Electric heating – Metal heating – By arc
Reexamination Certificate
1999-04-19
2001-04-10
Walberg, Teresa (Department: 3742)
Electric heating
Metal heating
By arc
C219S121540
Reexamination Certificate
active
06215088
ABSTRACT:
The invention relates to a plasma torch.
In known plasma torches a substantially continuously flowing plasma is used, mostly for hardening the surface of objects made of steel.
A laser beam or an electron beam is mostly used for other methods, e.g. for welding, in particular for soot welding thin sheets, or for producing a breakthrough in thinner metallic objects. This leads to the disadvantage, however, that laser welding processes require a very laborious preparation of the parts to be welded, which must be joined with a very high precision in order to enable their welding by means of a laser beam. The same also applies with respect to methods using electron beams. Moreover, the equipment required for performing such methods is very complex in a constructional respect.
It is the object of the present invention to- avoid such disadvantages and to provide a plasma torch which allows a simple machining of objects, in particular the production of spot welds or the burning of breakthroughs.
This is achieved in accordance with the invention by a plasma torch for locally heating an object, which comprises an anode having an axially extending bore, and a cathode having an end section extending coaxially through the bore in the anode and another section passing through a chamber communicating with a gas connection, the anode and the end section of the cathode defining an annular gap in communication with the chamber and forming a nozzle. Means is provided for connecting the anode and the cathode in an electrically insulating manner, a gas source is connected to the gas connection, a voltage supply is connected to the anode and the cathode, the voltage supply delivering intermittent voltage pulses exceeding the arc-over voltage in the annular gap, and a ring encloses the nozzle and projects axially therefrom to define a front chamber between the nozzle and the object upon which the ring is placed.
As a result of the proposed measures it is possible with relatively roughly prepared parts to join the same by means of spot welding. Measures will substantially suffice as are also required in electric resistance spot welding.
A very high ejection speed of the plasma pulse is secured by the ignition of the arc by exceeding the arc-over voltage of the anode-to-cathode gap, so that this pulse will impinge upon the parts to be welded with a high kinetic energy. The plasma pulses thus produced will reach very high temperatures of 20,000 to 50,000° C. and will cause adequate fusion of the mutually adhering surface areas of the parts to be joined despite a short action period of e.g. 10
−5
to 10
−0
seconds and will thus cause a secure connection.
The front chamber helps avoid the formation of oxide layers on the subjects, with the gas used for the production of the plasma, mostly argon or helium, appropriately being simultaneously used as inert gas.
The production of single plasma pulses is ensured in a simple manner by the proposed measures. The front chamber of the plasma torch also leads to the possibility to place the same on the subjects to be joined, thus ensuring keeping constant the distance between the electrodes and the subjects in a very simple manner. Moreover, this also allows producing the individual welding spots under a protective gas atmosphere in the simplest possible way, as while maintaining a plasma gas flow said plasma gas such as argon or helium fills the front chamber.
Such a device can also be used to produce seams consisting of individual welding spots. In such a case it is merely necessary to provide a forward feed device which forces a relative movement between the plasma torch and the subject and to ensure a continuous sequence of voltage pulses applied to the electrodes of the plasma torch.
Preferably, the voltage supply comprises a capacitor battery having an input connected to a charging circuit and an output connected to the anode and cathode.
In such a device it is possible in a simple way to charge the subject(s) to be machined with a sequence of very short plasma pulses. In the course of charging the capacitor battery the arc-over voltage of the anode-to-cathode gap will be exceeded and thus an arc will be formed through which there will be a discharge of the capacitor battery. The arc will extinguish as soon as the voltage of the capacitor battery drops below the arc drop voltage. As a result of a respective dimensioning of the charging circuit and the discharge circuit of the capacitor battery with respect to the time constants it is possible to determine both the arc duration in each cycle as well as the repetition frequency. The arc which thus burns only very briefly produces plasma pulses which, as a result of the very rapid heating of the ambient gas, exit with a very high speed from the outlet opening of the chamber of the plasma torch and impinge upon the objects to be joined or the object to be provided with a breakthrough and as a result of their high temperatures ensure the fusion or the melt-through of the object(s).
The device in accordance with the invention allows achieving the short pulse durations of the plasma pulses of 10
−5
to 10
−0
seconds for example and a repetition frequency of 7 to 100 Hz which are required for the careful treatment of the objects to be machined. As a result of these short operating times of the individual plasma pulses, the thermal stress on the objects is kept low and thus the danger of distorting the mostly very thin or thin-walled objects is substantially avoided.
In order to allow producing particularly short plasma pulses it is preferable to provide a separate ignition set connected to the anode and cathode. These measures allow initiating the ignition of the arc even before reaching the arc-over voltage of the anode-to-cathode gap, which allows keeping the arc duration, and thus the plasma pulses, extremely short without having to make any particularly great efforts concerning a particularly low-resistance arrangement of the discharge circuit of the capacitor battery.
It is principally also possible to also use a technical AC network or a voltage source supplying a high-frequency AC current in conjunction with a phase controller instead of the capacitor battery as a voltage supply for the plasma torch. In this respect it must be ensured in the case of electrodes made of different materials that merely equally polarised half-waves are partly connected through so that voltage pulses with the same polarity are always applied to the different electrodes and substantially the same ratios as in the supply of the plasma torch with DC voltage pulses, like from a capacitor battery for example, are obtained.
In cases in that both are electrodes made from the same material, pulses with different polarity can be applied to each of the two electrodes.
As electrodes which are made of different materials for the purpose of achieving a longer service life are usually charged with the same polarity in plasma torches, the terms “anode” and “cathode” are generally used in the description and the claims.
Particularly for applications where more powerful plasma torches are required, such as in joining stronger sheets with weld seams formed from a number of welding spots, it is preferable to provide a mains apparatus including a rectifier circuit having a negative pole connected to a negative pole of the capacitor battery output and a positive pole connected to the object.
The proposed measures allow making do with smaller capacitor batteries because a portion of the required energy is supplied by the mains apparatus. Moreover, the connection of the subject(s) to the positive pole of the mains apparatus leads to the advantage of a narrow convergence of the plasma pulse and a concentration of its energy on a very small surface area. With a relatively small application of energy it is possible to ensure the secure welding of two parts in the zone of a small surface area and the heating of the ambient border zones can be strongly minimised, which thus reduces the danger of distortion of the subject.
REFEREN
Collard & Roe P.C.
Inocon Technologie Gesellschaft m.b.H.
Van Quang
Walberg Teresa
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