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Explosive welding-History, Definition, and principle

Explosive welding

Explosive welding belongs to the solid phase (state) welding process in which coalescence is affected by high-velocity agitation produced by a controlled explosion.

History of explosive welding

The origin of the explosive welding process dates back to the First World War when it was observed that fragments of steel balls of bombs sometimes stuck to metal objects around the explosion. This incident was taken as an example of explosive welding.

Possibly the first utility of the explosive welding process was publicly recognized in the United States in 1957 by Flip Chuck. Since then, considerable work has been done in many countries, notably the United States, the USSR, West Germany, Czechoslovakia, Japan, the UK, etc.

Definition

Basically, explosive welding involves a high velocity (oblique) impact between(flyer) a plate propelled by an explosive charge and a (second)stationary rate when two plates are to be explosively welded.

Explosion welding (EXW) is a solid state (solid-phase) process where welding is accomplished by accelerating one of the components at extremely high velocity through the use of chemical explosives.

Wikipedia

Mechanism of explosive welding

As shown in the figure, a parallel arrangement
is adopted with an explosive whose detonation velocity is less than the sonic velocity in the base metal and flyer plate material.

The flyer plate has to be joined with the base plate. there is one Buffer over the flyer plate, which can be rubber, cardboard, or similar material to protect the top surface of the flyer plate from damage caused by the explosion of an explosive charge. on top of the buffer is a layer of explosives which is detonated from the lower edge. The parent plate rests on the anvil to limit the deformation(distortion) of the final job.

Principle of operation

As the explosive is ignited, the detonation wave advances directly and smoothly across the surface of the front flyer plate. The explosive impulse provides both an extremely high normal pressure and a slight, relatively shearing or sliding pressure between the flyer plate and parent plate.

At the point of impact “S”, as shown in the figure, high instantaneous pressure is generated which is large compared with the shear strength of the materials. Consequently, the metals behave as inviscid fluids which obey the law of fluid mechanics.

Arrangments for Explosive welding

There are two common arrangments are used to produce explosive welds. which are as follows:

Parallel arrangement

Paralled-arangment

Parallel arrangement(Direct stand-off method) is also known as contact explosive welding. Here the weld conditions are controlled by the stand-off distance, the charge density, the detonation velocity, and the deformation characteristics of the flyer plate. In this arrangement, the detonation velocity of the explosive should be less than the velocity of sound in the materials to be welded in order to satisfy the criterion that the collision point velocity must be subsonic.

As this is a difficult criterion to fulfill from the point of view of the explosive, it is better to use an Inclined or angular stand-off arrangement.

Emplimention of explosive

Some of the explosives employed and their properties are as follows:

ExplosivesDetonation velocityDensity
gm/cm3
PETN81901.70
TNT66001.56
RDX81001.65
Metabel69001.47
Tetryl78001.71
Du Pont70201.40

Metals to be joined by explosive welding

The main application of explosive welding is welding(joining) and cladding of metals.

  • A number of dissimilar metal combinations have been joined
  • successfully with the help of explosive weldings, such as:
    • Aluminum to steel
    • Titanium alloys to Cr-Ni steel
    • Tungsten to steel
    • Mo-Ti alloy to cr-Ni steel
    • Aluminum to stainless steel
    • Aluminum to nickel alloys
    • Copper to stainless steel
    • Stainless steel to nickel, etc.
  • Zirconium Titanium, Stainless steel Copper, and nickel alloys Hastelloy and Stellite can clad by this process.
  • Explosive cladding is finding use in the die-casting industry for nozzles, ie-cast biscuits, and other components.
  • Explosive elding has been used for plugging of nuclear heat exchangers.

Other applications of the explosive welding/cladding process are in :
(a) Chemical process vessels,
(b) Transition joints (e.g. aluminum to steel),
(c) Electrical industry,
(d) Shipbuilding industry, and
(e) Cryogenic applications.

Advantage

  • The simplicity of the process.
  • Extremely large surfaces can be bonded.
  • Welds can be produced on heat-treated metals without affecting
  • their microstructures.
  • These foils can be bonded to heavier plates.
  • A wide range of thicknesses can be explosively clad together
  • Explosives bonds have a solid-state joint that is free from heat affected zone
  • Lack of porosity, phase changes, and structural changes impart better mechanical properties to the joints.

Disadvantage

  • In industrial areas, the use of explosives will be severely restricted by the noise and ground vibrations caused by the explosion.
  • The regulations relating to the storage of explosives and the problem of preventing them from falling into unauthorized hands. may well prove to be the main obstacle to the use of explosive welding.
  • A limitation of explosive cladding is concerned with the brittleness of the alloys. Metals to be bonded by this process must possess some ductility and some impact resistance. Metals harder than about 50 RC are extremely difficult to weld.
  • Metal thicknesses greater than 62 mm of each alloy cannot be joined easily and require high explosive loads.
  • (v) Materials such as beryllium, tungsten, boron, glass, and ceramics
  • are not normally processed by explosive welding

References

Wikipedia Welding technology