b zhang paper exposive forming.pdf
TRANSCRIPT
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Applications and capabilities of explosive forming
D.J. Mynors*, B. ZhangDepartment of Systems Engineering, Brunel University, Uxbridge, Middlesex UB8 3PH, UK
Received 28 February 2002; accepted 3 March 2002
Abstract
All processes, after sufficient time, are visited by a new generation of workers that contemplates process merits and demerits for specific
applications. The process that is presently being revisited by academics and industry together is explosive forming. For over 100 years, it has
been recognised that explosives can be used in a controlled way in the manufacture of profiled metal components. The required profile results
from the explosive force that directly or indirectly deforms the metal. Explosive forming is a broad term covering many process variations.
Early patents relating to explosive forming appeared at the end of the 19th and at the beginning of the 20th century. An increasing number of
economically successful applications were being seen in the early 1970s, with the manufacture of large aluminium and high strength steel
parts. The work presented in this paper results from a global review of activities undertaken in the area of explosive forming, explains the
reason for the work, examines explosive forming applications, the associated metallurgy and reviews manufacturing requirements.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Explosive forming; Applications; Metallurgy
1. Introduction
Even after 11 September 2001, the European aerospace
industry is still one of the European communitys leading
industrial strengths, competing successfully in world mar-
kets and ensuring the employment of some hundred thou-
sand people across the European member states. However,
one of the major problems facing the European aerospace
industry relates to manufacturing capabilities. As the size of
aero-engines increase so does the size of individual engine
components. This increase in size means that key manu-
facturing capabilities typically reside outside the European
community, and for some of the larger components the
manufacturer has a monopoly. The situation is not strategi-
cally viable. First, a position where the supplier can dictate
terms to the customer does not make sound economic sense.
Secondly no or insufficient control over the political and
economic environment when dealing with external sup-
pliers means in difficult times the supplier may choose not,
or may not be able to, supply components to the customer.
It is anticipated that using integrated fabrication processes
will facilitate the production of components within the EC
and obviate the requirement for costly imports. On 1 March
2000 a project funded under the Competitive and Sustain-
able Growth Programme [1] of the ECs Framework Five
Programme commenced. The project, Manufacturing and
Modelling of Fabricated Structural Components (MMFSC)
[2], seeks to enable a step change in the process of design
and manufacture of aero-engine structures with an obvious
focus on fabrication.
The project is developing a framework within which
manufacturing and analysis techniques may be integrated
to develop methodologies for the design and manufacture of
fabricated structural components. This is particularly rele-
vant to the design of aero-engine structures, where there are
demands:
to reduce manufacturing lead times; to increase material utilisation and reduce waste; to reduce cost; to increase the manufacturing competitiveness of the EC
countries;
to operate with integrity in a high temperature environ-ment;
to maximise stiffness while reducing weight; to design with regard for aero-dynamic efficiency.
The project is also addressing the commercial risk
involved whenever a large number of technologies are
required to complete a project. This has been the main
reason why fabrication of structural components has so
far remained underdeveloped.
Journal of Materials Processing Technology 125126 (2002) 125
* Corresponding author.
E-mail address: [email protected] (D.J. Mynors).
0924-0136/02/$ see front matter # 2002 Elsevier Science B.V. All rights reserved.PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 4 1 3 - 2
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Two of the technical objectives of the MMFSC project
are:
to develop a new design for manufacture methodologybased on a practical knowledge of manufacturing pro-
cesses to give right first time designs and components that
are designed within process capability;
to obtain up to date capability surveys of fabricationprocesses [3].
The MMFSC project is a partnership of 19 participants.
The partners, drawn from five member states, bring together
five of the leading aerospace companies in Europe, one
small enterprise and six research organisations, which have
expertise in materials joining technology, materials charac-
terisation, fabrication processes, testing, data processing,
software engineering and technology transfer. The industrial
partners may together be classified as the European aero-
space industry. The partnership also includes eight univer-
sities which all have significant and complementary areas of
expertise including laser optics, sensors, processing, all
aspects of manufacturing technology, modelling of engi-
neering materials processing, optimisation for engineering
design applications and mechanical engineering for aero-
engine transmissions and structure applications.
The project which was originally coordinated by Rolls-
Royce plc from the UK and now by Industrial de Turbo
Propulsores SA of Spain, 47% Rolls-Royce owned, is split
into five technical work packages. Work package 1: design
for manufacture, work package 2: process modelling, work
package 3: welding technology and related control, sensors
and non-destructive testing, work package 4: fabrication
and machining of components and testing, work package 5:
fabrication of the high temperature material Inconel 939.
With the exception of work package 5 the project focuses on
Inconel 718 as the material to be fabricated.
A typical aero-engine is a complex structure with several
structural components on which the MMFSC project could
focus. The main component identified for examination dur-
ing the project is the tail bearing housing (TBH) examples of
which are shown in Fig. 1.
The main functions of the TBH are to:
maintain alignment of the rotor system within the staticstructures;
Fig. 1. TBHs [4,5].
Fig. 2. Engine mount [4].
2 D.J. Mynors, B. Zhang / Journal of Materials Processing Technology 125126 (2002) 125
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transmit loads across the gas flow path; provide engine structure spring elements and help main-
tain airfoil tip clearances;
support bearings and sumps; provide aero-dynamic turning of the flow path gas; provideenginemountandgroundhandlingfeatures (Fig.2); support other major engine components.
As part of the MMFSC project, a simplified but repre-
sentative version of the TBH has been designed to ensure a
range of manufacturing processes and simulation techniques
can be tried and tested. The design and manufacture of the
simplified component is the responsibility of work package
4, which is broken into three strands: welding and heat
treatment, forming and forging, and machining. The tasks
associated with each strand include capability surveys,
testing definitions, generating validation data and manufac-
turing the simplified component.
The simplified component will consist of an outer ring, an
inner ring, and nine struts plus additional struts for a range of
tests (Fig. 3). Two configurations for the outer ring and two
more for the inner ring have been analysed based on the
desire that the work should be driven by manufacturing
rather than design alone.
The need to investigate possible forming processes for
manufacturing all parts of the simplified component has
resulted in explosive forming being reviewed. The accessi-
bility of information has resulted in more questions being
generated than answered. As much detail as possible is
provided below but the authors would welcome any addi-
tional information about any aspect of explosive forming.
The information found during this work has been of sig-
nificant interest to ensure than trials are carried out as part of
the MMFSC project.
2. Introduction to explosive forming1
For over 100 years it has been recognised that explosives
can be used in deforming metals [6]. It was reported [7] that
the first application of explosives to metalworking was
undertaken by Daniel Adamson of Manchester in the United
Kingdom in 1878. Adamsons technique, free forming was
developed to assess the strength of boilerplates. Later Walter
Claude Johnson of Kent also in the United Kingdom,
developed the forming of metal against a die through the
application of explosives. The authors believe that this
resulted in one of the first patents, British Patent no.
21840, 23 September 1898, for the explosive expansion
of metal tubes for bicycle frame manufacture. A short time
later on 9 November 1909, Patent no. 939,702 was filed for
the explosive forming of sheet metal in the United States. In
the early 1950s, Johnsons invention [7] was adapted by the
Moore Company of America to make large fan hubs, costing
15% less than conventional mechanical shaping.
From Fig. 4 it can be seen that the number of publications
about explosive forming slowly increased from 1961, before
the authors were born, to 1972, after which time they
decreased dramatically [8]. Fig. 7 indicates that although
explosive forming was the dominant process, variants
evolved as different energy sources were investigated.
The discharge of capacitors typically into water, electro-
hydraulic forming (EHF), was one variation. Another was the
application of electromagnetics, electromagnetic forming
(EMF). All three processes, electromagnetic, electrohydraulic
Fig. 3. Representation of the simplified component.
1 Information taken from references conforms to a range of material
standards and measurement systems. The material specifications have been
left as quoted in the original documents. Metric equivalents to the
measurement systems have been provided as appropriate.
D.J. Mynors, B. Zhang / Journal of Materials Processing Technology 125126 (2002) 125 3
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and explosive forming, are available commercially today,
and in some cases compete with each other. Before going
on to describe what may truly be considered as explosive
forming, brief descriptions of electromagnetic and EHF are
presented.
2.1. Electromagnetic forming
EMF is dependent on the electrical properties of the
material being formed. Typically, the material to be formed
must have an electrical resistivity of no more than 15 mO cm.Suitable materials include copper, aluminium, mild steel,
brass, most precious metals and stainless steel. To undertake
forming a capacitor bank is discharged into a coil surrounding
the workpiece (Fig. 5). The current that flows through the
coil generates a magnetic field the strength of which is
proportional to the current flowing, Biots law. The magnetic
field generated around the coil in turn generates an electric
current in the workpiece. The generated current in turn
generates a magnetic field around the metal workpiece.
The two magnetic fields repel each other. The force causes
the workpiece to deform. This system is a non-contact
forming process and hence does not require lubricant. As
described here the process is often used as an assembly
method but other applications exist. One example being in
the automotive industry, Chrysler as was, Ford and General
Motors under their United States Council for Automotive
Research, have been manufacturing aluminium car door
liners using a hybrid technique of conventional stamping
augmented with EMF. Additional references relating to
EMF are provided [911].
2.2. Electrohydraulic forming
EHF can be considered as the link between EMF
and chemical explosive forming. Charged capacitors are
Fig. 4. Explosive forming activity over time [8].
Fig. 5. An EMF assembly.
4 D.J. Mynors, B. Zhang / Journal of Materials Processing Technology 125126 (2002) 125
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discharged (Fig. 6); into an energy transfer medium typically
water. The die is usually placed underwater with the sheet or
tube in place and a vacuum between it and the die. Two large
electrical wires run parallel through the water. A potential
across the wires causes a current to flow, sometimes aided by
the presence of a fuse wire or initiating filament across the
two wire ends. The current flow results in the water breaking
down into oxygen and hydrogen which then explodes pro-
ducing a shock wave that pushes the material onto the die
surface at a high rate. The inclusion of fuse wire may affect
the deformation. The location of the electrodes in the water
depends on the shape of the product to be formed. There are
a considerable number of applications including the forming
of boat hulls [12].
Daehn [13] of Ohio State University reports that General
Electric made missile components from aluminium, using
360 mF capacitors and up to 10 kV. Vickers of Newcastle inthe United Kingdom used capacitor banks of up to 40 kJ to
produce various parts including stainless steel swivel joints;
to combine piercing and forming of thin aluminium sheets;
assemble components using various aluminium, copper,
Nimonic 75 and titanium alloys. Daehn also states that
reports indicate using EHF successfully eliminated forming
problems with copper, 6061 TO aluminium and stainless
steel. Cincinnati Shaper manufactured EHF machines,
with energy values ranging between 25 and 150 kJ, under
the trade name of Electroshape; while Rohr produced
Soniform machines with an energy range of between 15
and 60 kJ.
The limit of the process resides with the amount of energy
available from the capacitor banks. In reality, the physical
size of the capacitor banks are the limiting factor. Conse-
quently, the process is typically regarded as being for small
and medium sized tube and sheet components of relatively
thin gauge material, for example Fig. 7. It is stated [14] that
with suitable tooling and electrode arrangements the EHF
technique can be applied to most of the stand-off operations,
described later, carried out with chemical explosives.
2.3. Explosive forming
The main driver for explosive forming appears to have
been the aerospace industry. In 1960 there were at least 80
government sponsored programmes running simultaneously.
This pattern typically fits with evidence of commercial
activities. For example, Daryl G Mitton founded a company
Fig. 6. An electrohydraulic configuration. Courtesy of Miller (see Table 1).
Fig. 7. EHF examples from the Miller (see Table 1).
D.J. Mynors, B. Zhang / Journal of Materials Processing Technology 125126 (2002) 125 5
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specialising in chemical milling and explosive forming,
which was eventually sold in 1968 [15]. During the com-
panys existence projects included those with NASA where
experimental and production work was undertaken for the
space rockets Mercury, Gemini and Apollo; which included
producing two-thirds of the skin on the Apollo. The com-
pany also explosively formed 33 ft (10 m) diameter domes
on the first stage Saturn Booster Engine and completed work
on many of the US ballistic missile programmes [16]. It was
this type of activity that led to the routine production, at
North American Aviation, of large 2014 aluminium alloy
gore (triangular) segments by explosive methods (Fig. 8).
At Aerjet, 54 in. (1.37 m) diameter domes of 0.125 in.
(3.175 mm) thick AMS 6434 high strength steel, were
explosively formed on a production basis. An additional
number of successful large scale applications emerged.
These included the explosive forming of 2014 aluminium
into 10 ft (3.05 m) diameter domes for an American Air
Force Missile Improvement Programme and 5 ft (1.52 m)
diameter domes on a production basis.
In a research report of the US Defense Advanced
Research Projects Agency (DARPA) [17] explosive forming
is recorded as being a mid-1960s DARPA project. The
project resulted in the development of a cost-effective
process for forming a variety of metals and metal alloys.
The result was a high degree of reproducibility for complex,
large metal structures to tight tolerances. The process was
used extensively in US Department of Defence projects,
the applications included making afterburner rings for the
SR-71, jet engine diffusers for Rohr, Titan manhole covers,
rocket engine seals, P-3 Orion aircraft skins, tactical missile
domes, jet engine sound suppressors and heat shields for
turbine engines.
In addition to the work being undertaken in the United
States it is known that work was being performed in the
United Kingdom at both Queens University, Belfast, and in
Manchester. It is believed that as a result of the research work
at the University of Manchester Institute of Science and
Technology (UMIST) that the company Northern Energetics
Company (NEC) a subsidiary of Reverse Engineering [18]
was formed. The company, which exists today, Table 1, states
that it has a fully instrumented explosive test facility, which
can be used for explosive forming and explosive cutting
operations. There also appears to have been, and still is, an
extensive amount of work undertaken in Ukraine and China.
In trying to determine why the number of publications
associated with explosive forming declined after 1972 Groe-
neveld [19] of Exploform BV suggested a few reasons. The
Fig. 8. Aluminium 2014-0 gore segment for 10 m diameter bulkhead of Saturn rocket, manufactured by North American Rockwell [6].
6 D.J. Mynors, B. Zhang / Journal of Materials Processing Technology 125126 (2002) 125
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year 1972 appears to be the year in which explosive cladding
was discovered, or at least started to be exploited as a
commercial process. At that time the cladding process
was thought more commercially promising and technically
straightforward than explosive forming. Two-thirds of the
American explosive forming capabilities were eliminated
when companies decided to concentrate on cladding even
though it is a very different process. At the same time what
may be called, other non-conventional technologies
appeared, including rubber pressing, stretching and super-
plastic forming. These techniques can be applied in a
standard workshop while the complexities associated with
explosives, the fact that the process is labour intensive
requiring a highly experienced and skilled workforce
ensured there was an obvious interest in these alternative
processes. Groeneveld also thought that many of the com-
panies undertaking explosive forming were often using the
process to manufacture their own products and the process
was eliminated through product redesign.
Laurion of Northwest Technical Industries [20], suggested
that the decline in publications associated with explosive
forming occurred as the first two commercial explosive form-
ingcompanies in theUSstarted toprovideaservice.This ties in
with the increasing number of commercial operations pre-
viouslymentioned. It isbelieved that oneof the twocompanies
ceased to provide explosive forming capabilities in 1998.However, over 30 years of commercial activity it is believed
that companies undertook a significant amount of private
research. The authors believe that these trends were seen
elsewhere except perhaps in China and what was the USSR.
In the 1970s, TNO Prins Maurits Laboratorium (TNO),
Holland, experimented with explosive forming. However, it
was not until 1992 that they defined a research project with
six Dutch companies. From the project, undertaken between
1995 and 1997, it was concluded that explosive forming was
a useful technology for small batches, complex shapes and
difficult materials. Personnel from TNO published a paper
covering some of the work. The paper [21] claims to have
Table 1
Establishments believed to undertake explosive forming commercially
Establishment Product Location
Miller Company Electrohydraulic-forming, produce thin
gauge components from sheet or tube,
see Fig. 7 in text, for examples
2065 S. Burleson Blvd., Burleson, Texas 76028,
USA, [email protected]
Exploform B.V. The product range is extensive and includes:
saddle-shaped products, ring-shaped products
P.O. Box 45, NL 2280 AA Rijswijk,
The Netherlands, Tel.: 31-15-284-36-64,fax: 31-15-284-39-50,email: [email protected]
TNO-Prins Maurits Laboratory The product range is extensive and includes:
parabolic antenna segments, rocket frames,
strongly double curved glare demonstrator parts
P.O. Box 45, 2280 AA Rijswijk, The Netherlands,
Tel.: 31-15-2843695, fax: 31-15-2843954,e-mail: [email protected]
Northwest Technical Industries (NTI). Forms parts for all of the major industries
including the aerospace and medical industries
2249 Diamond Point Road, Sequim, Washington
98382, USA, e-mail: [email protected]
Beijing Explosive Forming Factory Spherical and hemispherical vessels, single
and double-layered (diameters: 3004000 mm,
applications: chemical storage and architectural)
Beijing Explosive Forming Factory, Beijing,
P.O. Box 142-80, Beijing 100854, PR China
Inner Mongolia Polytechnic University Dieless explosive forming of spherical containers
(explosive die-forming of grooved rings)
Department of Materials Engineering,
Inner Mongolia Polytechnic University,
Hohhot 010062, PR China
National Aerospace University
in Kharkov
Significant capabilities, produced
stellarator components
National Aerospace University in Kharkov,
Ukraine
Kharkov Institute for Physics
and Technology
Significant capabilities, produced
stellarator components
Kharkov Institute for Physics and Technology,
Kharkov, Ukraine
Ukranian Kharkov University Aircraft components: aluminium window
frames, turbine housings, wing components
which operate at elevated temperature
Ukranian Kharkov University, Ukraine
Dynamic Materials Corporation Specialise in components that cannot be
formed by traditional means because either
the components are too large or the shapes
are not amenable to traditional tooling,
customers include Boeing and Rocketdyne
551 Aspen Ridge Drive, Lafayette, CO 80026,
USA, www.dynamicmaterials.com,
e-mail: [email protected]
Reverse Engineering Limited Explosive forming and fabrication capabilities Reverse Engineering Limited, Armstrong House,
Brancaster Road, Manchester M1 7ED, UK,
Tel.: 44-0161-2883210, fax: 44-0161-2883211Shock Wave and Condensed
Matter Research Centre
Non-die explosive forming of spherical vessels,
shock consolidation of advanced materials
Shock Wave and Condensed Matter Research
Centre, Kumamoto University, Kurokami
2-39-1, Kumamoto 860-8555, Japan
Oak Ridge Nuclear Facilities The explosive forming facility is frequently
used to form large reactor components
The North of Building 9204-4, Oak Ridge Nuclear
Facilities, US Department of Energy, USA
D.J. Mynors, B. Zhang / Journal of Materials Processing Technology 125126 (2002) 125 7
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demonstrated that explosive forming is a competitor to
superplastic forming and stretch forming. The paper also
confirms that the Ukraine has considerable expertise in this
field. In order to be able to offer explosive forming as a
commercial production technique, TNO and others started a
new company, Exploform B.V. [19].
Even before the initiation of Exploform B.V. others were
actively using explosive forming. In 1981 a car manufacturer
in the former German Democratic Republic started explo-
sively forming rear axle housings. Reporting a 50% invest-
ment cost saving and a 10% material saving when compared
to alternative conventional processes [13].
In the last few years, the fusion research community has
been preparing to build a series of stellorators. They have
undertaken analyses of materials and fabrication techniques
by which to manufacture the vacuum vessels. Fig. 9 [22]
provides an indication of the geometry of the sections
making up a vacuum vessel. In 1998, Oak ridge National
Laboratory contemplated using either 316L stainless steel or
Inconel 625, Fig. 10, with the fabrication process being
brake bending, explosive forming or some form of casting.
In 2000, the Lawrence Berkeley National Laboratory
decided to explosively form the HSX stellorator vacuum
vessel, part of which is shown in Fig. 11 [23]. Finally, in
April 2001 the Princeton Plasma Physics Laboratory [24]
decided to form the NCSX stellorator vacuum vessel. An
insightful quote from the project is as follows:
Fabricating the large, highly contoured vacuum vessel
is one of the technical challenges facing NCSX. A
vacuum vessel forming study was recently completed
by Aleksandr Georgiyevskiy and a team of engineers
and scientists from the National Aerospace University
in Kharkov, Ukraine (KhAI) and the Kharkov Institute
for Physics and Technology (KIPT). The study recom-
mended explosive forming as the preferred process for
vacuum vessel forming. KhAI and KIPT have extensive
experience in explosive forming. They hold eight
patents on the process and have successfully employed
it in numerous applications, including the helical wind-
ing shell on the Uragan-3 stellarator. In explosive
forming, springback in forming individual panels
is avoided by multiple, high impulse cycles. Weld
distortions during assembly of the individual panels
are removed by explosively forming the welded article
back to the geometry of the die after welding. It is
anticipated that fabrication of one or more sub-scale
(perhaps 1/3 scale) vacuum vessel segments will be
funded in FY02 prior to selection of the vacuum vessel
forming process and fabricator. Inconel 625 (or equiva-
lent) 0.375 in. thick.2
In addition to the applications mentioned above a variety
of other forms have been fabricated including:
dome shapes (Fig. 12); beaded panels; large shallow reflectors;
Fig. 9. Vacuum vessel schematic [22].
2 From Barbara Sobel, Weekly Highlights for 20 April 2001 (23 April
2001, Monday, 13:46:02 EDT), [email protected].
8 D.J. Mynors, B. Zhang / Journal of Materials Processing Technology 125126 (2002) 125
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Fig. 10. Schematic diagram of the Oak Ridge vacuum vessel.
Fig. 11. HSX vacuum vessel section [23].
D.J. Mynors, B. Zhang / Journal of Materials Processing Technology 125126 (2002) 125 9
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shallow and deep rectangular boxes; manhole access covers; equipment covers; large cylinder parts; turbine housings, Fig. 13, made from heat resistant steel,
of 1.5 mm thickness and 540 mm diameter [21];
Ti5Al2.5Sn alloy Turbojet engine cases formed fromconical performs;
exit cone components from Inconel 718 sheet.
Fig. 13 shows a large cylindrical, 5052 H32 aluminium
alloy, part that forms the liquid oxygen manifolds for the
booster of a large missile and has been formed by a series of
explosions. The first firing improved the circular dimensions
of the part. Next smaller bags filled with water and contain-
ing small charges were used to flange each conventionally
cut hole [25].
Spherical vessels of diameters ranging from 300 to
4000 mm have been produced using dieless explosive form-
ing. Applications include chemical storage vessels, architec-
tural decorations, hemispherical domes and double-layered
vessels for storing dangerous and poisonous materials [26]
(Fig. 14).
Although explosive forming is often utilised for large
component production it is just as applicable to smaller parts
including, for example, the production of stainless steel
denture bases and metal implants for dental and orthopaedic
surgery.
In general according to the publications seen by the
authors and reported comments of the Dynamic Materials,
Table 1, every metal that has been subjected to explosive
forming has been successfully formed. Although of course
this may not be the case, the renewed interest in explosive
forming is likely to stem from an increased use of materials
which are hard to form, since the high strain rates as
experienced in explosive forming can improve the form-
ability of some materials. This effect can be seen in Fig. 15
where uniform stain is plotted for a range of forming
velocities and Fig. 16 which provides an indication of the
relative formability, under explosive conditions, of a range
of metals.
What is apparent from examining the accessible data is
that although explosive forming publications declined for a
range of reasons after 1972 the process is still used. Table 1
contains details of several companies and other establish-
ments that appear to undertake explosive forming on a
commercial basis.
Having examined some examples of the type of compo-
nents that can be produced, the next step is to examine the
type of equipment required to undertake explosive forming.
3. Explosive forming equipment and its operation
The main elements associated with explosive forming are
the explosives, the dies, the energy transfer medium and the
Fig. 12. Dome shaped components courtesy of Dynamic Materials (Table 1).
10 D.J. Mynors, B. Zhang / Journal of Materials Processing Technology 125126 (2002) 125
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Fig. 13. Explosive flanged 5052 H32 aluminium alloy cylinder.
Fig. 14. Spherical vessel produced using dieless forming [26].
D.J. Mynors, B. Zhang / Journal of Materials Processing Technology 125126 (2002) 125 11
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physical site. These can be arranged in a variety of ways
depending on the component geometry and the numbers
required. Here the configurations used for tubular and sheet
components will be examined, as will aspects of the main
elements associated with the process.
3.1. Explosives
An explosive can be described as a substance or device
that can produce a sudden high pressure burst of gas. The
most widely used type of explosives in forming are chemical
explosives which can be classified into two general cate-
gories depending on the speed of the chemical reaction:
deflagrating or low explosives and detonating or high explo-
sives. Low explosives such as gunpowder or cordite are
mostly used as propellants in guns or rockets [6] and develop
pressures of 0.28 GN/m2 sustained over relatively long
periods. Low explosives have not found much use in explo-
sive forming.
High explosives are of two types, primary and secondary.
Small quantities of primary explosives, which are quite
sensitive and easily detonated, are used in detonating caps.
Secondary explosives have a much higher energy content
than primary explosives, but are much less sensitive, and can
be detonated only by a sudden and intense shock, as
provided by a detonating cap.
Explosive metalworking exclusively employs secondary
explosives such as dynamite, PETN (pentaerythritol tetra-
nitrate, C5H8N12O4), TNT (trinitrotoluene, C7H5N3O6), and
RDX (cyclotrimethylene-trinitramine, C3H6N6O6). These
tend to produce a relatively short pulse in the high pressure
range of 13.827.6 GN/m2. Primacord and sheet explosive
have been widely used in forming as they are easily handled
and can be cut to size with a knife. However, for large metal
forming applications requiring 10 kg or more of explosive,
pressed or cast explosives are often more convenient as they
can be machined to very close weight and dimensional
Fig. 15. Effect of forming velocity on the maximum uniform strain [27] (1 in./s (ips) is 0.0254 m s1, 100 ips is 2.54 m s1, 700 ips is 17.78 m s1, etc.).
Fig. 16. Relative formability of a range of metals.
12 D.J. Mynors, B. Zhang / Journal of Materials Processing Technology 125126 (2002) 125
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tolerances. Accurate explosive processing is often exploited
in scaled down forming trials.
A simple comparison between low explosive, high explo-
sive and a typical forming press can be provided as: 1 kg of
high explosive provides as much useful energy as 5 kg of
low explosives; and 1.5 kg of high explosive provides as
much useful energy as a 7.5 MN press [16].
Generally, the chemical explosive, a cylindrical or point-
charge is located near the centre line of a tubular workpiece
or at a distance from a sheet workpiece. In the forming of
large parts, a number of charges can be distributed appro-
priately. Typically, the aim is to achieve the required defor-
mation in the least number of operations, using the largest
permissible weight charge. Although referring to forming
route described for the NCSX stellorator previously multiple
operations can be advantageously used.
During the detonation of a mass of solid secondary high
explosive, a detonation wave travels from the point of
initiation through the explosive, converting it almost instan-
taneously into a mass of gas with pressures in the order of
34 106 lb/in.2 (23 104 MPa) in close proximity tothe charge. The explosive reaction also releases large quan-
tities of heat. The expansion of this high temperature, highly
compressed gas bubble against its surroundings provides the
energy for explosive forming. In very imprecise terms, the
volume of gas liberated by an explosive is approximately
1000 cm3/g of explosive.
3.2. Experimental configurations for tubular component
manufacture
Explosive forming of tubes often occurs in what is called a
closed die system, Fig. 17, often with a split die arrange-
ment. In this closed system arrangement the die completely
encloses the explosive, with the workpiece positioned
between the forming surface of the dies and the explosive.
The location of the explosive during the forming of a tube is
critical and is normally along the central axis of the tube or
tubular perform. A vacuum is produced between the work-
piece and the die surface to ensure that air is not trapped
during the forming process. Trapped air or lubricant pro-
vides resistance to the workpiece movement and when
compressed can explode damaging the die and the work-
piece. Under normal explosive forming conditions the die
cavity deteriorates progressively as a result of gas erosion.
The debris that results may mark the surface of the work-
piece. In extreme cases catastrophic die failure, which is
hazardous, can occur.
It was reported [14] that die costs become uneconomical
for the forming of tubular shapes of diameters greater than
50 mm. Despite this, there are several examples of produc-
tion uses and complex operations. In 1973 it was reported
that split dies made from tool steel were being used in the
routine high volume [28] production of artificial limbs. The
limb components were produced to a high degree of accu-
racy with good surface finish from spun aluminium alloy
performs with a basic tubular form.
Single multi-impression dies have been reported as being
used for cup production from tubes at a rate of 2436 h1.
Although, just as with all forming operations it is the design
of the performs and dies that is important, the explosive
selection and configuration [28] are also significant. The
explosive forming of tubular parts typically allows the
formation of any final profile desired as a single piece.
The elimination or reduction of the need for joins allows
components to be used more reliably in many corrosive
environments.
When comparing explosive tube forming with tube hydro-
forming [29] there are a considerable number of similarities,
including reduced joint numbers in complex components
and the capability of both processes to simultaneously form
and punch as part of a single operation. An example of this is
the explosive forming of a 316 stainless steel tube [27] of
inner diameter 130 mm, of wall thickness 1 mm and length
2.3 m which was sized, formed and punched in a single
operation. The final formed tube had 27 countersunk and
perforated rivit dimples, two circular holes, of diameters 19
and 25 mm, and an elliptical slot 15.7 mm wide and
59.5 mm long. The final internal diameter was 132 mm.
The operation was completed in a submerged split die with a
vacuum between the workpiece and the tooling.
What is not evident from the literature are the techniques
used to manufacture the tubes to be explosively formed. This
is very surprising as in hydroforming a considerable empha-
sis is placed on the tube manufacturing process, normally
extrusion or the welding of rolled sections. The tube proper-
ties and, if present, the quality of the weld being significant
factors in the formability and quality of the final product.
3.3. Experimental configurations for components
manufactured from sheet
In the simplest of terms, the formation of parts from sheet
requires that the sheet is forced on to the die and the most
common arrangement is an open die system as shown in
Fig. 18. The variables in the process are similar to those
associated with deep drawing. The workpiece is clampedFig. 17. Closed die forming system [14].
D.J. Mynors, B. Zhang / Journal of Materials Processing Technology 125126 (2002) 125 13
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over a die; the air in the die cavity is evacuated. The vacuum,
typically 3 mmHg, is drawn from the lowest point of the die.
The workpiece and die arrangement is lowered into a water
filled tank. An explosive charge is then positioned at a
prescribed height and detonated. Other configurations are
possible with either a container full of water, Fig. 19, or a
plastic bag full of water, Fig. 20, placed on top of the
workpiece. The process is often referred to as stand-off
explosive forming, with the stand-off distance being the
distance between the workpiece and the explosive. As
explosives dissipate energy in all directions, only a fraction
of is directed towards the workpiece. Under normal circum-
stances sufficient energy is available if the distance from the
charge to the surface of the water is twice the distance from
the workpiece to the charge. The female dies shown in
Figs. 18 and 19 are generally the most often used config-
uration. Male die configurations, Fig. 20, cannot be sub-
merged. The energy transfer medium can only be placed
above the workpiece. Water below the deforming metal
would cause the metal to bounce of a barely compressible
medium. A third configuration is the free forming die,
Fig. 19, where the workpiece is formed through an orifice.
This cannot be submerged in water for similar reasons to
those stated for the male die arrangement. There has also
been some work using dieless forming arrangements in this
case formed sheet pieces are welded together to form the
container into which an energy transfer medium, typically
water, is placed. The explosive charge is then lowered into
the container, centred and detonated deforming the con-
tainer. This technique, still a stand-off process, has been used
in the forming of very large spherical vessels (Fig. 14). In
addition to stand-off forming contact forming is also used.
As the name suggests the explosive is placed in contact with
the workpiece and detonated resulting in a shock wave being
generated directly in the metal as opposed to the energy
transfer medium.
Fig. 18. Schematic diagram of a stand-off explosive forming process.
Fig. 19. Schematic diagram of a stand-off explosive forming using a free
forming die.
Fig. 20. Schematic diagram of a stand-off explosive forming using a
male die.
14 D.J. Mynors, B. Zhang / Journal of Materials Processing Technology 125126 (2002) 125
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3.4. The energy transfer media and the forming dies
It is important to be able to estimate the amount of energy
delivered by an explosive charge to a metal workpiece. For a
given type and size of charge, the efficiency of energy
transfer depends greatly on the configuration of the work-
piece relative to the charge and the properties of the energy
transfer medium between the charge and the blank. A range
of transfer media have been identified including water, the
most common, air, plasticine effective in operations invol-
ving the deformation of localised areas of the workpiece, and
jelly [30] which reduces the physical working restrictions.
Water is more commonly used than air as it has a much
higher energy transfer efficiency.
In the forming of a component from a sheet, Fig. 18, the
chemical explosive is detonated underwater and a gas bubble
is produced under high pressure. A primary shock wave
travels out from the gas bubble through the surrounding
water. At a short distance from the source, this primary
shock wave carries with it about 50% of the total energy of
the charge. The gas bubble expands until its internal pressure
drops below that of the surrounding water pressure. The
expansion eventually ceases and the bubble begins to con-
tract until it reaches a minimum size. The reduction in size
results in increased pressure within the bubble. When the
pressure increases sufficiently the reduction in size stops.
The pressure is greater than its surroundings and it begins
to expand again. At the beginning of the second expansion,
a secondary shock wave is generated in the water. A
secondary shock wave is emitted each time the bubble
reaches a minimum size. The secondary shock waves carry
only a small fraction of the total energy, and contribute little
to a metal forming operation.
The primary shock wave in the fluid impinging on a blank
imparts to it an initial velocity. The blank motion in turn
sends a rarefaction wave back into the water. This lowers the
pressure in the water adjacent to the blank until cavitation
occurs. The blank slows due to the restraints at its boundaries
and the water between the gas bubble and the cavitated area
catches up with it. The resulting impact is referred to as the
reloading phenomenon and delivers even more energy to the
blank than the primary shock wave; this has been verified
experimentally. For reloading to occur, the head of the water
above the charge must be greater than twice the stand-off
distance. If this is not the case the bubble bursts at the water
surface and the energy transfer process stops. The primary
shock wave and the reloading do the majority of the work in
a metal forming operation.
The complex phenomenon of energy transfer from an
underwater explosion has not yet yielded a complete math-
ematical description. Approximate methods are available for
estimating the total energy delivered to a blank, the three
common methods are:
1. The geometrical method, based primarily on the specific
energy of the explosive and the configuration of the
workpiece relative to the charge. The influence of the
energy transfer medium is expressed relatively by an
empirical factor.
2. The energy method, based on empirical energy density
formulae derived from measurements of underwater
explosions that include the reloading effect. The energy
density is integrated over the area of the workpiece. The
use of this method is limited to explosives for which the
appropriate empirical energy constants have been
determined.
3. The impulse method, also based on empirical formulae
obtained from the measurement of shock pressures from
underwater explosions can only be used when the
energy transfer medium is water and when the reloading
phenomenon is absent.
Both the first and the second methods provide upper
bound estimates of the explosive energy delivered to the
workpiece from both the primary shock wave and reloading
phenomenon. The third method gives a lower bound esti-
mate based on empirical pressure and impulse formulae and
does not include the energy delivered in the reloading
phase.
As an example the peak pressure P generated where the
transfer medium is water, can be given by the expression [6]:
P k w2=3
R
a(1)
where P is the peak pressure in pounds per square inch, k a
constant that depends on the explosive: 21600 for TNT, w
the weight of explosive in pounds, R the distance of the
explosive from the workpiece (stand-off) in feet and a a
constant, generally taken as 1.15.
An important factor in determining peak pressure is the
compressibility of the energy-transmitting medium and its
acoustic impedance, the product of the mass density and
sound velocity in the medium. The lower is the compres-
sibility, the higher is the density of the medium and the
higher are the peak pressures. To estimate compression
under the forming conditions it is useful to know that
detonation speeds are typically 22.2 ft/s (6.8 m s1), while
the speed at which the metal is formed is estimated to be in
the order of 100600 ft/s (30200 m s1).
Here only simple stand-off configurations have been
considered, the wave propagation phenomena is consider-
ably more complex in closed die systems where there is a
greater proportion of reflective surfaces although the energy
density of the reflected wave will also depend on the quantity
of explosive.
The difference between the energy radiated by the explo-
sive and that absorbed by the workpiece will be absorbed by
the forming system, mainly the die. To ensure the die
behaves elastically the die must absorb a considerable
fraction of the remaining energy, without yielding, and so
the duration of the loading and the rate of loading felt by the
die must be considered.
D.J. Mynors, B. Zhang / Journal of Materials Processing Technology 125126 (2002) 125 15
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One way that has been used to establish the time duration
of the die loading, on a cylindrical explosive forming die was
to instrument it with electric resistance strain gauges mea-
suring the hoop strains on the outside of the die. The die was
made of thick walled steel tubing with an inside diameter
of 6.75 in. (171 mm) and an outside diameter of 10.50 in.
(267 mm) and a length of 1.62 in. (41 mm). A 2 in.
(50.8 mm) diameter length of 400 g/ft PENT primacord
explosive was placed symmetrically in the die and deto-
nated. The resulting strain trace showed an oscillatory
structural vibration of the die as a high frequency ripple
superimposed on a much longer period of expansion. The
period of the structural vibration was about 124 ms/cycle,approximately 8066 Hz. The general trend of the expansion
period persisted for over 1 ms. The relatively gentle rise of
the straintime curve in the early stages shows that a strong
shock wave was not generated in the die. This suggested that
the impact of the workpiece on the die was not a violent
phenomenon and that the rate of onset of the reloading was
well behaved. The amplitude of the ripple due to the
vibration of the die was a little less than 20% of the
amplitude of the overall expansion. Thus, the existence of
the ripple can be accounted for by using a modest safety
factor consequently; the use of a quasi-static analysis to
design is justified. This conclusion has been validated for
cylindrical forming dies in which there is considerable strain
of the workpiece.
The choice of die material depends on the service con-
ditions, the number of components, the surface finish and
tolerances required. For a very few parts or to finalise the die
design or the inter stage heat treatment requirements, weak
inexpensive materials such as concrete can be used. How-
ever, if small explosive forces are being used glass fibre
reinforced epoxy resins are an alternative die material.
Fabricated mild steel dies supported in concrete, can be
used for larger components. To form many parts, more
durable materials are required. Ductile cast iron is desirable
for high pressure intensities and frequent use. Cast steel will
give longer production runs but with poor surface finish. For
a high quality surface finish and long production runs then
precision machining of tool steel is recommended, an exam-
ple of which is shown in Fig. 21. The die is used to form the
vacuum bags used during the brazing cycle of the space
shuttle engines. The bags are mated to the nozzle and a
vacuum pulled to hold 1080 tubes in place during the brazing
cycle where tolerances are critical. The die is a parabolic
shape, 36 in. (914.4 mm) in diameter at the bottom, 91 in.
(2311.4 mm) diameter at top, and approximately 150 in.
(3810 mm) high. It was gas metal arc welded from
1.25 in. (317.5 mm) thick plate and 2.25 in. (57.15 mm)
thick rolled and welded rings using 0.045 in. (1.143 mm)
diameter wire for all welds. The final wall thickness of the
die is 1/2 in. (12.7 mm) minimum. However, the contour is
such that after the die was welded together a considerable
Fig. 21. A formed, welded and machined explosive forming die.
16 D.J. Mynors, B. Zhang / Journal of Materials Processing Technology 125126 (2002) 125
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amount of material was removed by machining to achieve
the finish tolerances [31].
When designing explosive forming dies the key thing to
remember is that shock waves will be passing through the
metal resulting in compressive and tensile conditions. In
addition, waves are reflected from surfaces and hence
reflecting surfaces should not result in the focussing of
waves to a high degree of intensity especially if the result
is sufficiently intense that possible die splitting will occur. If
the die material is strong in compression but weak in tension,
for example concrete, it should be encased in a denser and
stronger material with a higher specific acoustic impedance.
This ensures that compressive shock waves reflected off the
boundaries of the die will be reflected back as compression
rather than tension waves.
3.5. The actual site and associated costs
The requirements for explosive forming depend on the
size of the components to be formed, the die configuration
and the transfer medium. In the case of large components
using a female die then very often a water filled tank sunk
into the ground is required. The die, workpiece and charge
assembly are lowered into the tank. Generally, the tank is
built by excavating a hole and lining it with concrete. In
some cases this is sufficient and can be used as the com-
pleted tank. However, concrete is likely to contract and
expand as a result of repeated use, resulting in the formation
of cracks. Typically, a waterproof liner is required normally
rubber or mild steel. If a steel liner is used then often, the
resulting tank has a diameter equal to commercially pro-
duced cylindrical tube with the tank base being a separate
steel plate. Building the tank into the ground ensures any
failure in the tank, backing layers or die are contained within
the system and the surrounding soil.
Teotia [32] in the early 1970s undertook a very compre-
hensive study into the cost of building an explosive forming
facility. Included in the study were the complete design rules
for tanks expected to withstand the blast from between 5 and
500 lb (1 lb 454 g) of TNT, the cost of excavation, posi-tioning the steel liner, surrounding it with concrete and
backfilling with soil. Also in the analysis were the costs
associated with a water heating system and all the associated
filter considerations if the ambient temperature was below
13 8C and likely to affect the workpiece material properties,the safety fence around the facility, etc. In addition, consid-
eration was given to the need for the equipment at the site
capable of lowering the die workpiece assembly into the tank,
producing a vacuum between theworkpiece and the die and an
area to prepare and store explosives. Hence, costs associated
with mechanical handling facilities, vacuum-pumping equip-
ment and a building for the preparation and storage of
explosives were considered. The analysis was then compared
with the actual costs of building three such sites in the US.
The capital cost of an explosive forming facility are
reported as being less than that of a conventional facility
of equal capability by a factor ranging from 10:1 to 50:1. On
the other hand, labour costs per part can be appreciably
higher for explosive forming.
At the other end of the scale small components can be
manufactured using water filled plastic bags on top of the
ground with no more site preparation than that required for
the safe handling of explosives and the relative positioning
of the die and the workpiece. If very small parts are being
considered such as dental fixtures then the forming can be
completed almost in a vessel the size of a mug placed on a
table. There are reports that for dies up to 500 mm in
diameter and 100 mm deep the Explo-forma machine can
be used for automatic production of components [33,34].
4. Achievable tolerances
The precision obtainable with explosively forming can be
illustrated by the rectangular tubes with corner radii of
1.3 mm that have been formed [20]. In addition, tolerances
of 0.025 mm have been obtained on small explosivelyformed parts. However, working tolerances are directly
related to the amount, distribution and duration of the
pressure acting on workpiece. In general as a result of the
difference in springback behaviour [53] tolerances achieved
by explosive working can be equal to or better than those
obtained by conventional forming.
For a given die forming operation, it appears that the
amount of springback can be controlled to a certain extent,
depending on the material, by varying the amount of explo-
sive and the stand-off distance. Increasing the charge size or
reducing the stand-off distance increases the force and hence
deformation seen by the workpiece. The increased force is
transmitted through to the die whose elastic deformation will
be greater than before. Hence, the additional deformation
seen by the workpiece will compensate for its elastic recov-
ery and the final workpiece dimensions will be closer to
those of the elastically recovered die. The difference
between the final workpiece dimensions and the die dimen-
sions after explosive forming metal sheet into a die first
decreases, almost linearly, with an increase in charge weight
and then becomes approximately constant at a value which
depends on material characteristics such as hardness and the
modulus of elasticity.
Table 2 shows the tolerances achieved in forming large
domes with diameters in the range of 10001500 mm from
AMS 6434 high strength steel, with thicknesses ranging
Table 2
Tolerances obtainable when explosive forming large domes
Dimension Tolerance (mm)
Normal Possible
Diameter 0.254 0.128
Thickness 0.100 0.050
D.J. Mynors, B. Zhang / Journal of Materials Processing Technology 125126 (2002) 125 17
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from 2.3 to 3.8 mm [25]. As a comparison tolerances of
0.030.2 mm have been reported for the deep drawing of
components with diameters of 500 mm [36].
5. The mechanical properties of metal after explosiveforming
Having looked at the type of products and the experi-
mental configurations it is necessary to examine the proper-
ties of metals after they have been explosively formed. This
section summarises the hardness, tensile behaviour, fatigue,
ductility, and the fracture toughness of the deformed metals,
thus providing an indication of the mechanical properties of
explosively formed components.
5.1. Hardness
Listed in Table 3 are some comparative hardness values.
The comparison is between uniaxially statically strained
materials, iron [37], mild Steel [38,39] and aluminium with
those that have been dynamically strained. For iron and
steel, the data indicates, with one exception, that hardness
and hence workhardening is less as a result of dynamic
deformation in the strain rate range of 10103 s1 than
during static deformation to an equivalent strain.
Similar trends are reported to occur under forming con-
ditions. The hardness of 0.025% C mild steel free-formed
explosively to a 10 and 40% reduction in thickness was
found to be 8 and 4%, respectively, lower than that of their
hydraulically formed equivalents [40]. Williams [41] also
reported less strain hardening in explosively formed mild
steel, presumed to be 0.08% C, and, as an aside, in commer-
cially pure titanium. The one reported exception is the
increased hardness reported for the 0.05% C steel used by
Harris and White [39] following dynamic loading. It has
been suggested that ageing or non-uniform strain distribu-
tions may be possible factors contributing to this anomalous
result.
Conversely, aluminium, Table 3, copper [64], nickel alloys
[41] and austenitic CrNi stainless steel [43,44,6567] tend
to workharden more during explosive forming than during
forming at lower strain rates. The hardness after precipita-
tion-hardenable aluminium alloys have been either statically
or dynamically strained is almost identical for both
[45,46,65,68]. Thus indicating that when the dislocation
substructure and other lattice defects contribute little to
hardening then hardness is not very sensitive to strain rate.
The behaviour of the CrNi austenitic steels is further
complicated by the effects associated with the rate depen-
dence of the formation of the hexagonal closed packed e-phase and body centred cubic martensite.
5.2. Strength
In order to compare material strengths, resistance to
deformation or fracture, a comparison of flow stress mea-
surements, resistance to frictionless plastic deformation, has
been made [8,36]. Samples of each material were separately
strained either dynamically or statically to an agreed value,
the pre-strain column in Table 4. Each sample was then used
to traditionally measure static flow stress. The total resultant
percentage strains, pre-strain plus the strain as a result of
flow stress determination, are shown in the total strain
column of Table 4. The flow stress values for the statically
and dynamically pre-strained samples are shown in columns
four and five, respectively.
As seen from Table 4, iron and mild steel with static pre-
straining, invariably leads to a higher flow stress than those
with dynamic pre-straining. This links directly to the hard-
ness results described in the previous section. The work-
hardenable AlMg alloy exhibits behaviour similar to that of
iron and mild steel. However, in line with the hardness
information the majority of the data for high purity alumi-
nium indicates a reverse trend. The greatest benefit to the
final strength of aluminium from a higher rate, dynamic, pre-
strain can be seen to be at small additional static strain, for
example 99.95% pure aluminium with 14.2% pre-strain
and 15.0% final strain or 99.99% pure aluminium with
5.5% pre-strain and 7.0% total strain (Table 4).
Relative ductilities, as measured by total elongation and
reduction in area at fracture, have been investigated. No
general rule has been formulated to relate ductility to pre-
strain rate for an alloy group. Nor is there a general relation-
Table 3
A comparison of hardness after static or dynamic uniaxial pre-strain
Material Method used to
apply static strain
Percentage
strain (%)
Method of
measuring hardness
Hardness values Difference in
hardness (%)a
Statically
applied strain
Dynamically
applied strain
Armco iron Compression 2.6 Vickers 105 95 10Mild steel (0.05% C) Tension 5.5 Vickers 162.4 212.0 49.5Mild steel (0.2% C) Tension 8.0 Vickers 155 151 4Mild steel (0.24% C) Compression 4.1 Brinell 126 113 13Aluminium Not reported 35 Vickers 32.3 33.8 1.5
a Defined as % difference in hardness hardness of dynamically strained sample-hardness of statically strained sample=hardness of statically pre-strained sample 100%:
18 D.J. Mynors, B. Zhang / Journal of Materials Processing Technology 125126 (2002) 125
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ship between the pre-strain ratestrength link and ductility
for a given alloy.
5.3. Fracture toughness
From the available evidence, summarised below, explosive
forming does not appear to have any appreciable effect upon
fracture toughness. Agricola et al. [49] compared unformed
and explosively formed materials that had experienced the
same heat treatment. The heat treatment was applied after
forming. Plane strain fracture toughness, KIc, values were
established for solution heat treated and artificially aged
aluminium 7039-T62, HP9-4-25, D6AC alloy steels,
12Ni5Cr3Mo and 18% nickel maraging steels. Forming
was conducted using a flat bottomed female die. Some
variation in the fracture toughness values were noted for
the 7039-T62 aluminium. However, no adverse trends were
apparent as a result of the forming operation. A statistical
analysis conducted on the results obtained with the 18%
nickel maraging steel indicated the difference in KIc between
parent and formed stock was marginal. Similar conclusions
were drawn from the D6AC steel results. Explosive forming
appeared to have no effect on the fracture toughness values of
HP9-4 steel and 12Ni5Cr3Mo nickel maraging steels.
5.4. Fatigue behaviour
It is important for a designer to know the effect a manu-
facturing process has on the fatigue life of a component.
In addition to absolute values of fatigue it is important
that a comparison of the results of specific processes be
made. Mikesell [50] compared slow rubber pressed and
explosively formed 2014-T6 aluminium alloy. A statistical
analysis of the results demonstrated that the fatigue life
was not influenced significantly by the deformation pro-
cess, irrespective of the process type as the fatigue life of
the deformed materials differ little from the undeformed
material.
Similarly, Eftestol et al. [51] compared the effect of
conventional hydraulic forming with stand-off explosive
forming on the endurance limit of austenitic, 304, 316,
and 347 stainless steels. They found that the explosively
formed components had an increased, beneficial, endurance
limit of more than 10% for 304 and 347 stainless steels when
compared with the conventionally formed components. The
data for 316 stainless steel indicated that, at worst, there was
no difference between the fatigue strengths of the material
formed by the two methods.
The low-cycle fatigue studies completed by Baudry and
Cooper [52] revealed an improvement in the fatigue proper-
ties of austenitic manganese steels after explosive forming as
compared with conventional forming.
Williams [53] reported that the fatigue strength of 13
explosively cupped metals and alloys was generally similar
to that following similar static forming. Investigated were
seven basic metals, five heat-treatable alloys, and two non-
heat-treatable alloys representing the three common struc-
tures: face centred cubic, body centred cubic and hexagonal
Table 4
A comparison of flow stresses after static or dynamic pre-straining
Material Pre-strain (%) Total
strain (%)
Static flow stress
values from samples
subjected to static
pre-straining (MPa)
Static flow stress
values from samples
subjected to dynamic
pre-straining (MPa)
Difference in flow
stress: dynamically
and statically
pre-strained samples
Difference
in flow
stresses (%)a
Armco iron 2.5 2.7 224.1 206.2 2.6 8.0
Mild steel (0.025% C) 7.8 8.0 262.0 229.6 4.7 12.415.7 15.9 308.9 252.4 8.2 18.3
Mild steel (0.2% C) 2.9 4.4 328.9 266.8 9.0 18.92.9 5.9 356.5 334.4 3.2 6.2
Mild steel (0.2% C) 1.2 1.6 362.7 288.2 10.8 8.02.0 383.4 337.9 6.6 3.8
4.2 5.0 532.3 490.2 6.1 20.57.0 600.6 572.3 4.1 11.9
Stainless steel (AISI 304) 5.0 5.2 343.4 339.9 0.4 0.8
15.0 15.2 468.9 519.2 7.3 10.7
Aluminium (99.95%) 14.2 15.0 45.2 47.2 0.30 4.6
30.0 48.3 49.6 0.19 2.7
Aluminium (99.99%) 5.5 7.0 48.7 54.1 0.77 10.9
11.0 56.8 58.8 0.29 3.5
Al2.5Mg alloy (5056-O) 5.0 5.2 241.3 228.2 1.9 5.415.0 15.2 363.4 355.1 1.2 2.3
a Defined as: % difference in flow stress flow stress of dynamically pre-strained sample flow stress of statically pre-strained sample=flow stressof statically pre-strained sample 100%:
D.J. Mynors, B. Zhang / Journal of Materials Processing Technology 125126 (2002) 125 19
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closed packed. A random examination of other explosively
formed parts tended to confirm this result [53].
In summary, the evidence reported suggests that explosive
forming produces material fatigue properties, which either
differ little, aluminium alloys, from those produced by more
conventional processes, or represent a significant improve-
ment, stainless steels, in fatigue strength so alleviating
designers qualms.
6. Microstructure after explosive forming
In order to understand the microstructure of explosively
formed material it is necessary to isolate the factors that
influence microstructure. There are effects from the shock
waves passing through an energy transfer medium and
impacting on the metal and the effects that result from
shock waves generated in the metal when contacting the
die or when explosives are detonated while in contact with
the metal. To examine the effect from media transmitted
shock waves a stand-off free forming die configuration is
typically used as this eliminates a considerable amount
of die workpiece contact during forming. In this free form-
ing arrangement the shock wave peak pressure will be of
order of 100500 MPa and the ensuring strain rate range is
102103 s1. The difference between the microstructure fol-
lowing explosive forming and that after lower-rate forming
can be seen in the variations in the density and distribution
of lattice defects such as dislocations, vacancies, stacking
faults, mechanical twins and the amount of strain-induced
transformation products in alloys normally susceptible to
such transitions. In the following sections twinning, point
defects and phase transitions will be examined.
6.1. Twinning
It is commonly accepted [6] that as the deformation rate
increases twinning can become the preferential deformation
mode in many metals so releasing local internal stresses
while at lower strain rates slip relieves local internal stress.
Generally body centred cubic metals predominantly form
twins as a result of explosive free forming [27]. Twins have
been observed in explosively formed mild steel but not in
steels with higher carbon contents. It has been postulated
that iron and mild steel with less than 0.2% carbon will twin
at dynamic strain rates characteristic of explosive forming.
Williams [41] and Hollingum [42,54] observed twins in
explosively free-formed mild steel while no twins were
detected in the slowly pressed material. Twins have also
been perceived in explosively formed 0.1% carbon steel
[55]. Further more, deformation by twinning is expected in
tungsten (bcc) and chromium (bcc) but not in molybdenum
(bcc), niobium (bcc), vanadium (bcc), and tantalum (bcc) at
the rates associated with explosive forming. Low and high-
alloy high strength steels should also be immune to this
mode of deformation during high energy rate forming.
Considerable controversy still exists concerning the exis-
tence of twins in dynamically deformed, low stacking-fault
energy, austenitic NiCr stainless steels (AISI 300 series).
On the one hand, van Wely and Verbraak [56] discovered
markings in explosively die-formed 304 stainless steel
dishes, which were interpreted as deformation twins gener-
ated by the high strain rate, not die impact. On the other hand
there are no twins in 301, 304, 316, 347, type stainless steel
after explosive forming. The free forming of 321 stainless
steel resulted in 1030% of the microstructure being classi-
fied as twinned, contrasting with the 14% after hydrostatic
forming. Twins have also been detected by transmission
electron microscopy [57] in 321 stainless steel pans which
had been stand-off formed into a die.
In the case of commercially pure titanium (hcp) Williams
[58] revealed only somewhat less twinning after slow press-
ing than after explosive forming to the same strain.
The different twining behaviour of metals may be attrib-
uted to the difference in lattice friction and stacking fault
energy. A lower interfacial energy, stacking fault energy,
ought to reduce the initiation energy for twin nucleation.
Explosive forming inputs a greater amount of energy than
low rate forming and it is possible that for some metals the
input energy will exceed the energy required for the initia-
tion of twin nuclei. While for other metals which have a
higher stacking-fault energy, which require higher energy
for the formation of twin nuclei, the input energy from
explosive forming may still not be sufficient to induce twin
nucleation.
Another factor is the lattice friction of a material. The
higher is the lattice friction the greater is the local internal
stresses generated when the material is explosively formed,
and hence the likelihood of twinning is enhanced.
6.2. Point defects
It is well known that the faster dislocations move the
greater the number of vacancies, interstitials and other point
defects [40] are present. It has often been assumed [54,56] to
explain enhanced diffusion and, thereby, ageing effects,
that a higher density of vacancies is produced during explo-
sive forming than during conventional forming. van Torne
and Otte [59] did argue that the growth of dislocation
loops during the annealing of explosively formed 2219-O
aluminium was strong evidence for the existence of a non-
equilibrium vacancy concentration. Since no comparison was
made with material deformed at a lower rate, no strain rate
history dependence could be definitely established.
6.3. Phase transitions
Those alloys which are normally subject to strain-induced
phase transformations will respond differently at different
rates of deformation. When 301 stainless steel was deformed
dynamically in uniaxial tension [60], transformation pro-
ducts were not detected even in electron micrographs of
20 D.J. Mynors, B. Zhang / Journal of Materials Processing Technology 125126 (2002) 125
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surface replicas. However, the presence of body centred
cubic martensite was revealed by X-ray diffraction, in a
greater amount than in material deformed slowly to the same
strain. Conversely, uniaxial tensile loading of 302 [41] and
304-type [42] corrosion resistant stainless steels resulted in
less martensite after dynamic than after static straining.
More extensive studies of 321 stainless steel have dis-
closed similar discrepancies. Williams [58] observed lath-
shaped body centred cubic martensite as one of the main
kinds of martensite in explosively formed material. He
concluded that less martensite was present after slow press-
ing of the same material. However, Hollingum [42] did not
find evidence that the amount of this phase was dependent on
forming rate. Van Wely and Verbraak [56] also perceived a
body centred cubic martensitic phase both in 321 pans
explosively die-formed with stand-off and in 321 sheet
shock-loaded by explosives in contact with the surface.
In contrast, DAguanno and Pfanner [47,48,61] attributed
similar deformation features in explosively and electrohy-
draulically formed 321 to mechanical twins.
Murr and Grace [62] discovered that the deformation
features typical of statically compressed, cold-rolled, or
explosively shocked 304 stainless steel are dislocation
pile-ups, body centred cubic martensite plates, or micro-
twins. Moreover, twinning did not occur below shock pres-
sures of 1:5 104 MPa. At these low pressures, an increasein pressure was characterised by an increase in stacking-fault
and dislocation density only. Accordingly, it is suggested
that the application of high strain rates by explosive forming,
compared with those encountered during more conventional
forming to the same strain, will result in a higher stacking-
fault density, more hexagonal closed packed e-phase, lessbody centred cubic martensite. This is still subject to experi-
mental verification, and would appear to be a possible area
of future research.
There is a contradiction between the strength properties
and the microstructure behaviour for 304 stainless steel. As
mentioned there is less martensite after dynamic straining
than after static straining. Since martensite has a significant
strengthening effect, it is anticipated that the strength of the
304 stainless steel after dynamic deformation could be less
than that after static deformation However, as indicated in
Table 4, the strength of the 304 stainless steel after dynamic
deformation is greater than that after static deformation.
This indicates that there must be other microstructural
factors influencing the properties. No detailed investigation
of this appears to have been undertaken.
As stated above in forming operations where the explosive
is not in contact with the workpiece and when there is no
impact with a hard die, the peak pressure will be of order of
100500 MPa. The ensuring strain rate will be in the range of
102103 s1. On the other hand, when the explosives are
placed in contact with the workpiece, shock pressure can
range from about 103 to 2 104 MPa. The ensuring strainrate is in the range of 104107 s1. The next section examines
the effect of shock waves generated at the surface of a metal.
7. Metallurgical effect of shock waves in metals
Stress waves can be classified as elastic, plastic and shock
waves. Elastic waves produce only elastic deformation in
metal. However, when the amplitude of an elastic wave
exceeds a critical value for the yield stress of material, at that
specific strain rate the dimensions of the body are changed.
These are called plastic waves, longitudinal or shear. If the
geometry of the body is such that uniaxial strain occurs then,
the propagation velocity of the plastic wave increases with
increasing pressure as there cannot be any lateral flow of
metal. The wave has a sharp front and is defined as a shock
wave and requires a state of uniaxial strain which allows the
build-up of the hydrostatic component of stress to high
levels. When this hydrostatic component reaches levels that
exceed the dynamic flow stress by several factors, it can be
assumed that the solid has no resistance to shear and the
shear modulus is zero, the hydrodynamic assumption. The
microstructure and properties of materials that result from
shock are very important in evaluating the performance of
explosive formed materials.
Without exception, dislocations are generated at the shock
front. The dislocations generated remain as a relatively
stable microstructure behind the shock pulse, although some
rearrangement, multiplication, or annihilation can occur in
the relief portion of the pulse, i.e. behind the peak pulse. At
high stacking-fault energies (>70 mJ/m2), dislocations can-
not extend appreciably and cross-slip is predominant. With
sufficient time available in the shock pulse, this produces
dislocation arrays, which from a cell-like structure particu-
larly prominent in shock-loaded nickel. As alloying reduces
the stacking-fault energy, cross-slip is impeded or impos-
sible, and dislocations from planar arrays, which include
extended stacking faults in the {1 1 1} planes of face centred
cubic materials. In many materials, regular or periodic
arrays of stacking faults can produce new phase regimes
or twins. The microstructural density, variations in micro-
structure, increases with increasing peak pressure. In the
case of face centred cubic materials with high stacking-fault
free energy, increasing dislocation density with increasing
peak shock pressure will cause an increase in the cell
dislocation density. This can only occur by if the mean cell
size decreases or the number of dislocations composing the
cell walls increases.
Particularly significant is the pressure-induced phase
transformation undergone by iron and steel. At 13 GPa
the body centred cubic a-phase transforms into the hexa-gonal closed packed e-phase. The kinetics of this transfor-mation is rapid enough for it to be produced by a shock
wave. These phase transformations have been studied in
detail [64,65].
Shock loading is unique since significant hardening and
strengthening arises from shock wave propagation, while the
residual strains are small or even negligible. When compar-
ing traditional cold forming techniques such as cold rolling
with shock deformation as a means of enhancing specific
D.J. Mynors, B. Zhang / Journal of Materials Processing Technology 125126 (2002) 125 21
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mechanical properties of a material and hence the micro-
structure, significantly greater true strains are required in
cold rolling than in shock loading. In addition, it appears that
no specific texture is produced by shock waves which is in
contrast to most conventional deformation processes.
The creation of second phase by ageing and other treat-
ments followed by shock loading, or similar thermomecha-
nical shock treatment schedules could provide unique
metallurgical properties. This has been demonstrated with
creep properties in nickel-based superalloys such as Udimet
700 [35,66] and Inconel 718 [67,68] (Fig. 22). The benefits
arise mainly from the development of a high volume fraction
of finely dispersed precipitates (g0), and a finely dispersedthermally stabilised dislocation substructure.
8. Models and modelling of explosive forming
As with all processes understanding is improved through
the use of modelling and simulation. The modelling and
simulation relating to explosive forming can be divided into
three categories: physical modelling, shock wave modelling
and molecular-dynamic modelling.
8.1. Physical modelling
Small-scale trials are often used before full sized dies are
manufactured. For small-scale trials to be successful the
scaling factor from trial to real size must have a reasonable
degree of accuracy. To achieve this the following scaling law
and similitude requirements must be followed.
The scaling law requires that the mass of full-scale
explosive charge must be n31 times the mass of small-scale
charge. Where n1 is the ratio of the full-scale die opening to
the corresponding small-scale value. The similitude require-
ments are:
1. Completed geometrical similitude must be provided.
2. The blank material for both the model and the full-
scale trial must be the same.
3. The same explosive, pressed or cast to the same
density, must be used for both the model and the full-
scale trial.
4. The energy transfer medium must be the same for the
model and the full-scale trial.
5. The stiffness of the full-scale die clamps restraining the
perimeter of the blank must be n1 times the
corresponding stiffness for the model.
6. If the die is shock mounted, the stiffness of the
supporting springs for the full-scale die must be n1times the corresponding value for the model.
7. The hold-down force in the full-scale die clamps must
be n21 times the small-scale value.
8. The mass of the full-scale die should be n31 times the
mass of the small-scale die.
9. The die materials and strength should be the same for
model and full-scale trial.
10. The air pressure in the die cavity before forming must
be the same for model and full-scale trial.
11. The coefficient of friction between the blank and the
die surfaces over which it slide should be approxi-
mately the same.
Fig. 22. Constant-load creep curve for Inconel 718 at 649 8C.
22 D.J. Mynors, B. Zhang / Journal of Materials Processing Technology 125126 (2002) 125
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12. The ductility of the blank material in the model should
not be greater than the full-scale trial value.
13. Temperature differences between the model and the
full-scale trial should not appreciably affect the
material properties of the blank or the energy release
from the explosive charge.
The scaling law and similitude requirements outlined
above have been verified by developing an explosive form-
ing process on a 24 in. (609.6 mm) diameter model and
scaling it up, to form 120 in. (3048.0 mm) diameter domes.
The resulting domes were successfully processed. The
12.2 kg full-scale explosive charge was scaled up from
the 97.5 g scale model charge according to the scaling
law. A comparison of predicated and observed strain is
summarised in Table 5.
It can be seen that the scaling laws and corresponding
similitude requirements for explosive forming must be used
with an understanding of their associated uncertainties.
Although a scaling law is very useful in predicting charge
weights for large installations using either free or die form-
ing, it can only be considered as a first approximation to the
amount of charge necessary, since the effects of some
parameters can be very complex, particularly if a difficult
shape is to be formed. For details about the derivation of
scaling laws, similitude requirements and scale factors for
explosive forming, please see Ref. [6].
8.2. The modelling of shock waves in metals
The treatment developed by Hugoniot and Rankine for
fluids is commonly applied to the treatment of shock waves.
Essentially, it is assumed that the shear modulus of the metal
is zero and that it responds to the wave as liquid; hence, this
treatment is restricted to higher pressures.
The state of uniaxial strain generates shear stresses, and
these cannot be ignored in a more detailed account. Another
problem in the mathematical treatment of shock waves is the
discontinuity in particle velocity, density, temperature and
pressure across the shock front. The differential equations
describing these processes are non-linear and trial-and-error
computations are required at each step. Nenmann and
Richtnyer introduced an artificial viscosity term and
achieved much better mathematical results [63]. This arti-
ficial viscosity term had the purpose of smoothing the sharp
shock front and rendering it tractable in differential equa-
tions and finite difference techniques. The shock front was
made some what larger than the grid in the finite difference
network. Dissipative mechanisms take place at the shock
front and they can be represented by a mathematical visc-
osity term. This method of analysis has been applied to a
variety of problems involving dynamic propagation of dis-
turbances.
The simulation results for the detonation of a high explo-
sive in contact with a copper block indicated that the
cratering effect is