Patent Application
20170320174
The present invention relates to a method for repairing a turbine engine part, which does not exclude manufacturing it.
Some turbine engine parts, such as, in particular, turbine blades, are subject to erosion or wear causing major damage which must be repaired after a certain number of cycles.
The damage may in particular be in the form of a lack of material. The repair then consists in restoring the original (or very close) shapes and dimensions of the worn part.
To achieve this, several techniques are used in the prior art, including producing a preform by sintering a superalloy powder and a brazing powder (whose melting point is lower than that of the superalloy powder), then bonding the preform onto the part to be repaired by diffusion brazing (hereinafter the term “metal” shall include alloys).
It should be recalled that brazing is a process which consists in assembling, for example, two metal parts of identical or different materials by means of a filler metal, whose melting point is considerably lower than the melting points of the materials of the parts. The solder contained in the filler metal is supplied in the liquid state and the parts are heated by the filler metal, but remain solid.
Diffusion brazing (or transient liquid phase bonding) is an assembly operation of two metal parts similar to brazing, but in which the difference in composition between the filler metal and the parts to be assembled is progressively resorbed by a diffusion heat treatment. This treatment leads to the formation of a quasi-chemically homogeneous bond and whose characteristics are close to those of the parts to be assembled. Diffusion brazing could thus be considered as conventional brazing to which diffusion treatment has been added.
When assembling two parts, the filler metal used has a chemical composition that is close to that of the parts to be assembled, but its melting temperature is lower due to the solder. During diffusion brazing, the solder melts and wets the surfaces to be assembled and then solidifies isothermally by diffusion of the alloying elements in the filler metal into the material of the parts, the composition of which changes and homogenizes with that of the brazing seam thus formed. At the final stage of the diffusion brazing process, the filler metal forms part of the material of the parts and is indistinguishable from it.
Such a method allows for several parts to be assembled, as indicated above, while providing the assembled parts and their bonds with mechanical and metallurgical characteristics comparable to those of the original parts. Moreover, the temperatures used in such a method are compatible with the superalloys commonly used to produce these parts, in particular in the aeronautical field.
However, repairing a part using a substantially flat preform limits the applications of this method. Therefore, in the case of turbine blades, in which the area to be repaired can have a three-dimensional profile with an amount of material to be added which may not be constant over the entire area (e.g. variable thickness), for example, FR 2 978 070 proposes the following:
However, such parts where the area to be repaired can therefore have a three-dimensional profile, with an amount of material to be added which may not be constant over the entire area, are not feasible with the expected quality. In direct manufacturing (producing the preform, layer by layer, through selective melting), it has been found that the molten material is liable to crack severely upon cooling, thus altering the production.
A purpose of the invention is to avoid these situations. For this reason, it has been thought of controlling the shrinkage of the material. More specifically, it has been thought of that the solder should only have one main transformation peak during heating (and/or cooling). To be even more specific, it is proposed that the amplitude of the heat flux of the main transformation peak of the brazing material used to produce the preform is at least twice the respective amplitudes (within 20%) of the heat fluxes of the secondary transformation peaks of this brazing material, as shown in the accompanying figures.
Thus, after mixing the brazing and superalloy powders (including in the case of prealloyed powders, i.e. previously mixed), not only will this compound include two main transformation peaks (one for melting the solder and the other for melting the superalloy), but the amplitudes of the secondary transformation peaks for melting the solder will be much smaller than the amplitude of the main peak.
The method according to the invention can thus be applied to the production of various turbine engine parts and in particular to the repair of turbine blades.
The preform can be produced with a controlled roughness: the preform is more easily brazable when it has a certain roughness, since the solder can then more effectively wet the surfaces to be assembled.
The base material is identical or similar to that of the part to be repaired so as to favour the assembly of the preform by diffusion brazing. Two “similar” materials have at least the same base (e.g. nickel, cobalt, titanium, etc.).
An application of the invention relates to the metallurgical production of the powders of shaped parts intended to be assembled by self-brazing to metal parts capable of receiving them and called receptors. Self-brazing is the autogenous brazing of the shaped part onto the receiving metal part, the brazing elements being contained in the shaped part (which will therefore favourably be three-dimensional).
The self-brazing of this shaped part on the metal part may comprise or be followed by a solid-state diffusion heat treatment, thereby constituting what is commonly referred to as a brazing/diffusion operation, a treatment intended to homogenize the composition and structure of the shaped parts and of the self-brazing bond (bonding area).
Particularly within this context, it is recommended that the chemical composition of the aforementioned base material (used to produce the preform by selective melting) corresponds to a Ni, Co, Ti or Fe-based superalloy and that that of the brazing material corresponds to a Ni, and/or Co, and/or Fe-based alloy in which the melting element is Si and/or B (as known per se, a melting element, for example silicon or boron, is an element that substantially lowers the (solidus) melting temperature of the alloy into which it is introduced).
As to the compound to be supplied to the brazing material so that said amplitude of the heat flux of the main transformation peak of the brazing material is therefore equal to at least twice the respective amplitudes of the heat fluxes of the secondary transformation peaks of this brazing material, it is recommended that it be chosen from Cr, Co, Mo, and Fe. The recommended weight percentage of this compound ranges from 7 to 23%.
These choices are particularly suitable for producing turbine parts, and in particular turbine blades.
In this regard, it is even advisable that, in nominal composition and in weight percentages, the brazing material is a Ni-based alloy with between 9 and 19% of Co, Si, B, but also Cr (it being specified that all percentages in the present description are provided in percent by weight).
By using the so-called NiCoSiB 1060 brazing powder based on Ni and Co 20, Si 4.5, B 3, a balanced result will be obtained between low risk of cracking, resistance to hot corrosion, and final qualities of the base material/brazing material pair, in particular with regard to resistance to high temperature creep.
Advantageously, the amount of chromium in the brazing material is 14%. This particular quantity offers a good compromise between limitation of cracking and strength of parts.
In fact, adding too small an amount of chromium, i.e. less than 9%, would not allow for adequately limiting the occurrence of cracks on the part, whereas too great an amount, i.e. more than 19%, would increase the melting temperature of the brazing material and bring it too close to the melting temperature of the base material, which would embrittle the part.
According to another characteristic of the invention, the preform is produced by selectively melting a base material powder and a brazing powder, the melting temperature of which is less than the melting temperature of the base powder. The dimensional tolerances of the preform will thus be greatly reduced and the final assembly optimized.
The preform which already contains a brazing material can be soldered directly to the part to be repaired, depending on the quantity of brazing material.
Preferably, the preform will contain at least 60% of base material, so as to impart adequate mechanical characteristics to the preform.
In a variant of the invention, the preform is produced by selectively melting a powder containing only the base material.
A brazing material may then be deposited on the assembly surface of the preform.
This deposition is, for example, performed by laser spraying or by plasma spraying a brazing powder, or by co-depositing (electrodepositing) in an aqueous medium.
The thickness of such a deposition will, for example, range from 20 to 200 μm.
The invention also relates to a turbine engine part produced by executing the aforementioned method.
The invention will be better understood and other details, characteristics, and advantages of the invention will appear on reading the following description given by way of non-limiting example and with reference to the accompanying drawings, in which:
In a first embodiment, the repair method according to the invention first of all consists in producing, layer by layer, a sintered preform 1 by selectively melting a mixture of powders including a base material powder and a brazing material powder, regardless of whether these materials having been premixed.
The melting temperature of the brazing material is lower than that of the base material. By way of example, the melting temperature of the brazing material will range from 1,000 to 1,300° C., while the melting temperature of the base material will range from 1,200 to 1,600° C.
The base material is preferably a superalloy, e.g. a nickel-based superalloy. In this case, the brazing material also is nickel-based and also comprises melting elements, such as silicon and/or boron.
The selective melting is performed using a plant such as that shown in
The plant further comprises a scraper 9 for supplying powder from the tank 2 to the vessel 6 by moving along a horizontal plane A, and means 10 for generating a laser beam or an electron beam coupled to a computer-controlled device 11 to direct and move the beam 12. Adjacent to the vessel 6, a vat 13 may also be provided to collect excess powder 14.
This plant operates as follows: First, the bottom 4 of the tank 3 is moved upwards so that a certain quantity of powder 3 is situated above the horizontal plane A. The scraper 9 is moved from left to right so as to scrape said layer of powder 3 into the vessel 6 and deposit a thin layer of metal powder onto the horizontal flat surface of the plate 7. The quantity of powder and the position of the plate 7 are determined so as to form a layer of powder of a selected and constant thickness. A laser beam 12 or an electron beam perpendicular to plane A then scans a specific area of the layer formed in the vessel so as to locally melt the brazing powder (and not the base powder). The melted areas then solidify by agglomerating the particles of the base powder and by forming a first layer 15 of a sintered preform 1, this layer 15 having, for example, a thickness of the order of 10 to 150 μm.
More specifically, the thickness of the layer 15 ranges from 10 to 45 μm, respectively from 45 to 150 μm, when the powder is melted by means of a laser beam or respectively by means of an electron beam.
The plate 7 is then lowered and a second layer of powder is supplied, in the same manner as previously, onto the first layer of powder. Through controlled displacement of the beam, a second layer 16 is formed by sintering on the first layer 15.
These steps are repeated until the preform 1 is completely formed. The layers 15, 16 have substantially the same thickness.
In the case where the preform 1 is built layer by layer by selectively melting the powder using a laser beam, the powder has an average particle size ranging from 10 to 45 μm. The particle size distributions of the two powders are not necessarily identical. Preferred are near averages, i.e. the case where the two powders each have an average particle size of between 10 and 45 μm, in order to facilitate mixing of the powders.
In the case where the preform 1 is built layer by layer by selectively melting the powder using an electron beam, the powder has an average particle size ranging from 50 to 100 μm.
This preform 1, which contains an adequate quantity of brazing material, can be brazed directly onto the part to be repaired 17 (
To achieve this, the surfaces of the preform 1 to be brazed and the part to be repaired 17 are degreased and/or pickled and then the preform 1 is placed on the surface of the part to be repaired (
The preform 1 is then tacked (laser tacking, condenser discharge, etc.) to the part to be repaired, in order to keep it in place on the part to be repaired 17.
The preform 1 and the part to be repaired 17 are then placed in an oven where they will undergo a diffusion brazing cycle.
For a base material of type NK17CDAT, also known as Astroloy, and for a NiCrB braze material, the diffusion brazing may include a temperature rise to 1,205° C. lasting about 2 hours 30 minutes, a first stage at 1,205° C. lasting 15 minutes, followed by a second stage at 1,160° C. lasting 2 hours, followed by a temperature decrease from 1,160° C. to 20° C. lasting about 1 hour.
During brazing-diffusion, the brazing material melts first. The liquid phase to which it gives rise is retained by capillarity and moistens the surfaces of the part to be repaired 17 and of the preform 1.
After cooling, a solid intermediate layer is formed between the preform 1 and the part to be repaired 17 and has a homogeneous metallographic structure diffusion-bonded to the surfaces of these parts.
The repaired part thus has mechanical characteristics identical or similar to those of a new part.
The repaired part finally undergoes a finishing step in which the repaired surfaces are adjusted or machined in such a way that the part recovers the dimensions of a new part (
In order to further increase the mechanical characteristics of the preform 1 and hence of the repaired part, said preform 1 may include a reduced or null proportion of brazing material at its core, whereby powder rich in brazing material can then be deposited on the surface to be brazed.
Thus, the preform 1 can be produced by selectively melting a mixture of base powder and brazing powder in which the proportion by weight of the base powder is greater than 90%. The preform 1 may also be produced by selectively melting a base powder only.
In this case, a layer of powder enriched with brazing material must be formed on the surface of the preform. This layer can be produced by laser spraying or by plasma spraying, electrodeposition.
The powder used to form this layer may comprise 60 to 90% by weight of base powder and 10 to 40% by weight of brazing powder.
The principle of laser spray deposition is shown in
To achieve this, the preform 1 is placed in an enclosure 21 containing argon, for example. Means 22 for generating a YAG laser beam produce a laser beam 20 directed toward the surface 18 of the preform 1, through a nozzle 23 directed perpendicularly to this surface 18. The nozzle 23 and the laser beam 20 can be moved relative to the surface (or vice versa) by means of a control system and appropriate means 24.
One or several successive layers 31, enriched with brazing material, can thus be formed on the corresponding surface 18 of the preform 1.
The principle of plasma spray deposition is shown in
The melting of the powder grains 19 is due to the very high temperatures within the plasma, making it possible to deposit materials with a high melting point.
As it solidifies, the material of the powder forms a deposit on the surface 18 of the preform 1.
The high velocities of the gases and particles allow obtaining strong adhesion of the deposit, low porosity, and a reduced level of chemical transformation.
Typical examples of the chemical composition of the base material are provided below. The reference and the corresponding chemical composition (percent by weight) are provided for each material:
Note that Astroloy is the material giving the best results during the tests.
In the same manner, the chemical composition of the brazing material is provided below:
By way of example, the mixture of powders (base material/brazing material) may comprise 75% by weight of SYP3 or Astroloy powder (base material) and 25% by weight of TY 134b powder (brazing material). Alternatively, this mixture may comprise 70% by weight of base material powder and 30% by weight of brazing material.
In fact, on the illustrated Differential Thermal Analysis graph of the above mixture of powders, showing the development of the part's heat flux, i.e. the voltage U as a function of time, where the mixture reaches more than 1,300° C. after more than 1 hour of heating, the following can be distinguished:
Later, in a comparable temperature range (between 900° C. and 1,140° C.), the occurrence of the peak(s) (referred to as a whole 42) of transformation of the base material, in this case a superalloy.
The precise temperature and time values at which they occur can be read in
Measurement of the heat flux of the material concerned can be achieved using an “RDF Micro-Foil”® sensor. This sensor is connected to a microvoltmeter (voltage U). The whole then provides a direct measurement of the rate of transfer of heating or cooling through both the sensor and the mounting surface. There is a direct relationship between the output of the microvoltmeter and the heat flow. In
In a graph of this nature, the two examples with curves 43a and 43b in
Again, at higher temperatures (for temperatures above 1,200° C.), one finds the transformation peak(s) (referred to as a whole 49) of the base superalloy.
The three dashed curves (refer to the legend of the figure to identify each one) show the behaviour of the mixture of the base material and the brazing material when the brazing material comprises 9%, 14% and 19% chromium respectively.
It is observed that the higher the concentration of chromium the less the thermal amplitudes of the transformation peaks are pronounced for temperatures below 1,185° C. Thus, the risk that the parts will crack will indeed be limited.
The reference 55 identifies the curve showing the changes in the base material, i.e. the selected superalloy.
For the sake of clarity, we shall define the following:
Thus, after mixing the solder and superalloy powders, this compound comprises two transformation peaks (one for the melting of the solder and the other for the melting of the superalloy). The decrease in the number of transformation peaks during heating as well as during cooling makes it possible to limit the stresses that the part is exposed to during cooling and to avoid any cracking.
However, it is also noted in
The addition of chromium to the brazing material increases the melting temperature of the brazing material, which comes closer to the melting temperature of the material that the part to be repaired is made of.
Preferably, the brazing material is determined so that its melting temperature is at most 1,210° C. and preferably less than 1,210° C.
While it would allow limiting the occurrence of cracks during cooling, any amount of chromium above 19% would generate a problem in the behaviour of the material the part is made of. In fact, the melting temperature of the brazing material will then be close to that of the material the part is made of so that, when the part is heated, the material of which the part is made will react to the heat required to melt the brazing material.
The behaviour of the material that the part to be repaired is made of will be as shown schematically in
By way of example based on a favourable embodiment, a given part may in particular be manufactured from a nickel-based superalloy using the powder metallurgy method thus using a base powder A and a brazing powder B. The base powder A may be that known under the trade name Astroloy (NK17CDAT according to the AFNOR designation). This material is fully compatible with the superalloy called René77 used to manufacture a blade, specifically in as far as solidus temperature and mechanical characteristics are concerned.
The solidus temperature of the base powder A is 1,240° C. Its liquidus temperature is 1,280° C. The brazing powder B used to perform the sintering of the Astroloy powder and the self-brazing with the blade is a 1060 Ni—Co—Si—B alloy powder containing 17% Co, 4% Si, and 2.7% B by weight. The solidus temperature of the brazing powder B is 965° C. Its liquidus temperature is 1,065° C. and is lower than the solidus temperatures of the base powder A and that of the blade. These data can be used to define a self-brazing temperature of 1,200° C., which is higher than the liquidus temperature of the brazing powder, but which is lower than the solidus temperature of the blade made of Rene 77 and that of the Astroloy powder A.
Thus, the self-brazing temperature will be higher than the liquidus temperature of the brazing powder and lower than the solidus temperatures of the base powder and of the receiving part (such as the aforementioned part 17), while the sintering of the blank (such as the aforementioned preform 1) will have taken place at a temperature higher than the liquidus temperature of the brazing powder but lower than the temperature of the subsequent self-brazing treatment. It will thus be possible to obtain a part of a shape that is suitable for self-brazing, the relative density of which is at least equal to 95%.
As taught in FR 2785559, it is advisable that, in practice, in the application intended for aeronautical turbine engines and in particular for turbine elements, more specifically vanes and/or distributors for low-pressure turbines, the following is observed, regardless of whether in combination or not, in order to optimize the quality of the production, accounting for the known general state of the art:
The method according to the invention makes it possible to repair various turbine engine parts. In fact, since the preform is built layer by layer by selectively melting powder, the preform may have a three-dimensional shape and, if necessary, a variable thickness.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1461035 | Nov 2014 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2015/053091 | 11/16/2015 | WO | 00 |
