High-energy-rate operations

Introduction
The last three decades have seen the development of industrial forming .techniques based on the utilization of high-energyrate
dissipation. The dynamic aspects and mechanisms of these operations make it possible to manufacture, usually
fairly economically, either large components. such as radar dishes, rocket nose cones and pressure vessels, or to produce
smaller semifabricates (in sheet or tubular form) to be processed further in conventional operations. Some examples of
this approach were given in Sections 16.2.6 and i6.2.7.

The usual sources of energy available are:

1. chemical explosives,
2.electrostatic fields (hydroelectric forming). and
3. magnetic fields (magnetic forming).
These sources of energy are used in:
1. forming (sheet and tube),

• These sources of energy are used in:
  forming (sheet and tube),
• welding (sheet sandwich semifabricates, and multimetallic
  tubular components),
• powder compaction (to form prefabricates),
• forging, and
• hardening

The potential use of any of these operations is conditional on the formability of the given metal in the dynamic condition.
An indication of this, based on 1100-0 A1 as standard, is given in Figure 16.60.

Sources of energy

Explosives The proper utilization of the energy evolved when an explosive charge is detonated depends on the degree
of understanding of both the properties of explosives and of the detonation phenomena. From a practical point of view, commercia! chemical explosives will generally fulfil the requirements of the welding, forming and compacting processes. On detonation, the detonating front travels through the explosive converting its mass to a high temperature, high pressure gas which is then used in the generation of a stress wave and, depending on the conditions, in the appearance of shock and release waves.
Chemical explosives are subdivided into ‘high’ and ‘low’ (deflagrating) materials. High explosives are characterized by
very high detonation pressures and high rates of reaction. Because of this. high explosives are subdivided further into
two groups.

1. Primary or detonating explosives which are sensitive and may be detonated by slight impact. flame, static electric
charge, or simple ignition. They are normally used in detonators. but seldom as a source of energy in metal
working.

2. Secondary high explosives are used mainly in metal working and other industrial applications. They require a
detonator to initiate the reaction and sometimes a booster charge to reinforce the detonation wave. They have a
higher energy content than primary explosives.

Deflagrating or low explosives burn rather than detonate when the reaction is initiated and produce much lower pressures.
They usually contain their own oxygen supply and, therefore, burn easily, but in some materials the reaction is
difficult to initiate. Their low rate of burning makes them excellent propellants, but fire risk is considerable when handling
such chemicals.

The charges are initiated by detonators. Electric detonators are widely used and are safe to handle if reasonable precautions
are taken. Commercial detonators normally consist of a thin metallic container protecting the contents of an initiating
primary high explosive, and a small amount (about 1 g) of a sensitive secondary explosive, e.g. PETN or tetryl. Initiation
is achieved electrically, using an exploding bridge wire. Some types contain a slow-burning material to provide a time delay
when many charges are fired at different time intervals. The following explosives are commercially available and are
used in industrial applications.

Primary high explosives

1. mercury fulminate,
2. lead azide,
3. diazodinitrophenol,
4. lead styphnate, and
5. nitromannite.
Secondary high explosives
1. TNT (trinitrotoluene).
2. tetryl (trinitrophenylmethylnitramine),
3. RDX (cyclotrimethylenetrinitramine),
4. PETN (pentaerythritol tetranitrate),

5. ammonium picrate,
6. picric acid
7. ammonium nitrate,
8. DNT (dinitrotoluene),
9. EDNA (ethylenediaminedinitrate),
10. NG (nitroglycerine), and
11. nitro starch.
Low explosives
1. smokeless powder;
2. nitrocotton;
3.black powder (potassium nitrate, sulphur, charcoal); and
4. DNT (dinitrotoluene ingredient).

black powder (potassium nitrate, sulphur, charcoal); and Explosive materials are available in different forms and
some of them in more than one. Many of them can be melted allowing other explosives to be added in the furm of slurries.
Powdered, granular, solid, liquid and plastic explosives can be used. One of the most useful types is so-called ‘Datasheet’ or
‘Metabel’, which is essentially a PETN explosive combined with other ingredients to form a tough, flexible waterproof
sheet that can be cut and shaped to the required size for contact and stand-off operations. It is available in different
thicknesses and can be glued together or used as shaping back-up material if a shape charge is required. Another very
useful type is powder explosive, particularly various mixtures of TNT with aluminium powder, which can fill a container of
any form and then be compacted to attain higher densities.

Sealed containers are not required, thus offering an advantage over liquid explosives. Cord explosives, e.g. Cordtex, are also
available. These consist of a flexible cord containing a core of explosive. Cords are very useful when continuous long charges
are needed and give reasonable accuracy in a number of forming operations.

The effectiveness of a charge depends on the characteristics of the explosive, as reflected by the pressure-time function,
velocity of detonation, explosive/specimen mass ratio, standoff distance and the transmitting medium.
‘Detonation’ is a term used to describe the process in which an explosive charge undergoes a chemical reaction accompanied
by a characteristic type of shock wave (or detonation wave). Depending on the properties and type of the explosive
material, the velocity and intensity of the characteristic shock wave varies, but remains constant for a given type of explosive
and for a charge of uniform geometry and density. This simplifies the mathematical solution of the hydrodynamic
theory which applies to the process. The general behaviour of a primary explosive during reaction is characterized by a slow
combustion process at the beginning, and then by deflagration, up to a point of sudden transition to detonation. The
whole process is completed in a few microseconds. However, the rate of build up of the reaction and the transfer to
detonation when a secondary explosive is detonated without a detonator is much slower and burning before detonation may
occur.

Low explosives are characteriized by the absence of the transition period. They react at rates which are proportional to
the build-up pressure which, in turn, increases as the chemical reaction speeds up. This cycle leads to explosion within a
fraction of a second, but the rate of reaction is usually much slower than 1% of that in detonation, and peak pressures are
also lower. However, energy comparable with that obtained from high explosives can be generated by low explosives when
they are adequately confined or used in sufficient quantity, and pressure distribution is easily controllable.
As far as metal-working processes are concerned, the most important parameters of the detonation process are:

Explosive materials are available in different forms and some of them in more than one. Many of them can be melted
allowing other explosives to be added in the furm of slurries. Powdered, granular, solid, liquid and plastic explosives can be
used. One of the most useful types is so-called ‘Datasheet’ or ‘Metabel’, which is essentially a PETN explosive combined
with other ingredients to form a tough, flexible waterproof sheet that can be cut and shaped to the required size for
contact and stand-off operations. It is available in different thicknesses and can be glued together or used as shaping
back-up material if a shape charge is required. Another very useful type is powder explosive, particularly various mixtures
of TNT with aluminium powder, which can fill a container of any form and then be compacted to attain higher densities.
Sealed containers are not required, thus offering an advantage over liquid explosives. Cord explosives, e.g. Cordtex, are also
available. These consist of a flexible cord containing a core of explosive. Cords are very useful when continuous long charges
are needed and give reasonable accuracy in a number of forming operations.

The effectiveness of a charge depends on the characteristics of the explosive, as reflected by the pressure-time function,
velocity of detonation, explosive/specimen mass ratio, standoff distance and the transmitting medium.
‘Detonation’ is a term used to describe the process in which an explosive charge undergoes a chemical reaction accompanied
by a characteristic type of shock wave (or detonation wave). Depending on the properties and type of the explosive
material, the velocity and intensity of the characteristic shock wave varies, but remains constant for a given type of explosive
and for a charge of uniform geometry and density. This simplifies the mathematical solution of the hydrodynamic
theory which applies to the process. The general behaviour of a primary explosive during reaction is characterized by a slow
combustion process at the beginning, and then by deflagration, up to a point of sudden transition to detonation. The
whole process is completed in a few microseconds. However, the rate of build up of the reaction and the transfer to
detonation when a secondary explosive is detonated without a detonator is much slower and burning before detonation may
occur.

Low explosives are characteriized by the absence of the transition period. They react at rates which are proportional to
the build-up pressure which, in turn, increases as the chemical reaction speeds up. This cycle leads to explosion within a
fraction of a second, but the rate of reaction is usually much slower than 1% of that in detonation, and peak pressures are
also lower. However, energy comparable with that obtained from high explosives can be generated by low explosives when
they are adequately confined or used in sufficient quantity, and pressure distribution is easily controllable.
As far as metal-working processes are concerned, the most important parameters of the detonation process are:

1.the tnergy released by the detonation.

2.the detonation? velocity (Le the velocity of propagation of

3.the detonation front), and the pressure exerted by the gaseous products of detona-
?ion on the specimen.

Other aspects of the process, such as the thermal stability
and sensitivity of the explosive, the temperature, the heat
generation and the ionization phenomena in the gaseous
products, as well as their composition, are of no importance.
To carry out a successful metal-working operation the
optimum strain rate for the metal must be ascertained, and the
amount of energy dissipated must be adjusted to give the
desired rate.
The adjustment of the amount of energy available for a
given operation depends on the source used and the method of
application. Regardless of the type of source used, the energy
can be delivered in one of two ways: through 2: transmitting
medium, or directly to the metal. Apart from some special
appiicatioms, delivery through a transmitting medium is more
usual.
Hydroelectric forming The principle of operation is based on
the rapid dissipation and transmission of energy evolved when
an electrostatic field is suddenly discharged. Two different
techniques of using the energy stored in a bank of condensers
are employed: underwater discharge, and exploding wire. In
the formler case, the discharge across two submerged electrodes
produces a shock wave in the transmitting medium,
accompanied by heating and vaporizing of the adjacent layers
of the medium. The plasma created by the spark expands as a
gas bubble, transmitting the force of explosion to the work
piece. The efriciency of the operation depends on the conductor
material, losses of energy in the circuit, and the
geometry and surface conditions of the electrodes.
The second method consists of connecting submerged electrodes
with an initiating wire. The transmitted energy vaporizes
the wire and converts it into plasma, creating a pressure
wave. The increase in volume of the vaporized wire is of the
order of 25 000 times its original volume. The exploding-wire
method possesses certain distinct advantages over the sparkdischarge
method in that the process can be more rigidly
controlled. The shape of the shock wave can be determined by
the shape of the wire, and a long arc discharge can be obtained
as opposed to a point source one. The amount of energy can
be controlled by the dimensions and material of the wire. For
instance, tungsten produces more energy than tantalum, niobium,
molybdenum, titanium, nickel or aluminium (in that
order).
Electrom!agnetic forming The principle of electromagnetic
forming is, basically, the same as that of an electrohydraulic
operatioo. The energy stored in a bank of condensers is
rapidly discharged through a magnetic coil, which surrounds,
is placed inside, or is in the proximity of the work piece
(Figure l6.61). A high intensity magnetic field is thus created
and, providing that the material of the work piece is conductive,
electric current is induced in the specimen. The current
interacts with the coil field and produces high trznsient forces.
The specimen thus acts as a secondary short-circuited coil. The
energy level produced by the magnetic field depends on the
conductive properties of the formed metal, its shape and mass
and the duration of the initial current pulse. These factors can
quite easily be controlled by suitable choice of the materials
and geometry of the coils. Materials of low conductivity are
sometimes lightly coated with copper. The shape of the
impulse wave can be modified by using 'field shapers' which
consist of shaped beryilium-copper pieces inserted in the coils.
Electrically, shapers help to depress or concentrate the intensity of the magnetic flux in those sections of the work piece
which may require a lower or higher degree of deformation.
The life of a coil depends on the magnitude of the force to
which it is subjected. This being equal to the force generated
on the surface of the work piece, the pressures can be very
high. The usual practice consists, therefore, of using one-shot
disposable coils for more complicated operations involving
only a few parts, and of limiting the pressures to the value of
the compressive strength of the coil material for massproduced
parts.

Forming systems

Explosive forming Explosives are used primarily for shaping sheet. plate and tubes, and for sizing and flanging, all these
operations being of the stand-off type. Close-contact operations are used to a lesser extent and for more sophisticated
processes such as extruding, cladding (welding), powder compaction and controlled surface hardening of Hadfield steels.
The particular applicability of explosives to forming arises from the fact that virtually any shape in any size can be
obtained without recourse to plant or machinery. The forming tool consists of a die which is comparatively light and does not
require any foundations since the inertia of the tool mass is sufficient to counteract the applied force. It is essential,
however, to realize that explosive forming is not economically viable when a very large number of components is required.
Its advantage over the conventional methods lies in the possibility of producing complex parts very accurately and
with very little or no machining, but in small quantities. The saving is due to the fact that there is no capital cost of presses,
tooling, etc. The dies for stand-off operations are very often made from cheap cast materials such as epoxy or concrete.
The range of metals successfully worked includes: aluminium and its alloys, stainless steels, magnesium and some of its
alloys, titanium and its alloys with aluminium, vanadium and manganese, refractory metals, copper and its alloys, and
special alloys such as stellite, iron-nickel, nickekopper, chromium-nickel and cobalt-iron. Carbon and low-alloy
steels are less often used because of their low formability in dynamic conditions.

 Three essential types of techniques characterize the standoff operations: (i) free forming (cups, flanging and deep
drawing), (ii) cylinder forming, and (iii) bulkhead forming (sheet and plate). These techniques are shown diagrammatically
in Figure 16.62. Cylinder shaping is done in an open system using high explosives. A transmitting medium other
than air is used to sustain the pressure for a longer period, thus increasing the impulse delivered to the work piece. In general,
the forming of metals can be carried out either in a tank sunk in the ground and filled with water as the transmitting
medium, or in an empty tank with the water being contained in a polythene bag (in the case of a cylindrical component) or
in any suitable, disposable container. The first method is used

for large components that require large charges, where the confinement of explosion within a large volume of water
serves both as a safety measure and as a means of reducing noise. In such a case, the space between the undeformed metal
and the die must be sealed-off and evacuated to enable full deformation to take place. In the second case, it may not be
necessary to evacuate the air, providing that the die is fitted with a suitable system of ventilating holes.
A very important aspect of this method of forming is correct die design. The near absence of spring-back depends on the
proper balance of tool profile, avoidance of sharp edges and deep narrow grooves and the provision of reasonably smooth
transition sections.

Bulk forming of sheet is carried out by using either a single blank subjected, possibly, to a series of shots until the die is
filled completely, or by using a mechanically shaped preform which is then given its final accurate dimensions in an explosive
operation (Figure 16.63).

Forming without a die is also possible when relatively simple shapes are required, e.g. an impeller. An initially circular
specimen can be formed to the shapes shown in Figure 16.64. Although explosive forming cannot be regarded as a substitute
for existing processes, it is an extension of the techniques now in use in the sense that it enables a number of
difficult materials to be formed to a high degree of accuracy and at low cost. In the general field of metal shaping, the
process is very advantageous when large parts are formed, often in a single operation, as costly heat treatment, tooling
and machining are not required. The process is not competitive when parts can be produced in bulk using conventional
equipment in a small number of operations. The use of explosives requires special precautions when storing and handling,
and for safety reasons forming cannot be done in a congested enclosed area. Hydroelectric forming In the application of this controlled and repeatable electrical force, the most obvious difference from explosive forming is the design of the forming apparatus. The dies are completely enclosed, since the volume of plasma generated is very small compared with the gas bubble created by a chemical explosive. By closing the die, full advantage can be taken of the pressure wave and, in the case of tubular parts, the shock wave is reflected from the ends, thus giving rise to additional energy. Each die has its own set of electrodes. In
either technique the discharge takes place in a matter of microseconds, and is repeatable within 30 s. Operations can
be automated and, unlike explosive forming, are eminently suited for mass production. Industrial forming machines are
commercially available that have a basic output of, for example, 15 kJ, the amount of energy sufficient to form

ordinary commercial materials using blanks of 12-100 mm diameter and wall thicknesses of 0.2-2.5 mm. The output can
be increased to 60 kJ. For most purposes, dies used in hydroelectric forming are made of aluminium or cheap castable materials. The serious disadvantage of this technique is the comparatively high cost of the electrical equipment needed.
Electromagnetic forming The dies used in this process are made from cheap materials and, in some cases, are not
required at all since the magnetic fields produce uniform pressures sufficient to expand cylindrical parts. The technique
is very suitable for shaping sheet and tubular parts, swaging and the production of finned components. The process can be
fully automated and forming machines are available. These are capable of producing magnetic flux densities of the order
of 3 X lo5 G, corresponding to pressures of over 340 MPa. The duration of the pulse is short, because of the permeability
of metals, and is usually limited to 10 ms. Magnetic fields are quite sufficient for forming copper, aluminium and similar
materials, but for metals like tungsten and for large parts, fields of the order of 5 X lo5 to 5 X lo6 G are needed.
The major disadvantage of the process is, again, the relatively high cost of the electrical equipment, but this is offset to
some extent by the very high reliability, repeatability, speed of production and accuracy obtainable.