Explosive Welding

To form an explosive weld the following conditions need to occur:
• Two surfaces that need to be joined are initially spaced at a small distance (standoff
distance).
• An explosive force brings these two surfaces together progressively at a collision
front. The collision front's velocity must be lower than the speed of sound in the
materials, so that the shock wave precedes the bond being formed. If not, the
shockwave would interfere with the contacted surfaces preventing a bond occurring.
• The interfacial pressure at the collision front must exceed the yield strength of the
materials, so that plastic deformation will occur.
A jet of metal is formed just ahead of the collision front, comprising of the two component
surfaces, which is finally ejected from the interface. The surfaces and any surface
contaminants are removed in the jet. Behind the collision front, the now clean surfaces
bond, under extreme pressure, in the solid state. This dynamic welding situation is shown
in Fig.1. In cross section, the materials usually bond together in an undulating wave form
and the process can weld a parent plate of thickness 0.025mm to over 1m (the maximum
flyer plate thickness is one third that of the parent plate). Up to 30m2 can be welded in
one explosion.
Fig.1. Dynamic situation at the
collision front showing the jetting
mechanism
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Current Status
Explosive welding was first recognised as a solid state process in 1944 when solid state
welding had occurred between two metallic discs which had been in contact with a
detonator. It was not until the 1960s that the process was exploited commercially
throughout the world. The process was mainly used to clad large areas of one metal with
another. The materials that are commonly clad are shown in Fig.2.
Fig.2. Common clad material
combinations
Current issues
More recently the process has moved away from simple cladding and is used to produce
corrosion resistant pressure vessels, transition joints for shipbuilding, electrical busbars
and heat exchangers for nuclear installations. Future developments of the technique might
create advanced composite structures for the aerospace industry.
Benefits
The advantages of explosive welding over conventional welding techniques are as follows:
• Used to join dissimilar metals
• Achieves high bond strength
• Maintains parent metal qualities
• Achieves welds over large areas
• Requires low capital outlay
• Produces minimal distortion of parent metals
• Enables remote welding to take place
• Enables welding in hostile environments
Introduction
Explosive welding is a solid state welding process, which uses a controlled explosive
detonation to force two metals together at high pressure. The resultant composite system
is joined with a durable, metallurgical bond.
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Explosive welding under high velocity impact was probably first recognized by Garl in
1944. Explosive welding was first recognized as a possibility in 1957 in the United States
when it was observed by Philipchuck that metal sheets being explosively formed
occasionally stuck to the metal dies. Between that and now the process has been
developed fully with large applications in the manufacturing industry.
It has been found to be possible to weld together combinations of metals, which are
impossible, by other means.
The Process
This is a solid state joining process. When an explosive is detonated on the surface of a
metal, a high pressure pulse is generated. This pulse propels the metal at a very high rate
of speed. If this piece of metal collides at an angle with another piece of metal, welding
may occur. For welding to occur, a jetting action is required at the collision interface. This
jet is the product of the surfaces of the two pieces of metals colliding. This cleans the
metals and allows to pure metallic surfaces to join under extremely high pressure. The
metals do not commingle, they are atomically bonded. Due to this fact, any metal may be
welded to any metal (i.e.- copper to steel; titanium to stainless). Typical impact pressures
are millions of psi. Fig. 1 shows the explosive welding process.
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Explosives
The commonly used high explosives are –
Explosive Detonation velocity , m/s
RDX (Cyclotrimethylene trinitramine, C3H6N6O6 8100
PETN (Pentaerythritol tetranitrate, C5H8N12O4) 8190
TNT (Trinitrotoluene, C7H5N3O6) 6600
Tetryl (Trinitrophenylmethylinitramine, C7H5O8N5) 7800
Lead azide (N6Pb) 5010
Datasheet 7020
Ammonium nitrate (NH4NO3) 2655
Applications
1. Joining of pipes and tubes.
2. Major areas of the use of this method are heat exchanger tube sheets and pressure
vessels.
3. Tube Plugging.
4. Remote joining in hazardous environments.
5. Joining of dissimilar metals - Aluminium to steel, Titanium alloys to Cr – Ni steel, Cu to
stainless steel, Tungsten to Steel, etc.
6. Attaching cooling fins.
7. Other applications are in chemical process vessels, ship building industry, cryogenic
industry, etc.
Advantages
1) Can bond many dissimilar, normally unweldable metals.
2) Minimum fixturing/jigs.
3) Simplicity of the process.
4) Extremely large surfaces can be bonded.
5) Wide range of thicknesses can be explosively clad together.
6) No effect on parent properties.
7) Small quantity of explosive used.
Limitations
1. The metals must have high enough impact resistance, and ductility.
2. Noise and blast can require operator protection, vacuum chambers, buried in
sand/water.
3. The use of explosives in industrial areas will be restricted by the noise and ground
vibrations caused by the explosion.
4. The geometries welded must be simple – flat, cylindrical, conical.