EP2776599B1 - Method for manufacturing a part made of a ta6zr4de titanium alloy - Google Patents

Description

The invention relates to a thermomechanical method for manufacturing a part made of a titanium alloy TA6Zr4DE, and a part resulting from this process.

The invention is particularly, but not exclusively, applicable to rotating parts of turbomachines, such as discs, journals and wheels, and in particular to high-pressure compressor discs.

Currently, according to the technique used by the applicant, the high pressure compressor discs are obtained by forging comprising a forging stage of the blank in the alpha / beta domain and a hot stamping step in the beta domain of the invention. titanium alloy. This stamping is performed at about 1030 ° C.

This press stamping step is followed by a heat treatment cycle comprising a solution step in the alpha / beta domain of the alloy at a temperature of 970 ° C, corresponding to the beta-30 ° beta transus temperature. C, for an hour. This dissolution step is followed by a quenching step in oil or in a water-polymer mixture.

Thereafter, a 595 ° C income treatment was performed for eight hours followed by air cooling.

Without taking into account any particular conditions in the implementation of this manufacturing method, an alloy having areas of coarse microstructure which are not favorable to good strength of the titanium alloy is obtained, in particular according to a test of imposed-pressure olygocyclic fatigue maintained for a certain holding time compared to the same type of fatigue test without holding time, in particular for a temperature range of use between -50 ° C and +200 ° C. The loss of life observed during this fatigue test following the introduction of a holding time at the maximum load leads to the phenomenon called "dwell effect". More precisely, it is a creep at low temperature (below 200 ° C.) which coupled with the oligocyclic fatigue, causes an internal damage of the material until the premature failure of the part.

In particular, an alloy called "6242", which comprises approximately 6% aluminum, 2% tin, 4% zirconium and 2% molybdenum. It is more specifically the TA6Zr4DE alloy according to the metallurgical nomenclature.

The type of structure conducive to the phenomenon of "dwell effect" is represented on the figure 1 non-entangled needles having the same orientation are located on either side of a grain seal 10. In this configuration called "feather" structure, the needles are parallel to each other.

On the other hand, when the alpha phase needles are well entangled, that is to say that they do not cluster in parallel needle packets but are arranged and distributed with very different orientations (see FIG. figure 1 : zone 20 or whole of the figure 2 ), we obtain a preferred structure that is not conducive to the phenomenon of "dwell effect".

Thus, the application in the aeronautical field, and in particular for a high-pressure compressor disc is very sensitive to this phenomenon of "dwell effect" because during the take-off and landing phases, the engines are subjected to operating conditions in the temperature and stress range corresponding to this phenomenon. This phenomenon can lead to ignition premature fatigue cracks, or even the rupture of the room.

This phenomenon of "dwell effect" is very well identified by the turbomachine manufacturers and it is the subject of numerous studies; moreover, it concerns all temperature-stabilized titanium alloys: beta, alpha / beta, almost alpha and alpha titanium alloys.

FR2936172 discloses a method of forging a thermomechanical titanium alloy part and FR2899241 discloses a heat treatment process of a thermomechanical titanium alloy part.

The present invention aims to provide a method of manufacturing a thermomechanical part made of a titanium alloy TA6Zr4DE which can be used industrially and to overcome the disadvantages of the prior art and in particular to provide the possibility of limiting the extent of the "dwell effect" phenomenon.

The present invention aims to improve the thermomechanical manufacturing process to obtain parts whose life time to the phenomenon of "dwell effect" is increased, despite the cyclic stresses undergone at low temperatures.

For this purpose, the present invention relates to a method of manufacturing a thermomechanical part made of a TA6Zr4DE titanium alloy comprising a forging step of a blank in the alpha / beta domain to form a preform, a stamping step to heat of the preform to form a blank, in the beta domain of the titanium alloy, and a heat treatment, characterized in that during the stamping step, the blank undergoes at all points a local deformation ε superior or equal to 1.2, this mastering step ending in an immediate cooling at an initial cooling rate of greater than 85 ° C / min, and preferably greater than 100 ° C / min.

The idea underlying the present invention corresponds to the fact that it has been found that there exist within the material of parallel needle zones or colonies, conducive to the phenomenon of "dwell effect". Such colonies are found to consist of elongated primary alpha phase needles which are relatively coarse and contiguous with each other. Such colonies may have a length of up to several millimeters over a thickness of the order of 0.1 to 1.5 mm.

Such colonies constitute locations at which, when the material is under stress, a large concentration of dislocations occurs which, when activated, without any particular thermal effect, can cause slips between the needles, which can lead to breaks.

The present invention proposes to implement a manufacturing method making it possible to limit the size of grains and "colony-like" structures, in particular by aiming at obtaining "entangled" type structures, in order to minimize the effects of dwell effect ", and this by decreasing the range of free movement dislocations, to minimize their accumulation and the risk of breakage of the room.

It is for this reason that, according to the present invention, not only a local minimum deformation of the part is achieved to obtain a fine microstructure at the end of the stamping step, but also ensures to preserve this fine microstructure by performing an immediate and sufficiently fast cooling of the blank at the end of the stamping step.

For example, the cooling ending the stamping is performed by quenching with water, especially with a water whose temperature does not exceed 60 ° C.

Advantageously, in this manufacturing method according to the invention said heat treatment comprises a solution in the alpha / beta domain of the alloy immediately followed by cooling at a cooling rate greater than 100 ° C / min throughout the entire process. room.

Preferably, the cooling ending solution dissolution is carried out by a quenching step of the room at a cooling rate greater than 150 ° C / min, and in particular between 200 and 450 ° C / min.

Advantageously, the cooling ending solution dissolution is carried out by quenching with oil or in a water / polymer mixture.

Thus, thanks to this rapid cooling, the state of the microstructure is frozen in its situation at the end of the dissolution stage and a new evolution of this microstructure is avoided with a magnification of the needles of the suitable alpha phase colonies. to the phenomenon of "dwell effect".

Also, this choice of high tempering speed makes it possible to promote the martensitic type transformation (which results in a rather fine microstructure) of the beta phase in the alpha phase with respect to the germination / growth phenomenon (which results in a rather coarse microstructure). ).

• on réalise, après l'étape de trempe terminant la mise en solution, une étape de revenu à une température de l'ordre de 595°C pendant une durée de l'ordre de 8h, avec un refroidissement ultérieur à l'air.

Also, preferably, at the end of the manufacturing method according to the invention, the method further comprises the following steps:

• after the quenching step ending the dissolution, a recovery step is carried out at a temperature of the order of 595 ° C. for a period of about 8 hours, with subsequent cooling in air.

Advantageously, the manufacturing method according to the invention further comprises, between the stamping step (followed by cooling with water) and the solution step, a machining step, and in the pre-machining occurrence, aimed at reducing the massiveness of the part. Other machining operations will follow to rectify the dimensions of the part and reach the final geometry.

After the quenching step, the cooling rate should preferably be greater than 350 ° C / min if the pre-machining step is added.

In this way, the volumes of material to be treated during the heat treatment can be reduced and the whole room cooled more quickly.

The inventors have found that this manufacturing method which makes it possible to refine the structure does not have the consequence of affecting the thermomechanical properties of the material.

Also, the present invention relates to a thermomechanical part made of a titanium alloy TA6Zr4DE with the manufacturing method which has just been presented.

Preferably, this titanium thermomechanical part forms a rotating part of a turbomachine, and in particular a compressor disk, especially a high-pressure compressor.

Finally, the present invention also relates to a turbomachine equipped with a thermomechanical part according to one of the definitions given above.

• la figure 1, déjà décrite, montre la microstructure obtenue selon le procédé de fabrication classique de l'art antérieur ;
• la figure 2, déjà décrite, montre la microstructure type obtenue selon le procédé de fabrication selon la présente invention;
• la figure 3 illustre les étapes du procédé de fabrication selon l'art antérieur et selon l'invention, et,
• la figure 4 montre les résultats de durée de vie d'un test de fatigue (cycles « trapèze » avec temps de maintien) à température ambiante, pour une pièce résultant du procédé de fabrication de l'art antérieur et pour une pièce obtenue par le procédé de fabrication conforme à l'invention et sur deux zones (référencées 3 et 5) de massivité différente de la pièce.

Other advantages and characteristics of the invention will become apparent on reading the following description given by way of example and with reference to the appended drawings in which:

• the figure 1 , already described, shows the microstructure obtained according to the conventional manufacturing method of the prior art;
• the figure 2 , already described, shows the typical microstructure obtained according to the manufacturing method according to the present invention;
• the figure 3 illustrates the steps of the manufacturing method according to the prior art and according to the invention, and
• the figure 4 shows the lifetime results of a fatigue test (trapezoidal cycles with holding time) at ambient temperature, for a part resulting from the manufacturing process of the prior art and for a part obtained by the manufacturing process according to the invention and on two zones (referenced 3 and 5) massiveness different from the room.

In relation to the figure 3 , it is recalled what is the conventional heat treatment of the prior art used in particular by the applicant company for high pressure compressor discs made in titanium alloy TA6Zr4DE or "6242".

Initially, a blank or billet of material is forged in the alpha / beta domain for example at 950 ° C and followed by air cooling to form a preform.

This preform then undergoes a hot stamping step in the beta domain of the titanium alloy at a temperature of 1030 ° C., corresponding to the temperature of beta transus +30 ° C., followed by cooling with water after forging. hence the obtaining of a blank (also called "milled stock") intended to form a disk.

This mastering step is followed by a heat treatment comprising a solution step in the alpha / beta domain of the alloy at a temperature of 970 ° C., corresponding to the temperature of beta-trans-30 ° C., during a hour.

This dissolution step is followed by an oil quenching step or in a water-polymer mixture (minimum initial cooling rate of the order of 200 ° C./min and between 200 and 450 ° C. min).

Subsequently, a 595 ° C. income treatment was carried out for eight hours with cooling in air.

A material having the microstructure visible on the figure 1 , having at certain sites colonies consisting of alpha phase needles parallel to each other and located on either side of a grain boundary. These needles have an elongate section visible in the figure often extending over several hundred micrometers.

• lors de l'étape de matriçage, la pièce brute subit en tout point une déformation locale ε supérieure ou égale à 1,2. Avantageusement, cette valeur de déformation locale minimale ε est de 1,5, de préférence supérieure à 1,7, voire de 1,9, avec une majorité de point dépassant 2.

On the figure 2 , the visible microstructure corresponds to that of a titanium alloy identical to that of the figure 1 , having undergone the above-mentioned manufacturing process with the following difference:

• during the stamping step, the blank undergoes at all points a local deformation ε greater than or equal to 1.2. Advantageously, this minimum local deformation value ε is 1.5, preferably greater than 1.7, or even 1.9, with a majority of points exceeding 2.

In this case, parallel needle colonies are fewer in number and smaller in size. The majority of the needles are entangled and are, moreover, dissimilar in size. Indeed, as is apparent from the figure 2 , the needles are all smaller in section, their length remaining less than 100 micrometers, and generally of the order of 20 to 50 micrometers.

Consequently, it can be expected that the absence of parallel aligned large needles prevents the "dwell effect" by preventing accumulations of dislocations likely to cause risks of rupture.

Indeed, the decrease in the size of the needles is accompanied by a decrease in their volume and contiguous surfaces between needles, which hampers the ability to move defects such as dislocations or gaps, which travel distances weaker and have fewer possibilities to accumulate.

According to the present invention, the term "local deformation" means the equivalent generalized deformation in the sense of Von Mises calculated by simulation software Forge 2005. The calculation equation:

With [ε] ^pl which corresponds to the tensor of the plastic deformations.

To ensure that the minimum local deformation value has been obtained at all points after the mastering step, a simulation is used, using CAD (computer-aided design) means.

In particular, the material resulting from the entire manufacturing process has thermomechanical characteristics, and in particular the fatigue properties of olygocyclic fatigue under imposed deformation, which are no less than those of the materials resulting from the manufacturing process of the invention. prior art.

An imposed constraint olygocyclic fatigue withstand test was conducted using a trapezoidal signal (1 s without constraint, 40s with stress, 1s without stress), with a maximum stress of 772 MPa, at room temperature, for a high pressure compressor disk.

The results in terms of the number of cycles before rupture for tests carried out in an area 3 (corresponding to a bore) and in a zone 5 (corresponding to the web) of the disks, and visible on the figure 4 , are mentioned in the following table 1: Table 1 Zone 3 average Zone 5 average Standard range - prior art 88278 91754 30003 28000 95235 25997 Invention 150903 139443 (+ 52%) 105213 120821 (+ 331%) 127903 136430

Thus, there is an increase in the lifetime and therefore a resistance to the phenomenon of "dwell effect" ranging from a factor 1.5 (in zone 3) to a factor 4 (in zone 5), which is very significant.

Of the other comparative mechanical tests carried out which have demonstrated a resistance at least as high for a part resulting from the manufacturing method according to the invention as for a part from a standard range, tensile tests (at 20 ° C and 450 ° C) and creep-elongation tests at 500 ° C.

A lifespan increased by a factor of 3 for a part resulting from the manufacturing method according to the invention compared with a piece from a standard range, has also been noted, in the case of a stress vibration fatigue test. imposed at room temperature, at a frequency of 80 Hz.

Claims (12)

1. A thermomechanical method of fabricating a part made of TA6Zr4DE titanium alloy, the method comprising a step of forging a blank in the alpha/beta domain to form a preform, a step of hot die-stamping the preform in order to form a rough part in the beta domain of the titanium alloy, and heat treatment, the method being characterized in that during the hot die-stamping step, the rough part is subjected at any point to local deformation ε greater than or equal to 1.2, this hot die-stamping step terminating with immediate cooling at an initial cooling rate faster than 85°C/min, wherein the local deformation is defined by

   

   Where [ε]^pl corresponds to the plastic deformation tensor.
2. A fabrication method according to claim 1, characterized in that said heat treatment includes solution heat treatment in the alpha/beta domain of the alloy immediately followed by cooling at a rate faster than 100°C/min.
3. A fabrication method according to any preceding claim, characterized in that the cooling that terminates the die-stamping is performed by quenching in water.
4. A fabrication method according to claim 2 or claim 3, characterized in that the cooling terminating the solution heat treatment is performed by a step of quenching the part at an initial cooling rate faster than 150°C/min.
5. A fabrication method according to claim 4, characterized in that the cooling terminating the solution heat treatment is performed by quenching in oil or in a water/polymer mixture.
6. A fabrication method according to claim 4, characterized in that the rate of cooling during the quenching step terminating the solution heat treatment lies in the range 200°C/min to 450°C/min.
7. A fabrication method according to any preceding claim, characterized in that it further includes the following step:

   • after the quenching step terminating the solution heat treatment, performing an annealing step at a temperature of about 595°C for a duration of about 8 h, with subsequent cooling in air.
8. A fabrication method according to any preceding claim, characterized in that it further includes, between the die-stamping step and the solution heat treatment step, a machining step seeking to reduce the massivity of the part.
9. A thermomechanical part made of a TA6Zr9DE titanium alloy using the fabrication method according to any one of claims 1 to 8.
10. A thermomechanical part according to claim 9, characterized in that it forms a rotary part of a turbomachine.
11. A thermomechanical part according to claim 9 or claim 10, characterized in that it forms a high pressure compressor disk.
12. A turbomachine including a thermomechanical part according to any one of claims 9 to 11.

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