CN109202017B

CN109202017B - Casting method for producing a blade for a gas turbine

Info

Publication number: CN109202017B
Application number: CN201810698606.5A
Authority: CN
Inventors: W.贝勒; K.施瓦茨
Original assignee: Ansaldo Energia Switzerland AG
Current assignee: Ansaldo Energia Switzerland AG
Priority date: 2017-06-30
Filing date: 2018-06-29
Publication date: 2022-05-13
Anticipated expiration: 2038-06-29
Also published as: EP3421156B1; CN109202017A; EP3421156A1

Abstract

The invention provides a casting method for producing a blade for a gas turbine, comprising the steps of: a) producing a model of a blade made of wax; b) covering the mold with a shell made of slurry; c) heating the pattern covered by the shell to separate the wax from the shell; d) pouring molten metal inside the enclosure; e) the outer shell is removed from the inner solidified metal. Wherein, between steps c) and d), the method comprises the additional step of inserting a packing element inside the casing; the packing element is made of a material adapted to dissolve upon contact with molten metal poured inside the casing.

Description

Casting method for producing a blade for a gas turbine

Priority declaration

The present application claims priority based on european patent application 17179205.4 filed 2017, 6, 30, the disclosure of which is incorporated by reference.

Technical Field

The invention relates to a method for producing a blade for a gas turbine. In particular, the invention relates to a casting method for producing blades for gas turbines. More specifically, the present invention relates to a casting method for producing shrouded blades for gas turbines.

Background

Casting techniques for producing blades for gas turbines are commonly used today. According to the practice of the prior art, such a casting process for producing blades for gas turbines comprises a series of steps consisting in:

a) producing a blade made of wax (called "model") having the same shape as the metal blade to be realised;

b) covering the model with slurry material;

c) heating the covered pattern to separate the wax from the crust in the slurry;

d) pouring molten metal inside the enclosure;

e) the outer shell is removed from the inner solidified metal.

In general, the above process is referred to as a "lost wax casting" process.

The step a) of producing the blade model consists of a process of generating blades made of wax by injection. The mould also includes a feeder gate or opening to be used during the pouring step and a pouring element as a structure for manipulating the mould. The mould structure made of wax may comprise more than one blade mould connected to the casting element in order to achieve a tree assembly, wherein the casting element forms a channel through which the molten metal will flow to all blade moulds. The process may also include a preliminary step of manufacturing a core to be integrated inside the wax pattern.

Step b) of covering the pattern of wax with a slurry material (i.e. a slurry of fine ceramic particles) is typically performed by dipping the pattern into a slurry tank, so that once the slurry is dried, a shell is created around the pattern of wax. This dipping step is repeated until the shell is thick enough to withstand the molten metal.

The step c) of heating is performed by placing the pattern of wax covered by the casing of slurry in a furnace box at a temperature suitable for melting the wax. In this condition, the molten wax leaves the shell, which becomes a hollow shell. Such a shell is preheated up to about 1000 ℃ before step d) of pouring the molten metal inside the shell.

The step d) of pouring the molten metal inside the housing is typically performed by feeding the molten metal to a feeder gate. Molten metal fills the hollow shell from the feeder gate. Typically, molten metal is fed from the root portion of the blade to the inside of the mud shell and, under gravity, reaches the opposite end of the blade (i.e., the tip portion). Once filled, the outer shell is placed outside the oven box to allow the molten metal to cool and solidify into the final metal blades. The cooling time depends on the thickness of the part and on the metal used.

The step e) of removing the outer shell from the inner solidified metal can be performed by using a water jet. Once the shell is removed, the metal blade is separated from the casting element by sawing or by being able to break (using liquid nitrogen) or other methods. The last step of the casting process consists of a finishing operation, such as a grinding or sandblasting operation.

Currently, aerodynamic considerations call for the realization of turbine blades with long, smooth shapes. However, this particular shape involves some problems during the casting process of the blade, wherein the molten metal flows from the root portion to the tip portion of the blade. Specifically, when designing blades having shrouded tips, casting problems arise. Indeed, during the casting process of this particular kind of blade, molten metal flows from a smaller area (the airfoil portion of the blade) to a larger area (the shrouded tip). The metal volume required to fill the shroud is higher than that required in the "feed" airfoil region.

Such blades incur the creation of a porous structure along the airfoil portion. Indeed, during the filling of the shrouded tip, the molten metal on the airfoil portion cools too rapidly, and such rapid solidification generates a porous structure. In other words, during the casting process for realizing the blade, this drawback occurs, in which the molten metal passes through a smaller volume into a large volume, as in the bottleneck passage.

Disclosure of Invention

It is therefore a main object of the present invention to provide a casting method for producing blades for gas turbines, which is suitable for solving the aforementioned problems of the practice of the prior art.

In order to achieve the above-mentioned object, the present invention provides a casting method for producing a blade of a gas turbine, comprising at least a series of steps consisting of:

a) producing a model of the blade made of wax;

b) covering the mold with a shell made of slurry;

c) heating the pattern covered by the shell to separate the wax from the shell;

d) pouring molten metal inside the enclosure;

e) the outer shell is removed from the inner solidified metal.

According to a main aspect of the casting method according to the invention, between steps c) and d), the method comprises a step f) of: inserting a packing element into the inside of the shell, wherein the packing element is made of a material adapted to dissolve upon contact with molten metal poured into the inside of the shell.

Advantageously, according to the method of the invention, it is possible to achieve a better smooth aerodynamic blade (i.e. a blade with less porosity at the airfoil portion) using casting techniques. In practice, during step d) of pouring the molten metal inside the shell, such shell is placed in a vertical configuration so as to pour the molten metal from the root and, under the action of gravity, the molten metal flows to the tip. According to this vertical configuration of the housing, the filler element inserted inside the housing reaches the tip portion under the effect of gravity before the pouring step. Such a filler element liquefies when the molten metal comes into contact with the filler element at the tip portion, and thus, facilitates filling of the blade tip. As a result, the packing element allows for rapid filling of the tip portion, and thus, rapid backfilling at the airfoil portion of the casing disposed upstream of the tip. The term upstream refers to the direction of molten metal flow from the root to the tip. Such backfilling slows the cooling of the molten metal at the airfoil portion of the shell and, therefore, precludes the formation of too rapid solidification of the shrinking cellular structure. This advantageous effect can be noted in particular for smooth blades, i.e. with long (10-20 cm) parallel faces upstream of the tip, or for blades with shrouded tips. Indeed, for such kind of blades, the molten metal must fill the "large" tip region before backfilling the airfoil portion. In the absence of the inventive filler element, the backfill is delayed, and such delay causes excessively rapid solidification and the generation of a shrinking cellular structure at the airfoil portion.

Preferably, between steps f) and d), the process of the invention comprises a step g) as follows: the housing containing the packing elements is heated.

Advantageously, according to this embodiment, the packing element reaches a high temperature, and therefore, it can liquefy rapidly in contact with the molten metal poured inside the casing.

Preferably, the packing element is a sponge element (e.g., a wire mesh sponge).

Advantageously, according to this embodiment, the packing element presents a large volume with a small amount of material.

Preferably, the packing elements are made of the same material of the molten metal.

Advantageously, according to the present embodiment, the blade produced does not comprise any impurities with respect to the molten metal material. For example, the packing element can be made of Ni, Co or Fe. Alternatively, the filler element can be made of a Ni-based, Co-based or Fe-based alloy or superalloy.

Preferably, the step of heating the envelope containing the packing element is performed in a vacuum oven.

Advantageously, according to this embodiment, the filler material does not realize any oxide layer during the heating step.

Alternatively, to avoid oxidation, the method comprises the steps of: the filler material is plated with an inert material prior to the step of inserting the filler element into the inside of the housing. According to this embodiment, the plating material can be Pt deposition, Au deposition, or W deposition. Advantageously, the W deposition avoids oxidation and does not require any temperature limitation during the heating step.

Alternatively, to generate the oxidation, the heating is performed in a furnace box under the atmosphere during the step of heating.

Advantageously, according to this embodiment, the filler material realizes oxide particles capable of cracking during the pouring step. These oxide particles can be used for the shroud function. In fact, the shrouded tip becomes wear resistant and, therefore, suitable for achieving a sealing surface during use of the blade.

Preferably, the model produced in step a) (and therefore the metal blade produced by the casting method of the invention) comprises an airfoil provided with parallel faces upstream of the blade tip. The length of the parallel surface is at least 10 cm. According to this embodiment, the filler element allows a suitable back-filling of the molten metal at the airfoil parallel faces.

Preferably, the mould produced in step a) (and therefore the metal blade produced by the casting method of the present invention) comprises a shrouded tip.

Finally, the invention also relates to a blade directly obtained by the aforementioned casting method.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed. Other advantages and features of the present invention will become apparent from the following description, the accompanying drawings, and the claims.

The features of the present invention, which are believed to be novel, are set forth with particularity in the appended claims.

Brief Description of Drawings

Further benefits and advantages of the invention will become apparent upon a careful reading of the detailed description with appropriate reference to the drawings.

The invention itself, however, will be best understood by reference to the following detailed description of the invention when read in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic perspective view of a shrouded blade for a gas turbine;

FIG. 2 is a schematic view of a blade configuration with respect to gravity during a casting process;

fig. 3 is a schematic view of a flow chart of an embodiment of a casting method according to the present invention.

Detailed Description

The technical content and the detailed description of the invention are described below, in cooperation with the accompanying drawings, according to preferred embodiments, which are not intended to limit the scope thereof. The invention as claimed covers all equivalent variations and modifications as may be made in accordance with the appended claims.

Now, the present invention will be described in detail with reference to the accompanying drawings.

Referring to FIG. 1, FIG. 1 is a schematic perspective view of a shrouded blade for a gas turbine. In general, shrouded blades (i.e., blades having shrouded tips) are used to minimize flow leakage between the blade tips and the surrounding stator frame. Typically, the shrouded tip comprises a platform extending in a plane substantially parallel to the opposing stator frame and comprises one or more fins.

According to the example of fig. 1, the represented blade 1 comprises an airfoil 2, the airfoil 2 extending along a longitudinal axis 3 from a root 4 to a tip 5 portion, wherein the longitudinal axis 3 is a radial axis with respect to the turbine rotor. The tip portion 5 comprises a platform-shrouded section 6. Once installed within the gas turbine, such shrouded sections 6 abut similar shroud sections of adjacent blades to achieve an annular shroud. The shrouded section 6 is provided with a plurality of fins 7, the fins 7 defining parallel channels 8.

Referring to fig. 2, fig. 2 is a schematic view of the configuration of the blade with respect to the direction of gravity 10 during the casting method of the present invention.

According to fig. 2, at least during the pouring step (to be explained below), the blade 1 is arranged substantially vertically so that the molten metal introduced at the root portion 4 flows through the airfoil 2 to the tip portion 5 (i.e. the shrouded tip portion) under the action of gravity. In fig. 2, reference portion 9 represents a portion of the airfoil connected to the tip 5, wherein the portion has a parallel shape in order to achieve a smooth blade.

Referring to fig. 3, fig. 3 is a schematic view of a flow chart of an embodiment of a casting method according to the present invention. According to this example, the casting method comprises the following series of steps:

A) producing a mould of shrouded blades by wax injection, the mould being provided with a casting element and a feeder opening;

B) dipping the mold in a slurry solution so as to cover the mold with a shell made of slurry;

C) heating the pattern covered by the shell to separate the wax from the shell;

D) sintering the hollow shell;

E) inserting a sponge element inside the housing;

F) heating the housing containing the sponge element;

G) pouring molten metal inside a housing containing a sponge element;

H) removing the outer shell from the inner solidified metal;

I) removing the casting element and the feeder from the blade;

l) finishing the blade.

The above casting method steps will now be described in detail.

During step a), a model of the shrouded blade is generated by wax injection. The model discloses the same outer shape of the metal blade to be produced. According to fig. 2, the blade comprises a shrouded tip and an airfoil portion having a parallel shape connected to the tip. As known in the lost wax casting process, the mould also comprises a pouring element and a feeder with an opening.

During step B), the pattern made of wax is immersed in a slurry solution (e.g. a slurry of fine ceramic particles) so as to cover the pattern with a shell.

During step C), the mould of wax covered by the shell is heated to the melting temperature of the wax. Once melted, the wax flows out of the shell, which becomes a hollow shell.

During step D), the above hollow shell is sintered in order to achieve a solid structure.

During step E), at least the sponge element is introduced inside the hollow casing. The sponge elements are introduced from the root of the shell into the shrouded tip and are distributed as evenly as possible. As presented in the flow chart of fig. 3, the sponge element may be plated, e.g. by W deposition (step O), in order to avoid oxidation during the remaining steps of the method.

During step F), the housing containing the sponge element is heated. As represented in the flow chart of fig. 3, step F can be performed in two different ways. In the first case (F'), the heating step is carried out in a vacuum oven to avoid oxidation. Alternatively, in the second case (F "), a heating step is performed in the oven box under ambient conditions, so as to produce an oxidation which will be beneficial to the shield function.

During step G), molten metal is poured inside the housing containing the sponge element. Upon contact with the molten metal, the sponge element dissolves. In this manner, the shrouded tip portion is rapidly filled and, as a result, the molten metal reaches the parallel portion of the airfoil before it solidifies.

During step H), and after solidification of the molten metal, the outer shell is removed from the inner solidified metal.

Finally, the casting element and the feeder are removed from the blade (step I) and the blade is finished (step L).

According to a different embodiment, represented by hatching in figure 3, the method comprises a preliminary step N) of producing cores and a penultimate step M) of leaching such cores.

Although the invention has been explained in relation to the preferred embodiment(s) of the invention as mentioned above, it will be understood that many other possible modifications and variations could be made without departing from the scope of the invention. It is therefore contemplated that the appended claim(s) will cover such modifications and variations as fall within the true scope of the invention.

Claims

1. A casting method for producing a shrouded blade for a gas turbine, the shrouded blade comprising a root and a shrouded tip, the method comprising the steps of:

a) producing a pattern of the shrouded blades made of wax;

b) covering the mold with a shell made of slurry;

c) heating the pattern covered by the shell to separate the wax from the shell;

d) pouring molten metal inside the housing; the housing is placed in a vertical configuration so that the molten metal is poured from the root and flows under gravity to a shrouded tip;

e) removing the outer shell from the inner solidified metal;

characterized in that, between said steps c) and d), said method comprises a step f) of inserting a filler element inside said casing at least at said shrouded tip, wherein said filler element is made of a material suitable for dissolving when in contact with said molten metal poured inside said casing.

2. Casting method according to claim 1, wherein, between said steps f) and d), said method comprises a step g) of heating said shell containing said filler element.

3. The casting method according to claim 2, wherein the packing element is a sponge element.

4. Casting method according to claim 3 or 2, wherein the filler element is made of the same material of the molten metal.

5. Casting method according to claim 1, wherein the filler element is made of Ni or Co or Fe, or of a Ni-based or Co-based or Fe-based alloy or superalloy.

6. Casting method according to claim 2 or 3, wherein the filler element is made of Ni or Co or Fe, or of a Ni-based or Co-based or Fe-based alloy or superalloy.

7. Casting method according to claim 6, wherein said step g) of heating the enclosure containing the filler element is performed in a vacuum oven.

8. Casting method according to claim 6, wherein said step g) of heating the enclosure containing the packing element is performed in an oven box under atmosphere.

9. Casting method according to claim 6, wherein, before said step g), said method comprises a step of plating said filler element.

10. The casting method according to claim 9, wherein the plating step is performed by W deposition.

11. The casting method as recited in claim 3, wherein said filler element is plated with an inert material prior to said step f).

12. A casting method according to any one of claims 1-3, wherein the blade produced by moulding in step a) comprises parallel faces upstream of the blade tip.

13. The casting method according to claim 12, wherein the length of the parallel face is at least 10 cm.

14. A turbine blade for a gas turbine, realized by a casting method according to any one of the preceding claims.