The Explosion Welding and it's Mechanism
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.
INTRODUCTION
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.
Forge welding is a solid-phase process and in recent decades the age of old process of hammering together soft metal has led to cold pressure welding ; other recently developed solid state processes are ultra sonic friction, flash-butt and explosive welding and diffusion welding.
For the two surfaces to unite it is necessary to bring the two surfaces sufficiently close together to be within the range of the inter-atomic attractive forces.
The essential problem in joining two metals is how to achieve virgin surface without oxide or other contaminant surface film. Such a state of cleanliness is impossible to achieve by normal mechanical or chemical processes, as the surface if cleaned are immediately subject to re-oxidation.
It is possible to produce the required state of cleanliness under very high vaccum, and if surfaces produced under these conditions are brought together under vaccum then adhesion will occur between the asperities. So essentially welding must provide a way of obtaining intimate contact between the components to be joined. Basically this can be achieved in two ways, which form the two distinguishable groups of welding processes.
Friction welding and Solid phase welding.
( Here Explosion welding which belongs to Solid phase welding is going to be explained)
In explosion welding there is a high-velocity oblique impact between two components being welded which causes the metals to behave like fluids.As a result a high-velocity jet is formed from the two surfaces of both components, which leaves two virgin clean surfaces which are pressed together to form a weld.
It appears that the realisation of the potential commercial value of explosive welding stimulated early work in Russia, Japan and United Kingdom.
Explosive welding has mainly found commercial application in large plate cladding of one metal on another , tube to tube plate welding, cladding one tube on another, plugging of heat exchangers, various electrical connectors, especially those between copper and aluminum , transition pieces, especially for pipework in cryogenic systems, pipe to pipe welding etc.
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.2-2.
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.2.3 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.
An observer moving with A would see P coming towards him with a velocity represented by vector DP which will be
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.
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
while the absolute velocity of the slug willbe
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.2-14a 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.2-14b. and ultimately it completely blocks off the re-entrant jet. 2-14c.
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.2-14d 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. 2-14e and as a consequence the re-entrant 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.