Explosive Metal Forming - Hani Aziz Ameen

Explosive metal forming Dr. Hani Aziz Ameen

Technical College – Baghdad – Iraq- Dies and Tools Eng. Dept.

((Explosive metal Forming))

Dr. Hani Aziz Ameen

Asst. Prof. in Mechanical Engineering

Technical College - Baghdad

Dies and Tools Engineering Department

E-mail: [email protected]

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Introduction:

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.

Since explosive forming transmits the explosively-generated energy through water, it can simulate a variety of other conventional metal forming techniques.

has been an accepted metal-forming technique for almost 50 years. It has been used in a wide variety of applications in the automotive, aerospace, and maritime industries. PA&E has extensive experience using this technique to form a wide variety of metals. Typical forming projects yield very close tolerances and very high degrees of repeatability.

It can offer significant cost savings on short-run parts because it often only requires a one-sided tooling die. In the explosive hydro-forming process, the water slug applies force evenly over the surface of the work piece, as it forms into the cavity of the forming die.

Properties of materials at high rates of deforming are mostly always different of respective properties at static loading. This causes much influence on a great number of practical advantages. The use of dynamic effects and increase of materials plasticity are possible in the certain interval of deformation velocities, which determine technological process parameters for each kind of the prototypes manufacturing. To choose optimal parameters of deformation the relationship of material deformability and it’s properties should be expressed with a physically substantiated regularity, that will enable to consider different practical cases.

Benefits of Explosive Forming

Explosive-forming has many benefits. It employs lower tooling costs and uses stamping type applications which only require a one-sided tooling die. Explosive energy can be transmitted differently across the part, in order to concentrate force onto specific forming features. It has a large size capability and is suited to difficult configurations.





Explosively-formed parts can range up to 6 feet and have very few limitations. Explosive-forming can simulate many aspects of all other conventional forming methods, without their respective limitations.

improved quality of parts (by high-strength materials), perfecting the installation through adapting active media to different shapes of part, simply adapting to production process, reduction of production stages, flexibility of the process due to quick and simple transformation of the tool elements, low production costs. By Explosive forming as against traditional practice only one of two tools is required either a die or a punch. By this means labour consuming and expensive mutual matching of toolings is excluded. 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.

Fig. 1





(Fig. 2) Explosive forming set up

Set up

The system used for standoff distance operation consist of :-

• An explosive charge. • An energy transmitted medium (water, air, oil). • A die assembly. • The workpiece.

Figure 2 shows an arrangement of standoff distance explosive forming operation. The die assembly is put together on the bottom of the tank. Workpiece is placed on the die and blankholder placed above. A vacuum is then created in the die cavity. The explosive charge placed in position over the center of the workpiece. 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 workpiece, the metal is displaced into the die cavity.






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. 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. Kriksite and plastic faced dies are employed for light forming operation, cast steels, and ductile iron for medium requirements, fiberglass and kiksite 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. 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 workpiece. Water is more desirable medium than air for producing high peak pressures to the workpiece. Formability aspects Formability has been defined as the ability of a sheet metal to be deformed by a specific sheet metal forming process from its original shape to a defined shape without failure. In normal explosive forming operations, the major characteristics of the workpiece metal that determine formability are ductility and toughness. It is general practice not to exceed the elongation, as






determined by tension testing, in forming a part from the same metal that. The 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.

A. Strain energy of deformation In 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 workpiece:- U = σ ϵ ………………………… ( 1 )

Where U is strain energy

σ = √ (σ − σ ) + (σ − σ ) + (σ − σ ) …….(2)

ε = √ (ε − ε ) + (ε − ε ) + (ε − ε ) …….(3) For most strain hardening materials:- σ = K ε …………………………………………………….(4) K is constant and n is the strain hardening exponent For strain hardening material U = ε ……………………………………………(5) Effect of explosive standoff distance on strain distribution in the explosive forming of flat circular blanks 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

Estimating of total strain energy and the weight charge required for an explosively formed dome in single shot.

a- Computing the value of Do and Bo Do= D + re ………………………………………………(6) Bo= √4w + B ………………………………………..(7)

R

w

W1






= 1− 1− 4( ) ………………………(8)

b- Calculating of max. permissible depth of draw

w = (2n + 1)(4n + 13) …………………….(9)

A- If w ≥ this means we are needing only one shot.

B- If w < this means we are needing multiple shot (more than one shot).

c- If A condition is satisfied then effective draw depth(w∗) should be

calculated as follows:- U = (Bo − D ) ……………………………..(10)

w∗ = w − |U | + 0.57 r ………………………..(11) U = D h ln 1 + 4( ∗ ) ………..(12) U = π √ ( ) h Bo − ….(13)

d- Estimating of the charge weight using the geometrical method:- W= 2UT ( Ф) …………………………………….(14) e = specific energy of the charge = 4.23 − 3.7 for ≤ 0.5 …………..(15)

= 4.02− 3.7 for ≤ 1 …………….(16) cosФ = tan …………………………..(17)






Ф = tan …………………………….(18) e- If B condition is satisfied one must compute the depth of the draw

of the first shot and the required weight exactly as in the step c and d then must compute the depth of the dome and weight of the charge for second shot as follows:- Multiple shot explosive forming If h1 is a material thickness after first shot and h2 is a material thickness after second shot then:-

= ………………………………………….. (19)

= ………………………………………(20)

∴ the thickness strain εt is then given by:-

εt = ln h1h2 = ln 1+4w12D21+4w22D2 ……………………………….(21) ∴ ε = −εt

w2 w

1

Ф2

charge

Fig.5 Multiple shot forming sequence

h1

h2






∴ ε = ln ……………………………………….(22)

Using Hill formula ( = (2 + 1)) for max. permissible bi-axial strain energy and equating it to the max. effective strain at the end of the second shot we get:-

= xp (2n + 1) …………………………………(23)

Solving for we have

= 1 + exp (2n + 1) − 1 ………………(24) After obtaining w2 , now we can calculate strain energy per unit volume for the second shot:- U = Kn+1 ln (1+4w22D21+4w12D2

n+1 ……………………………….(25)

∴ the total strain energy of the deformation can be calculated by:-

(U ) = D h ln ( …………………………(26)

The explosive energy (ET)2 delivered to the blank by the second shot may be estimated as follows:-

(E ) = ζ (1− cosФ )W e ………………………………………….(27)

Where Ф is defined in figure 5, w is the weight of second charge and e is the specific energy of the explosive i.e. energy per unit weight of the charge.






Case study of domed shape forming Estimate the required (TNT) charge weight (W) to produce flanged dome shape made of Aluminium alloy 2014-0 with the following dimensions:- B =3.3m , D = 3m, w = 1m, ho = 0.02m and re=0.01m. ----------------------------------------------------------------------------- Solution From tables:

K = 320.4Mpa, n = 0.244, = 4.23− 3.7 = 3.6 and

e = 0.393 × 106 m.N/Kg and chose = = 0.167. 1- Do= D + 2 x re = 3.02 m.

2- Bo= √4w + B = 3.86 m, is a blank diameter.

3- To calculate max. permissible depth:

w = (2n + 1)(4n + 13) =1.072 m.

w > w so we need just a single shot to achieve the process. 4. w∗ = w − (B − B) + 0.57 r = 0.64 m.(but we apply w = 1m). 5. To calculate the energy required to form the dome:

U = D h ln 1 + 4( ) = 10.5 × 106 N.m. 6. To calculate the energy required to form the flange: U = π √ ( ) h Bo − = 0.98 × 106 N.m The total energy required to form the dome: ET = UT = U + U = 11.48 × 10 N. m.






So the explosive charge (W) can be calculated : W = ζ1(1−cosФ1) e

Ф = tan Since = 1.67 L= 0.5 m So W required is 17 kg.

Conclusions and disruptions

In explosive metal forming chemical energy from the explosive is used to generate shock waves through a medium which are direct to the workpiece 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 workpiece 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 informations are not available and forbidden. To provide a fully picture about explosive forming there must be additional information available.





Examples that have been produced by explosive forming





References

1. Herold, K.P.; Vovk, V.; Taran, V.; Vovk, A: Explosive forming of high-

strength sheet material. 10-th International conference Sheet Metal, Ulster, E; April, 2003.

2. Hydroforming - Wikipedia, the free encyclopedia. 3. Taran, V.*; Vovk, V.* ; Sabelkin, V.**; Vovk, A.* . * Otto-von-Guericke-

University Magdeburg, Magdeburg, Germany** IMP, Mexico(Explosive Forming of Metal Blanks).

4. Principals and practice of explosive metal working by A.A. EZRA.

The Author

Dr. Hani Aziz Ameen , Birth date 1971 in Baghdad- Iraq, has Ph.D. in

Mechanical Engineering – Applied Mechanics – from the University of

Technology –Iraq in 1998. He has more than 50 published papers and he is an

expert in the ANSYS software and finite element analysis.

Working in several universities and colleges (Technology University-

AlNahreen University- Tikrit University – Technical College AlMusaib).

And now he is Asst. Professor in the Technical College – Baghdad / Dies and

Tools Engineering Department.

E-mail: [email protected]


mailto:[email protected]



Explosive Metal Forming - Hani Aziz Ameen

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