Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Post sintering operation
Reexamination Certificate
2001-01-24
2003-04-15
Mai, Ngoclan (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Post sintering operation
C419S036000, C419S037000
Reexamination Certificate
active
06548013
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to a method for manufacturing products from shape memory alloys, particularly nickel-titanium (Ni—Ti) alloys such as nitinol, using injection-molding techniques. Moreover, the method relates to the molding of nitinol into complex shapes and imparting the desired properties of nitinol, namely shape memory and superelasticity.
2. Description of the Prior Art
Shape memory metal alloys are combinations of metals that possess the ability to return to a previously defined shape when subjected to an appropriate temperature. Although a wide variety of shape memory alloys exist, only those that can recover from a significant amount of strain, or those that generate significant force while changing shape are considered commercially valuable. Examples of such alloys include nickel-titanium alloys (Ni—Ti) such as nitinol and copper based alloys. In the medical device community nitinol has received a great deal of attention not only for its shape memory and superelastic properties but also its biocompatibility.
Nitinol has two temperature-dependent forms. The low temperature form is called martensite. The martensitic form of nitinol is characterized by a zigzag-like arrangement of microstructure referred to as “self-accommodating twins”. Martensitic nitinol is soft and easily deformed into new shapes. When martensitic nitinol is exposed to higher temperatures, it undergoes a transformation (sometimes called the thermoelastic martensitic transformation) to its stronger, high temperature form called austenite. The austenite form of nitinol is less amenable to deformation.
The unique properties of shape memory alloys such as nitinol, particularly shape memory and superelasticity, are inherent based upon a phase transformation from a low temperature martensite form to the stronger, high temperature austenite. This transformation occurs not at a specific point, but rather over a range of temperatures. The key temperature points that define the transformation, beginning with the lowest temperature, are the martensitic finishing temperature (M
f
), the martensitic starting temperature (M
s
), the austenite starting temperature (A
s
), and finally the austenite finishing temperature (A
f
). At temperatures above A
f
, nitinol possesses the desired properties of shape memory and superelasticity. Moreover, the transformation also exhibits hysteresis in that the transformations occurring upon heating and cooling do not overlap.
Shape memory is a unique property of shape memory alloys that enables a deformed martensitic form to revert to a previously defined shape with great force. An illustrative example of how shape memory properties work is a nitinol wire with its “memory” set as a tightly coiled, unexpanded spring. While in the martensitic form, the spring is easily expanded and if a constant force is applied to the spring, such as a weight pulling downward on a vertically placed spring, the spring expands. But, when the temperature of the spring is increased above the transformation temperature, the spring “remembers” its predefined state and returns to its uncoiled state. This can occur with significant force. For example, the force could be enough to lift the weight.
The mechanism of shape memory is based upon the crystal fragments or grains. When the memory is set, the grains assume a specific orientation. When martensitic nitinol is deformed, the grains assume an alternate orientation based upon the deformation. Shape memory takes place when deformed martensitic nitinol is heated above its transformation temperature so as to allow the grains to return to their previously defined orientation. When this occurs, the nitinol “remembers” its predefined state, based upon the grain orientation, and returns to its predefined state.
Superelasticity is a second unique property of shape memory alloys. This property is observed when the alloy is deformed at a temperature slightly above the transformational temperature and the alloy returns to the original orientation. An illustrative example of this effect is a nitinol wire wrapped around a cylindrical object, such as a mandrel. When nitinol exhibiting superelastic properties is coiled around a mandrel multiple times and then released, it will rapidly uncoil and assume its original shape. A non-superelastic nitinol wire would tend to yield and conform to the mandrel. Superelasticity is caused by the stress-induced formation of some martensite above its normal temperature. Therefore, when nitinol is deformed at these elevated temperatures, the martensite reverts to the undeformed austenite state when the stress is removed.
Manufacturing of molded metal or metal alloy products traditionally has been accomplished by casting, powder metallurgy, or powder injection molding techniques. Casting involves the melting of the metal or alloy and forming the product in a mold or die. Powder metallurgy generally involves the molding of particulate metal, often by using die and piston compaction. Powder injection molding is a refinement of powder metallurgy wherein atomized or particulate metals or alloys are molded by injection into a mold. Powder injection molding requires smaller particulate matter than other powder metallurgy techniques and generally results in parts that have greater density.
Traditional powder metallurgy techniques have generally not worked in the formation nitinol products. To better understand the reasons for this, the importance of crystal fragments, or grains, need to be further considered. Grains are the fundamental microscopic units of metal structures. The arrangement and size of grains can have a major impact on both the desirable properties of nitinol and the ability to thermo-mechanically process nitinol so as to impart the desirable properties. For example, when traditional powder metallurgy techniques are used on standard alloys, the result is grains of a random orientation. Nitinol with this grain configuration does not have shape memory or superelastic properties. In order to impart the desired properties, cold or hot working must occur so as to align the grains in a specific orientation amenable to thermo-mechanical processing.
Casting results in similar observations and, therefore, cast nitinol does not have shape memory or superelastic properties. Casting of nitinol also results in enlarged grains. In order to impart the desirable properties into cast nitinol, cold or hot working again needs to occur so as to align the grains in a conformation suitable for thermo-mechanical processing. When typical preparations of nitinol, such as wire, are manufactured, a cast nitinol product is used that is then drawn or rolled so as to appropriately align the grains. Using these techniques, which are well known in the art, nitinol wire can be readily produced.
Because working is required to impart shape memory and superelasticity into cast nitinol, the ability to form complex shapes using traditional casting techniques is limited. The manufacturing of finished parts from nitinol has generally been accomplished by starting with preshaped, semifinished nitinol in the form of a rod, tube, strip, sheet, or wire. The preshaped, semifinished nitinol can then be cold worked to produce the desired object. A novel method for manufacturing shape memory alloys, such as nitinol, into complex shapes while imparting the desired properties would prove beneficial.
In addition to the drawbacks related to grain structure, another difficulty associated with manufacturing formed nitinol parts is the high reactivity of nitinol with oxygen. Atomization of nitinol complicates this difficulty by increasing the surface area where oxidation can take place. When nitinol reacts with oxygen, its properties can vary greatly. Partially oxidized nitinol has a differing transformation temperature, different sintering requirements, and may lack shape memory or superelastic properties. Additionally, partially oxidized nitinol may become brittle and difficult to work. High oxygen react
Baumgarten Donald C.
Kadavy Thomas D.
Fish & Richardson P.C.
Mai Ngoclan
Sci-Med Life Systems, Inc.
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