Method for producing forged TiAl components

Abstract

A method for producing a forged component from a TiAl alloy is provided, in particular a turbine blade (10), in which method a blank of a TiAl alloy is provided and deformed by forging into a forged, semi-finished part (9). A usable volume is defined within the forged, semi-finished part, the usable volume corresponding to the forged component to be produced. The shape of the blank is selected such that within the usable volume of the forged, semi-finished part, the degree of deformation resulting from forging deviates by no more than 1 from a defined value.

Claims

1. A method for producing a forged component from a TiAl alloy, comprising: providing a blank of a TiAl alloy; deforming the blank by forging into a forged, semi-finished part, a usable volume being defined within the forged, semi-finished part, the usable volume corresponding to the forged component to be produced; and selecting a shape of the blank such that within the usable volume of the forged, semi-finished part, a degree of deformation .sub.g has a selected defined value and a deviation from the selected defined value resulting from the forging is no more than 1 from the selected defined value over the usable volume; where .sub.g=|.sub.max|=(|.sub.x|+|.sub.y|+|.sub.z|) and where .sub.x, .sub.y, .sub.z are degrees of deformation in the x, y and z directions and are each defined as the natural logarithm of a ratio of a finished dimension in the x, y or z direction after the deformation to an original dimension in the corresponding x, y or z direction.

2. The method as recited in claim 1 wherein the degree of deformation .sub.g deviates from the selected defined value by no more than 0.25.

3. The method as recited in claim 1 wherein the selected defined value of the degree of deformation .sub.g is greater than or equal to 0.7, the degree of deformation .sub.g being no less than 0.7 within the usable volume.

4. The method as recited in claim 1 wherein the selected defined value of the degree of deformation is less than or equal to 2.5.

5. The method as recited in claim 1 wherein the selected defined value of the degree of deformation is less than or equal to 2.0.

6. The method as recited in claim 1 wherein a rate of deformation lies in the range of from 0.01 to 0.5 per second.

7. The method as recited in claim 1 wherein a rate of deformation lies in the range of from 0.025 to 0.25 per second.

8. The method as recited in claim 1 wherein the shape of the blank is selected such that the blank is divided into three portions of equal size along the longitudinal axis of blank to define a first and a second end portion as well as a middle portion, the following holding: M.sub.M<M.sub.E1M.sub.E2, where M.sub.M is the mass of the blank in the middle portion, M.sub.E1 is the mass of the blank in the first end portion and M.sub.E2 is the mass of the blank in the second end portion.

9. The method as recited in claim 8 wherein M.sub.MM.sub.E2/1.25.

10. The method as recited in claim 1 wherein the TiAl alloy includes niobium and molybdenum.

11. The method as recited in claim 10 wherein the TiAl alloy contains 27 to 30 percent by weight of aluminum, 8 to 10 percent by weight of niobium, and 1 to 3 percent by weight of molybdenum.

12. The method as recited in claim 10 wherein the TiAl alloy contains 0.01 to 0.04 percent by weight of boron.

13. The method as recited in claim 10 wherein the TiAl alloy, in addition to unavoidable impurities, contains at least one additional constituent selected from the group including carbon, oxygen, nitrogen, hydrogen, chromium, silicon, iron, copper, nickel and yttrium.

14. The method as recited in claim 13 wherein concentrations of the TiAl alloy include 0.05 percent by weight of chromium, 0.05 percent by weight of silicon, 0.08 percent by weight of oxygen, 0.02 percent by weight of carbon, 0.015 percent by weight of nitrogen, 0.005 percent by weight of hydrogen, 0.06 percent by weight of iron, 0.15 percent by weight of copper, 0.02 percent by weight of nickel and 0.001 percent by weight of yttrium.

15. The method as recited in claim 13 wherein the TiAl alloy is used whose chemical composition contains titanium in an amount which, together with niobium, molybdenum, any additional constituents selected from the group including carbon, oxygen, nitrogen, hydrogen, chromium, silicon, iron, copper, nickel and yttrium, and unavoidable impurities, makes up 100 percent by weight of the alloy.

16. The method as recited in claim 1 wherein the deformation is accomplished by isothermal forging in the temperature range of the ++ phase region of the TiAl alloy.

17. The method as recited in claim 16 wherein the forging temperature is between 1150 C. and 1200 C.

18. The method as recited in claim 16 wherein the forging is closed-die forging.

19. The method as recited in claim 1 wherein the deformation is accomplished by isothermal forging, and after the deformation by the isothermal forging, the TiAl alloy is subjected to a two-stage heat treatment, the first stage of the heat treatment including recrystallization annealing for 50 to 100 minutes at a temperature below the / transition temperature, and the second stage of the heat treatment including stabilization annealing in the temperature range of from 800 C. to 950 C. for 5 to 7 hours, and the cooling rate during the first heat treatment stage in the temperature range of between 1300 C. and 900 C. being greater than or equal to 3 C./s.

20. The method as recited in claim 19 wherein the recrystallization annealing is performed for 60 to 90 minutes or the stabilization annealing is performed in the temperature range of from 825 C. to 925 C. or for 345 to 375 minutes.

21. The method as recited in claim 20 wherein the recrystallization annealing is performed for 70 to 80 minutes, or the stabilization annealing is performed in the temperature range of from 850 C. to 900 C.

22. The method as recited in claim 19 wherein during the two-stage heat treatment, the temperature is set and maintained at an accuracy of 5 C. to 10 C. of upward and downward deviation from the setpoint temperature.

23. The method as recited in claim 1 wherein the blank is provided from raw stock and produced using at least one method selected from the group including casting, metal injection molding, powder-metallurgical methods, additive methods, 3D printing, deposition welding, hot isostatic pressing, and material-removing machining processes.

24. The method as recited in claim 1 wherein the deformation is performed in a single-stage deformation step.

25. The method as recited in claim 24 wherein the deformation is performed in a forging die set.

26. The method as recited in claim 24 wherein the deformation includes an isothermal forging performed as a closed-die forging with a heated die set.

27. The method as recited claim 1 wherein the blank provided is unforged and is formed into the semi-finished part in only one forging step.

28. The method as recited in claim 27 wherein the only one forging step is performed by pressing two dies of a die set toward one another, each in only one respective direction, so as to deform the blank located therebetween into the semi-finished part.

29. The method as recited in claim 1 wherein the forged, semi-finished part is subsequently machined using a material-removing machining process so as to produce the forged component.

30. The method as recited in claim 29 wherein the material-removing machining process includes mechanical machining or electrochemical machining.

31. The method as recited in claim 29 wherein the mechanical machining includes milling.

32. The method as recited in claim 1 wherein the forged component is a blade of a turbomachine.

33. The method as recited in claim 32 wherein the blade is a turbine blade.

34. The method as recited in claim 33 wherein the turbine blade is a low-pressure turbine.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying drawings show purely schematically in

(2) FIGS. 1a and 1b a process sequence for manufacturing a turbine blade in accordance with the present invention; in

(3) FIG. 2 a diagram for illustrating possible mass distributions in a blank for the forging operation; and in

(4) FIG. 3 an equilibrium diagram for a TiAl alloy as may be used in the present invention, indicating the phase field in which forging or deformation takes place.

DETAILED DESCRIPTION

(5) Other advantages, characteristics and features of the present invention will become apparent from the following detailed description of the exemplary embodiments. However, the present invention is not limited to such exemplary embodiments.

(6) FIGS. 1a and 1b a show the sequence of process steps performed in an exemplary embodiment of the method according to the present invention.

(7) Initially, a blank 5 is produced by pouring a molten TiAl alloy into a casting mold 1 having a cavity 2 which corresponds to the shape of the blank 5 to be produced.

(8) After the TiAl alloy has been cast in mold 1 and solidified, cast blank 4 may be hot-isostatically pressed in a machine 3 for hot isostatic pressing in order to densify cast blank 4 and to close possible casting voids or the like. Thus, the hot isostatic pressing is not used for deforming cast blank 4, but only for densifying the material.

(9) Thereafter, blank 5 may in addition be subjected to material-removing machining, for example, by mechanical machining processes or by electrochemical machining.

(10) The blank 5 so produced is forged to a near-net-shape, semi-finished part 9 in a drop forge 6, the drop forge 6 having two drop-forge dies 7 and 8 defining a cavity therebetween which corresponds to the shape of the semi-finished part 9 to be forged, as indicated in dashed lines in FIG. 1b. By pressing drop-forge dies 7 and 8 together, with blank 5 located therebetween, the TiAl alloy is deformed into the forged, semi-finished part 9. By suitably heating drop-forge dies 7 and 8, the deformation of blank 5 into forged, semi-finished part 9 can be performed by isothermal forging at as constant a temperature as possible. The pressing together of drop-forge dies 7 and 8 is indicated by arrows in FIG. 1b.

(11) After isothermal forging, a near-net-shape, forged semi-finished part 9 is present which may be formed into the finished component, namely a turbine blade 10, by subsequent, material-removing machining. The subsequent machining by removal of material may be performed using mechanical machining processes or electrochemical machining processes.

(12) After the subsequent machining, a finished turbine blade 10 is present which has an airfoil 13, a blade root 11 and a shroud 12.

(13) As is apparent from FIGS. 1a and 1b, in the method according to the present invention, a near net shape of the component to be produced can be obtained in a single deformation step through isothermal forging in a drop forge 6, which makes it possible to minimize subsequent machining. Because the present invention uses, for isothermal forging, a blank 5 whose shape is adapted for isothermal closed-die forging, it is ensured, in particular, that during the deformation of blank 5 into the forged, semi-finished part 9, as uniform a deformation as possible takes place throughout the component, without failing to achieve a minimum deformation, but allowing the deformation to be kept as small as possible. This makes it possible to obtain a homogeneous microstructure for the TiAl alloy, so that the material properties are homogeneous throughout the finished turbine blade 10.

(14) FIG. 2 shows, in examples 1 through 3, different mass distribution profiles along the longitudinal axis of blank 5 as may be used in the present invention. FIG. 2 illustrates that a blank 5 may be divided into sections of equal size along the longitudinal axis of blank 5, these sections containing different masses of the blank, namely more mass at the two ends of the longitudinal axis than in a middle portion. The masses in the respective portions at the ends may be equal or unequal.

(15) FIG. 3 shows a so-called pseudobinary phase diagram of a TiAl alloy as may be used in the present invention. The term pseudobinary means that in the phase region shown, only the percentages of two constituents, here Ti and Al, change, while the other alloy constituents, here Nb and Mo, remain constant. The dashed phase field 14 in which processing occurs lies in the ++ phase region and indicates the temperature range within which isothermal forging can be performed for the respective composition of the TiAl alloy. In the phase diagram, the / transition temperature corresponds to the line between the + phase region and the ++ phase region.

(16) Although the present invention has been described in detail with reference to the exemplary embodiments thereof, those skilled in the art will understand that it is not intended to be limited thereto and that modifications may be made by omitting individual features or by combining features in different ways, without departing from the protective scope of the appended claims.

LIST OF REFERENCE NUMERALS

(17) 1 casting mold 2 cavity 3 machine for hot isostatic pressing 4 cast blank 5 blank 6 drop forge 7 drop-forge die 8 drop-forge die 9 forged, semi-finished part 10 turbine blade 11 blade root 12 shroud 13 airfoil 14 phase field in which processing occurs