High Velocity Forming Part II

8/3/2019 High Velocity Forming Part II

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72 ~~ ~,DAMENTALS OF METAL DEFORMATION UNDER IMPULSIVE LOADINGREFERENCES

I. Mohr, O. Z., Vafil l Deutscher lngrnirure (1


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76 ELECTROHYDRAULIC FORMING

(0) (b)Fig. 3-2. Wire in process of vaporization. (Courtesy, Republic Aviation Corporation)

The average wire temperature can be determined from the energy input andthe mass of the wire if thermodynamic equilibrium exists. In the solid and liquidstate, thermodynamic equilibrium is probably reached in short time intervals,compared to a millimicrosecond, since electron-relaxation times are typicallyon the order of 10-13 sec. At ten millimicroseconds, enough energy has beenintroduced into the wire to melt it completely. At 12 millimicroseconds, the wirebegins to vaporize at 1.0 atm. and, at 17 millimicroseconds, enough energy hasbeen developed to completely vaporize it.

The current reaches a peak value, then rapidly falls off with the rate of fallabout twice as great as the current rise. During this period, conduction mustoccur under conditions predominantly above the critical point. Large amountsof energy arc introduced into the wire during this phase and, even though thecurrent is rapidly decreasing, the voltage remains at a high level to furnishsizable power inputs. Large magnetic pressures, caused by the large currentnow, raise the temperature sufficiently for vaporization to occur from a mini-mum at the wire's surface to a peak along its axis. This vaporization occurspreferentially along the wire surface. Initially, the surface begins to vaporizeinto the surrounding medium at a little more than 1.0 atm., but it appears thatvaporization occurs so rapidly that the outer vapor does not move an appreci-able distance before the next underlying layer isvaporized. Consequently, hugepressures arc built up, resulting ill progressively higher vaporization tempera-

77QUIPMENTtures. As the inner layers of the wire aloefurther heated to a state well above thecritical point by continuous current flow, some interesting phenomena mustoccur.

There is evidently much more energy introduced into the wire during postdwell (region of low energy input) conduction but, eventually, energy loss proc-esses such as radiation, the work of pushing away the outer, colder vapor andwater layers, and diffusion cause the vapor column to cool and break away fromthe shock wave and fall behind.

EQUIPMENTThe equipment required for electrohydraulic forming isproduced by several

manufacturers in units ranging in energy capacities from 6 to 150 kilojoulesand more. Examples of these units are shown in Figs. 3-3 through 3-6. Themachine shown in Fig. 3-3 consists of two die handling facilit ies and a 172,000joule capacitor bank. The energy unit services both die facilities, but only thetube forming facility is shown. Additional facilities can also be serviced fromthe energy unit.

The die handling unit shown is suitable for forming tubular configurationsup to 12 in. diameter and 24 in. long. The other unit, not shown, will formflat parts up to 24 in. diameter and up to 8 in. deep.

The machine shown in Fig. 3-4 is a 150,000 joule unit with a 64 in. by 48 in.useful die area. The clamping unit develops 100 toris' of force. Smaller modelsare available with C frame die facilities. /'

Fig. 3-3. Electrohydraulic forming fa-cility unit providing 172.000 joules ofenergy. (Courtesy, Electro-Form, lnc.}


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ELECTROHYDRAULIC FORMING

Fig. 3-4. Electrohydraulic forming fa-cility unit providing 150,000joules. (Cour-tesy, CillcinTUlti Shaper Company)

Fig. 3-5. Electrohydraulic and electromagnetic forming unit, 36,000 joules. (Courtesy,Gulf General Atomic. Incorporated]

The machine shown in Fig. ~-5 requires 8 sec. for charging to its ratedenergy level of 36,000 joules. The die holder opens and closes in the hori-zontal direction. Clamping force is adjustable up to 125 tons. The formingmachine accommodates parts up to lOin. diameter and 36 in. long. Specialfeatures of the machine are a magnetic forming station operable from the sameenergy source, and provisions in the head of the die machine for acceptingmagnetic expansion roils.

EQUIPMENT 79

Fig. 3-6. Electrohydraulic forming unit. 60,000 joules. (Courtesy, Rohr Corporation)The 60.000:joule machine shown in Fig. 3-6 can accommodate tubular parts

up to 8 in. in diameter and ~6 in. long if the part is formed in a horizontalposition, and up to 12 in. in diameter and 16 in. long if the.part is formed inthe vertical position. The machine uses spark gap transducers. Charging time. required for the 60,000 joules is less than 25 sec.Machine Design

Designing equipment for clectrohydraulic forming requires an under-standing of: the pressures generated by electrical spark discharge, the mannerin which the pressures dissipate, efficiency of energy conversion, energy re-quirements to form various types of paris and materials, how to control theprocess, and how the pn)Cess is influenced by the various parameters involved.Stress Analysis

The stresses which occur during electrohydraulic forming are diflicult todescribe because they are subject 10 so many process variables. The mostsignificant of these variables will be discussed individually. The basic energyequation, U = 1/2 CV2, Eq. (1). discussed ea rlier, represents the potential energyof the system. Upon transformation to mechanical energy, the energy systembecomes dynamic. and kinetic energy is expressed as

U=i t l Il V2 or 1 /2 ~ v2Where: 11 1 =Mass of moving body

tv = Body's weightv =Velocity o r the moving body at impactg = Accelerat ion of gravi ty

This energy changes to another form at the point of impact, and all the energyin the system must be dissipated into the workpiece insofar as possible, or intothe tooling and equipment without causing- damage, or as heal. The energ-y

(2 )


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84 ELECTROIIYDRAULIC FORMINGWhen metal is shaped by cieclroilydraulic forming, not all of the energy

in the discharge goes into part deformation. Some is given up in heat duringthe process of conversion to shock, some is dissipated into the machine or thetooling, or both. Some of the energy moves in such a direction as to completelymiss the part to be formed. When all these factors are taken into consideration,total part deformation will quite commonly be found to be only I () t o 20 per centof the energy released. The efliciency of energy conversion is affected by manythings such as: wire mate rial, wire size, equipment efficiency, length of wire,length of gap, charge voltage, etc. The effects of these parameters will be dis-cussed later. Two methods of approximating efficiency of conversion are:

Example No. J. A 21,000 joule energy release (2640 microfarads at 4000 v.)was made inside a .25 in. hy 3.25 in. 0.0. by 5.5 in. long 5052 aluminum alloytube. The release was discharged into an aluminum wire .065 in. diameter,loaded across a .5 in. gap. The energy of deformation was calculated to be26,775 in-Ibs (3025 joules) in the following manner (for definitions of symbolssee "Li st of Symbol s" ):

U; =F X s; F ='ID

U ; =rrl'lD ( rM a x - 1 ' 0 )

U"=i-n [~tlV + r i\ - fO ]rr lI' "" (SMI" + SMax) I

rMax + YoWhere: Vr = 21,120 joules x 8.856 = 187,000 in-lbs

MJ=2.'i0 in.tlV =65 g. of water =3.9 in'1= . 'i .5 in.

fO =1 .375 in.P =3320 ps i.u,=2rr(3320)(5..'i)(2.75) [~:.9+ (1.37.'i)2 - 1.37.'i].).5

=35,000 l.226 + 1.88 - 1.375)Un =2fi,775 in-Ibsu; 26,775 3 Ii .- =8700 =14. per cent efficiencyU , I , 0S =(see Fig. 3-12 [or values)

EQUIPMENT

,0

10 -- - , . . . - !---- - - - - -~.-, VIL : MATERIAL: ALUMINUM 5052-0,,:I

36

J1

20

16

12

.02 . o . c .06 ,08 .10 .20 .30STRAIN(IN/IN)

Fig. 3-12. True srress-true strain diagram for 5052-0 aluminum.

Example No.2. A test piece, shown in Fig. 3-13, was expanded .Ofi i n. over its15 in. length by mounting an .045 in. diameter aluminum wire through thecenter of the tube. The tube was filled with water, and a IO,oon joule (4,()()O v.)discharge released into the wire. .

Energy release = 10,000 joules = 8R,560 in-IbsStrain = .06 in. = .02 in.3.0 in. in.T 1 . .02 in.ota str am = -.--X3 in. (rr) = .1 885 in.Ill.

Fr om true str ess -strain diagr am, s tres s required 10 obtain a .02 in/in strain =20,000 p si .

P ., . I I (20,000)(.125)ressure act ll lg II1SI( e tu )e = =1667 psiIh :JCross section area of tube =45 in'Total force applied inside tube =45 X Ifi67 =75,025 I ll s.Force acting circumf erentially in wall of tulx- = '/, X 75,02.'i = 37,512 lbs .Work = F X s; F= 37,512 Ibs.; s = .188 in.

={7,512 X .188 =9,052 in-lbsEfficiency = 9,052/88,560 = 10 per cent

85


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86 ELECTROIIYDRAULIC FORMINGf T ' ' ' '31N,TUBE00

1== l = z = = = = i _ l::::::::~:::::::::::::::::::::j-rI. ''__SH . . I L,;o Fig. 3-13. Deformation by 10 kilojouleenergy release.Design Criteria. Electrohydrauhc forming systems consrsung of ener gy

storage and discharge units integrated with functional die actuators are com-mercial ly avai lable a s i llus tra ted in Figs. 3-3 through 3-6. Good design demandsthe coordinated efforts of electrical, mechanical, and hydraulic engineers.Commercial units must comply with the standards of: (I) National ElectricalManufacturers Association (NEMA), (2) Joint Industrial Council (JIC), (3)National Machine Tool Builders' Association (NMTBA). Intended applica-tions will infl uence t he basic configuration while energy requirements, controlof t he process, and electrical aspeCis must recei ve individual att ention.

The material, size, and shape of the parts to be formed will establish theenergy requirements of the system. Energy requirements are also influenced byparameters such as critical impact velocity of the material to be formed and thesystem's efficiency of energy conversion. Depending upon equipment efficiency,selected operating parameters, and the manner of applying energy to partforming, part deformation has been found to be approximately 10 to 30 percent of energy released. If a minimum efficiency of energy conversion of 10per cent is arbitrarily selected, the equipment designer can readily calculateenergy requirements for producing any desired deformation. In tests, a 9600joule (1200 microfarads at 4000 v.) discharge through an aluminum wire of.045 in. diameter produced deformation in a 5052-0 aluminum tube as illus-trated in Fig. 3-I 4. The energy of deformation ill this case was approximately1150 It-lbs. Calculated efficiency of energy conversion was approximately 16per cent.M " " ' ~'"


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88 ELECTROIIYDRAULIC FORMING

Where: F = (T x A(T= Average stress acting, psi.A = Area over which (T is actings = Displacement (strain x distance)

l00r-r;~~-r--------r--------r--------r--------r--------r-~

g~ ~ r l r i - t - + - r - - ~ ~ ~~ - - - - - - - r - - - - - - -- ~ - - - - - - ~ - - - - - - -- ~ ~t;;

M AT ER IA L ST A1H lE SS ST EE L 32 1~~-+'_+-r-------r-------~------~------~------+-~1 0 ~ - t ' _ + - r - - - - - - - r - - - - - - - r - - - - - - - ~ - - - - - - ~ - - - - - - + - ~.02 .06 .10 .20 .30

STRAIN (IN/IN).' 0 .50 .60

Fig. 3-17. True stress-true strain diagram for Type 321 stainless steel.From Fig. 3-17, (T i s found to be approximately 50,000 psi . Thus:

A =.1 in. X 2 ill. = .2 ill". I in.s = strain X distance =-.- X 41 T4 Ill.

= .25 X 12.56() = 3.141 in. = .26 ft.IF = 50,000 x'2 = 10,000 lbs.U =10,000 X .26 = 2600 ft-Ibs

Assuming the cflicicnry of the system is 10 per cent, then:lJl l",.uin'" = 2()OOX 10= 26,000 It-lbs2(),OOOIt-Ibs.738 = ;15,OOOjouics

The designer may ('ondude [rom the foregoing that energy of no less rhan:\6 kilojoules is desirable to form 'I ill. diameter 321 stainless steel tubing of0.1 in. wall thickness. If such paris are to be formed cont inuously, then gooddesign would require a 72 kilojoule machine so as to avoid continuous dutycycle at maximum energy.

Control of the clccuohydraulic forming process is readily accomplished,even though it is allcctcr] by many variable parameters. Some of the moreimportant ones arc: energy levels , energy containment, air escape, wire position,

.:. .~. .'

EQUIPMENT 89

and wire diameter. Less critical parameters include: liquid media, wire material,wire length, and gap length. Reflectors are sometimes used as a variable pa-rameter to imparl additional specific control to the process. One companyuses them extensively to shape and concentrate the pressure in specific areas (6).A selection of such devices is shown in Fig. 3-18 while the effects of using ashock reflector are vividly demons trated in Fig. 3-19.

Fig. 3-19. Free formed tubes. Left tubewas formed without reflector, centertube with reflector, and right tube with areflector but subst itut ing equal explosiveenergy. (Court,sy, Gmeml Dynamics/FortWorth Division)

Operator understanding of these parameters is more important to success-ful implementation than is operator technique or finesse. Fig. 3-20 shows asimple shape produced in 5052-0 aluminum tubing of 2 in. diameter and .035in. wall thickness that required knowledge of most of the par-ameters mentionedabove. The part was formed by c1ectrohydraulic forming with two energy re-leases of 2400 joules each. The 2.75 in. diameter of the bulged section repre-sents a :17.5 pCI' cent increase over the tube start ing diameter.

One of the most important of the process variables is energy level. Thisvariable is a complex one resulting from combinations of capacitance and vol-tage; even so, it is one of the easiest of the parameters to control. Vohage was


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90 ELECTROHYDRAULIC FORMING

Fig. 3-20. Electrohydraulic formed tubewith two energy releases of 2400 joule.each. 5052-0 aluminum. 2 in. diameterby .035 in. wall bulged to 2.75 in. diam-eter. (Courtesy. General Dynamics/ForiWorth Division)

identified earlier as the parameter most often used to control energy levels.Energy levels of electrohydraulic forming systems are based upon a certainoperating voltage. selected as maximum. and sufficient capacitance at thatvoltage to deliver the specified maximum energy. The voltage is then variedfrom zero to 100per cent of maximum to develop any desired energy level with-in the rating of the system. Fig. 3-21 shows a part damaged byan attempt toaccomplish too much deformation with one energy release. When the energylevel was lowered. the part wassuccessfully formed in two energy releases. Someparameters which necessitate increased energy levels are: high strength partmaterial. increases in part material thickness. increases in part diameter. andany increase in the severity of part deformation required.Energy containment is an important factor which contributes to process

efficiency. Research shows deformation energy for a given energy release to beconsiderably greater when contained in a closed chamber than deformationproduced by an equal energy release in an open chamber (I).Some effects of trapped air between a part and the die are depicted in Fig.

3-22. Trapped air can produce reverse dimples as shown by Fig. 3-22a. orsevere air bums as shown by Fig. 3-22b. These detrimental conditions can beprevented byevacuating the air from between the part and die. or providing airholes through which air may be expelled concurrently with the forming action.

Fig. 3-21. Aluminum duct damaged byovershooting. (Courtesy. General Dynamics/Fort Worth Division)Fig. 3-%2. Damage resulting from trap-ped air. (Courtesy. General Dynamics/FortWorth Division)

EQUIPMENT 91The latter technique is illustrated in Fig. 3-23. Use of vacuum to evacuate theair is the preferred method for maximum efficiency;however, when productionrequirements are low, the expulsion method may be preferred. This methodpresents no sealing problems and is therefore very easy to apply. The disad-vantage is that water gets between the part and die each time an energy dis-charge is made. To prevent damage to the part, the water must he removedbefore subsequent discharges are made.

Fig. 3-23. Die with provisions for airescape.

EXPELLEDAIR

DIE

Fig. 3-24. Effects of wire posiuon.(Courtesy, General Dynamics/Fori WorthDivision)The sensitivity of the e1ectrohydraulic process to wire positioning is shown

by Fig. 3-24. Each of the three parts was formed by a 5400 joule energy re-lease. A wire was positioned .25 in. below center to form a; a crack appearednear the lower radius. A wire was positioned .25 in. above center when formingc; a crack appeared near the upper radius. A wire was positioned at the centerwhen forming b; and a good part was produced.

Wire position is synonymous with standoff which is expressed as a lineardimension. As this dimension increases, the total deformation resulting from agiven set of parameters willdecrease. This statement implies that standoff dis-tances should be kept as low as possible in order to attain maximum efficiency;however, the standoff can be reduced to a point such that the deformation pro-duced is severely localized. Such localized deformation is usually undesirable.In general, the best results are obtained when standoff distances allow most ofthe part area requiring forming to be impinged upon bythe spherically shapedwave front of energy as it moves outwardly from the point of release.Other process parameters which influence results are liquid media, wire

material. wire diameter, wire length, and gap length. Researchers have studiedall of these parameters and others (I) and found that liquid media and wirematerial are not critical. but wire diameter, gap, and wire length are very im-portant. Their findings also show that most of these parameters have optimum


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92 ELECTROIIYORAULIC FORMINGvalues for particular energy levels. Thus, it is evident that there is still disa-greement among researchers as to which parameters are the most critical.

The higher peak pressures produced in water as compared to similar energyreleases in air are shown in Fig. 3-7. Some liquids are more effective and someles s effect ive than water. For example, when methyl chloride was used in placeof water, an increase in deformation of approximately 50 per cent resulted(I); however, because of convenience and other economic factors, water is theliquid nearly always used.

Aluminum, magnesium, ti tanium, copper, and Pyrofuze* have been evalu-ated as materials for initiating wires based upon amount of deformation workproduced by the different materials with 12 kilojoule energy releases. The graphin Fig. 3-25 shows that, for the test conditions employed, aluminum wire pro-duces more deformation work than the other materials tested, and that, in

C OWMON CG lO IT I G4S :I) AX IAL E L E CT R OOE -2) 1.01N.GAPDISTANCf(T - ~ ( E XC E PT H O TE D )) ) T AP W AT ERj 10(Vs to' 5 ) 2 40 M I CR O fA R AD S e-

I I ~6) J..(IN.)O.O. x 8 -( lN .) lONG X

. 00 2 W A LL , 5 05 10 A LU M IN U MTUllE

0 I I\ LEGENO:s V A lUM~ r-,,' ~ - . . . . . . . . C OP PE R: o MAGNESIUMK ~ :" . . MAGNESI lJ, ! (4 IN. GAP)/ o PYROFuze0 T IT AN IU M -

~ VD "'I, V "\\ . . . . . . . . . _ :--__.s

.015 .04' .1lSllO030 ..." .075 .090W IR E D IA ME TE R ( IN .)

Fig. 3-25. Effect of wire diameter on deformation work for several wire materials. (AfterPlanner, et al. (1addi tion, an optimum wire diameter exists for each material. Also illustrated isa trace showing deformation produced by a magnesium wire with a 4 in. gapdistance. Comparison of this trace with the one for magnesium in a 1.0 in.gap indicates that a wider range of wire diameters can be tolerated for a 1.0in. gap than for a 4 ill. gap. The trace for the 4 in. gap is rather peaked and thework output falls off sharply on either side of the optimum diameter, whilethe trace for the 1.0 in. gal' is fairly flat over a relatively wide range of wirediameters. It is also readily apparent that much more work is obtained with a4 in. gap than with a 1.0 ill. gap (I).

Rrg. TM. P)wfme COpt>fOtinn.

EQUIPMENT 93The effect of wire diameter on deformation work is shown in Fig. 3-26. The

data indicate that when discharging at 8 kv. (7680 joules), a .03 in. diameter isoptimum. When voltages are increased to 10k v. (12,000 joules), a wide range inthe wire diameter can be tolerated with very little loss in deformation work.The graph also illustrates that, as the wire size is increased, a point is reachedat which the deformation work produced drops very rapidly.

Wire length and gap length have independent and interrelated effects upondeformation produced by specific energy releases. Research has shown that "an.optimum wire length exists for a given gap distance and a given wire material,wire diameter, energy level, and workpiece specimen" (I).

2.'

w~ 183~i5~ 1.2ga

.6

C O ,W .. .OO COND IT I ONS : f--I) A XIA L E LE CT RO DE1 ) TAP WATER. 3) 240 MICROFARADS4) GAPLENGTH WIRELENGTH .4 IN I--5 ) ' HI RE M AT ER IA L, M A GN E SI UM6 ) WORK 1MHRIAL- 5052-0 ALUMINUM I--V 1 \ 2 IN . DIAMETER BY B/N .BY . f * 2 IN. WAll! \ -./1'\/ \ o 8 KY, DISCHARGE010 KV. DISCHARGE1 / 1 \

\ \\ \\ "tr-;- -, -015 .030 .150900


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94 ELECTROHYDRAULIC FORMINGoperating efficiency if this condition could always be attained, However, criticallydamped discharges for electrohydraulic forming are not pract ical to generate,Discharge circuits with R > 2VTJC are easy to design, on the other hand, andthe resulting overdamped discharges extend capacitor life, but diminish peakcurrents in such a way that efficiency of the electrohydraulic forming process is

R < 2 \ [ f.. .zw'" TIME'">u(.)

I~ R = 2 ' / T ~ TIME(b)

i~ R > 2 V ~ TIME Fig. 3-27. Discharge modes of RLC cir-(0) cuits.

greatly affected. A pract ical solut ion is to des ign the energy discharge system sothat discharges are slightly oscillatory, The designer must strive to so design asys tem that i ts res istance approaches but never exceeds 2ViJC.

Voltage. A major factor in selecting an operating voltage range is whetheror not initiating wires are to be used. Operating levels of 10,000 to 30,000 v.are desirable when using spark transducers; levels of 2,000 to 8,000 v. sufficewhen initiating wires are used. Charged voltage can be readily controlled by avariable voltage transformer in the primary of the power supply. For thisreason, voltage is the parameter most often used to control the desired energylevel. Usually, the maximum operat ing voltage of a sys tem will be established at75 to 80 per cent of the rated voltage of the capacitors in the discharge system.A derating to 75 per cent results in 850 per cent of the rated voltage life ex-pectancy (7). Othe ... factors such as percentage of voltage reversal and time atvoltage will also affect capacitor life.

Capaci tance. Voltage and capaci tance arc interrelated with respect to elec-trical energy. The capacitance is dependent upon selected voltage and de-sired energy and is treated as a constant in the equipment design.

Inductance. When using energy storage capaci tors for fas t dis charge applica-tions such as c1cctrohydraulic forming, inductance must be kept to a minimum

EQUIPMENT 95so that relatively high peak currents can be produced in very short t ime spans .A 36 kilojoule energy bank of good quali ty will have a natural ringing frequencyof no les s than 35 kilohertz, and an inductance of no more than 24 nanohenries.Fig. 3-28 shows the effect of inductance upon the amount of deformation workproduced.

2CClMMON CONDITIONS:I ) A X IA L E L EC T RO D ESi\ 2) 1 .0 IN. GAP DISTANCE3) TAP WATER4) . 062 tN. D iAMETERMAGNE SI UM WR E

8 \ 5) 1 0 KV6) 240MICROFARADSr-, 7) J IN . DIAM ETE R B Y 8 IN .L ON G B Y . 062 IN . W ALL,


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96 ELECTROHYDRAULIC FORMINGDies for drawn shapes will have provisions for sealing the liquid away from

the cavityof the die and means for removing air from behind the part. The diewill require no punch; otherwise, the die willbe similar to a die for forming adrawn shape by mechanical means.

Electrohydraulic forming is particularly convenient for forming hollow bodyshapes. Operations such as beading, bulging, piercing, and trimming can beaccomplished. Dies for hollow body or tubular shaped parts will usually be ofa split configuration to permit removal of the part after forming. The splitconfiguration necessitates an accurate indexing means such as dowel pins toprevent mismatch or the die halves. During operations, the halves must be firmlyclamped together to prevent unwanted marking of the pan at the split line. Atypical die is shown in Fig. 3-29.

BEFORE FORM lNG ~ AFTER FORM ING

SEALPLUG../' " " 0 " RINGFig. 3-29. Typical die for e1ectrohydraulic forming of tubular shapes.

Internal finishmust be of a quality equal to or better than that required onthe workpiece. The process seems to work best when the forming action isinitiated at the bottom of the part and successivelyworked up asadditional shotsare required. This procedure places the smallest inside diameter of the die,which isa slip fi t for the tubular blank, at the bottom. The diameter must re-main constant for a distance sufficient to accommodate a sealplug for-the liquid.The seal plug can be made of rubber (a hardness or approximately 60 du-rorneter) for small parts, but should he a metal plug with an 0 ring for partsof 2 in. diameter or more. If the part shape permits, air vent holes at the topof the die will suffice for air removal. But if the shape of the part iscomplex,vacuum is required to prevent air entrapments.

It is a good practice to provide excess length of at least one-half the tubediameter at the top of the part toinsure fullforming and a good definition of the

';. , ~ .. '

EQUIPMENT 97part in that area as wellas to provide space outside of the end of part for the airvent holes which tend to mark the part. The excess area isalso an ideal placefor installing the vacuum hole when required. Dieshape and finish in the ex-.cessarea should be such that friction between the part and the die is held to aminimum during forming so that the length of the part blank is allowed todraw inward. This action minimizes thinning which results from the expansion.The less resistance there is tochanging of the length, the lessthinout there willbe in the walls of the part.Materials. Die materials are dictated bythe operations to be performed andthe quantity of parts to be produced. Low carbon steel is the most common

material for dies used for forming only. Except when the forming issevere orwhere cutting actions are required, dies made from this material willsuffice forforming several hundred parts. If cutting actions are required, or iflong die lifeisdesired, a heat treated tool steel should be used aswould be the case in designof dies for conventional processes. Aluminum can be used as a die material ifspecial care is used not to "overshoot" when forming the part. Spark dischargesof sufficient energy intensity can be released to carry the material being formedagainst the face of the die with an impact capable of compressing the die surface.

Dies for electrohydraulic forming can be fabricated from epoxy castingresins (see Fig. 3-30) and are inexpensive to make. They require little or nomachining because they are molded to a pattern of the required shape of the

Fig. 3-30. Plastic die for electrohy-draulic forming. (Courtesy, General Dy-namics/Fort Worth Division)part to be formed. Physical requirements for plastic dies are basically the sameas those for metal dies. Dies of this type last for a dozen or more parts ifthe re-quired forming isgentle and the dies are handled with care. Care must alsobeexercised during casting and curing to avoid porosity. Voids leave weakspots inthe die which will break down under the impulsive loading of the formingoperation. If severe transitions are required in the part, the die mayproduce nomore than two or three parts before breakdown. Plastics provide an economicalmethod for making feasibility studies of the process capability to form a partconfiguration in question. This type of die can also be recommended where onlytwo or three parts may be required for experimental purposes or tests.Dies cast from SAE 925 die metal have performed satisfactorily for short

runs of aluminum parts (8). This material, used extensively for drop hammerdies, has a melting point of 727F. and a compressive strength of 153,000psi. (9).


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98 ELECTROHYDRAUI.lC FORMINGWhen tooling for severe forming operations, cutting operations, or part

quantities in excess of a few hundred, dies for electrohydraulic forming shouldbe manufactured from alloysteels and heat treated. SAE 4130 and 4340 steelsare excellent for most applications. These materials cost less per pound thanspecial purpose tool steels, and have optimum heat treat characteristics whichpermit their being machined with metal cuuing tools after heat treating. SAE4130 isusually heat treated to a strength level of 160,000 to 180,000 psi.; SAE4340, after an optimum heat treat has a strength level of around 220,000 psi.If a die providing maximum life for cutting action on tough materials is re-quired, then special purpose tool steels such asAISI Type S2or AISI Type L6should be used.

APPLICATIONS AND LIMITATIONSMany parts are producible by e1ectrohydraulic forming as shown in Fig.

3-3 I. A wide range of part sizes can be accommodated, and most materials canbe worked. The process has received its broadest acceptance in the aerospaceindustry. The parts shown in Fig. 3-31 illustrate the ability of electrohydraulicforming to accomplish bulging, forming, beading, drawing, blanking, andpiercing. All of these operations are normally performed by sheet-metal work-ing equipment such as rolls and presses. The reason for selecting the electro-hydraulic process isthat many designs have some peculiarity which isnot withinthe capability of conventional equipment, at least not without some special pro-visioning or tooling. When electrohydraulic forming is used, the cost of toolingwill almost always be less than for conventional equipment. The nature of theprocess issuch that large amounts of energy can be directed into isolated areasas required by some piercing operations.Shapes Produced

Hollow bodies, unusual or irregular pan shapes, hemispheres, bulkheads,and conical parts can be advantageously produced using e1ectrohydraulic tech-nique and are discussed below.Hollow Bodies. Electrohydraulic forming is especially well suited to form

hollow body shapes. It also offers advantages in the forming of some unusualpan shapes and deep recesses. Hollow bodies formed by the process includethose having shapes which are: round and symmetrical, round and non-sym-metrical, and irregular cross sectioned. One of the most important advantagesthat e1ectrohydraulic forming offers is that it uses a liquid punch. This permitsthe reproduction of intricate detail in parts from one-piece tooling in lieu of ,more expensive matched tooling. Electrohydraulic forming also provides thecapability to pierce parts simultaneously with the forming action.

Parts which can be described as bodies of revolution, especially those havingonly slight changes in cross sectional dimensions, are readily shaped byelectro-hydraulic forming. The beaded duct in Fig. 3-32 tapers from1n. diameter to5 in. diameter and is 11.5in. long. Wall thickness of the tube is .035 in. Thebeads are formed simultaneously with the shaping of the duct's body. The crosssectional shape of the part shown isround all along its axis; the changes in diam-eter along itsaxis are mild. The most severe aspect of the forming requirements

APPLICATIONS AND LIMITATIONS 99

Fig.3-31. Typical applications of electrohydraulic forming. (Courtesy. General Dynamics!Fori Worth Diinsion]for this part is to form the beads at the lower end. Forming operations aresimplified, however, because the part material is 5052-0 aluminum alloy whichpossesses excellent formability properties.The part shown in Fig. 3-33 isround all along its axis, but the axisis not

straight. The joggled offset is too severe for tube bending. The difficulty in


15/41

ELECTROHYDRAULIC FORMING

Fig. 3-32. Beaded duct of 5052-0aluminum alloy. Part is 11.5in. long andtapers from 4 in. diameter to 5 in. diam-eter. (Court ",. Genera l Dynam ics/ FortW orth Division)

Fig.3-33. Round duct withjoggled off-set. Final diameter of 4 in. is producedfrom 3.5 in. diameter 6061 aluminumtubing. (Cour te sy . General Dynamic s/ For iWorth Division)

forming this pan isgreater than for the previously discussed part. but the bene-fits derived are also greater. Electrohydraulic forming of this part eliminatedno less than 12 in. of welding per part.The unusual shape shown in Fig. 3-34 represents a more difficult levelof

forming. because not only docs the axis of the part change direction but theexterior profile of the part is constantly changing. The stainless steel partshown in Fig. 3-35 has a very irregular shape. The centerline of the duct isconstantly changing. and the cross sectional shape is also continually changing.The stainless steel rocket nozzle shown in Fig. 3-36 represents one of the mostsevere electrohydraulic forming deformations. The part isformed from 4.5 in.diameter tubing. The 52 per cent thin-out which resulted in the area of maxi-mum deformation issmall when considering the amount ofexpansion produced.The maximum diameter of 13.75 in. is 300 per cent of the original tube size.Hemispheres. The hemisphere isa very simple shape. but isvery difficult to

produce with conventional forming techniques. Hemispheres formed by con-

Fig . 3-34. Angular duct t ransi tionfrom 3 in. diameter at one end to 4 in.diameter at the other. 6061 aluminum.( Court esy, Gener al Dyrwmics /Fort WorthDivision)

Fig. 3-35. Engine exhaust duct formedfrom stainless steel. Type 321. Centerlineand cross sect ional shape constantlychanged. (Courtesy, General Dynamics/ForiWorth Division)

. '~

APPLICATIONS AND LIMITATIONS 10.Fig. 3-36. Rocket nozzle formed from4.5 in. diameter welded tube. Largediameter is 13.75 in. (Cour te sy . Rohr Cor -poration)_

Fig. 3-37. Four-inch diameter hemi-sphere of .036 in. stainless steel shapedby electrohydraulic forming. (Courtesy.General Dynamics/ Fort Worth Division)

ventional punch and die techniques arc characterized by excessive thinningunless several steps are used to produce their final shape. When attempting todraw a hemispherical part such asthe one shown inFig. 3-37 using press equip-ment. the center of the blank iscontacted first by the punch where pressuresbegin to concentrate and ultimately excessive thinning results. In addition,compression wrinkles in the area of the draw radius are very difficult to elimi-nate. Electrohydraulic forming of this type of part is easily accomplished. Tofacilitate the forming. a wire (to be exploded) can be loaded as a loop in thevicinity of the draw radius and thus create maximum pressures at the drawradius where they are needed'. and smaller pressure at the center of the blankwhere equal or greater pressure would cause thinning of the blank.Bulkhead type parts shown in Fig.3-38 arc readily formed on equipment of

the general type illustrated in Fig.3-4. The part was formed from 7075 alumi-num .062 in. thick in one operation. Forming by e1ectrohydraulics produces a

Fig. 3-38. Bulkhead formed in oneoperation from 7075 aluminum .062 in.thick. (Courtesy. Cincinllati ShaptrCompallY)


16/41

It ELECTROI IYDRAUL IC FORMINGwrinkle-free part which requires no subsequent handwork. Thin gage skindetails of aluminum alloys requiring compound contours arc readily produced(see Fig. 3-39). The same techniques al so produce such difficult-to-form shapesas that shown in Fig. 3-40. Fifty-six holes were pierced in the part shown inFig. 3-41 while the coni cal part was simultaneously formed.

Fig. 3-39. Skin for bonded panel. Cen-ter of part isapproximately 14 in. diarn-eter. (Courlesy, 1I0hr Corporation)

Work Materials

Fig. 3-40. Clam shell with deep recessmade from Type 347 stainless steel .032in. thick. (Courtesy, Cincinnati Shaper Com-pany)

Fig. 3-41. Forming and piercing Type310 stainless steel, .025 in. thick. (Cour-t'5Y, Electro-Form.Lnc.]

Almost any material that can he formed hy conventional mechanical proc-esses can also he formed hy electrohydraulic forming. Successful applicationsof the process have been made in: aluminum alloys; a wide range of stainlesssteels, including heat treatable alloys; titanium; high strength steels, such asSA E 4130; high nickel alloys, such as Rene 4 1 *, I nconel 600t and I nconel Xf ;. .Reg. TM. Vacuum Melted Allo) ,l I .Metal lurgica l Products Depar tment, General Elect ric Company.t Rcg IM, Hunungron Allo), Irc}(lun.\ Ili\'i~ion. The f nn-rrumonnl Nkkd Company. Inc.t Rt')(. 1 " 1 \ 1 , Huntington Alloy "rotU I~ Divivion. Tla t, Int IIl,IIHllill Nkkd Compauj -, ( ,, ,

- " .

APPUCAT IONS AND L IMITAT IONS 103and lIlany others. Parts elecu-ohydraulically formed from heat treatable stain-less steel (17-7), titanium. Inconel 718, l I astclloy", etc., arc shown in Figs.3-42 through 3-45.

In general, this forming process should not be expected to produce anymore part deformation than can he produced by other processes; however,

Fig. 3-42. Exhaust duct detai l, 17-7stainless steel .032 in. thick. (Collrtesy,General Dvnamicsl Fort IVorth Division)

Fig. 3-43.in. thick commercially pure titanium.(COlirtesy, General D Y T l G l l l i C J / Fort IV orthDiuision]

Fig. 3-44. Duct formed from Inconel"718. (Courtesy, Rohr Corporation)

Fig. 3-45. Duct formed from Hastelloy",(Courtesy, Rohr Corporation}

Reg. TM. IIuutiugton Alloy Producl1 Division.The I nt er na ti onal N ic ke l Compa ny . I nc .

Reg. TM. Union Car bi de Corporauou. Stdli\~Division.

strains can he produced more uniformly in some parts. As indicated in Fig.3-46, a natural pressure front or wave as generated by an e lec tri cal ener gyrelease will tend to be spherical in shape which is the natural shape desired toproduce hemispheres. This shape Gin he modified to assume the shape of anellipsoid, or a ' torus. The capability to shape the discharged energy gives elec-trohydraulic forming some advantage over conventional forming methods.Control ling these shock profiles inside of t ubes will produce impressive defor-mations in hollow body shapes. Intricate forms can also he produced by com-bining the control of these shock profiles with restraints presented by dies .

Reg. 'IM. lIunling\f)n Allny Products Division. The Interuauonal Nil"kd Company. IIIL"Re g. TM, Uni on Car bi de Corporauon. SIt_.lIilt'Dinsion.


17/41

J(k ELECTHOHYDRAULIC FORMING

UNI FORM SPHER ICAL EXPANS! ON UNRESTRAI NED EXPANSI ON RESTRAINED EXPANSION

Fig. 3-46. Energy patterns.

Forming can be controlled to specified dimensions within variation limits of.005 in. and less.

Design of Preforms. Most parts designed for forming by the electrohy-draulic process can be made from straight tubes or flat blanks. A great manymore parts can be produced [rom special preforms. The 15-7 stainless steelparts shown in Fig. 3-17 were formed from a rolled and welded t apered-cone.The Type 321 stainless steel part shown in Fig. 3-18 was formed from a 15 in.diameter bent tube. The special shape in Fig. 3-49 was formed and pierced froma preformed hemisphere. The unusual shape in Fig. 3-50 was made from astraight tube which had been swaged down on one end; the larger end was thenflattened and further expanded. After an annealing operation. the part wasfinished formed by electrohydraulics in a female cavity die.

Fig. 3-47. Housing formed from arolled and welded tapered cone madefrom heat treatable PH 15-7 stainlesssteel. (Court es , Genera l Irmam ics ] ForiII'orth Division)

Fig. 3-48. Hot a ir duct ing formed fromType 321 stainless steel prebent tubing.1.5 in. in diameter. (Courtesy. GeneralDynamics/Fori Worth Division)

;I'

APPLICATIONS AND LIMITATIONS 10 5

Fig. 3-49. Drawn shape in Inconel*600, s ized and pierced. (Courtes),. Electro-Form, lnc.)

Fig. 3-50. Transi tion duct made f romstraight tube swaged down on one endand fl attened and further expanded onthe other. After annealing, the duct wasf inished formed by e1ectrohydraulics in afemale cavity die. Reg. TM, Huntington Alloy Products Division.The I nt er na ti onal N ic ke l Compa ny , I nc .

Some Advantages of the ProcessElectrohydraulic forming can direct large amounts of energy into isolated

areas of the workpiece. Originally, the nozzle shown in Fig. 3-51 required 15min. to make before electrohydraulic forming was used to reduce manufactur-ing time. The procedure now is to swage one end of a 1.0 in. diameter Type321 stainless steel tube down to . 625 in. The p iece i s then elec trohydraul ica llyformed, in one shot, to the shape shown in the center of Fig. 3-51. Trimmingis the only remaining operation. Time elapsed is II. hr. This procedure pro-duces excellent parts with practically no spoilage. The orifice at the smalldiameter end is held within a .004 in. toler ance . and i ts r oundness i smaintained.

Before using electrohydraulics to make the part, many parts were scrappeddue to cracking and thinning from the spinning operation. The procedure usedprior to electrohydraulic forming was to swage a 1.25 in. diameter tube down to.875 in .. then complete by spinning the small end down to .625 in. diameter.Besides the high scrap rate which resulted. the small diameter of the part wasexcessively thinned and, consequent ly. very fragile. This end was seldom withinthe specified tolerance.

Fig. 3-51. Nozzle. (Courtesy, General Dy-namicsl t'ort Worth Division]


18/41


19/41

108 ELEr.TROIIYDRAULlC FORMINGD. L. Bagn~li a~d I~. J Moldla, "Applications of High velocity j lydro-Elecrrical Discharge as aNoudesu -uct ive res ting and Metal Forming Too l, " Technical Uf'/JOTt 3238 (New Jersey: PicaunnyArsenal. 1%5). pp. 13. ~9. .

4. ",t igh F . 1 l C ' r g y Rate Electro-Hydr aufic Forming," Filial Uti,ort FAIR 56- J72 (Texas: For t \Vo rthDivision or General Dynamics Corp . HUll) . P: R.5. ToolEngineers Handbook (Znd ed. : New York: McGraw- if il l Rook Co. , Inc ., 1959) .6. G. C. Cadwell. "Production Forming with Shod Waves." ASTME Paper No. SP64-/ /5 (1961).7. "Energy Discharge Capac.itors," Ruth/in T~C20H (Illinois: Sangamo Electric Company).8 . K. F . Smi th , "Fo rming o f Tubular Air cr af t Par ts S impl if ied by Elcctrohydraulic Shot," Afachilll'Tj(September. 1964).

9. Metals Handbook (8th ed.: Ohio: American Society for Metals, 1961).

C H A P T E R 4

~~~g~E X P L O S I V E F O R M I N G

The required energy for the deformation of metal s, shock hardening, bond-ing, and the compaction of powders can be obtained in several ways. Chemicalexplosives have probably received the greates t attent ion , mainly because of theversati li ty afforded by the many types and forms available. However, consider-able progress has also been made in the use of gaseous mixtures during the pas tfew yeals. Since both of the energy sources arc important to the fabr ication ormetal s, th is chapter will present detai ls of each process , applications and l imi-tat ions, and considerations important to the selection or tool ing and facili ties.Deformation process ing with the use of high explosives di ffers somewhat

from gas sources. The major difference is in the rate at which metal i ss trained.In gas systems the rate or straining is on the order of a few inches per inch persecond while , in explosive systems, deformation results in stra in rates on theorder of 400 to 600 in/in/sec. Another significant difference between propellantforming and high explosive forming is the time involved in the application ofene rgy. Propellant systems are ope rative for lip to several tenths or a secondwhile explosives release useful energy within only a few milliseconds at the most.Thus the heat of reaction in a propel lant sys tem has sufficient time to affect theworkpiece by elevating i ts temperature during deformation. This i snot t rue inthe use of high explosives.Although explos ive and propellant methods for fabrication have some basicdifferences they are both high energy techniques and wil l he considered in thefollowing sections with regard to theory, equipment, and applications. By point-ing out the salient features of each process it is hoped that the reader will beable to recognize which process is the best to lise for his specific f abricationproblems.

THEORET ICAL CON S IDERATI ON SEnergy Sources

Explosives and propellants are energy sources. General ly, chemical explo-sives are available in a variety of shapes and vary incomposition and subsequent

10!)


20/41

110 EXPLOSIVE FORMINGenergy release. Table IV-I shows the properties of several selected high ex-plosives used for metal fabrication (I).

Table IV-2 presents information on how to detonate some of these selectedexplosives that require special ini tialing devices (I). Space does not permit adetailed presentation of the chemistry of explos ives or the theory of detonation;however, cons iderat ions of shock wave phenomena and impulse relat ionshipswill be presented later in the chapter.

The selection of fuels, oxidizers, and diluents for high energy gas formingis somewhat more restrictive than for high explosive sources. Generally, ac-ceptable fuels must satisfy the following conditions:

I) Stable detonation with controlled pressure release2) Combustion products must not be toxic3) Gases should be permanent gases at charging pres sure and temperature

to assure thorough mixing and uniform combus tion4) Fuel gas , oxidizing gas, and diluent gas should be of practical cost . Table

IV-3 lists some of the common gases used for metal forming.High Explosives

In using explosives to work metals, large differences in the energy require-ments and in the resulting behavior of the workpiece will occur depending onwhether' the energy is released from a standoff charge or a contact charge ofexplosive. Because of the large differences which exist between these two bas ictypes of operations, they are normally treated as two separate problems.

In ei ther' of these operational types (standoff or contact), the behavior of thesystem can be divided into two general areas of study. One area treats theenergetics of energy release and the mechanisms of energy transmission fromexplosive to workpiece. The other area considers the response behavior of theworkpiece and any associated components. While it has been cus tomary to con-sider each area separately, in any actual metalworking operation the two areasare interdependent , and a complete coverage of an operat ion involves an under-standing of hoth areas and the manner in which they are interrelated. The fol-lowing sect ions treat the dynamic behavior of both standoff and contact opera-t ions in terms of the energy relat ions between explosive and workpiece.

Standoff Operation. In a standoff operation. the explosive energy is releasedsome distance from the workpiece and is propagated mainly in the form of apres sure pulse through an intervening medium, usually water. Peak pressuresat the workpiece range [rom several thousand to several hundred thousand psi .,with most operations performed at the lower end of the pressure range. Themotion and subsequent deformation of the workpiece are primari ly dependenton the external forces associated with the action of the pressure pulse. withsome additional work occasionally resulting from secondary effects such asbubble behavior and underwater reflections.

In an underwater' s tandoff operat ion where the explosive charge releases itsenergy some distance from the workpiece, an understanding of the mechanismsof energy transmittal through the water, and the quantitative values of pres-sure and impulse which exist at a given distance from the energy source at agiven t ime is necessary. Data of rhis type come from relationships determined

THEORETICAL CONSIDERATIONS III

+-" ' ' ' ' 0 0 ] ' ' ' 0 0 ' ' '~ocio~ciOO~ZZZenZZZ

+-ca ]'OJ0,,-Zen

0 0 ' "o 0ZZ' " 0 0c c-r 0Z~Z

Ct It ) I I " ' ) r-f..OC.f~-C'i~"';~

8g88g888OU")C'lOt--.:t


21/41

" ;;~ >~~~~~g 00 CO~ 00..,..001(') '"~! 0>0> : ~ = o-~" ~~ : e - :" i r-. "t'f: "~'-l

. . . ....~ ;:2_ g IJ 4 ": >< l ~ . 51} + + . . . . . ::: [" zzzzxz ~ - : et ; t ; t ; t ; Q t ; ~~.~]~o..o..o..":~ ~8~~ .8"O~'%C:~ 1 !E g1: '" '" 0 t!

0- ]i~~~u " .!:! f .08l- v ~ z g t ! : : . . : : :.fl g~~~ . . 1 : .~~:~~0:1I' ~ w w ' t r ::; ~gQ~


22/41

I EXPLOSIVE FORMINGthe water with a velocity of about 4800 Ips. in fresh water. When the pressurepulse impinges against the workpiece, the metal is displaced into the die cavitywith an initial velocity of several hundred fps., thus deforming and shaping theme ta l to the desi red configura tion.

Following the detonation of the explosive, a gaseous bubble is producedwhich in thc general behavior pattern expands to an initial maximum diameter,collapses, and then goes through an oscillatory process as it rises or migratesthrough the water. Each oscillation produces an additional pressure pulse, butof much lower intensity than that of the primary pulse. The bubble will eventu-ally rise to the surface and vent to the atmosphere throwing up water in a highplume. This simple picture of system behavior can be complicated by such con-siderations as having the bubble "break" over the workpiece or by introducingpressure pulse reflections from thc sides of the forming tank.

Thc pt'essure pulse gcncratcd by a detonating explosive will propagate out-ward through the water with an cxpanding spherical front, constantly reducingin peak pressure and unit impulse due to encrgy losses to the water and thedivergent nature of thc pulse. At any given distance from the charge center, thepulsc will have a pressure-time profile of the type shown in Fig. 4-2. The high-intensity portion can be approximately represented by the expression:

6000 \ --- THEORET,IC ALI-- EXPERiMENTAL\J~\\ ' " /'\ ~\ - .\


23/41

116 EXPLOSIVE FORMING THEORETICAL CONSIDERATIONS.(reduced energy flux densi ty) are each independently plotted agains t W'/3/R ona log-log scale. Each curve then appears as a straight line, leading 10 the form ofequation cited above. The explosives' constants are obtained from the slopeintercept and the slope of the pertinent experimental plot. The positive natureof the constants a, {3 , and y indicates that the slopes of the lines in the log-logplots are positive and, that for a given charge weight, the values of peak pres-sure, unit impulse, and unit energy flux all decrease as the distance from thecharge increases.

If a similar plot is made for O/W,/3 (reduced time constant) and W'J3 /R, theresult is a curved line with the degree of curvature depending on the type ofexplosive used. However, for design purposes, any portion of the reduced limeconstant plot can be well approximated by a straight line which leads to a gen-eral expression:

10

[ ' 1 3 ] 1 . 1 3Pm'" 2 .16) t 10" i- - .2eXPLOSIVE , T H T

100,000 . ..5.6

.7

( W 1 l 3 ) - BO=DW' J 3 R r.8 ~.9 '"5 )For a given charge weight, the value of the time constant 0 increases with in-creasing distance from the charge. for any given explosive, the values of D anda will vary somewhat depending on the charge weight and distance limits withinwhich the equation is to be applied. For a 1.0 lb. charge of TNT and a distanceof 5 It., appropriate values of D=.045 and a =.361, with 0 given in milliseconds.

A useful way of presenting data from Eqs. (2), (3), (4), and (5) is in the formof nomographs (6). In this way, the relative effect produced by a variation inone of the parameters can he quickly determined. An example of such a nomo-graph for determining peak pressure as a function of charge weight and dis-tance for TNT is given ill Fig. 4-3. The dashed [ine represents the condi tion fora 1.0 lb. charge at a distance of 5 ft. Similar nomographs can be constructed forI>m, I, E, and 0 for any explos ive for which the explosives' constants have beendetermined.

Another useful, but somewhat less precise, expression for determining peakpressure as a function of charge weight, distance, and explosive type is given byRoth (7). The curves in Fig. '1-4 show the manner in which peak pressure varieswith distance for three different charge weights using Roth's equation for anexplos ive such as Composi tion C-3 with a detonat ion veloci ty of 25,000 fps.

The impulse, I, i s the t ime interval of the pressure as given by the expression:

Fig. 4-3. Nomograph to determine peak pressure as a funct ion of charge weight anddistance (TNT). (After Rinehart and Pearson (6time as experienced by the target may be different than the free water valuesobtained for the pulse profi le, since they are determined by the coupling proces swhich exists between the impinging pulse and target. This coupling process isdependent in a rather complicated manner on the geometry of pulse incidenceand reflection. properties of the target material, and the possible occurrence ofcavitation at the target surface (2,6).

The Gas Bubble. Following the detonation of the explosive, a high-pressuregaseous bubble is produced which goes through an oscillatory motion until itvents at the surface. With each oscillation, energy is released to the surroundingwater. Measured data for large charges reported by Cole (2) indicate that thepres sure pulse accounts for 60 per cent of the energy available, the first oscilla-tion for 25 per cent, and the remaining behavior of the bubble for the other 15per cent. These values will probably vary with the charge size and type of ex-plosive.

1 = L /'(1) dt ( 6 )In computing I, the integration should theoretically be carried out over an

infini te interval of time. However, in pract ice this is nei ther possible nor reason-able. Experience has demonstrated that a realistic estimate of the impulse fordesign purposes is obtained if the integration is carried out over a time intervalequal to about five times 0, Values for the appropriate explos ives' cons tantsgiven in Table IV-4 are based on integration times of 50 and 6.70.

The discussion in this section has been restricted to the determination ofparametric data for pressure pulse propagat ion through water. If this pressurepulse encounters an underwater target , the load characteris tics of pressure and

117


24/41

1.0

~w'""~ 30,000"~

20.000

10,000

.,.""'il![

't.!11"I iI i) .Ir\!rI,ilr '"I,

EXPLOSIVE FORMING

U N OE R W" " T ER E X PL OS IV ES0" : :15,000 (FPS}

II:.

10D IS TA NC E ( FT .)

THEORETICAL CONSIDERATIONS

Theoretical curves were generated to establish the force velocity vectors for anumber of charge types and configuratiollS. Fig. 4-:' illustrates a typical shockwave front generated by a disc charge cut from sheet explosive, To determinethe expected part velocities at different standoff distances. a series of curveswere developed using equat ions involving water velocity, charge detonat ion

FRONTLEGEND;

F YD'"' F OR CE V ELO CITY V EC TO RFd "FOR CE VELOCITY VEC TOR

OIS TAN CE AT CH ....N GE ' _ _a ..OR CE VELOCITY V ECTOR AN GLERc : , O tAR GE R ADI ISd .. STANDOFF 0 1 5 T . . ..NCE

(5H


25/41

EXPLOSIVE FORMING

1,000

0u,

.>~o@~~-c0

100

C ENHR J 5C CHARGE :- ! S HE fT OF) I N. DIAMETER A3 PETN

.\ \.\.~\\.~ " "~ ~" . . . . .I..... '" K\ '\., N""-'" ~ ' \...\ ~ --\__~ ~1151N.S H E E T\'~ \~501". PlATE10

C HA RG E S TA ND OF F D IS TA NC E, O N. )

Fig. 4-6. Part velocity curves for Type 4130 steel with center disc charge.,1,"iI'f :

7) The coefficient of friction must remain constant,8) The holddown force will vary as the sguare of the scale factor (5000 Ibs.

on a 6 ill. die would yield 500,000 Ibs. on a 60 in. die).Accurate prediction of full scale performance is possible if these require-

ments arc observed. For example, successful pans were produced Oil both a24 in. diameter and a 120 ill. diameter die from data generated on a 6 in. diarn-cter die using 2014-0 and 7039-0 aluminum sheet and plate (10, 11). Similarresults were obtained for HI' 9-4-25 steel, D6AC steel, and 12 per cent and 18per cent nickel mar-aging steels when using 6 in. and 24 in. diameter dies (II).The application of scaling laws to explosive forming has heen a significant ad-vancement in the st.ue-of-rhe-nrt and has permitted the economical develop-ment of mall)' parts .

Computer Prediction. In the past, explosive forming has been primarily atrial and error process, The application of scaling laws to the process has re-duced the trial and enol' procedures to subscale sizes only, Further steps to re-fine the technology hy computerizing the major parameters affecting metaldeformation have resulted in usable, though still not completely accurate, com-puter programs (13). Figs. 4-8 and 4-U are typical results from an IBM 1620computer program using an aluminum alloy, Further refinement is required

"iI:btI '\IIII1t '

THEORETICAL CONSIDERATIONS 121

RING CHARGE: 400-GRA,rN, 10tN. DIAMETER

'\.

" l ' " K '\..K;0..~ ~~ ~0~\

~ ~ ~12S INSHEET. . . . . . . . . . _\ ~~ ~ , . . _ . s o , " -PLATE~~IOO

J ,C HA RG E S TA N(X )F F D IS TA NC E, ( IN .)

Fig. 4-7. Part velocity curves for Type 4130 steel with ring charge,

(. - R AD IAL S TR AIN IN /INc ~ CIRCUMFERENTIAL STRAIN, IN/IN

EXPERIMENTALTHEORETICALFig. 4-8. Experimental vs, theorctical

strain for Type 2014-0 aluminum sheer. do

to permit prediction of forming results and to translate the rest~lls from onealloy systcm to another. Once perfected, the computer techniques shouldgreatly reduce the amount of empirical work necessary to describe and set upa spec if ic p roduct ion proc ess.


26/41

EXPLOSIVE FORMING

.0r------------.....;~!_-.:io; : i . 3w0[o~ .6o

Fig. 4-9. Experimental vs, theoreticalcontour data for Type 2014-0 aluminumsheet.

L2

Equations fOT Contact Operations. When detonation is initiated in an ex-plosive charge that is in contact with a metal surface, a detonation front is es-tablished which proceeds through the explosive at a constant, reproducibleveloci ty. A simple model of the detonation proces s would show the shock frontfollowed by a reaction zone in which the high detonation pressures are gener-ated. Behind this is a region in which the high pressures are sustained with theregional parameters of distance or time being mainly determined by the ge-ometry of the system. The high pressure region is terminated by a release frontbehind which the pressure drops drastically and the detonation productsrapidly expand toward the atmosphere. A discussion of the physics of detona-tion (14, 15) and the simplified working equations for the detonation process(15) are valuable for reference at this point .

As the detonation front comes in contact with the metal body, a high pres-sure is induced in the metal at the contact surface, and a high-intensity trans-ient stress pulse is ini tiated in the body. The magnitude of the induced pressuredepends on the orientation at which the detonation front impinges on the metalsurface, and on the relative properties of the metal-explosive combination.

The two extreme cases by which system orientation can affect the induced.pressures are shown in Fig. 4-10. In (a) the detonat ion front is s triking normalto the metal surface (normal incidence); while in (b), the detonation front ismoving in a direction parallel to the metal surface (sweeping incidence). For agiven metal-explosive combination, the manner in which the detonation frontis incident to the surface of the workpiece can introduce a factor of two orgreater into the value of the induced pressure. Normal incidence produces thegreatest pressure, sweeping incidence the lowest.

Fig. 4-11 shows the values of pressures induced in several materials withdifferent explosives under condi tions of normal incidence. The curves repre-sent the Hugoniots (16, 17) for the inert materials and the reflection character-istics for the several explosives based on the Chapman-Jouguet data (16, 17).The point of intersection of two such lines for a given' explosive-metal com-bination satisfies the requirements of continuity of pressure and continuity ofparticle velocity at the metal-explosive interface. A similar approach can be usedto determine pressure values when the detonation front impinges tinder con-di tions of other than normal incidence.

Simpler equations are also available for determining pressure magni tudesin metal by normal incidence detonation, but they are somewhat less exact(16, 17).


EXPLOSIVECHARGE

\ \ \ / / / EXPLO~VE PROOVCTS" '7 .AT HIGHPRE'SUREr---"I' - . . . . . 1 I{oj

E X PL OS IV E P RO OU CT SA T H IG H P RE SS UR E

(hi

U N D ET O O . . .. . E OEXPLOSIVE

Fig. 4-10. Normal ( and sweeping b detonation fronts.

1 2 r - - - - - - - - - , - - - - - - - - - ~ - - - - - - - - ~ r _ - - - - - - - - , _ - - - - - - - - _ ,

PARTICLE VELOCITY (103 FP~dFig.4-11. Induced pressures for several explosive materials under conditions of normal. incidence. (After Duvall (18

If the detonation pressure of the explosive is not known, an approximatevalue can be obtained from the working relat ion (15):

p =;rf)~.r 4 (7)


27/41

124 EXPLOSIVE FORMINGThis equation is based on the assumption that the density of the explosion

products is 30 per cent greater than the initial density of the explosive. Forthe high-energy explosives such as those listed in Table IV-5, Eq, (7) appearsto give values to within about 6 per cent of published data.

The combined effects of system geometry and system properties on inducedpressure is shown in Table lV-G, which compares pressure values produced inseveral metals by contact charges of Composition B for the two extreme orien-tations of normal and sweeping incidence. For the explosive-metal combinationsgiven in Table IV -6, pressure ratios, P " / P ! ) O , vary from 1.85 for aluminum to2.34 for i ron.

Table IV-5. Representat ive Explosive Values for Equat ion.Px DrExplosive (g/ee) (ips.)

50/50 Amatol 1.55 21,100Composition B 1.66 25,600Composition C-3 1.60 25,000Composition C-4 1.59 26,40050/50 Pentolite 1.65 24,450Tetrrl 1.71 25,75070/30 Tetrytol 1.60 23,950TNT 1.56 22,600

Table IV-6. Comparison of Pressures Induced by Normal and SweepingIncidence Detonat ion Us ing Composi tion B Explosive*,

PressurePressureRatiopolp ..aterial

Normal Incidence1'0

SWef/';"g IncidencePoo

AluminumCopperIron

5,200,0007,000,0006,800,000

2,800,0003,000,0002,~IOO,000

Aftcr Duvall (UJ)

Representative Impulse Data for Contact Operations. The impulse in acontact operation is related to two basic considerations: (I) the metal-explosivecombination which determines the peak pressure induced in the metal; and(2) the geometry of the system which determines the pressure-rime relation forthe loading period.

A simple tool for considering impulse in a general sense is the detonation-head concept. In the detonation-head model, it is assumed that there is a re-gion between the detonation front and the release front in which the pressureis a constant value; this usually is taken as the detonation pressure of the ex-plosive. The spatial or temporal dimensions of this model are determined by


1.852.332.34

the geometry of the metal-explosive system, and a relat ion that the release fronttravels with a velocity of about two-thirds that of the detonation front (19).

In general, two different types of problems are encountered in contactoperations: one type develops when the impulse is uniform over the loadedsurface; in the other type, the unit impulse is nonuniform. l n the first case,the body can be expected to follow a uniform behavior pattern; while in thesecond case, variations in the behavior can be expected. At the present time,the use of impulse studies in explosive contact operations is restricted mainlyto the general relative behavior of a body rather than to quantitative valuesof impulse.

The system shown in Fig. 4-) O b is an example of an essentially uniformunit impulse over the loaded surface. Here a thin, unconfined layer of explos iveof uniform thickness is placed in contact with a metal plate and detonated fromone end. The detonation-head model appears as a wedge, or triangular region,with its base on the plate surface. At any instant, the metal in contact with thisbase is considered to be subjected 10 essentially the same pressure magnitude.The time for which any point on the surface is subjected to the high, constantpressure of the detonation head is determined by the detonation velocity of theexplosive and the base length of the detonation head. The thicker the layer ofexplosive, the longer the pressure will be sustained at any point on the surfaceand the greater the unit impulse, although the principal pressure!nagni tudewill be independent of thickness. For a copper plate loaded with a 3 /, in. thicklayer of Composition B explosive in the manner shown in Fig. 4- lOb, the metalat any point at the interface would be subjected to a principal pressure of about3,000,000 psi. with a duration of about three microsec. (6).

Fig. 4-101, illust rates an example of a contact operat ion with nonunifonnimpulse. Here an unconfined cylindrical charge placed on end against a metalsurface is detonated at the free end. Pressure relief, controlled by lateral ex-pansion of the detonation products, establishes a detonation-head model witha conical configuration. While the entire contact area will be subjected to apressure of the same magnitude, that pressure will be sustained much longerat the center of the charge than near the periphery. Thus, the unit impulse isgreatest at the center of the charge, and decreases in value with increasing radialdistance from the center. For a long cylinder of explosive, increasing thediameter of the charge will increase the unit impulse at the charge center butwill not affect the principal pressure. For a copper plate loaded with a 2 in.diameter by 5 in. long cylinder of Composi tion B. the metal surface at the centerof the charge would experience a principal pressure of about 7,000,000 psi.for a duration of about 4 microsec. (IG). The remainder of the contact areawould experience the same principal pressure, butthe duration of the pressurewould be less depending on the radial distance from the charge center.

Determination of the approximate impulse values related to Ihe action ofthe explosive load during the principal pressure phase is relatively easy if thesimple approach in the preceding examples is followed. After the release frontpasses, the impulse values related to the pressure release phase are moredifficult to determine. The main value of the detonation-head concept is toprovide a ready means for making behavioral comparisons between systems andfor judging the effect of changes in the des ign parameters.


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EXPLOSIVE FORMING

Theory of the Combustible-Gas-Mixture ReactionWhen a mixture of combustible gases is confined within a closed vessel and

ignited. the reaction Illay occur in three phases:I) Adiabatic combustion: the velocity of the name front is subsonic and thechanges in pressure. temperature. and volume follow the laws of an adi-abatic reaction.

2) Unstable or transient detonation: an extremely turbulent reaction isaccompanied by severe shocks and over-pressures. This is a transientphase and is usually followed by stable det onation.

3) Stable detonation: the reaction occurs at a fixed supersonic velocity withina thin segment of the name front. and with the unburned gas ahead ofthe shock front at rest.

The following thermochemical reactions for hydrogen and oxygen indicatewhat happens in the adiabatic combustion reaction:

I) H, + 'I, 0, ~ H20 + 57.107 cal/g mol. at 00 K.2) OH + 'I, H2 ~ H,O + 67.107 callg mol. at 0 K.3) 2H ~ H, + 103.240 callg mol. at 0 K.4) 20 ~ 0, + 117.172 cal/g' mol. at 00 K.I.e Chateliers' principle' on equ il ib rium mixture s a ssi st s in an unders tanding

of the end result. This principle may he summarized as follows: (I) Increase intemperature favors action that uses heat; (2) Increase in pressure favors actionthat reduces the volume of the mixture.

Both temperature and pressure increase in an adiabatic combustion in aclosed vessel. Since temperature increases drive the reactions to the left whilethe pressure increases drive rhcm to the right. an intermediate point of equilib-rium is reached. Consequently. the end products are not pure H,O. but H20,. 01 I. H. and O. Analytical techniques arc available for calculating theseproducts and the final pressure. although the techniques are relatively complex(20). Regardless of this complexity. the calculation technique presents lessdifliculty than actual test measurements since: (I) the frequency sensitivity ofcurrently available high-presslII'e sensing devices presents problems, and (2)the disassociation of gases that occurs at peak pressure makes the analysis ofcombustion products inaccurate. Fig. 4-12 charts the calculated pressure valuesof the H101 mixture.A further consideration is the physical phenomenon of name propagation.I n a closed vessel. the name will normally propagate itself in a spherical flamefront from the point of ignition until it is quenched upon the vessel wall.

l n adiabatic combustion. as the first increment of gas burns and expands.the unburned gas is c ompressed slightly. The next increment of hurning furthercompresses the unburned gas and partially recompresses the previously ex-panded and burned gas. This process continues in the spherical flame frontuntil quenching occurs. Due to geometric relationships of unburned and burnedgas volumes to the flame front area thl'Oughoutthe entire range of increments.the end result will he a transient pressure gradient; i.e . the pressure at the point

I(;,[) Hodgman, bl. r t a t. in the IJfllldho(J/( o/ ( ;h{ 'mi.l ilJ {/1I/1Ph.pin (44th ('(I.; Ohio: The Chemical Rubber PublishingCompany . IW; :1 ) s ta re s t ha i " II s ome ~In:~!is hl'OlIghl 10bear "POll it SplCIII in equil ibrium, a change ( )C(1I1~,such tha ithe equililn inm i~displaced in OJ direction wili( h 1 I lU Is 1 0 undo Ihe effcct or (he 5IU:r.s,"


P.Pi

0 1 " " - .9 .. .r--, ""- .. .r - : . . ~

~ . . . . . . . . .


29/41

128 EXPLOSIVE FORMINGor ignition will be higher than that at the tool surface. Equilibrium is quicklyreached.The flame propagation phenomena also involve velocity. Until it reaches

sonic velocities, the propagation of flame isundergoing a continual acceleration.The acceleration force is a function of the atomic attraction forces of the par-ticular atoms involved, their separation distances, and the degree of agitation.The velocity at any time, therefore, will depend upon the accelerating forceand the distance traveled. This becomes of interest since the velocityand certainother quantum-mechanical considerations (21,22,23) determine when unstableand stable detonation occur. Since detonation is normally avoided for formingand because of safety considerations in larger parts, knowledge of the theoreticalaspects is helpful. Both the accelerating force and distances traveled can bemanipulated.

EQUIPMENTOne of the most important aspects of explosive and gaseous metal forming

is the proper design and selection of equipment. Many components have beenlost as a result of poor design or wrong material selection. The following sec-tions present important information relating to tooling, preparation of charges,and facility requirements.High Explosives

Die Design. A variety of die configurations have been used for explosiveforming which have revealed the optimum features for successful production ofmetal shapes. Dies designed for conventional metalworking processes, i.e., forhydraulic or mechanical presses, are generally not adequate for explosiveforming. Because of the shock loading imposed by the high energy explosivesource, sharp corners, improperly located vacuum ports, and brittle die ma-terials must be avoided. The detonation of a high explosive produces a shockwave which passes into the die and clamping system to be reflected byboundaryor intermediate surfaces. Cracks can form at the corners of a die as a result ofintensive stresses created by reinforced tensile waves from rebounding shockfronts. In addition, closely spaced vacuum holes (if more than one isused) canpermit fracture of material between the holes which will propagate into the diebody. This fracture is a result of die deflection during the initial high shockload on the system. Brittle materials such as ice, Kirksite,* or Cerrobend.]concrete, can catastrophically fail due to high bending loads or impact stressescaused by blank contact with the die surface.

Because of problems encountered with a variety of dies and the successfuluse of others, certain general ground rules have been established with respect todie design. These ground rules depend upon the particular material beingformed and the type of operation to be performed. Table IV-7 shows recom-mendations made for deep drawing dies. .

For a few parts made from a material having lowstrength, plastic, concrete,and even hardwood are adequate die materials. Naturally the tolerances ob-

. .Reg TM, Morris l l. Kirk and Son.tReg. TM, Cer rn Corporation.

EQUIPMENT 129Table IV-7. Recommended Die Materials for Explosive Forming (24).

Yield Strength of Mater ial to beFormed (ksi.)Quantityof Parts 10 /0 25 25/075 75/0150 150+I to 10 A B C D10to 100 B C D D100to 500 C D D EOver 500 D D E ELegend: . -f " '" Plastic and plastic faced

B "" KirksitrC - B oi ler p la te o r ca st s te elD - Alloy steel (4130, 4340, etc.)E - To ol "eel (H I l , etc.)

tained are a direct function of the die hardness, rigidity, and surface finish.When high strength alloys are formed, i.e., materials whose yield strengths aregreater than 75,000 psi., it is generally necessary to use cast steel, alloy steel,or tool steel dies.For sizing dies, the general requirements for deep drawing operations still

govern the die design and material selection. High strain hardening materialssuch as IN 718 have been sized in a composite die consisting of an epoxy face,fiberglass shell, concrete back-up, and steel container. Fig. '4""13 shows a dieused for the fabrication of exhaust cone components. The die remained intact

Fig. 4-13. Composite die used for fab-.:rication of exhaust cone components.(Courtesy, Explosive Fabricators Corporation)

after repeated loading of over 50 shots using charge sizes up to about 2500'. :grains. The only problem encountered was epoxy face spalling which resulted. 'from using too brittle an epoxy material. Composite dies show high promise foreconomical tooling. In addition, short lead times are possible, e.g., the sizingdie shown in Fig. 4-13 was fabricated and in use within three weeks from thetime of design initiation. Successful use of fiberglass, concrete, and steel for,deep drawing was reported byJue and Giannoccolo (25). Two 1.0 in. thicksteel domes were made for the Sealab II. Tolerances of '/4 in. on contour and' / ' 6 in. on diameter were maintained. The significant point is that from thetime of design initiation until completion of forming only 30 days elapsed.The massive dies normally used for the explosive forming of parts up to

about 60 in. in diameter become unwieldy when larger sizes are produced. Thisunwieldiness has led many investigators to study lightweight construction.


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EXPLOSIVE FORMING EQUIPMENT

Beyer was one of the earliest to report the use of plastic shell dies (26). Heused a pendulum concept in which a shell die is suspended with a blank in placeand a charge located above the surface. The charge was detonated with theblank normal to the water surface. This type of forming is possible since wateracts as a solid support for the die shell because it isessentially an incompressibleliquid. More recent innovat ions of the shell die concept have been succes sfullyapplied (10). One piece. 10 ft. diameter domes from 2014-0 aluminum platewere produced using a 2 in. thick fiberglass shell die mounted on a steel under-s tructure having spring shock mounts. Fig. 4-14 shows the arrangement. Under

A rather unique concept for forming hemispheres. ellipsoids. or completespheres was recently reponed by A. Frey (27). Basically the concept involvesthe use of charges placed inside of a hollow welded preform. A provision ismade for water entrance to the inside of the hollow body. Shape is controlled. by the placement. size. and type of explosive charge. The exact conditionsnecessary to form a given shape must be established empirically. The concepthas considerable merit since no die is necessary. but a trade-off is needed toestablish the costs of preform preparation vs. the die costs saved. The methodiscurrently being used in production in France.

Fig. 4-15. Flange wrinkling caused hyinsufficient stiffness of damping system.(CouTtesy . Martin Manella COT/JOWlioll)

Fig. 4-14. Fiberglass shell die used for explosive forming of 120 in. diameter tankdomes. (Courtesy, Marlin Marietta CorporationsU.S. Air Force contract (II). metal shell dies were evaluated for the forming ofhigh strength steels. Although the die shells were capable of withstanding theloads of forming. the major problem encountered was inadequate clampingst iffnes s. Because of the tendency of high strength metals to wrinkle. especiallyin thin sections. significant st iffness must be provided in the hold down system.Insufficient s ti ffness results in wrinkling of the type shown in Fig. 4-15. In orderto provide high sti ffness with at tendant low weight . hollow clamping rings withchannel stiffeners can be used. A metal shell die for use with high strength steelis shown in Fig. 4-16. Other concepts or variations may be used. but more devel-opment is needed to optimize a heavy duty cycle. lightweight die system forlarge components.

Fig.4-16. Metal shell die used for explosively forming high strength steel and aluminumalloys. (Courtess , Martin Marietta Corporation]I-Lieberman and Zernow (28) reponed the novel concept of using ice dies.

-The technique involves the use of dry ice to freeze water to form a forming sur-.: f ace on small dies and the installation of refrigerant coils for large diameterparts . Tolerances produced on these dies were good. and several advantageswere apparent: lowered die weight. rapid and economical repairs. short leadtime for fabrication. low cost fabrication. and readily available equipment .Although ice dies have merit. little has been done with them in this country .. I n Germany. W. Simmler (29) reports that additions of sand or aggregate to


31/41

EXPLOSIVE FORMING

ice improve s trength and permit addi tional lis e. Further developments shouldbe forthcoming from Europe over the next year.

Facility Design.The faci li ties needed to implement explosive forming tech-niques are not expensive when compared to mechanical or hydraulic pressrequirements. Most fi rms engaged in research and development or productionforming use steel lined concrete pools with vertical s ide walls. Pool s izes rangefrom about 6 ft. to 25 ft. in diameter. The steel liner thickness and propertiesgenerally determine the explosive charge limitation. Charges up to about 25lbs, of I'ETN have been used in the vertical wall pools. A large installation isshown in Fig. 4-17. Charge capaci ty can be increased by the use of a perforatedring at the bottom of the pool or around the periphery of the die through whichair is blown. The continuous stream of bubbles between the expanding shockfront and the pool wall greatly reduces the shock pressure and thus protectsthe pool from damage.

The largest known pool in thc United States is in Denver, Colorado, at the'Martin Mariet ta Corporat ion. This pool differs in concept from others availablein that a sloping wall is used to permit larger explosive charges for forming.Charges up to 26.8 lbs. of waxed RDX have been used to date. Another devia-tion from "conventional" concepts is a motorized dolly which supports the dieand travels down a steel track to straddle a concrete pedestal on the bottom of .the pool. The die weight is transferred from the dolly to the pedestal priorto forming. Other pools use bridge cranes, jib cranes, monorail systems, or A- .frame hoists to lower and raise die loads.

A convenient source fill' forming is the ocean since virtually unlimited depthand size are possible. The Navy successfully formed 12 ft. diameter steel end

EQUIPI>.IENT 13:..closures in San Francisco Bay. Over 100 lhs. of C-3 plastic explosive was deto-nated to effect forming. Similar use of large bodies of water may be necessaryin the future as the need for larger componcnts increases.

Explosive Preparation. The variety of types and forms of explosives avail-able makes explosive fabricat ion versat ile. Explos ives are available which havedetonat ion velocit ies ranging from about 7000 fps. up to nearly 30,000 fps.They come in the form of bulk powders, cord, sheet, liquid, or plastic. Thepowders can be further modified by casting or pressing. Fortunately, most ofthe explosives useful in explosive forming are quite insensitive to shock andheat. Thus handling and safety problems are greatly reduced. The powderedexplosives such as TNT, PETN, and RDX can be made even less sensitive bycasting or pressing. Naturally, better shape control and density control can beeffected by consolidat ing methods . For consis tent and controllable energy re-lease, pressing and casting should be used. TNT can be readily pressed or cast,but to assure consistent, first order detonation time after time, the use of RDXboosters is recommended. Dynamite contained in friable containers is a con-venient explosive, but the repeatabili ty and control of energy are poor.

Probably the most convenient forms of explosive for forming are Deta-* and Primacord+, However, because of their special shape they are notusually used for deep drawn contours having symmetry. They are highly de-sirable for large surfaced parts where uniform pressure over a significant area'is needed.

Information and literature about liquid explosives (other' than ni troglyc-is scarce. This scarci ty also applies to the use of liquid explosives for form-purposes . The s ilence is unwarranted because at least one liquid explosivePatent Nos. 3,132,060 and 3,239,395) has proved eminently sui table for

forming purposes and has been used for nearly ten years. This liquid explosive'. consists basically of a mixture of two liquids, one of which acts as a sensitizer' for the other. Taken separately, each liquid is only a combust ible liquid and notan explosive; this safety feature is perhaps the most significant practical ad-.vantage of the liquid. The safety requirements for shipping, storage, and hand-ling of these components are, therefore, considerably less rigorous than the re-quirements for a high explosive .. ; The prepar-ations for useof the liquid explosive consist only of measuring'.the required quantities, pouring them together, placing the liquid in the ex-plosive container, and installing a detonator. To dispose of surplus explos ivemixture, i t is only necessary to desensi tize i t by adding water. Comparat ive ex-plosive forming tests of the liquid explosive have shown its forming ability tobe somewhat above that of TNT, and approaching PETN. The crit ical diameterisbelow ' /. in.

The moldable explosives such as C-3 and C-4 arc based on RDX and arevery insensi tive. Various charge shapes .can be produced merely by molding byhand or into preshapcd containers. Although there are many advantages inthe use of plas tic explosives , i t is difficult to accurately control charge densi ty.Air entrapment causes uns table detonat ion and variable energy release-un-desirable for production forming requirements .

"R eg . TM, E . L du Pont d e N emou rs an d C ompa ny . I nc .t Reg. TM. Ensign-Bickstord Company.


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I~ EXPLOSIVE FORMING

Automation. Explosive forming has always been considered as a short runproduction or prototype technique. However, some past and present experienceshows that high volume product ion can be cons idered if sui tably designed equip-ment is used. I n 19tH the Ccrrnans reported (30) the production of sheet metalparts using an arrangement whereby the explosive was located at the center ofacircle, the periphery of which contained up to six dies. Thus the uniform shockfront emanating from the charge was fully utilized in forming. Up to 60 partsper hour were produced using the multiple die set-up. Metal cups were alsoproduced from tubes, six at a time by using a single die with six cup shapesmachined from the one die block. About two to three dozen cups per hourcould be produced.

Preliminary development of a machine showed the feas ibi li ty of semi-auto-mated operation using chemical explosives (31). Two facing dies were used withthe explosive located between them. Pneumatic controls permitted openingand clos ing of the system and pan eject ion was effected by bleeding air throughthe vacuum port of each die block. Blank and charge placement were accom-plished manually. Semi-automated operat ion of the system shown in Fig. 4-18yielded 24 to 36 parts per hour. Prepackaged charges, automatic blank feed,

Fig.4-19. Diagram of automated chem-icalexplosive machine developed by theUniversity of Birmingham, England.

W A T E R

Fig. 4-18. Semiautomatic explosive forming machine for deep drawing of thin sheets.(;o"r".I), Martin Metal: CompallY) HIGH EXPLOSIVE

charge feed, and part removal could easily permit the production of up to 30nparts per hour of simple shapes. More development is needed to perfect thesystem hut the feasibility has been dcmunsuarcd.

S. A. Tobias (32) in England has been very active with the automation of achemical explosive system. Present models have been semiautomatic in opera-tion, but work is in progress to completely automate the machine to perform allnecessary functions. The machine, shown in Fig. 4-19, is the only one of itskind in the free world and will be capable of forming parts up to 22 in. indiameter.

EQUIPMENT 135

Fig. 4-20. Energy sources for integrated high energy forming machine.

Documents

High Velocity Forming Part II