Composite medical device having a titanium or titanium based alloy section and a ferrous metal section

Abstract

A composite medical device having a titanium, and titanium based alloy, section welded to a ferrous metal section. The weld provides supplementary filler material to alter the proportions of various elements in the weld pool to ensure a strong and reliable weld. Certain fillers, such as nickel or iron, added to the weld pool enable high quality welds to be fabricated utilizing a wide variety of fusion welding techniques between the titanium, or titanium based alloy, section and the ferrous metal section. The sections may include nickel-titanium, also known as nitinol. The sections may be in the form of wires, bars, ribbons, and sheets. The composite medical device of the present invention may include guidewires, stents, right-angle needles, suture passers, retractors, graspers, baskets, and various retrieval devices.

Description

REFERENCE TO RELATED DOCUMENTS [0001] This application is a continuation of a previous application filed in the United States Patent and Trademark Office on Mar. 19, 2003, titled “Method of Welding Titanium and Titanium Based Alloys to Ferrous Metals,” and given Ser. No. 10 / 391,921, all of which is incorporated here by reference as if completely written herein.TECHNICAL FIELD [0002] The present invention relates to the field of medical devices; particularly, to a composite medical device having a titanium, and titanium based alloy, section welded to a ferrous metal section. BACKGROUND OF THE INVENTION [0003] Titanium and titanium alloys have become important structural metals due to an unusual combination of properties. These alloys have strength comparable to many stainless steels at much lighter weight. Additionally, they display excellent corrosion resistance, superior to that of aluminum and sometimes greater than that of stainless steel. Further, titanium is one of the most abu...

AI Technical Summary

Problems solved by technology

Since the nickel in these alloys is chemically bound to the titanium in a strong intermetallic bond, risks of human tissue reaction have been shown to be low.

A major limitation in the use of nickel-titanium alloys has been the difficulty of joining this material, both to itself, and to other materials.

Because of its high cost, it is often desirable to limit the use of nickel-titanium to the actual moving parts of a device, while fabricating supporting members of such materials as stainless steel.

However, welding of nickel-titanium to stainless steel has proved particularly troublesome, as disclosed by Ge Wang, in a review “Welding of Nitinol to Stainless Steel.”

Fusion welding has been fraught with difficulties, particularly, problems surrounding issues of solidification, or “hot,” cracking, and cracking due to intermetallic formation, or so-called called “cold” cracking.

This tensile force causes cracks to form at the liquid metal filled grain boundaries, and these cracks then propagate through the weld zone.

The larger the mushy zone, the more severe the solidification cracking problem.

“Cold” cracking is a particular problem when attempting to weld nickel-titanium to other materials, and is responsible for common observations in the art that welding is generally not an acceptable method of joining nickel-titanium to other materials, e.g., stainless steel, because brittle intermetallics are formed in the weld zone.

Ti and Fe form the brittle intermetallic compounds TiFe and TiFe2, both of which can cause cold cracking at welded joints.

Techniques such as direct fusion welding cause intermetallic formation at the bond line, and consequential failure of the weld.

Even solid state bonding techniques which do not require melting at the weld interface, while they initially form a stronger weld, are susceptible to solid state diffusion of intermetallics into the weld line, and consequential weakening.

On the other hand, titanium rich nickel-titanium alloys, such as those composed of approximately 51.5% titanium, are susceptible to solidification cracking.

As a result, cracks form at the weld metal centerline.

However, the difficulty of joining nickel-titanium to other materials, such as stainless steel, has remained exceedingly limiting to the art.

Many techniques have been employed with limited success.

These techniques are not without their problems.

Epoxies and adhesives are not suitable for all manufacturing techniques and types of uses to which these nickel-titanium products are directed.

Mechanical fastening may cause overdeformation and cracking of the nickel-titanium.

This approach suffers from the inherent complexity of a multilayer approach, which is disclosed in some embodiments to employ even more layers, consisting of tungsten and platinum, added to the vanadium and chromium / nickel / or iron layer.

Additionally, the nature of diffusion welding makes the process quite slow and cumbersome, requiring approximately 90 minutes at a pressure of 10 Newtons per square millimeter to achieve a satisfactory diffusion weld.

This prevents intermixing of the ferrous and titanium elements and the consequent prevention of intermetallic formation; however, welding conditions must be strictly controlled in order to prevent the liquefaction of the vanadium interlayer, making this technique less suitable for production use.

Such a method suffers from the additional steps involved in the complex manufacture of the tripartite weld metal.

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