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Page 1: Explosive Metal Forming, Hani Aziz Ameen

Explosive metal forming

Page 2: Explosive Metal Forming, Hani Aziz Ameen

أو لم یرى الذین كفروا أن السماوات ].30: األنبیاء[} واألرض كانتا رتقا ففتقناھما

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Introduction:

•is the shaping of blocks or sheet metal by quickly applying energy to them for a relatively short interval of time.

•is a metal forming technique that uses the energy generated by an explosive detonation to form the metal work piece. This process can deliver a great deal of flexibility in the metal-forming process.

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•To cold shape sheet or plate into a die and indeed to free-form it. A charge of high explosive (e.g TNT) either as a point or a distributed charge is “ Stood –off” from the work piece and then detonated. This operation is carried out under water. Large steel plates of 3.6m diameter and 25 mm thick were dished by this method using about 50 kg of explosive .

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•The simplified scheme of the process is pictured in Fig. 1. The energy releasing upon explosion of high explosive substance acts directly or through a conductive medium (water) on a sheet billet and deform it to fit the die profile. •The waves generated at dynamic forming contribute to oscillatory pressure change in a basin that produce beneficial effect on deformation process.

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Fig. 1

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(Fig. 2) Explosive forming set up

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Set upThe system used for standoff distance operation consist of :-•An explosive charge.•An energy transmitted medium (water, air, oil).•A die assembly.•The work piece.Figure 2 shows an arrangement of standoff distance explosive forming operation. The die assembly is put together on the bottom of the tank. Work piece is placed on the die and blank holder placed above. A vacuum is then created in the die cavity. The explosive charge placed in position over the center of the work piece. The explosive charge is suspended over the blank at a predetermined distance. The complete assembly is immersed in a tank of water.After the detonation of explosive , a pressure pulse of high intensity is produced. A gas bubble is also produced which expands spherically and then collapse until it vents at the surface of the water. When the pressure pulse impinges against the work piece, the metal is displaced into the die cavity.

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Explosives

Explosives are substances that undergo rapid under chemical reaction during which heat and large quantities of gaseous products are created. Explosive can be solid (T.N.T-trinitrotoluene), liquid (Nitroglycerine) or gaseous (Oxygen and acetylene mixtures).Explosive are divided into two classes; Low explosive in which the ammunition burns rapidly rather than exploding, hence pressure build up is not large, and High explosive which have high rate of reaction with a large pressure build up. Low explosives are generally used as propellants in guns and rockets for the propelling of missile.

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Die materials

Different materials are used for the manufacture of the dies for explosive working, for instance high strength tool steels , plastics, concrete. Relatively low strength dies are used for short run items and for parts where close tolerance are not critical, while for longer runs higher strength die materials are required. plastic faced dies are employed for light forming operation, cast steels, and ductile iron for medium requirements, fiberglass for low pressure and few parts, fiberglass and concrete for low pressure and large parts and epoxy with concrete for low pressure and large parts.

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Transmission medium

Energy released by the explosive is transmitted through medium like water, air, oil, gelatin and liquid salts. Water is one of the best media for explosive forming since it is available readily, inexpensive and products excellent results. The transmission medium is important regarding pressure magnitude at the work piece. Water is more desirable medium than air for producing high peak pressures to the work piece.

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Formability aspectsFormability has been defined as the ability of asheet metal to be deformed by a specific sheetmetal forming process from its original shape toa defined shape without failure. In normalexplosive forming operations, the majorcharacteristics of the work piece metal thatdetermine formability are ductility andtoughness.

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Case study of domed shape formingThe explosive forming of domes

The explosive forming of metal blanks is accompanied by large plastic deformation at high rate of strain, under bi-axial or tri-axial states of stress.

Strain energy of deformationIn order to reach at a rational method for predicting the amount of explosive charge it is necessary to compute the strain energy of plastic energy of plastic deformation of the metal part of work piece:-

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Where U is strain energy

For most strain hardening materials:-

K is constant and n is the strain hardening exponentFor strain hardening material

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Using centrally located charge as shown in the fig.3 to explosively form of a flat circular blank, the distribution of strain across a diameter of the blank will vary with the ratio of standoff distance, (L) to the diameter of the die opening, (D).

Fig.3

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Estimating of total strain energy and the weight charge required for an explosively formed dome in single shot.ho = initial blank thickness w1 = max. deflection R = radius of curvature of the segment of the sphere h = final thickness of deformed blank

R2= (D/2)2+(R-w1)2

The surface area of a segment of sphere = 2πRwVolume of deformed material is constant è (πD2/4) ho =2πRwh hence ..

22

2

4wDD

hh

o +=

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rε = radial strain

θε = circumferential strain

tε = thickness strain

For incompressible rε +

θε +tε =0

+−=⇒=

2

41lnDw

hdhd tt εε

rε θεWe assume uniform strain =

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+=

2

41lnDw

effε

122

41ln14

+

+

+=

n

o Dw

nKhDU π

In order to determine the quantity of explosive required . The total strain energy of the deformation U should be equated the total explosive energy ET .

WeET )cos1(21

1 φη −=

Where W=mass of charge e=specific energy of charge (from table)

)/(7.323.41

1

DL−=η

for L/D <0.5

)/(83.202.41

1

DL−=η

for L/D <1

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Conclusions and disruptionsIn explosive metal forming chemical energy from the explosive is used to generate shock waves through a medium which are direct to the work piece at very high velocities. This process was mostly used to form large and bulky components typically for military and aerospace components and now can be used for small parts with complex shapes and low capital investment also the ability to vary energy levels over wide ranges provides greater capabilities than conventional forming methods. The process can be achieved by the simplest requirements such as one side die, the work piece and an explosive charge.Beside all advantage with some limitations and although it is not new process but one feel that the information about explosive forming is still not enough, may be due to the difficulty of understanding what happens exactly during the process so someone trying to simulate the method, another reason is dealing with explosive so the information are not available and forbidden.

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To provide a fully picture about explosive forming there must be additional information available.