Abstract:
A method and apparatus for inducing a physical change in at least one metal workpiece are provided. Energy is applied to at least a portion of the metal workpiece, the energy being a combination of energies of at least two distinct sources. One of these energies is a pulsed magnetic force (PMF) energy which induces a rapid movement in a portion of the metal workpiece. The second energy is one which acts synergistically with the first energy to impart the desired physical change.

Description:
The present application is the national stage under 35 U.S.C. 371 of PCT/IL99/00322, filed Jun. 14, 1999. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to methods and apparatus for inducing physical changes in metal objects in general and making use of pulsed magnetic force (PMF) energy therefor, in particular. 
     BACKGROUND OF THE INVENTION 
     The employment of PMF, for forming, joining or welding of metal objects is known in the art. In this process, current is rapidly discharged through a coil, which is proximal to the workpiece. Owing to eddy currents in the workpiece, an intense magnetic pressure builds up, which may perform work on at least a portion of the workpiece. Typically, the portion of the workpiece, which is proximal to the working coil of the PMF apparatus, is induced into a rapid movement and the kinetic energy associated with this movement gives rise to said physical change. The change may be forming namely a change in the shape of the object. At times, the rapid movement of said portion is induced only on a microscopic scale. For example, in the case of joining or welding two metal workpieces, portions thereof are placed into close proximity and they in fact touch one another. But nevertheless notwithstanding this close proximity some clearance between the two remains even if only on a microscopic scale which permits a short rapid movement of a portion of at least one of the workpieces (obviously over only a very short distance). This suffices for the build-up of kinetic energy, which dissipates upon impact with a portion of the other workpiece and may give rise to joining or welding of the two workpieces to one another. 
     GENERAL DESCRIPTION OF THE INVENTION 
     The present invention is directed to a method and apparatus for working, i.e. inducing physical change in at least one metal workpiece. The term “physical change” denotes structural change in form or shape, cutting off of a portion of a workpiece, perforation of a workpiece, joining of two or more workpieces to one another or welding two workpieces or portions thereof to one another. The term “joining” used before and further below means to denote a tight engagement of two workpieces to one another, for example, tight fitting of a generally cylindrical object over a tubular object fitted within it; and the term “welding”, in distinction from joining, means to denote a very tight interaction of the surfaces of two objects, e.g. metalurgical bonding of at least a portion of the two workpieces. 
     The method and apparatus of the invention make use of pulsed magnetic force (PMF) energy. In accordance with the invention, PMF energy is combined with another energy from a separate energy source which acts synergistically with the PMF energy to impart a physical change. The other energy source (to be referred to herein at times as the “auxiliary energy source”) may be another PMF energy, may be a mechanical energy source, and many others. 
     In accordance with a first aspect of the invention there is thus provided a method for inducing a physical change in at least one metal workpiece, comprising: 
     (a) transferring at least one pulse magnetic force (PMF) energy to at least a portion of the at least one workpiece for inducing an intense magnetic pressure therein; and 
     (b) transferring an auxiliary energy at least partially co-extensive with the PMF energy to the at least a portion of the workpiece whereupon the PMF and auxiliary energies combine to yield the physical change. 
     In accordance with another aspect of the invention there is provided an apparatus for inducing a physical change in at least one metal workpiece, comprising: 
     (i) at least one PMF unit with a forming coil for the discharge of current therethrough to induce an intense magnetic pressure in at least a portion of the metal workpiece; and 
     (ii) at least one auxiliary energy source for transferring, at least partially co-extensive with the current discharge through the forming coil, an auxiliary energy to the at least a portion of the workpiece such that said pressure and auxiliary energy synergize to yield said physical change. 
     The term “transferring energy” or any similar term which may be used herein, means to denote the transfer of energy to a workpiece in a form which at least partially acts to yield said physical change. The term “metal workpiece” refers to any metal object which is to be worked by the method or apparatus of the invention, which may be a cylindrical object which is to be joined or welded to a tubular or another cylindrical objection; a metal plate which is to be formed, cut or perforated; etc. As will be appreciated, the invention is not limited to a workpiece of any specific kind but rather can be applied for a myriad of different workpieces and for inducing a wide variety of different physical changes. 
     The term “combination of energies” or the like means to denote the timed activation of the at least two energy sources to transfer their respective energies to the at least one workpiece such that the periods of their transfer at least partially overlap, whereby the combined effect of the two energies, as far as inducing the physical change, is substantially larger than the physical change which may be imparted by only one of the energy sources. The combination of energies in accordance with the invention gives rise to a variety of improvements over the use of PMF alone. For example, in the case of welding or joining of two metal workpieces to one another, the same result may be achieved with a lower PMF energy as compared to PMF alone. This may have design consequences as the generation of a large PMF necessitates large capacitor banks and thus an apparatus in accordance with the invention may have an overall smaller size. 
     Additionally, the present invention allows the achievements of results, which may be difficult to achieve in accordance with the prior art. For example, in the case of welding two tubular workpieces to one another, with one enveloping the other, the strong impact of the external one onto the internal one, may give rise to squeezing or crushing of the internal tubular workpiece. However, in accordance with the invention, which permits use of smaller PMF pulses for achieving the same effect, such an undesired effect may be eliminated or reduced. In the case of welding, the combination of energies in accordance with the invention may give rise to an increase of the area of the weld. 
     At least for the case of welding, it is preferred that the time period overlap be substantial, meaning that at least one of the energies is applied such that throughout most of its period of application, it overlaps the other applied energy. This may mean, for example, that both may begin and end simultaneously, both may begin simultaneously and end in succession, both may begin in succession but end simultaneously, one beginning and ending during the period the other energy is applied, and various combinations of these scenarios. 
     While one energy source is always a PMF energy source from a PMF device, to be referred to herein at times as “primary PMF energy source”/“primary PMF device”, the other energy source may be selected from a wide variety of different sources. In accordance with one preferred embodiment, the other energy source is an auxiliary PMF energy source of a different specification. In accordance with other embodiments, the other energy source is a device for generating a mechanical shock wave or mechanical vibrations in the workpiece; a device which can generate ultrasonic vibrations within the object, e.g. a device as customary used in ultrasonic welding; a device for transferring an electric current through an interface between portions of workpieces which are to be welded to one another, such as that used in resistant welding; etc. 
     It should be noted that the term “primary” used above and further below, does not mean to denote that this energy source is of prime importance as compared to other sources of energy used in accordance with the invention. Similarly, the term “auxiliary” should not be understood as meaning secondary in importance. Rather, these terms are used merely for the sake of convenience. It should be appreciated that in order to yield any efficient physical change in accordance with the invention is achieved by the combination of the primary and the auxiliary energy sources, as mentioned above and further below. 
     The primary and the auxiliary energies which are used in combination in accordance with the invention, are superimposed, namely they are generated such that there are applied for periods of time overlapping one another over substantial portions thereof. In accordance with one embodiment, the primary PMF energy is generated simultaneously with the auxiliary energy. In accordance with another embodiment, the auxiliary energy is initiated prior or after the primary PMF energy. 
     At times, the total amount of the transferred primary energy may be larger than the total amount of the transferred auxiliary energy, at times they may be about the same or at time the total of the auxiliary energy may exceed the total of the primary energy. 
     In accordance with one embodiment, the auxiliary energy is PMF energy. The apparatus in accordance with this embodiment comprises two discharge circuitries, one for discharging a primary electric current through the primary working coil, and another for discharging an auxiliary electric current through an auxiliary working coil, which may be the same or different than the primary working coil. The auxiliary electric current has a frequency which is substantially different (larger or smaller) than that of the primary electric current, typically larger, e.g. 5-100 times larger. For example, the primary pulse may have a period of 100-200 sec (a frequency of 10,000-5,000 Hz, respectively) while the period of the auxiliary pulse may have a period of about 6-10 sec. The auxiliary pulse in such a case is typically initiated after a certain period of time from the initiation of the primary pulse, e.g. after about 20-40 sec, respectively (when the primary pulse is close to its pick). The auxiliary pulse is timed so as to synergize with the primary pulse to yield said physical effect. 
     The amplitude of the primary current may be larger than the primary current, e.g. 2-50 times that of the primary current, or at times may be about the same. 
     By another embodiment, said auxiliary energy is a mechanical energy imparted by inducing rapid movement in at least one of the two metal objects. Such movement may be a mechanical shock or vibrations. In accordance with one embodiment, the mechanical energy is imparted onto the at least one metal object by means of a mechanical waveguide. The waveguide may be associated with an auxiliary working coil connected to a current discharge circuitry which, by means of a PMF, generates mechanical vibration waves in said waveguide which are then transmitted to the metal object. Alternatively, the waveguide may be associated with a discharge-in-fluid (DIF) device, which generates a shock wave, which is then transmitted by the waveguide to at least one of the objects. The DIF device comprises a fluid chamber with discharge electrodes provided within the chamber for discharging an electric current between them and through the fluid. When an electric current is discharged between the electrodes, a plasma forms within the fluid which generates a shock wave within the fluid which is then transmitted to the waveguide. The fluid is typically an aqueous solution. 
     In accordance with another embodiment, the mechanical energy is an ultrasonic energy. 
     While the present invention is applicable in general for yielding a more efficient physical change of at least one workpiece, it is particularly applicable to welding of two metal workpieces to one another (“the welding embodiment”). The welding embodiment is a preferred embodiment of the present invention. In accordance with this preferred, welding embodiment, there is provided a method for welding two metal objects to one another, comprising: 
     (a) during a first period of time, generating a primary pulsed magnetic force (PMF) energy by discharging a primary electric current through a primary working coil so as to induce an intense magnetic pressure on at least one portion of a first of the two metal objects to cause said portion to move towards an impact at least one other portion of the second metal workpiece; 
     (b) during a second period of time, transferring to at least one of the two metal objects, an auxiliary energy which is other than said primary PMF energy, said first time period and said second time period overlap one another over at least a substantial portion of one of said first or said second time periods, whereby said auxiliary energy in combination with the PMF energy causes welding of the at least two portions to one another. 
     In accordance with the welding embodiment there is further provided an apparatus for welding of two metal objects to one another comprising: 
     a primary electric discharge circuitry with a primary working coil for generating, during a first period of time of a working cycle of the apparatus, a pulsed magnetic force (PMF) to cause at least a portion of a first of the two metal objects to move towards an impact of this said portion of a second of the two metal objects; and 
     a device for generating, during a second period of time of the working cycle of the apparatus, an auxiliary energy and transferring it to at least one of the two metal objects which said first period of time and said second period of time overlap one another over at least a substantial portion of one of said first or said second period of time, whereby said auxiliary energy in combination with the primary PMF energy, causes welding of the two portions to one another. 
     For welding, a combination of PMF energy with any of the energies discussed above may be applicable. In addition, in the case of welding, the auxiliary energy may also be an electric current induced to pass through the interface between portions of the two metal workpieces, which are to be welded to one another. This embodiment may make use of electrodes of the kind customarily used in resistant welding, which are generally known per se. 
     The auxiliary energy, in accordance with another embodiment, particularly applicable to the case of welding, but may also be applicable to other embodiments dealing with different kinds of physical changes, mentioned above, is an auxiliary energy source which causes heating of the at least a portion of the metal workpiece. Such heating may be achieved by an ultrasound energy source, generally known per se, may be achieved by induction heating which is also generally known per se, and by a variety of other heating means known per se. In this case, after the workpiece has been heated, the effect of the PMF may be more pronounced than without heating. 
     As may be appreciated, the auxiliary energy source used in the method and apparatus of the invention may be a combination of the auxiliary energy sources discussed above. Thus, in such a case the energy applied to the metal workpiece is a combination of three or more superimposed energies. For example, a combination of induction heating and an auxiliary PMF. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to understand the invention and to see how it may be carried out in practice, the invention will now be described, by way of non-limiting example only in the following detailed description, with reference to the accompanying drawings, in which: 
     FIG. 1 is a scheme illustrating an embodiment of an apparatus of the invention for discharging a primary and auxiliary current through a working coil to yield a superimposed PMF for welding of two metal objects one to another. 
     FIGS. 2A-2C show three different embodiments on the manner of superimposing a primary electric current and an auxiliary electric current to yield a superimposed PMF current. 
     FIGS. 3 and 4 show schemes of two other embodiments of an apparatus combining a primary and an auxiliary discharge currents to yield a super positioned PMF forming current. 
     FIG. 5 shows another embodiment of an apparatus in accordance with the invention, which is a modification of that shown in FIG. 1, wherein the auxiliary current is generated by a high frequency generator connected to the working coil through a high voltage breaker. 
     FIG. 6 is a scheme of an apparatus in accordance with another embodiment of the invention wherein the auxiliary energy is provided by means of a mechanical waveguide with an associated working coil. 
     FIG. 7 illustrates an embodiment of the implementation of an apparatus of the kind shown in FIG. 5 for welding two metal tubes to one another. 
     FIGS. 8 and 9 show schemes of two embodiments of apparatuses in accordance with the invention wherein the auxiliary energy is mechanical energy transmitted to the welded object through a waveguide associated with a working coil. 
     FIG. 10 shows a scheme of an apparatus in accordance with another embodiment of the invention, which in this specific case is fashioned for welding together two metal tubes, wherein the auxiliary energy is a mechanical shock energy generated by a DIF device transmitted to one of the tubes through a mechanical waveguide. 
     FIG. 11 shows a scheme of another apparatus in accordance with the invention wherein the auxiliary energy is an ultrasonic energy. 
     FIG. 12 shows another embodiment of an apparatus in accordance with the invention wherein the auxiliary energy is a resistance heating energy caused by current generated between two current transmitting electrodes. 
     FIG. 13A shows, in cross section, an apparatus in accordance with another embodiment of the invention for simultaneous forming, cutting and perforation of a metal plate. 
     FIG. 13B shows the mold of the apparatus of FIG. 13A with a physically changed metal plate. 
     FIG. 13C shows the physically changed metal plate after working by the apparatus of FIG.  13 A. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference is first being made to FIG. 1, showing a scheme of an apparatus  20  in accordance with an embodiment of the invention comrprising a working coil  22  and two discharge circuitries  24  and  26 . Discharge circuitry  24  consists of a power supply  30 , a capacitor battery  32 , which may consist of a single or a plurality of capacitors and a high current switch  34 . Similarly, discharge circuitry  26  consists of a power supply  36 , capacitor battery  38  and a switch  40 . Switches  34  and  40  are controlled by means of an ignition circuitry  42  which provides a trigger to these switches. Switches  34  and  40  may be any one of a variety of high current switches known per se, such as a controlled vacuum discharger of the kind disclosed in PCT Application No. PCT/IL 97/00383. 
     Circuitry  24  is designated here as the primary circuitry and circuitry  26  as the auxiliary circuitry. In this specific embodiment, as can be seen, both the primary discharge current of circuitry  34  and the auxiliary discharge current of circuitry  26  are discharged through the single working coil  22 . 
     FIGS. 2A-2C show several different combinations of a primary and auxiliary discharge currents to yield a combined, superimposed PMF-generating current The primary discharged current, which is represented by the upper curve of FIGS. 2A-2C, has typically an amplitude of about 10-200 kA, typically 100 kA and an initial oscillation frequency of about 10-100 KHz. The auxiliary discharged current, illustrated as the middle curve in each of FIGS. 2A-2C has typically a frequency of about 50-1000 KHz and an amplitude of about 1-10 kA. The two different current may be discharged simultaneously (FIG.  2 A); or the auxiliary current may be discharged after (FIG. 2B) or prior (FIG. 2C) the primary current. The superimposed current is illustrated as the lower curve in each of FIGS. 2A-2C. It has been realized in accordance with the invention that such a superimposed PMF current gives rise to efficient welding, without the need to substantially increase the current intensity and fine tune the current discharge parameters, as was needed in the prior art PMF processes. 
     Superpositioning of the energies from two different sources may be performed in a similar manner also in the embodiments illustrated below in FIGS. 3-13, namely both sources may be activated simultaneously or one with a delay after the other. 
     FIGS. 3 and 4 show a scheme of apparatuses  50  and  60  respectively, in accordance with two other embodiments of the invention. In FIGS. 3 and 4, like reference numerals to those of FIG. 1 were used to show like elements. The apparatus  50  of FIG. 3 differs from that of FIG. 1, in that coil  52  which is connected to both circuitries, is in inductive relationship with forming coil  54 , in this specific embodiment a single wind coil. In the case of apparatus  60  of FIG. 4, the primary discharge circuitry  24  and the auxiliary discharge circuitry  26  are independent and are provided with coil  62  and  64  respectively, which are in inductive association with forming coil  66 . 
     An apparatus  70  in accordance with another embodiment of the invention is shown in FIG.  5 . Here again, like elements to those of FIG. 1 have been given like reference numerals. In apparatus  70 , discharge circuitry  26  is provided with a high frequency generator  72 , typically capable of generating current at a frequency of about 100-1000 KHz, which is connected to coil  22  through high voltage breaker  74 , e.g. a Fe-controlled vacuum switch. 
     The forming coils  22  of FIGS. 1 and 5,  54  of FIG. 3,  66  of FIG. 4 and 90 of FIG. 6, may have a design of a forming coil as disclosed in PCT Application, Publication No. WO 97/22426 and PCT Application, Publication No. WO 98/23400. However, as will no doubt be appreciated, the invention is not limited to these types of coils. The type of coil and its design will obviously depend on the type of workpiece to be worked: the coil may have a ring structure or be cylindrical in the case of making a cylindrical object, may be planar for working a metal plate, etc. Furthermore, the design of the coil will also depend on the result to be achieved, namely whether the physical change intended is forming, cutting, perforation, joining or welding. 
     An apparatus  80  in accordance with another embodiment of the invention is seen in FIG.  6 . The apparatus comprises a power supply  82 , a capacitor battery  84 , switch  86 , triggering circuitry  88 , a primary working coil  90  and an auxiliary working coil  92 , associated with a mechanical waveguide  94 . Upon trigger from triggering circuitry  88 , electric energy previously charged into capacitor  84  by power supply  82 , discharges through coils  90  and  92 . Coil  90  induces high velocity movement in at least a portion of one of the two objects to be welded whereas coil  92  generates vibrations in waveguide  94  which are transmitted therethrough to at least one of the two metal objects. 
     An illustration of the arrangement of the two coils and the waveguide in an embodiment of the invention for welding together two tubes, is seen in FIG.  7 . In FIG. 7, corresponding elements to those seen in FIG. 6, have been given like reference numerals. In this case, a current is discharged simultaneously through coils  90  and  92 , whereby coil  92  generates vibrations in waveguide  94 , illustrated by arrow  96  and these vibrations then travel into metal tube  98  as illustrated by arrows  100 . At the same time, coil  90  causes portion  102  of metal tube  98  to move towards and impact portion  104  of metal tube  106  as illustrated by arrows  108 . Typically the discharging current will have an initial frequency of about 10-100 KHz. These combined mechanical forces facilitate welding of the two tubes to one another. 
     A scheme of two apparatuses  110  and  120  in accordance with two other embodiments of the invention are shown in FIGS. 8 and 9, respectively. These embodiments, similar to that shown in FIG. 6, also comprise a primary working coil  90  and an auxiliary working coil  92 , with the latter being associated with a mechanical waveguide  94 , (like reference numerals to those used in FIG. 6 have been used here for like elements). The difference between apparatus  110  to apparatus  80  of FIG. 6 is in that in the former, coils  90  and coil  92  are connected in parallel and as a result, whereas in the case of apparatus  80  the same current flow discharges in both coils, the current in the case of apparatus  110  is divided between the two coils, in an inverse proportion to the respective impedances of coils  90  and  92 . 
     In the case of apparatus  120 , the primary coil  90  and the auxiliary, waveguide-associated coil  92 , are included in independent circuitries  122 , 124  provided with respective power supplies  125 ,  126 , capacitor batteries  127 ,  128  and switches  129 ,  130 , controlled by means of discharge control circuitry  132 . 
     Here again the structure or design of the primary coil depends on the discreet result and the type of metal workpiece to be worked and may be, but not limited to a coil of a kind disclosed in WO 97/22426 and WO 98/23400, already mentioned above. 
     An apparatus  140  in accordance with another embodiment of the invention can be seen in FIG. 10, in this specific example, fashioned so as to be suitable particularly for welding together two tubes  142  and  144 . Apparatus  140  comprises two discharge circuitries, a primary discharge circuitry  146  and an auxiliary discharge circuitry  148 . Primary discharge circuitry  146  comprises a coil  150 , a capacitor battery  152 , a switch  154  and a power supply  156 . Discharge circuitry  148  comprises a DIF device  160 , a capacitor battery  162 , a power supply  164  and a switch  166 . Switches  154  and  166  are controlled by circuitry  168 . 
     DIF device  160  comprises a chamber  170  defined between rigid wall portions  172  and an elastic wall  174  and accommodating a fluid, which may be a gas or a liquid, and is typically an aqueous solution. A plurality of pairs of electrodes  176  are provided and upon closing of switch  166 , an electric current is discharged between the electrodes (represented by arrow  178 ). Such a discharge causes formation of plasma within the fluid which yields a shock wave travelling towards flexible wall  174  (represented by arrows  180 ). Wall  174  is in contact with mechanical waveguide  182  and the shock waves then travels through the waveguide (represented by arrows  184 ) and condense to yield a higher amplitude shock wave at its tapered end (represented by arrow  186 ). 
     A scheme of an apparatus  200  in accordance with another embodiment of the invention is shown in FIG.  11 . The apparatus comprises a primary discharge circuitry  202  and an assembly  204 . Primary discharge circuitry  202  comprises a power supply  206 , a capacitor battery  208 , a switch  210  and a primary coil  212 . Assembly  204  comprises an ultrasound energy generating device  214 , a power generating device  216  and a switch  218 . By the use of this apparatus, the resulting physical change, preferably welding, is a combined result of an ultrasonic energy and PMF energy. 
     The switches  210  and  218  are controlled by means of discharge control circuitry  220 . 
     An apparatus  230  for welding in accordance with another embodiment of the invention can be seen in FIG.  12 . By the use of apparatus  230 , welding is a combined PMF and resistance welding process. Apparatus  230  comprises a primary discharge circuitry  232  and an assembly  234 . Primary discharge circuitry  232  comprises a coil  236 , a capacitory battery  238 , a switch  240  controlled by triggering circuitry  242  and a power supply  244 . Assembly  234  comprises a power supply  246  and a pair of resistance welding electrodes  248 . These electrodes pass current in the direction represented by arrow  250  and as a result of increased resistance at the interface  252  between the two metal objects  254 ,  256 , the interface  252  is heated. As known in the art, electrodes  248  are typically cooled by water circulation. In apparatus  230 , high welding is achieved by a combination of a PMF and resistance welding processes. 
     An apparatus generally designated  300  for working of a metal plate is shown in FIG.  13 A. The apparatus comprises a mold  302  with a central inverse dome-shaped recess  304 . Recess  304  is defined within an annular ridge  306  accommodating an annular groove  308 . Peripheral of ridge  306  is a shoulder portion  310  defined by an upright wall  312 . Defined at the bottom of recess  304  are vertical bores  316 . 
     The apparatus further comprises a planar forming coil  320  connected to a discharge system  322  and a coil-support member  324 . 
     Discharge system  322  is in principle similar to the circuitry seen in FIG.  1  and thus functionally like elements have been given like reference numerals and the reader is requested to the description of FIG. 1 for explanations of their function. 
     The apparatus holds a metal plate  330  which is to be formed, cut and perforated. 
     Upon current discharge, a superimposed PMF current is generated having, for example, have a shape of the kind shown in FIGS. 2A-2C. Consequently, a magnetic pressure is generated which induces rapid movement of portions of plate  330  in the direction represented by arrows  340  and  342  in FIG.  13 B. Consequently, the main central portion of the plate is reformed to yield a general shape defined by mold  302 . In addition, the upright walls  312  function essentially as a knife and consequently the plate is cut along a line defined by upright wall  312  yielding a shaped plate  330 A, seen separately in FIG. 13C, and a cut-off rim portion  330 B. 
     Similarly to upright wall  312  also the rims of bores  316  act as knives and once the shaped plate  330 A impacts the walls of recess  304 , a portion  348  corresponding in diameter to the diameter of bore  316  is being cut-off and continues to move rapidly through bore  316  as represented by arrows  350  in FIG.  13 B. The resulting plate  330 A is formed with a central recess  360 , a peripheral annular groove  362 , with a cut rim  364  and with perforations  366 .