Patent Application
20130108460
Priority is claimed to Swiss Patent Application No. CH 01755/11, filed on Oct. 31, 2011, the entire disclosure of which is hereby incorporated by reference herein.
The present invention relates to the configuration and manufacturing of components or coupons (i.e. a part of a component, preferably used for repairing the component), especially for gas turbines, which are used under extreme thermal and mechanical conditions and to a method for manufacturing such component or coupon.
Components of gas turbines or other thermal machines, e.g. rotating blades or the like, are subject to severe operating conditions. In general, grain size has an impact on the lifetime of a component made of metal and/or ceramic alloys. Depending on the operating temperatures or stresses, the component can suffer from various failure mechanisms that are described as Low-Cycle Fatigue (LCF), Thermo-Mechanical Fatigue (TMF), Creep, Oxidation, as well as High-Cycle Fatigue (HCF) damages. In terms of the operation condition, the designed system can be loaded by one or more damage mechanisms, as they are mentioned above. However, other damage mechanisms may also be taken into account.
In accordance with common criteria for the best lifetime arrangement, small or big grain sizes of the applied alloy are convenient for minimizing damage rate of LCF or creep mechanism, respectively. Since the temperature and stress distribution within the mechanical component are non-uniform, like for instance in a gas turbine blade, some more specific rules for grain size in terms of a local component loading seems to be more adequate than these common well-known criteria.
Frequently, different parts of the same component can suffer either from LCF or from creep, and then more specific criteria of the grain size dependence of minimum LCF or creep rate are expected. Concerning only the creep mechanism, the creep rate can perform with respect to grain size in different manners as it is schematically illustrated in
For higher temperatures above 0.5Tm, (where Tm denotes the absolute melting temperature of the alloy) and intermediate stress magnitudes σ, the creep rate decreases up to the specific value, which then remains constant independent on increasing grain size (see a solid curve A in
In the range of intermediate temperature varying between 0.4 and 0.5Tm, and higher stresses, the creep rate shows a minimum value at a particular grain size of the alloy (see the dotted curve B in
These considerations apply to the situation in a gas turbine.
The gas turbine blade 15, which is schematically shown in
The performance of a gas turbine engine increases with higher firing temperature in the combustor, and therefore vane 16 and blade 15 operate in the range of high temperatures close to Tm. To protect the blades and vanes from oxidation damage, they are covered by a thermal barrier coating (TBC) and in addition cooled internally by a coolant, such as either air provided from the compressor, or steam injected from other systems, like a steam turbine (in a combined-cycle environment). The coolant is redistributed under platform 14 of blade 15 to reduce the temperature of the shank section 13 and root part 12, where the stresses reach their maximum values due to the centrifugal loading (see
The complex geometries of blade 15 and vane 16 match with requirements of the aerodynamic and mechanical integrity. Therefore, many geometrical notches are present within the blade and vane, thus inducing local stress concentrations.
The stresses σ and temperatures T acting on the blade 15 under the nominal boundary condition can be computed with a numerical approach, like e.g. the Finite Element Method (FEM), Boundary Element Method (BEM), and others. In addition, the temperatures and stresses are frequently measured in a prototyping process of the engine, and those experimental results are used for validation of the numerical values.
A metallurgical investigation of a component, which has been in service, provides an empirical assessment of the real temperatures in the system, which is also considered in the validation of the numerical model and its thermal boundary conditions. These three approaches or at least one of them can be used for creating a detailed map of the temperature and stress distribution within the whole component for the assessment of its lifetime.
Based on the described variation of the temperature T and mechanical stress σ (or/and strain ε) magnitudes within blade 15, which may lead either to LCF or creep damages, a controlled variation of optimal grain sizes of the alloy is a beneficial parameter for maximizing lifetime capability of the component made of the same alloy or different alloys.
Document U.S. Pat. No. 5,649,280 A describes a method of high retained strain forging for Ni-base superalloys, particularly those which comprise a mixture of gamma and gamma prime phases, and most particularly those which contain at least about 30 percent by volume of gamma prime. The method utilizes an extended subsolvus anneal to recrystallize essentially all of the superalloy and form a uniform, free grain size. Such alloys may also be given a supersolvus anneal to coarsen the grain size and redistribute the gamma prime. The method permits the manufacture of forged articles having a fine grain size in the range of about ASTM 5-12.
Document U.S. Pat. No. 5,759,305 A discloses a method of making Ni-base superalloy articles having a controlled grain size from a forging preform, comprising the steps of: providing a Ni-base superalloy preform having a recrystallization temperature, a gamma prime solvus temperature and a microstructure comprising a mixture of gamma and gamma prime phases, wherein the gamma prime phase occupies at least 30% by volume of the Ni-base superalloy; hot die forging the superalloy preform at a temperature of at least about 1600° F., but below the gamma prime solvus temperature and a strain rate from about 0.03 to about 10 per second to form a hot die forged superalloy work piece; isothermally forging the hot die forged superalloy work piece to form the finished article; supersolvus heat treating the finished article to produce a substantially uniform grain microstructure of about ASTM 6-8; cooling the article from the supersolvus heat treatment temperature.
Document U.S. Pat. No. 7,763,129 B2 teaches a method of forming a component from a gamma-prime precipitation-strengthened nickel-base superalloy so that, following a supersolvus heat treatment the component is characterized by a uniformly-sized grain microstructure. The method includes forming a billet having a sufficiently fine grain size to achieve superplasticity of the superalloy during a subsequent working step. The billet is then worked at a temperature below the gamma-prime solvus temperature of the superalloy so as to form a worked article, wherein the billet is worked so as to maintain strain rates above a lower strain rate limit to control average grain size and below an upper strain rate limit to avoid critical grain growth. Thereafter, the worked article is heat treated at a temperature above the gamma-prime solvus temperature of the superalloy for a duration sufficient to uniformly coarsen the grains of the worked article, after which the worked article is cooled at a rate sufficient to reprecipitate gamma-prime within the worked article.
Although these documents teach various methods for achieving a certain optimized grain size within a gas turbine component, or the like, there is no intent to establish, or knowledge about the advantages of, a specified local variation of the grain size within the component in accordance with the locally varying thermal and mechanical loads on that component.
In an embodiment, the present invention provides a component or coupon for use in a thermal machine under extreme thermal and mechanical conditions. The component or coupon comprises an alloy material having a controllable grain size (d). A grain size distribution (d(X,Y,Z)) of the component or coupon corresponds to at least one of an expected temperature distribution (T(X,Y,Z)), an expected stress distribution (σ(X,Y,Z)) and an expected strain distribution (ε(X,Y,Z)), which vary with geometrical coordinates (X,Y,Z) of the component or coupon, such that a lifetime of the component or coupon is improved with respect to a similar component or coupon having a substantially uniform grain size.
The present invention will be described in even greater detail below based on the exemplary figures. The invention is not limited to the exemplary embodiments. All features described and/or illustrated herein can be used alone or combined in different combinations in embodiments of the invention. The features and advantages of various embodiments of the present invention will become apparent by reading the following detailed description with reference to the attached drawings which illustrate the following:
In an embodiment, the present invention provides a component or coupon which is optimized in its internal structure with respect to the locally different thermal and mechanical loads.
Another embodiment of the invention provides a method for manufacturing such a component or coupon.
The component/coupon according to an embodiment of the invention is made of an alloy material with a controllable grain size, and is in service subjected to an expected temperature and/or stress and/or strain distribution, which varies with the geometrical coordinates of the component/coupon. It is characterized in that it has a grain size distribution, which depends on said expected temperature and/or stress and/or strain distribution such that the lifetime of the component is improved with respect to a similar component with a substantially uniform grain size.
According to another embodiment of the invention said component is a part of a gas turbine.
According to another embodiment of the invention said component is a rotating turbine blade.
According to a further embodiment of the invention said component or coupon is made of a superalloy.
According to another embodiment of the invention said component or coupon is made by an additive manufacturing process, especially selective laser melting (SLM).
The inventive method for manufacturing a component or coupon according to an embodiment of the invention comprises the steps of.
generating 1D or 2D or 3D parameter distribution data of one or more grain-size-relevant and lifetime-determining parameters for said component/coupon being under operating conditions; and
controlling during manufacturing of said component or coupon the grain size distribution within said component or coupon in order to maximize the lifetime of said component.
Grain-size-relevant means that the effect of these parameters on the component can be controlled by grain size.
According to an embodiment of the inventive method 3D parameter distribution data comprising a computed 3D temperature distribution and von Mises stress distribution are generated by a calculation, especially with a Finite Element Method (FEM).
According to another embodiment said component or coupon is manufactured by means of an additive manufacturing method, and the desired lifetime-maximizing grain size distribution is directly generated during said additive manufacturing process.
Preferably, said additive manufacturing method includes selective laser melting (SLM) of a suitable powder with a first laser beam, whereby the grain size is controlled by controlling the cooling rate of the melt pool within the SLM process.
Especially, the cooling rate of the melt pool within the SLM process is controlled by controlling the local thermal gradients at the melting zone.
Especially, the local thermal gradients at the melting zone are controlled by a second laser beam and/or a radiant heater.
According to another embodiment a substrate plate for the SLM process is used, which is heated or cooled by a heating or cooling medium to lower or increase said thermal gradients.
According to a further embodiment said component is manufactured with a homogeneous microstructure, and the desired lifetime-maximizing grain size distribution is generated after said manufacturing process.
Preferably, said lifetime-maximizing grain size distribution is generated by locally heating and/or cooling said component.
The present invention recognizes that the grain size has an impact on the lifetime of the component operating at elevated temperatures. An additive manufacturing process (selective laser sintering or melting (SLS or SLM), electron beam melting (EBM), 3D printing or other additive manufacturing processes) of the entire component or only its repair coupon is controlled in terms of the three-dimensional temperature T, strain ε or/and stress σ distribution obtained from numerical, experimental, or/and empirical approaches. In the numerical or/and lifetime model of the component, the stress field is described with a vector of 6 stress components such as:
σxx the normal stress in the X-direction of the Cartesian reference system,
σyy the normal stress in the Y-direction of the Cartesian reference system,
σzz the normal stress in the Z-direction of the Cartesian reference system,
σxy the shear stress on the XY-plane of the Cartesian reference system,
σyz the shear stress on the YZ-plane of the Cartesian reference system, and
σzx the shear stress on the ZX-plane of the Cartesian reference system.
Also, the strain state is defined in the same manner like the stress by using 3 normal εxx, εyy, εzz, and 3 shear εxy, εyz, εzx strain components referred in the Cartesian reference system. By using the matrix notation, the relation between the stress and strain at every point of the component is determined for the three-dimensional stress field based on Hooke's law by
{σ}=[C]{ε}, (1)
where {σ} and {ε} are vectors of the six stress and strain components, whereby [C] denotes a (6×6) matrix, called the elastic stiffness, which in the general case of anisotropic materials, contains 36 elastic constants Ci,j, where i=1, 2, . . . , 6, and j=1, 2, . . . , 6. In case of an isotropic material, the matrix [C] is determined with Poisson's ratio v and Young modulus E(T), which depends on the metal temperature T.
In general, the stress and strain components depend on displacements (deformations) of an arbitrary point of the deformed part. These deformations are driven by the thermal expansion and/or mechanical loadings that can act as a static or dynamic pressure and/or forces on the component. The deformations of an arbitrary point are defined with the displacement vector {q}=col{qx, qy, qz}, determining displacements of this point in the Cartesian reference system along the X, Y, and Z axis, respectively. The relation of the strain to the deformation at an arbitrary point (X,Y,Z) of the part is defined by:
εxx=∂qx/∂X, (2)
εyy=∂qy/∂Y, (3)
εzz=∂qz/∂Z, (4)
εxy=(∂qx/∂Y+∂qy/∂X)/2, (5)
εyz=(∂qy/∂Z+∂qz/∂y)/2, (6)
εzx=(∂qz/∂X+∂qx/∂Z)/2. (7)
In the design process, the stress {σ}, strain {ε}, displacement {q} and temperature T of an arbitrary point are computed with an engineering software based on the Finite Element Methods, Boundary Element Methods, and others. A typical example of these analyses is shown in
With respect to the temperature and stress distribution, these tools exactly predict the lifetime of the part using the failure mechanisms of creep, Low Cycle Fatigue, High Cycle Fatigue, Fracture Mechanic, Relaxation, and others. In order to include the grain size d of the polycrystalline materials, the general creep equation can be expressed by
dε
C
/dt=(C σm exp(−Q/kT))/db (8)
where εC denotes the creep strain, C means is a material constant of the specific creep mechanism, m and b are exponents dependent on the creep mechanism, Q corresponds to the activation energy of the creep mechanism, T is the absolute temperature at point (X,Y,Z), σ is the stress acting on the point (X,Y,Z) of interest, d is the size of the grain of the material, and k is Boltzmann's constant. In the literature, different models of the creep being dependent on the grain size are given are well-known.
Regarding the grain size d, the yield stress σp can be defined for instance by
σp=G b(ρ)1/2+K/(d)1/2, (9)
where G is the shear modulus G(T)=E(T)/[2(1+v)] dependent on temperature T, b denotes Burger's vector, K means Hall-fetch coefficient, and ρ is the dislocation density.
By using the equations (8)-(9), or similar equations given in the literature or obtained from an internal investigation, a trend of the strain behaviors in terms of grain size d can be calculated with respect to the arbitrary position (X,Y,Z) of the part. For the component of interest, whose stress and temperature fields are computed with respect to the service conditions for maximizing the lifetime, the required grain size d is transferred to the manufacturing machine or processing apparatus, which produces the component with the locally controlled grain sizes d(X,Y,Z) dependent on the temperature T(X,Y,Z), stress σ(X,Y,Z) or/and strain ε(X,Y,Z) or other parameter based on the lifetime model.
This process is illustrated in
The numerical model of temperature T(X,Y,Z), strain ε(X,Y,Z), stress σ(X,Y,Z) and other parameters is used for determining the demanded grain size d(X,Y,Z) resulting in the maximum lifetime of the part with respect to the desired operation conditions. The 3D parameter distribution data 24 are transferred to a processing apparatus 25, which processes the desired component 26.
In a preferred embodiment of this invention and representative for any potential additive manufacturing process, selective laser melting (SLM) is used to produce the mechanical component. Selective laser melting (SLM) is an additive manufacturing technology used to directly produce metallic parts from powder materials. As described for example in document U.S. Pat. No. 6,215,093 B1, thin powder layers with a thickness of typically between 20 μm to 60 μm are generated on a metallic base plate or the already produced fraction of an object, respectively. The cross-sections of a sliced CAD model stored in the SLM machine are scanned subsequently using a high power laser beam to compact the powder material. In general the STL-format is used to transfer the model geometry to the SLM machine.
In an embodiment of the present invention the STL file mentioned above will be replaced accordingly by a CAD file including not only the geometrical information but also the temperature T(X,Y,Z), stress σ(X,Y,Z) or/and strain ε(X,Y,Z) or other parameter distribution based on the lifetime model. The optimal grain size distribution can then be derived from the above-mentioned information and equations. The optimal grain size d(X,Y,Z) can either be already included in the mentioned CAD file or can be calculated on the SLM machine (27) during processing.
To achieve the desired grain size d(X,Y,Z), the process parameters of the manufacturing process have to be adapted accordingly. This can be done for the whole layer or selectively. In general, the grain size correlates to the cooling rate of the melt pool within the SLM process: the higher the thermal gradient the smaller the resulting grain size, and vice versa. Therefore, the precise local adaption of the process parameters, such as but not limited to, laser power, laser mode (continuous wave or pulsed), laser focus diameter, scan speed and scan strategy is crucial to achieve the desired thermal gradient and grain sizes, respectively.
Further process equipment or processing apparatus can be used to better adjust local thermal gradients. In a preferred embodiment of this invention, a second laser beam 32′ (
In another embodiment of this invention (
By using the described method and means a component or its part can be produced with locally optimized grain sizes in respect to the local 6 normal and shear stresses {σ(X,Y,Z)} or strains {ε(X,Y,Z)} obtained from the 1D, 2D, or 3D numerical simulations. Therefore, these components have superior lifetime compared to conventionally manufactured components.
The description of the stress and/or strain field of the component/part can be simplified by other approaches. For instance, the stress and strain distribution can be represented by the average normal and shear stress or/and strain instead of digitalized stress σ(X,Y,Z) state of the component/part. In this case, the stress, temperature, strain and other parameters vary with respect to the one direction of the reference system like it is shown in
If the component or its repair coupon is produced with a homogenous microstructure of the alloy for a constant grain size, a customized and locally varying heat treatment can be applied according to
A suitable and exemplary alloy for a component or coupon (repair part of the component) according to the invention may be IN738LC. Other Ni base superalloys or superalloys on a different basis are also suitable.
The process of the additive manufacturing produces the object for the controlled local optimal grain sizes with respect to the expected loading. Arbitrary approaches, such as: different sizes of the metal powder applied to the process, adjusting laser power, and others may be taken in consideration, but are not presented here in detail for each process.
While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. It will be understood that changes and modifications may be made by those of ordinary skill within the scope of the following claims. In particular, the present invention covers further embodiments with any combination of features from different embodiments described above and below.
The terms used in the attached claims should be construed to have the broadest reasonable interpretation consistent with the foregoing description. For example, the use of the article “a” or “the” in introducing an element should not be interpreted as being exclusive of a plurality of elements. Likewise, the recitation of “or” should be interpreted as being inclusive, such that the recitation of “A or B” is not exclusive of “A and B.” Further, the recitation of “at least one of A, B and C” should be interpreted as one or more of a group of elements consisting of A, B and C, and should not be interpreted as requiring at least one of each of the listed elements A, B and C, regardless of whether A, B and C are related as categories or otherwise.
| Number | Date | Country | Kind |
|---|---|---|---|
| 01755/11 | Oct 2011 | CH | national |
