EP3372700B1 - Method for making forged tial components - Google Patents

Description

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

The present invention relates to a method for producing forged components from a TiAl alloy, in particular components for gas turbines, preferably aircraft turbines and in particular turbine blades for low-pressure turbines.

STATE OF THE ART

Due to their low specific weight and their mechanical properties, components made of titanium aluminides or TiAl alloys are of interest for use in gas turbines, in particular aircraft turbines.

Titanium aluminides or TiAl alloys are hereby understood as meaning alloys which contain titanium and aluminum as main constituents, so that their chemical composition has constituents with the highest proportions of aluminum and titanium. In addition, TiAl alloys are characterized by the formation of intermetallic phases, such as y - TiAl or α ₂ - Ti ₃ Al, which give the material good strength properties.

However, TiAl alloys are not easy to process and the microstructures of TiAl materials need to be precisely adjusted to achieve the desired mechanical properties.

For example, from the DE 10 2011 110 740 B4 a method for producing forged TiAl - components is known, in which after the forging a two-stage heat treatment is carried out to set a desired structure. Also the documents DE 10 2015 103 422 B3 and EP 2 386 663 A1 disclose methods of making components from TiAl alloys.

In the European patent application EP 2 386 663 A1 Already the problem is addressed that with TiAl alloys the problem often arises that the structure is formed inhomogeneous and therefore also the properties of the TiAl material exhibit inhomogeneities. However, this is undesirable for use of the TiAl alloys in turbomachines, such as aircraft engines. The EP 2 386 663 A1 proposes for this a heat treatment of the transformed TiAl material to carry out a recrystallization. However, this does not completely solve the problem of inhomogeneous microstructure education.

DISCLOSURE OF THE INVENTION OBJECT OF THE INVENTION

It is therefore an object of the present invention to provide a production method for the production of components made of TiAl materials, which can be used in gas turbines, in particular aircraft turbines, preferably in the region of the low-pressure turbine, and have a homogeneous microstructure and thus a homogeneous property profile.

TECHNICAL SOLUTION

This object is achieved by a method having the features of claim 1. Advantageous embodiments are the subject of the dependent claims.

The invention proposes that, in the case of a forging process for the production of a forged component from a TiAl alloy, the forming by forging be carried out in such a way that a homogeneous deformation takes place for the entire component. It has been shown that with a uniform deformation over the entire component, in a simple manner a homogeneous microstructure of the forged component can be achieved, so that the property profile of the forged component over the entire component is homogeneous. Accordingly, a blank is provided for the forging, the shape of which is selected such that the deformation over the entire volume of the blank or of the blank forged from the blank is substantially the same. For this purpose, a defined degree of deformation is set, of which only for ± 1 over the entire usable volume of the forged Derived from the semi-finished product. The usable volume of the forged semi-finished product is understood to be the area of the forged semi-finished product which corresponds to the forged component to be produced, for example the area or the volume of a turbine blade to be produced. Accordingly, the usable volume of the forged semi-finished product is understood to be the area of the forged semi-finished product which remains after the forging as a finished component after a material-removing post-processing. A forged semi-finished product can thus be understood in particular to be a forged upper part or forged intermediate product, which can be processed via one or more processing steps to form a finished component, for example a turbine blade. Under a blank can be understood in particular a forging material, which can be processed by a forging process to the semifinished product.

wobei ϕ_x, ϕ_y, ϕ_z die Umformgrade in x - , y - und z - Richtung sind.In this case, the degree of deformation φ is defined as the natural logarithm of the ratio of final dimension x ₁ after deformation to initial dimension x ₀ in the case of a one-dimensional dimensional change in a Cartesian reference system. In the case of a three-dimensional transformation, the deformation is characterized by the greatest degree of transformation φ _g , which is given by: $${\mathrm{\phi }}_{\mathrm{G}}=\left|{\mathrm{\phi }}_{\mathrm{Max}}\right|=\mathrm{½}\left(\left|{\mathrm{\phi }}_{\mathrm{x}}\right|+\left|{\mathrm{\phi }}_{\mathrm{y}}\right|+\left|{\mathrm{\phi }}_{\mathrm{z}}\right|\right)$$

where φ _x , φ _y , φ _{z are} the degrees of deformation in the x, y and z directions.

The blank can now be shaped in such a way that the degree of deformation has a defined value in one of the directions of the reference system, for example the x, y or z direction of a Cartesian reference system, in one of the directions of the reference system, and only within that or that the degree of deformation in several directions of the reference system or in each direction, in particular each main direction of the reference system has a defined value and deviates from this only within the allowable fluctuation range. Moreover, it is also possible to form the blank in such a way that from the degree of deformation of different directions the value-wise largest and / or the value-wise smallest degree of forming fulfills the given conditions of the homogeneous forming.

In particular, the shape of the blank can be chosen so that the transformation to be performed has a defined degree of deformation, which is within the usable volume of the forged Semifinished from the defined value of the degree of deformation by a maximum of ± 0.5, in particular ± 0.25 deviates.

The defined value of the degree of deformation may in particular be greater than or equal to 0.7, so that a minimum deformation takes place to that extent. Preferably, the degree of deformation of 0.7 within the usable volume is not exceeded, so that the entire material of the forged semi-finished product undergoes a minimum deformation by forging.

In addition, the defined value of the degree of deformation can be kept as low as possible in order to keep the cost of forming low. Accordingly, the value of the degree of deformation may be less than or equal to 2.5, in particular less than or equal to 2.0.

The forming speed, ie the change in the degree of deformation per unit time, during forging can be in the range of 0.01 to 0.5 1 / s and in particular in the range of 0.025 to 0.25 1 / s.

In addition, the shape of the blank can be selected so that along the longitudinal axis of the blank, so the axis with the largest dimension, the mass is distributed so that more mass is present at the two ends than in the middle of the blank. For this purpose, the blank can be divided along its longitudinal axis into three equally long regions or sections, namely a first and second end region and a central region wherein the mass of the blank is distributed in the regions so that there is more mass in the end regions than in the central region , Accordingly, the blank may be formed such that M _M <M _E1 ≦ M _E2 where M _{M is} the mass of the blank in the central region, M _{E1 is} the mass of the blank in the first end region and M _{E2 is} the mass of the blank in the second end region.

Furthermore, the blank can satisfy the condition: M _M ≦ M _E2 / 1.25.

For the production of forged components from TiAl alloys, in particular for gas turbine components, such as, for example, low-pressure turbine turbine blades, alloyed titanium aluminide alloys which are especially suitable for niobium and molybdenum can be used. Such alloys are also referred to as TNM alloys.

For the present process, an alloy of 27 to 30 weight percent aluminum, 8 to 10 weight percent niobium, and 1 to 3 weight percent molybdenum may be used, the remainder being titanium.

In particular, the aluminum content may be selected in the range of 28.1 to 29.1 weight percent aluminum, while 8.5 to 9.6 weight percent niobium and 1.8 to 2.8 weight percent molybdenum may be added.

In addition, the alloy may be alloyed with boron in the range of 0.01 to 0.04 weight percent boron, more preferably 0.019 to 0.034 weight percent boron.

Further, the alloy may include unavoidable impurities such as carbon, oxygen, nitrogen, hydrogen, chromium, silicon, iron, copper, nickel and yttrium, the content of which is ≦ 0.05% by weight of chromium, ≦ 0.05% by weight of silicon, ≦ 0.08 wt% oxygen, ≤ 0.02 wt% carbon, ≤ 0.015 wt% nitrogen, ≤ 0.005 wt% hydrogen, ≤ 0.06 wt% iron, ≤ 0.15 wt% copper, ≤ 0.02 wt% nickel and ≤ 0.001 wt% yttrium , Further constituents may be contained individually in the range of 0 to 0.05 percent by weight or in total from 0 to 0.2 percent by weight.

The forging of the blank can be carried out in particular as isothermal forging, wherein only a single-stage forming, so only one forming step preferably can be carried out in only one forging without a further forming or forging takes place in another forging die. In this way, the cost of forming can be kept low.

In this case, one-stage means both that the forming process takes place in a single continuous process, and that only a single transformation takes place in the production process.

Accordingly, the forming of the cast, for example, not yet formed blank for semi-finished can be done in a single forging step, without further transformation to the finished component is necessary. So it does not have to be pressed several times and from different directions, but it is only a press or a die with two forms required between which the blank is inserted and formed during pressing of the two forms against each other. The forged part does not have to be moved or moved between different forging steps.

The forging of the corresponding components can be carried out by drop forging in the temperature range of the α + γ + β phase region, wherein the forging temperature in the range of 1150 ° C to 1200 ° C can lie. A corresponding die can be kept at the temperature by heating during the forging process. Depending on the material of the die, an inert ambient atmosphere may be adjusted during forging.

After forging, the forged semi-finished products may be subjected to a two-stage heat treatment, wherein the first stage of the heat treatment provides for recrystallization annealing below the γ / α transformation temperature for a period of 50 to 100 minutes. The annealing at a temperature below the γ / α transformation temperature, where α-titanium is converted to γ-TiAl in accordance with the phase diagram for the TiAl alloy used, can be as close as possible to the γ / α transformation temperature, with a temperature of 8%, in particular 4%, below the γ / α - conversion temperature should not be fallen below.

The recrystallization annealing may preferably be carried out for 60 to 90 minutes, especially 70 to 80 minutes.

The first stage of the heat treatment with the recrystallization annealing may be followed by a second stage of heat treatment with stabilizing annealing in the temperature range of 800 ° C to 950 ° C for 5 to 7 hours.

The stabilization annealing can be carried out in particular in the temperature range from 825 ° C. to 925 ° C., preferably from 850 ° C. to 900 ° C., with a holding time of from 345 minutes to 375 minutes.

The cooling in the recrystallization annealing can be done by air cooling, wherein in the temperature range between 1300 ° C and 900 ° C, the cooling rate ≥ 3 ° C per second should be to set a fine-lamellar microstructure of α ₂ -Ti ₃ Al and γ-TiAl, which required mechanical properties guaranteed.

The cooling in the second heat treatment stage, so the stabilization annealing, can be done with correspondingly lower cooling rates in the oven.

For the adjustment of the microstructure and reproducibility of a corresponding structural adjustment, it is important that the heat treatment steps are carried out as accurately as possible at the corresponding selected temperature. However, an increasingly accurate adjustment of the temperature and keeping the components at the appropriate temperatures with increasing Expenses connected, so that for an economically meaningful processing a compromise must be found. For the heat treatment of forged TiAl components, a temperature adjustment with a deviation in the range of 5 ° C to 10 ° C up and down from the setpoint temperature has proven to be advantageous. Accordingly, the selected target temperature for the heat treatment steps of the present invention can be set and held up and down in a corresponding temperature window of 5 ° C to 10 ° C deviation from the target temperature.

As blanks for forging cast and / or hot isostatically pressed blanks can be used. As an alternative to casting, the precursor material may also be made by metal injection molding (MIM), powder metallurgy, additive processes (e.g., 3D printing, cladding), or combinations thereof. Regardless of the production, the blanks or the starting material can be hot-isostatically pressed before forging. It may be advantageous to machine the starting material before forging on all sides or locally with a material-removing machining process in order to work off surface edge zones and / or to give the blank the desired shape for the subsequent shaping. Any suitable method can be used as the material-removing machining method, in particular metal-cutting methods or electrochemical machining methods.

The blanks can be produced by melting in vacuo or inert gas with self-consumable electrodes or in the cooled crucible by means of plasma arc melting, wherein a single or multiple remelting of the alloy can be performed. The remelting may be by vacuum induction melting or vacuum arc remelting (VIM vacuum induction melting), and the cast material may be hot isostatically pressed using temperatures ≥ 1200 ° C at a pressure ≥ 100 MPa and a holding time ≥ 4 hours can.

After forging and before or preferably after the two-stage heat treatment, the forged semi-finished product can be post-processed with a material-removing machining process to produce the finished component. Any suitable method can be used as the material-removing machining method, in particular metal-cutting methods or electrochemical machining methods.

BRIEF DESCRIPTION OF THE FIGURES

Figuren 1a und 1b

einen Verfahrensablauf zur Herstellung einer Turbinenschaufel gemäß der vorliegenden Erfindung, in

Figur 2

ein Diagramm zur Verdeutlichung möglicher Masseverteilungen in einem Rohling für das Schmieden und in

Figur 3

ein Zustandsdiagramm für eine TiAl - Legierung, wie sie bei der vorliegenden Erfindung eingesetzt werden kann, mit der Angabe des Phasenfeldes, in dem das Schmieden bzw. die Umformung stattfindet.

The accompanying drawings show in a purely schematic manner in FIG

FIGS. 1a and 1b

a method of manufacturing a turbine blade according to the present invention, in

FIG. 2

a diagram to illustrate possible mass distributions in a blank for forging and in

FIG. 3

a state diagram for a TiAl alloy, as can be used in the present invention, with the indication of the phase field in which the forging takes place or the transformation takes place.

EMBODIMENTS

Further advantages, characteristics and features of the present invention will become apparent in the following detailed description of the embodiments. However, the invention is not limited to these embodiments.

The FIGS. 1a and 1b show the sequence of process steps in the implementation of an embodiment of the method according to the invention.

Initially, a blank 5 is produced by filling a molten TiAl alloy into a mold 1 having a cavity 2 corresponding to the shape of the blank 5 to be produced.

After pouring the TiAl alloy in the mold 1 and solidifying the TiAl alloy, the casting blank 4 can be correspondingly pressed in a system 3 for hot isostatic pressing in order to compact the green cast iron 4 and to close possible cast blanks or the like. The hot - isostatic pressing thus does not serve the transformation of the casting blank 4, but only the material compression.

Thereafter, the blank 5 can be additionally subjected to a material-removing post-processing, for example by machining or by electrochemical machining.

The correspondingly produced blank 5 is forged in a drop forging 6 to a near net shape, forged semi-finished 9, wherein the drop forge 6 has two Gesenkhohlformen 7 and 8, which define between them a cavity corresponding to the shape of the forged semi-finished product 9, as in the dashed representation of the FIG. 1b is shown. By compressing the die cavities 7 and 8 with the blank 5 arranged therebetween, the TiAl alloy is formed into the forged semifinished product 9. By a corresponding heating of the die cavity molds 7 and 8, the deformation of the blank 5 for the forged semi-finished product 9 can be carried out by isothermal forging at a temperature which is as constant as possible. The compression of the Gesenkhohlformen 7 and 8 is indicated by the arrows in the FIG. 1b shown.

After isothermal forging, there is a forged, semi-finished product 9 near the final shape, which can be manufactured into a finished component, namely a turbine blade 10, by means of a material-removing finishing operation. Post-processing by material removal can be performed by machining or electrochemical machining processes.

After finishing, a finished turbine blade 10 with an airfoil 13, a blade root 11 and a shroud 12 is present.

As is clear from the FIGS. 1a and 1b results in the process of the invention can be prepared by a single forming step by isothermal forging in a drop forging 6 a near net shape of the component to be produced, so that the post-processing can be minimized. The inventive use of a blank 5 for isothermal forging, which is tuned in its shape to the isothermal drop forging, it is particularly ensured that takes place during the deformation of the blank 5 to the forged semi-finished product 9 over the component as uniform as possible deformation, wherein a Minimum deformation is not below, but the deformation can be kept as low as possible. As a result, it is possible to achieve a homogeneous microstructure adjustment in the case of the TiAl alloy so that the properties of the material are present homogeneously over the finished turbine blade 10.

The FIG. 2 shows in Examples 1 to 3 different courses of the mass distribution over the longitudinal axis of a blank 5, as they can be used in the present invention. FIG. 2 shows that a blank 5 can be divided into equal sections along the longitudinal axis of the blank 5, wherein within these sections different masses of the blank are present, namely at the two ends of the longitudinal axis respectively more mass than in a central region. The mass of the respective areas at the ends can be the same or different sizes.

The FIG. 3 FIG. 12 shows a so-called quasibinary state diagram of a TiAl alloy as can be used in the present invention. Quasi-binary means that in the state shown only the proportions of two components, in the present case Ti and Al change, and the other alloying constituents, in this case Nb and Mo, remain constant. The dashed working field 14 lies in the α + β + γ phase region and indicates the temperature range in which isothermal forging can be carried out with the corresponding composition of the TiAl alloy. Although the present invention has been described in detail with reference to the working examples, it will be understood by those skilled in the art that the invention is not limited to the γ / α transformation temperature is limited to these embodiments, but that modifications can instead be made in such a way that individual features can be omitted or other types of combinations of features can be realized, as long as the scope of the appended claims is not abandoned.

LIST OF REFERENCE NUMBERS

1

mold

2

cavity

3

Plant for hot isostatic pressing

4

cast blank

5

blank

6

Gesenkschmiede

7

Gesenkhohlform

8th

Gesenkhohlform

9

forged semi-finished product

10

turbine blade

11

blade

12

shroud

13

airfoil

14

field of work

Claims (19)

1. Method for producing a forged component (10) from a TiAl alloy, in particular a turbine blade, in which a blank (5) made of a TiAl alloy is provided and deformed into a forged semi-finished product (9) by forging, a usable volume being defined in the forged semi-finished product, which volume corresponds to the forged component which is to be produced, characterized in that the shape of the blank (5) is selected such that the degree of deformation ϕ_g, caused by forging, within the usable volume of the forged semi-finished product has a defined value which, over the usable volume, deviates from the defined value by at most ±1, where ϕ_g=^1/2(|ϕ_x|+|ϕ_y|+|ϕ_z|), where ϕ_x, ϕ_y, ϕ_z are the degrees of deformation in the x, y and z directions and are each defined as the natural logarithm of the ratio of the relevant final dimension in the x, y or z direction after the deformation to the relevant initial dimension in the x, y or z direction.
2. Method according to claim 1, characterized in that the degree of deformation within the usable volume of the forged semi-finished product (9) deviates from the defined value by at most ±0.5, in particular ±0.25.
3. Method according to either of the preceding claims, characterized in that the defined value of the degree of deformation is greater than or equal to 0.7, the degree of deformation in particular not falling below 0.7 within the usable volume.
4. Method according to any of the preceding claims, characterized in that the defined value of the degree of deformation is less than or equal to 2.5, in particular less than or equal to 2.0.
5. Method according to any of the preceding claims, characterized in that the deformation speed is in the range of from 0.01 to 0.5 1/s, in particular from 0.025 to 0.25 1/s.
6. Method according to any of the preceding claims, characterized in that the shape of the blank (5) is selected such that the blank is subdivided into three equal regions along the longitudinal axis of the blank, specifically a first and a second end region and a central region, where M_M < M_E1 ≤ M_E2 and M_M is the mass of the blank in the central region, M_E1 is the mass of the blank in the first end region and M_E2 is the mass of the blank in the second end region.
7. Method according to claim 7, characterized in that M_M ≤ M_E2/1.25.
8. Method according to any of the preceding claims, characterized in that a TiAl alloy comprising niobium and molybdenum, in particular an alloy comprising 27 to 30 wt.% aluminum, 8 to 10 wt.% niobium and 1 to 3 wt.% molybdenum, is used.
9. Method according to claim 8, characterized in that an alloy comprising 0.01 to 0.04 wt.% boron is used.
10. Method according to either claim 8 or claim 9, characterized in that an alloy is used which comprises, in addition to unavoidable impurities, at least one additional constituent from the group comprising carbon, oxygen, nitrogen, hydrogen, chromium, silicon, iron, copper, nickel and yttrium, it being possible for the content thereof to be ≤ 0.05 wt.% chromium, ≤ 0.05 wt.% silicon, ≤ 0.08 wt.% oxygen, ≤ 0.02 wt.% carbon, ≤ 0.015 wt.% nitrogen, ≤ 0.005 wt.% hydrogen, ≤ 0.06 wt.% iron, ≤ 0.15 wt.% copper, ≤ 0.02 wt.% nickel and ≤ 0.001 wt.% yttrium.
11. Method according to any of claims 8 to 10, characterized in that an alloy is used of which the chemical composition comprises titanium in an amount such that the alloy, together with the other constituents of claims 8 to 10, makes up 100 wt.%.
12. Method according to any of the preceding claims, characterized in that the deformation takes place by isothermal forging, in particular drop forging in the temperature range of the α+γ+β phase region of the TiAl alloy, in particular at a forging temperature of between 1150°C and 1200°C.
13. Method according to any of the preceding claims, characterized in that, after being deformed by isothermal forging, the TiAl alloy is subjected to a two-stage heat treatment, the first stage of the heat treatment being recrystallization annealing for 50 to 100 minutes at a temperature below the γ/α transformation temperature, and the second stage of the heat treatment being stabilization annealing in the temperature range of from 800°C to 950°C for 5 to 7 h, and the cooling rate in the first heat treatment step in the temperature range of between 1300°C and 900°C being greater than or equal to 3°C/s.
14. Method according to claim 13, characterized in that the recrystallization annealing is carried out for 60 to 90 minutes, in particular for 70 to 80 minutes, and/or the stabilization annealing is carried out in the temperature range of from 825°C to 925°C, in particular 850°C to 900°C and/or for 345 to 375 minutes.
15. Method according to any of the preceding claims, characterized in that the temperature during the heat treatment is adjusted up and down and maintained with a precision of a 5°C to 10°C deviation from the target temperature.
16. Method according to any of the preceding claims, characterized in that blanks (5) are used as a starting material for forging, which blanks are produced by at least one of the methods from the group comprising casting, metal injection molding (MIM), powder metallurgy methods, additive processes, 3D printing, deposition welding, hot isostatic pressing and material-removing machining methods.
17. Method according to any of the preceding claims, characterized in that the isothermal forging and/or the deformation takes place in a single-stage deformation step, in particular in a forging die, and/or the isothermal forging is carried out as drop forging using a heated die.
18. Method according to any of the preceding claims, characterized in that the blank (5) provided is unforged and is deformed into the semi-finished product by means of only one forging step, the one forging step being carried out in particular such that two halves of a die are each pressed in only one direction and against one other in order to deform the blank between the halves into the semi-finished product (9).
19. Method according to any of the preceding claims, characterized in that the forged semi-finished product (9), which in particular has been deformed using only one forging step, is finished using a material-removing machining method, in particular by chip-removing machining, preferably milling and/or electrochemical machining, in order to produce the forged component, in particular without further deformation, and/or in that the forged component is a blade of a turbomachine, in particular a turbine blade, preferably of a low-pressure turbine.

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