Apparatus, method, and use for strengthening the surface of a product.

Abstract

The present invention relates to a method and apparatus for strengthening the surface of a workpiece, particularly a metal workpiece, by the mechanical effect accompanying the impact of a small projectile or by the mechanical effect accompanying the impact of a shock wave caused by plasma generated by electrolytic deposition of a metal foil, the apparatus comprising a polymer strip (32) having a metal foil (71) on a surface facing away from the surface of the workpiece (10) on which a foil bridge (34) is formed to form a projectile (100), and further comprising two electrodes (33) and (66) adjacent to the metal foil (71) located on the polymer strip (32), the bridge (34) being formed between the contact surface areas (27) of the metal foil (71), the electrodes (33) and (66) between which the plasma is formed being attached to a support (41), through which the flat conductors (44) and (46) are connected to a switch (84) for switching a high current and high voltage with a high voltage source (83), the polymer strip (32) having the metal foil (71) being connected to the electrodes (33) and The electrodes (33, 66) together with the polymer strip (32) are firmly abutted against the support (41), and protrude above the upper surface of the support (41) to provide electrical contact with the contact surface area (27) of the attached metal foil (71). The method for strengthening the surface of a workpiece (10) using the device according to the invention consists in the fact that one cycle for strengthening the surface of the workpiece (10) comprises the action of a current pulse supplied by the conductors (44, 46) from a high voltage source (83) after closing a switch (84) to the electrodes (33, 66) between which a high voltage is applied, thereby short-circuiting the metal foil (71) at the location of the bridge (34), forming an expanding plasma which acts by a compressive force on the polymer strip (32), parts of which impact the surface of the workpiece (10) as projectiles (100), and the plasma generated by the current pulse is accelerated not only by the expansion pressure but also by the electromagnetic Lorentz force (20) caused by the passage of the current through this plasma in the generated magnetic field.

Description

Technical Field

[0001] On the one hand, the present invention relates to a method for strengthening the surface of a workpiece, particularly a metallic workpiece, by a mechanical effect accompanied by the impact of a small projectile or by a mechanical effect accompanied by the impact of shock waves caused by a plasma generated by the electro-deposition of a metal foil. The present invention further includes an apparatus for carrying out this method. The impact of the projectile induces plastic deformation in the surface layer of the workpiece, and the plastic deformation results in residual mechanical stresses. These bring about a significant improvement in the functional mechanical properties of the final product. The general application of the present invention is planned for engineering production, particularly in the aerospace industry and generally when components that are mechanically highly stressed and are endangered, for example, by fatigue failure are manufactured.

Background Art

[0002] Numerous technical procedures and methods are known for improving the mechanical properties of a product by acting on the surface layer of a manufacturing workpiece, particularly made of a metallic material. This enhances the mechanical, cavitation, adhesion, and corrosion resistance of the surface. These methods generally are based on the principle of changing the stress in the surface layer by introducing an appropriate level of compressive residual stress and its depth gradient beneath the surface. A method of surface strengthening such as shot peening, for example, is an accelerated steel ball as a projectile that impacts the workpiece surface. The method is very commonly used, but the achievable values of the residual stress and the depth affected by the material are not large. Therefore, today, more advanced methods such as surface strengthening by the action of a beam of light generated by a high-power laser that impacts the workpiece surface covered with a liquid have been introduced. The short irradiation of this high-power light beam generates a shock deformation wave on the workpiece surface that strengthens the material. Such new technologies have great potential for extending the service life of parts that are dangerously stressed, such as aircraft and rocket engines, spacecraft structures, steam turbine blades, high-performance pump impellers, reactor vessels, and many more.

Summary of the Invention

[0003] While the advanced laser technologies described above are already in use in industry, their wider adoption is hindered by extremely high investment and operating costs. These costs incurred in laser surface enhancement often account for a significant portion of the overall product price. Furthermore, several complexities must be overcome, including the need for direct optical access to the surface being treated and the requirement to coat the surface with a liquid layer. [Means for solving the problem]

[0004] The aforementioned problems relating to previously known techniques for improving the mechanical properties of a workpiece by acting on its surface are eliminated by a method for reinforcing the surface of a workpiece by the impact of a projectile and the introduction of associated residual stress into the surface layer, characterized in particular by a process in which a projectile made from a polymer material in the form of a polymer strip is directed toward the surface of the workpiece and allowed to impact the surface, wherein a metal foil is applied to the polymer strip beforehand, and an electric current pulse is introduced by the discharge of a capacitor, the pulse being large enough to melt the metal foil, causing it to evaporate and subsequently turn it into an expanding plasma, the plasma accelerating the projectile in the opposite direction to the surface of the workpiece being treated, at least partially by its expansion pressure.

[0005] According to the present invention, it is also convenient to carry out the process according to the present invention, and the essence of the present invention is that the plasma generated by the current pulse is accelerated not only by expansion pressure but also by the electromagnetic Lorentz force caused by the current passing through this plasma in the generated magnetic field.

[0006] According to the present invention, it is also convenient to carry out the process according to the present invention, and the essence of the present invention is that the projectile is guided in the direction of the workpiece surface through a channel inside the nozzle.

[0007] According to the present invention, it is also convenient to carry out the process according to the present invention, the essence of which is that the strengthening of the workpiece surface is carried out in a periodically repeated functional cycle, at the beginning of each cycle, a polymer object in particular in the form of a strip is pre-formed within a metal foil on the polymer strip and is displaced in the direction of its length by a distance of one gap between bridges created by locally narrowing the metal foil, and the movement of this strip is then stopped for the remainder of the cycle to create plasma by discharge.

[0008] In particular, it is convenient to use the apparatus according to the present invention to carry out this method, and the essence of the present invention is that a polymer object, in particular in the form of a polymer strip, is placed on the surface of a workpiece, and there is a metal foil having a bridge on the surface of the polymer strip on the side offset from the surface of the workpiece, and this metal foil is in electrical contact with two electrodes, on one side by a first electrode and next to the first electrode, in particular in the direction of the length of the polymer strip, with a bridge between them, each of the two electrodes is a conductor, the first electrode has a first conductor and the second electrode has a second conductor, one of the two capacitor electrode plates is on the opposing side of the dielectric, these conductors are connected, and one of the bodies is connected to the output of a high-voltage source via a switch.

[0009] This method is also convenient to implement with the apparatus according to the present invention, and the essence of the present invention is that a nozzle is positioned between the surface of the workpiece and the opposite part of the metal foil, with its opening directed toward the surface of the workpiece and its inner channel directed toward the surface of the polymer object, and the polymer object has the shape of a polymer strip through which the active part of the displacement mechanism passes.

[0010] This method can also be conveniently implemented in the apparatus according to the present invention, and the essence of the present invention is that the switch includes a spark gap input, a spark gap output, and an air gap between the spark gap input and the spark gap output.

[0011] This method is also convenient to implement with the apparatus according to the present invention, and the essence of the invention is that the conductor has a flat shape with a width greater than its thickness. A shock wave strong enough to cause plastic deformation of the material at the workpiece surface is generated in order to transmit the required mechanical stress to the surface layer of the metal object. To do this, it is necessary to achieve a very high compressive force. The energy required for this is first gradually stored in a capacitor, and after the capacitor discharges, the energy is transferred to a plasma formed by ionization from the conductive foil bridge in tens of nanoseconds. The expanding plasma converts the supplied energy into the kinetic energy of the projectile. Thus, it is accelerated to a high speed of several kilometers per second relative to the workpiece, and it transfers a large portion of this energy to the workpiece. Generally, processing of larger areas is required, which requires rapidly repeating the impact of many projectiles in a computer-controlled process.

[0012] The solutions according to the present invention, shown in Figures 12 to 14b and presented in Examples 5 and 6, are in particular the following modifications of the embodiments relating to Examples 1 to 4. - The switch includes a spark gap input connected to a first conductor on the one hand, and a spark gap output connected to the same conductor on the other hand. Furthermore, the switch has a gap between the spark gap input and the spark gap output, and / or - Between the surface of the workpiece and a portion of the metal foil, a polymer strip is in direct contact with the surface of the workpiece and mechanically connected to the metal foil. This polymer strip passes through the active part of the displacement device, and / or - The conductors connecting the capacitor to the electrodes are flat in shape, with a width greater than the thickness. One of these conductors is simultaneously the capacitor electrode plate in part, while the other conductor is blocked by a switch.

[0013] The advantage of the proposed solution is that the polymer strip protects the workpiece surface, so the workpiece surface is not contaminated with ablation products of foil bridging. The length of the pressure pulse caused by plasma expansion can be varied by the electrical parameters of the circuit, which allows the layer to be affected to a greater depth, along with the possibility of expanding the processing area with a single shock wave.

[0014] To introduce the required mechanical stress into the surface layer of a metallic object, the discharge of an electrical capacitor generates a shock wave powerful enough to cause plastic deformation of the material on the surface of the workpiece. Achieving this requires very high compressive force. The energy that makes this possible is initially gradually accumulated within the capacitor and, after the capacitor discharges, transferred to a plasma formed by ionization from the conductive foil bridge over a period of tens to hundreds of nanoseconds. The expanding plasma converts some of the supplied energy into a shock wave, which is delivered directly to the workpiece via a polymer strip; therefore, no plastic projectiles are formed, as in the embodiments of Examples 1 to 4. Generally, processing of larger areas is required, which necessitates rapidly repeating the impact of many shock waves on a pre-selected area of ​​the workpiece surface in a computer-controlled process. Rapid cycle repetition is ensured by pressing a new foil bridge onto a selected location on the workpiece before the capacitor discharge occurs, thereby causing the mechanical displacement of the polymer strip holding the new foil bridge and its fixation.

[0015] Due to the extremely fast discharge of the non-inductive capacitor into the foil bridge, plasma is formed by the evaporation and ionization of this bridge. The generated plasma, through its expansion, produces a shock wave that is induced through the polymer strip to the surface of the workpiece, which is in direct contact with assemblies 32, 34, 37, and 41. This shock wave causes plastic deformation of the surface layer of the workpiece, thus leaving residual compressive stress within the surface layer.

[0016] According to the present invention, it is also convenient to carry out the method according to the present invention, and the essence of the present invention is that the plasma generated by the current pulse is further trapped between the workpiece and the support, and therefore the plasma expansion pressure increases.

[0017] According to the present invention, it is also convenient to carry out the method according to the present invention, and the essence of the present invention is that the plasma generated by the current pulse is isolated from the workpiece and / or support by the polymer layer.

[0018] According to the present invention, it is also convenient to carry out the method according to the present invention, the essence of which is that the strengthening of the workpiece surface is carried out by a periodically repeated functional cycle, at the beginning of each cycle, assemblies 32, 34, and 37 are displaced in the longitudinal direction by a distance of one gap between bridges pre-created on the metal foil on the polymer strip formed by their local narrowing, and the movement of this belt is then stopped for the remainder of the cycle to achieve plasma formation by discharge. [Brief explanation of the drawing]

[0019] The attached figures schematically illustrate alternative embodiments of a shock wave generator for reinforcing the surface layer of a material according to the present invention. Furthermore, an explanatory diagram is provided in Figure 5, which illustrates the spatial direction of the Lorentz force used in the present invention. The first exemplary embodiment shown in Figure 1 serves primarily to clarify the functional principle, and therefore some details that are not essential to this explanation have been omitted for simplicity. Only in the additional figures of Figures 2, 3, and 4 are there configurations that are practically applicable. Other directly applicable embodiments are shown in Figures 6, 7, and 8 on the one hand, and in Figures 9, 10, and 11 on the other hand.

[0020] In an alternative preferred embodiment that overcomes some of the disadvantages of the solution according to FIG. 6, these shock waves are used for the strengthening of the surface layer of the metallic material according to the invention. These are schematic diagrams that mainly serve to clarify the functional principle of an example of the design of the device, and thus, for the sake of clarity and simplicity, some details that are not essential for this description are omitted, such as, for example, the mechanical displacement of the polymer strip with foil bridges, and traversers for the treatment of preselected locations on the workpiece surface.

[0021] For the sake of clarity, since the capacitor and the connecting tape lines are laminated with foils of a maximum thickness of several tens of micrometers, the scales are different on the x-axis and the y-axis, which would cause great confusion in the drawing.

[0022] FIG. 12 shows a diagram of a device according to a first preferred variant embodiment according to the invention.

[0023] FIG. 13 shows a diagram of a device according to a second preferred variant embodiment according to the invention.

[0024] FIGS. 14a and 14b show details of a polymer strip provided with a metal foil.

[0025] FIG. 14a shows the structure of a polymer strip 32 having a metal foil 71. In this case, the strip is always displaced between individual pulses by the distance between the bridges 34 in a direction perpendicular to the plane shown in FIGS. 12 and 13.

[0026] FIG. 14b shows an insulating layer 37 having openings for the electrodes 33 and 66 that protects the support 41 that is necessary for the correct functioning of the device further explained in Examples 5 and 6 according to both variant embodiments. In this case, all three layers are firmly joined to each other such that the metal foil 71 having the bridges 34 is between the layers of the polymer strip 32 and the insulating layer 37.

[0027] FIG. 14a shows only two metal foils 71 having a bridge 34 shown in FIG. 14b and its corresponding openings in the insulating layer 37. However, in reality, it is an elongated strip having a plurality of metal foils 71 having a bridge 34. Instead, it can be arranged longitudinally, but then it is moved in the x-axis direction, between pulses - FIGS. 12 and 13.

Embodiments for Carrying Out the Invention

[0028] Example 1 In this example of the simplified embodiment according to FIG. 1, at the upper part of the figure, there is a detailed part of the reinforced workpiece 10, while the device itself for applying the required residual stress to the surface layer of this workpiece 10 is shown at the lower part of FIG. 1. In this figure, the trajectory of the launched projectile 100 thus proceeds vertically upwards. The important components of the exemplary device of this embodiment have a circular shape that is symmetric along the axis. The components are depicted in the cross-section of a vertical plane passing through the axis of symmetry in the figure. In reality, these components are in close contact with each other and are firmly connected to each other during the strengthening process. Nevertheless, for the clarity of the illustration in FIG. 1, these components are depicted as being displaced perpendicularly to each other, and thus the voids are visible between the components in the figure. Looking from top to bottom, these components are first (a) the circular disk of the nozzle body 31 - although not necessary in principle, its use improves the functional parameters of the device. Below the nozzle 31, next (b) a polymer strip 32 is inserted, and the polymer strip 32 may have a different shape, but in this case, it has a circular disk shape that is substantially the same as the shape of the nozzle body 31. At the lower part, (c) the support 41 is preferably also in the shape of a circular disk.

[0029] The metal foil 71 contacts the polymer strip 32 from below. The metal foil 71 is placed at the bottom along with two electrodes, a first electrode 33 and a second electrode 66. Both are attached to a support 41. To guide and accelerate the formed projectile 100, the nozzle 31 has a through-hole in the center in the direction of its axis of symmetry. The nozzle 31 is oriented with respect to the surface of the workpiece 10, whose upper end is reinforced. Unlike other conventional nozzles that have a rounded entrance into the through-hole, here the nozzle body 31 has a sharp edge 93 at the entrance of the lower edge of this hole. The metal foil 71 may be in the form of a flat strip of a certain width and thickness, but preferably the metal foil 71 is locally narrowed between the two electrodes 33 and 66. The function of the apparatus is ensured by the electrical circuit depicted in the lower right of this Figure 1. In particular, there is a high-voltage source 83 made by known methods, and in known embodiments there is also a switch 84, which is usually made to switch between large currents and voltages. The current from the high-voltage source 83 is conducted to the first electrode 33 by the first conductor 44. In contrast, the current from the switch 84, from the second terminal of the high-voltage source 83, is similarly conducted to the second electrode 66 by the second conductor 46.

[0030] By closing switch 84, a high voltage is applied between the two electrodes 33 and 66. However, the circuit is short-circuited by the metal foil 71. This causes a high-intensity current to flow through the circuit for a short period of time. Due to ohmic heating, the metal foil 71 first melts for a very short time and then immediately evaporates. Subsequently, a plasma is formed from these metal vapors. The plasma expands, simultaneously applying a high compressive force upward to the polymer strip 32 relative to the nozzle 31. The center of the polymer strip 32 is cut off by a sharp edge 93, and only this portion moves upward. The cut-off portion, thus formed into a disc, is pushed through the through-hole of the nozzle 31. The cut-off portion then shoots out of this opening at the top like a projectile 100, impacting the surface of the workpiece 10, generating a shock wave therein, causing the desired deformation and leaving residual stress.

[0031] Example 2 The simplified first embodiment shown in Figure 1 is primarily intended to illustrate the basic principle of producing a projectile 100 within the apparatus according to the present invention. The basic principle is adapted for use in practical engineering production, as shown, for example, in Figures 2, 3, and 4.

[0032] As shown in Figure 1, part of the reinforced workpiece 10 is also shown at the top of Figure 2, while the actual apparatus for forming the projectile 100 is at the bottom of the figure. The apparatus performs the periodically repeated generation and acceleration of many projectiles 100 that are rapidly and continuously generated one after another. In this embodiment, in addition to the basic thermal expansion, there is also the effect of additional forces on the accelerated plasma. Finally, and in this exemplary embodiment, the electrical circuit is adapted to achieve a very high short-term electrical output at a given level of supplied electrical energy.

[0033] In this configuration, the formed and accelerated projectiles 100 are repeatedly launched in this manner so that they gradually move away from the nozzle 31 upwards. Although the nozzle 31 is not required, it improves the acceleration and direction of the flying projectiles 100 by concentrating the charge carrier in the plasma and preventing the charge carrier from leaking out to the sides. As shown in Figure 1, for this guidance and acceleration of the projectiles 100, the nozzle 31 has an opening in the center for the passage and acceleration of the launched projectiles 100. To rapidly repeat the generation of projectiles 100, a polymer strip 32 that supplies material to the projectiles 100 enters the apparatus from the left. Its upper side is in close contact with the lower side of the nozzle 31. There is also a thin metal foil 71, as shown in Figure 1. In this case, the metal foil 71 is firmly attached to the lower side of the polymer strip 32. The metal foil 71 is formed directly on the lower side of the polymer strip 32 by vapor deposition, pressing, or similar known techniques. The actual polymer strip 32 depicted at the top of Figure 3 has a constant width and thickness along its entire length. In contrast, as shown at the bottom of Figure 3, the metal foil 71 is locally narrowed within the bridges 34 by periodically repeating longitudinal distances. This can be done on the polymer strip 32, for example, by vacuum deposition of evaporated material through a properly formed mask, or by etching away unnecessary portions of the pattern. Also, as shown in Figure 3, the metal foil 71 is left between the individual bridges 34 with its entire width equal to the width of the polymer strip 32, forming a sufficiently large contact surface 27, on which the first electrode 33 rests on one side of the bridge 34, and next to it, particularly in the direction of movement of the polymer strip 32, the second electrode 66 is in contact.

[0034] Both electrodes are connected mechanically, but not electrically, to a stationary support 41, so that the electrodes remain stationary even when the device is operating. The electrodes protrude slightly above the top surface of the support 41 and thus make electrical contact with the metal foil 71 in the initial state of the functional cycle. The polymer strip 32 is long because it must contain material for forming many continuously formed projectiles 100. Therefore, to save space, this polymer strip 32, with the metal foil 71 formed on top, is wound in a helical shape as shown in the left portion of Figure 2. This helix gradually rotates and unfolds during the operation of the device. A displacement device 35 through which the polymer strip 32 passes helps to induce this movement. The movement is carried out in one of the commonly known configurations. Also in this case, below the supplied polymer strip 32 is the support 41, which is firmly connected to the stationary portion of the entire device. The polymer strip 32 is made from a non-conductive material such as biaxially oriented PET (polyethylene terephthalate) or polyimide (Kapton). Together with the attached or directly formed metal foil 71, the polymer strip 32 moves gradually in the direction of the length of the strip in steps corresponding to the distance between the two electrodes 33, 66. The conductors 44, 46 shown in Figure 2 are connected to the electrodes 33, 66 through openings formed in the support 41. The electrodes are the first electrode 44 and the second electrode 46, both of which are equipped with a very effective electrical insulator 85, as very high demands are placed upon them. Since the process involves high voltage and a very small distance between the two conductors 44, 46, it must be a material with extremely high dielectric strength. It must be ensured that plasma is generated only between adjacent pairs of electrodes. The two electrodes 33, 66 protrude slightly above the upper surface of the support 41 to ensure their electrical contact with the contact surface 27 of the supplied metal foil 71. At their opposite ends, these conductors are connected to the foil 53 of the capacitor 50. Here, the capacitor helps generate plasma and then projectile 100 by storing electrical energy for a short time in each repeated functional cycle, which will be used at the end of each cycle.To store this energy, the capacitor 50 is charged from the high-voltage source 83 during the initial stage of each cycle. In this case, the high-voltage source 83 is a relatively complex but generally known electrical device.

[0035] In this case, a critical requirement of the electrical circuit is the extremely low inductance of all circuit components. The lower the value, the shorter the discharge duration from capacitor 50 and the higher the instantaneous maximum power supplied to bridge 34. These are three components with potentially restrictive inductance that are primarily limited by design. On the other hand, capacitor 50 is not typically designed for extremely low inductance requirements, so it is only a part of it that will be present within capacitor 50. Furthermore, it is also the inductance component of the leads to electrodes 33 and 66. The leads need to be as short and flat as possible, minimizing the distance between them. Finally, it is the components present in the electrical switch 84 that are required at the output of capacitor 50 to initiate the plasma explosion process 90 by closing it. Therefore, it is very advantageous for both capacitor 50 and conductors 44 and 46 that electrodes 33 and 66 be as flat as possible and that the dielectric thickness between electrodes 33 and 66 be made as small as possible. Figure 2 shows a configuration in which this low inductance problem in the switching process is solved, such that switch 84 is designed as a spark gap. When the capacitor is charged, no current passes through this spark gap at all. Both parts, spark gap input 80 and spark gap input 81, are isolated from each other by air or another suitable gaseous dielectric. A very short-term electrical connection is established by the passage of current only when the voltage difference between the input of spark gap 80 and the input of spark gap 81 exceeds the electrical strength of air. Thus, the spark gap essentially functions as an auto-actuated switch 84. Alternatively, the spark gap can be controlled by an auxiliary electrode and switched by a high-voltage pulse.

[0036] The extremely low required inductance of the capacitor is achieved by its unconventional configuration, which is a thin, planar dielectric plate 52 with capacitor plates 53 on both sides.

[0037] The metallic material of the bridge 34 on which the plasma is formed may be copper, aluminum, or gold, and since only small amounts are required, cost is not an issue at all. Ideally, the surface density of the bridge 34 is comparable to the surface density of the polymer strip 32 on which the projectile 100 is formed. Thus, efficient momentum transfer is ensured.

[0038] A crucial role is played by the support 41, which may have the characteristics of a relatively thin plate. It helps to trap the recoil and prevent the plasma from expanding in the opposite direction from the workpiece 10. The presence of the support 41 increases the efficiency of energy transfer to the projectile 100. Since the thickness of the foil bridge 34 is typically small, ranging from units to tens of micrometers, the lateral gap between the surface of the support 41 and the polymer strip 32 is negligible as far as plasma leakage is concerned.

[0039] Furthermore, the nozzle 31 may also be made from polymer material, but its service life is very short, and the nozzle 31 must be replaced frequently. For example, a new nozzle 31 formed on a supplied polymer strip 32 together with a metal foil bridge 34 can be inserted into the device after each functional cycle. However, if the nozzle 31 must have a long service life without frequent replacement, the nozzle 31 must be made from a ceramic material with high density and toughness. Zirconium dioxide material is particularly suitable. However, if the capacitor 50 is well suited to the electrical load formed by the bridge 34, which is also desirable from the standpoint of the energy efficiency of the device, the discharge of the capacitor 50 occurs very quickly, so it is possible to make the nozzle 31 from metal so that the polymer strip 32 does not break before the end of the current pulse. Suitable materials for the structure of a metal nozzle 31 have a high melting point, high density, and high hardness. Tungsten and molybdenum are particularly suitable. If the coolant flow is used to cool through an internal cooling channel, it is also desirable that the material has high thermal conductivity.

[0040] To improve the properties of the workpiece 10 over a larger surface area, the workpiece must be moved in small steps between individual functional cycles. That is, at the beginning of each functional cycle, the workpiece 10 is moved by the traverser 61. Since the newly enhanced location is always in front of the mouth of the nozzle 31, the projectiles 100 ejected from the nozzle 31 continuously strike the determined location on the workpiece surface 10.

[0041] To illustrate the ongoing process of creating the projectile and inducing its movement, Figure 4 shows a magnified view of the central portion of the described apparatus, where the explosion 90 is currently occurring. Similar to the figure above, a portion of the reinforced workpiece 10 is shown at the top, while the apparatus itself for applying the required stress to the surface layer of this workpiece 10 is at the bottom of the figure. Conductors 44 and 46 are shown along with insulators 85 connected to electrodes 33 and 66.

[0042] This exemplary embodiment of a shock wave generator for reinforcing a surface layer of a workpiece 10 according to the present invention, shown in Figures 2, 3, and 4, operates on the principle of inducing mechanical stress by impact of a projectile 100. These are generated and accelerated by the expansion of plasma formed from a bridge 34, which is a narrowing of a metal foil 71. Just before or at the very end of an operating cycle, as a discharge from a capacitor passes between the spark gap input 80 and the spark gap output 81, this current also passes through the metal foil 71 where it is located between the two electrodes 33 and 66. Due to the low electrical resistance of the bridge 34 and the high voltage on the charged capacitor 50, this current has an intensity of several kiloamperes. Extreme resistive heating is generated to a temperature where the bridge 34 not only melts but instantly evaporates. Finally, it is ionized as the metal vapor atoms lose some of their electrons.

[0043] All of this is caused by the action of strong short-term pulses of current from the capacitor 50 passing through the bridge 34 connected to the circuit via electrodes 33, 66. This is repeated in a functional cycle that is equally long and periodically repeated. At the beginning of each cycle, there is no voltage between the two capacitor plates 53 on either side of its dielectric 52. Responding to the discharge from the previous cycle, the displacement device 35 begins to move the polymer strip 32, gradually displacing it in its longitudinal direction by a distance equal to the distance between electrodes 33, 66 and simultaneously the distance between the bridges 34. In this new position, the strip is then fixed. This must be the position where the bridge 34 is located between the first electrode 33 and the second electrode 66, both of which are in conductive contact with the metal foil 71 at both ends of the bridge 34. The switch 84 is closed at the beginning of each cycle, and current flows from the high-voltage source 83 through the switch 84. Simultaneously, the traverser 61 moves the workpiece 10 to a suitable position where the workpiece 10 remains stationary for the remainder of the operating cycle. However, the circuits of electrodes 33 and 66 are interrupted by the air gap between the spark gap input 80 and the spark gap output 81, so no current passes through electrodes 33 and 66. The capacitor 50 is charged from the high-voltage source 83, which is reflected in the gradual increase in the voltage between the capacitor plates 53 located on the opposing sides of its dielectric 52. This part of the operating cycle ends when the voltage difference across the spark gap between the spark gap input 80 and the spark gap output 81 exceeds the value of the electrical strength of the air. It is said to have a value of 3 MV / m. This means that if the gap between the spark gap input 80 and the spark gap output 81 is 0.7 mm, discharge can be expected when the voltage across the capacitor 50 typically reaches 2-5 KV. Since capacitor 50 can have a capacitance of approximately μF, if the discharge lasts for about 50 ns, it can transfer an output of approximately tens of MW to the plasma, while the energy supplied is only in the joule range.

[0044] Due to the high intensity of the current flowing, a strong magnetic field is generated around conductors 44 and 46. The lines of force of this magnetic field are in the shape of circles directed almost opposite to each other, as shown in the lower right portion of Figure 4. Similar lines of force are also formed around the electric arc 30 formed between the first electrode 33 and the second electrode 66. This causes a large force to act on the electrons in the conductors, thus directing them to separate from each other. Thus, the two metal conductors 44 and 46 are fixed together in the apparatus by a sufficient accumulation of force. However, this separating force also acts on the charge carriers of the electric arc 30 formed between the two electrodes 33 and 66. These charge carriers are pushed by this force acting on them, and thereby—along with the expansion caused by the explosion 90—onto the cut portion of the polymer strip 32 which is pushed into the through-hole in the nozzle 31. Inside the nozzle 31, the motion induced by these forces is desirable because it accelerates the projectile 100 toward the surface of the workpiece 10.

[0045] After each explosion 90, the polymer strip 32 is moved by a displacement device 35 so that there is always a new bridge 34 below the nozzle 31. The entire process can be rapidly repeated in this way so that large surfaces of the workpiece 10 can be processed in a short time. Due to the high repeatability, it is then necessary to ensure proper cooling of not only the nozzle 31 but also the support 41 and other switching elements such as the spark gap or other switching elements, including the cascade of IGBT transistors. The polymer strip 32 can be manufactured by commonly available flexible printed circuit board manufacturing techniques. The portion in which the projectile 100 is formed is separated by cutting at a sharp edge 93 formed at the entrance edge of the hole in the nozzle 31. The limiting residue of the polymer material is discharged from the apparatus as polymer waste 91 that can be recycled.

[0046] explanatory diagram Figure 5 essentially shows a schematic diagram of the apparatus according to the present invention, but in contrast to exemplary embodiments, the purpose of Figure 5 is not to show the actual shapes of the individual components of the apparatus. The purpose is to illustrate the forces generated during the operation of the apparatus. Specifically, Figure 5 illustrates the principle used to generate a Lorentz force that acts on the plasma and accelerates the plasma toward the nozzle 31. On the right side of Figure 5, for simplicity, the nozzle 31 is not depicted as a whole, but only the outline of the axial hole of the nozzle 31—that is, the hole for guiding the plasma toward the workpiece 10—is shown. On the right side of Figure 5, a capacitor 50 with a completely flat design is shown. This is chosen for its extremely low inductance. In the center of Figure 5, a first conductor 44 connecting the capacitor 50 to the first electrode 33, and a second conductor 46 connecting the capacitor 50 to the second electrode 66 are schematically shown. Above the two electrodes 33 and 66, a polymer strip 32 with a metal foil 71 on its underside is passed. The second conductor 46 is interrupted by a spark gap consisting of a spark gap input 80 and a spark gap output 81.

[0047] In the embodiment used, the capacitor 50 is a dielectric layer 52 having plates 53 on its two flat sides—that is, on the top and bottom, as shown in Figure 5. Both conductors, the first conductor 44 and the second conductor 46, are connected to these capacitor plates 53, and each capacitor has one of the capacitor plates. As soon as a discharge occurs from the capacitor through the spark gap, the movement of charge carriers occurs not only in the conductors 44, 46 but also in the plasma formed from the material of the bridge 34. There, the charge carriers directed from the first electrode 33 to the second electrode 66 form an electric arc 30. The two conductors 44 and 46 repel each other with great force and must be clamped together to prevent them from separating from each other. For further explanation, one point is selected from this arc 30 in Figure 5, where the vectors of the electromagnetic effects are then drawn. Tangentially to the movement of charge carriers in the arc 30, the current vector 21 is drawn at the selected point. According to Faraday's law, a magnetic field is generated around this vector of current 21. Here, the induction field lines of this magnetic field 23 have a nearly circular shape.

[0048] The strength of the magnetic field 22 at the observation point is indicated by another vector perpendicular to the current vector 21. Both vectors 21 and 22, which are perpendicular to each other, determine the Lorentz force 20. At a given point, that vector is perpendicular to the two previous vectors and therefore points upward in Figure 5. This means that the Lorentz force 20 is upward in this direction, i.e., repelling the charge carrier of the electric arc 30 inside the nozzle 31. Due to the high current strength, this is a very large force contribution to the total force accelerating the projectile 100.

[0049] Example 3 The effects at work represent spatially complex relationships that are difficult to depict in two-dimensional images, so the above-mentioned figures were primarily explanatory. Therefore, these figures inevitably involve considerable simplification. However, following these explanations, a third embodiment shown in Figures 6, 7, and 8 can now be described. This embodiment describes, in principle, a realized or feasible embodiment. The embodiment is characterized by providing good efficiency when creating and launching the projectile 100. To this end, several modifications are required, particularly modifications that lead to the absolute minimum inductance in the electrical circuit and thus shorten the discharge duration. This leads to the maximum achievable instantaneous power. It is desirable to flatten all conductors and minimize the dielectric height between them. At the same time, it is also desirable to have the shortest conductors 44 and 46 with the minimum length between the capacitor 50 on the one hand and the polymer strip 32 and active bridge 34 on the other. This second small distance coefficient is achieved in this case thanks to the fact that the capacitor 50 is an indispensable part of the generator of the projectile 100.

[0050] An exemplary embodiment is shown in Figure 6 in a cross-section through a vertical plane. The orientation of the components is the same as in the embodiment described above, i.e., the workpiece 10 is at the top, the nozzle 31 is below it, and the support 41 is at the bottom. Below the nozzle 31 is a notch, and the polymer strip 32 passes through the notch together with the attached metal foil 71. However, in this configuration, the polymer strip 32 moves differently through this slot in a direction perpendicular to the projection plane, as shown in Figure 5. In another figure of the same embodiment, i.e., Figure 7, a perspective view of the three conductive metal components of the generator of the projectile 100 is shown, indicating the direction and position of movement of the polymer strip 32.

[0051] Figure 8 then shows an embodiment of the polymer strip 32 to which the metal foil 71 is attached. In this exemplary embodiment, the polymer strip 32 has different, in this case vertical orientations of bridges 34 formed by the shrinkage of the metal foil 71. The dielectric 52 is not depicted, otherwise the display of the entire apparatus would be very complex due to its spatially complex arrangement.

[0052] According to Figure 6, the capacitor 50, as part of the generator of the projectile 100, is formed by a dielectric layer 52 of a certain thickness, which extends not only below the nozzle opening 31 on the left side of the figure, but also to the lower left of the upper edge of the support 41. Basically, the dielectric 52 has a rectangular shape complicated by a small protrusion that forms a spark gap input 80. The metal foil 71 on the upper side of the dielectric 52 extends to this gap input. A short metal protrusion of equal width, separated by a gap, is located relative to this protrusion that forms the input of the spark gap 80. The metal protrusion is held in place by a molded protrusion of the dielectric plate 52. Although this short protrusion is just a small fabricated part, the protrusion performs two tasks. On the one hand, the protrusion is the spark gap output 81 - at the same time, the part of it further away from the capacitor 50 functions as a second electrode 66. Also, on the lower side of the dielectric layer 52, there is a metal plate 53 which also has several functions. In addition to its role in charge storage within the capacitor 50, it also functions as a first conductor 44 connected to the first electrode 33. The polymer strip 32 extends over the dielectric 52 on both sides above the two electrodes 33 and 66. The corresponding points form contact surface regions 27 on the metal foil 71, as shown in Figure 8. Two contact surface regions 27 adjacent to each other in the width direction of the polymer strip 32 are always electrically interconnected via a bridge 34.

[0053] The functions of the generator and accelerator of the projectile 100 differ only in configuration from the embodiment described in relation to Figure 2, in that the polymer strip 32 is moved by a displacement device 35 in its longitudinal direction, while the final electric arc 30 of each functional cycle ignites in the width direction of the polymer strip 32, and therefore laterally with respect to its length. This is why the polymer strip 32 must be extremely wide, which can lead to some degree of complexity.

[0054] Example 4 In this last, fourth described embodiment, shown in Figures 9, 10, and 11, a polymer strip 32 with a narrower width than that essential in the arrangement shown in Figure 8 is achieved. The narrower dimensions are achieved by the diagonal arrangement of the elements of the metal foil 71, as shown in Figure 9. These elements, i.e., the contact surface areas 27 interconnected by the bridges 34, must be shaped by their circular edges so that there is sufficient open surface area 28 around each of the bridges 34, as shown in Figure 9. The reason for this open area 28 is the intensity of heat transferred from the bridges to the surrounding areas. However, a sufficiently large contact area 27 must remain above each of the two electrodes 33 and 66.

[0055] Figures 10 and 11 below show the arrangement of the capacitor plate 53, conductors 44, 46, and electrodes 33, 36 of the capacitor 50 in this diagonal embodiment of the portion of the metal foil 71. In both cases—in Figure 10 and Figure 11—they are essentially the same, differing only from each other by different angles of view. The orientation of Figure 10 is very similar to the arrangement shown in Figure 7 above. Also, the dielectric 52 is not depicted here, as this would make the image of the entire apparatus very complex. The narrower width polymer strip 32, which is induced as in Figure 7, is not depicted. Figure 10 shows an electric arc 30 passing between the first electrode 33 and the second electrode 66. It is shown how the Lorentz force 20 lifts the arc 30 acting on the charge carriers by the force pushing these carriers from the flow of electrons in the first conductor 44, which is therefore also diagonally arranged. Example 5

[0056] In the apparatus of the first modified embodiment shown in Figure 12, the workpiece 10 is shown in close contact with the polymer strip 32 at the top, and the polymer strip 32 is intended to electrically insulate the workpiece from the high-voltage circuit and protect the workpiece from contamination by products resulting from the evaporation of the foil bridge and the ablation of a portion of the polymer strip 32. A particularly suitable material for making the polymer strip 32 is polyimide, which is sufficient in thickness up to 25 micrometers. However, adhesion is unsuitable for connecting the polymer strip 32 to the foil bridge because the extra layer of adhesive unnecessarily adsorbs a portion of the shock wave energy. The metal foil 71, narrowed at the location of the bridge 34, is in direct contact with the electrodes 33 and 66 by the contact surface 27. The metal foil 71 and polymer strip 32 are molded as shown in Figure 14a. Figure 14b shows the openings for electrodes 33 and 66 within the insulating layer 37.

[0057] Electrodes 33 and 66 pass through holes made in the second insulating layer 37, thereby isolating the exploding bridge from the support 41, and thus protecting the support from plasma ablation. The second insulating layer 37 also allows the support plate 41 to be made from a conductive metallic material. The material for making the second insulating layer 37 may again be polyimide, but a cheaper and thicker material such as Bi-PET, which can be bonded to the polymer strip 32 and foil 71 by adhesive, may be advantageously used. Here, the thicker layer between the metal foil 71 and the support plate 41 is advantageous because it protects the support plate 41. A suitable material for the support 41 is molybdenum, for example, which has high wear resistance and is also suitable for electrodes 33 and 66, and consequently, the support plate 41 and electrodes 33 can be made as a single component. The spark gap formed by the lead wires 80 and 81 is controlled by a high-voltage switching pulse at a selected moment, as in the prior art embodiments, and further comprises an auxiliary trigger electrode 86 that allows the spark gap to operate at a voltage lower than the self-breakdown discharge voltage. This makes it possible to set the required discharge energy by adjusting the capacitor voltage and trigger the spark gap using the auxiliary trigger electrode 86.

[0058] Example 6 In the apparatus of the second modified embodiment shown in Figure 13, the workpiece 10 is again shown in close contact with the polymer strip 32 at the top, and as in the example of the first modified embodiment, the polymer strip 32 is intended to electrically isolate the workpiece from the high-voltage circuit and protect the workpiece from contamination by products resulting from the evaporation of foil bridges and ablation of a portion of the polymer strip 32. However, for clarity in the illustration, in Figure 13 these components are depicted displaced perpendicular to each other, and as a result there are gaps between the components in the figure.

[0059] The function is similar to that described in the embodiment of Example 1, except that the spark gap is moved to the opposite side, and its input 81 is connected as a whole to the support plate 41 and electrode 66 (e.g., by brazing). This allows for better cooling, and at the same time, the spark gap does not restrict access to the workpiece 10. The working current passes through the support 41, and high pressure appears on it for a short period of time, so it is necessary to mount it electrically insulated. The protective film 54 protects the dielectric of the stripline from burnout of the spark gap electrode, thus greatly extending its service life. Suitable materials are polyimide, polytetrafluoroethylene, or mica slices. The spark gaps 80 and 81 can be operated in air at normal pressure, but to extend the service life of the electrodes and insulators, it is very convenient to fill them with nitrogen to prevent oxidation of the electrodes and the formation of ozone that attacks the polymer parts. By increasing the working gas pressure, it is possible to further reduce the inter-electrode gap distance for a given dielectric breakdown voltage and reduce the parasitic inductance of the spark gap.

[0060] Industrial use The present invention is particularly applicable in mechanical engineering, especially in the aerospace industry, and generally when components subjected to high mechanical stress are produced. The present invention helps to strengthen the surface of a workpiece, especially a metal workpiece, by the mechanical effect of a small projectile impact, when a projectile impact causes plastic deformation in the surface layer of the workpiece and introduces residual mechanical stress, thereby resulting in a significant improvement in the functional mechanical properties of the final product, in particular, an increase in the mechanical cavitation, adhesion, and corrosion resistance of the surface. List of reference marks 10 workpieces 20 Lorentz force 21 Current 22 Magnetic field strength 23 Field lines of magnetic induction 27 Contact area 28 Free space 30 Electric Arc 31 nozzles 32 polymer strips 33 First electrode 34 Bridge 35 Displacement Mechanism 37 Insulating layer 41 Support 44 First conductor 46 Second conductor 50 Capacitors 52 Dielectrics 53 Capacitor Plate 54 Protective film 61 Traverser 66 Second electrode 71 Metal foil 80 Spark gap input 81 Spark gap output 82 Parasitic Inductance 83 High-voltage source 84 switches 85 Insulator 86 Auxiliary trigger electrode 90 explosions 91 Polymer waste 93 Sharp edges 100 projectiles

Claims

1. A device for strengthening the surface of a workpiece (10) by introducing compressive stress, The apparatus comprises a polymer strip (32) having metal foil (71) on the surface of a side of the workpiece (10) that is offset from the surface, where a foil bridge (34) is formed to form a projectile (100), and the apparatus further comprises two electrodes (33) and (66) that contact a metal foil (71) located on the polymer strip (32), and the foil bridge (34) is formed between the contact surface regions (27) of the metal foil (71), Electrodes (33) and (66) in which plasma is formed are attached to a support (41), and via the support (41), the electrodes (33) and (66) are connected to a switch (84) for switching between a high current and voltage and a high voltage source (83) using flat conductors (44) and (46). The polymer strip (32) comprising the metal foil (71) is firmly attached to the support (41) together with the electrodes (33) and (66), and the electrodes (33) and (66) protrude above the upper surface of the support (41) to provide electrical contact with the contact surface region (27) of the metal foil (71). The apparatus further includes a nozzle (31) having a sharp edge (93) located below its inlet opening for guiding and accelerating the projectile (100), wherein the nozzle (31) opening is directed toward the surface of the workpiece (10).

2. The apparatus according to claim 1, wherein the foil (71) is locally narrowed to form the foil bridge (34) at a longitudinal distance that is periodically repeated between the contact surface regions (27).

3. The apparatus according to claim 1 or 2, wherein the polymer strip (32) passes through a displacement mechanism (35) that shifts the polymer strip (32) between the electrodes (33) and (66).

4. The apparatus according to any one of claims 1 to 3, wherein the polymer strip (32) is made of biaxially oriented polyethylene terephthalate or polyimide.

5. The apparatus according to any one of claims 1 to 4, wherein the apparatus comprises a capacitor (50) charged from a high-voltage source (83) for storing energy necessary for generating plasma, the capacitor (50) comprises a plate (53) on its outer surface surrounding a dielectric (52) of a certain thickness, and the plate (53) is connected to conductors (44) and (46).

6. The apparatus according to any one of claims 1 to 5, wherein the conductors (44) and (46) are provided with an electrical insulator (85) having dielectric strength to resist high voltages of at least 2 to 5 kV.

7. The apparatus according to any one of claims 1 to 6, wherein the switch (84) has a spark gap configuration having a spark gap input (80) and a spark gap output (81), the spark gap input (80) and the spark gap output (81) are separated from each other by air or other suitable gaseous dielectric, the switch is provided with an auxiliary electrode for switching by high-voltage pulses to control the switch (84), or the switch (84) takes the form of a cascade of IGBT transistors.

8. The apparatus according to any one of claims 1 to 7, wherein the foil bridge (34) has a thickness in units of tens of micrometers.

9. The apparatus according to any one of claims 1 to 8, wherein the surface density of the foil bridge (34) matches the surface density of the polymer strip (32).

10. The apparatus according to any one of claims 1 to 9, wherein the apparatus includes cooling means for cooling the nozzle (31) and / or the support (41) and / or the switch (84).

11. The capacitor (50) is made as part of the generator of the projectile (100) such that it is at least partially positioned between the support (41) and the polymer strip (32) having the metal foil (71). At the locations of the electrodes (33) and (66), the polymer strip (32) is mounted perpendicular to the axis connecting the electrodes (33) and (66), and the polymer strip (32) extends on both sides above the two electrodes (33, 66) onto the dielectric (52). The apparatus according to claim 5, wherein a conductor (44) passing beneath the plate of the dielectric (52) is connected to an electrode (33) located above the plate of the dielectric (52).

12. The apparatus according to any one of claims 1 to 11, wherein the polymer strip (32) has foil bridges (34) formed by narrowing the metal foil (71) which are perpendicular or oblique to the longitudinal axis of the polymer strip (32), and as a result each foil bridge (34) electrically connects exactly one pair of contact surface regions (27) which are oriented laterally or obliquely to the longitudinal axis of the polymer strip (32).

13. An apparatus for strengthening the surface of a workpiece (10) by introducing compressive stress, wherein the apparatus comprises a polymer strip (32) having a metal foil (71) on the surface of a side of the workpiece (10) that is offset from the surface, where foil bridges (34) are formed to generate plasma, The apparatus further comprises two electrodes (33) and (66) that contact a metal foil (71) located on the polymer strip (32), and the foil bridge (34) where the plasma is generated is formed between the contact surface regions (27) of the metal foil (71). The electrodes (33) and (66) are attached to a support (41), and the electrodes (33) and (66) are connected via the support (41) to a switch (84) for switching between a high current and voltage and a high voltage source (83) using flat conductors (44) and (46). For the storage of energy necessary for plasma generation, the apparatus includes a capacitor (50) that is charged from the high-voltage source (83), The capacitor (50) has a plate (53) on its outer surface that surrounds a dielectric (52) of a certain thickness, and the plate (53) is connected to the conductors (44) and (46). The switch (84) has a spark gap configuration having a spark gap input (80) and a spark gap output (81), and the spark gap input (80) and the spark gap output (81) are separated from each other by air or other suitable gaseous dielectric. Both the spark gap input (80) and the spark gap output (81) are formed on the conductor (46), and the capacitor (50) is at least partially sandwiched between the support (41) and the polymer strip (32) having the metal foil (71). The apparatus is characterized in which a polymer strip (32) that electrically insulates the workpiece (10) from a high-voltage circuit is positioned in close contact with the workpiece (10). The apparatus is characterized in that it comprises a second insulating layer (37) that isolates the exploding foil bridge (34) from the support (41) and protects it from plasma ablation, and the electrodes (33) and (66) pass through holes in the second insulating layer (37).

14. The apparatus according to claim 13, wherein the polymer strip (32) is made from polyimide.

15. The apparatus according to claim 13 or 14, wherein the second insulating layer (37) is bonded to the polymer strip (32) and the metal foil (71).

16. The apparatus according to any one of claims 13 to 15, wherein the spark gap formed by the lead wires (80) and (81) includes a trigger electrode (86) for controlling switching by a high-voltage pulse.

17. The apparatus according to any one of claims 13 to 16, wherein the conductors (44) and (46) connected from the capacitor (50) to the electrodes (33) and (66) are flat in shape, with a width greater than the thickness, and one of these conductors is part of it, and at the same time the plate (53) of the capacitor (50) and the other conductors are blocked by the switch (84).

18. The apparatus according to any one of claims 13 to 17, wherein the dielectric (52) is provided with a protective foil (54) to protect the dielectric (52) at the locations of the spark gap input (80) and the spark gap output (81).

19. The apparatus according to any one of claims 13 to 18, wherein the space between the spark gap input (80) and the spark gap output (81) is filled with nitrogen.

20. A method for strengthening the surface of a workpiece (10) using the apparatus according to any one of claims 1 to 12, wherein one cycle of strengthening the surface of the workpiece (10) includes the action of current pulses supplied from the high-voltage source (83) to electrodes (33, 66) between which high voltage is introduced, after the switch (84) has been closed via the conductors (44, 46), thereby short-circuiting a high-voltage circuit on the metal foil (71) at the location of the foil bridge (34) and forming a plasma that expands and applies compressive force to the polymer strip (32), a portion of which collides with the surface of the workpiece (10) as the projectile (100), and the plasma generated by the current pulses is accelerated not only by the expansion pressure but also by an electromagnetic Lorentz force (20) caused by the passage of current through this plasma in a generated magnetic field.

21. The method according to claim 20, wherein, after the expansion of the plasma, a portion of the polymer strip (32) is cut by the sharp edge (93) of the nozzle (31) and then accelerated onto the workpiece surface (10) as the projectile (100) by a compressive force passing through the through-hole of the nozzle (31).

22. The method according to claim 20 or 21, wherein the surface of the workpiece (10) is treated in a periodically repeating cycle, and at the beginning of each cycle, the polymer strip (32) is moved using a displacement mechanism (35) to a position where the foil bridge (34) is located between the electrodes (33, 66), and both electrodes (33, 66) are in conductive contact with the metal foil (71) at both ends of the foil bridge (34).

23. The method according to any one of claims 20 to 22, wherein the switch (84) has a spark gap configuration having a spark gap input (80) and a spark gap output (81), and the switch (84) is closed by a voltage difference in the spark gap between the spark gap input (80) and the spark gap output (81) that exceeds the value of the electric intensity of the air or other suitable gaseous dielectric.

24. The apparatus according to any one of claims 1 to 19, used for strengthening the surface of a workpiece.

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