Method for preventing abnormal grain growth in beta-treated titanium alloys

Abstract

Method for avoiding abnormal grain growth in beta-treated titanium alloys. The present invention relates to a method for manufacturing a part made of β-treated titanium alloy without abnormally growing beta grains, characterized in that it comprises the following successive steps: a) first heat treatment of a titanium alloy blank at a core blank temperature higher than the β transus temperature of the titanium alloy so as to obtain a β-treated blank; b) final deformation step of the β-treated blank, this final deformation step being carried out at a core blank temperature lower than the β transus temperature of the titanium alloy with a local plastic strain level ≥ 0.1 and with a plane strain level ≤1 so as to obtain a titanium alloy part;c)- second heat treatment of the titanium alloy part at a core temperature higher than the transus temperature β of the titanium alloy so as to obtain a β-treated titanium alloy part.;

Description

Title of the invention: Method for preventing abnormal grain growth in beta-treated titanium alloys technical field

[0001] The present invention relates to the field of beta-treated titanium alloys, in particular intended for aeronautics. Previous technique

[0002] During the manufacture of titanium alloy parts, it may be necessary to subject them to a final heat treatment at a temperature above the beta transus temperature in order to improve the damage tolerance and fracture resistance of the part and thus obtain a microstructure suitable for applications in the field of aeronautics.

[0003] However, it has been discovered that beta treatment, i.e., heat treatment at a temperature above the beta transus temperature, can cause abnormal growth of beta grains, potentially impacting the mechanical properties of the part. This phenomenon is called abnormal grain growth (AGG) and is observed mainly in parts that have undergone plastic deformation during manufacturing, such as forging. It is described in particular in the article by Byres et al. ("Analysis of the development of abnormal grain structures during beta annealing of Ti-64 wrought product," MATEC Web Conferences, 321, 12043 (2020)).The literature has demonstrated that this phenomenon is linked to the development of a very pronounced crystallographic texture in the beta phase of the alloy. This can develop during forging operations performed at temperatures below the beta transus temperature due to the significant cumulative reduction in cross-section from the ingot to the forged blank and from the forged blank to the finished part, creating a substantial amount of localized plastic deformation within the volume of the forged part. In particular, it has been shown that a cube or cube-turned crystallographic texture—developed during a rough forging phase—can induce excessive grain growth during the final beta treatment, even if a finishing die-forging phase occurs between the rough forging and the final beta treatment.It has been shown that the presence of a few randomly oriented grains within a zone of strong cube or cube-turned crystallographic texture is a favorable condition for the abnormal growth of these randomly oriented grains, to the detriment of grains exhibiting the cube or cube-turned texture. Forged parts with microstructures composed of abnormally large grains may exhibit degraded mechanical properties and durability, a phenomenon known as grain initiation. and possibly the propagation of fatigue cracks being the most affected. (An investigation of the Development of coarse grains during beta annealing of hot-forged Ti-6A1-4V, NC Levkulich, S / L. Semiatin, EJ Payton, S. Srivatsa and AL Pilchak, Met. And Mat. Trans. A, 52A (2021) 1353).

[0004] US patent applications 2017 / 0175241 and 2019 / 0032184 propose a process for preventing abnormal grain growth by using a very specific final thermal cycle in which the beta treatment step is directly preceded by a recrystallization step at very specific temperatures. The abnormal grain growth phenomenon described in this patent is said to be linked to a localization of significant deformation within forged titanium alloy blanks. No mention is made of the grain growth phenomenon caused by the appearance of a zone with a strong rotated cube texture. The application of a recrystallization treatment below the beta transus temperature is recommended in this patent.However, this type of heat treatment has not been shown to remove the cube or rotated cube texture prior to the final heat treatment, and therefore has not been shown to prevent the AGG phenomenon described in this patent application.

[0005] It has also been proposed to solve this problem by avoiding critical deformation paths, such as plane deformation (article by Pilchak et al. MATEC Web Conferences, 321, 12007 (2020)). This type of solution is very complex and costly to manage industrially when several forging operations must be chained together to obtain a complex shape.

[0006] The inventors discovered that in certain cases it was possible to avoid the presence of abnormally large beta grains in the final part by applying a first heat treatment at a temperature above the beta transus temperature (first beta treatment) before the final deformation step, followed by a second heat treatment at a temperature above the usual beta transus temperature (second beta treatment). Indeed, the first beta treatment allows the abnormal growth of beta grains to develop on the blank in cases where the latter has developed a very pronounced cube or rotated cube crystallographic texture during the roughing steps. It should be noted that the abnormally growing grains have a random orientation, and that their growth eliminates the cube or rotated cube crystallographic texture from which they originate.The final deformation step then recrystallizes the grains to obtain a homogeneous structure in the resulting part; the second beta treatment of this part thus does not cause any new abnormal grain growth, and the resulting part therefore does not exhibit any abnormally growing beta grains.

[0007] This solution is much simpler than that of the prior art, and the implementation cost is moderate. Furthermore, it avoids restricting the deformation path of the forged blank before the first heat treatment, thus allowing greater flexibility in the part manufacturing process. This is a significant advantage when manufacturing parts with complex geometries, requiring a series of forming steps. Description of the invention

[0008] The present invention therefore relates to a method for manufacturing a part made of treated titanium alloy [3] not exhibiting abnormally growing beta grains, characterized in that it comprises the following successive steps: a)- first heat treatment of a titanium alloy blank at a core blank temperature (or core temperature of the blank) higher than the transus temperature [3 of the titanium alloy so as to obtain a treated blank [3; b)- final deformation step of the treated blank [3, this final deformation step being carried out at a core blank temperature lower than the transus temperature [3 of the titanium alloy with a local plastic strain level >0.1, in particular in areas likely to have anomalous grain growth, advantageously throughout the blank, and with a plane strain level <1 so as to obtain a titanium alloy part; c) Second heat treatment of the titanium alloy part at a core temperature (or part temperature) higher than the transus temperature [3] of the titanium alloy so as to obtain a treated titanium alloy part

[0009] In this application, the expressions "between ... and ...", "from ... to ..." and "included in the range .......", shall be understood to include the limits unless explicitly stated otherwise.

[0010] For the purposes of the present invention, the term "treated [3]" or "beta treated" means a part / blank that has undergone heat treatment at a core temperature higher than the beta transus temperature of the constituent alloy of the part / blank, so as to transform the microstructure of the alloy of the part / blank from a mixture of alpha and beta phases into a single beta phase at the treatment temperature. Following the beta treatment and after cooling, no nodular alpha phase (more commonly called primary alpha) remains.

[0011] For the purposes of the present invention, "beta treatment" means a heat treatment at a core temperature of the part / work blank that is higher than the beta transus temperature of the alloy constituting the part / work blank, so as to transform the micro structure of the alloy of the part / blank of a mixture of alpha and beta phases into a single beta phase at the processing temperature.

[0012] The "beta transus temperature" according to the present invention is the lowest temperature at which the titanium alloy according to the invention contains 100% beta phase (complete dissolution of the alpha phase). The beta transus temperature of the titanium alloy can vary from 700°C to 1050°C, depending on the alloy composition. The beta transus temperature of the Ti-6A1-4V alloy is generally between 980°C and 1020°C.

[0013] For the purposes of the present invention, "titanium alloy" means any titanium-based alloy, i.e., one containing titanium as the main chemical element (more than 50% by mass) and also containing other chemical elements.

[0014] In an advantageous embodiment, the titanium alloy according to the invention is a titanium alloy containing aluminum, advantageously a titanium alloy containing aluminum and vanadium, more advantageously the Ti-6Al-4V alloy. Particularly advantageously, the titanium alloy according to the invention is a metastable alpha+beta or alpha-beta or beta two-phase alloy. Indeed, this type of alloy generally includes combinations of alpha and beta stabilizing chemical elements, such as aluminum or vanadium. Thus, titanium alloys according to the invention can be the alloys Ti-6A1-4V, Ti-6A1-4V-ELI, Ti-6Al-6V-2Sn, Ti-8Mn, Ti-7Al-4Mo, Ti-6Al-2Sn-4Zr-6Mo, Ti-5Al-2Sn-2Zr-4Mo-4Cr otherwise called Til7, Ti-6Al-2Sn-2Zr-2Mo-2Cr, Ti-3A1-2.5V or Ti-4Al-4Mo-2Sn-0.5Si, Ti-6Al-2Sn-4Zr-2Mo otherwise called Ti6242.

[0015] Hillert demonstrated in 1965 that the largest grains resulting from normal grain growth are approximately twice the size of average grains. For the purposes of this invention, "abnormally growing beta grain" refers to grains in the beta phase of the alloy whose size is therefore more than twice the grain size of the surrounding matrix.

[0016] The process according to the present invention therefore comprises a first heat treatment of a titanium alloy blank according to the invention at a core temperature of the blank higher than the beta transus temperature of the titanium alloy so as to obtain a beta-treated blank (step a)). In the context of this application, this first heat treatment may also be referred to as: "first beta heat treatment". This step a) of the process according to the present invention thus makes it possible to trigger the potential abnormal growth of beta grains in the titanium alloy blank according to the invention.

[0017] In order to carry out step a), it is therefore necessary that the entire blank, including the core of the blank, be at a temperature higher than the temperature of beta transus of the titanium alloy. This is why the temperature indicated for this step is a core temperature of the blank, also called the core temperature of the blank.

[0018] The heat treatment of the titanium alloy in step a) can be carried out with any heat sources, heating devices and / or heat treatment systems known in the art of titanium forging. For example, vacuum furnaces, electric furnaces or gas furnaces can be used.

[0019] Depending on the heating technology used for beta treatment, a certain degree of temperature homogeneity is guaranteed within the furnace. To ensure that the temperature at every point of the blank, including in the core of the blank—that is, that the core temperature of the blank is higher than the beta transus during beta treatment—it is therefore necessary to target a core beta treatment temperature of the blank at least equal to the beta transus temperature of the alloy plus a safety margin which can be at least 14°C, advantageously at least 20°C, and in particular at least 25°C.Advantageously, the core temperature of the blank during step a) is at least 14°C higher than the transus temperature [3 of the titanium alloy, advantageously at least 20°C higher than the transus temperature [3 of the titanium alloy, more advantageously the temperature is greater than or equal to the transus temperature [3 of the titanium alloy + 25°C. .

[0020] It is also preferable not to use an excessively high core blank temperature during the implementation of step a) for economic reasons, since the aim is to ensure that the core blank temperature during this step is above the beta transus temperature of the titanium alloy. Therefore, it is preferable that the core blank temperature during step a) be less than or equal to the beta transus temperature of the titanium alloy + 50 °C. Thus, advantageously, in the case of the Ti-6A1-4V alloy, the temperature of step a) is less than or equal to 1055 °C.

[0021] The duration of step a), called the metallurgical holding time (i.e., the holding time at the target temperature in the core of the blank), must be sufficient to allow the entire alloy of the blank to transition from a mixture of alpha and beta phases to a single beta phase. Thus, advantageously, the duration of step a) according to the invention is at least 20 minutes, advantageously at least 30 minutes, in particular at least 45 minutes, and more advantageously it is 1 hour.

[0022] For reasons of economy, it is not necessary for the duration of step a) to be too long. Thus, advantageously, step a) lasts a maximum of 5 hours, more advantageously a maximum of 3 hours, and in particular a maximum of 2 hours.

[0023] The blank processed in step a) of the process according to the present invention can be obtained by any method known to those skilled in the art in the field of titanium alloys, in particular enabling the transformation of an ingot into a blank. Indeed, the deformation path leading to the blank according to the invention is irrelevant. Advantageously, the blank according to the invention is obtained by forging, extrusion, drawing, and / or die forging, more advantageously by forging, in particular at a core blank temperature lower than the beta transus temperature of the titanium alloy.Advantageously, the process according to the invention includes a step a0), prior to step a), of deforming the titanium alloy blank, in particular by forging, spinning, drawing and / or die-casting, more particularly by forging, advantageously at a core blank temperature lower than the transus temperature [3 of the titanium alloy, in particular to remain in the mixing zone of the alpha and beta phases in order to better control the beta grain size, more particularly in a core blank temperature range between the beta transus -10°C and the beta transus -80°C.

[0024] In an advantageous embodiment, the treated beta blank obtained in step a) can undergo water quenching before step b) of the process according to the invention. Thus, advantageously, the process according to the present invention comprises an intermediate step a1) between steps a) and b), of water quenching the treated blank [3] obtained in step a), and step b) is carried out on the treated blank [3] obtained in step a1). Indeed, the first beta heat treatment of step a) according to the invention can induce tears in the surface and subsurface of the treated beta blank according to the invention during step b). In this case, rapid cooling, and more specifically water quenching, prevents the precipitation of thick intergranular lines which are brittle, thus reducing intergranular decohesion, whereas after air quenching intergranular decohesion can remain very pronounced.

[0025] Step b) of the process according to the present invention consists of a final deformation step of the treated blank [3 (obtained in step a) or in step a1)), this final deformation step being carried out at a core blank temperature lower than the transus temperature [3 of the titanium alloy with a local plastic strain level > 0.1 and with a plane strain level <1 so as to obtain a titanium alloy part.

[0026] For the purposes of this invention, the "final deformation step" means a step during which a final plastic deformation is applied to the treated blank beta obtained in step a) in order to obtain the final part. Indeed, after this step b), there will be no further plastic deformation steps on the resulting part. Obviously, this final part can subsequently undergo one or more finishing treatments such as machining but such treatments do not introduce any plastic deformation to the part.

[0027] For the purposes of the present invention, "local plastic deformation" means the plastic deformation defined in Reference M600 of Techniques de l'ingénieur published on July 10, 1996] and noted:

[0028] [Math.l]

[0029] which has the following value:

[0030] [Math.2] è = iQèdt

[0031] where

[0032] [Math.3] è

[0033] is the plastic deformation rate defined by

[0034] [Math.4]

[0035] with

[0036] [Math.5]

[0037] (plastic flow law) where

[0038] [Math.6] 8

[0039] is the strain rate tensor, o the stress tensor, <p(o)la contrainte équivalente de von Mises et X un multiplicateur plastique positif ou nul, qui is incompetent in the field of plastics.

[0040] Advantageously the level of local plastic deformation of step b) is > 0.2.

[0041] For the purposes of the present invention, "plane strain level" means a deformation such that the strain rate tensor can be put in the form next, with a change of reference frame:

[0042] [Math.7] 0 è = “ê 0 0 .0 0 0 ■ 0 -1.

[0043] Advantageously, step b) of the process according to the present invention may consist of forging, spinning, drawing and / or die forging, in particular final forging, particularly die forging, more particularly finishing die forging.

[0044] Step b) of the process according to the present invention is carried out at a core blank temperature lower than the transus temperature [3 of the titanium alloy, in particular to remain in the mixing zone of the alpha and beta phases of the titanium alloy according to the invention, more particularly at a core blank temperature lower than at least 14°C compared to the transus temperature [3 of the titanium alloy, advantageously lower than at least 20°C compared to the transus temperature [3 of the titanium alloy, more advantageously at a core blank temperature lower than or equal to the beta transus temperature of the titanium alloy - 25°C.

[0045] The process according to the present invention finally includes a step c) consisting of a second heat treatment of the titanium alloy part at a core part temperature higher than the transus temperature [3 of the titanium alloy so as to obtain a titanium alloy part treated [3. This gives rise to a titanium alloy part comprising only beta grains without abnormal growth, in particular equiaxed beta grains.

[0046] Steps b) and c) of the process according to the present invention make it possible to completely recrystallize the beta grains of the titanium alloy according to the invention and in particular to recrystallize the abnormally grown beta grains that may have resulted from the forging phases of the blank. Indeed, step b) makes it possible to introduce sufficient energy into the titanium alloy so that the beta grains recrystallize during step c).

[0047] Step c) of the process according to the invention is carried out in a conventional manner for a beta treatment. The conditions for carrying out this step, in particular temperature and duration (i.e., metallurgical holding time), are advantageously as described previously in the context of step a). Advantageously, step c) of the process according to the present invention can be carried out under the same conditions, in particular temperature and duration, as the first beta treatment of step a) of the process according to the invention, the only difference being that it is applied to the final part obtained from step b) and not to the blank.

[0048] This step c) aims to transform the microstructure of the alloy of the part from a mixture of alpha and beta phases into a single beta phase.

[0049] In order to carry out step c), it is therefore necessary that the whole of the part, including the core of the part, be at a temperature higher than the beta transus temperature of the titanium alloy.

[0050] Advantageously, the titanium alloy part of the process according to the invention is intended for aeronautics, advantageously it is an aeronautical structural part, in particular for an aircraft.

[0051] The present invention will be better understood upon reading the description of the following examples, which are given by way of non-limiting illustration.

[0052] EXAMPLE

[0053] A Ti-6A1-4V alloy billet with a transus [3] of 1000°C, measuring 110 mm x 80 mm x 80 mm, was forged using a hydraulic press at a temperature of 960°C, followed by a finishing die at 970°C with a local plastic strain of 0.1 and a plane strain of <1 (final deformation stage) using a 300T hydraulic press, and then by a one-hour heat treatment at 1030°C (final beta treatment) (comparative example 1). Abnormally growing grains were observed in the center of the billet, with grains reaching a size of approximately -7 ASTM according to ASTM E12 dated 2021. This abnormal grain growth is linked to the presence of a cube-like crystallographic texture, created during the initial forging phase at 960°C.

[0054] Another Ti-6A1-4V alloy billet with a transus [3] of 1000°C, of ​​identical dimensions to that of Comparative Example 1, underwent the same treatment as described in Comparative Example 1 under the same conditions (forging at 960°C, final die forging at 970°C, final beta treatment at 1030°C for one hour), the only difference being the implementation of a first beta treatment at 1030°C for one hour between the forging and final die forging stages (Example 1). The absence of abnormally growing grains was observed, the microstructure of the billet exhibiting equiaxed beta grains with a size on the order of -4 ASTM according to ASTM E12 dated 2021. The first beta treatment thus enabled the eruption of abnormal grain growth before final die forging. The deformation during the finishing die-making process allowed the grains to recrystallize, resulting in a completely regenerated and homogeneous microstructure.

[0055] Another Ti-6A1-4V alloy billet having a transus [3 of 1000°C with dimensions diameter = 80mm, h=70mm and r=8mm, underwent a beta treatment of 45min followed by slow cooling, then a finishing die at a temperature of 850°C. Some areas show significant tears in and under the skin (example 3).

[0056] Another billet of Ti-6A1-4V alloy having a transus [3 of 1000°C and dimensions diameter = 80 mm, h = 70 mm and r = 8 mm, underwent a 45-minute beta heat treatment followed by water quenching, then a finishing die forging at a temperature of 850°C (example 4). [It is observed that the intergranular decohesion of example 3 is still present but greatly reduced. Thus, water quenching made it possible to avoid surface and subsurface tears as well as to reduce intergranular decohesion.

Claims

Demands

1. A method for manufacturing a treated titanium alloy part [3] without abnormally growing beta grains, characterized in that it comprises the following successive steps: a)- first heat treatment of a titanium alloy blank at a core blank temperature higher than the transus temperature [3] of the titanium alloy so as to obtain a treated blank b)- final deformation step of the treated blank [3], this final deformation step being carried out at a core blank temperature lower than the transus temperature [3] of the titanium alloy with a local plastic strain level > 0.1 and with a plane strain level < 1 so as to obtain a titanium alloy part;c)- second heat treatment of the titanium alloy part at a core temperature higher than the transus temperature [3 of the titanium alloy so as to obtain a treated titanium alloy part [3.;

2. A manufacturing process according to claim 1, characterized in that the titanium alloy is a titanium alloy containing aluminum, advantageously a titanium alloy containing aluminum and vanadium, more advantageously the Ti-6A1-4V alloy.

3. A method according to any one of claims 1 or 2, characterized in that it comprises an intermediate step a1) between steps a) and b), of water quenching the treated blank [3 obtained in step a) and in that step b) is carried out on the treated blank [3 obtained in step a1).

4. A method according to any one of claims 1 to 3, characterized in that the level of local plastic deformation of step b) is >0.

2.

5. A method according to any one of claims 1 to 4, characterized in that the titanium alloy part is intended for aeronautical use, advantageously it is an aeronautical structural part, in particular for an aircraft.

6. A method according to any one of claims 1 to 5, characterized in that the core temperature of the blank during step a) is at least 14°C higher than the transus temperature [3 of the titanium alloy, advantageously higher by at least 20 °C compared to the transus temperature [3 of the titanium alloy, more advantageously the temperature is greater than or equal to the transus temperature [3 of the titanium alloy + 25 °C.

7. A method according to any one of claims 1 to 6, characterized in that step a) has a duration of at least 20 minutes, advantageously of at least 30 minutes, in particular of at least 45 minutes, more advantageously it is 1 hour.

8. A method according to any one of claims 1 to 7, characterized in that step b) consists of a final forging, advantageously die-finishing.

9. A method according to any one of claims 1 to 8, characterized in that it comprises a step a0, prior to step a), of deformation of the titanium alloy blank, in particular by forging, advantageously at a core blank temperature lower than the transus temperature [3 of the titanium alloy.