Preliminary examination of high-velocity metal-shaping with electrical wire explosion

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1 Preliminary examination of high-velocity metal-shaping with electrical wire explosion M.Woetzel 1, 2, M.J. Löffler 1, E.Spahn 2, H.Ritter 2 1 University of Applied Sciences Gelsenkirchen, Energy Institute, Germany 2 French-German Research Institute of Saint Louis (ISL), France Keywords: metal shaping, electrical wire explosion, electro-explosive shaping Abstract The experiments aimed at shaping 1-mm aluminium sheets into cones by different electrically initiated ways. The goal of the experiments was the comparison between several electrical explosion methods. In the experiments copper and aluminium wires (l=20 mm, D=0.8 mm) were exploded or arcs were initiated between electrodes. The experiments (arc and wire explosion) were provided by the same capacitive power supply with a charging voltage of 9 kv and a stored energy of 1 kj. The wires as well as the arcs were ignited under air, oil, or water. Under the same electrical conditions, the experiences showed that the shaping effect using the explosion of an aluminium wire under water gives remarkably better results compared with other electro-hydraulic methods (like arc strike under water). This superiority can be attributed to the well known chemical reaction of the aluminium micro-spray with the surrounding water. The results are discussed together with considerations about explosives used for explosive forming of large metal sheets. INTRODUCTION Forming metal sheets with high velocity is a promising method to produce prototypes or build complex forms out of metal sheets. In the past high velocity metal shaping was accomplished by dynamite-explosions, electromagnetic or electro-hydraulic forming. High-velocity metal-shaping with electrical wire explosion is very close to the dynamite-explosion method but it is possible to include it in industrial production lines, in principle: There is no need for special security conditions compared to the use of dynamite. STATE OF THE ART High-velocity-shaping systems allow to shape metal with very high velocity (>>100 m/s) in a very short time (<100 µs). The systems are: Shaping with high velocity forge hammer, pneumaticmechanical shaping with expansion condensed gas, electromagnetic shaping by capacitor discharge in a coil, electro-hydraulic shaping by capacitor discharge in spark absorber under water and explosive shaping in liquid High-velocity-shaping has some remarkable advantages but there are severe arguments against explosive highvelocity-shaping in the industrial production: - safety regulations - bad efficiency - long preparation time - low productivity Sometimes shaping with high-velocity forge hammer, electromagnetic and pneumatic-mechanical systems is realized. Shaping with explosives is skipped by the most of westward users. The investigated process is close to explosive shaping but without its disadvantages. EXPLOSIVE FORMING Explosive forming is a high energy rate forming (HERF) process using active media with the energetic effects of sizing and stretch forming according to standard DIN The shock wave generated during the explosion or detonation, for example under water, is used to transmit energy through the medium and transfer it partially to the workpiece. The piece is accelerated to velocities of >>100 m/s, hits the rigid one-piece forming tool and is formed by kinetic energy. An efficiency of some per cent of the energy contained in the explosive charge is obtained. A time-delayed small increase in pressure of the gas bubble produced by the conversion products follows after the shock wave. Finally, the gas bubble breaks through the surface of the water tank in the form of a fountain. The shock wave s initial velocity depends on the used explosives: - Low explosives (slow explosives) have a conversion rate of m/s. - High explosives (blasting agents) have a maximum velocity of 8000 m/s which, after a certain propagation interval, decreases to the sound speed of the medium (water = 2000 m/s). Because of the high velocities during the forming process, especially due to sizing or stretch forming, the tool cavity has to be evacuated in order to perform forming operations without interruption, to limit the compression temperatures and to avoid the diesel effect generated in the presence of grease or oil. Explosive forming requires important safety measures, specially trained staff and facilities out of the way of

2 manufacturing and housing areas. Workpieces with small to very large dimensions (pipes of 2030 mm and more, bars of a diameter of mm and more at wall thicknesses ranging from 1-30 mm or more) may be manufactured. Most of the one-piece tools are made of steel, cast iron, concrete, plastic and, if necessary, ice. Explosive forming offers the following advantages for the production of small quantities of large workpieces, especially made of hardly formable materials: Comparatively low investment costs Low tooling costs Reduced number of operating cycles compared to conventional techniques But there are also a few disadvantages: Difficult definition of forming cycle and tool design Low-quantity production Considerable safety measures As a whole, the technical advantages seemed not to be very convincing and the economic disadvantages proved to be serious. As a result, explosive forming is only employed for a limited number of worldwide applications (for example: production of profiled sheet prototypes). Even the development of facilities allowing explosive forming in closed tools with cartridge explosives had no positive influence on this trend. EXAMINATION OF ELECTRO-EXPLOSIVE SHAPING ADVANTAGES OF THE METHOD An idea described in [2] shall be picked up for the electroexplosive forming: A piece of aluminum-wire put under water is burdened with a capacitively driven high current. As a result the wire is overheated by ohmic heating. The lightning-arc develops a first shock wave during the wire explosion. The sprayed aluminum reacts with the water and gets oxidized by its oxygen [7]. Thereby about the same amount of chemical energy is set free as electrical energy is necessary to evaporate (<15 kj/g). By the release of hydrogen, overheated water-steam as well as by electro-chemical energy an additional pressure is developed. The blast effect, is suitable to reproduce the effect of explosives under water, in principle, being suited for the explosive forming of sheet metal. The electro-explosive forming has the following advantages in comparison to the explosive-driven high-speed sheet metal forming: Wire, water and the electrical energy supply is not reacting with outside effects No special skilled personnel is necessary Storage and transportation of the required materials is easy and not hazardous No special safety measures are necessary against criminal attacks No toxic gases during and after the blast; only hydrogen generation has to be taken under consideration The affordable electrical energy is less than at magnetic or pure electrohydraulic shaping systems due to the exothermic reaction. Experiences in explosive shaping are directly applicable to shaping with reactive wire-explosion. TARGET OF RESEARCH PROGRAM Usually wire explosions are considered as a high energyconsuming process basically preventing their industrial usage also due to the high investment cost of the necessary energy supply. Experiments with reactive wire materials like aluminum are rarely done and, as far as the authors know, are done without any relation to the forming of sheet metal. The main experience came from numerous experiments with copper wire and other energy consuming materials. In spite of the numerous experiments with wire explosions [2] there exists no expressive direct comparison between the forming with wires of different materials under different environments at the same supplied electrical energy. Hence the aim of the investigations was such a comparison. Wire materials for the tests are: aluminum wire copper wire varnished copper wire electrical arc As environments are taken: water oil air For the tests the energy in the capacitors is kept constantly at 1 kj. EXPERIMENTAL SETUP Figure 1 shows the test assembly. Figure 1: Test assembly On the left side an exploded drawing of the device and, on the right side, the opened device together with its main dimensions are shown. The test device is made of aluminum. From the outside, two threaded steel rods with insulating sleeves made of polyamide are fixed to the device. Fastening screws designed to fix the wire between the threaded rods are mounted on their pointed ends. The clearance between the rods inside the test device amounts to 20 mm when using a wire. It is up to 2 mm for arc igni- 20

tion. Simple screw connections fixed to the outside parts of the threaded rods ensure current supply. The device may be filled up with 65 ml of tap water, mineral oil or air.

Mounting base with opening Test-box before final assembly Aluminum sheet fixed on mounting base Aluminum sheet on mounting base after dismantle Figure 3: Circuit diagram Decisive for the maximum

The charge voltage is ascertained directly on charger because the charging stops when the assigned level is reached. The current is measured by Rogowski-coil.

Four screws and one mounting plate are used to fix it to the test device. The mounting plate has an opening of 50 mm diameter. On the sheet metal side, the opening is curved with 2 mm radius.

3 tion. Simple screw connections fixed to the outside parts of the threaded rods ensure current supply. The device may be filled up with 65 ml of tap water, mineral oil or air. The countersinking hole in the test device is 30 mm deep. Mounting base with opening Test-box before final assembly Aluminum sheet fixed on mounting base Aluminum sheet on mounting base after dismantle Figure 3: Circuit diagram Decisive for the maximum current are circuit inductivity, capacity and charge voltage. The ohmic resistance in the circuit has only a small influence to the current peak. The charge voltage is ascertained directly on charger because the charging stops when the assigned level is reached. The current is measured by Rogowski-coil. TEST RESULTS Figure 2: Assembly and mounting base of the metal sheet. Disassembly. The square-shaped aluminum sheet to be formed is 1 mm thick with an edge length of about 130 mm (see figure 2). Four screws and one mounting plate are used to fix it to the test device. The mounting plate has an opening of 50 mm diameter. On the sheet metal side, the opening is curved with 2 mm radius. A copper sheet is used to seal the device (see drawing in figure 1, left side). After finishing the tests, the test device is dismantled by loosening the four screws and the formed aluminum sheet is re-examined. In the shown example, the aluminum plate has been stressed beyond its limit of elasticity and bursted in the middle area. ENERGY SUPPLY The circuit diagram is shown in Figure 3. The capacity is C=20 µf. The voltage for energy adjustment up to W 0 =1 kj is constant at u 0 =10 kv. From current oscillation in short circuit tests it is calculated that the inductivity in the circuit is L=372 nh and the ohmic resistance R=9.3 mω. The ohmic resistance of the wire R wire (t) (respectively of the spark discharge) is represented by solid state of the wire and he is much smaller than the circuit resistance (<100 µω). TYPICAL TEST SEQUENCE The pneumatically operated close switch is turned on after charging the capacitor battery to an operating voltage of 9 kv. An oscillating current begins to flow. Figure 4 shows a typical current wave form with delayed wire explosion (in this case: varnished copper wire). The same peak current of about 30 ka limited by the inductivity of the system is measured during the other tests. The prior measured variable is the forming result as a function of the different blast generating processes (wire or electric arc under water, oil or air) with constant device parameters. After dismantling the test device, the formed aluminum sheet is removed and the height of the formed metal cone is measured. Thus, the cone is placed against a scale (see figure 5) and its height (h) is determined by visual inspection.

Figure 4: Typical current wave form. Figure 5: typical result and evaluation steps. SUMMARY OF RESULTS Water As a rule, three tests are carried out for each parameter setting.

4 Figure 4: Typical current wave form. Figure 5: typical result and evaluation steps. SUMMARY OF RESULTS Water As a rule, three tests are carried out for each parameter setting. In case of intact cones, the reproducibility of the tests is surprisingly high. During the tests concerning the measurable maximum heights, a poor reproducibility is only obtained when the cone point is broken off. In this case, an average minimum height of the broken cone point may be defined. The used wires made of aluminum, copper or varnished copper have a diameter of 0.8 mm and a length of 20 mm. In case of a simple electrical breakdown between the rods the electric arc is 2 mm long. This reduced electrode spacing is necessary for an electrical breakdown in each surrounding medium, such as tap water, mineral oil and air. The tests show clearly that the forming result depends on a high degree on the used wire-or arc-material combination and the surrounding medium. During the tests carried out with an aluminum wire under water, considerable forming effects culminating in the destruction of the profiled cone point are generated. These tests show that the broken cone point reaching values of >19 mm is at least twice as high as in case of tests using an electric arc under water and oil or an aluminum wire under oil. During the tests using a copper wire under water and oil, considerably smaller forming effects are obtained. Wire explosions or electric arcs in the air show almost no forming effects. Aluminum Copper varnished Copper Arc (2 mm) Oil h > 19mm h = 4. 0 mm h = 4. 8mm h = 10. 5mm h = 9. 8mm h = 5. 5mm h = 10. 5mm Air h = 1. 0 mm h = 0. 1mm h 0 mm Figure 6 shows a summary of the test results. The different pressure generating wire/arc material Figure combinations 6: Summary INTERPRETATION of test results OF TEST RESULTS and the surrounding medium as well as the supervising illustrations of the corresponding test results and the height of the cone points of the forming result are summarized in a table. Tests including wire explosion or simple arc discharge in the air show clearly that the sheet metal forming process needs more than electric energy. Insignificant forming results may only be obtained in case of a blast transmitting medium (metallic fumes partially oxidized by atmos-

pheric oxygen) through the exploded wire material. Aluminum wire leads to considerably better forming results than copper wire or gas discharges.

If oil is used as pressure transmitting medium, comparable forming results are obtained with both aluminum wire and simple electrical breakdown.

5 pheric oxygen) through the exploded wire material. Aluminum wire leads to considerably better forming results than copper wire or gas discharges. This proves that a higher amount of energy is liberated chemically due to the oxidation of aluminum spray. If oil is used as pressure transmitting medium, comparable forming results are obtained with both aluminum wire and simple electrical breakdown. In this case, wire explosion offers no particular advantages or even disadvantages compared to conventional electro-hydraulic forming processes. Copper wire leads to poorer forming results than an electric arc due to extraction of electric energy used for heating, fusing and vaporizing of the copper wire. compared to aluminum copper requires more fusing and vaporizing energy. On the other hand, compared to electro-hydraulic forming processes, aluminum wire under water generates more powerful pressure effects enabling even the destruction of the profiled cone. The fact that the explosion of the aluminum wire under water results in more powerful pressure effects than under oil could be explained by the oxidation of the aluminum vapor or spray. The test using an electric arc under water and showing comparable results as under oil proves that, in this case, there is no other particular effect of the surrounding water that may be decisive. -Primary explosive 100 mg (lead azide detonator) -Explosive 185 mg (PETN) Figure 7: Experimental design explosive shaping RESULT AND COMPARISON????? COMPARISON WITH EXPLOSIVE FORMING EXPERIMENTAL DESIGN For explosive forming the experimental design is nearly the same like in electro-explosive forming. Electrodes are not used. The explosive detonator is inserted from the top inside a screw shown in figure 7. The explosive is given with: Primary explosive 100 mg Lead acide (Pb(N 3 ) 2 Explosion heat: 1.54 kj/kg 154 J Explosive 185 mg Nitroexplosive (PETN) Explosion heat: 6400 kj/kg 1184J speed of propagation: 8000 m/s The energy of the detonator is ~1.3 kj. Set-off of the explosion is made by electrical contact (0.8 A)[6]. CONCLUSION Sheet metal forming experiments with wire explosions (Al, Cu) and arc discharges under water, oil and air at peak currents of 30 ka are described. For ignition of the wire explosions and arc discharges the electrical energy is kept at 1 kj. For comparison also a forming experiment with a PETN explosive is accomplished. With concern to the forming results the experiments clearly show the advantage of the chemically reactive Alwire explosion under water compared to the other types of wire explosion or of arc discharges. At comparable energy input the forming of the reactive Al-wire explosion matches to the effect of a pure explosive forming method. Because the use of conventional explosives for industrial metal forming is excluded due to strong and expensive safety regulations the application of chemically reactive wire explosions for sheet metal forming comes to the fore as a promising alternative.

6 REFERENCES [1] [Gräfen, H.]: Lexikon Werkstofftechnik, ISBN [2] Wieland, H.A.: Elektrische Drahtexplosionen als Grundlage eines alternativen Sprengverfahrens. Dissertation. Hamburg ISBN [3] M. J. Löffler: Abschlussbericht: Orientierende Untersuchungen zur Hochgeschwindigkeitsumformung dünner Bleche mit Hilfe elektrischer Drahtexplosion unter verschiedenen Medien. FH Gelsenkirchen, Labor für Hochspannungs- und Hochleistungspulstechnik, [4] Rudolf Meyer: Explosivstoffe. Verlag Chemie [5] Fachgespräch mit Dr. Ritter (ISL France) 03/08/06 [6] Datenblatt Bleiazidsprengkapsel [7]Lee, W.M.: Effect of current pulse shape on driving metal/water chemical reaction. 8 th IEEE International Pulsed Power conference, 1991, pp

Electrical Wire Explosions as a Basis for Alternative Blasting Techniques? Abstract

Electrical Wire Explosions as a Basis for Alternative Blasting Techniques? M. Loeffler, H.A. Wieland University of Applied Sciences Gelsenkirchen Neidenburger Str. 10, D-45877, Gelsenkirchen, Germany Phone: