US 3409978 A
Description (OCR text may contain errors)
' Nov. 12, 1968 w. R. GRAMS 3,409,973.
METAL CLADDING PROCESS Filed Aug. 17, 1965 /n venfor: Wi/l/am R. Grams,
His A fiomey.
United States Patent 3,409,978 METAL CLADDING PROCESS William R. Grams, Ballston Spa, N.Y., assignor to General Electric Corporation, a corporation of New York Filed Aug. 17, 1965, Ser. No. 480,272 1 Claim. (Cl. 29494) ABSTRACT OF THE DISCLOSURE A titanium body is clad with tightly-bonded aluminum by heating the titanium body under conditions which result in adhering oxygen and oxide 0n the titanium body surface diffusing into the body so that metal-to-metal contact is established between the body and the aluminum cladding.
The present invention relates generally to the metal cladding art, and is more particularly concerned with a novel process of cladding bodies of tantalum, titanium, niobium and certain other metals with copper, aluminum and certain other metals.
This invention is predicated upon my surprising discovery that under certain conditions excellent bonds can quickly, easily and consistently be established between substrate bodies and cladding metals which heretofore have generally been considered either too difficult or entirely impossible to use in such combination. This discovery consequently opens the way to the improvement of various production operations such as, for example, those in which solderable copper claddings would long have been in general use but for the difficulty and uncertainty of providing adequately-bonded copper coatings on base or substrate metal parts. Thus, in the case of tantalum, it would be desirable in many operations to be able to join a copper part, for instance, by soft soldering and thereby avoid the necessity for having to carry out a welding operation or resort to an undesirably complicated design or construction.
For purposes of illustration, this invention will first be specifically described as it pertains to the cladding of tantalum wire with a solderable copper cladding but it is to be understood that this and other examples set out herein are merely by way of illustration and is not intended to limit the invention in any way.
Briefly described, the process of this invention comprises the following steps:
(a) Cleaning the surface of an oxide-diffusing metal to be clad so as to minimize the thickness of oxide film thereon; and
(b) Applying the coating metal to the resulting cleaned metal surface; in the absence of any reactive gases such as air, oxygen, nitrogen and hydrogen and while the substrate metal body is at a temperature conducive to the diffusion of oxide into said metal body.
While I do not wish to be bound by any specific theory, it appears that effective cladding can only be achieved for metals which in the solid state and under the process conditions applicable thereto, will diffuse the oxide materials from their surfaces into the bulk metal. Actually, in accordance with generally accepted theory, it is oxygen rather than oxide which so diffuses, the surface oxide first dissociating at elevated temperature and leaving a truly clean reactive metal surface as the oxygen is absorbed. For tantalum, titanium, zirconium, niobium and the like,
3,409,978 Patented Nov. 12, 1968 such swallowing of their surface oxygen or oxides occurs at elevated temperatures and at a rate depending upon the temperature. Moreover, in accordance with this invention, only a limited amount of oxide normally on the surface of a body of one of these metals is removed by oxide diffusion into the body. Thus, the amount of oxide swallowed by the metal body is limited, the major portion of the oxide coating which can be removed by heretofore conventional chemical or mechanical means or methods (i.e., the gross oxide) not being eliminated from the body surface by absorption action in the practice of this invention and preferably essentially none of such gross oxide is so swallowed.
In the preparation of lead wires to be attached to capacitors by soldering the following specific procedure was followed. Tantalum Wires (50 and mil diameter) were cleaned mechanically in some instances and chemically in others so as to minimize the thickness of the oxide coating thereon. The cleaned tantalum wires were then passed individually through a bath of copper at 1200 C. maintained under an argon atmosphere, the wires each being passed into the argon-filled furnace, thence into the melt and thence out of the furnace. The argon pressure in the furnace was sufficient that each wire entering and leaving the furnace was surrounded by argon and thereby protected against contact with the air while the wire is at a substantially elevated temperature conducive to oxidation of the wire or the freshly applied coating on it. Total residence time of the wire in the copper bath was only five seconds but was sufficient to produce a tenaciouslybonded copper clad onto the tantalum wire.
It appears that the diffusion of oxide from the surface to be clad may be facilitated by the heating to which the surface is subjected when it is brought into contact with the cladding metal. Thus, preheating of the substrate body to promote absorption of oxide prior to immersion in or contact with the coating metal bath may be advantageous when either the temperature of the bath is relatively low or the residence time of the substrate body in the bath is relatively brief so that substantially complete absorption and removal of oxide from the said body surface is not assured.
Coating or cladding metals which are suitable for the process contemplated include those metals which readily wet the truly clean metal body to be clad but which are not reactive in a detrimental way with the metal of the body. It would, for instance, not be desirable to use a coating metal which would react with the metal body to produce an amount of an intermetallic compound on the body surface which would impair the utility or value of the resulting body or article for its intended purpose.
The choice of combinations of base and coating metal which may be utilized in the practice of this invention will be readily apparent to those skilled in the art and will depend on the metallurgical compatibility of the various pairs as well as the relative melting points, the coating metal usually being one which melts at temperatures well below the melting point of the substrate being coated.
By suitably modifying the apparatus, it is possible to clad under a vacuum instead of a noble gas as described above.
Furthermore, instead of cladding with a melt, a heated vapor of the cladding metal may be used. In such an operation, the base metal will be preheated after removal of gross oxide so as to cause the residual oxide to diffuse into the base metal bulk, leaving the base metal surface clean and free from any barrier to the bonding of cladding metal to the base metal.
The following table sets forth exemplary treatments to remove the surface oxidation from the indicated metal (in each instance in the form of a 50-mil wire) while coating the same with the indicated metal by immersion in a molten bath of the coating metal.
Immersion Base Metal Coating Metal Temp. C.) (Durat1on in seconds) Those skilled in the art will gain a further and better understanding of this invention from the detailed description of the application of the present novel process to the production of capacitors, taken in conjunction with the drawings, in which:
FIG. 1 is a side view of a cylindrical capacitor in production in accordance with this invention, parts being bnoken away for purposes of illustration; and
FIG. 2 is :a semi-diagrammatic view of apparatus for processing a metal wire to provide it with a tightlybonded cladding or coating of another metal by the present method.
Capacitor 10 shown in FIG. 1 includes a cylindrical shell 11 of aluminum and end caps 12 and 13 of titanium disposed partially within and closing the ends of shell 11. Two tantalum wires 14 and 18 run axially through glass seals 15 in the end caps 12 and 13.
The end caps are sealed to shell 11 according to this invention by a bonding operation, the inner surfaces of the end portions of shell 11 being clean and substantially free from gross oxide. Thus, as indicated in FIG. 1 heat is applied (suitably in the form of :an electron beam, the capacitor and electron beam generator nozzle 16 being situated in a vacuum chamber in this operation) to the end of shell 11 to cause a portion of the aluminum shell to melt. Simultaneously, the residual oxide on the surface of the heated portion of the titanium end cap is absorbed into the interior of the end cap, leaving the titanium surface clean and amendable to the establishment of a tight bond with the shell as the heat source is removed and the molten aluminum is permitted to cool and freeze. In preferred practice, then, an annular seal 17 is established between cap 13 and shell 11 by turning the capacitor assembly to bring succesive portions of the circumference of the shell end portion into the range of the heat source 16, the rate of such rotation being regulated to insure the fusion of sufiicient aluminum to create the desired bond and seal. The same openation is applied to accomplish the joining and sealing of cap 12 in place as illustrated and the two sealing operations are desirably carried out simultaneously with two separate heat sources.
The wire coating operation of FIG. 2 is conducted following a suitable cleaning operation as described above, resulting in the removal of at least the major part of the gross oxide from the wire. In this instance, a molten bath 20 of copper at 1200 C. is contained in a molybdenum vessel 21 which is generally tubular and channel-shaped in transverse section with its open ends disposed above the level of bath 20. Vessel 21 is disposed within a housing 22 in which an argon atmosphere can be maintained to protect vessel 21 while it is at elevated temperature during the coating operation. Thus, housing 22 is closed except for a small opening 24 at one end to admit wire 25 and a larger opening 26 at its other end for withdrawal of coated wire product. Additionally, a conduit 27 connects the chamber of housing 22 to a source (not shown) of argon under pressure so that argon can be introduced into the housing continuously during the coating operation,
flowing from the inlet point at conduit 27 to opening 26 and to :a lesser extent to opening 24. Guides 30 and 31 are integrally-formed with the body of vessel 21 and located within the vessel to engage wire 25 of tantalum and maintain it in spaced relation to the vessel wall between these guides.
An induction coil 32 is disposed adjacent to housing 22 and connected to an electric power source (not shown) to heat copper both 20.
Tantalum wire 25 on reel 33 is run continuously through a cleaning station 34 and then into and through molten copper bath 20 continuously and collected on reel 36. The rate of travel of the wire over this course will preferably be substantially constant and at a rate which would have the wire immersed in bath 20 for eight seconds. In general, the lower the melt temperature, the longer the wire-melt contact period for achieving an adequate metallurgical bond. Consequently, in the practice of this invention, the operator has a comparatively wide latitude of choice as to melt temperature, melt vessel design and wire travel rate for any given coated wire product specifications.
Having thus described this invention in such full, clear, concise and exact terms as to enable any person skilled in the art to which it apertains to make and use the same, and having set forth the best mode contemplated of carrying out this invention, I state that the subject matter which I regard as being my invention is particularly pointed out and distinctly claimed in what is claimed, it being understood that equivalents or modifications of, or substitutions for, part of the specifically-described embodiments of the invention may be made without departing from the scope of the invention as set forth in what is claimed.
What I claim as new and desire to secure by Letters Patent of the United States is:
1. The process of cladding a titanium body with aluminum which comprises the steps of cleaning the titanium body and removing gross oxide from a portion of its surface, bringing the aluminum body into contact with the gross oxide-free surface portion of the titanium body in the absence of a reactive atmosphere, heating the titanium body and thereby causing residual oxide on the surface portion thereof in contact with the aluminum body to be absorbed into the titanium body and melting a surface portion of the aluminum body in contact with the resulting oxide-free surface portion of the titanium body and finaly cooling the titanium body and the aluminum bady and freezing the molten aluminum in contact with the said oxide-free surface portion of the titanium body.
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