Metal fusion bonding – Process – Using high frequency vibratory energy
Reexamination Certificate
2001-10-10
2003-12-09
Elve, M. Alexandra (Department: 1725)
Metal fusion bonding
Process
Using high frequency vibratory energy
C228S111000, C228S121000, C228S122100, C228S123100, C228S124500, C148S023000, C148S024000, C148S025000, C148S026000, C420S557000, C420S580000
Reexamination Certificate
active
06659329
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field
This invention relates to a soldering alloy made of tin and an active wetting promoting element such as aluminum and to a soldering technique that is useful for joining a wide variety of hard-to-wet and hard-to-join materials including titanium alloys (e.g., Nitinol), stainless steels, aluminum alloys, carbon steels, glasses, and ceramics.
2. Background
Nitinol is a trade name for a titanium alloy with a composition of nickel 50 atomic % titanium. Also known as “Tee-nee”, “Memorite”, “Tinel” or “Flexon”, this alloy is utilized for its superelasticity, or shape memory effect. Shape memory is the ability to fully recover plastic deformation, up to 8%, upon heating above a specific temperature. Superelasticity is the ability to completely recover large “pseudo-elastic” strains on the order of 5 to 8% upon removal of the loading stress. This superelasticity and/or shape memory, coupled with Nitinol's biocompatibility, corrosion resistance, and fatigue resistance, make the alloy a very attractive material for a variety of applications. Although Nitnol can be joined using brazing or welding techniques, it begins to dramatically lose its superelasticity, and/or shape memory characteristics, when heated above approximately 500° C., as occurs during welding and brazing. Subsequent heat treatment can only recover a small percentage of the properties.
Although some efforts have been made to use soldering techniques, which by definition use temperatures below about 450° C., such efforts have been less than successful. Titanium and titanium alloys are difficult to solder because they form a particularly tenacious surface oxide that is hard to wet. While this oxide imparts these alloys with exceptional corrosion resistance, it also makes them extremely difficult to solder. Although two methods for soldering Nitinol have been used previously: 1) soldering using halogen-based fluxes, and 2) electro/electroless plating, both have significant draw backs.
A number of manufacturers have developed fluxes for use on aluminum alloys, which develop a tenacious surface oxide similar to that of titanium and its alloys. Although these fluxes have been found to be useful with Nitinol, they are typically based on very aggressive halogen bearing inorganic acids that are hazardous to handle and dispose of. During the soldering operation the flux generates large amounts of toxic fume that must be vented to prevent exposure to personnel. The flux residue remaining on the solder joint must also be cleaned off with hot water and mechanical scrubbing. The cleaning water and residue is an environmental hazard and must be handled and disposed of accordingly.
In addition, the flux residues must be cleaned off completely to prevent subsequent corrosion of the solder joint, and potential in-service contamination. Leaching of toxic flux materials from the joint presents a formidable problem in the manufacture of medical devices. The complex joint geometry often necessary in surgical devices makes complete removal of flux residue difficult, time consuming, and expensive. If not completely removed, persistent flux residue can compromise the solder joint integrity and potentially contaminate a surgical patient.
Plating is another technique used for soldering difficult-to-solder alloys. In the case of nickel-titanium, nickel plating can be used. Nitinol can be nickel plated with both electroless and electrolytic processes. Often the part is plated with a secondary layer of a more noble metal, such as gold over the nickel. Soldering can then be done on the plated surface using an appropriate flux. The major drawback to this approach is that plating titanium is an involved and difficult process. Nickel plating is typically a multi-step process involving cleaning, etching, and plating. Plating titanium alloys is even more complex due to their tenacious surface oxides. Often, several intermediate plating steps may be necessary to facilitate the final nickel plating. For most manufacturers, it is very costly to develop extensive in-house plating capabilities and the expertise just to facilitate a soldering process. Furthermore, plating quality can vary greatly and plating vendors are often reluctant to work with titanium alloys due to issues of handling and storing the aggressive chemicals involved such as hydrofluoric acid. An additional drawback is that even though fluxes that are less aggressive than the halogen based ones can be used to solder the plated surface, the flux residues must still be removed completely.
While the concept of ultrasonic soldering has been around for half a century, it has had very limited commercial success. It did show some potential for soldering aluminum heat exchangers, but this effort was largely dropped in favor of several competing approaches. While its use remains relatively limited, the most common commercial application appears to be in pre-tinning, or solder coating, of copper electrical leads and components. Limited success has been achieved in using ultrasonic soldering to solder difficult to solder materials. Although, it has been used in conjunction with indium-based solder alloys to aid slightly in the soldering of oxide ceramics, these joints can also be made easily without the use of ultrasonic soldering. Furthermore, joints made with indium-based alloys, with or without ultrasonic soldering, typically have very low strength, on the order of only several hundred pounds per square inch. Prior attempts to ultrasonically solder titanium and its alloys, using conventional as well as custom solder alloys have shown very limited success producing weak joints that result from a primarily mechanical bond instead of a true chemical bond.
In order to overcome the various problems encountered with prior art methods of joining hard-to-wet materials, it is an object of the present invention to provide a soldering method and solder alloy for wetting and joining these hard-to-join materials.
It is an object of the present invention to join hard-to-join materials at a low temperature.
It is an object of the present invention to join hard-to-join materials without the need for corrosive fluxes.
It is an object of the present invention to avoid the production of hazardous workplace fumes and joining by-products
It is an object of the present invention to avoid the production of environmentally dangerous fumes and joining by-products.
It is an object of the present invention to join hard-to-join materials without the need for tedious and complex plating steps.
It is an object of the present invention to join hard-to-join materials without the need for use of corrosive plating chemicals.
It is an object of the present invention to join hard-to-join materials using a minimum of processing steps.
It is an object of the present invention to join nitinol parts without loss of superelasticity or shape memory characteristics.
It is an object of the present invention to clean hard-to-join materials without the use of a flux.
It is an object of the present invention to provide high-strength joints for hard-to-join materials.
It is an object of the present invention to join hard-to-join materials using inexpensive equipment.
It is an object of the present invention to join hard-to-join materials with little if any joint cleanup after the joining process.
It is an object of the present invention to join hard-to-join materials with no flux residue cleaning after the joining process.
It is an object of the present invention to join hard-to-join materials without the use of a vacuum, or reducing gas environment.
It is an object of the present invention to form solder wetted areas on hard-to-wet materials.
It is an object of the present invention to provide a tin-based alloy solder with an active wetting promoting chemical element.
It is an object of the present invention to use aluminum as an active wetting promoting element in a tin alloy.
The foregoing and other objects, features and advantages of the invention will become apparent from the following disclosure in whi
Dawsey David J.
Edison Welding Institute Inc
Edmondson L.
Elve M. Alexandra
Gallagher Michael J.
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