Explosion welding

Explosion Welding (EXW) is a solid state process where welding is accomplished by accelerating one of the components at extremely high velocity through the use of chemical explosives. This process is most commonly utilized to clad carbon steel plate with a thin layer of corrosion resistant material (e.g., stainless steel, nickel alloy, titanium, or zirconium). Explosion welding can produce a bond between two metals that cannot necessarily be welded by conventional means. The process does not melt either metal, instead it plasticizes the surfaces of both metals, causing them to come into intimate contact sufficient to create a weld. This is a similar principle to other non-fusion welding techniques, a group that includes friction welding and inertial welding. Large areas can be bonded extremely quickly and the weld itself is very clean, because the surface material of both metals is violently expelled during the reaction. Explosion Welding can also be referred to as explosive welding, explosive bonding, or explosive cladding.

Development

Unlike other forms of welding such as arc welding (which was developed in the early 1800s), explosion welding was developed relatively recently, in the decades after World War II. Its origins, however, go back to World War I, when it was observed that pieces of shrapnel sticking to armor plating were not only embedding themselves, but were actually being welded to the metal. Since the extreme heat involved in other forms of welding did not play a role, it was concluded that the phenomenon was caused by the explosive forces acting on the shrapnel. These results were later duplicated in laboratory tests and, not long afterwards, the process was patented and put to use.

In 1962, DuPont applied for a patent on the explosion welding process, which was granted in 1964 and resulted in the use of the "Detaclad" trademark to describe the process. On July 22, 1996, Dynamic Materials Corporation completed the acquisition of DuPont's Detaclad operations for a purchase price of $5,321,850.

The Process

In explosion welding, explosives are detonated over a metal plate, called a clad plate or flyer plate. The explosion then accelerates the clad plate through a thin gap between the metals and into the base plate. The contact forces between the two plates are extremely high and as the flyer plate is propelled into the base plate, the kinetic energy fuses the two together. The kinetic energy of the flyer plate striking the base plate produces a wavy interface at the bondline. This intense impact interlocks the surfaces and pressure welds them by plastic deformation. The flyer plate is positioned at an angle relative to the base plate so that any oxide films are expelled from the interface. Consequently, the resulting weld has a very high bond strength. Due to the nature of this process, producible geometries are very limited. They must be simple. Typical geometries produced include plates and tubing.

Explosives

The explosives used in explosion welding can vary. In form, they range from a flexible plastic sheet or cord to granulated or liquid structure. Different explosives will yield different detonation velocities. Some commonly used explosives are:

* RDX
* PETN
* TNT
* Tetryl
* Lead azide
* Detasheet
* Ammonium nitrate

The detonation speed is usually ranges from 2400 to 3600 m/s and is dependent upon the type, thickness, and packing density of the explosive. A standard commercial blasting cap is used to detonate the explosives.

Applications

Applications for explosive welding are varied. A few examples of explosion welding applications include:

* Joining of dissimilar metals (e.g., Aluminium to steel, Titanium alloys to Cr – Ni steel, Cu to stainless steel, Tungsten to Steel, etc.)
* Remote joining in hazardous environments.
* Joining pipes and tubes.
* Heat exchanger tube sheets and pressure vessels.
* Tube Plugging.
* Attaching cooling fins.

Other applications are in chemical process vessels, ship building industry, cryogenic industry, etc.

Advantages of Explosion Welding

* Many dissimilar, normally unweldable metals are capable of being bonded together.
* Extremely large surfaces can be bonded.
* A wide range of thicknesses can be explosively clad together.
* There is no effect on parent properties.
* High bond strength is attained through explosion welding.

Disadvantages of Explosion Welding

* The metals must have high enough impact resistance, and ductility.
* The noise and blast of the process can require special precautions to be taken. The blast zone is sometimes buried underground or situated in another such remote location.
* The use of explosives in industrial areas will be restricted by the noise and ground vibrations caused by the explosion.
* The geometries welded must be simple (e.g., flat, cylindrical, or conical).
* Expansive knowledge of explosives is needed before the procedure may be attempted. Explosion welding is therefore far less commonly used than fusion welding alternatives.

References

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