EP3421156A1

EP3421156A1 - Casting method for producing a blade for a gas turbine

Info

Publication number: EP3421156A1
Application number: EP17179205.4A
Authority: EP
Inventors: Wolfram Beele; Karsten Schwarz
Original assignee: Ansaldo Energia Switzerland AG
Current assignee: Ansaldo Energia Switzerland AG
Priority date: 2017-06-30
Filing date: 2017-06-30
Publication date: 2019-01-02
Anticipated expiration: 2037-06-30
Also published as: EP3421156B1; CN109202017A; CN109202017B

Abstract

A casting method for producing a blade of a gas turbine, the method comprising the following steps: a) producing a pattern made of wax of the blade; b) covering the pattern with an outer shell made of slurry; c) heating the pattern covered by the outer shell for separating the wax from the outer shell; d) pouring a molten metal inside the outer shell; e) removing the outer shell from the inner solidified metal. wherein between the steps c) and d) the method comprises the additional step of inserting a filler element inside the outer shell; the filler element being made of a material suitable to go in solution when in contact with the molten metal poured inside the outer shell.

Description

Field of the Invention

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

Description of prior art

It is common today to use the casting technology for producing the blades of a gas turbine. According to the prior art practice, such casting process for producing the blades of a gas turbine comprises the following sequence of steps:

a) producing a blade made in wax, called "pattern", having the same shape to the metal blade to be realized;
b) covering the pattern with a slurry material;
c) heating the covered pattern for separating the wax from the outer shell in slurry;
d) pouring a molten metal inside the outer shell;
e) removing the outer shell from the inner solidified metal.

In general, the above described process is known as "lost wax casting" process.
The step a) of producing a blade pattern consists in creating by injection a blade made in wax. This pattern comprises also a feeder gate or opening, that will be used during the pouring step, and a gating element that is a structure used for manipulating the pattern. The pattern structure made in wax may comprise more than a blade pattern connected to gating element in order to realize a tree-like assembly wherein the gating element forms a channel though which the molten metal will flow to all blade patterns. The process may also comprises a preliminary step of manufacturing a core that will be integrated inside the pattern in wax.
The step b) of covering the pattern in wax with a slurry material, i.e. a slurry of fine ceramic particles, is usually performed by dipping the pattern in a slurry tank in order to create, once dried the slurry, an outer shell around the pattern in 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 in wax covered by the slurry outer shell into an oven at a temperature suitable for melting the wax. At this condition, the melted wax leaves the outer shell that becomes a hollow outer shell. Before the step d) of pouring the molten metal inside the outer shell, such outer shell is preheated until to about 1000°C.
The step d) of pouring a molten metal inside the outer shell is usually performed by feeding the molten metal to the feeder gate. From the feeder gate, the molten metal fills the hollow outer shell. Usually the molten metal is fed inside the slurry outer shell from the root portion of the blade and, by gravity, the molten metal reaches the opposite end of the blade, i.e. the tip portion. Once filled, the outer shell is placed outside the oven to allow the molten metal to cool and solidify into the final metal blade. 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 water jets. Once removed the outer shell, the metal blade is separated from the gating element by sawing or could breaking (using liquid nitrogen) or other methods. The last step of the casting process consists in the finishing operations such grinding or sandblasting operations.
Currently, the aerodynamic considerations require to realize turbine blades having a long sleek shape. However, this particular shape involves some problems during the casting process of the blade wherein the molten metal flows from the root to the tip portion of the blade. In particular, the casting problems occur when the blade is designed having a shrouded tip. Indeed, during the casting process of this particular kind of blades the 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 the metal required within the 'feeding' airfoil area.
This kind of blades is exposed to the creation of porosities along the airfoil portion. Indeed, during the filling of the shrouded tip the molten metal on the airfoil portionools down too quickly and such quick solidification creates porosities. In other words, this defect occurs during the casting process for realizing a blade wherein the melted metal arrives in a large volume passing through a smaller volume, as in a bottleneck passage.

Disclosure of the invention

Accordingly, a primary object of the present invention is to provide a casting method for producing a blade for a gas turbine suitable for overcoming the problem foregoing described of the prior art practice.
In order to achieve the objective mentioned above, the present invention provides a casting method for producing a blade of a gas turbine comprising at least the following sequence of steps:

a) producing a pattern of the blade made of wax;
b) covering the pattern with an outer shell made of slurry;
c) heating the pattern covered by the outer shell for separating the wax from the outer shell;
d) pouring a molten metal inside the outer shell;
e) removing the outer shell from the inner solidified metal. According to the main aspect of the casting method of the invention, between the steps c) and d) the method comprises the step f) of inserting a filler element inside the outer shell wherein the filler element is made of a material suitable to go in solution when in contact with the molten metal poured inside the outer shell.

Advantageously, according to the method of the present invention it is possible to realize with the casting technology a better sleek aerodynamic blade, i.e. a blade having less porosity at the airfoil portion. Indeed, during the step d) of pouring a molten metal inside the outer shell, such outer shell is placed in a vertical disposition so that the molten metal is poured from the root and it flows to the tip under the action of the gravity force. According this vertical disposition of the outer shell, the filler element, inserted inside the outer shell before the pouring step, reaches the tip portion under the action of the gravity force. When the molten metal goes in contact with the filler element at the tip portion, such filler element liquifies and therefore helps the filling of the blade tip. Consequently, the filler element allows to obtain a quick filling of the tip portion and therefore a quick back-filling at the airfoil portion of the outer shell arranged upstream the tip. The term upstream refers to the molten metal flow direction from the root to the tip. Such back-filling decelerates the cooling of the molten metal at the airfoil portion of the outer shell and therefore the too quick solidification that creates shrink porosities is eliminated. In particular, this favorable effect can be noted for sleek blade, i.e. having long (10-20 cm) parallel surface upstream the tip, or for blade having a shrouded tip. Indeed, for such kinds of blade the molten metal has to fill a "large" tip area before back-filling the airfoil portion. In absence of the inventive filler element, this back-filling is delayed and such delay causes a too quick solidification and the creation of shrink porosities at the airfoil portion.
Preferably, between the steps f) and d) the method of the present invention comprises a step g) of heating the outer shell containing the filler element.
Advantageously, according to this embodiment the filler element reaches a high temperature so it can liquify quickly under the contact with the molten metal poured inside the outer shell.
Preferably, the filler element is a sponge element, for instance a wire mesh sponge.
Advantageously, according to this embodiment the filler element presents a high volume with a small amount of material.
Preferably, the filler element in made of the same material of the molten metal.
Advantageously, according to this embodiment the produced blade does not comprise any impurities with respect to the molten metal material. For instance, the filler element can be made of Ni, Co or Fe. Alternatively, the filler element can be made of an alloy, or superalloy, Ni-based, Co-based or Fe-based.
Preferably, the step of heating the outer shell containing the filler 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, in order to avoid the oxidation, the method comprises the step of plating the filler material with an inert material before the step inserting a filler element inside the outer shell. According to this embodiment, the plating material can be a Pt-deposition, an Au-deposition or a W-deposition. Advantageously, the W-deposition avoids the oxidation and does not require any temperature restriction during the heating step.
Alternatively, in order to create the oxidation during the step of heating is performed in an under-atmosphere oven.
Advantageously, according to this embodiment the filler material realizes oxide particles that can be cracked during the pouring step. These oxide particles can be used for the shroud function. Indeed, the shrouded tip becomes abradable and, therefore, suitable for realizing a sealing surface during the use of the blade.
Preferably, the pattern produced in the step a), and therefore the metal blade produced through the casting method of the invention, comprises an airfoil provided with parallel surfaces upstream the blade tip. The length of the parallel surfaces is at least 10 cm. According to this embodiment, the filler element allows to realize a suitable back-filling of the molten metal at the airfoil parallel surfaces.
Preferably, the pattern produced in the step a), and therefore the metal blade produced through the casting method of the invention, comprises a shrouded tip.
Finally, the invention also relates to the blade directly obtained by the casting method foregoing described.
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 invention will be apparent from the following description, drawings and claims.
The features of the invention believed to be novel are set forth with particularity in the appended claims.

Brief description of drawings

Further benefits and advantages of the present invention will become apparent after a careful reading of the detailed description with appropriate reference to the accompanying drawings.
The invention itself, however, may be best understood by reference to the following detailed description of the invention, which describes an exemplary embodiment of the invention, taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a schematic prospective view of a shrouded blade for a gas turbine;
Fig. 2 is a schematic view of the blade disposition with respect to gravity force during the casting method;
Fig. 3 is a schematic view of a flow chart of an embodiment of the casting method according the invention.

Detailed description of preferred embodiments of the invention

In cooperation with attached drawings, the technical contents and detailed description of the present invention are described thereinafter according to preferred embodiments, being not used to limit its executing scope. Any equivalent variation and modification made according to appended claims is all covered by the claims claimed by the present invention.
Reference will now be made to the drawing figures to describe the present invention in detail.
Reference is made to Fig. 1 that is a schematic prospective view of a shrouded blade for a gas turbine. In general, a shrouded blade, i.e. a blade having a shrouded tip, is used to minimize the flow leakage between the blade tip and the surrounding stator frame. Usually, the shrouded tip comprises a platform extending in a plane essentially parallel to the opposite stator frame and comprises one or more fins.
According to the example of figure 1, the represented blade 1 comprises an airfoil 2 extending along a longitudinal axis 3, that is a radial axis with respect to turbine rotor, from a root 4 to a tip 5 portion. The tip portion 5 comprises a platform-like shrouded segment 6. Once mounted within the gas turbine, such shrouded segment 6 abuts against similar shroud segments of adjacent blades to realize a ring-like shroud. The shrouded segment 6 is provided with a plurality of fins 7 defining parallel channels 8.
Reference is made to Fig. 2 that is a schematic of the disposition of the blade with respect to gravity force direction 10 during the casting method of the present invention.
According to figure 2, at least during the pouring step that will be explained in the following, the blade 1 is substantially vertically arranged so that under the action of the gravity force the molten metal introduced at the root portion 4 flows to the tip portion 5, i.e. to the shrouded tip portion, passing through the airfoil 2. In the figure 2 the reference 9 represents a portion of the airfoil connected to the tip 5 wherein this portion has a parallel shape in order to realize a sleek blade.
Reference is made to Fig. 3 that is a schematic view of a flow chart of an embodiment of the casting method according the invention. According to this example, the casting method comprises the following sequence of steps:

A) producing by wax injection a pattern of a shrouded blade, this pattern is provided with a gating element and feeder opening;
B) dipping the pattern in a slurry solution in order to cover the pattern with an outer shell made of slurry;
C) heating the pattern covered by the outer shell for separating the wax from the outer shell;
D) sintering the hollow outer shell;
E) inserting a sponge element inside the outer shell;
F) heating the outer shell containing the sponge element;
G) pouring a molten metal inside the outer shell containing the sponge element;
H) removing the outer shell from the inner solidified metal;
I) removing the gating element and the feeder by the blade;
L) finishing the blade.

The above casting method steps will be now described in details.
During the step A), a pattern of a shrouded blade is created by wax injection. The pattern discloses the same outer shape of the metal blade to be produced. According to figure 2, this blade comprises a shrouded tip and an airfoil portion connected to the tip having parallel shape. The pattern, as known in the lost wax casting process, comprises also a gating element and feeder with an opening.
During the step B), the pattern made of wax is dipped in a slurry solution, for instance a slurry of fine ceramic particles, in order to cover the pattern with an outer shell.
During the step C), the pattern in wax covered by the outer shell is heated up to the melting temperature of the wax. Once melted, the wax flows out from the outer shell that becomes a hollow outer shell.
During the step D), the above hollow outer shell is sintered in order to realize a solid structure.
During the step E), at least a sponge element is introduced inside the hollow outer shell. The sponge element is introduced from the root to the shrouded tip of the outer shell and as far as possible evenly distributed. As presented in the flow chart of figure 3, the sponge element may be plated (step O), for instance by a W-deposition, in order to avoid oxidation during the remaining steps of the method.
During the step F), the outer shell containing the sponge element is heated. As represented in the flow chart of figure 3, the step F may by performed in two different ways. In the first case (F') this heating step is performed in a vacuum oven to avoid oxidation. Alternatively, in the second case (F") the heating step is performed in a under-ambient oven in order to produce oxidation that will be beneficial for the shroud function.
During the step G), a molten metal is poured inside the outer shell containing the sponge element. The sponge element goes in solution under the contact with the molten metal. In this way the shrouded tip portion in quickly filled and the, consequently, the molten metal reaches the parallel portion of the airfoil before its solidification.
During the step H), and after the solidification of the molten metal, the outer shell is removed from the inner solidified metal.
Finally, the gating element and the feeder are removed from the blade (step I) and the blade is finished (step L).
According to a different embodiment, represented in figure 3 by hatched lines, the method comprises the preliminary step N) of producing a core and the second last step M) of leaching such core.
Although the invention has been explained in relation to its preferred embodiment(s) as mentioned above, it is to be understood that many other possible modifications and variations can be made without departing from the scope of the present invention. It is, therefore, contemplated that the appended claim or claims will cover such modifications and variations that fall within the true scope of the invention.

Claims

A casting method for producing a blade of a gas turbine, the method comprising the following steps:
a) producing a pattern made of wax of the blade;

b) covering the pattern with an outer shell made of slurry;

c) heating the pattern covered by the outer shell for separating the wax from the outer shell;

d) pouring a molten metal inside the outer shell;

e) removing the outer shell from the inner solidified metal. characterized in that between the steps c) and d) the method comprises the step f) of inserting a filler element inside the outer shell wherein the filler element is made of a material suitable to go in solution when in contact with the molten metal poured inside the outer shell.
Casting method as claimed in claim 1, wherein between the steps f) and d) the method comprises the step g) of heating the outer shell containing the filler element.
Casting method as claimed in claim 2, wherein the filler element is a sponge element.
Casting method as claimed in claim 3 or 2, wherein the filler element is made of the same material of the molten metal.
Casting method as claimed in any one of the foregoing claims 4-2, wherein the filler element is made of Ni, or Co or Fe or is made of an alloy or superalloy Ni-based, or Co-based or Fe-based.
Casting method as claimed in claim 5, wherein the step g) of heating the outer shell containing the filler element is performed in a vacuum oven.
Casting method as claimed in claim 5, wherein the step g) of heating the outer shell containing the filler element is performed in a under-atmosphere oven.
Casting method as claimed in claim 5, wherein before the step g) the method comprises the step of plating the filler material.
Casting method as claimed in claim 8, wherein the plating step in performed by a W deposition.
Casting method as claimed in claim 3, wherein the filler element in made of an inert material.
Casting method as claimed in any one of the foregoing claims, wherein the blade mould produced in the step a) comprises parallel surfaces upstream the blade tip.
Casting method as claimed in claim 11, wherein the length of the parallel surfaces is at least 10 cm.
Casting method as claimed in any one of the foregoing claims, wherein the blade mould produced in the step a) is a shrouded blade.
Turbine blade for a gas turbine realized through the casting method as claimed in any one of the foregoing claims.