Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
2001-09-06
2003-12-30
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S565000
Reexamination Certificate
active
06669794
ABSTRACT:
FIELD OF THE INVENTION
The present invention concerns a treatment method for an object made of a material exhibiting martensitic transformation, in particular a shape memory material.
The invention applies to the manufacture of active or passive monolithic structures (i.e. one single piece of material), made of shape memory material, and in particular to the manufacture of monolithic actuators, connectors, active components for fixing and grippers, of very small dimensions, made of shape memory material.
DESCRIPTION OF THE PRIOR ART
The following two documents should be consulted as regards shape memory materials:
[1] Engineering Aspects of Shape Memory Alloys, T. W. Duerig et al., Ed. Butterworth-Heinemann, 1990
[2] Shape memory materials, Edited by K. Otsuka and C. M. Wayman, Cambridge University Press, 1998, chapter 10, pages 21 to 237
The following document which divulges particular applications of these materials should also be consulted:
[3] French Patent Application No. 9615013 of Dec. 6, 1996, “Dispositif de préhension en matériau à mémoire de forme et procédé de réalisation”, an invention by Y. Bellouard, J. E. Bidaux and T. Sidler—see also International Application No. PCT/EP97106966, International Publication No. WO 98/24594.
It will be recalled that shape memory materials have several solid phases in mestastable equilibrium. The change of phase from a solid phase to another may be induced under stress (super-elasticity) and/or by temperature change (shape memory effect).
When the change of phase is heat induced, it may be accompanied by a macroscopic shape change. Thus a shape memory material which is apparently plastically deformed in its low temperature phase, called the “martensite phase” can return to its initial shape by being heated to its high temperature phase, called the “austenite phase”.
The characteristic temperatures of the beginning and end of the austenite-martensite transformation are respectively designated M
a
and M
f
. The characteristic temperatures of the beginning and end of the martensite-austenite transformation are respectively designated A
s
and A
f
.
Particular care must be taken that there exists only one “memorised” shape in a shape memory material, namely the austenite shape: the phenomenon is thus not intrinsically reversible.
Obtaining an intrinsic reversible effect for such a material requires either the use of a very particular manufacturing method of the material (for example the method known by the name of “Melt Spinning”), or the use of a thermo-mechanical treatment commonly called the “education method” which will in a way “memorise” a preferred martensite shape.
Another known technique consists in exploiting the fact that the mechanical characteristic of the material evolves with the change of phase. Thus, a mechanical assembly including on the one hand an element made of such a material and on the other hand another element whose characteristic remains constant will have two stable operating points corresponding to the temperature and stress zones defining the solid phases of this shape memory material.
However, when one wishes to obtain an actuator of very small dimensions, it is very difficult to make such an assembly. This is why, a known technique consists in creating a single piece structure which is also called a monolithic structure: the actuator is then manufactured in a single same element made of a memory shape material.
In this regard, the following document should be consulted:
[4] Y. Bellouard et al., “A concept for monolithic SMA microdevices”, Journal de Physique IV, n
o
11, p.603-608 (1997).
The difficulty is thus to be able to obtain a reversible effect and for such purpose to obtain different mechanical properties in this same element: In order to do this, it is necessary to heat the latter locally so that only a part thereof can have a shape memory effect while the other part remains passive.
However, in order for a displacement to occur, it is imperative to achieve mechanically an initial pre-deformation of the element (except if there is a two-way memory effect).
EP-A-0 086 357 A discloses a method for manufacturing a crank case wherein the surface layer material of the case is transformed from a ferritic state to a substantially martensitic state.
MESSER K ET AL: “STAND DES LASERSTRAHLHAERTENS” HAERTEREI TECHNISCHE MITTEILUNGEN, DE, CARL HANSER VERLAG, MUNCHEN, vol. 52, no. 2, Mar. 1, 1997 (1997-03-01), pages 74-82 discloses the laser beam hardening of iron based alloys.
MIGLIOREL L R: “HEAT TREATING WITH LASERS” ADVANCED MATERIAL'S & PROCESSES, US, AMERICA SOCIETY FOR METALS. METALS PARK, Ohio, vol. 154, no. 2, Aug. 1, 1998 (1998-08-01), pages H25-H29 discloses the treatment by laser of steels.
FR-A-2 393 075 discloses the annealing of a non ferrous metal part by means of a laser.
CHEMICAL ABSTRACTS, vol. 126, no. 24, Jun. 16, 1997 (1997-06-16) Columbus, Ohio, US; abstract no. 319898, VILLERMAUX, F. ET AL: “Corrosion kinetics of laser treated NiTi shape memory alloy biomaterials” & MATER. RES. SOC. SYMP. PROC (1997), 459 (MATERIALS FOR SMART SYSTEMS II), 477-482, 1997 discloses the laser treatment of shape memory alloys.
WO-A-89 10421 discloses heat treatment of shape memory alloys.
SUMMARY OF THE INVENTION
The object of the present invention is to overcome the problem of local change (i.e. at least in a pre-defined zone) of the microstructure of an object made of a material exhibiting martensitic transformation, in particular a shape memory material.
“Local change of the microstructure” of such an object means:
the local crystallisation of the object when the material is amorphous
or the local recrystallisation of the object when the material is work-hardened
or the secondary local crystallisation of the object when the material is already crystallised (for example to induce locally a transformation temperature change)
or the controlled formation of precipitates or the annihilation of crystalline faults, locally, in the object (with a view to locally changing the mechanical properties of the latter).
More precisely, the present invention concerns a treatment method for an object made of a material able to undergo martensitic transformation, in particular a shape memory material this method being characterised in that at least a pre-defined zone of this object is irradiated by a laser beam able to heat this zone sufficiently, to a temperature lower than the melting temperature of the material, to cause, in said zone, a microstructure change selected from among a crystallisation, recrystallisation, secondary crystallisation controlled formation of precipitates and annihilation of crystalline faults, said zone being heated to a temperature and for a time which are not able to cause amorphisation of the material.
This laser beam is thus used to anneal the object locally by bringing the latter to a much higher temperature T than the temperature A
f
of the shape memory material of which the object is made.
However, it should be noted that the temperature and the annealing time are such that amorphisation of the material cannot be obtained.
It should also be noted that the material could even have been annealed in a furnace prior to implementing the method of the invention.
Irradiating a zone of an element made of shape memory alloy by means of a laser beam is of course known, from European Patent document Nos. 0360455A and 031 0294A (Catheter Research Inc.). The use of a laser to modify and alter the crystalline structure so that the martensitic transformation can no longer occur is divulged therein. The notion of altering is important in the sense where, in the case of these documents, the laser is used to “destroy” and not to “construct” a crystal lattice. This means that the element is annealed beforehand then locally “amorphised” to prevent the migration of contaminating ions such as the silver ions in the NiTi matrix of the element. It is thus a method with an object contrary to the object of the method of the present invention. Indeed, the object of loca
Bellouard Yves
Bidaux Jacques-Eric
Clavel Reymond
Lenhert Thomas
Eta sa Fabriques d'Ebauches
Sughrue & Mion, PLLC
Wyszomierski George
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