Abstract:
A method for locally heating objects, in particular thin sheet metal, by charging the objects by means of a plasma ignited between two electrodes. In order to keep the thermal stress of a subject as low as possible outside of the zone to be heated it is provided that the machining such as spot welding or burning through a breakthrough occurs with merely one plasma pulse which is produced by applying a voltage pulse exceeding the arc-over voltage of the gap between the electrodes.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a division of U.S. patent application Ser. No. 09/294,612, filed Apr. 19, 1999, now U.S. Pat. No. 6,215,588 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method for the partial fusion of objects. 
     2. Description of the Prior Art 
     In known such methods 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 spot 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. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to avoid such disadvantages and to provide a method of the aforementioned kind 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 machining with merely on plasma pulse, which is produced by applying a voltage pulse exceeding the arc-over voltage of the gap between the electrodes. 
     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. 
     Machining in a protective gas atmosphere 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. 
     If the plasma pulse has a duration of about 10 −5  to 10 −0  seconds, preferably 10 −4  to 10 −1  seconds, relatively compact devices or plasma torches may be used which can be operated at a relatively high output over a short period. 
     For a weld seam from a number of welding spots, the object to be joined are charged with a number os successive plasma pulses while the objects are moved relative to the electrodes and the electrodes are kept at a constant distance from the objects, a repetition frequency of the plasma pulses of 5 to 100 Hz being provided. 
     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). 
     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 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. 
     Initiating even before reaching the arc-over voltage of the anode-to-cathode gap 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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be explained in closer detail by reference to the enclosed drawing, wherein: 
     FIG. 1 shows a sectional view through a device with a plasma torch in accordance with the invention; 
     FIG. 2 shows a top view on the holder plus a plasma torch in accordance with FIG. 1; 
     FIG. 3 shows a sectional view through the plasma torch in accordance with FIGS. 1 and 2 on an enlarged scale; 
     FIG. 4 shows a sectional view through a coolant chamber of the anode contact part; 
     FIG. 5 shows a sectional view through the centering sleeve; 
     FIG. 6 shows a first embodiment of a voltage supply for a plasma torch and 
     FIG. 7 shows a further embodiment of a voltage supply for a plasma torch. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A holder  1 ′ is provided in the embodiment in accordance with FIG. 1, which holder is provided with bores  4 ′ for receiving contact pins  9 ′, with the contact pins  9 ′ being axially bored through. The contact pins  9 ′ are provided with an outside thread  29  in a zone outside of the holder  1 ′ on which terminal nuts  30  are screwed and between which cable lugs  31  of connecting lines  6  (FIG. 2) are clamped. 
     The rear end of the contact pins  9 ′ is arranged for the connection of tubes through which cooling water can be supplied. 
     Furthermore, a gas supply line  3 ′ is held in the holder  1 ′ which—as can be seen from FIG.  2 —is connected with a gas tube  36  through a radial duct  32  which is outwardly occluded with a grub screw  33  and through an axial bore  34  which opens into the same and into which a hose nozzle  35  is screwed. A gas required for producing the plasma can be supplied through said gas tube. 
     The gas supply line  3 ′ is provided in the zone of the radial duct  32  with slots  37  through which the gas can flow into the interior of the gas supply line  3 ′. The gas supply line  3 ′ is secured in its position by means of the screw  39  which engages in the same. 
     As can be seen from FIG. 1, the contact pins  9 ′ project in their spring-loaded idle position beyond the face surface  38  of the holder  1 ′ and engage in the jacket surface of a plasma producer  11 ′ which is arranged as a module. The same also applies for the gas supply line  3 ′ which, when the plasma producer  11 ′ is mounted, engages in the same. 
     The plasma producer  11 ′ which is arranged as a module is held by means of a pipe bracket  40  whose rigid part held on the face side  38  of the holder  1 ′ is held with pins  42 . Pipe bracket  40  is provided with a joint  43  whose axis extends perpendicularly to the axis of holder  1 ′. 
     The holding part  18 ′ of cathode  19 ′ is formed by a collect chuck in plasma producer  11 ′, which chuck is made from an electrically well-conducting material. Said collect chuck is held in the usual manner in a receiver  44  which is screwed into a contact part  45 . 
     Said contact part  45  is provided with a coolant chamber  46  which is connected with a connecting opening  48  through a radial duct  47 . Said connecting opening  48  is in true alignment with the contact pins  9 ′ when holder  1 ′ is mounted in plasma producer  11 ′. 
     An adjusting nut  49  is provided for tensioning and loosening the collect chuck  18 ′, which adjusting nut rests on the upper face surface of receiver  44  through two seals  50 , as a result of which any escape of coolant liquid is prevented. Receiver  44  is also supported on the contact part  45  through a seal  51  for sealing the coolant chamber  46 . 
     An O-ring  52  is provided for further sealing the coolant chamber of the contact part  45 , which O-ring is inserted into a groove of a bore  53  which is penetrated by receiver  44 . 
     In order to secure the axial setting of the cathode  19 ′ during the tensioning of the collect chuck  18 ′, adjusting nut  49  is provided with a threaded through-bore  90  into which a stop  91  is screwed which engages into the collect chuck  18 ′. Said stop  91  is provided with a smooth head  94  in which a circular groove is incorporated for receiving an O-ring  95  which is used for sealing the interior of the collect chuck  18 ′. 
     A counternut  92  is provided to secure the position of stop  91  which is adjustable by means of screwdriver which is inserted into the face-sided slot  93 . Counternut  92  simultaneously ensures a torsionally rigid connection between the stop  91 , on which rests cathode  19 ′, and the adjusting nut  49 . 
     Stop  91  ensures that during the tensioning of the collect chuck cathode  19 ′ can no longer be axially moved with respect to anode  15 ′ by collect chuck  18 ′, because the adjusting nut  49  rests on the face side of contact part  45  and anode  15 ′ is fixed with respect to the same. 
     Contact part  45 , which is used for making the contact of cathode  19 ′, rests on an intermediate part  55  by interposing a seal  54 , which intermediate part is made from an electrically insulating material such as ceramic. Said intermediate part  55  determines the chamber  27 ′ which is connected with a connecting opening  57  through a radial duct  56 . 
     The radial ducts  47  and  56  are provided with circular grooves  58  in which O-rings  59  are arranged. They are used for sealing the contact pins  9 ′, which engage in these ducts, and the gas supply line  3 ′. 
     A distributor ring  59 ′ is arranged in chamber  27 ′ which is provided with bores  60  which are arranged distributed over the circumference and whose diameters in both directions of rotation increase with an increasing angle towards the radial duct  56 . The axial bore of the distributor ring  59 ′ is penetrated by the cathode  19 ′. An annular space  61  remains between the inner wall of the intermediate part  55  and the distributor ring  59 ′. 
     The intermediate part  55  rests on the anode contact part  63  supported through a seal  62 . A clamping sleeve  64  is screwed into an inner thread  65  in said anode contact part  63 , with a sealing  66  being interposed between the anode contact part  63  and the face surface of the clamping sleeve  64 . 
     The clamping sleeve  64  is provided in the zone of its one end with a conical bearing surface  67  on which rests a diametrically opposed conical jacket surface  68  of a head  69  of an anode  15 ′ which, like the clamping sleeve  64  and the anode contact part  63 , is made of an electrically well-conducting material. 
     Anode  15 ′ is supported with its end averted from head  69  with a further head  70 , which by interposing a seal  71  rests on a shoulder of the anode contact part  63 . Anode  15 ′ penetrates a coolant chamber  46  of the anode contact part  63 . 
     Anode  15 ′ is bored through in the axial direction, with a sleeve  73  made from an electrically insulating material such as ceramic is disposed on bore  72  and is penetrated by cathode  19 ′. 
     Moreover, a centering sleeve  74  is inserted in bore  72  in the zone close to the orifice of anode  15 ′, which sleeve is illustrated in closer detail in FIG.  5  and whose guide surfaces  75  provided on guide ribs  89  rest on the jacket surface of cathode  19 ′. 
     As is shown in FIG. 4, anode  15 ′ is provided with radially projecting guide ribs  76  which extend from the anode  15 ′ having a hexagonal cross section up to the inner wall of the clamping sleeve  64  and stand perpendicular to the axis of the radial duct  47 . Guide ribs  76  extend away from head  70  against the head  69  of anode  15 ′, with a flow gap  77  remaining between the head  69  and the guide ribs  76 . 
     In this way the coolant chamber  46 , which is limited on its part by the anode contact part  63  and the clamping sleeve  64 , is subdivided by the guide ribs  76 . 
     The two coolant chambers  46  of the contact part  45  and the anode contact part  63  are mutually connected through a transfer duct. 
     Which is substantially composed of the axial bores  79  in the contact part  45  and the anode contact part  63 , respectively, and radial bores  80  which are coaxial to the radial ducts  47  and open into the axial bores  79  bore  78  in intermediate part  55 . 
     Seals  82  are provided in the zone of the bore  81  of the intermediate part  55 . 
     An insert  83  is provided in the orifice zone of anode  15 ′, which insert is made of a wear-resisting materials such as a tungsten-cerium oxide alloy and delimits a nozzle aperture  16 . The section of anode  15 ′ projecting from the clamping sleeve  64  is encompassed by a ring  96  which is made of a wear-resistant material and projects axially beyond the nozzle aperture  16  of anode  15 ′ and defines a pre-chamber  97 . 
     The two contact parts  45  and  63  are encompassed by rings  84  made of an electrically insulating material and rest on collars  85 . 
     As can be seen from FIG. 1, the pipe bracket  40  is provided in the zone of the collars  85  of the contact parts  45  and  63  with recesses  86 , thus preventing a short between the two contact parts  45  and  63 . 
     Cathode  19 ′ is arranged conically at its two ends. 
     The two contact parts  45  and  63  and the intermediate part  55  are mutually connected by means of the screws  87  shown in FIG.  2  and represent the connecting parts which thus ensure a modular arrangement of the plasma producer  11 ′. 
     As soon as cathode  19 ′ is worn off, the plasma producer  11 ′, which is arranged as a module, can be removed by loosening the straining screw  88  and by opening the pipe bracket  40 , whereupon the adjusting nut  49  can be loosened and the cathode  19 ′ can be removed from the collect chuck. Thereafter the cathode can either be turned round or its conical ends can be re-ground. Then the cathode can be adjusted by means of a caliber with respect to anode  15 ′. Then the stop  91  is adjusted while the collect chuck  18 ′ is opened and the cathode  19 ′ is fixed again in the collect chuck  18 ′ by means of adjusting nut  49 , whereupon module  11 ′ can be mounted again. 
     During the operation a gas such as argon, helium, nitrogen or the like is blown into chamber  27 ′ and an arc between the cathode  19 ′ and the anode  15 ′ is ignited through a voltage pulse which after a brief period of time drops below the arc drop voltage, so that the arc goes out. The plasma pulse thus formed exits through the nozzle aperture  16 , passes through pre-chamber  97  and impinges upon the subject(s) to be machined. They are fused by the action of the plasma pulse, thus melting a breakthrough or fusing two subjects to be welded, depending on the energy of the plasma pulse. In the latter case there will be a secure connection of the two parts during the following solidification after the plasma pulse has gone out. In this process these parts are sufficiently pressed together by the kinetic energy of the plasma pulse exiting with a high speed, whereby speeds of 2000 m per second are achieved, thus ensuring a secure connection. 
     The pre-chamber  97  allows in a very simple way charging the subjects to be machined with plasma pulses under a protective gas atmosphere. For this purpose it is merely necessary to supply the plasma torch  11 ′ with a substantially constant flow of plasma gas such as argon, helium or nitrogen. Nitrogen can only be used if the subject to be machined is compatible with a nitrogen atmosphere in the fused stated. 
     Furthermore, the plasma torch  11 ′ can be placed on the subject to be machined with the face side of ring  96  during the production of individual welding spots, thus simultaneously defining the distance between the electrodes  15 ′,  19 ′ and the upper side of the subject. 
     For special applications such as the production of breakthroughs with very small diameters it is possible to provide nozzles  16  with very small diameters, as small as 10 μm for example. As in such plasma torches  11 ′ it is possible to reduce the output appropriately, one can omit cooling ducts in such plasma torches. 
     FIG. 6 shows a voltage supply for a plasma torch  11 ′ in accordance with FIGS. 1 to  5 , with the voltage supply being provided for the production of a pulse plasma. 
     A capacitor battery  130  is connected by way of a charging resistor  131  with the connections X 1  of a controllable DC voltage source  132 . The capacitor battery  130  is provided with a fixedly connected capacitor  1 C 1  and a capacitor  1 C 2  which is connectable parallel to the same through a switch  1 S 1 . Groups of capacitors can be concerned in both cases. 
     Said capacitor battery  130  is connected through connecting lines  133 ,  134  with the cathode and anode of plasma torch  22  (not illustrated in FIG.  6 ). 
     An RC module is switched in parallel to the capacitor battery  130  which is formed by a capacitor  1 C 3  and a resistor  1 R 1 . This RC module forms an HF block circuit in conjunction with a choke  1 L 1  switched in the connecting line  134 , which choke is provided for the protection of the capacitor battery  130  against HF signals. 
     The outputs of an ignition set  135  are further connected to the connecting lines  133 ,  134 . Said ignition set  135  is connected on the input side with an AC voltage source X 2  and provided with a trigger switch  1 S 2  by which an ignition pulse can be initiated when actuated. 
     During operation, the capacitor battery  130  is charged according to the set voltage of the DC voltage source  132  which is adjustable between 50V and 300V and the time constant which is co-determined by the capacity of the capacitor battery  130  and the line resistances and the charging resistance. 
     Once the capacitor battery  130  reaches a voltage which corresponds to the arc-over voltage of the anode-to-cathode gap  15 ′,  19 ′ of the plasma torch  11 ′, an ignition of an arc between anode  15 ′ and cathode  19 ′ (FIG. 1, FIG. 3) and thus the formation of plasma in the orifice zone of the anode  15 ′ of the plasma torch  11 ′ will occur. 
     At the same time the capacitor battery  130  will discharge according to the time constant given by its capacity, the line resistances and the resistance of the arc. If as a result of this discharge the voltage of the capacitor battery  130  drops below the arc drop voltage, the same goes out and the capacitor battery  130  charges up again, as a result of which the described process is repeated and a frequency is obtained which is determined by the charging and discharging time constants. The operation of the ignition set is not required. 
     For certain applications it can be desirable to determine the ignition time of the arc precisely or to initiate such a one prior to reaching the arc-over voltage of the anode-to-cathode gap  15 ′,  19 ′ in order to enable the production of particularly short plasma pulses. 
     In this case an ignition pulse is initiated by actuating the trigger switch  1 S 2  which leads to the ignition of an arc between the anode  15 ′ and the cathode  19 ′ of the plasma torch  11 ′ without the capacitor battery  130  having reached a voltage corresponding to the arc-over voltage of this gap. In this way the pulse-duty factor, which can be selected between 1:10 and 1:100 and even beyond this figure, can be changed respectively and the ratio between the arc duration and its pause during a cycle can be changed in the sense of an extension of the arc pause, since the energy of the high-frequency ignition pulses of the ignition set  135  is sufficient for igniting the arc, but not for maintaining the same when the voltage of the capacitor battery  130  drops below the arc drop voltage. 
     The embodiment of the voltage supply for the plasma torch  11 ′ in accordance with FIG. 7 is distinguished from the one in accordance with FIG. 6 only in the respect that a mains apparatus  136  is provided in addition to the capacitor battery  130 , which mains apparatus is connected to an AC voltage network and is provided with a rectifier circuit. The illustration of the blocking circuit and the choke was omitted. 
     The connecting line  133 ′, which is connected with the negative pole of the output of the mains apparatus, is connected to the connecting line  133  which is connected to the negative pole of the capacitor battery  130  and the connecting line  134 ′, which is connected to the positive pole of the mains apparatus  136 , is connected with a subject  138 . 
     An automatic current controller  137  is further connected to mains apparatus  136 . 
     In operation the mains apparatus  136  will also supply current to plasma torch  11 ′ once an arc has been ignited between anode  15 ′ and cathode  19 ′, with the electric circuit for the mains apparatus being closed through cathode  19 ′ of the plasma torch, the plasma and the subject  138  as well as the connecting lines  133 ′,  133 ,  134 ′. 
     As soon as the arc in the plasma torch  11 ′ goes out because of the drop of the voltage of the capacitor battery  130  below the arc drop voltage, the electric circuit for the mains apparatus  136  is also interrupted, as its output voltage is not sufficient to maintain an arc between the cathode and the subject  138 . 
     A pulse plasma is also used in a voltage supply pursuant to FIG.  7 .