Explosive Forming

 

 

 

 

October 22nd, 2003

 

Todd White

 

The University of Kansas

3rd Annual KU Aerospace Materials and Processes “Virtual” Conference

“Materials and Manufacturing for Tomorrow’s Engineer”

 

 


Table of Contents


Table of Contents. i

List of Figures. ii

List of Tables. ii

1.     Introduction and Overview.. 1

1.1        History of Explosive Forming. 1

1.2        Other forms of Explosive Metal Working. 2

1.3        Types of Explosive Forming. 2

2.     Contact Operations vs. Standoff Operations. 3

2.1        Contact Operations. 4

2.2        Standoff Operations. 6

3.     Charge Types, Die Materials, and Formability of Metals. 10

3.1        Charge Types. 10

3.2        Die Materials. 10

3.3        Formability of Materials. 11

4.     Engineering Applications. 13

4.1        Flexibility of Explosive Forming Examples. 13

4.2        Cost and Suitability. 17

5.     Research and Future Applications. 20

6.     References. 22

Appendix A:  Conference Proposal Abstract 23

 

 

 

 

Cover page image from Reference 1, illustrating the unitization of a nose cone structure made possible with explosive forming.


List of Figures [top]


Figure 1.1 Explosively engraved brass plate (Ref 1) 1

Figure 2.1 Differences between Standoff and Contact Operations (Ref 1) 3

Figure 2.2 Failures and Fracture propigation in a Contact Operation (Ref 1) 3

Figure 2.3 Interaction of Detonation Shock and Metal Surface, Before and After (Ref 1) 4

Figure 2.4 Worked region thickness as function of Explosive thickness (Ref 1) 5

Figure 2.5 Hardness of 1020 Steel plate after Contact Operation (Ref 1) 5

Figure 2.6 Standoff Operation Explosive Forming Water Tank setup (Ref 1) 6

Figure 2.7 Pressure-distance Curves for different Charge sizes (Ref 1) 6

Figure 2.8 Effect of Standoff Distance on cup profile (Ref 1) 7

Figure 2.9 Effect of Medium on cup profile, Standoff distance = 1/2" (Ref 1) 7

Figure 2.10 Large Explosive Forming Tank (Ref 1) 8

Figure 2.11 Schematic of Large Explosive Forming Tank (Ref 1) 8

Figure 2.12 Explosive Forming of Tubing (Ref 4) 9

Figure 3.1 Relative Formability of metals with Explosive (Ref 1) 12

Figure 4.1 Waffle Panel for Saturn V Rocket (Ref 3) 13

Figure 4.2 Example of Corrugated Jet aircraft Panel (Ref 1) 14

Figure 4.3 Jet engine Fuel Filter (Ref 1) 14

Figure 4.4 Re-entrant Shaped Part and Preform (Ref 1) 15

Figure 4.5 Explosively Perforated Cylinder (Ref 1) 15

Figure 4.6 2024-T3 Aluminum formed into Ring Explosively (Ref 3) 16

Figure 4.7 PM1000 (High Temperature Resistant Alloy) beaded structure for use in Crew Return Vehicle from Space (Ref 3) 16

Figure 4.8 Titanium (Ti6Al4V) formed explosively (Ref 3) 17

Figure 4.9 Aluminum Nose Cones, by (left) conventional means, and (right) by explosive forming. (Ref 1) 18

Figure 4.10 Generalized Cost Comparison Curves (Ref 1) 18

Figure 5.1 Numerical Analysis and Simulation of Pressure Calibration (Ref 3) 20

Figure 5.2 Numerical Analysis of Engine cowling for Explosive Forming Study (Ref 3) 20

Figure 5.3 Glare® material contains glass fibers, formed by TNO Research (Ref 3) 21

 

 

List of Tables [top]


Table 3.1 Features of Low and High Explosives (Ref 2) 10

Table 3.2 Properties of some Explosives used in Explosive Forming (Ref 2) 10

Table 3.3 Common Die Materials (Ref. 2) 11

 

 

 

 

 

 


1.         Introduction and Overview [top]


Explosive Forming is a manufacturing technique that uses explosions to force metal into dies and molds.    The explosives are typically either detonated underwater or in direct contact with the materials.  The technique is useful for short production runs of conventionally difficult-to-manufacture parts.

Explosive forming can be used for forming parts on the scale of a few inches to up to 15 feet—the main limitations are the initiation of uniform detonation and the size of the holding tank.  Because of this flexibility, explosive forming has historically been employed in the aerospace industry for prototyping of complex parts.

 

1.1         History of Explosive Forming [top]

 

Explosive forming was first documented in 1888.  It was used in the engraving of iron plates.  In this engraving, the explosive was placed in direct contact with metal, and the thickness of the explosive layer on the plate determined the depth of indention made after detonation.  Figure 1.1 shows an example of explosive engraving.

 

Figure 1.1 Explosively engraved brass plate (Ref 1)

 

Over time, many other applications for explosives have been found.  The research performed on the effects of explosives and shockwaves on metals has had its roots in military applications.  During World War I and II, especially, many programs investigated such phenomena for the development of torpedoes and other weapons designed to attack armored vehicles.

The development of explosives, propellants, and other exothermic chemicals, has been intimately tied to military weapon development.  The need for more powerful guns and projectiles drove research of propellants and mechanisms of explosives.  The in-depth analysis of modern gun propellants has been crucial to explosive forming, and today’s engineer will find these propellants both well characterized and readily obtainable.

            As early as the 1950’s, aerospace companies in the United States, such as Rocketdyne, Aerojet General Corporation, and Ryan Aeronautical were using explosive forming for the manufacture of complex curved aerospace components.  Explosive forming was especially important in the development of short-production-run missile components—particularly for the curved domes of missiles and rocket nose cones.

            Other aerospace components were produced through explosive forming: complex corrugated panels for aircraft, and fuel filters and asymmetrical exhaust tubes for jet engines.  During this time, the Soviet Union also began using explosive forming in their rocket industries for large curved panels.

 

1.2         Other forms of Explosive Metal Working [top]

There are numerous types of metalworking done with explosives.  This paper will focus only on explosive forming.  However, some other metal working techniques are:

 

·         Explosive welding

·         Explosive cutting

·         Explosive powder forming

·         Explosive sheet lamination

·         Explosive coating

·         Explosive stress relieving

·         Explosive compaction

·         Explosive cleaning

 

1.3         Types of Explosive Forming [top]

Explosive forming can be separated into two different categories:  contact operations and standoff operations. 

 

·         Contact operations place the explosive in direct contact with the metal.

·         Standoff operations places the explosive charge some distance away from the workpiece.

 

Contact operations require specific studies of the interactions between the metal, the explosives, and the by-products.  Standoff operations typically use some buffer medium, such as air, oil, or water.  The difference in pressures, loads, and speed of forming developed in these two different methods is significant.  This will be discussed in more detail in Section 2.

 

 

 


2.         Contact Operations vs. Standoff Operations [top]


Figure 2.1 Differences between Standoff and Contact Operations (Ref 1)

 

Figure 2.1 depicts the range of explosive forming that can be done, either by contact or standoff operations.  From Ref 1, page 5:

 

“Conceptually, each explosive metal working operation can be placed in a spectrum of operations as a function of strain rate or pressure at the workpiece...The scale is divided into the two basic areas of standoff and contact operations.  The pressure range over this spectrum can run from several thousand psi to several million psi.  Most of the operations are performed in the microsecond to millisecond range....Large differences in material behavior patterns can be expected over this operations spectrum.”

 

As stated, the material properties and behavior will vary considerably, depending on the distance of the charge from the metal.  The mechanisms of plastic flow for explosive forming are similar to other methods of metalworking.  Microstructural plastic deformation like slip and grain distortion occur, as with any other metal forming operation.

 

It is the control of the explosion and the shockwave-induced pressures that determines the probability and variation of extreme localizations in the microstructure of the formed piece.  Such localizations can lead to unpredicted material behavior and can contribute to fracture and failure if not predicted and monitored properly (Figure 2.2).

 

Figure 2.2 Failures and Fracture propigation in a Contact Operation (Ref 1)

 

Because of the increased risk of failure involved with contact operations, the standoff operation is more frequently used in forming.  The standoff method has other advantages as well. First, because of the greater distance between the charge and the workpiece, the energy the shockwave transfers to the metal is typically lower then in contact operations.  Larger components can also be produced with a smaller amount of explosives.

Other explosive working methods, besides forming, use contact operations, (See Section 1.2.)

 

2.1         Contact Operations [top]

 

The stresses developed by a contact operation charge detonation are very short in duration, yet are “at least an order of magnitude higher than are normally encountered in static loading” (Ref 1).  Such stresses are similar to stresses encountered during conventional forming operations, such as punching.  However, the details of the transmission of stress from the detonating explosive to the metal are considerably different from those encountered in other manufacturing operations.

 

Figure 2.3 Interaction of Detonation Shock and Metal Surface, Before and After (Ref 1)

Figure 2.3 shows a before-and-after diagram of the interaction between the detonation front and the metal.  The details of the compression and shock propagation will not be discussed in this paper (Reference 1 offers a good theoretical guide to this subject).  It is important to note that the explosion causes a shockwave to move through the metal, and that some displacement in the metal occurs almost instantaneously as the detonation front reaches the workpiece.

 

Figure 2.4 Worked region thickness as function of Explosive thickness (Ref 1)

 

As the compression wave propagates, the metal experiences “working”.  Figure 2.4 shows the thickness of the metal that is highly worked as a function of the charge thickness.  Obviously, the more explosives present, the larger the high-worked region of the workpiece.

One advantage of explosive forming is the ability to use the finished metal in an annealed/treated condition before the forming operation.  Nevertheless, the working done by the shock wave can drastically change the physical properties of the metal, including work-hardening or work-softening.  From Ref 1,

 

“The working of metals by intense transient stress disturbances manifests itself as changes in physical properties in the metals.  The hardness is increased, the tensile strength goes up and yield and plastic flow characteristics are altered.”

 

For example, mild steel shows a marked change in hardness due to explosive forming.  Figure 2.5 shows the change in hardness of 1020 steel after contact operations.

 

Figure 2.5 Hardness of 1020 Steel plate after Contact Operation (Ref 1)

However, mild steel is an exception. Nickel, for example, is not so strongly affected by contact explosive forming operations.  The crystal structure (body centered cubic, face centered cubic, etc.) of the formed metal has a strong affect on the magnitude of changes in material properties after forming.

2.2         Standoff Operations [top]

Figure 2.6 Standoff Operation Explosive Forming Water Tank setup (Ref 1)

 

Both the location of the charge and the method by which pressure loads are transmitted to the unformed workpiece are different in standoff operations and contact operations.  Figure 2.6 above shows a typical standoff operation water tank, (in this case for forming a cone, possibly for the top of a missile).

The study of shock and wave propagation through different mediums is very important in understanding standoff operations.  The shockwave induced by the charge detonation instantaneously converts the liquid buffer from low density, temperature, and pressure to a high density, high temperature, and high-pressure fluid.

 

Figure 2.7 Pressure-distance Curves for different Charge sizes (Ref 1)

 

 

 

Figure 2.8 Effect of Standoff Distance on cup profile (Ref 1)

 

 

Figure 2.9 Effect of Medium on cup profile, Standoff distance = 1/2" (Ref 1)

 

The distance between the charge and the metal piece is called the “standoff distance.”  The standoff distance and the amount of charge determine the amount of pressure transmitted to the metal.  Other factors, like as the explosive type, shape of explosive, and type of buffer medium also affect the pressure.  Figure 2.7 shows the relationship between explosive weight (TNT in this case), standoff distance, and pressure.  Figures 2.8 and 2.9 show the differences that standoff distance and medium have on the cupping operation of 2024-O material.

Based on Figures 2.8 and 2.9, the selection of water (cheaper and more easily available than either kerosene or glycerin) at a standoff distance of ½” (most accurate cup shape) would be appropriate for forming this particular piece.

An understanding of the compression waves and rarefactions developed in detonation is extremely important in predicting the forming metal’s reaction.  Compression wave topics are well understood in the field of aerospace, making standoff explosive forming operations an appealing manufacturing technique, particularly in the aerospace industry.

Standoff Operations can also be used in producing large unitized components.  Figures 2.10 and 2.11 show two large tanks, used in the forming of large aerospace components.

Figure 2.10 Large Explosive Forming Tank (Ref 1)

 

Figure 2.11 Schematic of Large Explosive Forming Tank (Ref 1)

Another type of standoff explosive forming is shown in Figure 2.12.  It can be used to make modified tube-like structures from drawn tubing.  Figure 2.12 shows a schematic of the die and explosive charge placement, where the buffer medium is sand instead of water.

 

Figure 2.12 Explosive Forming of Tubing (Ref 4)

 

The limitations of short production runs and the formability of different types of metals by explosions will be covered in the subsequent chapters.  Examples of parts formed in contact and standoff operations will also be shown in the Chapter 4, “Engineering Applications.”

 


3.         Charge Types, Die Materials, and Formability of Metals [top]


 

3.1         Charge Types [top]

The types of explosives used in explosive forming operations vary.  Table 3.1 below compares the methods of igniting high and low explosives, as well as the time scales involved and the maximum pressures attainable with each.  Clearly, the pressures needed to mold each specific part will determine the selection of the explosive.

Table 3.2 shows some attributes of the types of explosives used in industry for explosive forming.  Both Tables 3.1 and 3.2 are taken directly from Reference 2.

 

Table 3.1 Features of Low and High Explosives (Ref 2)

Property

High Explosives

Low Explosives

Method of initiation

Primary HE-ignition, spark, flame, or impact

Ignition

 

Secondary HE-detonator, or detonator and booster combination

 

Conversion time+

Microseconds

Milliseconds

Pressure

up to about 4,000,000psi

up to about 40,000psi

 

 

Table 3.2 Properties of some Explosives used in Explosive Forming (Ref 2)

 

 

3.2         Die Materials [top]

 

The material for the die (or mold) is selected based on several factors:

 

·         Production Run Duration

·         Type and thickness of the metal being worked

·         Configuration (radii, depth of pockets, beads, etc.)

·         Tolerance desired in finished part

·         Explosive weight

·         Standoff Distance

 

From Reference 3, on the selection of die assembly material:

 

“Relatively low strength dies are used mainly for short run items and for parts where close tolerances are not critical, while for longer runs higher strength die materials are required....[For] long runs the die [should] be designed from materials with a higher yield strength than the material from which the pieces are being formed.”

The difficulties encountered in making a mold for explosive forming are similar to those in making molds for conventional forming operations.  If the die material is to have higher yield strength then that of the workpiece metal, it is very likely that it will be very difficult to machine and form into the desired shape.

For very short production runs or prototyping, when the mold is used to produce few parts, materials such as Kirksite or Fiberglas may be used.  Table 3.3, from Reference 2, lists some die materials and their application areas.

 

Table 3.3 Common Die Materials (Ref 2)

Material of Die

Application Area

Kirksite

Low pressure and few parts

Fiberglass and Kirksite

Low pressure and few parts

Fiberglass and Concrete

Low pressure and large parts

Epoxy and Concrete

Low pressure and large parts

Ductile Iron

High pressure and many parts

Concrete

Medium pressure and large parts

 

3.3         Formability of Materials [top]

 

TNO, a Netherlands-based research group (Ref 3), has used explosive forming for the following materials:

 

Materials; all metals, including:

·         Titanium alloys (CP, Ti6Al4V, Ti6-2-4-2-, Ti-B21S, etc.) 

·         Nickel alloys (718, 625, Hastelloy-X, etc.)

·         Aluminum alloys (e.g. 2024, 7075, 6xxx, 5xxx, etc.)

·         (Stainless) Steels

·         Special, extremely strong or brittle alloys, e.g. ODS alloy PM1000

 

Reference 1, “Explosive Working of Metals” contains the following comparison of formability of metals by explosives shown in Figure 3.1.

Figure 3.1 Relative Formability of metals with Explosive (Ref 1)

 

Materials used frequently in the aerospace industry, such as Aluminum (2XXX, 5XXX, 6XXX, and 7XXX series), titanium, and nickel can all be formed explosively.

 

 


4.         Engineering Applications [top]


4.1         Flexibility of Explosive Forming Examples [top]

 

 

Figure 4.1 Waffle Panel for Saturn V Rocket (Ref 3)

 

Figure 4.2 Example of Corrugated Jet aircraft Panel (Ref 1)

Figure 4.3 Jet engine Fuel Filter (Ref 1)

 

Figure 4.1-4.3 show the range of sizes of components that can be accurately formed explosively.  Figure 4.4 and Figure 4.5 demonstrate the flexibility offered, using tubes or similar structures as performs (a cone, in the case of Figure 4.5), and subsequently explosively forming from within.

 

Figure 4.4 Re-entrant Shaped Part and Preform (Ref 1)

 

Figure 4.5 Explosively Perforated Cylinder (Ref 1)

 

Figure 4.6 2024-T3 Aluminum formed into Ring Explosively (Ref 3)

 

 

Figure 4.7 PM1000 (High Temperature Resistant Alloy) beaded structure for use in Crew Return Vehicle from Space (Ref 3)

 

Figure 4.8 Titanium (Ti6Al4V) formed explosively (Ref 3)

 

Figures 4.6 through 4.8 illustrate the variety of aerospace materials that can be formed with Explosive forming. TNO (Ref 3) states, concerning Figure 4.8:

 

“Titanium alloys like Ti6Al4V are hard to form. This sample shows that by imposing an appropriate forming scheme (based on deep drawing), remarkable shapes can be achieved. The next step will be hemispheres.”

 

TNO is currently performing research into using explosive forming techniques for a variety of specialized alloys and non-metals.  Chapter 5, “Research and Future Applications” will briefly cover these topics.

 

4.2         Cost and Suitability [top]

 

Explosive forming can be used to easily produce difficult-to-manufacture components.  The ability to quickly prototype a unitized (Figure 4.9), complex, high tolerance component can be a tremendous advantage in industry.  Explosive forming, however, is not necessarily a replacement for conventional metal forming techniques. 

 

Figure 4.9 Aluminum Nose Cones, by (left) conventional means, and (right) by explosive forming. (Ref 1)

 

Explosive forming can be characterized by (Ref 2):

§         Very large sheets with relatively complex shapes, although usually axisymmetric.

§         Low tooling costs, but high labor cost.

§         Suitable for low-quantity production.

§         Long cycle times.

In general, the unit cost of an explosively formed part is higher than that of conventional methods for large production runs, as shown in the below figure (Figure 4.9)

 

Figure 4.10 Generalized Cost Comparison Curves (Ref 1)

 

Figure 4.9 indicates that at some point P, the unit cost of using explosive forming (if practical and possible) is significantly lower than that of conventional methods.  This is due largely to the low tooling costs.  Consequently, explosive forming can be very useful as a low cost alternative to otherwise expensive conventional means of prototyping.  Beyond this point P, though, conventional methods are cheaper.  The application of explosive forming in mass production is very limited.

Because of the high labor costs and experience necessary involved in explosive forming, many companies have turned to smaller specialist companies for prototyping operations.  One such company is Exploform (Ref 4), a joint effort of a three companies/research laboratories, ANTONIUS Vesselheads BV, TNO Prins Maurits Laboratorium, and Fabriek van plaatwerken Van Dam B.V.

 

 


5.         Research and Future Applications [top]


Research is being performed by TNO to develop numerical simulations of explosive forming operations.  TNO (Ref 3) foresees explosive forming being applied to the following fields in the future:

 

·         Gas turbine exhaust structures

·         Engine cowlings

·         Fiber metal laminates

·         Propellant pressure tanks

·         Other Gas turbine applications

 

Figure 5.1 Numerical Analysis and Simulation of Pressure Calibration (Ref 3)

 

 

Figure 5.2 Numerical Analysis of Engine cowling for Explosive Forming Study (Ref 3)

 

 

Fiber metal laminates can already be formed explosively. Figure 5.3 shows a Glare ® containing component made from metal with glass fibers.   Subsequent inspection must be performed to ensure that the rapid forming does not result in delamination.

 

Figure 5.3 Glare® material contains glass fibers, formed by TNO Research (Ref 3)

 

Explosive forming remains a useful prototyping manufacturing technique.  In industries where ”rapidly changing technology causes many system concepts to become obsolete before going into production.” (Ref 1) or in industries where the prototype metal parts have very complex shapes, explosive forming may be a viable alternative to expensive conventional metal forming operations.

 

 

 

 

 


6.         References [top]


1.       Rinehart J.S and Pearson, John, Explosive Working of Metals, Pergamon Press, 1963.  New York.

2.       EXPLOSIVE FORMING - An Overview, by Amit Mukund Joshi, Junior Research Fellow at Indian Institue of Technology, Bombay. (http://www.metalwebnews.com/howto/explosive-forming/explosive-forming.html) Accessed 9/22/03.

3.       TNO PML Website, "Explosive Forming for the Aerospace market" (http://www.pml.tno.nl/en/products_services/emb/explosive_forming_technology_aerospace.html) Accessed 9/22/03.

4.       Exploform Company Website.  (http://www.exploform.com/old/Zindex2.htm) Accessed 9/22/03.

 

 


Appendix A:  Conference Proposal Abstract [top]


Todd White

9.22.03

AE 510 Conference Proposal Abstract

 

 

Explosive Forming

Objective

This presentation will focus on the different uses and methods of Explosive forming in the Aerospace as well as automotive industry.  Explosive forming involves placing a metal plate submerged in water, above a die or mold.  Detonating a charge above the plate rapidly forces the metal into the desired shape.

 

Results

The paper will discuss the current and past applications of explosive forming.  It will also cover what applications that are suitable for explosive forming, as well as the relative hardware and labor costs of the process, and for what materials it is an appropriate technique.

 

Significance

Explosive Forming has historically been used in creating otherwise difficult to machine parts as a less expensive alternative to superplastic forming.  There already exists information on explosive forming, as Ukrainian and German manufacturers have employed it in limited production.  Understanding the costs, availability and feasibility in regards to Aerospace manufacturing applications would be very significant to tomorrow’s Engineer.



 

Resources:

http://www.pml.tno.nl/en/products_services/emb/explosive_forming_technology_aerospace.html

 

http://www.metalwebnews.com/howto/explosive-forming/explosive-forming.html

 

http://www.exploform.com/old/produktie.htm

 

http://www.tinmantech.com/html/faq__explosive_forming.html