron oxide can be used:
- magnesium – 31%;
- silicon – 21%;
- calcium – 43%;
- titanium – 31%.
The combustion temperature of the metal powder reaches 2400-2700 degrees Celsius. This is enough to melt most of the metals.
It is important to pay attention to the fact that to maintain the combustion process does not require an influx of oxygen: it is sufficiently contained in the mixture itself. Therefore, thermite welding can be performed in an inert environment.
Welding cast iron is associated with the need to adhere to special requirements. Termite, designed to connect cast iron parts, must necessarily contain silicon.
Provided that all the requirements are met, a strong and durable connection is obtained. There is no manganese in cast iron welding additives.
Types of CAD Welding
There are two different methods of CAD Welding or Thermit welding –
- Crucible and
- Muffle welding.
Their use is determined by the working conditions and the materials to be connected.
Crucible welding is also called aluminothermic and is used in the assembly of various metal structures and grounding circuits.
Termite for this type of welding is prepared from aluminum powder and iron oxide. The ratio is 23:70 respectively. In the process of burning, iron is reduced from scale, the melt of which connects the blanks.
In addition, aluminothermic welding is very often chosen to repair metal surfaces by surfacing. The main advantage of this method is the possibility of connecting cast iron blanks without joints.
The formation of seams on aluminum elements using this method of connection is impossible. To connect them, it is better to resort to another type of welding.
For muffle welding, magnesium-based termite is taken, since aluminum evaporates under the influence of high temperature. Magnesium has a high melting point, so it does not spread on the surface.
As a result, a strong welded joint without a seam is formed. When aluminum billets are welded, a special flux is used to get rid of the oxide layer.
Methods of CAD or Thermit Welding
Thermit welding is performed in one of four ways:
- Butt Joint Welding: Thorough preparation is required beforehand. The surface of the workpieces is thoroughly cleaned. To avoid unwanted deformation of workpieces, the parts are wrapped with a special film. After the burning of the termite, a melt is obtained. From the crucible, it is poured into the butt space between the two blanks. The parts are tightly compressed together.
- Intermediate casting: The technique is used to connect parts fixed in a certain spatial position. This is the easiest connection option. Pre-welded in the crucible melt fills the space between the two blanks.
- Combined Welding: The method combines two types of welding. It is used to repair the railway track. The ends of the rails are pre-cleaned, after which the joint is formed using metal form plates. A closed cavity is formed, into which the melt is poured. The rails are compressed. After the melt hardens, the joint along the perimeter is boiled again.
- Duplex Welding: Classic crucible welding plus additional pressing of joints.
CAD Welding Equipment
Technologically, the process is not difficult. However, for the normal course of the reaction and obtaining the desired result, special tools and equipment are required:
- Crucible: It is made either of refractory metal or of ceramics. The tank must ensure the safety of the molten metal discharge.
- Molds will be required to create castings. They can be disposable or long-term use. So that the liquid metal does not spread, the surface of the mold is coated with a special clay for thermite welding.
- Vices, clamps and other similar devices for compressing parts of molds.
- Oxygen lance. It will be required in many welding or metal cutting operations.
- Abrasives for grinding the surface of the welding zone. It can be a grinder with a grinding wheel, sandpaper, a brush on metal.
- A set of auxiliary tools: strip type thermometer, metal hacksaw, gas burner, oxygen cutter and others.
Uses of CAD Welding
The technology allows you to combine all types of metals – non-ferrous and ferrous – without expensive equipment and complex tooling. Compared to other methods, CAD welding has significant advantages:
- Lower labor costs to perform a large amount of work.
- hHigh quality of welded joint, impeccable aesthetics of joints.
- Operations take little time to complete.
- The cost of consumables is low, and it is easy to purchase them in specialized stores.
- No electricity or gas is required for welding.
As a disadvantage, it demands high safety requirements. Thermite mixture is a flammable substance. Therefore, special storage and transportation conditions are required.
When performing work, the operator must be careful to prevent liquid from entering the burning thermite. If this happens, you will get an explosive mixture that can detonate at any time.
It is strictly forbidden to initiate the ignition of the charge from the electric arc, since an explosion can also occur. Welding work is allowed to be performed at an ambient temperature not lower than 10 degrees Celsius.
Application of CAD Welding
Most often, the CAD welding based on aluminum powder is resorted to in cases of surfacing on structural elements, if it is necessary to connect cast iron or other brittle alloys.
The method is ideal for repairing railway tracks: it is inexpensive, it is carried out quickly enough and provides a strong connection with a long service life.
Thermit welding is in demand in other areas of production. For example, in mechanical engineering, this is the most acceptable option in the manufacture of propellers for heavy-duty marine vessels.
Most crankshafts of road transport are made using this method. Industries related to metalworking, thermite welding is used in the repair of rolling mills, rotary shafts and a number of other large-sized equipment and tooling.
Another important area where this technology is used is energy supply and data transmission. In communication and transportation lines of electricity, large-diameter cables are often used.
To repair them and lay new lines, they resort to thermite welding. The consumables for such cases are produced by their magnesium.