US3120054A - Method of making tungsten coils - Google Patents

Description

United States Patent 3,120,054 METHOD OF MAKING TUNGSTEN COILS John ll. Fitzpatrick, Bloomfield, N.J., assignor to Radio Corporation of America, a corporation of Delaware N0 Drawing. Filed Jan. 21, 1960, Ser. No. 3,731 7 Claims. (Cl. 29-547) The present invention relates to a method of making coils of tungsten and particularly concerns a novel combination of steps for translating an ingot or slug of sintered tungsten into a relatively closely wound coil of fine tungsten wire wherein the wire is free from objectionable fissures or slivers.

Coils of tungsten wire find wide use in electron tubes. For example, some type of heaters for indirectly heated cathodes, and helices of traveling wave tubes, comprise coiled structures of relatively fine tungsten wire wherein the turns are relatively closely spaced.

Prior methods of fabricating coils of this type have involved first die-drawing a swage rod formed from an ingot or slug of sintered tungsten, to desired wire size, and then coiling the wire so produced to form a coiled structure. The drawing operation usually involves a plurality of drawing steps for reducing the thickness of the swage rod to the desired wire size, which may, for example, be seven mils. The ingot or slug referred to, is initially an elongated structure having a rectangular cross-section. Prior to drawing, the initial slug is processed in conventional manner to round off its corners to produce a swage rod of circular cross-section for convenient engagement by dies having circular openings.

During each drawing step, dies engage the surface first of the swage rod and then of the wire produced therefrom. The engagement is sliding in character and is effected with sufficient pressure to produce contraction in the transverse dimensions of the swage rod and wire. The tungsten crystals in the surface region of the wire engaged by the die, respond in elongation to the aforementioned sliding pressure engagement. Consequently, after repeated drawing steps, the resultant relatively fine wire will be characterized by a surface including appreciably elongated tungsten crystals.

So long as the wire is isolated from stress tending to bend the elongated crystals referred to, the crystals will remain in orientations determined by the drawing steps and will contribute to a definition of the normal contour of the wire. However, when the wire is bent, as by coiling, bending stresses are applied to the elongated surface crystals. Such bending stresses result in severe strain at bonded portions of the crystals, such as end portions of the crystals, thereby urging such end portions outwardly of the normal wire contour. Such strain is relieved when one end portion of a crystal subjected to such bending stress becomes displaced from the normal wire contour. With the strain thus relieved, the other end portion remains anchored in the wire.

The displacement of a portion of an elongated surface crystal from the normal wire contour is characterized by several serious disadvantages. The portions so displaced result in the formation of slivers or projections extending from the normal wire contour and which'in some cases are of such magnitude as to bridge the space between adjacent turns of a subsequently formed coil and thereby produce electrical shorts therebetween. Furthermore, the spaces from which portions of the elongated crystals have been displaced constitute depressions or fissures in the wire which involve appreciable reduction in diameter of portions of the wire. Where the wire serves as a carrier of electrical power such reduction in diameter may produce hot spots resulting in rupture of the Wire.

Accordingly, it is an object of the invention to provide an improved method of making a coil of tungsten.

3,120,054 Patented Feb. 4, 1964 Another purpose is to fabricate a coil of relatively fine tungsten wire wherein the turns of the coil are relatively closely spaced and wherein the wire of the coil is substantially free from slivers and fissures.

A further aim is to make a closely wound coil of relatively fine tungsten wire, having a relatively long life as a conductor of electrical power.

In accordance with one way of practicing the method of the invention, a relatively fine tungsten wire, which may be of the order of seven mils, is made by subjecting a swage rod of tungsten to a novel combination of drawing, etching and annealing steps, and winding the resultant Wire to coil shape without the evolution of objectionable slivers and fissures in the wire. As a consequence, the adjacent turns of the coil may be relatively closely spaced, without danger of electrical shorts between the turns, Furthermore, the absence of fissures in the wire avoids hot spots thereby contributing to long life of the coil as an electrical conductor.

The novel combination of drawing, etching and annealing steps referred to includes a first group of drawing steps to reduce the tungsten swage rod to an intermediate size. The tungsten swage rod to be drawn, constitutes a generally cylindrical body of sintered tungsten powder which may have a diameter of about 150 mils. The first group of drawing steps involves reduction of the swage rod to a wire form of say 17 mils thickness. It will be appreciated that the accomplishment of this reduction requires an appreciable working of the rod. Such working is effected by drawing the swage rod axially through a plurality of dies of successively reduced size, with appreciable force. The force exerted by the die on the swage rod is both axial and transverse of the rod. As a consequence, both surface and subsurface crystals in the rod are flattened and elongated axially of the rod. In the example under consideration, it is estimated that the length of the elongated crystals is about mils, and their thickness is approximately five microns.

Some of the crystals so elongated are firmly anchored throughout their length to the wire body so that they are effectively restrained against displacement from the normal wire contour, even when subjected to further drawing and coiling. However, others of the crystals have had their bonds to the wire body weakened, as by a microscopic displacement of a portion thereof from the wire body. Obviously, if the entire crystal were so displaced no bond at all would exist and the crystal would drop from the wire body. During subsequent coiling of the wire, the microscopic displacement above referred to of a surface crystal will be increased appreciably, to produce slivers extending from the wire body and depressions in the regions vacated by the slivers.

To avoid such formation of slivers and depressions, the novel combination of steps aforementioned includes an etching step carried out after the first group of drawing steps described have been performed and prior to coiling the Wire. The etching step comprises immersing the 17 mil wire produced by the first group of drawing steps in an electrolytic caustic bath. The caustic bath may include about 25% sodium hydroxide in a suitable carrier and involving current density of 20 amperes. The wire is permitted to remain in this bath for about from 10 to 15 minutes, to reduce its size to about 15 mils. This etching step serves to remove the loosely bonded crystals from the wire surface by penetrating the region of partial displacement of a surface crystal as aforementioned, and attacking the loosely bonded crystal from all sides thereof.

The foregoing etching step effects removal of surface crystals from the wire. It does not appreciably afi ect subsurface crystals whioh have also been elongated by the first group of drawing steps. However, the elongation of such subsurface crystals is not as severe as in the case of the surface crystals, due to their remoteness from the die and the shielding afforded by the surface crystals. But the subsurface crystals exposed by the etching step are sufiiciently long, so that further elongation thereof during the second group of drawing steps would nullify the effects of the etching step in that the newly exposed sur- [face crystals would have the objectionable lengths produced by the first group of drawing steps.

The etching step is not relied on as the final step in the method of making a tungsten wire suitable for bending to coil shape without incidents of slivers. This is because the etching step produce depressions in the wire surface, as aforementioned, which give rise to hot spots and rupture of the wire when serving as a carrier of electrical power. Therefore, the final group of drawing steps is employed, according to the invention, to flatten out such depressions.

In order that the second or final group of drawing steps maybe performed without re-introducing the objectionably long surface crystal formations removed by the etching step, the etched wire is subjected to an annealing step prior to the final drawing steps. The annealing step may be accomplished by passing an electric current of suitable value, through adjacent lengths of the wire in an atmosphere of hydrogen, to heat the wire to a temperature at which the elongated crystals contract in length and form bodies of concentrated mass. Such transformation of the crystal shapes, however, adversely affects the ductility of the wire so that it would be incapable of withstanding bending without rupture.

It will be appreciated, therefore, that while the annealing step is advantageous in reducing the length of the crystals in the wire, it is accompanied by the disadvantage of reducing the ductility of the wire mate-rial. However, thi disadvantage is overcome by the same means according to the invention, that is relied on for flattening out or filling the depressions in the wire produced by the etchin step aforementioned.

This means comprises the final group of drawing steps referred to before herein. This final group of drawing steps not only flattens out depressions produced in the Wire by the etching step, but also moderately elongates the aforementioned contracted crystals to impart ductility to the finished wire. To avoid excessive elongation of the crystals during the final drawing step, critical control of such steps is employed. One way in which such control is manifested is in the magnitude of reduction in wire diameter effected by the final drawing steps. In the example under consideration, this reduction in diameter was eight mils, or a reduction of wire size from mils after the etching step, to seven mils after the final dralwing step. However, successful results have been obtained when the magnitude of final reduction in wire size falls within a range, 'as pointed out in the following.

It is important in practicing the invention that the etching step be carried out when the size of the wire is from about 100 to about 150% above that of the finished wire. If the wire after etching and annealing is smaller than about twice the size of the finished wire, it limits the magnitude of the final drawing operation to a value that may be inadequate to produce the degree of crystal working required for ductility, and to remove the depressions formed by displaced surface crystals. If the size of the Wire after etching and annealing is more than 150% of the finished wire, the final drawing will involve an excessive working of the surface crystals which may elongate them excessively and adversely affect their bond to the wire body, thereby giving rise to slivers during a subsequent coiling of the wire.

After the wire is formed to the desired size aforementioned, it is coiled by suitable means to form a desired coiled structure, such as a coiled heater for an electron tube, or a helix for a traveling wave tube. In neither case are objectionable slivers or fissures in the wire evident in the coiled structures.

It is to be understood that during the several drawing operations, the wire is coated with a suitable lubricant, such as aquadalg.

It will be appreciated from the foregoing that I have provided a novel and useful method of fabricating a tungsten wire according to which two groups of drawing steps are separated by an etching and annealing step, for simultaneously reducing the wire diameter and controlling the working thereof to a degree suflicient for satisfactory ductility of the wire required for bending to coil form, while avoiding the formation of crystal structures which give rise to slivers and fissures in the finished coil.

While the method of the invention has been described in association with tungsten, it also has utility in the drawing to wire form of swage rods made of metals other than tungsten. Thus, the invention is of advantage in situations when the drawing of a swage rod of any metal, to wire form is accompanied by the formation of surface crystals elongated to a degree to produce slivers and fissures in the finished wire when the latter is subjected to a forming operation involving the application of a bending stress to the crystals referred to.

What is claimed is:

1. Method of making a tungsten wire coil comprising drawing a tungsten swage rod having a thickness greater than five times that of the finished wire to reduce its thickness to a magnitude of from two to two and one-half times as great as the thickness of the finished wire, whereby elongated surface crystals are formed in the resultant intermediate size Wire, etching said intermediate size Wire to remove at least portions of loosely bonded ones of said elongated surface crystals, annealing said etched intermediate size wire after said etching step and prior to further working of said intermediate size wire, then drawing the etched and annealed wire to still further reduce the thickness thereof to the size of said finished wire, and winding said finished wire to coil form.

2. Method of making a bent tungsten wire structure comprising the steps of die-drawing a tungsten swage rod having a size of from about one and a half to about eight times the size of an intermediate wire size to form a wire of said intermediate size, whereby relatively long and loosely bonded surface crystals are formed in said intermediate wire, etching a surface portion of said intermediate wire including said loosely bonded surface crystals, annealing said etched intermediate wire immediately after said etching step, further die-drawing said etched and annealed intermediate wire to from two-fifths to onehalf the size of said intermediate Wire, toform relatively short and firm-1y bonded surface crystals and to reduce the thickness of the intermediate wire to the size of a desired wire, and bending said desired wire to the form of said structure, whereby said desired wire is free from slivers.

3. Method of reducing the thickness of a tungsten swage rod, having a diameter of about rnils, to the size of a desired wire, comprising first die-drawing said rod to effect a predetermined reduction in the size thereof and to produce a wire of intermediate size of about 17 mils in diameter, ctchingly removing a surface layer of said intermediate wire, annealing the etched intermediate wire prior to further working thereof after said etching step, and then die-drawing said etched wire to effect a smaller reduction thereof of from two-fifths to one-half the size of said intermediate wire, to produce a wire of final size, whereby said final Wire is substantially free from loosely bonded surface crystals and the surface of said final wire is free from slivers and fissures when bent to desired shape.

4. Method of reducing the size of a tungsten swage rod to that of a relatively fine wire of ductile tungsten and substantially free from loosely bonded surface crystals tending to produce slivers and fissures therein when the fine Wire is subjected to bending stresses and wherein said swage rod is greater than five times the size of said fine wire, comprising die-drawing raid rod to reduce the size thereof to a magnitude from two to two and one-half times the size of said fine wire and to produce a wire of intermediate size, etchingly removing a surface portion of said inter-mediate wire, annealing the etched intermediate wire whereby ductility of said intermediate wire is impaired, and re-drawing said etched and annealed wire to produce said fine wire in ductile state, said etching and annealing steps being carried out in [the order named, said intermediate wire being free from any processing step between said etching and annealing steps.

5. Method of making a tungsten wire coil comprising drawing a tungsten swage rod more than five times as thick as the finished wire to form a wire of from two to two and one-half times as thick as the finished Wire, etching the wire so produced to remove a surface layer therefrom about one mil thick and including elongated surface crystals, annealing the etched wire to contract surface crystals exposed by said etching step as the next adjacent step to said etching step in :the processing of said wire, drawing the annealed Wire to the size of said finished Wire, and bending said finished Wire to coil shape.

6. Method of making a closely wound coil of tungsten wire drawn to a final size, comprising drawing a tungsten swage rod, having a thickness greater than one and onehalf times the thickness of a predetermined thickness, to form an intermediate wire having said predetermined thickness, whereby said intermediate wire is provided with appreciably elongated surface crystals and less elongated sub-surface crystals, removing said surface crystfls and exposing said sub-surface crystals, then in an intermediately adjacent step annealing said intermediate wire with said exposed sub-surface crystals to transform said subsurface crystals to bodies of such concentrated mass that further drawing of said annealed intermediate wire to said final size elongates said bodies to a magnitude less than the length of said appreciably elongated surface crystals, said magnitude being suflicient to render said final size wire sufilciently ductile to facilitate winding. to said coil form while preserving said coil from slivers extending from the normal Wire contour, drawing said annealed intermediate wire to reduce the thickness thereof to from two-fifths to one-half of said predetermined thickmess to form said final wire size, and closely winding said final size wire to coil form.

7. Method of making a closely wound coil of tungsten wire drawn to a finished size, comprising drawing a tungsten swage rod having a thickness more than five times that of said finished wire, to an intermediate wire size having a thickness of from two to two and one-half times as large as the thickness of said final wire size, whereby surface and sub-surface crystals are elongated in a descending order of length in said intermediate Wire, etching said intermediate wire to remove said surface crystals and to expose said subsurface crystals, annealing said etched intermediate wire in a step immediately following said etching step to transform said exposed subsurface crystals to bodies of such concentrated mass that further drawing of said intermediate wire to said finished size only moderately elongates said sub-surface crystals to impart desired ductility to said finished wire and to avoid slivers when said finished wire is wound to produce said coil, drawing said etched and annealed intermediate wire to said finished wire size, and closely winding said final wire to coil shape.

References Cited in the file of this patent UNITED STATES PATENTS 1,077,696 Fuller Nov. 4, 1913 1,663,564 Rich Mar. 27, 1928 2,792,627 Thall May 21, 1957

Claims (1)

1. METHOD OF MAKING A TUNGSTEN WIRE COIL COMPRISING DRAWING A TUNGSTEN SWAGE ROD HAVING A THICKNESS GREATER THAN FIVE TIMES THAT OF THE FINISHED WIRE TO REDUCE ITS THICKNESS TO A MAGNITUDE OF FROM TWO TO TWO AND ONE-HALF TIMES AS GREAT AS THE THICKNESS OF THE FINISHED WIRE, WHEREBY ELONGATED SURFACE CRYSTALS ARE FORMED IN THE RESULTANT INTERMEDIATE SIZE WIRE, ETCHING SAID INTERMEDIATE SIZE WIRE TO REMOVE AT LEAST PORTIONS OF LOSELY BONDED ONES OF SAID ELONGATED SURFACE CRYSTALS, ANNEALING SAID ETCHED INTERMEDIATE SIZE WIRE AFTER SAID ETCHING STEP AND PRIOR TO FURTHER WORKING OF SAID INTERMEDIATE SIZE WIRE, THEN DRAWING THE ETCHED AND ANNEALED WIRE TO STILL FURTHER REDUCE THE THICKNESS THEREOF TO THE SIZE OF SAID FINISHED WIRE, AND WINDING SAID FINISHED WIRE TO COIL FORM.