Metal working – Method of mechanical manufacture – Utilizing transitory attached element or associated separate...
Reexamination Certificate
2000-04-28
2002-09-24
Bryant, David P. (Department: 3726)
Metal working
Method of mechanical manufacture
Utilizing transitory attached element or associated separate...
C072S368000, C072S370250
Reexamination Certificate
active
06453536
ABSTRACT:
The invention concerns a method of producing hollow profiles, in particular tubes, having a small external diameter and/or small wall thickness, made from a nickel titanium alloy by shaping a composite block.
Alloys having approximately the same amount of titanium and nickel atoms exhibit special effects leading to their designation as shape memory alloys. The effects are based on a thermoelastic martensitic phase change, i.e. a temperature-dependent change in the crystal structure: at high temperatures the alloy is austenitic; at low temperatures it is, however, martensitic. According to T. W. Duerig and H. R. Pelton (“TI-NI Shape Memory Alloys”, in: Materials Properties Handbook: Titanium Alloys, 1994, pages 1035-1048, ASM International 1994), shape memory alloys have two properties which should be distinguished from another. Alloys with a titanium content between 49.7 and 50.7 atom % have a thermal shape memory, also called shape memory, and alloys with a titanium content of 49.0 to 49.4 atom % have a mechanical shape memory, also called super-elasticity.
In addition to binary nickel titanium alloys, other alloys can also have these properties. A shape memory alloy can contain ternary components (e.g. iron, chrome or aluminium). The ratio between nickel and titanium and the presence of ternary additions strongly influence the markedness of the thermal and mechanical shape memory. Even slight concentration variations cause large changes in the material properties.
When using thermal shape memory for building components, an alloy of suitable composition is transformed, without diffusion, from an austenitic to a martensitic structure by cooling. Subsequent shaping of a component produced from this alloy can be reversed by thermal treatment thereof (heating to temperatures above a certain transition temperature). The original austenitic structure is thereby reproduced and the component adapts to its original shape. The transition temperature is usually the temperature at which the martensite is completely changed to an austenite. The transition temperature depends largely on the composition of the alloy and the loading of the component. Components having a thermal shape memory can cause movements and/or exert forces.
The mechanical shape memory effect occurs in a component made from a suitable alloy of austenitic structure when the construction unit is shaped within a certain temperature range. It is thereby energetically more favorable for the austenitic structure to change into a martensitic structure when loaded, wherein elastic expansions of up to 10 percent can be achieved.
Upon load relief, the structure returns to the austenitic phase. Components made from such an alloy can therefore store shaping energy.
Conventional alloys having the above-described properties are designated as Nickel Titanium, Titanium Nickel, Teenee, Memorite®, Nitinol, Tinel®, Flexon® and Shape-Memory-Alloys. These terms do not refer to one single alloy of a certain composition, but to a family of alloys having the described properties.
Many technical fields, e.g. medical technology and mechanics, are highly interested in use of components made from shape memory compositions due to the particular properties of nickel titanium alloys. In mechanical applications, they can e.g. be used for switching elements, actuating elements or valves. Shape memory alloys are also used to an increasing degree in medical technology, since components made from such alloys are biologically acceptable, fatigue-resistant and have also good flexibility as superelastic alloys.
Stents, catheters and endoscopic and laparoscopic instruments for minimum-invasive diagnosis and therapy are examples for use of nickel titanium alloys in medical technology, the intermediate product being a nickel titanium tube. Intermediate products in the form of tubes, in particular, of small external diameter, are also required for other applications.
Large-scale use of nickel titanium tubes and instruments is curtailed inter alia by the currently high price thereof which, in turn, results from the conventional methods for producing the tubular intermediate product.
Conventionally, nickel titanium tubes are produced by drilling forged bars. The tubes typically have an external diameter of between 12 and 25 mm. Due to the poor cutting property of nickel titanium alloys, the deep hole drilling method is difficult and results in short service life for the tools, long processing times and high production costs for the tubes. In addition, there is large material loss, in particular, when producing thin-walled tubes. The cuttings produced during drilling or turning on the lathe are waste material.
European patent document 0459909 describes manufacture of seamless tube from a corrosion-resistant alloy, consisting almost exclusively of titanium, using a tube extrusion method. In the method, a perforated press block is pressed through a gap between a press mandrel and a die using punch pressure. After subsequent shaping, the tubes generated in this manner serve e.g. for heating salt water in sea water desalination plants and as heat exchanger tubes in chemical production plants.
Due to the unfavorable shaping behavior of nickel titanium alloys, only tubes with a large external diameter (more than 40 mm) can be extruded economically using such a method. The extrusion of tubes having a smaller diameter is expensive since, due to the lack of cooling, it is not possible to achieve a sufficiently long tool service life in the temperature range dictated by the material. Moreover, the mandrels break off easily during extrusion, leading to a large amount of waste. The very large formation resistance of nickel titanium alloys at very high formation temperatures prevents production of small, thin tubes, since the press mandrel cannot withstand the high thermal and mechanical tensile loads which occur. According to prior art, therefore tubes having a large external diameter are initially pre-fabricated by tube extrusion and subsequently shaped into tubes of the desired small diameter using additional expending processing steps, e.g. drawing and rolling. Due to the advantages of nickel titanium alloys, in particular of shape memory alloys, these disadvantageous costs associated with the demanding production procedures are accepted in prior art.
WO 96/17698 discloses a method for lost core extruding of composite blocks without using a press mandrel. A block hollowed-out by drilling is filled with a steel core and both are extruded once together. The geometrical shape of the hollow extruded product depends on the geometrical shape of the extrusion die and of the core. The larger the core relative to the extrusion die, the thinner the wall. When thin-walled tubes are produced, this type of block preparation therefore results in considerable material waste, which is a substantial disadvantage with nickel titanium alloys. Moreover, in this method, the shaping process disadvantageously results in the metal core forming an intimate metallic connection with the nickel titanium material in the extruded product such that an additional processing step is required to remove the core material for obtaining a hollow extruded product, e.g. by drilling out and/or chemically removing the core material. Moreover, a desired small profile dimension cannot be achieved in all cases in a single extrusion of the composite block.
It is the underlying purpose of the invention to provide a method for the production of hollow profiles or tubes made from a nickel titanium alloy having a small external diameter and/or a small wall thickness in an inexpensive and effective manner. The hollow profiles or tubes may have any cross-sectional shape. The designation tube therefore refers to any profiled tube or hollow profile.
This object is achieved with a method for producing hollow profiles, made from a nickel titanium alloy, having small external diameters and/or small wall thickness through shaping of a composite block, wherein, in a first step, a composite block is formed comprising a solid core
Müller Klaus
Nusskern Hans
Basov Daniel
Bryant David P.
Chadbourne & Parke LLP
G. Rau GmbH & Co.
Schaefer Ira J.
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