Abstract:
Method of producing superconducting cables by using cold plastic deformation operations only including the step of obtaining a bar-like semi-finished product of prefixed dimension through the steps of: forming round-section, mono- or multifilament, superconducting copper bars of relatively long length; assembling the bars about a cylindrical copper core of substantially the same length, using assembly templates, the templates having through holes arranged in a circle to support the bars, and a central through seat for supporting the core; tying the bars onto an outer lateral surface of the core; sliding onto one end of the bar/core assembly a number of metal supporting rings, while sliding the templates off the opposite end thereof; sliding a copper tube onto the bar/core assembly while cutting the ties in axial sequence and sliding off the supporting rings; and subjecting the assembly to a number of drawing operations.

Description:
The present invention relates to a cold composition method for obtaining a bar-like semifinished product from which to produce superconducting cables, particularly of niobium-titanium (hereinafter indicated “NbTi”). 
   The invention also relates to a superconducting cable produced from such a bar-like semifinished product. 
   BACKGROUND OF THE INVENTION 
   At present, superconducting, in particular NbTi, cables are produced from an assembly comprising a cup-shaped copper ingot, into which are inserted, in orderly manner, the ends of bars having a core of superconducting material, defined by one or a number of wires of NbTi, and a sheath of copper (and/or other noble metal). The bars are short (at most about 800 mm long), and are hexagonal in cross section to “fit” easily inside the copper ingot. The free end of the ingot is then sealed by welding on a copper cap, a vacuum is formed inside the assembly so formed, and it is subjected to one or more hot extrusion steps (at temperatures of around 500° C.) to reduce it to the size of a 60–80 mm diameter bar (of over 10 m in length). At this point, possibly after being heat treated, the bar-like semifinished product is cold drawn gradually to form a superconducting cable. 
   A major drawback of superconducting, particularly NbTi, cables produced as described above lies in their having a fairly low critical current (Jc) with respect to the capacity of the alloy. 
   The Applicant&#39;s technicians, however, have found that eliminating any hot extrusion from the processing cycle of NbTi superconductors increases critical current (Jc) by over 25% for a given chemical composition of the superconductor and for given alpha values (Cu to NbTi volume ratio of the cable). 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention to provide a method of producing superconducting, in particular NbTi, cables, which provides for increasing the critical current of the superconducting cable. 
   It is a further object of the present invention to provide a cold composition method for obtaining a bar-like semifinished product from which to produce superconducting cables, and which is cheap to implement, and provides for simpler, faster processing, while at the same time ensuring a high quality standard. 
   According to the present invention, there is provided a method of producing superconducting, in particular NbTi, cables from mono- or multifilament superconducting bars, characterized by comprising exclusively cold plastic deformation steps. 
   More specifically, the invention also relates to a “cold” composition method, i.e. employing exclusively cold plastic deformation operations, for obtaining a bar-like semifinished product, and comprising the steps of: forming round-section, mono- or multifilament, superconducting copper bars of relatively long length; assembling said bars about a cylindrical copper core of substantially the same length, using assembly templates which open book-fashion and are fitted to and slide along an assembly bench, the templates having through holes arranged in a circle to support the bars, and a central through seat for supporting the core; tying the bars onto an outer lateral surface of the core; sliding onto one end of the assembly so formed a number of metal supporting rings resting on the assembly bench, while sliding said templates off the opposite end of the assembly; sliding a copper tube onto the assembly so formed, while at the same time cutting the ties in axial sequence and sliding off said supporting rings; and performing a number of drawing operations on the finished assembly to gradually reduce the cross section and increase the length of the assembly to obtain a bar-like semifinished product of the required dimensions, from which, after salt bath heat treatment, a superconducting cable is obtained by cold drawing. 
   By eliminating any hot extrusion, the superconducting cable obtained by cold drawing the bar-like semifinished product described has 25% higher critical currents than the same type of superconductor, in which the starting bar is composed in conventional manner, i.e. assembled in the form of an ingot and then compacted and hot extruded. 
   Moreover, the composition method described starts from round-section superconducting bars, which are easier to make, and which are already of considerable length (about 5 m), so that the bar-like semifinished product (about 14 m long) is obtained faster and more cheaply. 
   Finally, the bar-like semifinished product produced according to the invention by exclusively cold processing (i.e. at substantially ambient temperature) can be substituted for currently used semifinished extruded bars on conventional drawing systems, with no alterations required to the finished superconductor cable production systems. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A non-limiting embodiment of the invention will be described by way of example with reference to the accompanying drawings, in which: 
       FIG. 1  shows a flow chart of the complete processing cycle for producing superconducting cables using the method according to the present invention; 
       FIG. 2  shows a schematic view in perspective of one step in the cold composition method according to the present invention, and of part of the special equipment employed; 
       FIG. 3  shows a schematic view of a further step in the  FIG. 2  method, and a side view of a resulting assembly; 
       FIG. 4  shows a schematic view of a further step in the  FIG. 2  method, and of the rest of the special equipment employed; 
       FIG. 5  shows a detail of a final step in the composition method according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With reference to the accompanying drawings, number  1  ( FIG. 5 ) indicates as a whole a bar-like semifinished product produced by cold composition, according to one aspect of the invention, and which can be used in a method of producing superconducting cables, characterized, according to a further aspect of the invention, in that any plastic deformation step in the method is performed exclusively cold, i.e. at ambient temperature, as shown in the  FIG. 1  flow chart. 
   The invention starts with bars  2  formed in known manner and each comprising a core defined by a mono- or multifilament of superconducting material, in particular NbTi alloy, and a copper sheath, possibly with a barrier layer of noble metal or metal alloy interposed between the superconductor and copper. In the case of multifilament bars, the core comprises 121 monofilaments of superconducting alloy. 
   Though, here and hereinafter, specific reference is made to NbTi superconductors and copper, the object of the invention is clearly not limited to these materials, but also extends to the use of other materials, in which the superconducting material has critical current values also depending to some extent on the degree of work hardening of the material, and in which copper is replaced by another metal or metal alloy of substantially equivalent performance in terms of operation of the superconducting cable obtained according to the invention. 
   Monofilament bars  2  preferably have an alpha (copper to superconductor volume) ratio of about 0.5, and multifilament bars  2  a higher alpha ratio which may be around 2. 
   In accordance with known technology, bars  2  must be assembled inside a copper shell, and the resulting assembly is subjected to a number of successive plastic deformation steps until the required size cable is obtained. 
   According to a first characteristic of the invention, as opposed to a hexagonal cross section, bars  2  have a round cross section, e.g. of roughly 4 mm diameter for final monofilaments of about 50 micron diameter, or final multifilaments of 4 micron diameter or over. 
   According to the invention, bars  2  are assembled inside a copper shell using a bar-like semifinished product composition method, the main steps of which are shown in detail in  FIGS. 2 ,  3  and  4 , the rest of which is shown schematically in  FIG. 1 , and which forms part of said semiconductor cable production method according to the invention, characterized by comprising exclusively cold plastic deformation steps. 
   The composition method according to the invention employs bars  2  having the characteristics described above, and further characterized by being relatively long (axially), e.g. about 5 m, i.e. at least 7–8 times the length of conventional hexagonal-section superconducting bars used in known methods. 
   Once formed, bars  2  are subjected to conventional chemical treatment comprising successive immersion in various degreasing and pickling baths, and drying. Dealing with such long bars, however, the bars are treated in bundles, which are inserted into a powered open-sided rotary basket designed to support the full length of bars  2 , are treated chemically by immersing the rotary basket in the baths, and are dried by inserting the whole rotary basket inside a drying furnace. 
   The rotary basket—not shown for the sake of simplicity—is defined, for example, by a metal supporting structure, which hooks onto a lifting device and supports the motor high up so that it remains dry even when the supporting structure is immersed in the treatment baths; and by a number of rotary disks fitted idly to the metal supporting structure, connected angularly to one another, and each supporting a number of circumferentially oriented rollers (e.g. three, 120° apart) for supporting the bars. By means of the motor, a mechanical transmission rotates the disks and the rollers, which constitute the actual rotary basket, with respect to the metal supporting structure. 
   The treated bars  2  are then sent to an assembly bench  3  shown schematically in  FIGS. 2 and 4 . Similarly, a solid cylindrical copper core  20  of substantially the same length as bars  2  is also treated in the rotary basket described above, and then also sent to assembly bench  3 . 
   Bench  3  comprises a bed or actual bench  4  with straight, e.g. cylindrical, guides  5 , along which slide a number of assembly templates  6  (only one shown in  FIG. 2 ) which open book-fashion. Each template  6  comprises a bottom half-member  7  which engages and slides along guides  5 ; and a top half-member  8  located alongside and turned over 180° with respect to half-member  7 , the top half-member  8  being hinged either directly to corresponding half-member  7 , as shown schematically in  FIG. 2 , or, preferably, to a further guide (not shown) parallel to guides  5  and along which half-member  8  also slides. 
   Half-members  7  and  8  are saddle-shaped so as to define, when half-member  8  is turned 180° over onto half-member  7 , a cylindrical through seat  9  through which core  20  is housed and supported, and so supported by templates  6  on bench  3  with its axis of symmetry parallel to guides  5 . Similarly, half-members  7 ,  8  have a number of through holes  10  arranged concentrically in a circle about central cylindrical seat  9  (when half-members  7 ,  8  are assembled one on top of the other to form template  6 ) and of such a diameter as to house bars  2 , one through each hole  10 , so that bars  2  are also supported by templates  6  in a circle about core  20  on bench  3  and parallel to guides  5 . 
   Holes  10  may be arranged in one circle about seat  9 , or, as in the example shown, in two concentric circles, the radially inner one indicated  11 , and the radially outer one indicated  12 . 
   To begin with, a number of bars  2  are inserted inside holes  10  in the outer circle  12 , with templates  6  still open, and with half-members  7 ,  8  side by side and turned over with respect to one another; the remaining bars  2  are then inserted inside holes  10  in the inner circle  11 ; and, finally, core  20  is placed on half-members  7 , and half-members  8  (with the bars inserted inside them) are turned over in the direction of the arrow ( FIG. 2 ) to close, and so grip core  20  inside, templates  6 . 
   Bars  2  are thus supported in orderly manner about core  20 , at which point, the bars are bound onto an outer lateral surface  21  of the core by means of ties  30  ( FIG. 3 ), e.g. tied manually and made of copper wire. Ties  30  are tied successively, one at a time, working gradually along guides  5 , from one end  32  ( FIG. 4 ) to the opposite end  33  of bench  3 . 
   At the same time, still starting from end  32 , a number of rings  34 , e.g. of copper, are fitted to bench  3 , so as to rest on and slide along guides  5 , and so as to enclose core  20  and bars  2 , with bars  2  contacting surface  21 . The above operations are obviously performed some distance from each template  6  to allow bars  2  to flex and contact surface  21 . More specifically, starting with a first tie, as the first ring  34  is assembled at end  32 , templates  6 , still in the closed position, are slid back gradually towards end  33  along guides  5 . A second ring  34  is then assembled, which therefore takes over from the slid-back templates  6  in supporting core  20  and bars  2  tied to core  20  by ties  30 , while the first ring  34  is slid further along guides  5 , and further ties  30  made. 
   At the end of the above steps, an assembly  40  is obtained, supported on bench  3  by rings  34  and defined by bars  2  assembled in a circle against core  20  and retained by ties  30 , while templates  6  are by now all released and moved to end  33 , where they are gradually slid off guides  5  as they are released from bars  2  and core  20 . 
   In other words, a step is performed in which rings  34  are slid onto a first end  41  of assembly  40  adjacent to end  32  of bench  3 , while templates  6  are slid off a second end  42  of assembly  40  (shown partly in cutaway section in  FIG. 3 ) opposite the first end and therefore adjacent to end  33  of bench  3 . 
   At this point, a copper tube  50  ( FIG. 4 ) is slid onto assembly  40 , starting from first end  41  of assembly  40 ; and, at the same time, ties  30  are gradually cut as they are reached by tube  50 , and supporting rings  34  are gradually slid off second end  42  of assembly  40 , to eventually obtain an assembly  40 /copper tube  50  assembly, in which bars  2  are held in position against copper core  20  solely by copper tube  50  fitted concentrically and coaxially with core  20 . 
   The above step is performed with the aid of two devices  52  and  54  ( FIG. 4 ) fitted to bench  3  at ends  32  and  33  respectively. 
   More specifically, device  52  is a so-called “pinch-roll” device fitted removably to end  32  of bench  3  (e.g. so that it can be moved aside), and which comprises two rollers  55 ,  56  mounted parallel with an adjustable centre distance. Rollers  55 ,  56  are pressed against each other by compression means  57  defined, for example, by a hydraulic or pneumatic cylinder, pinch tube  50  between them as shown in  FIG. 4 , and at least one of which (roller  56  in the example shown) is rotated by a motor. 
   Device  54  is defined by a counter-head movable axially (e.g. along guides  5 ) towards “pinch-roll” device  52 , and comprising a counter-plate  60 , and a hydraulic or pneumatic cylinder  61  acting parallel to guides  5 . 
   During said step, copper tube  50  is slid onto assembly  40  by “pinch-roll” device  52  at first end  41  of assembly  40 , by virtue of the axial thrust exerted frictionally by powered roller  56  on tube  50 ; and, at the same time, assembly  40  is held resting axially against counter-head  54  by counter-plate  60 , and the sliding movement of counter-head  54  along guides  5  is prevented at this step by stops or brakes not shown for the sake of simplicity. 
   The final stage in the fitting of copper tube  50  onto assembly  40  (i.e. when tube  50  is almost entirely fitted onto assembly  40 , as shown in  FIG. 4 ) is performed by stopping rollers  55 ,  56  to arrest copper tube  50 , and by moving counter-head  54  axially forward in the example shown, by moving counter-plate  60  forward by means of cylinder  61 —so as, this time, to insert assembly  40  inside tube  50  as opposed to vice versa. 
   As it is being fitted onto assembly  40 , tube  50  comes into contact with the ring  34  closest to end  41  and pushes it towards end  42  and into contact with the next ring  34 , and so on until rings  34  are all pushed gradually towards end  42  as copper tube  50  is fitted gradually onto assembly  40 . To improve this step and also hold assembly  40  together when ties  30  are removed, rings  34 , according to the invention, have substantially the same radial dimensions (inside and outside diameter) as copper tube  50 . 
   At this point, the tube  50 /assembly  40  assembly is removed from bench  3  and, according to the invention, undergoes a number of cold drawing operations to gradually reduce its cross section and so increase its length to eventually obtain a bar-like semifinished product  1  of the required dimensions. 
   According to a further aspect of the invention, after being cold drawn, bar-like semifinished product  1  is salt bath heat treated, but is first closed in substantially fluidtight manner at opposite ends by caps  70  (only one of which is shown in  FIG. 5 ). 
   Caps  70  are cup-shaped to fit on the opposite ends of bar-like semifinished product  1 , and are made of material having a lower thermal expansion coefficient than copper, e.g. iron, so as to be self-sealing. When heated, to perform the heat treatment, in fact, the iron expands less than the copper, thus resulting in a perfectly fluidtight, forced interference fit of caps  70  to bar-like semifinished product  1 . 
   More specifically, the assembly  40 /copper tube  50  assembly undergoes a first drawing step to achieve a relatively small reduction in section ranging between 4% and 9%, and so lock copper core  20 , copper tube  50 , and bars  2  mechanically integral with one another; and then a number of successive drawing steps, each resulting in a constant reduction in section, until the required dimensions are obtained. 
   Each successive drawing step is performed to reduce the section of the assembly  40 /copper tube  50  assembly by approximately 18% to 24%. 
   Before being drawn, the opposite ends of the assembly  40 /copper tube  50  assembly may be airtight sealed, e.g. by disposable polyamide or polyethylene seals, to protect the parts against oxidation. Unlike conventional assemblies, however, a vacuum is no longer required, in that, during drawing, the air inside tube  50  or between core  20  and bars  2  can escape from the ends of tube  50 , the seals at this stage being destroyed or removed. In any event, at the first drawing step, tube  50  has been found to undergo a greater increase in length than core  20  and bars  2 , thus forming “compensating” chambers for receiving the air as it escapes from the components. 
   At the end of the steps described, a 10–14 m long, 60–80 mm diameter bar-like semifinished product  1  is obtained, which, following salt bath heat treatment, can be subjected to a conventional cold processing cycle of successive drawing operations to obtain a superconducting cable of the required dimensions. 
   The superconducting cable, however, is characterized by a relatively high critical current (Jc), normally at least 20% higher than that of superconducting cables of the same section and chemical composition, but formed from hot extruded semifinished products. 
   Using the composition method described, the products (bar-like semifinished products  1 ) have a roughly 30% higher quality index value “n” with respect to extrusions, and axially constant alpha values, i.e. with none of the roughly 20% variations at the ends typical of extruded semifinished bars, thus eliminating wastage.