EXPLOSIVE
WELDING
HIREN YADAV

Abstract
Explosion welding or bonding is a solid-state welding process that is
used for the metallurgical joining of dissimilar metals. The process uses the
forces of controlled detonations to accelerate one metal plate into another
creating an atomic bond. Explosion bonding can introduce thin, diffusion
inhibiting interlayers such as tantalum and titanium, which allow conventional
weld-up installation. In addition, explosive welding is considered a coldwelding process, which allows metals to be joined without losing their prebonded properties.
This paper describes work carried out to numerically analyse a two plate
welding process using a finite element method (FEM) and the verification of
the results using experimental data. The numerical simulations identify factors
such as the level of strain induced in the plates and the direction of the shear
stress at the collision zone, in the surface of flyer plates as indicators of bond
strength. The phenomenon of jetting is computationally reproduced.
Explosion welding is a solid-state process that produces a high velocity
interaction of dissimilar metals by a controlled detonation , Oxides found on
material surfaces must be removed by effacement or dispersion, Surface
atoms of two joining metals must come into intimate contact to achieve
metallic bond

Introduction to Explosive Welding
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.
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.

Principle of Explosion
• Cladder metal can be placed parallel or inclined to the base plate
• Explosive material is distributed over top of cladder metal
• Upon detonation, cladder plate collides with base plate to form weld
It has been found to be possible to weld together combinations of
metals, which are impossible, by other means. 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. shows the explosive welding process.
The process can be divided into three basic stages.
(i) The detonation of the explosive charge.
(ii) The deformation and acceleration of the flyer plate.
(iii) The collision between the plates.
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
(stand- off 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. Dynamic situation at the collision front showing the jetting mechanism
It is now generally accepted that jet formation at the collision point is an
essential condition for welding. The jet if formed sweeps away the oxide layers
on the surface of the metals leaving behind clean faces which are more likely
to form a metallurgical bond by making it possible for the atoms of two
materials to meet at interatomic distances when subjected to the explosively
produced pressure waves. The pressure has to be sufficiently high and for a
sufficient length of time to achieve inter-atomic bonds. The velocity of the
collision point, Vc sets the time available for bonding. This high pressure also
causes considerable local plastic deformation of the metals in the bond zone.
As the bond is metallurgical in nature it is usually stronger than the weaker
material. The quality of the bond depends on careful control of process
parameters such as surface preparation, plate separation, detonation energy
and detonation velocity Vd . While various welding mechanisms have been
proposed for the explosive welding, they all almost agree that it occurs as a
direct result of high velocity oblique collision. Experimental results indicate
that there are certain critical values forboth the collision and the geometrical
parameters which have to be observed. These can be summarized as follows:

Since a jet is required at the collision region, a collision angle βis
critical. For a given metal, βis a function of the collision velocity. It has been
shown experimentally that the collision velocity Vc and the plate velocity Vp
must be less than the velocity of sound in either metal. El-Sobky and Blazynski
used this to define the conditions necessary for the reflected stress waves not to
interfere with the incident wave at the current collision point. It is also known
that at supersonic velocities, the dynamic pressure is not held for a sufficiently
long period to create physical conditions conducive to the adjustments of
interatomic diffusion and of equilibrium within the collision region. While
Wylie et al. suggested that the velocity of sound maybe exceeded by up to 25%;
the subsonic collision appears to be more satisfactory. As Vc is related to Vd
and β, it may be adjusted by introducing an initial angle of obliquity. The
velocity of sound or more precisely, the velocity of stress wave propagation
provides an upper limit for Vp and Vc.

A minimum impact pressure must be exceeded (and hence a minimum
Vp), in order that the impact energy should be sufficient to produce a weld. It
has been suggested by Wylie et al. [3] that the impact energy required is related
to the strain energy and the dynamic yield strength of the flyer material. An
upper limit for the energy is also required to avoid excess heating and possibly
melting by viscous dissipation and thus the formation of brittle layers.
Obviously, such an upper limit should be sought in terms of the melting energy
of the lower melting point of the weld combination.

A sufficient stand-off distance has to be provided in order to flyer plate
can accelerate to the required impact velocity. Therefore, the critical parameters
used to establish a weld ability window are
• The critical collision angle for jet formation.
• The collision velocity, Vc.
• The kinetic energy and impact pressure in the collision region associated
with the impact velocity, Vp.
Welding can be defined as the process of joining two or more metals , often
matallic , by localized coalescence or union across the interface.
Forge welding and soldering were the only forms of welding until the
nineteenth century when the discovery of electric batteries, generators,
acetylene and oxygen let to fusion welding process.
During this century tremendous advances have been made and many new
processes introduced , such as flux -shielded arc welding, gas -shielded are
welding , percussion welding , resistance welding and lazer welding.
 Most welding processes involve melting of the surfaces of components to
be joined or the melting of the a low melting point metal to join two
components, but several welding processes have been developed which
do not involve melting and these are known as phase welding process.

THE MECHANISM OF EXPLOSIVE WELDING
It is generally accepted that explosion occurs under high velocity oblique
impact, though it is conceivable to use explosive energy to form a conventional
cold pressure welding.
It is fairly obvious that there are several problems involved in analyzing the
dynamics of process.
Firstly it is necessary to understand the interaction between the explosive and
the flyer plate. This requires a knowledge of the equation of state for the
explosive, but for the explosives commonly used in explosive welding this is
not yet established.
Secondly, it is necessary to understand the response of the velocity and
deformation of the flyer plate to the explosive loading, so as to determine the
velocity and deformation of the flyer plate prior to collision with the parent
plate .
Lastly it is necessary to understand what occurs during the collision between the
flyer and parent plates.
It is in this final stage that the major difficulty in understanding occurs, as high
instantaneous pressures are generated with associated very high strain rates, and
under these conditions it is near to impossible to define material properties.
It is under these circumstances that it is necessary to make some simplifying
assumptions in order to begin to understand what is happening.
The mechanism of the linear collapse for wedge-shaped cavity is shown in Fig.
The very high pressure exerted by the detonation wave on the liner causes the
liner to collapse . The forces are so great that the strength of the liner material
can be neglected, and the material maybe assumed to behave as an inviscid
fluid, though this does not imply that it is necessarily in a fluid state, and indeed
there is evidence that it is not.
To the left behind moving apex is a section of completely collapsed wedge
which, in fact, contains metal from the outer surface of the wedge liner. The
inner part of the wedge forms a jet which is squeezed out from the inner apex of
the liner and travels at a very high speed along the axis, to the right. At zero
time it will be assumed that the detonation wave has reached the plane passing
through point P in Fig. and in unit time it will have moved to the plane through
Q, so the horizontal distance between planes will be equal to the detonation
velocity, VD.
At zero time the liner is in the position APQ and atfer unit time it is in position
BQ, so the apex of the wedge has moved from A to B.
V1={V0cos {1} over
{2}(¥â-¥á)} OVER
{sin¥â}
An observer moving with A would see P coming towards him with a velocity
represented by vector DP which will be
V2=V0[{cos{1} OVER {2}(¥â-¥á)} OVER
{tan¥â}+sin {1} OVER {2}(¥â-¥á)]
As viewed by the observer at A the process appears to be in a steady state and
as the liner material may be assumed as incompressible, then Bernoulli's
equation can be applied.
p={1} OVER
{2}¥ñ{¥ì}^{2}=const
So the pressure at any point in the fluid determines the velocity of the fluid at
that point If it is assumed that the pressure of the exploded gases rapidly decays,
then the pressure on the collapsing surfaces remote from P will be very low and
can be considered constant.
The absolute velocity of the jet will be
Vj=V1+V2=V0[{cos{1} OVER {2}(¥â-¥á)} OVER {sin¥â}+{cos {1} OVER
{2}(¥â-¥á)} OVER {tan¥â}+sin{1} OVER {2}(¥â-¥á)]
while the absolute velocity of the slug willbe
Vs=V1-V2=V0[{cos{1} OVER {2}(¥â-¥á)} OVER {sin¥â}-{cos {1} OVER
{2}(¥â-¥á)} OVER {tan¥â}-sin{1} OVER {2}(¥â-¥á)]
The slug is an example of explosion welding as it is formed from the two sides
of the wedge liner. Basically the liner surface of the liner have been removed
from the jet, leaving two virgin clean surfaces which are forced together to form
a solid state weld.
However , in the hollow charge situation the explosive loading is much greater
than is necessary for welding, as the objective is to produce an effective jet, and
as a result the slug will be very severely damaged.
In the analysis considered so far the collision problem has been treated as one of
classical hydro-dynamics of an incompressible perfect fluid, and the role of
shock waves within the flow has been ignored.
The analysis rests on the assumption of fluid-like behavior under high velocity
impact conditions, and this appears to be valid for the extreme conditions
achieved in a hollow charge.
And this collision can be divided into two categories, namely , jetless and jet
forming collisions .
Supersonic collision Jetting occurs downstream from the detached shock waves
due to the pressure acting on the unsupported flanks of the main jet. Since the
detached shock waves tend to deflect the flow away from the collision point, the
collision angle is somewhat less,and consequently the mass in the jet may be
expected to be less than for the subsonic case for the same angle ¥â.
These assumption was expanded it's way to more specific conditions. In
asymmetrical case, we assume that it involves the collision of plates of different
materials and thickness, and travelling at different flow velocities relative to the
collision region. For the parallel set-up they have calculated the critical angle
for jetting for various metals and combination of metals, as a function of the
detonation velocity,
Vd(=Vw=Vf).
If the two plates are of different materials or approach the collision points at
different velocities, then the stagnation pressures of the two streams are
different.
It has to be realized that the flow patte±º ·¢ ideal fluids may not even be
meaningful for real fluids, much more for metals, for at least two reasons.
Firstly , the flow patte±º envisaged contain regions where viscous effects could
be especially important in modifying the fluid flow, such as the region of high
relative velocity between the re-entrant jet, and the parent plate.
Secondly, and perhaps more importantly the flow observed is frequently
unstable and oscillations occur at the interface.
There are two provided physical descriptions of wave formation.
Fig. a shows the combination of the deformation of parent plate, caused by the
momentum of the flyer plate jet, coupled with the forward velocity of the parent
plate relative to S, together with the shearing of the surface of the parent plate
by the re-entrant jet. As a consequence a hump is formed ahead of the point of
impact. This hump deflects the re-entrant jet upwards into the flyer [late jet.
Fif.b. and ultimately it completely blocks off the re-entrant jet. Fig.c.
The trapped re-entrant jet forms a vortex at the back of the hump, in which high
temperatures are likely to be created by the dissipation of the kinetic energy of
the trapped jet, and this can result in phase changes and local melting.
When the re-entrant jet is completely chocked the stagnation point moves from
the trough to the crest of the wave, Fig.d and the high pressure associated with
the stagnation point will be depress and elongate the hump so that a forward
trunk is formed. As the hump continues to move downstream the stagnation
point depends the forward slope of the hump .Fig.e and as a consequence the reentrant jet will be increased angle of inclination between the jet and the inclined
side of the hump. At the same time the velocity of the re-entrant jet is reduced.
As the re-entrant jet descends the forward slope of the hump a second
stagnation point is formed at S'. and part of the jet enters the cavity under the
trunk causing another vortex.

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

Experimental tests and procedures
Some welding trials were performed with two PETN- based explosives in
inclined and parallel arrangements, but most welding experiments were
carried out using ANFO mixtures with the plates mounted in parallel.
Preliminary experiments were undertaken to determine the flyer plate and
collision velocities and the detonation velocity of the explosives using a pin
contact test method. All the welding tests performed were simulated. list the
explosive welding tests carried out. The explosive used was either a 77/23
ANFO/perlite mixture or PETN. The majority of the tests were performed with
the flyer and base plates set up in a parallel arrangement. A few tests were
made with a 15° angle between the flyer and base plate The parallel plate tests
used a 50 mm thick EN1 mild steel base plate (Metals Handbook, 2000; [29])
resting on sand. Tests were performed for several stand-off distances (from
half to twice the flyer plate thickness) and for different thicknesses of explosive
and flyer plate.
The ANFO detonation velocities as measured using a detonation velocity meter
and the pin contact method [10], [30], [31], [32] and [33], varied between 1800
and 2600 m/s.
The test program was designed to determine the effect of changes in the
operational parameters of the process (contact velocity, flyer plate velocity
and dynamic angle) on the physical parameters such as effective stress, strain
and contact pressure. Loading is due to detonation of the low explosive, which
is in the form of 80 mm and 235 mm high powder mixtures covering the upper
surface of the flyer plate for the different cases considered.
Most of the low explosives cannot generally be initiated by a detonator alone.
A booster made of about 50–500 g of high explosive such as Composition C4,
which can be initiated by a detonator, is used to start the detonation.
Following the tests two samples were cut from the central region of each
welded plate, one sample parallel to the longitudinal axis of the plate, (i.e. in
the direction of detonation), the other sample perpendicular to the direction
of detonation. Each sample was polished and metallography of the region
around the bond lines performed.
It was found that using an inclined plate arrangement with high explosive
(PETN) the plates only partially bonded, whereas with the parallel arrangement
and ANFO, bonding was almost 100% complete. In the case of the inclined
plate set up bonding did not occur until approximately half way along the plate
i.e. about 12 cm from the booster charge.
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.
The commonly used high explosives are
Explosive
RDX (Cyclotrimethylene trinitramine,
C3H6N6O6
PETN (Pentaerythritol tetranitrate,
C5H8N12O4)
TNT (Trinitrotoluene, C7H5N3O6)
Tetryl
(Trinitrophenylmethylinitramine,
C7H5O8N5)
Lead azide (N6Pb)
Datasheet
Ammonium nitrate (NH4NO3)
Detonation velocity , m/s
8100
8190
6600
7800
5010
7020
2655

Common industries that use explosion welding
• Chemical Processing
• Petroleum Refining
• Hydrometallurgy
• Aluminum Smelting
• Shipbuilding
• Electrochemical
• Oil & Gas
• Power Generation
• Cryogenic Processing
• Pulp & Paper
• Air conditioning & Chillers
• Metal Production

Applications
 Joining of pipes and tubes.
 Major areas of the use of this method are heat exchanger tube sheets and
pressure vessels.
 Tube Plugging.
 Remote joining in hazardous environments.
 Joining of dissimilar metals - Aluminium to steel, Titanium alloys to Cr
– Ni steel, Cu to stainless steel, Tungsten to Steel, etc.
 Attaching cooling fins.
 Other applications are in chemical process vessels, ship building
industry, cryogenic industry
 Can weld large areas of metal
 Can weld inside and outside surfaces of pipes
 Transition joints can be made

Advantages
 Can bond many dissimilar, normally unweldable metals.
Minimum fixturing/jigs.
 Simplicity of the process.
 Extremely large surfaces can be bonded.
 Wide range of thicknesses can be explosively clad together.
 No effect on parent properties.
 Small quantity of explosive used.
 No heat-affected zone (HAZ)
 Only minor melt
 Material melting temperatures and coefficients of thermal expansion
differences do not affect the final product
 The shock front compresses and heats the explosive material which
exceeds the sonic velocity of undetonated explosives

Limitations
 The metals must have high enough impact resistance, and ductility.
 Noise and blast can require operator protection, vacuum chambers, buried
in sand/water.
 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 – flat, cylindrical, conical.

Acknowlagment
An understanding of explosive welding depends on numerous related
disciplines such as explosives, the interaction of explosives with metal surface
they are in contact with, the behavior of metals under high velocity oblique
impact and the parameters which control the overall process.
The application of explosive welding has unique advantages in joining different
metals which would frequently be incompatible , but there are great limitation
on the geometries which may be satisfactorially welded due to stress
intensification arising from reflection of shock waves from changes in section.
Large plate cladding to form composite plates for use in pressure vessel
construction and for tube plates in heat exchangers is well established and
accepted.
And it's applications are being widely expanded because of it's unique physical
properties.
The mechanical and metallurgical properties of explosive welds are important.
The strength and quality of the bond between a thin cladded layer and the parent
plate is difficult to determine whatever the method of fabrication, and explosive
welding is no exception. Though explosive welding does not compete with
conventional welding process it is a useful additional process in the welding
engineer's armorny.
This book is aimed at providing an understanding of this new welding process.