WO2000020146A1

WO2000020146A1 - Nickel-titanium seamless tubes

Info

Publication number: WO2000020146A1
Application number: PCT/US1999/023558
Authority: WO
Inventors: Albert Sickinger; Gerald J. Julien; Gary A. Hislop
Original assignee: Promet Technologies, Inc.
Priority date: 1998-10-08
Filing date: 1999-10-08
Publication date: 2000-04-13
Also published as: AU6423399A

Abstract

A process of producing seamless tubes of a nickel-titanium intermetallic compound by creating a plasma stream of ionized gasses and entraining small particles of Nitinol (99) heated to a partially molten state in the plasma stream or by creating molten nickel-titanium intermetallics with an electric arc that burns between two Nitinol wires in an argon gas stream, which atomizes the liquid nickel-titanium intermetallic, compound and creates a stream of molten droplets and particles, wherein the stream of particles is directed toward and impacted against a mandrel (85) and deposits a tubular layer on the mandrel. The mandrel is removed from the interior of the tubular layer, leaving a seamless nickel-titanium tube or other hollow structures, the shape of which is defined by the shape of the mandrel. For example, if the mandrel is tubular or rod-like mandrel, a tubular layer of Nitinol is deposited on the mandrel, and when the mandrel is removed from the interior of the tubular layer of Nitinol, a seamless Nitinol tube is left.

Description

NICKEL-TITANIUM SEAMLESS TUBES

Technical Field

This invention pertains to manufacturing of hollows, seamless structures made of shape memory alloys such as nickel-titanium intermetallic compounds by thermal spray deposition.

Background Art

In 1961 Buehler, W. and Wiley R. of the U.S. Naval Ordinance Laboratory developed a series of engineering alloys that possess a unique mechanical property: shaped memory. The generic name of the series of alloys is 55-Nitinol, where Nitinol stands for Nickel Titanium Navel Ordinance Laboratory. These alloys are based on the intermetallic compound NiTi with a chemical composition in the range of 53 to 57 weight percent nickel, and the balance is titanium. NITINOL (an acronym for Nickel Titanium Naval Ordinance Laboratory) is a family of intermetallic materials which contain a nearly equal mixture of nickel (55 wt. %) and titanium. Other elements can be added to adjust or "tune" the material properties. Nitinol exhibits unique behavior. The two terms used to describe this behavior are "Shape Memory" and "Superelasticity".

Shape memory alloys (SMAs) exhibit two unique characteristics: (1) Shape Memory: Shape memory effect describes the process of restoring the original shape of a plastically deformed sample by heating it. This is a result of a crystalline phase change known as "thermoelastic martens itic transformation". Below the transformation temperature, Nitinol is martensitic. The soft martensitic microstructure is characterized by "self-accommodating twins", a zigzag like arrangement. Martensite is easily deformed by de-twinning. Heating the material converts the material to its high strength, austenitic condition (see atomistic model). The transformation from austenite to martensite (cooling) and the reverse cycle from martensite to austenite (heating) does not occur at the same temperature. There is a hysteresis curve for every Nitinol alloy that defines the complete transformation cycle. The shape memory effect is repeatable and can typically result in up to 8% strain recovery. Thus, shape recover effect manifests itself in a remarkable restoring force and/or recovery of a tremendous amount of plastic strain by means of a reversible cystalline phase transformation. SMAs generate recovery stresses in excess of 690 Mpa (- 100 ksi) or recover their original heat treated strain (up to 8%) when heated above a critical transformation temperature.

(2) Superelasticity: Martensite in Nitinol can be stress induced if stress is applied in the temperature range above A/(austenite finish temperature). Less energy is needed to stress-induce and deform martensite than to deform the austenite by conventional mechanisms. Up to 8% strain can be typically accommodated by this process. Since austenite is the stable phase at this temperature under no-load conditions, the material springs back to its original shape when the stress is removed (see atomistic model). This extraordinary elasticity is also called "pseudoelasticity" or transformational "superelasticity". The typical stress-strain curve of a properly processed Nitinol alloy shows the lading and unloading plateaus, recoverable strain available, and the dependence of the loading plateau on the ambient temperature. The loading plateau increases with the ambient temperature. As the material warms above the austenite finish temperature, the distinctive superelastic "flag" curve is evident. Upon cooling, the material displays less elasticity and more deformation until it is cooled to where it is fully martensite; hence, exhibiting the shape memory property and recovering its deformation upon heating. Nitinol alloys are superelastic in a temperature range of approximately 50 degrees above the austenite finish temperature. Alloy composition, material processing, and ambient temperature greatly effect the superelastic properties of the material . Fortunately for the medical device community, binary Nitinol alloys, when processed correctly, are at their optimum superelastic behavior at body temperature. Superelasticity, the elastic modulus changes from 27 Gpa (3.9 Msi) to 110 Gpa (15.9 Msi) when the material transforms from the martensite phase to the austenite phase (NiTiCu SMAs) . These characteristics of SMAs can be exploited to design smart mechanisms capable of shockless separation, shape control, precision pointing, and controlled structural deployment. Materials with A 's ranging from -15C to 100C are available. Many applications of tubes require functional features that are rare, even in modern materials, in the areas of mechanical characteristics (such as elasticity), corrosion, erosion, and cavitation resistance and vibration damping, sometimes in the same part. For example, materials for making vascular and coronary stents need to have extraordinary elasticity and also body compatibility.

Many industrial applications would be well served using seamless tubing that has high tensile strength, and is extremely elastic and chemically non-reactive. In addition, vibration of parts in a mechanical systems present fatigue problems in fluid lines that have required awkward solutions. The use of seamless tubing that is erosion, cavitation, and corrosion resistant, and the property of vibration damping in a tubing element to lessen the destructive influence of vibration would be an extremely welcome development in the hydraulics industry, as well as many other industries.

Temperature sensitivity of conventional tubing has been a serious problem in industry. Cryogenic applications have been forced to use rigid tubing, usually made of stainless steel, to achieve the strength, temperature tolerance and chemical resistance needed, but rigid tubing limits the design options for the application and is expensive. Likewise, high temperature applications of tubing are unable to use plastic tubing and often have serious problems with chemical compatibility. These industries would all be well served with flexible, high strength seamless tubing that is chemically inert and tolerant of both high and low temperatures.

Plasma spraying systems are well known and the present invention envisions using such conventional systems that are modified to the extent disclosed herein.

Disclosure of Invention

Accordingly, it is an object of this invention to provide an improved process for making seamless tubular components and systems that have cavitation, erosion, and corrosion resistance, and have damping properties. Another object of this invention is to provide seamless tubes of nickel-titanium intermetallic compound, and for providing a process for making such tubes. Yet another object of this invention is to provide thin wall seamless tubing of nickel-titanium intermetallic compound and an improved process for making such tubing. Still another object of this invention is to provide vascular and coronary stents made of Nitinol, and a process for making such stents. These and other objects of the invention are attained in a process for thermal spray deposition of Nitinol on a wire or rod mandrel. The thermal spray deposition process uses two distinct spraying devices, namely plasma spraying using a plasmatron and electric arc spraying using a wire arc spraying system. The plasma spraying process includes entraining a powder of nickel-titanium intermetallic compound such as Nitinol in an argon or helium gas, or mixtures of argon and hydrogen or argon and helium gasses heated and ionized in a plasmatron, creating plasma. The particles heated to a molten or partially molten state are ejected at high velocity from the plasmatron in the plasma gas mixture. The particles impact against the mandrel surface where they cool and freeze to produce a deposition of nickel-titanium intermetallic compound on the mandrel.

The electric wire arc spraying process includes using nickel-titanium intermetallic compound such as Nitinol in form of wire which is melted with an electric arc burning between the two wires and an argon gas or a mixture of argon and hydrogen gasses atomizing the molten wire in small particles or droplets. The molten particles are ejected at high velocity from the wire arc spray gun with the argon gas stream. The particles impact against the mandrel surface where they cool and freeze to produce a deposition of nickel- titanium intermetallic compound on the mandrel.

The wire or rod mandrel is then removed, leaving the seamless tubular structure of nickel-titanium intermetallic compound. The seamless tubular structure is processed by heat-treating and cold working either before or after removing the mandrel to produce the desired properties.

Description of the Drawings The invention and its many attendant objects and advantages will become better understood upon reading the description of the preferred embodiment in conjunction with the following drawings, wherein:

Fig. 1 is a sectional elevation of a low pressure plasma spray apparatus used in the process of this invention; Fig. 2 is a schematic sectional elevation of a plasmatron and a plasma stream to a substrate; Fig. 3 is a perspective view of a fixture for holding and rotating wire or rod mandrels on which nickel-titanium intermetallic compound is deposited for making the tubular structures; and

Fig. 4 is a schematic sectional view of an apparatus for making continuous lengths of tubing of nickel-titanium intermetallic compound.

Fig. 5 is a schematic sectional elevation of an electric wire arc spraying system and a particle stream to a substrate;

Best Mode for Carrying Out the Invention Turning now to the drawings, wherein like reference numerals designate like or corresponding parts, and more particularly to Fig. 1 thereof, an apparatus is shown for plasma deposition of materials onto a substrate. The apparatus includes a low pressure plasma spray system (LPPS) 30, originally made by Electro-Plasma, Inc. in Irvine, California, and available now in similar design from Sulzer Metco Company in Switzerland. It is sold for use primarily for depositing nickel alloys and other specialized material on turbine blades and vanes for protection from the high temperature corrosion and erosion influences found in jet turbine engines.

The low pressure plasma spray system 30 includes a pressure vessel 32 having main chamber 35 within which a low pressure, inert atmosphere can be established. The vessel 32 also includes a transfer chamber 40 having a gate or pass-through structure illustrated by a gate 45 through which parts can be passed between the transfer chamber 40 and the main chamber 35 without contaminating the atmosphere in the main chamber 35 or affecting the low pressure gas atmosphere therein. Gas feed and exhaust lines connect to fittings on the main chamber 35 and the transfer chamber 40 for exhausting and purging to establish the desired atmosphere composition and pressure.

A spraying device 50, preferably a plasmatron 20 (Fig. 2) or an electric wire arc spraying gun 200 (Fig. 5), is disposed in the main chamber 35, preferably on an end fitting 52 of a robotic arm 55 by which the spraying device 50 can be manipulated remotely within the chamber 35 by conventional robotic controls outside the chamber. The plasmatron spraying device 20, shown schematically in Fig. 2, has a nozzle 60 having a conical cavity 65 within which a cathode 70 is suspended centrally, creating an annular passage 72 of about 0.150" between the cathode 70 and the wall of the conical cavity 65 in the body of the nozzle 60 which serves as an anode.

A fixture 80, shown in Fig. 3, is installed in the chamber 35 for holding multiple mandrels 85 suspended in a vertical orientation. The fixture includes a motor (not shown) driving a drive shaft 88 to which a sun gear 90 is keyed. A planet gear carrier 92 is mounted on a bearing on the end of the drive shaft 88 for free rotation relative to the drive shaft 88. A connecting shaft 94 extends between the planet gear carrier 92 and a disc 96 so they rotate together. A plurality of planet gears 98 is mounted on the planet gear carrier 92 in engagement with the sun gear 90 and also in engagement with a ring gear 100 surrounding the planet gears 98. Two planet gears are illustrated, but in practice a large number, 12-20, planet gears would be used to achieve maximal efficiency and minimal waste of material. The ring gear 100 is grounded to the vessel by way of a shroud 101 (Fig.1) which also surrounds and protects the planetary gear set from deposition of Nitinol from the spraying device 50. The disc 96 has a plurality of holes 102 through which the mandrels 85 extend, and weights 104 hang from the lower end of mandrels 85 to hold them straight. The holes in the disc 96 prevent the mandrels 85 from swinging during deposition of Nitinol by the spraying device 50.

In operation, the main chamber 35 is evacuated to a pressure of about 50 millitorr through a gas line 74 by one or more vacuum pumps 75, and then backfilled with clean (99.995% pure) argon to 300 torr through another gas line 76 connected to a source of argon. The chamber 35 is again evacuated to 50 millitorr using the vacuum pumps 75 and recharged with argon to an operating pressure of about 30 torr. The preferred atmospheric conditions for processing are now established.

In case of using a plasmatron 20 as spraying device 50 two powder feeders (only one of which is illustrated in Fig. 2 at 91) of known design and commercially available for the LPPS system are filled with nickel-titanium intermetallic compound (commercially available in various blends and referred to as Nitinol powder) and evacuated to 50 millitorr, then backfilled with pure argon to 4 psig. This process is preferably repeated two more times to minimize the oxygen content in the powder feeders and in the powder. The preferred powder is a gas atomized Nitinol powder with particles in the range of 10-45 micrometer diameter. It is commercially available from Special Metals Corporation in New Hartford, New York. A plasma gas consisting essentially of a mixture of 95 % argon and 5 % hydrogen is passed through the annular passage 72 at a rate of about 150 scfh argon and 8 scfh hydrogen. A conventional DC plasma power supply 95 is energized to create an arc in the passage 72, and 71.5 kW of power is applied at 1300 amp and 55 volts. The fixture 80 is energized to rotate the drive shaft 88 and rotate the sun gear 90 which in turn rotates the planet gears 98 about their own axes, and which also revolve around the inside of the ring gear 100. In this way, the outer peripheries of all the mandrels 80 are uniformly exposed to the plasma stream. The plasma gas exits the nozzle 60 in a plasma gas stream at high temperature and supersonic velocity, and impinges on the mandrels 85, positioned preferably between 12 and 17 inches in front of the nozzle 60.

The most preferred embodiment of the present invention uses a cylindrical mandrel thus yielding seamless tubes. However, mandrels of different configuration may be used, such as for example triangular, square and other curvilinear configuration. Hollow, seamless structures produced by the process of the present invention, regardless of their shape, can also be made with multiple layers of the Nitinol compounds, each layer having a different Nitinol composition.

The powder feeders 90 are then turned on to feed powder at a rate of about 50 grams/minute each with a carrier gas flowing at a rate of approximately 15 scfh. The powder is entrained in the plasma gas flow 99 and ejected from the nozzle 60 at subsonic speeds. It travels with the plasma gas stream in a diverging or conical flow and impacts against the substrate surface at high speed. The high energy (kinetic and thermal energy) and partially melted state of the powder cause the powder particles to flatten when they impact against the mandrel surface. The deposition is allowed to continue until the nickel-titanium layer on the mandrels 85 has formed a seamless tube about each mandrel and reached the desired wall thickness, which is preferably in the range of 0.002" to 0.500".

The electric wire arc spraying system 200, shown schematically in Fig. 5, includes a nozzle 205 having a conical cavity 65 within which two wires with wire contact jets 207 are suspended centrally, creating an annular passage 201 between the contact jets 207 and the wall of the conical cavity 206, which serves as a nozzle 205. When using the electric wire arc spray system 200 as spraying device 50 the preferred operation is as follows: the main chamber 35 is evacuated to a pressure of about 50 millitorr through a gas lines 74 by one or more vacuum pumps 75, and then backfilled with clean

(99.995 % pure) argon to 300 torr through another gas line 76 connected to a source of argon.

The chamber 35 is again evacuated to 50 millitorr using the vacuum pumps 75 and recharged with argon to an operating pressure of about 100 to 300 torr. The atmospheric conditions for preferred processing are now established.

The two wire feeding reels of known design are loaded with the desired nickel-titanium wire commercially available from Special Metals Corporation in New Hartford, New York. The atomizing gas consisting essentially of argon (99.995% pure) or a mixture of 95 % argon and 5 % hydrogen is passed through the passage 201 at a pressure of about 95 psi. A DC plasma power supply 202 is energized to create an arc between the wires 202, and 2.8 kW of power is applied at 100 amp and 28 volts and the wire feeding mechanism 204 is turned on. The fixture 80 is energized to rotate the drive shaft 88 and rotate the sun gear 90 which in turn rotates the planet gears 98 about their own axes, and which also revolve around the inside of the ring gear 100. In this way, all sides of all the mandrels 80 are uniformly exposed to the particle stream. The atomizing gas with the molten droplets 208 exit the nozzle 205 at subsonic velocity and impinges on the mandrels 85, positioned preferably between 6 and 10 inches in front of the nozzle 205.

The wire feeders 204 feed wire material at a rate of about 80 to 100 grams/minute.

The atomized wire droplets travel with the argon gas stream in a diverging or conical flow 208 and impact against the substrate surface at high speed. The kinetic and thermal energy of the melted droplets cause the particles to flatten when they impact against the mandrel surface.

The deposition is allowed to continue until the nickel-titanium layer on the mandrels has formed a seamless tube about each mandrel and reached the desired thickness, preferably in the range of 0.002" to 0.500". The Nitinol material is available from several suppliers, such as Duriron Corporation,

Timet, Special Metals, Oremet Wah Chang and others that are known to those in the art.

Specific blends of these materials are prepared by the suppliers in accordance with specifications determined by the user. The typical specification required is known as the cold transition austenitic final temperature, usually referred to as the "Af" temperature or the cold transition temperature Af. Furthermore, the terms superelastic and pseudo-elasticity are generally understood to be synonymous in this field and the terms are used herein in that sense. Thus, for a specific end use of the nickel-titanium tubes of the present invention, a desired austenite final transition temperature for the nickel-titanium intermetallic compound is chosen so that the end product tube remains in its austenitic state for all temperatures expected to be found in the environment of use of the end product. For example, products that may be used within the human body are chosen so that the austenite final transition temperature is always below the temperature found in the living human body. In this way, the end product, for example a vascular stent, will always exhibit superelastic properties in use.

After deposition, the coated mandrels 85 are removed from the fixture 80, preferably by a remotely operated manipulator, and withdrawn from the apparatus 30 through the transfer chamber 40. Without removing the mandrels, the nickel-titanium tubes formed on the mandrels are heat treated and cold worked to provide desired pseudo-elasticity properties, i.e. , a rubber like behavior, as is well known. Depending on the properties required, a superelastic Nitinol tube can be made using a Nitinol powder or wire in the spraying device 50 having a cold transition temperature Af of superelastic composition available from Nitinol suppliers such as Duriron Corporation, Timet, Special Metals, Oremet Wah Chang, and some foreign suppliers. This gives an austenite final transition temperature of about -30°C to about 0°C so that the material remains in its austenitic state for all temperatures in the normal operating range of vascular and coronary stents.

The coated mandrels may be heat-treated and/or cold worked, if desired, and the mandrels 85 are then removed from the coating, leaving the nickel-titanium deposition as a thin- walled seamless tube. Removal of the mandrels 85 is facilitated by preventing diffusion bonding of the deposited nickel-titanium material on the mandrel 85 during deposition. Bonding of the nickel-titanium to the mandrel 85 can be inhibited by polishing, hard surfacing, and/or by placing coating the mandrel with a release material such as boron nitride or by a thin aluminum foil sleeve on the mandrels. The mandrels 85 can be made of a material with a low coefficient of thermal expansion, such as ceramic or carbon-carbon composite that can be easily extracted from the deposited Nitinol tube when the coated mandrel is cooled. Alternatively or in addition, the mandrels or aluminum foil sleeves can be removed from the Nitinol tubes by electrochemical or acid etching using conventional solutions used for etching aluminum, such as hot hydrochloric acid or other etching solutions effective for removing the materials of which the mandrels or mandrel sleeves are made as is known. Another mandrel removal option uses a mandrel made of a material, such as molybdenum tubing or wire, that is removed by sublimation. Heating the nickel-titanium coated mandrel to a temperature above about 600°C in air will oxidize the molybdenum so rapidly that it sublimes or smokes off. A process of making continuous lengths of seamless nickel-titanium tubing is performed in an apparatus illustrated in Fig. 4. A pressure vessel 132 has a main chamber

135 within which a low pressure, inert atmosphere can be established. The vessel 132 also includes an input transfer chamber 140 having input and output seals 141 and 143, and an output transfer chamber 145 having input and output seals 147 and 149. The input and output seals in the input and output transfer chambers allow continuous movement of mandrel material 185 into the main chamber 135 from a supply real 160, and continuous movement of nickel-titanium coated mandrel material out of the main chamber 135 without compromising the quality of the atmosphere inside the main chamber 135. Gas feed and exhaust lines connect to fittings on the main chamber 135 and the transfer chambers 140 and 145 for exhausting through an exhaust pump 175 and purging to establish the desired atmosphere composition and pressure as described earlier for the apparatus 30 of Fig. 1.

A first bank of spraying devices 150 is disposed in the main chamber 135 around the line of travel of the mandrel 185. The spraying devices 150 in the first bank are illustrated as lying in a single transverse plane perpendicular to the longitudinal axis of the vessel 132, although they could be staggered axially to avoid conflicting spraying patterns and currents. The first bank preferably has three or four spraying devices 150 for depositing nickel-titanium onto the mandrel 185. A second bank 151 of spraying devices, and, optionally, additional banks of spraying devices will be needed, depending on the speed of travel of the mandrel 185 and the diameter and wall thickness of the seamless tubing to be made.

To prevent damage to the output seal in the output transfer chamber; the tubing may be cooled in the output transfer chamber. The input seal 147 in the output transfer chamber 145 is preferably a temperature tolerant type, such as a labyrinth seal or the like, unless the tubing is sufficiently cooled within the main chamber 135 before reaching the seal 147.

After exiting the output transfer chamber 145, the tubing is taken up on a take-up reel 165. The take-up real is removed and taken to a post-processing facility for heat treating and cold-working to produce the desired superelastic properties as is well known for these materials. Alternatively, the post processing can be performed in line with the plasma- deposition equipment before the tubing is taken up on the take-up reel 165.

The options for extracting the mandrel 185 from the continuous length of nickel- titanium tubing are limited because of the difficulty in physically withdrawing the mandrel from a roll. The use of molybdenum wire or tubing that can be removed by heating and passing air or oxygen through the tube are preferred embodiments. In another preferred embodiment an aluminum foil mandrel sleeve is formed around a cylindrical mandrel form extending through the input chamber 140 and into the main chamber 135. In this embodiment, the foil mandrel sleeve is formed from a strip of aluminum foil on a supply real 160 and formed around the forming mandrel by a series of forming dies, and then lap welded by resistance or laser welding into a tube at the entrance of the input seal 141 of the input transfer chamber 140. The foil mandrel sleeve slides along and is supported by the forming mandrel through the main chamber 135 and then slides off the end of the forming mandrel after exiting the output seal of the output transfer chamber. The aluminum foil sleeve mandrel is then easily removed from the continuous length of Nitinol tubing by passing an etching solution through the tubing, which quickly dissolves the thin aluminum foil.

Nickel-titanium tubing exposed to erosive influences can be made with an interior surface layer of Type 60 Nitinol and superelastic Nitinol as the principle portion of the tubing wall thickness. Type 60 Nitinol is very hard, on the order of 55 to 62 on the Rockwell C scale, and may be heat treated for an even harder and more erosion and corrosion resistant surface. The dual composition Nitinol tubing, having Type 60 Nitinol at the interior surface and superelastic Nitinol for the main portion of the tubing sidewall, can be made in the apparatus shown in Fig. 4 by initially spraying Type 60 Nitinol in a thin layer onto the mandrel 185 in the first spraying devices 150, and then spraying superelastic composition Nitinol in the second spraying devices 151. Typically, and preferably, the wall sickness of the inner, relatively hard layer will be in the range of 0.002" to 0.004" . The typical, and preferred wall thickness of the outer, super-elastic layer is then sprayed on in a thickness sufficient to make the entire wall thickness range from 0.002 + " to 0.500". In this way, a tube having a very hard inner surface and overall exhibiting super-elasticity can be made with the process of the present invention. The Nitinol is preferably used in powder form in the plasmatron spraying device 20 because it produces a plasma with a fine particle or globular structure, at a relatively high cost. Alternatively, the use of wire in the spraying device 150 is preferred for relatively low cost, or industrial applications. The stream of nickel-titanium particles or globules tends to be larger in wire-fed electric arc spraying devices, but for thicker walled, larger diameter tubing (0.5 to 4.0 inch inner diameter), the post processing can be done at higher temperatures to help consolidate the deposited nickel-titanium structure, and a hot drawing step may be added for the same purpose. Hot isostatic pressing (HIP) may also be used for consolidation heat treatment. The disadvantage of a wire-fed electric arc spraying device may be reduced or negated, with consequent improvement in efficiency.

Obviously, numerous modifications and variations of the preferred embodiment described above are possible and will become apparent to those skilled in the art in light of this specification. For example, many functions and advantages are described for the preferred embodiment, but in many uses of the invention, not all of these functions and advantages would be needed. Therefore, we contemplate the use of the invention using fewer than the complete set of noted features, benefits, functions and advantages. Moreover, several species and embodiments of the invention are disclosed herein, but not all are specifically claimed, although all are covered by generic claims. Nevertheless, it is our intention that each and every one of these species and embodiments, and the equivalents thereof, be encompassed and protected within the scope of the following claims, and no dedication to the public is intended by virtue of the lack of claims specific to any individual species. Accordingly, it is expressly intended that all these embodiments, species, modifications and variations, and the equivalents thereof, are to be considered within the spirit and scope of the invention as defined in the following claims, wherein we claim: While the present invention has been described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but to the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit of the invention, which are set forth in the appended claims, and which scope is to be afforded the broadest interpretation so as to encompass all such modifications and equivalent structures .

Claims

I claim:

1. A process for making a seamless nickel-titanium hollow structure, comprising: creating a plasma stream of ionized gasses; entraining nickel-titanium intermetallic compound in said plasma stream to form partially molten particles; directing said partially molten particles in said plasma stream toward, and impacting said partially molten particles against a mandrel having a predetermined length and a predetermined cross-sectional shape to deposit a layer of said nickel-titanium intermetallic compound about said mandrel; removing said mandrel from the interior of said tubular layer of said nickel-titanium intermetallic compound; and leaving a seamless nickel-titanium intermetallic hollow structure.

2. The process of claim 1 wherein the mandrel is a cylinder and the hollow structure is a tube.

3. A process for making a seamless structure of a nickel-titanium intermetallic compound as defined in claim 1, further comprising: providing a surface on said mandrel that inhibits bonding of said nickel-titanium intermetallic compound to said mandrel.

4. A process for making a seamless a nickel-titanium intermetallic compound as defined in claim 1 structure wherein: said mandrel is of a continuous length; and after removing said mandrel from said layer, leaving a continuous length of the seamless hollow structure.

5. A process of claim 1 wherein: removing said mandrel from the interior of said layer includes: shrinking said mandrel away from the interior walls of said layer; and axially separating said layer from said mandrel .

6. The process of claim 5 wherein: said shrinking of said mandrel away from the interior walls of said layer is by differential thermal expansion and contraction of said layer and said mandrel.

7. The process of claim 4 wherein: said mandrel is conveyed through said plasma stream more than once; and at least two different nickel-titanium compositions are entrained in said plasma stream to form at least two layers of nickel-titanium intermetallic compound, the two layers having a different composition of nickel and titanium.

8. The process of claim 7 wherein: one layer is of Type 60 Nitinol plasma sprayed onto the mandrel: and the second layer is of superelastic Nitinol composition plasma sprayed and diffusion bonded onto said first layer.

9. The process of claim 1 wherein an electric arc wire spraying device creates said plasma stream and entrains said compound in said plasma streams.

10. The seamless, hollow structure formed by the process of claim 1.

11. The seamless tube formed by the process of claim 2.

12. The seamless tube formed by the process of claim 2, further including an outside surface of said tube having a smooth finish produced by cold working.

13. The seamless rube formed by the process of claim 2 wherein: said cold working includes one or more of the processes selected from the group consisting essentially of drawing, grinding, polishing, planishing and particle blasting.