THERMAL TREATMENT FOR THE STRESS-RELIEF OF TITANIUM ALLOY PARTS

Abstract

The invention relates to a process for the preparation of a part made of titanium alloy, comprising a thermal treatment for relaxing the internal stresses of the part, the thermal treatment comprising maintaining at a temperature “T1” greater than the beta transus (beta transition) temperature, referred to as “Tbt”, and the part being free to deform by creeping. The invention also relates to a tool for carrying out this process.

Description

The invention relates to a stress thermal relaxation treatment and to a shaping tool for carrying out this relaxation thermal treatment. The invention relates in particular to a thermal treatment process for slender parts or alternatively parts which are more massive but which comprise significant variations in cross section made of titanium alloy, in particular of the alpha-beta type, that is to say for which the microstructure exhibits, at ambient temperature, both the alpha phase and the beta phase. A widespread alpha-beta alloy is in particular the alloy known under the trade names TA6V or TA6V4 or also Ti-6Al-4V.

STATE OF THE ART

Alpha-beta titanium alloys and in particular of TA6V type make it possible to produce parts having a good compromise between mechanical strength and toughness, in addition to their low density and their good resistance to corrosion. As a result of the mechanical properties which it is possible to achieve, alloys of TA6V type are used in various fields of application, in particular in the aeronautical field, to produce large parts, such as spars of engine pylons, aircraft door frame elements or also structural frame elements for aircraft.

These parts are typically obtained by a sequence of stages of forging and stamping, thermal treatments and machining.

The abovementioned large parts are typically either:

• • slender parts, that is to say which are very slender, which is in particular such that the length/mean thickness or length/mean diameter ratio is greater than 10. Slender parts of the targeted type can reach a length of 5 meters for a mean thickness of less than 100 mm, with or without variation in cross section; or
  • parts which are more massive but with large differences in cross section, that is to say with variations in cross section having cross section ratios greater than 2/1. Part “which is more massive” is understood to mean a part for which the dimensions (diameter, thickness, length, and the like) are such that the diameter of the largest sphere capable of being included within the volume of the part is greater than 150 mm. Reference is also made to “equivalent diameter” greater than 150 mm. In particular, the parts concerned can be structural frame elements, the maximum equivalent diameter of which can reach 250 mm.

The specific geometries of these large parts promote, after thermomechanical treatments, normal machining and/or thermal treatment operations, the appearance of internal stresses of thermal or crystallographic origin and also distortions and deformations of the part.

These are real problems encountered by manufacturers employing thermomechanical transformation and machining processes with the purpose of obtaining, from titanium alloy billets, large parts which have to exhibit superior mechanical properties and observe precise size tolerances

This is because the more slender the part and/or the more it comprises large differences in cross section, the more the part will have a tendency to deform during the cooling stages or to crack and break prematurely as soon as it is thermomechanically stressed, For example, the more the part comprises large differences in cross section and the greater the heterogeneity in the cooling rates from one section to another (faster cooling rate at the core of a small cross section than at the core of a more massive cross section), the greater the concentrations of stresses at the variations in cross section.

As a result of these problems, it is to date difficult, for these types of parts, to observe precise size tolerances and to achieve superior mechanical properties.

In order to overcome these disadvantages, it is known to carry out, during or after the thermomechanical transformation stages (forging and/or stamping), slow cooling operations, either:

• • by performing a controlled cooling while keeping the parts in a furnace, or else
  • by leaving the parts to cool in thermally insulated chambers.
    These slow cooling operations, which are carried out at rates of less than 5° C./min, are expensive in terms of energy used and in terms of immobilization of material and tools, The fact of having to slowly cool the parts considerably lowers the productivity of the plants.

Before the rough and final machining operations and in order to overcome these disadvantages, it is also known to carry out a “stress-relieving” alpha-beta thermal treatment (less than 730° C.) also know under the name stress relaxation treatment, in order to reduce the stresses of thermal or crystallographic origin inside the part in order to prevent the part from deforming during machining operations.

However, this type of relaxation treatment does not make it possible to eliminate all the thermal stresses and can sometimes introduce new stresses if the cooling is too fast and as a function of the size of the variations of cross section (differences in mass) and/or of the differences in cooling rate inside the Part.

For these large parts requiring significant mechanical properties (and specifically properties of tolerance to damage (that is to say, of resistance to crack propagation)), it is known to additionally carry out a thermal treatment for improving the mechanical characteristics. The purpose of this treatment is to improve the mechanical characteristics of the alloy and generally comprises the maintenance of the part in a predetermined position (often by virtue of clamping tools). This is a treatment named “beta treatment”, that is to say the treatment above the beta twists temperature. This treatment makes it possible to obtain a structure having coarse grains, making it possible in particular to improve the characteristics of resistances to crack propagations. However, this treatment, also carried out before the machining operations, has the known disadvantage of deforming the part, either by creeping, during the treatment itself above the beta transus, or during the cooling from this temperature.

Thus it is that, in order to avoid this problem of uncontrolled deformation during the beta treatment, the part is stressed by clamps or any other tool which makes it possible to prevent relative variations in the dimensions of the part. However, a quid pro quo results in the introduction of new stresses inside the part which are difficult to eliminate, even on carrying out afterwards an alpha-beta relaxation thermal treatment such as that mentioned above. The consequence is that, during subsequent machining operations, the internal stresses which have accumulated in addition to those resulting from the preceding thermomechanical treatments can be released and result in greater deformations of the part or result, under the effects of the additional pressures exerted by the cutting tools, in microcracks or tears of the part at the regions of the part under the greatest stresses.

Furthermore, and as soon as the clamps are released after beta thermal treatment, the declamped part has a tendency to be deformed again under the effect of the accumulated internal stresses. Thus, before carrying out the machining operations, the parts are generally deformed and in a different way from one part to another, so that the machining parameters and references have to be adjusted from one part to another in order to avoid obtaining a part which does not observe the final size tolerances required.

Purposes of the Invention

A purpose of the invention is to overcome the abovementioned disadvantages and to provide a stress-relaxation thermal treatment process which makes it possible to obtain parts which observe the final size tolerances required and which are devoid of residual internal stresses while making it possible to achieve the prescribed mechanical properties.

A second purpose of the invention is to solve the new technical problem consisting in providing a shaping tool which makes it possible, with parts made of titanium alloy, to conform, at treatment temperature, to a calibrated pattern cavity and to cool it without generating internal thermal stresses and without deformation, or with a minimum of stresses and deformation, that is to say significantly less than the stresses brought about by a conventional stress-relaxation treatment.

Another purpose of the invention is to solve the new technical problem consisting in providing a process for the preparation of a titanium alloy part comprising a stage of thermomechanical transformation and a machining but without it being necessary to carry out beforehand a relaxation treatment stage.

DESCRIPTION OF THE INVENTION

To this end, a first subject matter of the invention is a process for the preparation of a titanium alloy part, characterized in that the process comprises a thermal treatment for relaxing the internal stresses of a titanium alloy part, for example having been subjected beforehand to one or more thermomechanical transformation stages, in particular bringing about internal stresses, the process being characterized in that the thermal treatment comprises maintaining at a temperature “T1” greater than the beta transus (beta transition) temperature, referred to as “Tbt”, and in that the part is free to deform by creeping.

According to one alternative form, the titanium alloy is of alpha-beta type and in particular of TA6V type.

Advantageously, maintaining at the temperature T1 is carried out for a time sufficient to make possible the complete transformation of the microstructure of the alloy from a dose-packed hexagonal structure to a body-centered cubic structure.

Preferably, the temperature T1 is greater by at least 5° C. than Tbt and is preferably greater by at least 10° C. than Tbt, and preferably again the process comprises maintaining at the temperature T1 for a period of time of 5 to 120 minutes, preferably for 15 to 60 minutes, and preferably at temperature from 1010° C. to 1060° C., in particular for an alloy of TA6V type.

Subsequent to maintaining at the temperature Ti, it is possible to carry out cooling with a cooling rate of greater than 5° C./min, preferably of greater than or equal to 10° C./min, preferably of between 10 and 20° C./min, such as, for example, in the air, preferably outside the treatment furnace.

According to one alternative form, the titanium alloy part is positioned for the stress-relaxation treatment in a shaping tool comprising one or more pattern cavities calibrated to receive a part to be relaxed, said shaping tool preferably being made of at least one single or composite material, the thermal inertia of which is greater than that of titanium or of the titanium alloy used and the variations in size of which related to creeping at the temperature T1 are virtually nonexistent (less than 2 mm rising), indeed even nonexistent.

According to a specific embodiment of the invention, the shaping tool is made of concrete or composite concrete, preferably comprising curved stainless steel fibers, the distribution of which in the material is isotropic.

The invention also relates to a process for the preparation of a titanium alloy part comprising one or more rough machining stages prior to the stress-relaxation thermal treatment stage which comprises maintaining at a temperature “T1” greater than the temperature “Tbt”.

In the present patent application, “one or more rough machining stages” is understood to mean machinings which make it possible to remove, on one or more given surfaces of the part, at least 70% of material allowances. In a different way and as described below, final machining stages are, for their part, carried out in order to obtain the final dimensions of the part and the final surface condition required by eliminating the remaining allowances, i.e. less than 30% with respect to the starting allowances before the 1^st machining stage.

Advantageously, the rough machining stage or stages are carried out in order to treat virtually all, indeed even all, of the surfaces of the parts.

Advantageously, the rough machining is carried out on a part which has not been subjected to stress-relaxation treatment, in particular by maintaining at a temperature lower than Tbt and typically lower than 730° C. Advantageously, during the machining stage, the part is flattened and shaped against at least one reference support.

According to a specific embodiment, the part is flattened and shaped against said reference support by flattening, against the reference support, one or more flashes formed around the part and resulting from an upstream stamping stage.

After the stress-relaxation thermal treatment according to the invention, the process can comprise one or more stages of final machining of the titanium alloy part which make(s) it possible to treat one or more surfaces of the part in order to remove areas of surface contamination, to obtain a specific roughness and to achieve the final dimensions of the part.

Typically, the part is a slender part, such as a spar of an engine pylon, a door frame element or a structural frame element for an aircraft.

The invention also relates to a shaping tool comprising a shaping region comprising one or more pattern cavities calibrated in order to shape, by creeping, one or more slender parts and/or parts with large differences in cross section of titanium alloy, said shaping tool being composed of at least one single or composite material, the thermal inertia of which is greater than that of titanium or of the titanium alloy and the variations in size of which related to the creeping at a temperature of 1060° C. are virtually nonexistent, indeed even nonexistent.

According to one embodiment, the tool is composed of concrete and optionally comprises, in addition, curved stainless steel fibers distributed isotropically in the concrete.

Advantageously, the tool has dimensions so that the cooling rates are substantially constant from one slice, part plus tool, to another.

According to a preferred embodiment, the pattern cavity region is calibrated in order to shape, by creeping, a slender part exhibiting a slenderness of greater than 10 and/or different cross sections, the variation in cross section of which is greater than 2/1, and preferably in order to receive a spar of an engine pylon, a door frame element or a structural frame element for an aircraft.

Advantageously, the titanium alloy is of alpha-beta type and is preferably a TA6V alloy.

According to a specific embodiment, the pattern cavity comprises at least two supporting surfaces on which the part to be relaxed can at least partially rest, said supporting surfaces preferably being positioned so that, when a titanium alloy part is maintained at a temperature T1 greater than its temperature Tbt, said part can be positioned by creeping with greater contact, virtually complete contact, indeed even complete contact, on the supporting surfaces, in order to correct the deformations, such as the curving or twisting defects related to a prior stage of preparation of the part to be shaped, in particular during a thermomechanical transformation, a cooling or a machining.

Advantageously, the pattern cavity comprises a positioning stop formed in each calibrated mold, the other end of the mold being free, that is to say without a stop, in order to allow the part to freely deform by creeping, or comprises a stop positioned by taking into account the creeping and the thermal expansion coefficient of the part to be relaxed before and after relaxation thermal treatment, in particular in order to make possible free deformation by creeping.

Alternatively, the pattern cavity comprises a visual marker which makes it possible to position the part in the calibrated pattern cavity.

The invention thus also relates to a process for the preparation of a titanium alloy part employing the tool of the invention and in particular as defined in any one of claims 17 to 24.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a thermal treatment process for relaxing thermal stresses in a titanium alloy part which has been subjected to one or more thermomechanical transformation stages, characterized in that the part is free to deform by creeping and the thermal treatment is carried out at a temperature greater than the beta transus (beta transition) temperature of the alloy in order to relax the stresses, that is to say above the point where the alpha phase completely disappears to the advantage of the beta phase and where the microstructure is transformed into a structure of body-centered cubic type.

In the invention, “creeping” is understood to mean the deformation of a part brought about, during maintenance at the temperature greater than the beta transus temperature, by the dead weight of the material or also by exerting a constant stress on the part by applying a load to the latter. The thermal treatment according to the invention makes it possible simultaneously to obtain a coarse-grained structure in order to improve the characteristics of resistances to crack propagations and the relaxation of the internal stresses of thermal and crystallographic origins by leaving the part free to deform without use of specific tools, such as clamps, which, by an external pressure on the part, make it possible to force the latter into a predetermined form. “Coarse-grained structure” is understood to mean a structure which typically has a structure of virtually isotropic grains with a size of less than approximately minus 3 ASTM with regions of grains less than minus 5 ASTM.

Thus and by virtue of the thermal treatment (THT) according to the invention, all the residual internal stresses can be eliminated, so that no specific management of the stresses is necessary before the thermal treatment as the thermal stresses are eliminated during the thermal treatment according to the invention. It is, for example, not necessary to take care to eliminate or limit the appearance of internal stresses, to carry out slow coolings or specific relaxation treatments before (rough) machining operations.

By heating the alloy to a temperature which is greater than the beta transus temperature, the alloy is subjected to an allotropic transformation where the alpha phase of close-packed hexagonal crystallographic structure is transformed into a body-centered cubic phase referred to as beta phase. During this transformation, the residual stresses, which appeared in particular in the part during the preceding thermomechanical transformation stages and which are of thermal and crystallographic origins, are released. Furthermore, in a body-centered cubic structure, the mobility of the dislocations is greater, which also promotes the release of the internal stresses.

This complete relaxation of the stresses contributes to preventing, during the cooling from the treatment temperature according to the invention and subsequently when the part is mechanically and/or thermally stressed, deformations of the part, stress cracks or the premature appearance of cracks in the parts.

For the implementation of the thermal treatment according to the invention, the part is deposited in a calibrated pattern cavity formed on a shaping tool supporting at least one part and, during the thermal treatment, the part conforms, by creeping, to the calibrated pattern cavity.

In addition to the abovementioned advantages, the body-centered cubic structure is favorable to the creeping of the part, Thus, the thermal treatment according to the invention is, contrary to practice, carried out by leaving the part free to deform by creeping, so that, under the effect of its own weight or, if appropriate, by applying an additional load, the part can deform in order to perfectly match the shape of a calibrated pattern cavity produced in the tool supporting the part and corresponding to the required final shape of the part.

As will be understood, during the thermal treatment according to the invention, the part is not prevented from deforming by constraining it using, for example, clamps but, on the contrary, the part is left to freely deform by creeping, so that the part comes to freely (without constraint) match a pattern cavity which is it calibrated. Thus, the parts which deform by creeping can be easily shaped to the calibrated pattern cavity in order to take the shape of the targeted final part while making it possible to eliminate internal stresses and while avoiding the generation of new stresses.

In the context of the invention, the calibrated pattern cavities make it possible to control the deformation of the parts by creeping. The calibrated pattern cavities have dimensions in order for the shape and the dimensions of the part obtained after shaping to be those of the final parts, minus the final machining.

The dimensions of the calibrated pattern cavity are determined by digital simulation according to the shape of the final part to be obtained and of the kinetics of creeping of titanium or of the titanium alloy and by taking into account the expansion differential, in the vicinity of 1000° C., between the concrete and titanium alloy under consideration.

Advantageously, the thermal treatment temperature is maintained for 5 to 120 minutes above the beta transus temperature +5° C. and preferably between 15 and 60 minutes from 1010° C. to 1060° C., for example for an alpha-beta alloy of TA6V type, as defined below, the beta transus of which is, according to the exact composition of the alloy, between 980 and 1000° C. The treatment temperature and time depend on the exact composition of the alloy.

A widespread α+β alloy is in particular the abovementioned alloy known under the TA6V or TA6V4 or also Ti-6Al-4V trade names, the composition of which, described as percent by weight with respect to the total weight of the alloy, is typically as follows:

5.50≦Al≦56.75

3.50≦V≦4.50

traces≦Fe≦0.30

traces≦O≦0.20

traces≦C≦0.08

traces≦N≦0.05

traces≦H≦0.0125

traces<Y<0.005

the remainder being composed of titanium and of purities resulting from the preparation;
and preferably a titanium alloy in accordance with AMS 4928 GR5 (Aero Material Specification).

Furthermore, the minimum treatment temperature is determined in order to make possible the complete allotropic transformation from a dose-packed hexagonal microstructure to a body-centered cubic microstructure and in order to obtain a beta-grain mesostructure. For its part, the maximum treatment temperature is determined in order to avoid coarse-grain growth kinetics with, in some regions of the microstructure, an explosion in the size of the grains which would be harmful to the achievement of good mechanical properties, such as the tensile strength and fatigue strength, and the like.

The treatment time depends on the massiveness of the part. Specifically, the greater the equivalent diameter of the part, the longer the treatment time,

Typically, for a TA6V part having an equivalent diameter of 15 to 30 mm, the treatment time is from 20 to 40 minutes, so as to make possible the complete dissolution of the beta phase in the part and the elimination in the part of the residual thermal stresses and in order to allow the part to creep and to perfectly conform to the calibrated pattern cavity of the tool. In another example, for a part with an equivalent diameter of 80 mm, the treatment time is approximately 1 hour.

The treatment time corresponds to the time during which the core of the part will be at thermal stationary phase temperature.

Preferably, the part is cooled from the treatment temperature according to the invention down to ambient temperature at a rate of greater than 5° C. per minute and preferably of greater than 10° C. per minute. The parts are typically cooled with air outside the treatment furnace.

During the cooling of the treated part from a temperature greater than the beta transus, the alloy will pass through the alpha-beta transformation domain during which the alpha phase reappears. The inventors have been able to observe that, in alloys of alpha-beta type, such as TA6V, and when the cooling rates are too slow, the germination of the alpha phase takes place essentially at the beta grain boundaries (intergranular germination) in the form of plates initiated at the beta grain boundaries (known under the name of intergranular Widmanstätten morphology) and as thick intragranular plates. At cooling rates greater than 5° C./minute, the alpha phase appears with an intragranular and intertwined fine needle morphology. The fine needles intertwine randomly, so that the alpha phase forms a network which counteracts any new dislocation movements, thus preventing the part from deforming during the cooling thereof or when it is subsequently mechanically stressed, Furthermore, this morphology confers, on the part, better mechanical characteristics, in particular in tensile strength (Rm>900 MPa), and confers good crack propagation behavior as the pathway for propagation of the cracks is more tortuous, and also good toughness.

A second subject matter of the invention is a shaping tool on which the parts to be shaped are positioned. The tool is characterized in that it is made of a material which has a greater thermal inertia than that of titanium or the titanium alloy used and which does not creep up to temperatures of approximately 1060° C.

This is because the inventors have been able to find that, by virtue of such a material, it is possible to obtain rapid cooling of the part (more than 10° C. per minute, e.g.: in the open air) which is homogeneous inside the part, making it possible to obtain homogeneous microstructures of the alloy as fine entangled needles and making it possible to avoid, during the cooling, the appearance of internal thermal stresses and uncontrolled deformations of the part during or subsequent to the cooling.

In a preferred form according to the invention, the tool is made of concrete, which can optionally comprise curved stainless steel fibers which have a greater coefficient of expansion than the concrete. Such fibers make it possible to stiffen the structure of the tool (by contraction of the fibers), so as to prevent it from being deformed at high temperatures under the weight of the parts. Use is preferably made of a refractory concrete. The concrete can be a concrete of the MFRRC (Metal Fiber-Reinforced Refractory Concrete) type having a thermal conductivity of approximately 3.5 W.m⁻¹.K⁻¹ (watt/(meter*degree kelvin)), a specific heat, measured at 500° C., of approximately 1000 J.kg⁻¹.K⁻¹ (joule/(kilogram*degree kelvin)) and a density of approximately 3000 kg/m³.

The tool comprises calibrated pattern cavities, in each of which is deposited a part to be shaped, in order to obtain, after a heat treatment according to the first subject matter of the invention, a part of predetermined shape, the shape of the part minus the final rnachinings, devoid of residual internal (thermal or crystallographic) stresses.

A third subject matter of the invention is a process comprising, after thermomechanical transformation, on the one hand, a thermal treatment in accordance with the first subject matter of the invention, in order to relax the residual stresses and to shape the part according to a predetermined shape, and, on the other hand, a rough machining schedule, characterized in that the machining schedule precedes said thermal treatment schedule and in that, during the machining, the part is flattened and shaped against a reference support.

Contrary to current practice, the inventors have been able to find that, by virtue of the thermal treatment according to the first subject matter of the invention, it is possible to carry out the rough machining before the thermal stress-relaxation treatment in the part. This is because practical experience would have it that the rough and final machining be carried out after the relaxation thermal treatments, below the beta transus temperature, for example at temperatures below 730° C., in order to machine a part having, at the start, a minimum of residual internal stresses, so as to prevent the part from cracking and from being deformed during the machining operations, Furthermore, the machinings are carried out after the final thermal treatments in order to eliminate the contaminated surface regions (e.g.: surface oxidations) subsequent to the thermal treatments. Before rough machining, practical experience would have it that the part also be deformed as little as possible and in a constant predetermined shape in order for the machining reference frames and parameters to be reproducible from one part to another and in order to allow the size tolerances required for the final part to be observed, it is known, during the stages which precede the rough machining, to attempt to avoid deforming the part by carrying out slow codlings and, during the thermal treatment phases, above the beta transus, targeted at obtaining the mechanical properties of the alloy, to maintain the part in a predetermined shape by constraining the latter, for example using damping tools.

However, these slow coolings and these relaxation treatments carried out to date were not sufficient to remove all the internal stresses in the part and to shape the part according to a predetermined shape. Furthermore, the fact of constraining the part according to a predetermined shape during the thermal treatments carried out above the beta transus temperature did not prevent the part from relaxing and from again deforming once the part was released from its damps. It thus frequently happens that, before machining, the part to be machined still contains internal stresses and is further deformed despite the precautions taken upstream. It then becomes very difficult to observe the final size tolerances targeted.

Furthermore, during the machining operations, the cutting tools exert a high pressure on the part, which generates additional internal stresses, in particular when rough machining operations are concerned, where the machining parameters apply greater forces on the part than in “final” machining. This is because, for the abovementioned types of parts and during the rough machining operations, the depths of machining cuts are generally greater than those carried out during the final machining operations. In rough machining, the rates of feed are also faster than in final machining. These additional stresses can increase the deformations of the part or cause premature cracks and/or breaks in the part, in particular when the latter is subsequently subjected to operational thermomechanical stresses.

After rough and final machining, it is therefore frequently necessary to carry out a new stress-relaxation thermal treatment.

The inventors have been able to observe that, when the heat treatment makes it possible to efficiently shape the part according to a predetermined shape and makes it possible to sufficiently reduce the internal stresses so as to prevent the part from deforming again after thermal treatment or from cracking prematurely during the use thereof, it is not necessary, before the thermal treatment, to specifically manage the stresses. Consequently, the inventors have been able to determine that it is possible to carry out the rough machining before the relaxation thermal treatment according to the invention provided that, for each part (or at least for the main surfaces of each part), during the rough machining operations, a predetermined and constant size is observed from one part to another, so that the machining parameters and reference frames are repeatable from one part to another and so as to be able to observe the size tolerances required for the final part.

This is obtained according to the invention by flattening and shaping the part against a reference support. More specifically, and as represented in FIG. 7 (diagrammatic view where the stamping flashes (5) are deliberately exaggerated to enhance understanding), the flash or flashes formed around the part and resulting from the final stamping operation are concerned, which flash or flashes are gripped against one or more reference supports in order to make it possible to shape the whole of the part. This is because, by exerting a combination of localized pressures on the flashes, so that the parts are flattened and deformed against the reference support or supports, the combined part is indirectly shaped according to a geometry which is predetermined and constant from one part to another, so that:

• • the parts are always machined in a given position and starting from a given geometry,
  • the machining references are repeatable from one part to another, and in order that
  • each machined part can observe the final size tolerances required.
    The flash or flashes of the part are flattened against the reference support or supports using (mechanical or hydraulic) clamps or any other tools which make it possible to apply a series of localized pressures in order to grip the stamping flashes against localized bearings formed on the support or supports. The position of the localized bearings thus determines the geometry of the part.

Contrary to normal practice, in the context of the invention, the flashes are not removed after the final stamping operation in order to provide regions for gripping between the clamps and the reference support(s) and so as to shape the part as mentioned above without interfering with the machining operations, which can he carried out on either side of the part and flashes on a single occasion, that is to say without being obliged to separate and reposition the part between two machining stages. Furthermore, by gripping the flashes of the part, marking/damaging the ape rational/working surfaces of the part is avoided.

At this stage, it is specified that it is known from the prior art to use clamps and tools to fix and hold parts to be machined but, until now, it was not known to flatten and shape these parts against one or more reference supports, furthermore by using the stamping flashes as gripping region. The flashes are removed after the rough machine stages and preferably before the stress-relaxation thermal treatment. The flashes are eliminated, for example, by machining.

After machining, the part includes a significant amount of internal stresses which originate:

from the residual thermal and crystallographic stresses resulting from the upstream thermomechanical and thermal stages,

from the stresses due to the flattening and to the shaping (at ambient temperature) of the part against its reference support during the machining, and

from the stresses introduced by the cutting tools during the machining operations proper.

As a result of these stresses and after machining, when the part is released, the latter relaxes and is deformed again. However, and by virtue of the effectiveness of the thermal treatment according to the invention, the latter defects are corrected during the thermal treatment according to the invention, which allows the part to regain a calibrated shape with a significant reduction in the internal stresses so that the part is not deformed again and does not crack prematurely during the cooling or subsequently when it is subjected to operational thermomechanical stresses.

During the relaxation thermal treatment, the parts are shaped according to the calibrated pattern cavity formed in a shaping tool.

The thermal treatment is followed by a final machining schedule in which the machining parameters (depth of cut, cutting speed, feed motion, and the like) are determined so as not to generate stresses inside the part which might bring about deformations or premature cracks in the part.

The final machining operations generate few stresses inside the part. The depths of cut and the rates of feed are lower than during the rough machinings. After such a final machining, it is not necessary to carry out a stress-relaxation treatment. Furthermore, it is preferable to carry out a final machining after the thermal treatment according to the invention in order to obtain a good surface condition (roughness and hardness) and to eliminate the areas of surface contamination generated during the final thermal treatment.

Other purposes, characteristics and advantages of the invention will become more clearly apparent to a person skilled in the art following the reading of the explanatory description, which refers to examples and figures which are given only by way of illustration and which cannot in any way limit the scope of the invention.

The examples and figures form an integral part of the present invention and any characteristic which appears novel with respect to any prior state of the art from the description taken in its entirety, including the examples and figures, forms an integral part of the invention in its function and its general nature,

Thus, each example or figure has a general scope, Moreover, in the description, the temperature is expressed in degrees Celsius, unless otherwise indicated, and the pressure is atmospheric pressure, unless otherwise indicated.

The invention will now be described in more detail as nonlimiting example, with reference to the appended drawings, in which:

FIG. 1 is a flowchart showing successive stages employing a process in accordance with the invention in order to obtain a titanium alloy part having the required final mechanical properties and dimensions,

FIG. 2 is a diagrammatic isometric view in 3 dimensions of the tool according to the invention.

FIG. 3 corresponds to the tool according to FIG. 1 on which is positioned a door frame element to be treated of simplified form.

FIG. 4 corresponds to the tool according to FIG. 1 represented according to a transverse cross section and on which several door frame elements are positioned.

FIG. 5 represents two tools according to the invention on a car hearth taken out of a treatment furnace.

FIGS. 5a, 5b and 6c correspond to the tool according to FIG. 1 represented along a longitudinal cross section passing through a calibration pattern cavity.

FIG. 7 represents a part (71) which is flattened and shaped against one or more reference supports (715) by gripping its stamping flash (75). The clamping means or damps (710a—open position (A), and 710b—closed position (B)), make it possible to hold the part (71) by gripping against the reference support(s) (715). The stamping clamps (710b) exert a series of localized pressures in the closed position (B). It is noted that the reference support or supports (715) exhibit two localized bearings. The use of one or more reference supports (715) generally depends on the size of the part.

The processes and tool according to the invention have been employed to produce elements forming part of a door frame for an aircraft. These elements have a curved and slender shape, as represented diagrammatically in FIG. 3, with a mean thickness typically of 25 mm, an equivalent diameter of 50 mm and a length of 4 meters and exhibit a single curve. It should be pointed out, at this stage, that the process according to the invention is suitable for shapes of parts which are more complex, for example exhibiting several curves, with optionally corkscrewed surfaces,

The elements were prepared from ingots of titanium alloy TA6V produced in a standard manner (FIG. 1, stage “a” of the flowchart).

The precise composition of the TA6V produced was as follows:

Al: 6.15

V: 3.82

Fe: 0.17

O: 0.15

C: 0.05

N: 0.02

H: 0.005

the remainder being composed of titanium and impurities resulting from the production.

The ingots were subsequently transformed by a known thermomechanical treatment (cf. FIG. 1, unit “b”):

Sequences of upsetting/drawing between 1100 and 1160° C., followed by

Sequences of upsetting/drawing at approximately 1050° C., followed by

Alpha-beta kneading reduction of 4 (reduction in cross section and increase in the height H to 4*H) between 900 and 980° C.

After forging, the billets obtained were cut up in order to obtain smaller billets of predetermined volume corresponding to the volume of the preform of the door frame element before machining.

Subsequently, the billets, of circular cross section, were forged in order to obtain square bar products which were stamped in one or more stages in order to obtain half-finished products already having a shape close to that of the finished parts, hereinafter denoted the “preform” of the door frame element.

Contrary to what was known, the flashes resulting from the stamping are not removed subsequent to the final stamping stage but are retained in order to provide gripping/clamping regions on a reference support during the machining.

At this stage and due to the various thermomechanical transformation stages, the preforms are deformed and include internal stresses.

In accordance with the invention, the preforms are prepared in order to be subjected to rough machinings. Contrary to normal practice, the rough machinings of the preforms are carried out in the context of the invention before the thermal stress-relaxation and shaping treatment (FIG. 1, unit “c” and then “d”). This is because, although the preforms include internal stresses resulting from the preceding thermomechanical transformation stages, the inventors have been able to observe that, after a succession of transformation stages which are standard for parts of this type, the internal stresses were sufficiently weak not to cause damage to the preforms nor to risk premature breakages of the preforms during the rough machining operations.

During the machining operations, the preforms (71) are flattened via their stamping flash (75) against several reference supports (715) using clamps (710a, 10b) (cf. FIG. 7). Each flash (75) is gripped between clamps (710b, closed position B) and reference supports (715) with a sufficient force to shape the preforms (71) according to a predetermined geometry. The fact of gripping the flashes (75) and not directly each preform (71) makes it possible to shape the latter without marking the functional surfaces of the preform (71) and without interfering with the movements of the cutting tools during the machining proper. This configuration makes it possible to retain an unchanging geometry from one preform to another so that the machining parameters and reference frames are repeatable from one prefrom to another and from one series of preforms to another, in order to observe the size tolerances required on the final part,

Once the rough machinings have been carried out, the preforms (71) are released from the clamps (710a, open position A) and relax in order to resume a deformed shape (see clearance “J”, FIGS. 6a and 6b).

The machined preforms (20, 320, 420, 620) are subsequently prepared in order to be subjected to the thermal stress-relaxation and shaping treatment according to the first subject matter of the invention (FIG. 1, unit “d”). The machined preforms (20, 320, 420, 620) are deposited (cf. FIGS. 2, 3 and 4) in calibrated pattern cavities (15, 315, 415, 615) formed in a shaping tool (10, 310, 410, 610). The calibrated pattern cavities (15, 315, 415, 615) comprise predetermined surfaces (317, 318, 417, 418, 617) which correspond to the shapes of the final door frame element to be obtained. In the present example and as may be observed in FIG. 4, each calibrated pattern cavity (415) exhibits two main supporting surfaces (417, 418) on which each machined preform (420) rests at least partially (as the part is deformed at this stage, cf, FIGS. 6a and 6b). Depending on the more or less complex geometry of the part to be shaped, there may be one or more main supporting surfaces.

In accordance with the second subject matter according to the invention, the tool is made of concrete composite material in which curved steel fibers are embedded, which fibers reinforce the tool at high temperatures. This tool makes it possible, as indicated above, to carry out rapid and homogeneous cooling of the machined-treated preforms, making it possible to obtain homogeneous microstructures of the alloy as fine entangled needles, conferring improved mechanical properties and making it possible to prevent, during the cooling, the appearance of internal thermal stresses and uncontrolled deformations of the part during or subsequent to the cooling.

In order to facilitate the longitudinal positioning of the machined preforms, the tool comprises a positioning stop formed in each calibrated pattern cavity. The other end of the pattern cavity is free, that is to say without a stop, in order to allow the part to freely deform by creeping. A visual marker on the tool may also be sufficient to position the preforms on the tool for thermal treatment.

The tool has dimensions so that the cooling rates are substantially constant whatever the cross section (or slice), part plus tool, under consideration. The thicknesses of the concrete tool are thus given dimensions by taking into account the variations in thickness of the part and the ratio of the thermal diffusion coefficients of the part with respect to the tool.

It is easily understood that, when the region of the part under consideration is thick, the corresponding thickness of the tool under this region is quite small compared with the mean thickness of the tool and, when the region of the part under consideration is thin, the corresponding thickness of the tool under this region is quite large compared with the mean thickness of the tool,

The shaping tool on which the machined preforms rest can, for example, comprise about ten pattern cavities. The shaping tool (510) is placed with the preforms to be treated in a thermal treatment chamber (550). The shaping tool (510) can be placed on a car hearth (530) with one or more other shaping tools (510) (cf. FIG. 5, showing two shaping tools).

The relaxation and shaping thermal treatment (FIG. 1, unit “d”) was carried out as follows:

rise in temperature at a rate of approximately 5° C/min (the faster the rise, the less the need to remain for a long time at beta transus in order to relax the part in order to avoid enlargement of grains in regions which are cooled more slowly) up to the thermal stationary phase of between 1020-1030° C. These temperatures are above the beta transus which, for TA6V, is in the present case approximately 1000° C., depending on the exact composition. The machined preforms were maintained at the temperature of the stationary phase for 25 minutes.

As represented in FIGS. 6a and 6b, before the thermal treatment according to the invention, the machined preforms (620) exhibited significant deformations, in particular twisting and curving defects, with clearances “J” with respect to the calibrated form (615) of several millimeters (5 to 30 mm, depending on the preforms) in some regions of the main supporting surfaces (617a, b).

During the thermal treatment above the beta transus, the preforms (620) were subject to free creeping under the effect of their own weight in order to take the shape of the calibrated pattern cavities (615) and thus to rest on the main supporting surfaces (617c) (cf. FIG. 5c).

Furthermore, at temperatures greater than the beta transus temperature, the alloy is subjected to an allotropic transformation where the alpha phase of close-packed hexagonal crystallographic structure is converted into a body-centered cubic phase, referred to as beta phase. During this transformation, the residual stresses, which appeared in particular in the part during the preceding thermomechanical transformation stages and which are of thermal and crystallographic origins, are released. Furthermore, in a body-centered cubic structure, the mobility of the dislocations is greater, which also favors the release of the internal stresses, These stress relaxations make it possible to avoid, during the cooling and at lower working temperatures, new deformations of the part or cracks, including stress cracks, in the parts.

The preforms were subsequently cooled in the air outside the treatment furnace, i.e. at a cooling rate of between 10 and 30° C./min.

The cooling rates inside the preforms were homogeneous, in particular by virtue of the use of the composite tool made of concrete,

Thus, the rapid cooling of the machined preforms from temperatures greater than the beta transus made it possible to bring about the appearance, in the alpha-beta domain, of the alpha phase with a morphology as fine intragranular and intertwined needles. The needles, which are intertwined randomly, counteract any new movement of dislocations and also counteract propagations of cracks, thus preventing the part from deforming during the cooling thereof or at ambient temperature and furthermore conferring, on the part, better mechanical characteristics, in particular of mechanical strength, good crack propagation behavior, as the propagation pathway for the cracks is more tortuous, and good toughness.

The homogeneous cooling of the machined preforms obtained in particular by virtue of the material used for the tool makes it possible to prevent the generation of internal thermal stresses and to prevent deformations of the preforms during the cooling or subsequently when the door frame elements are subjected to normal thermomechanical stresses.

An advantage of the concrete is also its thermal expansion coefficient, which is dose to that of titanium, allowing the calibrated pattern cavities to have variations in dimensions due to thermal expansions, for example at 1060° C., which are close to those of the parts made of titanium or of titanium alloy according to the invention. Thus, the dimensions of the preforms obtained after treatment are better controlled.

The concrete used to produce the tool according to the invention preferably has a thermal expansion coefficient of between approximately 3.5 and 7*10⁻⁶° K⁻¹, whereas that of titanium is between approximately 8 and 11*10⁻⁶° K⁻¹, for a T° range of between 100° C. and 1100° C.

The thermal treatment associated with the form of the calibrated pattern cavities (15, 315, 415, 615) comprising, in the present case, two main supporting surfaces (317, 318, 417, 418, 617) which intersect has made it possible to correct twisting and curving defects (it is preferable to have at least two supporting surfaces inclined with respect to one another in order to correct twisting defects). After the thermal treatment according to the invention and cooling down to ambient temperature, the differences measured with respect to the dimensions of the targeted part were less than 4 mm, indeed even less than 2 mm. The preforms thus obtained after treatment according to the invention were thus virtually devoid of deformations, thus making it possible to observe the final size tolerances required.

The parts obtained are removed from the concrete shaping tool and placed on the final machining tool.

Thus, after thermal treatment, the preforms are subjected to final rnachinings in order to obtain the final form of the door frame elements. Because the machining parameters (depth of cut, cutting speed and rates of feed) during the finishing are not very stressful for the part, it is not necessary to carry out a new stress-relaxation treatment.

Claims

1. A process for the preparation of a titanium alloy part, wherein the process comprises a thermal treatment for relaxing the internal stresses of a titanium alloy part which has been subjected beforehand to at least one thermomechanical transformation stage, the process being characterized in that the thermal treatment comprises maintaining at a temperature “T1” greater than the beta transus (beta transition) temperature, referred to as “Tbt”, and in that the part is free to deform by creeping.

2. The process as claimed in claim 1, wherein titanium alloy is of alpha-beta type.

3. The process as claimed in claim 2, wherein maintaining at the temperature T1 is carried out for a time sufficient to make possible the complete transformation in the alloy of a close-packed hexagonal microstructure to a body-centered cubic microstructure.

4. The process as claimed in claim 3, wherein the temperature T1 is greater by at least 5° C. than Tbt.

5. The process as claimed in claim 1, wherein it comprises maintaining at the temperature T1 for a period of time of 5 to 120 minutes.

6. The process as claimed in claim 1 wherein, subsequent to maintaining at the temperature T1, cooling is carried out with a cooling rate of greater than 5° C./min.

7. The process as claimed in claim 1, wherein the part made of titanium alloy is positioned for the stress-relaxation treatment in a shaping tool comprising one or more pattern cavities calibrated to receive a part to be relaxed.

8. The process as claimed in claim 7, wherein the shaping tool is made of at least one single or composite material, the thermal inertia of which is greater than that of titanium or of the titanium alloy and the variations in size of which related to creeping at the temperature T1 are virtually nonexistent.

9. The process as claimed in claim 8, wherein the shaping tool is made of concrete or composite concrete.

10. The process as claimed in claim 1, wherein it comprises, prior to the stress-relaxation thermal treatment stage, at least one rough machining stage.

11. The process as claimed in claim 10, wherein the rough machining stage or stages are carried out in order to treat virtually all, of the surfaces of the parts.

12. The process as claimed in claims 10, wherein, during the rough machining stage, the part is flattened and shaped against at least one reference support.

13. The process as claimed in claim 10, the part is flattened and shaped against said reference support by flattening, against the reference support, one or more flashes formed around the part and resulting from an upstream stamping stage.

14. The process as claimed in claim 1, wherein it comprises, after the stress-relaxation thermal treatment, one or more stages of final machining of the titanium alloy part.

15. The process as claimed in claim 1, wherein the titanium alloy is a TA6V titanium alloy.

16. The process as claimed in claim 1, wherein the part is a slender part.

17. A shaping tool comprising a shaping region comprising one or more calibrated pattern cavities in order to shape, by creeping, one or more slender parts and/or parts with large differences in cross section of titanium alloy, said shaping tool being composed of at least one single or composite material, the thermal inertia of which is greater than that of titanium or of the titanium alloy and the variations in size of which related to the creeping at a temperature of 1060° C. are virtually nonexistent.

18. The tool as claimed in claim 17, wherein it is composed of concrete and optionally comprises, in addition, curved stainless steel fibers distributed isotropically in the concrete.

19. The tool as claimed in claim 17, wherein it has dimensions so that the cooling rates are substantially constant from one slice, part plus tool, to another.

20. The tool as claimed in claim 17, wherein the pattern cavity region is calibrated in order to shape, by creeping, a slender part exhibiting a slenderness of greater than 10 and/or different cross sections, the variation in cross section of which is greater than 2/1.

21. The tool as claimed in claims 17, wherein the titanium alloy is of the type comprising an alpha-beta phase.

22. The tool as claimed in claim 17, wherein the titanium alloy is a TA6V alloy.

23. The tool as claimed in of claims 17, wherein the pattern cavity comprises at least two supporting surfaces on which the part to be relaxed can at least partially rest.

24. The tool as claimed in claim 17, wherein the pattern cavity comprises a positioning stop formed in each calibrated pattern cavity, the other end of the mold being free, in order to allow the part to freely deform by creeping, or comprises a stop positioned by taking into account the thermal expansion coefficient of the part to be relaxed before and after relaxation thermal treatment, in particular in order to make possible free deformation by creeping.

25. The process as claimed in claim 1, employing the tool as defined in claim 17.

26. The tool according to claim 23 wherein said supporting surfaces are positioned so that, when a titanium alloy part is maintained at a temperature T1 greater than its temperature Tbt, said part can be positioned by creeping with greater contact on the supporting surfaces.