Impact Wave Process Modelling and Optimization in High Energy Rate Explosive Welding

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Mlardalen University Press Licentiate Theses

No. 106

IMPACT WAVE PROCESS MODELING AND OPTIMIZATION IN HIGH

ENERGY RATE EXPLOSIVE WELDING

APPLIED MECHANIC

Mohammad Tabatabaee Ghomi

2009

School of Sustainable Development of Society and Technology


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Copyright Mohammad Tabatabaee Ghomi, 2009ISSN 1651-9256ISBN 978-91-86135-35-5

Printed by Mlardalen University, Vsters, Sweden


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Abstract

Impact waves are used in many different industries and are classified according to whether they

cause plastic or elastic deformations. In the plastic deformation mode, these waves can be used toproduce special electrical joints. In the elastic deformation mode, they can be used to detect leakage

or to measure the thickness of pipes. Both modes have applications in offshore technology.

In this thesis the application of impact waves in the plastic deformation mode and explosivewelding are discussed. In the explosive welding (EXW) process a high velocity oblique impact

produced by a carefully controlled explosion occurs between two or more metals. The high velocity

impact causes the metals to behave like fluids temporarily and weld together. This process occurs in

a short time with a high rate of energy.

EXW is a well known method for joining different metals together. It is a multidisciplinary research

area and covers a wide range of science and technology areas including wave theory, fluiddynamics, materials science, manufacturing and modeling. Many of the important results in EXW

research are obtained from experimentation.

This thesis is mainly based on experimental work. However, it begins with a review of the

fundamental theory and mechanisms of explosive welding and the different steps of a successful

welding operation. Many different EXW tests are done on horizontal and vertical surfaces with

unequal surface areas, and on curved surfaces and pipes. The remainder of the thesis evaluates the

results of these experiments, measures the main parameters, and shows the results of simulations to

verify the experimental results.

The thesis ends with a number of suggestions for improving and optimizing the EXW process. One

of these improvements is a model for joining metallic plates with unequal surface areas. An Al-Cu

joint based on this model is used in the ALMAHDI aluminum factory, a large company in southern

Iran that produces more than 200,000 tons of aluminum per year. Improved methods are also

suggested for joining curved surfaces. These methods may have extensive applications in pipelines

in oil and gas industries, especially in underwater pipes.


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Sammanfattning

Impact vgor anvnds i mnga olika branscher och klassificeras beroende p om de orsakar plasteller elastiska deformationer. I plastisk deformation lge, kan dessa vgor anvndas fr att tillverka

speciella elektriska skarvar. I elastisk deformation lge, kan de anvndas fr att upptcka lckageeller att mta tjockleken p rren. Bda lgena har tillmpningar inom offshore-teknik.

I denna avhandling tillmpningen av effekten vgor i plastisk deformation mode och explosivasvetsning diskuteras. I den explosiva svetsning (EXW) behandla en hg hastighet sned anslaget aven noggrant kontrollerad explosion intrffar mellan tv eller fler metaller. Den hga hastigheteneffekten gr att metaller att bete sig som vtskor tillflligt och svetsa ihop. Detta sker under en korttid med hg energi.

EXW r en vlknd metod fr att g olika metaller tillsammans. Det r ett tvrvetenskapligtforskningsomrde och tcker ett brett spektrum av vetenskapliga och tekniska omrden inklusivevgrrelselra, strmningsmekanik, materialvetenskap, tillverkning och modellering. Mnga av deviktigaste resultaten i EXW forskningen erhlls frn experiment.

Denna avhandling r huvudsakligen baserad p experimentellt arbete. Dremot brjar det med engenomgng av grundlggande teori och mekanismer av explosiv svetsning och de olika stegen i enlyckad svetsning operation. Mnga olika EXW tester grs p horisontella och vertikala ytor medojmn yta, och p krkta ytor och rrledningar. terstoden av avhandlingen utvrderar resultaten avdessa frsk, tgrder viktiga parametrar och visar resultaten av simuleringar fr att verifiera de

experimentella resultaten.

Avhandlingen avslutas med ett antal frslag fr att frbttra och optimera EXW processen. En avdessa frbttringar r en modell fr att g metallplattor med ojmna ytor. En Al-Cu gemensamt

baserade p denna modell anvnds i ALMAHDI aluminium fabriken, ett stort fretag i sdra Iransom producerar mer n 200.000 ton aluminium per r. Frbttrade metoder fresls ocks fr att gmed krkta ytor. Dessa metoder kan ha mnga olika tillmpningar i rrledningar i olje-ochgasindustrin, srskilt i vattnet rr.


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Acknowledgements

The work described in this thesis was carried out at the School of Sustainable Development of Society

and Technology, Mlardalen University, Sweden.

I would like to thank my supervisor, Professor Jafar Mahmoudi for his encouragement, guidance,

scientific help and unlimited support.

I would like to express my gratitude to Professor Erik Dahlquist and Professor Jinyue Yan.

I would like to express my gratitude to Professor Gholamhossein Liaghat from Tarbiat Modares

University, Professor Mohammad Mahjoob from Tehran University and Professor A. Darvizeh fromGilan University in Iran for their scientific help.

I would like to express my gratitude to Dr. Adel Karim at Mlardalen University.

The author would like to acknowledge Professor Dobroshin from PATON institute in Ukraine, Mr.

Chavideh from Chime-Tec Company in Germany, ACECR, and the TDI organization in Iran for

help with experiments.


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List of Publications

This thesis is based on the following papers and technical reports:

Appended Papers:

Paper 1:Mohammad Tabatabaee, Jafar Mahmoudi and Gholamhossein Liaghat, An Applied Method forwelding Metals of Unequal Surface Area Using Explosive Energy, Submitted to InternationalJournal of Impact Engineering, ISSN: 0734-743XPaper2:

Mohammad Tabatabaee, Jafar Mahmoudi and Gholamhossein Liaghat,Removing Leakage from oiland gas low pressure Pipes and vessels by high energy explosive welding method, ScientificConference on Energy systems with IT in connection with the Energiting 2009, March 11-12 atlvsj fair, Stockholm, ISBN number 978-91-977493-4-3.Paper3:

Mohammad Tabatabaee, Jafar Mahmoudi and Gholamhossein Liaghat, Effect of Explosive Layerthickness on detonation velocity in a high energy process, Submitted to High Energy PhysicsJournal, ISSN: 1126-6708.

Report

Mohammad Tabatabaee,Impact wave process control in explosive welding application, Technicalreport No. 3, 20 July 2008, Mlardalen University

Papers not appended:

Paper 4:

Mohammad Tabatabaee and Jafar Mahmoudi, Finite element simulation of explosive welding, The49th Scandinavian Conference on Simulation and Modeling (SIMS2008), ISBN-13: 978-82-579-46326Paper 5:

Mohammad Tabatabaee and Jafar Mahmoudi, FEM method simulation for Aluminum - Iron -Copper bonding using explosive welding method, IASTED International Conference on AppliedSimulation and Modeling, June 25, 2008, at Corfu, Greece. (ASM 2008), ISBN- 978-0-88986-748-2Paper 6:

Mohammad Tabatabaee and Jafar Mahmoudi,An advanced method for Aluminum - Iron-

Copperbonding using explosive welding method, SSSEC2008 conference, Stockholm, March 12-13, 2008ISBN-978-91-977493-2-9Paper 7:

Mohammad Tabatabaee and Jafar Mahmoudi, An advanced method of explosive weldingsimulation, The 16thAnnual (International) Conference on Mechanical Engineering, ISME2008,May 14-16, 2008, Shahid Bahonar University of Kerman, Iran


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Nomenclature and abbreviations

Latin letters

P pressure

VD, Vd detonation velocity

Vp, Vf velocity of flyer plate

Vc, Vw collision velocity

Ei strain energy

Re Reynolds number

E Youngs modulus

C speed of sound

H hardness

A amplitude

T temperatureT thickness

Greek letters

initial angle

dynamic angle

wave length

tensile stress

density

Abbreviations

EXW explosive welding

BSEW bond strength explosive welded

WW welding window

SEM scanning electron microscope

UTS ultimate tensile stress


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Table of ContentsAbstract ........................................................................................................................................... 3

Sammanfattning .............................................................................................................................. 5

Acknowledgements ......................................................................................................................... 6

List of Publications ......................................................................................................................... 7Nomenclature and abbreviations ..................................................................................................... 8

List of Figures ............................................................................................................................... 10

List of Tables ................................................................................................................................ 11

1. Introduction......................................................................................................................... 131.1. Background ................................................................................................................................. 13

1.2. Literature review ........................................................................................................................ 13

1.3. Motivation and objective ............................................................................................................ 14

1.4. Research approach ...................................................................................................................... 14

1.5. Limitations .................................................................................................................................. 151.6. Methodology ............................................................................................................................... 15

1.7. Thesis Outline ............................................................................................................................. 16

2. Theory ................................................................................................................................... 172.1. Mechanism and set up ................................................................................................................ 17

2.2. The impact wave in explosive welding ...................................................................................... 18

2.3. Predicting the wavelength .......................................................................................................... 19

2.4. Bonding criteria .......................................................................................................................... 20

2.5. Welding window ........................................................................................................................ 21

2.6. Governing equations ................................................................................................................... 24

2.7. Testing methods .......................................................................................................................... 24

2.8. Simulation of the explosive welding process ............................................................................. 27

3. Experimental Data, Results and Calculations ....................................................................... 293.1. Experiment setup ........................................................................................................................ 29

3.2. Experimental Results .................................................................................................................. 33

3.3. Calculation, numerical and simulation results ............................................................................ 35

3.4. Test Results ................................................................................................................................ 40

4. Discussion and future work .................................................................................................. 454.1. Discussion ................................................................................................................................... 45

4.2. Future work ................................................................................................................................ 47

5. Conclusions ........................................................................................................................... 495.1. Concluding remarks .................................................................................................................... 49

5.2. Practical Output .......................................................................................................................... 50

6. References ............................................................................................................................. 53

7. Papers summary .................................................................................................................... 55


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List of Figures

Figure 1. a) Basic set up for explosive welding [1] b) The explosive bonding process [4] ....... 17

Figure 2: Geometry of the process during the collapse [1] ............................................................... 17

Figure 3. Impact wave reflection [ 1] ................................................................................................. 19

Figure 4. Shape of the wave at the interface of the plates [ 1] ........................................................... 19

Figure 5. An example of a WW diagram in the Vc-plane .............................................................. 22Figure 6. Determination of detonation velocity by Dutrich method [ 4] ........................................... 25

Figure 7. The chisel test [ 1]. ............................................................................................................. 26Figure 8. Pressure simulation by AUTODYN software [ 34]............................................................ 27

Figure 9. Setup for Fe-Fe welding ..................................................................................................... 29

Figure 10. Unequal surface area set up ............................................................................................. 30

Figure 11. Al-Cu vertical unequal surface area and dual method set up ........................................... 31

Figure 12. Set up for filling a small hole .......................................................................................... 31

Figure 13. Set up for welding on a curve .......................................................................................... 31

Figure 14. Set up for repairing a leak and filling a small hole in a pipe ........................................... 32

Figure 15. Dutrich method setup using a thin Aluminum plate ........................................................ 32Figure 16. Dutrich method setup using a Brass plate ....................................................................... 32

Figure 17. Results of EXW on horizontal flat surfaces ..................................................................... 33

Figure 18. EXW of unequal surface areas ......................................................................................... 33

Figure 19. EXW tests for filling a hole .............................................................................................. 34

Figure 20. EXW with a detonator on a pipe for filling a small hole .................................................. 34

Figure 21. Dutrich method test results ............................................................................................... 34

Figure 22. WW for Cu-Fe joint .......................................................................................................... 36

Figure 23. Welding window for Fe-Fe joint ...................................................................................... 37

Figure 24. WW for Al-Cu joint .......................................................................................................... 39

Figure 25. Effect of explosive material thickness on detonation velocity ......................................... 41

Figure 26. Effect of explosive material thickness on detonation velocity in general ........................ 41Figure 27. Chisel test for 2 types of weld .......................................................................................... 41

Figure 28. BSEW in width direction .................................................................................................. 42

Figure 29. BSEW in length direction ................................................................................................. 42

Figure 30. Metallographic results ...................................................................................................... 43

Figure 31. Results of simulation ........................................................................................................ 44

Figure 32. Bonded areas measured by ultrasonic test for Al-Cu joint ............................................... 46

Figure 33. A set up for filling a hole before welding ......................................................................... 50

Figure 34. AlCu joints transmit the electric power of anode rods in the aluminum factory............ 51


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List of Tables

Table 1. Summary of papers ............................................................................................................. 15

Table 2. Specification of Fe-Fe horizontal setup ............................................................................... 29

Table 3. Specification of Al-Cu horizontal setup .............................................................................. 29

Table 4. Specification of Fe-Cu horizontal setup ............................................................................. 29

Table 5. Specification of Al-Cu, unequal surface area, horizontal set up .......................................... 30

Table 6. Specification of Al-Cu Vertical unequal surface area and dual method set up ................... 30Table 7. Specification of Stage 3 experiments ................................................................................... 31

Table 8. Specification of experiments in Stage 4 ............................................................................... 31Table 9. Specification of experiments in Stage 5 (Dutrich method) .................................................. 32

Table 10. Result of experiments described in Table 9 ....................................................................... 34

Table 11. Calculation for line (a-a) .................................................................................................... 35

Table 12. Calculation for line (f-f) ..................................................................................................... 35

Table 13. Calculation for line (g-g) ................................................................................................... 36

Table 14. Calculation for line (a-a) .................................................................................................... 36


Table 16. Calculation for line (g-g) ................................................................................................... 37Table 17. Calculation for line (a-a) .................................................................................................... 38


Table 19. Calculation for line (g-g) ................................................................................................... 38

Table 20. Calculation of VD for different thicknesses of explosive material.................................... 40

Table 21. Results of testing on measuring detonation velocity for different kinds of explosive

materials ............................................................................................................................................. 40

Table 22. Results of mechanical test in width direction .................................................................... 42

Table 23. Results of mechanical test in length direction ................................................................... 42


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1. Introduction

Impact mechanics is a branch of applied mechanics that deals with high rates of energy and load ina very short time. This important process has applications in many different industries. One of the

most useful ways of producing high energy rate impacts is by use of explosive materials. A small

quantity of explosive material can shape a tank, build a large crankshaft, has the power to weldmany parts of a heat exchanger and improve the mechanical properties of a rail.

Explosive Welding (EXW) is one application of impact waves. The impact waves are of the tension

wave types that produce elastic and plastic deformations in the solid material. Explosive welding

and shaping occurs in the plastic deformation region. Measurement by ultrasonic and impact waves

is done in the elastic deformation region.

In EXW, an oblique impact occurs between two parts such that they behave like fluids and weld

firmly together. Because of the high velocity of impact, a jet is formed that cleans the two surfaces,

presses them together and produces a joint. This joint has an acceptable resistance that is equal or

greater than the resistance of the weaker plate.

1.1. Background

Explosive materials were first used in manufacturing shortly after the Second World War. However,the first observations of their potential uses in manufacturing date back to the First World War. It

had been observed that a bullet did not only pierce metal but also welded to it. This phenomenon

was subsequently reproduced in the laboratory and applied commercially in industry. Advances in

the aerospace industry and the close tolerances necessary for manufacturing complex parts drove

the use of the EXW method on an industrial scale. By the mid 1950s, EXW was being applied in

manufacturing.

In the following years, it was quickly accepted that EXW methods could be applied to a number of

other industries. EXW processes were adapted and refined to serve the needs of the automotive,

shipbuilding, material processing, mining, and construction industries, among others. Over three

hundred joint between similar and dissimilar materials have been produced until now. The first

experiments with the EXW technique were carried out on horizontal surfaces, but many commercial

tests have subsequently been done on curved surfaces such as pipelines and heat exchanger

components.

1.2. Literature review

This section reviews research that predates this thesis.One of the major reference works in this field is the book by Blazynski [1]. In the work, Blazynski

described clearly the method of explosive welding, explained wave phenomena and the overall

EXW procedure. The basic method is also described in the book by Crossland [2]. The PATON

Institute [3] and professor Darvizeh [4] have performed many EXW experiments. The fundamentals

of the EXW process have also been explained in a number of handbooks [5], [6] and [7] and the

mechanism of the wave interface has also been described in the literature [8], [9] and [10]. A

number of researchers consider EXW to be fundamentally a fusion welding process (Phillipchuk,

[11]) which relies on the kinetic energy at the interface. Crossland and Williams [12] look at the

method as a pressure weld process.

Otto and Carpenter [13] proposed that interfacial shear occurs during welding, and attributed the

weld to the result of heat generated by shearing at the boundary. The process reaches a very hightemperature at the interface, above the melting point of the welded parts, for a short period on the

order of microseconds. Onzawa [14] reached a similar conclusion in his study. He performed


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interface observations using scanning electron microscopy (SEM). It is generally accepted, based on

experimental data, that jet creation makes an important contribution to welding. The jet cleans the

surfaces by removing a thin layer of metals and other contaminants. The investigative solutions of

the pressure and jet velocity of the impact of liquid drops were found by Lesser [15] and Lesser and

Field [16], [17], and Field [18] provided the first photographic evidence of the effect. Wilson and

Bronzing [19] studied the waves that form in the interface. The theories proposed for the

mechanism of the wave formation can be classified as indentation mechanism, flow unsteadinessmechanism, vortex shedding mechanism and stress wave mechanism (Reid, [20]. Bahrani and

Crossland [21], Bergman [22], Bahrani et al. [23] and Abrahamson [24] have worked on groups of

these categories. Another theory of wave formation was proposed by Hunt [25] and Robinson [26],

who suggested that the explosive welding wave forms when there is a velocity difference between

adjacent streams. The flow instability mechanism was described by Robinson, who proposed that

the waves are created behind the collision zone because of a velocity across the interface which

involves a jet. This is different to the flow instability mechanism expressed by Hunt. Cowan [27]

and Kowalik and Hay[28] pointed out the parallels between the waves in explosive welding and the

Von Karma's vortex generated by a barrier. A stress wave mechanism of wave formation was

proposed by El-Soky and Blazynslki. This wave formation mechanism was recognized by Plaksin

[29].

Lazari and Al-Hassani [30] studied the behavior of metal plates under explosive pressure using a

finite element method. They used the theory of virtual displacement of the Lagrangian deformation

to derive the equations of motion. Oberg [31] simulated the explosive welding process using

Lagrangian finite difference computer code. The process was also modeled by Akihisa [32]. Finally,

the results of simulation provided by Alhasani [33] and Akbari Mousavi [34], [35], have been

reviewed.

1.3. Motivation and objective

Application of explosive welding in industry is the main motivation:The southern Iranian aluminum company ALMAHDI had a requirement for special copper-

aluminum joints with unequal surface areas. Joints they had made previously were unsatisfactory.

As a result of this thesis, more than 1000 successful EXW joints have been made and confirmed by

the factory. Another motivation for conducting this thesis was the problems faced by the oil and gas

industry in repairing and preventing the leakage in pipelines.

Improving some explosive welding method is the main objective of the project:

During the course of this research, many experiments have been performed on materials of varioustypes and shapes, and a number of new techniques have been applied to improving the EXW

process, including a new method for horizontal welding, a new method for curve welding, and a

useful curve of the velocity of explosive material versus its thickness.

1.4. Research approach

The main hypothesis of this thesis is that Materials can be bonded together by the high energy

transient pressure or impact waves produced by oblique collision at high velocity.

To test this hypothesis, explosive material was used to produce the high velocity or impact waves toweld metals, a process called explosive welding.


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The work described by this thesis includes studies of the process, review of previous work in the

field, design of experiments using different materials and shapes, manipulation and control of

parameters before and after welding, calculation of the weld parameters, process optimization,

simulation and comparison of the results. Following numerous experiments on different materials

with different geometries, such as horizontal and vertical alignment, different surfaces and curved

shapes, several methods for improving the process are proposed.

1.5. Limitations

EXW has several limitations in theory and in practice. Working with explosives is very dangerous

and the high levels of sound produced can be harmful to hearing. The plate surfaces must be clean

and the process is best performed in a vacuum. EXW is at present a manual process and has not

been automated. There are various analytical methods for calculating the process variables, and

many formulas are obtained empirically. Therefore, simulation and calculation of these methods is

very difficult. All of the experiments performed for this thesis were performed in a vacuum

chamber.

1.6. Methodology

This experimental work described in this thesis is divided into five stages:Stage 1:

Study, calculation and experimental work on explosive welding together of horizontal surfaces of

equal surface areas and different materials such as Fe-Fe, Al-Cu and Fe-Cu

Tests are first performed on flat surfaces because EXW of flat surfaces is easier than on rods and

curved surfaces.

Stage 2:

Study, calculation and experimental work on explosive welding together of horizontal surfaces ofunequal surface areas and different materials such as Fe-Fe, Al-Cu and Fe-Cu

Here experimental tests (horizontal, vertical and dual method) are carried out on flat surfaces with

different dimensions.

Stage 3:

Study, calculation and experimental work on explosive welding of curved shapes

Tests are performed on curved surfaces based on the results from the tests on flat surfaces. A steel

rod is used at this stage.

Stage 4:

Study on explosive welding of cylindrical surfaces with different materials such as Fe-Fe, Al-Cu,

and Fe-Cu

Experiments are performed on cylindrical surfaces such as pipes and tubes.

Stage 5:

Control of explosive parameters (explosive materials process parameters, mechanical testing)

EXW parameters such as explosion velocity and flyer plate velocity are measured.

The work is presented in papers 1-7 and the areas covered are summarized in Table 1.

Table 1. Summary of papersPaper Area discussed Stages

6, 7 Horizontal EXW 1

5, 4, 7 EXW Simulation 1

1 EXW of unequal surfaces 2

2 EXW of curved shapes 3 and 43 Explosive materials 5


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1.7. Thesis OutlineThis thesis describes research during which more than 100 experiments were performed and the

results were applied in a large aluminum production company. There are at least 3 patentable

technologies described in the results. The thesis is organised as follows:

Part 1 Introduction: including background, objectives, motivation, limitations of the studies,

presentation of the methodology, formulation of the problem and outline of the thesis.Part 2 Theory: including definitions and expressions used in the thesis, and presentation of theory

in preparation for the scientific discussion.

Part 3Experimental Data and Results: including the experimental set up, results and test reports.

Part 4 Discussion: including discussion of results, calculations, and future work.

Part 5 Conclusions:including conclusions and practical output.

Part 6ReferencesPart 7 Summary of Papers


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sin

cos 2

sin cos 2

2 sin

2 cos

2 cos 2

sin

(1)

From the sine equations in triangle SBD we can write:

cos 2

cos 2

sin

(2)

(3)

Where is the velocity of the flyer plate in relation to point s and is the velocity of welding,equal to the collision velocity.In a parallel set up =0 and = = , and from the previous equations:

2 sin

2

(4)

The selection of parameters is based on the mechanical properties, density, and shear wave velocity

of each component, and many of these are determined experimentally. Considerable progress has

been made in setting up the optimum parameters required to produce an acceptable bond.

The parameters involved in the process (such as ,,, , etc.) are defined in a specialdiagram called the Welding Window (WW) that has been proposed by various authors [36].

2.2. The impact wave in explosive welding

In EXW the pressure created in the region of the detonation front of the explosive charge is used toprovide rapid acceleration of the flyer plate to a high velocity prior to impact on the parent plate.

The flyer plate velocity depends on the amount of explosive charge and the stand-off distance. The

pressure produced in the detonation front transmits into the flyer plate as a stress compression wave.

When the compression wave reaches the back surface of the metal slab, it is reflected as a tension

wave, and the velocity of particles is doubled [1] (Figure 3a). The same phenomenon occurs at the

edges of the plates where the impact wave propagates horizontally. This edge effect creates

problems for the quality of welding at the edges.

The pressure waves in the flyer plate are oblique and when they are reflected from the back surface

of the plate, they give rise to both a dilatational wave and a shear wave (Figure 3b). Many authors

have written on this subject. Examples of their conclusions include:

a) A low stress wave transmits through the solid at the speed of sound, which can be readilyderived from the theory of elasticity [2].


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b) At high pressure, as in a shock wave, the shear strength becomes negligible compared to the

pressure and the material behaves like a fluid [3].

c) El-Sobky and Blazynski [1] suggested a mechanism for the stress wave, represented in

Figure 3d. In their model, a single compression wave (symbolized by a full circle) is generated

at the collision point, while successive reflections from both the flyer and parent plate

(represented by the dashed circles) generate the wavy shape at the contact point that can be

seen under the SEM microscope.

a

b

c dFigure 3. Impact wave reflection [1]

a) Reflection of a compressive stress wave from back surface of the metal plate

b)Detonation of a layer of explosive in contact with a metal platec) Stress wave mechanism of surface wave formation

d) The wavy shape of the contact point

2.3. Predicting the wavelength

Figure 4 shows the shape of the impact wave at the interface of the plates. The wavelength can be

derived from this figure as follows:

Figure 4. Shape of the wave at the interface of the plates [1]

= C . A (7)

Where A is amplitude and C is a constant.

2Axx


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The plastic work is therefore:

(8)

Where lis the length of the plate and Y is the ultimate yield point of the metal.

The kinetic energy can be expressed as:

1 2 (9)

whereis the kinetic energy andis the thickness of the flyer plate.The wavelength is obtained by equating the plastic work and the kinetic energy. The wavelengthcan therefore be determined from Equations 4, 8 and 9we could:

2 (10)

k has been measured experimentally at 28, therefore:

2 2

(11)

In addition, we know from experimental data that is obtained as follows:

2 (12)

2.4. Bonding criteria

Previous experiments have shown that there are critical values for the geometry and the collision

parameters which have to be observed for successful welding. These are summarized below.

Velocity limit

Because a jet must be formed at the collision point, the collision angle is critical and it is a

function of (the collision velocity). It has been shown experimentally that and (the velocityof the flyer plate) must be less than the speed of sound in both metals. It is known that at supersonic

velocities, the dynamic pressure is not sustained for long enough to support the changes of inter-

atomic flow and stability in the collision area. As is dependent on and , it can be adjusted bysetting up an initial angle of obliquity (). The speed of sound provides an upper limit for and.These velocities may be increased by increasing . is assumed constant in the welding area in anexplosive welding process [10].

Collision angle()

The collision angle must be between 5 and 25 [1].

5 25 (13)


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Pressure limit

A minimum impact pressure () is required to impart sufficient impact energy to produce a weld. It

has been suggested by Wylie et al.[1] that the impact energy required is related to the strain energy

and the dynamic yield strength of the flyer plate. The upper limit of the impact energy is determined

by the need to avoid excess heat and possible melting by viscous dissipation and the consequent

formation of weak layers. This upper limit depends to the lower melting point of the weld.

Stand-off

The stand-off distance must be sufficient to allow the flyer plate to accelerate to the required impact

velocity. The minimum stand-off distance has been empirically determined as half the thickness of

the flyer plate. An empirically determined formula for the suggested stand-off distance [1] is:

S=3 K Xe C/M (14)

Where S is the stand-off distance, K is between 4 and 7, depending on the impact velocity, M is the

flyer plate mass, C is the explosion mass and Xe is the thickness of explosive material.

Surface flatness

The flatness is very important in the EXW process. This is because imperfections in the surface

cause the jet to be concentrated at a point, which produces high temperatures at the contact points,

thereby preventing high quality welding. For a successful test, surface flatness in the range of 2 to 3

microns is sufficient.

Explosive material

High velocity explosive materials are not used in the EXW process because of the risk of damage to

the flyer plate during the test.

A special diagram called the welding window or weld-ability window is used to describe the state

of plates at the interface and in the weld area. The critical parameters used to establish a weld-

ability window are

The critical impact angle for jet formation

The collision velocity

The kinetic energy and impact pressure that is indicated by

2.5. Welding window

The welding window (WW) includes straight and curved regions. In order to draw the WW the

relationships between the initial conditions (the angles and and the characteristics of the

explosive) must be established. The WW lies within the boundaries of 7 parameters as shown in

Figure 5. The parameters , , , , ,, and the properties of the material determine the WW.

This diagram can be drawn in both the - and - plane and displays an area within which the

weld is available. In this thesis the WW is drawn only in the - plane.


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Figure 5. An example of a WW diagram in the Vc-plane

Critical angle limit for jet formation [line a-a]The most important condition for welding is jet formation. This must occur at the contact point for

successful welding to occur. Theoretically, jetting will occur if remains subsonic. However, inpractice a minimum angle is necessary to satisfy the pressure requirements. Jetting occurs to the left

of the line a-a in Figure 5, which represents the critical angle cwhich is necessary for jetting.

Abrahamson suggests the following relationship betweenand [24]:= 10(-5.5) (15)

Upper limit of [line b-b]Line b-b in Figure 5 describes the upper limit of , which is predictable at 1.2 to 1.5 times thespeed of sound, and which also limits the other WW parameters.

Lower and upper limit of[lines c-c and d-d]The lower and upper limits of the dynamic angle were experimentally obtained by Bahrani and

Crosslan [21]. They suggested a lower limit of between 2 and 3 and upper limit of 31 for in a

parallel geometry. Suggested minimum and maximum values of the initial angle in an inclined set

up are 3 and 18 respectively.

Lower limit of [line e-e]Equation 16 defines the lower limit of for bonding as proposed by Simonov [10].

2 (16)

Cowan [27] defined the lower limit of according to the fluid hypothesis as follows:

22

(17)


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25

Tests to be carried out before welding:

a) Measuring and

Dutrich method

One of the easiest and oldest methods for measuring the detonation velocity is the Dutrich method.

In this method wire of length L with a detonation velocity is held in contact at two differentpoints to an explosive material of unknown detonation velocity . The distance between thecontact points is L1 (shown in Figure 6). The middle of the wire is marked on a thin aluminum,

brass or lead plate. The explosive wave passes through both ends of the wire. They collide at a point

a distance L2 from the middle of the wire. This collision creates a mark on the abovementioned

plate.

Figure 6. Determination of detonation velocity by Dutrich method [4]

The time that it takes for the explosive wave to travel from each contact point to the collision point

is the same.

t1=t2

L1/ VD+ (L /2 L2)/ Vd= (L /2) / Vd+ L2/ Vd

L1/ VD=2 L 2 / Vd

VD= VdL1/ 2 L 2 (33)

L1, L2and are known - can therefore be calculated from equation 33.

Pin contactor method

can be measured using a complex recording system to record electrical pulses. In the EXW

process when the flyer and parent plates are parallel, is equal to . This provides the basis for

the pin contactor method.

Velocity probe method

and can be calculated directly by using measuring probes to draw the location-time curve.

Each probe includes a closed-ended aluminum tube and a sensitive insulating wire. The voltage

variation after the explosion, caused by the decrease in the length of the insulating wire is measured

and indicated by oscilloscope.

Slanting wire method (for measuring and )


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26

This is a simple electrical method which is used to obtain contact parameters using electrical probes

and an oscilloscope.

Photographic method (for measuring )

This method can be used to measure continuously.

Radiography method (for measuring )

This is similar to the photographic method except in that it uses Xrays. Radiography can be used to

show the location and the moment of contact. The angle is therefore measured directly.

b) Measuring explosive power

Ballistic mortar method

In this method an explosive charge of 10 gram is used to fire a standard shot at the end of a

pendulum. The angle of recoil following the explosion is measured and used to calculate the

explosive power.

Trauzl method

In this method the explosive is placed in a cylindrical hole in a lead block. The remainder of the

hole is filled with sand. The explosive power can be calculated by measuring the increase in thevolume of the hole after detonation.

c) Measuring sensitivity

The sensitivity is used to determine the necessary load to initiate detonation. A small booster charge

is placed between the detonator and the main charge. The mass of the booster charge required to

initiate a stable detonation is used to calculate the sensitivity.

Tests to be carried out after welding

a) Primary tests

These including sizing, cutting, machining and stress relief.

b) NDT tests

Non destructive tests, including ultrasonic and radiography tests.

c) Mechanical tests

Impact resistance test

The impact test is used to check the strength of a weld under dynamic load.

Chisel or peel off test

This is a simple but very important test, and is illustrated in Figure 7. A chisel is used to try to

separate two the plates. Easy separation of the plates indicates incomplete welding.

Figure 7. The chisel test [1]


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27

Tensile tests

Including shear and tensile tests.

d) Metallographic tests

Metallographic tests are performed using scanning electron microscopy (SEM). SEM is a useful

method for evaluating the quality of welding. Under SEM, a perfect weld has a uniformly wavy

geometry. Irregularities or cracks indicate problems with the weld.

2.8. Simulation of the explosive welding process

As most results of EXW have been obtained by explosive experiments, repetition of similar

experiments can be avoided by using modeling and simulation. In addition, result prediction,

parameter selection and wave distribution determination can be performed by simulation software.

The most important simulation software packages in impact mechanics are ABAQUS, ANSYS, LS-DYNA, AUTODYN, RAVEN [39] and COMSOL [40]. Stress, strain, pressure, temperature

distribution, behavior of materials and displacement of energy can be simulated in two or three

dimensions. Figure 8 shows a 2D simulation of pressure distribution in an EXW process in

AUTODYN software.

Figure 8. Pressure simulation by AUTODYN software [34]

COMSOL software is used to check results in some sections of this thesis [40].


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28


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29

3. ExperimentalData, Results and Calculations

3.1. Experiment setup

The set up has been done in 5 stages:Stage 1: EXW experiments on horizontal and flat surfaces with different materialsThe materials used and their specifications are listed in Tables 2, 3, and 4. The setup for an

experiment with a Fe-Fe joint is shown in Figure 9.

Fe-Fe

Table 2. Specification of Fe-Fe horizontal setupNo. Flyer plate Parent plate Stand-off . Explosive . VD Buffer Detonator

1 Fe-

Fe-

0 Ammonium-nitrate:50 gr.

dynamite : 10 gr.

.8

1.2

2000

3000

Al

foil

1

2 Fe-

Fe-

4 dynamite : 40 gr. 1.5 4000 Al

foil

1

Figure 9. Setup for Fe-Fe welding

Al-Cu

Table 3. Specification of Al-Cu horizontal setup

Flyer plate Parent plate Stand-off . Explosive VD Buffer Detonator

Al-

Cu-

0 Ammonium-nitrate:

110 gr.

Dynamite: 40 gr.

.8

1.2

2000

3000

Al foil 1

Fe-CuTable 4. Specification of Fe-Cu horizontal setup

Flyer plate Parent plate Stand-off . Explosive VD Buffer Detonator

Fe-

cu-

0 Ammonium-nitrate :50 gr.

Dynamite : 10 gr.

.8

1.2

2000

3000

Al foil 1

Stage 2: Experimental work on EXW experiments on unequal flat surfaces with different

materials

Horizontal, vertical and dual experiments were performed on unequal flat surfaces. These tests were

performed prior to the subsequent stage of welding on curved shapes because explosive welding on

cylindrical shapes is effectively welding on surfaces of unequal surface areas. The specifications of

the experiments are shown in Tables 5 and 6. Figures 10 and 11 show the experimental setups of thedifferent arrangements (horizontal, vertical and dual). In order to prevent problematic edge effects


30/55

when wel

in Figure 1

Horizont

No. Flyer

1 Al 30

100

2 Al 50

120

Vertical u

Flyer plate

Al 50

120

Al 50120

ing unequ

0 and will

l unequal

plate Par

Cu-

20

Cu-

20

nequal sur

Ta

Parent plat

Cu-100

200

10

Cu-100 200

10

l surfaces

e explaine

urface ar

Table 5. Sp

ent plate

100

10

100

10

a

face areas

ble 6. Specificat

e Stand-o

ifferent se

d in subseq

as (Al-Cu)

ecification of Al

Stand-off

Figure 10.a)

b)

(Al-Cu)

ion of Al-Cu Ve

ff .

0

0

tups were

uent sectio

-Cu, unequal su

Explo

0 Dynapowd

75

0 Dynapowd

75

Unequal surfacAl-Cu, unequa

Al-Cu, unequal

rtical unequal s

Explosive

Dynamite po

75

Dynamite po75

sed for so

s.

rface area, hori

ive .

iter

1.2

gr.

iter

1.2

gr.

area set upsurface area, h

surface area, h

rface area and

.

wder

wder

1.

g

1.g

e experim

ontal set up

VD

660m/s

660m/s

b

rizontal set up

rizontal set up

dual method se

VD

.2

./

.2./

660m/s

660m/s

ents. Thes

Buffer

0

1 mm

Paper

0 1 mmPaper

(first test)

(second test)

up

Buffer

0

0

1 mm

Paper

1 mmpaper

30

are show

Detonator

1

1

Detonator

2

2


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31

Figure 11. Al-Cu vertical unequal surface area and dual method set up

Stage 3: EXW of curved metal plates

EXW experiments were performed on curved metal plates by applying the results of Stage 2. The

experiments are summarized in Table 7. Figure 12 shows the experimental setup for filling a small

hole in a plate and Figure 13 shows the experimental setupfor welding on a curved surface.

Table 7. Specification of Stage 3 experiments

No. Flyer plate Parent plate Stand-off .

Explosive VD Buffer Fig.

1

0 Detonator(0.7 gr.)

7000 m/s Mastic 12

2 Cu 0.5

thickness

25 mmdiameter

Fe-80 mm

diameter 0 Detonator

(0.7 gr.)

7000 m/s Mastic 13

Figure 12. Set up for filling a small hole

Figure 13. Set up for welding on a curve

Stage 4: EXW on cylindrical surfaces with different materials

EXW experiments were performed on pipes and tubes. The materials and physical specification of

the experiments are shown in Table 8. Figure 14 shows the setup used to fill a small hole to repair a

leakin a pipe.

Table 8. Specification of experiments in Stage 4Flyer plate Parent plate Stand-off . Explosive VD Buffer Fig.

Cu 0.5 thickness25 mm diameter

Fe 80-3 mm Pipe 0 Detonator(0.7 gr.)

7000m/s

Mastic 14


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32

Figure 14. Set up for repairing a leak and filling a small hole in a pipe

Key: 1-flyer plate Cu, 2-pipe Fe, 3-stand-off, 4-filler Fe, 5-detonator holder, 6- detonator

Stage 5: EXW Control parameters(explosive materials process parameters, test results)The specification of experiments is shown in Table 9. Figures 15 and 16 show experimental setups

for measuring Vdby the Dutrich method.

Table 9. Specification of experiments in Stage 5 (Dutrich method)

No. Wire

specification

Plate

specification

Explosive

thickness

Explosive

material

. L1 L

1 Cortex 5gr/m

Vd=6500m/s

8 mm AZAR* 1.4

gr./

2 Cortex 5gr/m

Vd=6500m/s

8mm AZAR* 1.4

gr./

3 Cortex 5gr/mVd=6500m/s

10mm AZAR* 1.4

gr./

4 Cortex 5gr/m

Vd=6500m/s

12mm AZAR* 1.4

gr./

5 Cortex 5gr/m

Vd=6500m/s

15mm AZAR* 1.4

gr./

6 Cortex 5gr/m

Vd=6500m/s

20mm AZAR* 1.4

gr./

*AZAR is an explosive mixture that includes TNT and ammonium nitrate.

Figure 15. Dutrich method setup using a thin Aluminum plate

Figure 16. Dutrich method setup using a Brass plate


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3.2.

Stage 1

The result

Stage 2

Figure 18(horizonta

Stage 3Figure 19

horizontal

Experi

of explosi

a

d

shows th, vertical a

hows the r

and curved

ental Re

e welding

Fi

e resultsd dual me

a

cFigur

esults of ex

shapes.

ults

on horizon

ure 17. Results

f explosihod).

e 18. EXW of u

a, b) Res

c, d) Res

plosive we

al and flat

b

e

of EXW on hor

a, b, c )Experi

d, e) Experim

f) Experiment

e welding

equal surface a

lts of Al-Cu joi

lts of Al-Cu joi

ding for fil

surfaces ar

izontal flat surf

ents for Fe-Fe

nts for Fe-Fe w

s for Fe-Fe wel

on mater

reas

nt in horizontal

t in vertical an

ling a smal

shown in

ces

welding

elding

ing

als with

b

setup

d dual method s

hole using

igure 17.

c

f

nequal su

d

etup

a detonato

33

face areas

r charge on


34/55

Stage 4

Fig 20 sho

Stage 5

The result

the explos

at the mid

aluminum

ws the resu

of the exp

on. The po

le point of

and brass p

No.

1

2

3

4

5

6

lt of an EX

Figure 2

eriments ar

int is mark

the wire ac

lates.

a

T

Thicknes

material

8

8

10

12

15

20

aFigure 19. EX

a) R

b) R

test for

. EXW with a

e summari

d by the e

cording to

Figure 21.

ble 10. Result o

s of explosiv

mm)

tests for fillin

esult of welding

esult of welding

illing a sm

etonator on a p

ed in Tabl

fect of the

utrich me

Dutrich metho

a) Test on a thi

b) Test on a Br

f experiments d

L2

me

----

139

125

116

110

110

ba hole

for filling a sm

with a detonato

ll hole in a

ipe for filling a

10. Figure

xplosion, s

hod. The e

test results

Aluminum pla

ss plate

escribed in Tab

ark accordi

hod (mm)

----------------

.25

ll hole

r on a curved s

pipe.

mall hole

21 shows t

howing the

xperiment i

b

te

e 9

g to Dutrich

--------------

ape

e contact

length L2,

s performe

34

oint after

and M is

d on both


35/55

35

3.3. Calculation, numerical and simulation results

Welding windowWWs are drawn for the experiments. Results of some sample calculations are shown below.

Cu-Fe(Tables 7 and 8)

Flyer plate: Cu

0

9

4900

Parent plate: Fe 0

000 Line a-a

Table 11 shows the results of calculations using Equation 15.

Table 11. Calculation for line (a-a)

0 5 10 15 20 25 30 35 40 45

5.5 6 6.5 7 7.5 8 8.5 9 9.5 10

Line b-b

From Section 2.5 we get: 4900 0 Line c-c

From Section 2.5 we get: Line d-d

From Section 2.5 we get: Line e-e

Equation 17 gives

as follows:

0 0 Line f-f

Equation 21 gives as follows: 4 0

94900

00 Equation 22 gives as follows:

min=0 40 0 9

0

For an increased safety margin the higher value (300 m/s) is used and is calculated fromEquation 4. The results are shown in Table 12.

Table 12. Calculation for line (f-f)

3 5 10 20 30 40

5.7 3.52 1.72 0.86 0.57 0.44


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36

Line g-g

Equation 25 gives as follows:=

The results are shown in Table 13.

Table 13. Calculation for line (g-g)

0 5 10 15 20 25 30 35

------------- 12.25 7 .15 4.06 3.4 2.95 2.61

The WW for Cu- Fe is drawn using these results and is shown in Figure 22. The weld-able area is

indicated by the crosshatched area.

Figure 22. WW for Cu-Fe joint

Fe-Fe(Table 2)

Flyer plate: Fe

Parent plate: Fe

Line a-a

Table 14 gives the results of calculations using Equation 15.


0 5 10 15 20 25 30 35 40 45

5.5 6 6.5 7 7.5 8 8.5 9 9.5 10

Line b-b

From Section 2.5 we get: Line c-cFrom Section 2.5 we get:


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37

Line d-d

From Section 2.5 we get: Line e-e

Equation 17 gives as follows:

=1460

Line f-f


=(

Vc is obtained from Equation 4 and the results are shown in Table15.


3 5 10 20 30 40

5.1 3.18 1.55 0.77 0.51 0.4

Line g-g

Equation 25 gives as follows:=

These results are shown in Table 16.


0 5 10 15 20 25 30 35

-------- 12.25 7 .15 4.06 3.4 2.95 2.61

The WW for Fe-Fe is drawn from these results (Figure 23) and the weld able area is indicated.

Figure 23. Welding window for Fe-Fe joint

Al-Cu (Tables 5 and 6)

Flyer plate: Al


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38

Parent plate: Cu

Line a-a

Table 17 shows the results of the calculation using Equation 15.


0 5 10 15 20 25 30 35 40 45 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10

Line b-b

From Section 2.5 we get: Line c-c

From Section 2.5 we get:

Line d-dFrom Section part 2.5 we get: Line e-e


=1466 Line f-f

Equation 22 gives as follows:= (

= (

Equation 22 gives as follows:min=

For greater safety we use =200 and from Equation 4, can be obtained. The results are shown intable 18.


3 5 10 20 30 40

3.8 2.34 1.14 0.57 0.38 0.29

Line g-g


The result is shown in Table 19.


0 5 10 15 20 25 30 35 ---------- 12.25 7 .15 4.06 3.4 2.95 2.61


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39

Figure 24 shows the WW for Al-Cu drawn from these results. The weld able area is indicated.

Figure 24. WW for Al-Cu joint

Explosive material

EXW experiments (horizontal and vertical) are arranged in a parallel setup with =0 and =. Byconsidering WW curves, a suitable detonation velocity and collision angle can be obtained.

= = 2500-6000 m/s = 25-10= 6000, = 10The ratio e/m can be calculated from Equations 4 and 32:

0.612

2

2

2 0.1 0.

where e is the explosive mass and m is the flyer plate mass.

For example, for an iron flyer plate weighing 100 grams, the explosive mass is estimated to be 80grams.

An alternative calculation for uses Equation 31 as follows:

= 0.557 C0= = 0.5576000 = 3462 flyer plate Fe= = 0.557 4900 = 2822 flyer plate Cu= = 0.5576400 = 3692 flyer plate Al

can be calculated from using Equation 18 as follows:

For Al with =3692 1.1

.

=.09 radian =5.5

Stand-off distance

As described in Section 2.4, the stand-off distance is selected at half or equal to the thickness of the

flyer plate. The optimum energy for jet formation may be obtained by varying the stand-off

distance.


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40

Calculation for Stage 5

is given by Equation 33 according to the results from Tables 9 and 10 as shown below.L1=150 mm, L=950 mm, L2=139 mm, =6500 m/s

= L1/ 2 L2=2

Table 20 shows the results of calculation of for different thicknesses of explosive material.

Table 20. Calculation of VD for different thicknesses of explosive material

No. Thickness(mm)

L2 mark(mm)

VD(m/s)

1 -------- 8 ------

2 139.25 8 3500

3 125 10 3900

4 116 12 4200

5 110 15 4500

6 110 20 4500

Table 21 shows the measured detonation velocities in tests with different explosive materials of

different thicknesses at high, medium and low explosion velocities.

Table 21. Results of testing on measuring detonation velocity for different kinds of explosive materials

Thickness of

explosion (mm)

High velocity

explosionVd(m/s)

Medium velocity

explosionVd(m/s)

Low velocity

explosionVd(m/s)

0 0 0 0

5 5000 2600 1500

10 7300 3800 2200

15 7500 4500 2700

20 7500 5000 2900

25 7500 5000 3100

30 7500 5000 3300

35 7500 5000 3300

Wavelength

Equations 11 and 12 give the wavelength and amplitude for a 3 mm Fe plate and a dynamic angle=10.

2 2

2

2

Similarly, for a 5 mm Al plate and =6 we get

3.4. Test Results

Measuring explosive parameters before welding

Measuring Figures 25 and 26 show the effect of the thickness of explosive material on detonation velocity as

recorded in Tables 20 and 21, found by the Dutrich method.


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41

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 10 20 30

T mm

VD

m/s

Figure 25. Effect of explosive material thickness on detonation velocity

Figure 26. Effect of explosive material thickness on detonation velocity in general

(High, medium and low explosive velocity)

Measuring explosive parameters after welding

Chisel test

Figure 27 shows the chisel test for peeling of the metal pieces.

Figure 27. Chisel test for 2 types of weld

Mechanical tests

Tables 22 and 23 show the results of mechanical tests - bond strength of explosive weld (BSEW) -

in two directions (width and length) for the two setups described in Table 6 and Figure 15. These

results are illustrated in Figures 28 and 29.

0

2000

4000

6000

8000

0 10 20 30 40

T mm

Vd

m

/s


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42

Table 22. Results of mechanical test in width direction

L mm 0 5 10 20 30 40 45 50

T(test1) kg/ 0 7 18 18 18 18 7 0

T(test 2) kg/ 15 18 18 19 18 17 18 14

Table 23. Results of mechanical test in length direction

L, mm 0 15 35 60 80 115 135 150

T(test1)

kg/

0 8 18 17 18 18 7 0

T(test 2)

kg/

16 18 18 19 18 18 17 15

Figure 28. BSEW in width directionSeries1. BSEW in simple setup in width direction of flyer plate

Series2. BSEW with groove in width direction of flyer plate

Figure 29. BSEW in length direction

Series1. BSEW in simple setup in length direction of flyer plate

Series2. BSEW with groove in length direction of flyer plate

0

10

20

0 10 20 30 40 50 60BSEW

Kg/mm2 Series1 Series2

L mm

0

10

20

0 20 40 60 80 100 120 140 160

BSEW

Kg/mm2

Series1 Series2

L mm


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44

The simulations are performed for flat equal and unequal surfaces. Figure 31 shows a typical

simulation result for pressure distribution and temperature effects in unequal surfaces according to

the setup shown in Figure 18.

a

b

Figure 31. Results of simulation

a) Pressure contour in unequal surface welding

b) Temperature distribution during explosion


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45

4. Discussion and future work4.1. Discussion

Requirements for successful bonding

Suitable setups for successful bonding can be identified through tests to measure various variables.Like a series of carriages that make up a train, a series of diagrams can be constructed, and the WW

in this composite diagram describes the test space where a successful weld can occur. In the

absence of this composite diagram, it is assumed that the most pertinent relationship is the one

between the pressure P, the impact velocity , parent plate velocity , and the effective strain. The

flyer plate attains its peak velocity at the collision point. On impact the velocity of the parent plate

at the collision point increases while the flyer plate velocity decreases. Under certain conditions, the

velocities of the flyer and the parent plate are the same. When the situation stabilizes at the collision

point and the pressure is high, inter-atomic bonding occurs.

Metallographic Tests

A fine structure was observed in most of the welding boundaries in SEM tests performed following

the EXW process. Nearly all the experiments with different shapes showed successful bonding.

Microstructure results of bonding are shown in Figures 30, which also shows the wavy pattern. In

the tests performed here, was measured at 0.7 mm and A was 0.14. These values confirmed the

results of the theoretical calculations in Section 3.3.

Chisel or Peel off Test

The bond strengths were measured by peel off and chisel tests. In some experiments, perfect results

were achieved (Figure 27), and there were no physical differences between the metals. The rupture

in these cases was in the weaker metal and away from the contact point. This was a confirmation of

the high quality of welding. Other experiments showed less impressive results, with joints that

separated easily (Figure 17a and c). This is likely to be for one of two main reasons - insufficient

explosive material or dirty surfaces.

Tensile Test

BSEW were measured in two directions in Stages 2 and 3. These tests showed that the bond

strengths were constant in the improved test with grooves throughout the contact surface according

to the setup shown in Figure 10b. In experiments using surfaces that lack grooves BSEW is very

low at the edges and increases towards the middle of the welding area. Figures28 and 29 show that

the strength reaches the maximum value within 1 to 1.5 times the thickness of the flyer plate fromthe edge. In the second test the edges were uniform and produced suitable joints after jumping. This

method is unique and is suitable for jointing two metals with different surfaces and does not have

the edge strength problems seen in Figures 28 and 29.

Discussion of Stage 4 (removing leakage)

The setup is one of the main problems when plugging a hole to stop leakage. The problem is

particularly difficult when the weld has to be applied in a wet area. A simple package is therefore

very desirable. This package includes detonator, a piece of thin copper plate, a piece of mastic as

holder, a piece of iron as filler and small rods to provide the stand-off. Figure 19a shows a

horizontal set up with the parent plate on the ground, which acts as an anvil. Figure 19b shows a

curved set up with a rod as the parent plate. Figure 20 shows a curved set up with a pipe as theparent plate. If the explosive is too high, the pipe may be damaged. A detonator has a small amount


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46

of explosive material, is cheap, and is very useful for this application. The use of a detonator alone

is suitable for pipe applications and this is a good method for repairing leaks in metal pipes.

This method is not suitable for large holes and is used only for leakage and for low pressure liquid

pipes. Other methods of explosive welding are better suited to welding high pressure thin pipes.

Hardness test

In most experiments the hardness of the flyer plate is increased in a predictable manner because ofimpact waves.

Discussion of Stage 5 (controlling the parameters), Dutrich method

Aluminum plates were used initially during these tests, but these damaged too easily because of

their small thickness. Although they could be replaced by thicker aluminum plates, 3 mm Brass

plates were used in subsequent experiments.

The Dutrich method is very simple, but an error is inherent in the test because the effect of the

detonator is not taken into account. In these experiments a detonator is used to initiate the

explosion. This influences the result of the test. The undesired effects can be minimized by placing

the detonator outside of the explosive material. The experiments show that most of the explosive

velocity of detonation is constant if there is more than 30 mm of explosive between the detonator

and the plate (Figure 25). This is an important result that should be taken into account in future

setups. 3 types of explosives are tested, and the results of Dutrich method testing on high, medium

and low velocity explosions are shown in Figure 26. This figure can be used to derive a formula for

the region where the detonation velocity is constant. It is divided to three parts (over 10 mm for

high velocity, over 20 mm for medium velocity and over 30 mm for low velocity).

Discussion of simulation

The simulations show 3D maps and profiles of a number of physical parameters, such as contact

pressure, shear stress, normal stress, plastic strain, effective strain, strain rate, internal energy,

kinetic energy, temperature, velocity of the flyer plate at the point of contact and the angle ofcontact. Figure 31 shows the two important parameters of pressure and temperature. Plots of mesh

and material boundaries, and quantities as a function of time and distance for given coordinates are

also available. The maximum element size used is 1/35th (in 2D) of the maximum distance in the

geometry. In this case the element size used is 1/50. However, the maximum element size can be

explicitly specified in the Maximum element size edit field in the Custom mesh size area. A finer

mesh can also be created by selecting Custom mesh size and typing 0.01 in the Maximum element

size edit field. The mesh used here consists of 30758 triangular elements. The highest pressure

predicted is at the collision point and is on the order of 109Pa (Figure 31). In the case of the AZAR

explosive with high detonation velocity (6600 m/s), the pressure wave makes a 30 angle with the

horizontal surface in the base plate. This is because the velocity of detonation is higher than the

speed of sound in the material. Shock waves reflected at the end of the plates produce scabbing andspilling on the edges of metals. This phenomenon reduces the bond strength (BSEW) in the edges,

and is considered an important problem. Scabbing and spilling may be reduced by varying the

velocity of explosive charge and thereby regulating the welding, but the experiments show a

reduction in the strength of bonding in the edges when this is done, and formation of bonding in the

middle of welded metals (Figure 32). The simulation results confirm this and show that the highest

bond strength is in the middle of the plates (see Figure 31).

Figure 32. Bonded areas measured by ultrasonic test for Al-Cu joint


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4.2. Future workThis thesis considers the impact mechanics area in the explosive welding field in the plastic zone.

More complete studies are needed on explosive welding or impact waves in plastic and elastic

zones. The most important of these future studies are described below.

Future work on explosive welding

-Simulation and numerical study to predict, compare and control parameters and experimental

results.

-Underwater EXW experiments.

- EXW on pressurized pipes.

- EXW on large surfaces with unequal surface areas.

- EXW on special multilayer surfaces.

- Other applications of EXW such as special cladding and stress relief.

Future work on plastic zone of impact waves

- Forming and shaping of metals

- Deep drawing

- Powder metallurgy

Future work on elastic part of impact waves

- Ultrasonic waves

- Measurement by impact waves


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5. Conclusions

5.1. Concluding remarks

As described in this thesis, it is possible to join two or more metal parts using impact waves. Whenexplosive materials are used to generate the necessary impact, the process is called explosivewelding (EXW). This method does not require a heat source and no melt is created during the

process. Theinterface temperature throughout the process is lower than the melting points of either

material. EXW does not have the common problems of welding at the contact point and with

suitable control of the process parameters during the test, the welding is of the highest quality.

These process parameters are predictable and measurable. There are some limitations in EXW at

present and its use in mass production is still at a very early stage of research. This method covers a

multidisciplinary research area, many results have been reported, and many theoretical and

numerical methods have been invented in the last few decades. A number of numerical software

packages have been created to analyze the process but complete simulation of EXW process is not

yet possible.The most important conclusions of this thesis are as follows:

Stage 1:- The EXW joining of CuAl and Al-Al was successful and no fault was formed at the interface.

- A mixed explosion material with lower velocity for joining Al-Cu and Al-Al plates has been used

in the tests.

- The EXW joining of Fe-Fe was not successful at the first attempt, and the joint separated in the

peel off test. A satisfactory joint was obtained after cleaning the surfaces and replacing the

explosive material with a high velocity explosive material.

- For Fe-Fe joints, a medium or high velocity explosive material must be used.

- For Fe-Fe joints grinding the surfaces is recommended.- For all types of joints stress relief is recommended.

Stage 2:- The major result of these experiments was the discovery of a suitable solution for a uniform

contact in explosive welding of two metals with different surfaces. The solution to the problem was

the use of grooves on the flyer plates and jumping the edges during the test.

- An experimental distance to achieve uniform contact strength 1 to 1.5 t was confirmed.

- No joining fault was seen at the interface, and no melting voids or inter-metallic compounds were

observed in the SEM images. The joining of CuAl was successful after the explosive welding.

-The design of a successful vertical setup was another important result of these experiments. In this

set up there is no requirement for an anvil and the impact waves damped each other in the contactzone. Another advantage of this method was savings in time and cost in comparison to the

horizontal set up.

- Simulation results confirmed the edge problem. The groove model is suggested according to

simulation results.

Stage 3:- In curved welding it is best to place the explosion start point at the center of the flyer plate.

- It is possible to use the horizontal stand-off formula for the curved setup.

- In the curved setup the collision angle is variable, but if the flyer plate shape is changed to

resemble the parent plate, this angle remains constant and welding occurs as it does in the

horizontal set up.

- It is possible to use a detonator alone instead of explosive material for thin plates.


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Stage 4:

-For repairing very thin pipes, a welding fixture must be used, but for pipes and vessels that are

more than 10 mm thick, a plate up to 5 mm thick can be used instead of a fixture.

-A new technique and a simple solution has been used to stop leaking in pipes. This method is

useful only for low pressure and small leaks.- Primary mechanical tests such as peel off have been carried out.

- Both surfaces of the plates must be clean.

- A pressure test is recommended to check the reliability of the repaired leak.

- Suitable fillers must be used to fill the hole before welding. The setup must be arranged so that the

filler does not jump during the explosion. Figure 33 shows a useful set up for this operation. This

system is suggested for use when:

(34)

Where is the diameter of the hole and D is the diameter of the pipe.

Figure 33. A set up for filling a hole before welding

Stage 5:

- There is a relationship between the thickness of explosive material and the explosive velocity.

The experiments described here support this statement and confirm previously published results.

The velocity of material in the small layer is variable and there is a different curve for each material

and a minimum thickness to achieve constant velocity derived from this curve. In this research

work, a curve for AZAR explosive material was obtained by experiment using the Dutrich method,and the results were applied in later experiments.

- Experiments show that high velocity explosive materials reach constant detonation velocity in a

thin layer. Figure 26 shows the relationship is as follows:

High velocity materials thickness>10 = constant

Medium velocity materials thickness>20 = constant (35)

Low velocity materials thickness>30 = constant

- In this thesis brass has been introduced as an alternative plate material for the Dutrich method.

- The location of the detonator can be changed to decrease the error when using the Dutrichmethod. The smallest error was achieved when the detonator was placed outside the explosive

material.

5.2. PracticalOutput

Aluminum, copper, and steel are the most common metals used in high-current conductor systems.

Use of these metals in dissimilar metal systems often maximizes the effects of special properties of

each material. However, joints between incompatible metals must be electrically effective to

minimize power losses. Mechanical connections that include aluminum create high resistance

because of the presence of the self-healing oxide skin on the aluminum component. Because thisoxide layer is removed by the jet in EXW, the interface of an explosion-clad aluminum assembly

offers no resistance to the current.


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6. References

1.Experimental investigation of the mechanics of explosive welding by means of a liquid analogue.

El-Sobky, H., Blazynski, T. Z.Colorado : Fifth International Conference on High Energy Rate

Fabrication, 1975. Vol. 4.

2. B.Crossland.explosive welding of metals and its application. new york : oxford university press,

1982.3. the Academy of Science. Ukraine : E.O. Paton Electric Welding Institute .

4. Darvizeh, A.Research notebooks. Iran : Gilan University.5. Welding Handbook.

6. SME

; Handbook, Tool and Manufacturing Engineers. 1984. pp. 19-1. Vol. L.

7.Metals Handbook, Explosive welding. Vol. 6.

8. welding, Mechanism of wave formation at the interface in explosive. Chemin, C. and , No.2, ,.

May 1989, ACTA Mechanica sinca, Vol. 5. 2.

9.Permissible rang of parameters for interface wave formation in explosive welding; India. Gupta,

R.C and welding.India : s.n.10. Binding criterion for metals with explosive welding combustion, Explosion and shock wave.

Simonov, V.A.1991.

11.Explosive welding status. Phillipchuk, V.1961. ASTME Creative Manufacturing Seminar. pp.

P65100.

12. A. Williams, B. Crossland and J.D.Explosive welding. 1970. pp. 79100.

13. Explosive cladding of large steel plates with lead. Carpente, H. Otto and S.H. 1973. Met.

Mater. pp. 7579.

14.Microstructure of explosively bonded interface between titanium and very low carbon steel as

observed by TEM. Onzawa, T., Iiyama, T., Kobayashi, S., Takasaki, A., Ujimoto, Y.1985.

15.Analytic solutions of liquid-drop impact problems. M.B. Lesser, , Proc. ., ), pp. .London : R.

Soc, (1981. pp. 289308. A 377 .16. The geometric wave theory of liquid impact. Lesser, M.B., Field, J.E.1983. Sixth International

Conference on Erosion by Liquid and Solid Impact.

17. The impact of compressible liquids. Field, M.B. Lesser and J.E.1983. Ann. Rev.Fluid Mech.

Vol. 15, pp. 9712

Documents

Impact Wave Process Modelling and Optimization in High Energy Rate Explosive Welding