EP0382109A1 - Process for treating a work piece made from a metallic shape memory alloy offering two states of reversible shape memory - Google Patents

Abstract

The process for treating a piece (P) made of metal alloy with double-effect shape memory according to the invention comprises the operations O1 to O7 consisting - in shaping (O1) at the ambient temperature T1 the piece (P) in the shape constituting a first shape memory state, - in mechanically maintaining the piece in its first shape memory state (O2) and in heating the mechanically maintained piece to the temperature T3 to bring it into an austenitic crystallographic phase state (O3), - in abruptly lowering the mechanically maintained piece to a temperature T4 (O4) and in a stabilising thermal treatment (O5) while maintaining its austenitic state, and - in subjecting the piece to a process of education in order to shape it in the second shape memory state (O6, O7). <IMAGE>

Description

The invention relates to a method for packaging parts made of metal alloy capable of undergoing a reversible transformation from the state of crystallographic phase of austenitic type to a state of crystallographic phase of martensitic type, and relates in particular to the packaging of parts having complex configurations for the reversible memorization by these of two states of shape memory.

There is already known from patent application EP-A-161 952 a method of packaging a part made of an alloy of the type mentioned above, making it possible to give the latter a double effect of reversible shape memory.

The process can be broken down into two series of operations, namely the preparation of the part to be educated and the education of the part itself.

Indeed, before implementing the education process it is necessary to prepare the part, this one being at the start in a state of undefined crystallographic phase which does not allow its education. This preparation essentially comprises three successive operations during which the part is first shaped according to a configuration constituting a first state of shape memory, then heated in order to be brought into a state of austenitic phase and finally cooled and stabilized at a temperature close to room temperature.

The preparation as described above however presents some difficulties during its implementation.

It is in particular difficult, according to this preparation process, to precisely conform parts in their first state of shape memory, the difficulty of obtaining a precise configuration being all the greater as the geometry of the part is complex. This is explained by the fact that when the part is heated in order to reach its austenitic phase state, it is worn, for safety reasons, at a temperature slightly higher than the theoretical temperature at which the single-phase austenitic phase begins to appear. Now, at this temperature, it is close to the melting temperature of the alloy, it follows that the part is in a softening state in which it collapses under its own weight and consequently loses its initial shape. This is a major drawback in many applications such as the preparation of complex parts with small sections.

Furthermore, the education method comprises the operations consisting successively of deforming the part in order to bring it into the configuration constituting its second state of shape memory by subjecting it, at ambient temperature, to mechanical stress, to subject this part under mechanical stress to lower the temperature as it is brought into a martensitic phase state, to remove the mechanical stress, and to heat the part to a temperature such that it is again brought into a phase state austenitic so that it resumes the configuration constituting its first state of shape memory. This cycle can be repeated a number of times to complete the education.

The education process described does not bring complete satisfaction either. Indeed, the implementation of this process requires a large number of delicate manipulations, since during each cycle it is necessary to successively impose a mechanical stress on the part and remove this mechanical stress. Thus the education of a series of parts is time consuming and therefore expensive.

The main object of the invention is therefore to remedy the drawbacks of the above-mentioned prior art.

To this end, the subject of the present invention is a method of conditioning a piece of metal alloy capable of undergoing a reversible transformation from the state of crystallographic phase of austenitic type to state of crystallographic phase of martensitic type for memorization. reversible of two states of shape memory including the operations consisting
- to conform, at room temperature, the part to the form constituting the first state of shape memory,
to mechanically maintain the part in its first state of shape memory and to heat the part thus maintained to bring it into a state of austenitic crystallographic phase, and
- subjecting the mechanically maintained part to a sudden lowering of the temperature, then to a thermal stabilization treatment while retaining its austenitic phase state, and
- to submit the part to an education process, in order to conform it in the second state of shape memory.

Thus, according to this method the part is prepared while being maintained in a configuration corresponding precisely to its first state of shape memory so that it retains the desired initial conformation, whatever the complexity of its geometry.

According to an advantageous characteristic of the invention, the education method consists in subjecting the stabilized part in its austenitic state to a sudden lowering of temperature to bring the part into a martensitic state, while simultaneously imposing on it a mechanical stress intended for the conform in the second state of shape memory.

Preferably, this education method further comprises an operation consisting in subjecting the part in its second state of shape memory and maintained under said mechanical stress, to a series of thermal stresses to bring the part alternately from a martensitic state to an austenitic state.

This avoids the various manipulations of mechanical stressing of the part at each education cycle of the known method described above, so that education is simplified and facilitated.

Other characteristics and advantages of the invention will appear during the detailed description which follows in a possible, but not limiting, way of implementing the method according to the invention.

• - la figure 1 montre un graphique représentant les traitements thermiques que subit une pièce en fonction du temps, au cours de la mise en oeuvre du procédé selon l'invention,
• - les figures 2 et 3 montrent respectivement les configura­tions à haute température et à basse température d'un ressort réalisé selon le procédé de l'invention, et
• - les figures 4 à 10 montrent les différentes configurations du ressort aux différentes étapes du procédé de conditionnement selon l'invention.

This description will be made with reference to the accompanying drawings, in which:

• FIG. 1 shows a graph representing the heat treatments which a part undergoes as a function of time, during the implementation of the method according to the invention,
• FIGS. 2 and 3 respectively show the high temperature and low temperature configurations of a spring produced according to the method of the invention, and
• - Figures 4 to 10 show the different configurations of the spring at the different stages of the packaging process according to the invention.

The packaging method according to the invention allows the preparation and education of metal alloy parts with shape memory for the purpose of their memorization, in a reversible manner, of two states of shape memory.

These parts are produced in a known manner from a metal alloy of the type having the property of being able to undergo a reversible transformation from their austenitic crystallographic phase state (high temperature) to the martensitic crystallographic phase state (low temperature).

With such alloys, the transition from one phase state to another takes place in one direction as in the other in a temperature range. The temperature at which the austenitic phase begins to appear when the alloy is heated is called As and the temperature at which the phase formation is completed is called Af (Af> As). Similarly, during cooling of the alloy, the temperatures at the start and end of the martensitic phase transformation are called Ms and Mf respectively (Mf <Ms).

In general, it should be noted that Ms and Mf are significantly lower than Af and As respectively, the temperature intervals [As, Af] and [Ms, Mf] being dependent on the composition of the alloy.

We will now describe in connection with Figure 1 the method of packaging a part P according to the invention.

Figure 1 is a graph whose abscissa axis represents time and the ordinate axis of temperature. This graph schematically represents the thermal cycles and the configurations of a part P to be conditioned, during the successive operations 0₁, 0₂ ... O₇ of the process.

The first two operations 0₁ and 0₂ are carried out at ambient temperature T₁, that is to say from 0 ° to 50 ° C. approximately. Note that the reference temperatures As, Af, Ms, Mf can be higher or lower than ambient temperature, depending on the metal alloy used. These temperatures can be lower than 0 ° C, or higher than 0 ° C as shown on the graph.

During operation 0₁, the part P is shaped using appropriate configuration means according to a determined configuration. This configuration which constitutes a first state of shape memory corresponds to the configuration of the part at high temperature.

In particular in the case where the initial configuration of the part and the first state of shape memory are very far apart, it may be advantageous to proceed with the configuration of the part in several successive stages, each using a particular configuration means for progressively passing from the initial configuration to the first state of form.

The part thus shaped is then placed in a device in which it can be maintained under mechanical stress σ (tension, compression or other) and / or simply supported, for example by a template, depending on the complexity of its geometry (operation 0₂). Thanks to this holding and / or support, it overcomes the inherent elasticity problems of the deformed part and the mechanical strength problems of the part during heat treatments. As a result, the part precisely retains its first state of shape memory.

The part is then subjected to a rise in temperature to be brought into a state of austenitic crystallographic phase (operation 0₃). During this operation, heating is carried out at the heart of the part to a temperature T₃ comprised in a range extending from approximately 600 ° to 850 ° C depending on the alloy considered. This heating is carried out, for example, in a conventional chamber oven, the latter having been previously heated.

It should be noted in this regard that the passage time of the part in the oven must be as short as possible, taking into account the shape and size of the part, in order to avoid evaporation of the light metals of the alloy. Indeed, such evaporation results in a modification of the composition of the alloy and therefore a significant modification of the thermal (transition points, etc.) and mechanical (elastic limit, etc.) characteristics. which risks modifying the ability to educate the alloy on the one hand, and the temperature range of use of the part on the other hand.

Following this heating, the part still maintained and / or supported is subjected to a sudden cooling down to a temperature T₄ (operation 0₄). The lowering of the temperature achieved, for example, by means of quenching, allows the fixation of the austenitic phase. In all cases, the temperature T₄ reached after cooling must be higher than the temperature Af otherwise the education potential of the part is lost, the latter having, in this case, passed through its austenitic-martensitic phase transformation zone without changing the configuration. Furthermore, the temperature T₄ to which the part is cooled must be chosen so that any appearance of a parasitic phase, that is to say a phase other than austenite or associated with austenite, is avoided.

Once the austenitic phase has been fixed, heat treatment is carried out to stabilize the part (operation 0₅). This treatment consists in maintaining the part for a few tens of hours at a temperature T₅ higher than Af and, for example, equal to the temperature T₄ to which the part has been previously cooled. This treatment allows a structural reorganization of the alloy and in particular makes it possible to release the internal stresses and to eliminate the gaps and other punctual defects which could have appeared during the sudden cooling.

It will also be noted that during this stabilization the maintenance and / or support can be eliminated, since the part is already fixed in its first state of shape memory.

It is imperative to note that to keep the possibility of educating the room, the room temperature between the two operations 0₄ and 0₅ must remain a few tens of degrees above the temperature Af.

The part obtained being stabilized in its first state of shape memory, can then be subjected to an education process.

The part prepared according to the invention (operations 0₁ to 0₅) can be educated according to the education process described in patent application EP-A1-161 952. However, this education process requires, as mentioned above, numerous manipulations of the parts which makes it not very advantageous in the context of mass production.

To avoid these drawbacks, an educational method is advantageously used according to the invention in which the part is first subjected to a sudden lowering of temperature to bring it into a martensitic state, while simultaneously imposing on it a mechanical stress intended to conform it in the second state of shape memory (operation 0₆). At this time, the play is already educated. Again, by lowering the temperature to bring the part into a martensitic state is meant a lowering to a temperature T₆ lower than Mf.

To complete the education of the part according to the invention, it can be imposed on the latter an additional operation 0₇. This operation consists in subjecting the part mechanically maintained in its second state of shape memory to a series of thermal stresses to bring it alternately from the martensitic state to the austenitic state. The education obtained is all the more effective as the number of thermal stresses is large and / or as the metal alloy used is of good quality.

We will now successively describe the different operations of the conditioning method according to the invention by applying it to the conditioning of a helical spring with a view to memorizing by the latter two shape memory positions in connection with FIGS. 2 to 10 .

FIGS. 2 and 3 show a helical spring 2 respectively in its first and second shape memory states.

The first state of shape memory corresponds to the shape of the spring at high temperature (T> Af) while the second state corresponds to the shape of the spring at low temperature (T <Mf).

In the example described, the spring 2 has in its form at high temperature turns 4 spaced apart from one another by a pitch X and has in its form at low temperature, its turns 4 spaced apart by a pitch Y where X> Y. Of course, the choice of the shapes of the parts at high and low temperatures is arbitrary and depends essentially on the application of these.

The alloy used to make the spring is, without limitation, a metal alloy with shape memory comprising approximately 75% copper, 18% zinc and 7% aluminum and whose phase transition temperatures are substantially the following: As = 43 ° C, Af = 68 ° C, Ms = 56 ° C and Mf = 41 ° C.

Of course, the nuances of the alloy can vary depending on whether it is desired to obtain a spring having more or less high phase transition temperatures. It will also be noted that the process which will now be described more precisely is valid for other shape memory alloys such as the Ti + Ni, Ti + Ni + X, Cu + Al + X, Fe + X alloys; etc ... X belonging to the set of metallic dopants.

Referring more particularly to Figures 4 to 8, we see the spring 2 to the different successive operations constituting the preparation before its actual education.

In Figure 4, we see the spring at room temperature before its preparation. This spring has been shaped by rolling, or any other equivalent means, from a shape memory alloy wire of the type defined above.

The following operation, represented in FIG. 5, consists in energizing the spring 2 at ambient temperature so that it takes the configuration corresponding to its first state of shape memory. To do this, the spring is fixed, for example, by each of its ends to a support device 6. This support can be constituted by a gutter, the edges 8 of the walls of the latter being each engaged between two turns of a end of the spring. It is preferable to choose a support device having a thermal inertia lower than or equal to that of the spring so as not to disturb the effects of subsequent heat treatments. In the present example, the support has been produced from a stainless steel mesh in order to avoid diffusion of the materials constituting the support on the part to be packaged.

It will be noted that advantageously, the use of a support such as a gutter allows the tensioning of a large number of parts simultaneously.

During the operation illustrated in FIG. 6, the spring 2 placed on the support (that is to say under tension) is subjected to a temperature of approximately 750 ° C. in order to bring the spring in a certain manner into the state of austenitic phase.

To do this, the spring is introduced, for example, into a conventional chamber oven, the latter having been preheated for two hours at 750 ° C. The spring is then kept in the oven for a few minutes, this time actually corresponding to the time necessary to carry out an austenitic transformation at the heart of the spring. Consequently, the heating time depends on the shapes and dimensions of the spring, and for reasons already explained above, the heating time must be as short as possible.

According to the method of the invention, it is advantageously noted that the spring retains its configuration during heating, and this even at high temperature, the tension under which it is kept preventing it from sagging despite the state of softening of the material at this temperature.

Following this operation, the austenitic phase is fixed (FIG. 7). This fixing is carried out by suddenly cooling the part to a temperature above Af while avoiding the formation of parasitic phases. In the case of the spring, it is cooled to a temperature 20 to 30 ° C higher than the temperature Af of the alloy, that is to say about 90 to 100 ° C.

This abrupt lowering of the temperature consists in quenching the spring in a bath thermostatically controlled at approximately 100 ° C. This bath contains a heat-transfer fluid having characteristics of rapid and homogeneous cooling. Preferably, cryothermal types of oils are used in this temperature range, for example, a silicone oil of the type sold under the name Rhodorsil manufactured by Rhone Poulenc.

In the case where metal alloys with shape memory having transition temperatures below 0 ° C. are used, quenching can easily be carried out in water at ambient temperature.

Once the above-described operation has been completed, it is then necessary to eliminate point faults and internal constraints inherent in sudden cooling.

To do this, the spring 2 is subjected to a thermal stabilization treatment (FIG. 8) in order to reorganize the crystal structure of the alloy and to release the internal stresses. This treatment consists in keeping the spring in the bath in which it has been cooled for 10 to 20 hours, the latter having not been removed after the previous step. Since the configuration of the spring in its first shape memory state was fixed at the same time as the quenching, it is no longer necessary to keep the latter under tension.

The preparation of the part being finished, we proceed as illustrated in Figures 9 and 10 to educate the spring.

FIG. 9 illustrates the essential operation of education, this operation consisting in simultaneously subjecting the spring 2, on the one hand, to a mechanical compressive stress C, in order to conform it in its second state of shape memory. and, on the other hand, to a sudden lowering of the temperature, namely, to a temperature lower than Mf. In the case of the chosen alloy, the spring undergoes a so-called martensitic quenching at a temperature between 0 ° and 20 ° C, the spring being pinched, for example, between the edges 10 of a gutter 12 in order to reduce its pitch . Preferably, the conformation of the spring in its low temperature form is carried out in the temperature range between Af and Mf.

Finally, the spring while remaining subject to the above mechanical stress, is alternately heated to a temperature above Af, ie 90 ° to 110 ° C, then to sudden cooling to a temperature below Mf, ie from 0 ° to 20 ° C for the alloy in question this being repeated a few tens of times.

Advantageously, the support, allowing the spring to be held under stress in its second shape memory state, is designed to allow the education of a large number of springs simultaneously. This eliminates the manipulation of the springs inherent in the process of the prior art described above.

Claims (7)

1. A method of conditioning a piece of metal alloy capable of undergoing a reversible transformation from the crystallographic phase state of the austenitic type to the crystallographic phase state of the martensitic type for the reversible storage of two shape memory states, characterized in that it comprises the operations consisting
- to conform, at room temperature, the part to the form constituting the first state of shape memory,
- mechanically maintaining the part in its first shape memory state and heating the mechanically maintained part to bring it into a state of austenitic crystallographic phase,
- subjecting the mechanically maintained part to a sudden lowering of the temperature and to a stabilization heat treatment, while retaining its austenitic state, and
- to submit the part to an education process in order to conform it in the second state of shape memory. 2. Method according to claim 1, characterized in that the education method comprises the operations of subjecting the stabilized part in its austenitic state to a sudden lowering of temperature to bring the part into a martensitic state by simultaneously imposing on it a constraint mechanical intended to conform the part in the second state of shape memory. 3. Method according to claim 2, characterized in that the education method further comprises an operation consisting in imposing on the part mechanically maintained in its second shape memory state a series of thermal stresses to bring the part alternately. a martensitic state to an austenitic state. 4. Method according to any one of the preceding claims, characterized in that the operation of shaping the part at ambient temperature comprises several successive stages for progressively passing from an initial configuration of the part to the first state of shape memory . 5. Method according to any one of the preceding claims, characterized in that during the operation during which the mechanically maintained part is subjected to a sudden lowering of the temperature and to a stabilization heat treatment, the part is suddenly subjected to a temperature appreciably higher than the temperature (Ms) of the start of formation of the martensitic phase to fix the austenitic phase and is maintained at this temperature for 10 to 20 hours. 6. Method according to any one of the preceding claims, characterized in that, in the education method the mechanical stress intended to conform the part in the second state of shape memory is imposed on the part between the temperatures of start and end of the martensitic phase. 7. Method according to any one of the preceding claims, characterized in that during the operation during which the part is subjected to a heat treatment to bring it into a state of austenitic crystallographic phase, the part is brought to a temperature close to 800 ° C and is maintained at this temperature between 1 and 60 min.

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