b zhang paper exposive forming.pdf

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

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

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

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.


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).


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].


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


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].


Fig. 10. Schematic diagram of the Oak Ridge vacuum vessel.

Fig. 11. HSX vacuum vessel section [23].


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).


Fig. 13. Explosive flanged 5052 H32 aluminium alloy cylinder.

Fig. 14. Spherical vessel produced using dieless forming [26].


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.


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].


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.


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.


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.


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


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%:


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%:


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


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


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.


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

b zhang paper exposive forming.pdf

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