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Supersonic Liquid Projectiles: a Novel Materials Processing Tool

ERNEST S.GESKIN, O.PETRENKO, V.SAMARDZIC, K.KLUZ

Mechanical Engineering Department New Jersey Institute of Technology

University Heights, Newark, NJ 07102 USA

Key-Words: - launcher, supersonic, projectile, impact, forming, microforming, welding, rocks boring, coal combustion Abstract: Formation and practical applications of supersonic liquid projectiles were investigated. The projectiles were generated in the course of unsteady liquid (water) acceleration in a converging nozzle. The liquid was driven by a moving piston or by powder combustion (explosion). Several versions of the launcher were tested and process conditions were evaluated. Numerical modeling of fluid acceleration using a method of characteristics and a commercial package (FLUENT) was carried out. The results were applied to analysis of the mechanism of the supersonic acceleration and to evaluation of the effect of process conditions on the projectiles properties. The high-speed movie was used to determine velocity of a projectile. The major effort, however, involved the study of the projectile-target interaction and, thus, potential practical applications of the proposed technology. It was shown that the projectile affects a target similarly to an explosive deposited on the target surface. Explosion-free neutralization of non-dischargeable explosive setup demonstrated the rate of target deformation by the impacting projectiles, while crashing of heavy reinforced concrete plates showed the intensity of the impact. The further experiments showed feasibility of the projectiles applications for various forming and micro forming operations, welding and rock boring. It is suggested that potential applications will include nanoimprint technology, solid free form fabrication of heterogeneous parts and emission-free coal combustion. Key-Words: supersonic, projectile, converging nozzle, impact, forming, microforming, welding, rocks boring, coal combustion

1 Introduction One of the most effective avenues in the material processing is creation of high (> 1 GPa) hydrostatic stresses in a work piece. Tresca [1, 2] established that at a high hydrostatic pressure a solid could experience large homogeneous strains and behave like a liquid. This notion was experimentally demonstrated by P.W. Bridgeman [3, 4]. Bridgeman showed that under some conditions both brittle and ductile materials exhibit extremely high plasticity. For example, at a hydrostatic pressure of 3 GPa plastic deformation of B2O3 glass results in the reduction of the cross section area of a sample by 87%. This experimental study demonstrated that when subjected to a sufficiently high hydrostatic pressure any material, including extremely brittle ones, becomes ductile and can flow infinitely.

The effect of the hydrostatic pressure on the workpiece properties is utilized in high energy rate forming (HERF) processes such as explosive, magnetic-pulse and electro-hydraulic forming which are widely used for materials shaping, improvement

of materials properties, welding, sintering, composites fabrication, etc [5]. In HERF processes the punch is replaced by a compression waves emanating from the explosion site. Thus, only one die is needed. More important, high rate of deformation caused by these waves enables us to generate complex parts out of hard-to-process materials.

One of reasons impeding the adoption of HERF is complexity of the process control. The energy of an explosion is evenly distributed within a sphere surrounding an explosion site and directing of this energy is a complicated engineering problem. The only engineering device where the control of the explosion energy is attained at an acceptable cost is a gun. In a gun products of the powder combustion (explosion) are confined by the gun barrel, thus the direction of the forces exerted on a projectile are precisely controlled.

Unfortunately projectiles generated by a gun have no manufacturing use. First of all, the cost of a single bullet is unacceptable for industrial applications. Then, the energy efficiency of a gun is not sufficient.

Proceedings of the 3rd IASME / WSEAS International Conference on CONTINUUM MECHANICS (CM'08)

ISBN: 978-960-6766-38-1 72 ISSN: 1790-2769

The most important, however, is the mechanism of a solid-solid impact, which makes precise control of the workpiece deformation difficult if not impossible. The shortcomings above are addressed if a solid projectile is replaced by a liquid one, e.g. by water projectile. The energy efficiency of the process can be dramatically improved. The liquid-target interaction can be controlled in a wide range so that a desired process accuracy can be attained.

A device where the energy of an explosive media was converted into the kinetic energy of a liquid was suggested by Voitsekhovsky [6] and improved by other researchers [7-9]. However, while the extremely high projectile velocity (4650 m/s) and a high firing rate (2 Hz) were attained, none of the developed devices found practical applications.

The Waterjet laboratory of New Jersey Institute of Technology (NJIT) initiated a study of the formation and application of supersonic projectiles in 1997. This work was a continuation of the research of Atanov [9]. The work of the Waterjet laboratory included numerical investigation of the water acceleration in a converging nozzle and an experimental study of the external and terminal projectile ballistics [10-18]. As a result, several launcher prototypes were designed and constructed and the feasibility of the use of liquid projectiles as a forming and welding tool was shown. While the work was initiated as an attempt to improve the conventional waterjet technology, the study showed that the supersonic liquid projectiles constitute a unique tool. The potential applications of this tool range from micro or, perhaps, nano-manufacturing to underwater rock boring and emission-free coal combustion. A summary of the performed research is given in this paper.

2 Thermodynamics of the Launcher The operation of the proposed launcher is based on the use of a liquid, e.g. water, as a medium for conversion of available chemical, mechanical or electrical energy into the kinetic energy of a projectile. A liquid is an effective energy conversion medium. Because it is a condensed medium its energy density is much higher than that of a gas. At the same time, the liquid is a fluid, thus additional acceleration can be accomplished in a nozzle. Non-steady liquid acceleration in a converging nozzle is especially efficient technology. Unlike a steady flow, the acceleration of an unsteady stream in a nozzle is not limited to the sonic velocity. As it was shown by the previous studies internal-to-kinetic energy conversion and kinetic energy redistribution in an unsteady converging liquid flow enable us to achieve liquid velocity exceeding 3 M.

One of the simplest devices for direct internal-to-kinetic energy conversion is a gun. However, the thermal efficiency of a gun is determined by the length of a barrel. At the same time a converging nozzle dramatically enhances the rate of the energy converging and thus increases thermal efficiency without complication of a device design. Launchers designed in the course of this study utilized the above feature of the converging nozzle.

3 Launcher Operation A launcher used in this study for projectiles acceleration operates as follows. A liquid load is placed in the barrel as a conventional round and accelerated by the explosion of a propellant or any other energetic material. An electrical discharge, mechanical impact, vaporization of a cryogenic liquid, a laser, a magnetic field or any other energy sources suitable for rapid energy release can also be used for projectile acceleration. Propellant combustion (explosion) results in the formation of a rapidly expending gaseous cavity exerting driving forces at the gas-liquid interface. The driving forces can be generated at the liquid boundary by an impacting piston or magnetic field. The generated forces accelerate the liquid in a barrel and then in an attached nozzle. As a result a high-speed projectile is expelled from the launcher.

A schematic of a launcher used in the performed study is shown in Fig. 1. As it is shown in this figure a water load was placed in a barrel and impacted by the products of the propellant combustion which expelled water from the launcher. In another type of a launcher, tested in this work, powder combustion was used to accelerate a piston which subsequently impacted the water load and expelled it from the launcher. While the rate of the energy injection in the liquid in the course of the piston impact by far exceeds that of the powder combustion no significant differences in the operation of both launchers was detected. However, due to the maintenance

Figure 1. Schematic a launcher, water is driven by combustion products.


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difficulties the direct energy transfer from the combustion products to the liquid was used in further experiments.

3 Numerical Modeling of the Projectile Formation Numerical modeling of the flow in the launcher was used to investigate water acceleration by powder combustion. In the course of modeling the launcher was approximated by a fluid system consisting of gaseous and liquid phases separated by a non-penetrable boundary. Expanding combustion products exerted pressure on the liquid at the gas-liquid interface. This pressure expels liquid from the launcher via a cylindrical barrel and a conical nozzle. Numerical techniques describing processes which occur simultaneously in both phases were developed. The properties of the gas phase were assumed to be uniform. Position and pressure at the gas-liquid interface constituted the boundary conditions for the liquid phase. It was assumed that the atmospheric pressure was exerted at another liquid boundary. The pressure at the gas–liquid boundary determines the liquid displacement and thus the increase of the volume of the gas phase.

A numerical technique presented in [19] was used to describe powder combustion and to determine gas properties. Three different techniques were used to describe the liquid flow. It was assumed that the fluid is incompressible. In this case the process was described by one-dimensional equations of the mass and momentum balances. Another computational procedure represented the liquid phase as a one dimensional flow of a compressible inviscid fluid. A combination of the Finite Difference Method and the Method of the Characteristics (Godunov method) was used for simulation of this flow [18]. Finally, a commercial package FLUENT was used for modeling a two-dimensional flow of a compressible viscous fluids. This procedure also involved determination of the strain and stresses in a launcher body. The developed computational techniques were experimentally verified by measuring the projectiles velocity and strains in the barrel.

The performed computations show that the exit water velocity can be as high as 5 km/s while the water pressure might exceed 1 GPa. The examples of the computed variation of the velocity and pressure are depicted in Figs 2-5. As it follows from these figures, the fluid compressibility significantly affects projectiles formation. While an incompressible fluid yields higher values of pressure and velocity, the process patterns of both compressible and incompressible liquids are very much similar. Thus, a

more simple model of an incompressible fluid can be used at least for preliminary process examination.

Water load can fill the barrel partially so there is a free space between the water surface and the nozzle exit. In this case water is accelerated prior to the exit from the launcher. This way of the launcher operation was analyzed by the above procedure. Another mode of launcher operaration involves complete filling of the barrel and nozzle. In this case the first portion of the water are expelled at very low

Figure 3. Variation of water velocity in the course of the projectile formation at the water load 200 g.

Figure 2. Outflow velocity vs. time at different water loads.

0

200

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600

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1000

1200

1400

1600

0 0.2 0.4 0.6 0.8 1

Time, ms

Vel

ocity

, m/s

M0= 0.1 kg

M0= 0.22 kg

M0= 0.3 kg

Figure 4. Outflow velocity vs. time calculated for an enhanced launcher design at 115g water load.

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0 0.05 0.1 0.15 0.2

Time, ms

V, m

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speed. While Figs 2-4 describe the first mode of the operation, the second mode is described by Figs 5 and 6. As it shown by these figures, at the second mode of operation high amplitude fluctuations occur in the system. The graphs in Figs 5 and 6 are obtained using the FLUENT package.

4 Experimental Study of the External Ballistics. Of Projectiles Several experimental techniques, including high speed movie, laser and pendulum anemometries, etc. were used for examination of the projectile flight between the exit of the nozzle and a target. The results of this study were used to validate the numerical techniques describing investigations of the projectiles formation as well as for evaluation of the energy loss during the flight. A high speed movie showing the sequential development of the water projectile for the experiment at 20 cm standoff distance is shown in Fig.7. The filming speed was 100,000 frames per second and the shutter time used in this experiment was 1µs. Velocity measured in this experiment ranged between 1300 m/s and 1600 m/s.

5 Experimental Study of the Terminal Ballistics of the Projectiles. A number of experiments were carried out to evaluate the projectile-target interaction. The intensity of the projectile impact is illustrated by the demolition of heavy reinforced concrete plates (Fig. 8) and piercing of steel plates (Fig. 9) while the rate of the stress propagation was quantitatively estimated by the explosion free destruction of a non-dischargeable explosive device (Fig. 10). The explosive neutralization with no explosion was possible because the duration of the device destruction by an impacting projectile is much shorter thenneeded to setoff detonation.. Investigation of the impact-induced deformation of steel targets showed that maximal attained elongation was 120%, maximal thickness reduction was 93.7% and the maximal strain rate was 8.5*10-6 s-1. Strain rate generated by the solid projectile is 10-4 s-1.

6 Experimental Study of Material Processing Using Liquid Projectiles The performed experiments involved application of liquid projectiles for forming, welding and rock boring. In the course of forming and welding

Figure 6. Time variation of the water pressure at the entrance of the converging nozzle.

Figure 5. Time variation of the water velocity at the launcher exit.

Figure 7. Images of the water projectiles at different time after the exit from the launcher.

160 µs

320 µs

500 µs

Figure 8. Demolition of 16cm-thick reinforced concrete block using 230g high speed projectile.


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experiments the targets were mounted on a pendulum in order to measure a momentum of the impacting projectiles. 6.1 Metal forming In the course of metal forming experiments a projectile was used as a punch impacting a workpiece placed on a female die. The shape of the generated parts reproduced the shape of the die cavity. The experiments included metal punching, stamping and extrusion.

Punching steel plates having the thickness ranging from 4 mm to 10mm was carried out. In these experiments 250g were accelerated to the speed of 1500 m/s. The plates were placed on a die with openings of 16.5 and 25 mm. The standoff of the launcher was 16 mm. The results of experiments are depicted in Fig. 11. All experiments except for one resulted in formation of clear opening. The impact of a 10 mm plates at the die opening of 25 mm results

only in a partial metal separation. Formation of non-round openings using round projectiles (Fig. 12) as well as punching the highly elastic spring steel was also successful.

Precise formation of the complex images on a workpiece surface (coining) using the liquid impact is demonstrated by Fig 13. Other successful forming experiments included forging and extrusion. 6.2 Micro forming These experiments involved formation of parts having at least one micron scale dimension. The hydrostatic stresses generated by the projectile impact liquefy a metal in the impact zone and makes it amendable to forming. However, this state is maintained during the projectile-target interaction that is during very short time. In the course of the microforming material displacement has an order of microns, thus the available time is sufficient for filling the die cavities by a workpiece material.

A series of experiments involving the study of microforming was carried out. One of the performed experiments involved formation of thin metal rings (Figs. 14 and 15). As it is shown in these figures the liquid impact brought about micron size metal rings. The height of a ring was 1.5 mm, while the ring thickness was 15 micron. Thus the ratio between the height and the thickness of the generated rings (the extrusion ratio) was almost 100.

Figure 10. Successful explosion free neutralization of a soft-case non-dischargeable explosive setup.

Figure 9. Penetration of three placed together 4.8 mm steel plates with 240g of water

Die

4mm. 8mm. 6mm.

Figure 11. Formation of shaped openings using a round jet. Notice that the workpiece thickness does not affect the accuracy of the piercing.

8 mm 6 mm 4 mm

Figure 12. Impact side view of punched openings in plates 8, 6 and 4mm thick. Punching done against die opening 16.5 mm in diameter.

Figure 13. General view of a coin stamped on aluminum and cooper samples. Notice reproduction of fine details of the coin on the sample surface.


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Another microforming experiments involved formation a network of micro channels (Fig.16). In this experiment wires having diameter 7 micron were forged by the liquid impact into the surface of a steel target. A wire network was placed on the target surfaces and covered by a high strength plate. A plate was impacted by the projectile and a network of channels was generated.

The coining experiments (Fig. 13) involved the use of the millimeter scale objects. However the elements of the generated images had micron scale dimensions. Thus, these experiments also demonstrate feasibility of the projectiles applications to micro forming. Moreover, the compliance between the geometries of the workpiece and the die in this experiment indicates possibility of application of supersonic projectiles to the submicron forming.

6.3 Welding Totally 17 experiments involving joining similar and disimilar metals were carried out. The experiments involved impacting 2 or 3 nested plates by the water projectiles. In some experimensts spacers were used to provide a distance of 1 mm between the plates as it is done in explosive forming while in other experiments it was no such distance. The tested combinations included Cu-Ni-Cu, Steel-Ni-Steel, Brass-Ni-Brass, Cu-Cu, Cu-Ni, Cu-Steel and Steel-

Figure14. Extruded circular brass rings

Figure 15. 3-D image of a section of the extruded brass ring, the section is 1500 µ high, 150µ thick.

10µ

Figure 16. Intersection of two micro channels formed on the brass surface by 7µ diameter tungsten wire.

Figure 18. Copper plates welded by liquid impact of 1500 m/s. b) Zoomed-in section of micrograph of wavy interface of joined copper plates.

b

a

Figure 17. Profile of the channel formed by a 7µ diameter wire. Notice the compliance between profile of the channel and the die.

Figure 18. A profile of the channel formed on a brass sample by the 40µ wire. Notice that the depth and width of the channel are approximately equal to the wire diameter.


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Steel. The quality of joining was estimated by simple mechanical tests and, in some cases, by the ultrasound examination. The testing showed that in 8 experiments the joining was excellent, in 5 cases it was acceptable, only partial joining was attained in 2 cases and in a one case process failed. The speed of the projectiles was 750 m/s, 850 m/s and 1500 m/s. At the speed of 750 m/s the process failed, at 1500 m/s the joining was acceptable while at the speed of 850 m/s the results were excellent. The micrographs of welded samples (Figs 18 and 19) show wavy character of a seam typical to the explosive welding. 6.4 Granite boring Crashing of granite plates was used to determine feasibility of rock and concrete boring. Granite removal by an impacting projectile is shown in Fig. 20. The results of the performed experiments indicated that a channel 100 m long and 10 cm in the diameter could be generated by the high speed projectile in 1 hr using 5000 kg of a working fluid.

7 Potential Applications The performed experiments indicate a possibility to address a number of technological problems using the supersonic liquid projectiles. Imprint Lithography is a low-cost three-dimensional patterning process. It creates patterns by mechanical deformation of polymers. The use of supersonic projectiles will enable us to apply Imprint Lithography to metals, alloys and ceramic and, thus, to create a new generation of nano- and microscale devices. The performed experiments involving microforming indicate the feasibility of such applications of projectiles.

Feasibility to use the supersonic projectiles for welding and forming enables us to integrate these two technologies and develop a novel process for Solid State Fabrication of Heterogeneous parts.

A launcher generating supersonic projectiles is a simple inexpensive device which does not require sophisticated maintenance (the launchers used by the Waterjet laboratory were operated by graduate students). Thus these launchers can be used at extreme conditions, e.g. for underwater or extraterrestrial construction.

It is known that a molten metal blasted by oxygen can absorb various fuels at extremely high rate if adequate bath stirring is facilitated. The use of high-speed liquid projectile containing oxygen and coal powder will assure the desired rate of transport processes and subsequent coal combustion. The generated CO gas will be cleaned by the slag covering the metal layer. The additional oxygen will burn CO and, high-temperature clean CO2 gas will be generated. Because the process will be carried out in pressurized reactors the gas will be generated at a high pressure. This gas will be used as a working fluid in gas turbines and as a hot media in boilers. At the end of heat extraction the generated CO2 will be liquefied. Liquid carbon dioxide will dissolve all other off gas components and subsequently will be sequestrated. Thus projectiles will be used for emission-free coal combustion.

8 Conclusion It can be expected that the high speed projectiles will become a conventional, rather than exotic material processing tool. The performed experiments illustrate feasibility and comparative simplicity of formation of supersonic liquid projectiles. The first launchers used in the performed experiments constituted a modified household power tool. In some way the use of projectiles is an improved version of explosion-based processing. For example, the explosion welding requires maintenance a precise distance between parts to be joined. As it was shown by the performed experiments no such requirement exists in the case of

Figure 19. Micrograph of wavy interface of joined copper-nickel plates welded by the water projectile impact at the water velocity 1500 m/sec.

Figure 20. Impact of a granite plate at a standoff distance of =80mm. Notice intensive granite removal in the course of this impact.


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the use of liquid projectiles. While a wide area of applications of the liquid projectiles ranging from the rocks boring to micromanufacturing can be envisioned, it is possible that the most immediate application will be a mass production of inexpensive metal and ceramic micro-parts. Another immediate application might be precise structure demolition needed for structure renovation or urban construction. Significant body of knowledge, however, is to be acquired in order to elevate the processes in question to the commercial technologies. It is necessary to select process conditions (a working fluid, rate and mode of energy supply, launcher geometry, etc.) which assure the desired projectile momentum at an accepted pressure in the launcher. It is also will be necessary to determine the impact conditions which assure desired target deformation with no deformation of the supporting die. If optimal process conditions will be found, the liquid projectiles in a number of cases will replace explosive forming, welding and demolition, and what is the most important, could enable us to create novel manufacturing and construction technologies.

9 Acknowledgement Generous support of this study by US National Science Foundation, New Jersey Committee on Science and Technology and Civil Research and Development Foundation is acknowledged. Experimental study was partially performed in cooperation with Professors G. Atanov, A. Semko and V. Kovaliov, Donetsk National University, Donetsk, Ukraine. References: [1] K.A. Padmanabhan et. al. (2001) Superplastic

Flow: Phenomenology and Mechanics. Springer, , p.15

[2] J.F. Bell, (1973), Experimental Foundations of Solid Mechanics, Springer,

[3] .P.W. Bridgman. (1951), “The Effect of Pressure on the Tensile Properties of Several Metals and Other Materials”. Journal of Applied Physics, V 24, # 5. May, pp. 560-570,

[4] .P.W. Bridgman, (1952). Studies in Large Plastic Flow and Fracture with Special Emphasis on the Effect of Hydrostatic Pressure, McGrow-Hill

[5] T.Z. Blazynski, Explosive Welding, Forming and Compaction, Applied Science Publisher, 1983

[6] B.V. Voitsekhovsky, Devce for Building Up High Pulse Liquid Pressure, US Patent 3412554, Nov 1968

[7] Chermensky, G.P., (1976). Breaking Coal and Rocks with Pulsed Water Jets. Proceedings of 3rd International Symposium on Jet Cutting Technology, BHRA Fluid Engineering (pp. D4-33 – D4-50), Chicago, USA.

[8] Cooley, W. C., (1985). Computer Aided Engineering and Design of Cumulation Nozzles for Pulsed Liquid Jets. Proceedings of the Third U.S. Water Jet Conference, Pittsburgh, Pennsylvania

[9] Atanov G. A. Hydro-Impulsive Installations for Rocks Breaking, Vishaia Shkola, Kiev, Ukraine, 1987

[10] Petrenko, O., (2007). Investigation of Formation and Development of High-Speed Liquid Projectiles. Ph.D Dissertation, Department of Mechanical Engineering, New Jersey Institute of Technology.

[11] Samardzic V., (2007). Micro, Meso and Macro Materials Processing Using High Speed Water Projectiles. Ph.D Dissertation, Department of Mechanical Engineering, New Jersey Institute of Technology.

[12] V.Samardjic, E.S.Geskin; G.A.Atanov, A.N.Semko, A.V. Kovaliov (2007) Investigation of Metal Processing Using A High Speed Liquid Impact, Proceeding 2007 Conference of the Waterjet Technology Association, Houston, Texas, August, Paper 2D

[13] V.Samardjic, E.S.Geskin; G.A.Atanov, A.N.Semko, A.V. Kovaliov (2007) Investigation of Metal Microforming Using High Speed Liquid Impact, ,Paper 1G

[14] V.Samardzic, K.Kluz, O.Petrenko, E.S. Geskin, M. Mazurkievitz, , A. Berger, (2007) Investigation of Granite Boring Using High Speed Liquid Impact, Paper 4C

[15] K.Kluz, E.S.Geskin, O.P.Petrenko; Numerical Approximation of Propellant Driven Water Projectiles Launchers, Paper 1B

[16] V.Samardzic, E.S.Geskin, G.A.Atanov, A.N.Semco, A.V.Kovaliov. (2007) Liquid Impact Based Material Micro Forming Technology; Journal of Materials Engineering and Performance, pp. 375-389

[17] Petrenko O., Geskin E.S., Atanov G., Semko A., Goldenberg B., Numerical Modeling of Formation of High-Speed Water Slugs, ASME Transaction, Journal of Fluids Engineering, March 2004, pp. 206-209

[18] Cappock Advanced Interior Ballistic Model, the freeware software developed by Fabrique Scientific® in 1995


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