CATHODE STRUCTURE OF A PLASMA TORCH

Abstract: A cathode structure of a transfer-type plasma torch which is so designed as to enhance the cooling effect and to decrease the wear of the cathode member by constantly moving the point of electric discharge on a plane at the lower end of the cathode while the plasma arc is being generated. The cathode structure includes a cylindrical cathode (21) of a small diameter having a tiny recess formed at the lower end surface thereof symmetrically relative to the axis thereof, a cathode holder (20) of a structure capable of effecting cooling easily and highly efficiently, and a device (23) for generating lines of magnetic force which is arranged concentrically with the cathode (21) in order to generate the vector B of lines of magnetic force symmetrically relative to the cathode axis, such that E x B = &cir& / O with respect to the vector E of lines of electric force generated on the plane at the lower end of the cathode.

Description: The present invention relates to a cathode structure of a transferred arc plasma torch according to the preamble of claim 1. A cathode structure of such type is disclosed by JP-U-60-130799. As a cathode structure of a plasma-arc torch in the prior art, a rod-shaped cathode structure and a buried type cathode structure are known as shown, for example, in "PRAZUMA SETSUDAN NO KISO TO JISSAI (Fundamental and Practice of Plasma-arch Cutting)" (in Japanese) edited by NIHON YOSETSU KAI (Society of Welding in Japan) (published December 1, 1983 by KOSAIDO SANPOH SHUPPAN, page 48). The rod-shaped cathode is a cathode mainly making use of inert gas such as argon, nitrogen, hydrogen, etc. as a working gas, and it is employed in a torch having a relatively small capacity, whereas the buried type cathode is a cathode wherein hafnium, zirconium or the like is buried in a tip end portion of a water-cooled copper pipe, and it mainly makes use of an oxidizing working gas such as oxygen, air, etc. and is employed in a torch having a relatively large capacity. In addition, with regard to a cathode structure of non-transferred arc plasma torch, in the case of large-capacity torches, cathodes having various shapes and structures have been devised. For instance, besides the above-mentioned rod-shaped or buried type cathodes, cathodes having a ring-shaped or hollow type (hollow cathode) structure have been used. In common to the transferred and non-transferred arc plasma torches, one big problem is abrupt consumption of a cathode and a nozzle accompanying the generation of plasma arcs. Especially in the case where oxidizing gases such as air, oxygen, etc. are used as a working gas, their life would become extremely short, and as a cathode, the life is so short that it becomes necessary to be replaced at an interval of about 0.5 ∼ 3 hours. Heretofore, because of the necessity of frequent cathode replacement works due to such a short period of life of the cathode, for instance, version to numerical control (NC) of a plasma-arc cutting machine and a plasma-arc welding machine as well as popularization of these machines are remarkably lagging. Now, it has been well known that among the heretofore employed cathode structures and working gas feed systems in the transferred arc plasma torches, there are an axial flow type and a swirl flow type. And in these plasma-arc torches of either axial flow type or swirl flow type in the prior art, a discharge point exists always at the center of the cathode, and as the time when a plasma arc is generated elapses, consumption of the cathode proceeds remarkably from its central portion. As one method for resolving the problem of abrupt cathode consumption, it was devised by the inventor of this application to continuously move the discharge point on the cathode surface to a new position during generation of a plasma arc. The device is disclosed in the above-mentioned Japanese Utility Model Application No. 60-130799 filed on August 29, 1985. The cathode structure of the transferred arc plasma torch based on this device is the structure shown in Fig: 1 of the accompanying drawings. In Fig. 1, reference numeral 10 designates a cathode holder, and to the bottom end portion of this cathode holder 10 is mounted a cathode 11. The cathode 11 is provided with a hemispherical concave surface 12 on its bottom surface. In addition, the cathode holder 10 is provided with a generating device of lines of magnetic force 13 such as a coil, a permanent magnet or the like, and this generating device of lines of magnetic force 13 is disposed above the cathode 11 coaxially therewith. In the case where the generating device of lines of magnetic force 13 is a coil, a D.C. coil is used. Reference numeral 14 designates a nozzle for swirling a working gas, numeral 15 designates lines of magnetic force, numeral 16 designates lines of electric force, numeral 17 designates a discharge point, numeral 18 designates a plasma arc, and numeral 19 designates a nozzle. Now description will be made on an operation of the plasma-arc torch shown in Fig. 1. When the generating device of lines of magnetic force 13 operates, lines of magnetic force 15 as shown by dash lines are formed. In addition, sicne lines of the electric force 16 are formed in the perpendicular direction with respect to the bottom surface of the cathode 11, a vector product (E x B) of the both would take an effective value not equal to zero, this is, E x B ≠ 0 almost over the entire region of the cathode bottom surface except for the center axis (the axis of symmetry) on which the respective directions of the line of electric force vector E and the line of magnetic force vector B coincide with each other. It is expected that in the proximity of the discharge point 17 in the figure, the product value takes the maximum value. Accordingly, a Lorentz force serving as a force for driving the discharge point 17 F = j x B = σE x B would similarly take the maximum value. It is to be noted that σ represents a conductivity, and j represents a current density of a current flowing through the plasma. And since the array of the lines of magnetic force (the magnetic field arrangement) is symmetric with respect to an axis, a revolving motion of the discharge point 17 as shown in Fig. 17 is induced. In the above-described cathode structure in a plasma-arc torch shown in Fig. 1, since the volume of cathode material becomes large, a cooling effect by coolant medium was poor. Especially, in the case where oxidizing gas such as oxygen, air or the like is used as a working gas, often hafnium, zirconium or the like is used as cathode material. As hafnium and zirconium are metals having a very small thermal conductivity, the known cathode structure had a shortcoming that when these metals are used as cathode material, if its volume is made large, the temperature in the proximity of the discharge point would rise, and an amount of local consumption of a cathode would become large. An example of the above mentioned hollow cathode structure is disclosed by US-A 3 610 796. This cathode structure comprises a ring-shaped permanent magnet which is coaxially attached to the lower end face of an inner supporting tube, and a cathode formed by an annular shell composed of material having high thermal and electrical conductivity. The shell comprises a cylindrical outer wall of larger diameter and a cylindrical inner wall of smaller diameter which are arranged coaxially with each other. The shell further comprises a disk-shaped top wall of said smaller diameter which is coaxially attached to the upper rim of said inner wall, and an annular bottom wall extending between the lower rims of said inner and outer wall. The ring shaped permanent magnet fits such into the annular space defined between said inner and outer wall that a gap is always maintained between the respectively facing surfaces of the magnet and the shell. This gap serves as passageway for a cooling fluid. This passageway communicates at one end with the hollow space of said inner supporting tube, and at its upper end with a cylindrical passageway defined between the outer surface of said inner supporting tube and the inner surface of a coaxially mounted outer supporting tube. This outer supporting tube is attached at its lower rim to the upper rim of the outer wall of the shell. The downwardly facing surface of the annular bottom wall of the shell serves as the arcing surface which therefore is of annular shape, too. The outside and the inside annular surface of the permanent magnet form the opposite magnetic poles, respectively, so that the magnetic field lines are radially bent around the arcing surface of the cathode. Thus, the above mentioned Lorentz force drives the discharge point of the arc along the arcing surface around the vertical symmetry axis of said cathode structure. Since the shell forming the cathode extends around the permanent magnet on three sides thereof, the volume of cathode material in such cathode structure is larger than that of the aforementioned cathode structure of JP-U-60-130799. Therefore, the problem of cooling is aggravated, especially if the cathode material having high thermal and electrical conductivity is replaced by hafnium or zirkonium having a very small thermal conductivity. The GB-A 1 317 050 discloses a plasma discharge system having a cathode holder and a cathode inserted into the bottom end of the cathode holder. The lower surface of the cathode is plain. There is no magnetic field generating device for moving the arch discharge point over the discharge surface of the cathode for extending the lifetime of the cathode. It is therefore the object of the present invention to provide a cathode structure of a transferred arc plasma torch of the type mentioned at the beginning which achieves an enhanced cooling effect of the cathode so that the amount of consumed cathode material is remarkably reduced. This object is achieved by a cathode structure as defined in claim 1. Advantageous embodiments are described in the subclaims. In the following, preferred embodiments are described with respect to the accompanying drawings. Fig. 1 is a schematic longitudinal cross-section view of an essential part showing a cathode structure of a transferred arc plasma torch in the prior art; Fig. 2 and 3 are schematic cross-section views of an essential part respectively showing different embodiments relating to a cathode structure of a transferred arc plasma torch according to the present invention, and Fig. 4 is a graph showing an amount of consumption of a cathode with respect to an operation time when experiments were conducted by making use of the cathode structure of a transferred arc plasma torch according to the present invention shown in Fig. 2. Description of the Best Mode of the Invention The cathode structure of a transferred arc plasma torch according to the present invention will be explained in greater detail in the following, in connection to the respective different embodiments shown in Figs. 2 through 5. In each one of Figs. 2 and 3, reference numeral 20 designates a holder composed of a water-cooled type of copper rod, and it is adapted to be easily and effectively cooled by coolant medium such as coolant water. Reference numeral 21 designates a cathode buried in the central bottom end portion of the holder 20. This cathode 21 is formed in a circular column shape having a small diameter, and on its tip end surface is provided a minute axially symmetric hemispherical concave surface 22a as shown in Fig. 2, or a minute axially symmetric circular cone-shaped concave surface 22b as shown in Fig. 3. The holder 20 for holding the cathode 21 is provided with a generating device of lines of magnetic force 23 such as a coil, a permanent magnet or the like, and this generating device of lines of magnetic force 23 is disposed above the cathode 21 and coaxially therewith. In the case where this generating device of lines of magnetic force 23 is a coil, a D.C. coil is used. It is to be noted that the above-mentioned coil 23 could be replaced by a permanent magnet. It is just as explained already in connection to Fig. 1 that a Lorentz force (F = σE x B) is generated on the bottom end surface of the cathode 21 also in the case of the either embodiment shown in Fig. 2 or in Fig. 3, in the above-described construction. It is to be noted that in the above equation, F represents a Lorentz force, E represents a line of electric force vector, B represents a line of magnetic force vector, and σ represents a conductivity of plasma. And similarly to the cathode structure in the prior art in Fig. 1, in the respective embodiments of the cathode structure according to the present invention shown in Figs. 2 and 3 also, since the array of magnetic field and the shape of cathode are both formed axially symmetrically, during generation of an plasma arc 25, for a discharge point 24 of the plasma arc 25 on the cathode bottom end surface 22a or 22b a revolving motion is induced, and the discharge point 24 is continuously moved on the cathode bottom end surface. Example of Experiment Fig. 4 shows one example of experiment conducted for the purpose of confirming the effect and advantage of the cathode structure according to the present invention, the ordinate shows an amount mg of consumption of a cathode, and the abscissa shows an operation time t (min). The cathode used in this embodiment has the shape of the embodiment shown in Fig. 2, and the material is hafnium. For the generating device of lines of magnetic force, a D.C. coil is employed and its output is such that a magnetic flux density on the bottom end surface of the cathode amounts to about 70 gauses. As a working gas, oxygen is used, a flow rate thereof is 40 ℓ/min, and an opening diameter of a plasma arc nozzle 19 is 2.3 mm. Also, an operation current was 100 A. In the figure, C₁ represents the case where a magnetic field was applied, and C₂ represents the case where a magnetic field was not applied. It is to be noted that even in the case where the cooling system for the cathode was changed to an air-cooling system, a sufficient effect desired according to the present invention was confirmed.

Claims: A cathode structure of a transferred arc plasma torch comprising: a cathode (21,21a,21b) with an arcing surface (22a,22b) of rotation symmetry at its bottom end, said cathode being cooled; a holder (20) for holding said cathode (21,21a,21b) at its bottom; and a magnetic field generating device (23) disposed coaxially with said cathode (21,21a,21b), for generating a magnetic field (B) which is symmetrical with respect to the symmetry axis of said cathode (21,21a,21b); characterized in that said cathode (21,21a,21b) is of circular column shape and is smaller in diameter than said holder (20); said cathode (21,21a,21b) has an axially symmetric hemispherical concave or cone-shaped concave surface (22a,22b) on its tip end; and said holder (20) is of higher thermal conductivity than said cathode (21,21a,21b), contacts the entire circumferential surface thereof and is cooled. A cathode structure according to claim 1, characterized in that said cathode (21,21a,21b) is made of hafnium or zirconium and said holder (20) is made of copper. A cathode structure according to one of the preceding claims, characterized in that said holder (20) is a water-cooled type copper rod structure.

Inventors: SAKURAGI SHUNICHI, SAKURAGI, SHUNICHI
Applicants: KOMATSU MFG CO LTD, KABUSHIKI KAISHA KOMATSU SEISAKUSHO
Application year: 1987
Main classification: H05H1
IPC codes: B23K9, B23K10, H05H1, B23K35