Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
2001-07-25
2004-02-03
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S402000
Reexamination Certificate
active
06685783
ABSTRACT:
DESCRIPTION
The invention relates to a process for manufacturing, shaping and machining a shape memory alloy part, and the part obtained that has remarkable functional properties.
The technical field of the invention may be defined in general as being metallurgy and science of materials. More particularly, the invention is in the field of shape memory alloys, and their fabrication and shaping or machining.
Remember that a metallic alloy has a shape memory if, after permanent deformation when cold or during cooling under stress, it returns to its initial shape simply by heating. This phenomenon, called a single directional memory effect (EMSS) is caused by a martensitic transformation that occurs above a critical temperature called the “transition temperature”. This transition temperature may be adjusted between −200° C. and +170° C., by acting on the chemical composition and/or heat treatments.
Furthermore, martensitic transformation confers other particular properties on shape memory alloys (AMF) for example the capacity to generate a large force during heating; the super elastic effect, the rubbery effect, the assisted two-directional memory effect (EMDSA) and the two-directional memory effect (EMDS). The presence of the martensitic phase also significantly increases the damping capacities.
The performances of an AMF element can usually be estimated in terms of displacement amplitude and recovery force available when heating.
All specific properties of shape memory alloys explain the “functional materials” or “intelligent materials” terminology often used to qualify them.
The shape memory effect has been known in metallic alloys since 1930 and the first industrial application was in 1967, but due to the complexity of their behavior, their extreme sensitivity to fabrication conditions and their cost, their industrial and commercial development has remained very limited and mainly applies to sectors using state-of-the-art technologies such as the defense, space and medical equipment industries.
Fabrication procedures used to fabricate shape memory alloys depend on the required application and the final geometry required for the AMF part or element.
Shape memory alloy parts used for the main applications found so far have a simple geometry. There are usually bars, for example used in deployment mechanism actuators in space and actuators for robotics; for example, wires in dental fixtures, medical instrumentation, portable telephone antennas, spectacle frames, clothes; flats or strips in spectacle manufacturing, electrical activators in household automation equipment; or springs or “stents” for stenosis in the medical field and “telltales” for detecting a failure in the cooling system in the food processing industry, etc.
The shaping processes used to obtain parts with the geometries mentioned above correspond to the use of conventional transformation means such as extrusion presses for producing bars; drawing and wire drawing benches to produce wires; rolling mills to obtain sheet metal, and shaping dies to make springs.
Machining processes may also be used for the purposes of any shaping operation by the removal of material intended to confer dimensions and a surface condition on an AMF part, for example a difference in shape and roughness within a given tolerance range.
In general, machining of the AMF takes place within the sequence of shaping operations, mainly at two levels:
either as a blank cutting operation for a blank to be rolled, forged, stamped or machined;
or as a finishing operation for previously extruded, drawn, forged parts, and parts assembled by welding.
Among the shaping processes mentioned above, processes that make use of cold shaping are difficult to implement on shape memory alloys, particularly on titanium-nickel alloys, and particularly due to the hardness of these materials.
In general, cold deformation must be limited to avoid damage and must include annealing steps at a controlled temperature to restore the material.
However, cold deformation is essential for these materials, because it provides not only precision on the final dimensional characteristics, but it also confers work hardening on the material that improves the mechanical properties and enables optimization of functional properties such as shape memory effect, the recovery force and super elasticity. It has also been demonstrated that as the work hardening effect increases, in other words as the deformation rate increases, the functional properties, and particularly the recovery force and the amplitude of the memory effect improve. Furthermore, after the last cold deformation phase, a short-term “flash” heat treatment must be carried out within a defined temperature range to optimize the functional properties.
The above confirms that the fabrication of finished parts, in other words parts with a defined geometry made of a shape memory alloy, requires long, complex and therefore expensive fabrication and shaping procedures, including cold transformation operations that are difficult to control.
Therefore, there is a need for a simple, fast and economic process for fabrication and shaping of a shape memory alloy part, comprising a limited number of steps and capable of producing parts with mechanical and functional properties at least equivalent to what can be obtained with known, long, complex and expensive processes.
The purpose of this invention is to divulge a process for fabrication and shaping of a shape memory alloy part that satisfies all the needs mentioned above, that does not have the disadvantages, defects, limitations and disadvantages of processes according to prior art and that solves problems that arise with processes according to prior art.
According to the invention, this and other purposes are achieved by a fabrication, shaping and machining process for a shape memory alloy part comprising a single step during which the fabrication, shaping and machining of the said part are done simultaneously in a single operation by a cutting machining process.
The process according to the invention satisfies all the needs mentioned above, does not have any of the defects of processes according to prior art and solves problems that arise with processes according to prior art.
Surprisingly, the inventors found that by applying a specific machining process, namely a cutting machining process, to shape memory alloys it is possible to perform fabrication, machining and shaping operations in a single step. Cutting machining processes are the only known machining processes capable of achieving the purposes mentioned above and solving problems according to prior art.
Cutting machining processes have the special feature that they preferably produce chips, unlike processes that remove material by abrasion, or by chemical or electrochemical methods.
Surprisingly, the cutting machining process used in the invention confers very high work hardening on the part or chip made of shape memory alloy and is simultaneously accompanied by extremely fast temperature rise that performs the same function as the heat treatment, this “flash” heat treatment actually corresponding to the final heat treatment performed in processes according to prior art. Consequently, with the process according to the invention, it is possible to obtain remarkable mechanical and functional properties on the chip in a single operation. The inventors have demonstrated that the properties of parts obtained by the process according to the invention, for example springs, are better than the properties that can be obtained on an AMF part or element obtained by a long and complex fabrication process according to prior art.
The process according to the invention is a means of directly obtaining the required remarkable properties after a single step.
Unlike processes according to prior art that involve a long series of complex shaping and machining steps, in most cases with a final cold deformation step followed by another heat treatment, the process according to the invention includes only one step during which all operatio
Nunes Daniel
Olier Patrick
Commissariat a l'Energie Atomique
Pearne & Gordon LLP
Wyszomierski George
LandOfFree
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