A method for reducing micro-porosity in nickel-based single crystal superalloys during casting by adding trace / trace elements carbon

Abstract

The present application relates to single crystal alloy casting defect control technical field, especially to a kind of trace element / microelement carbon reduction method for reducing micro-pore in the casting process of nickel-based single crystal superalloy.The present application is aimed at different alloy system, and target carbon content is determined by thermodynamics-solidification simulation, so that carbide starts to precipitate when solid phase mass fraction is 80%~90% during solidification, thereby promoting interdendritic residual melt feeding at the end of solidification when feeding channel is not completely closed, and the purpose of significantly reducing the porosity of single crystal casting is achieved.The present application is not limited to all alloys with fixed content range, but determines the corresponding carbon content according to the solidification characteristics of each alloy system, so it can be applied to single crystal alloys of different systems.

Description

Technical Field

[0001] This invention relates to the field of single-crystal alloy casting defect control technology, and in particular to a method for reducing micropores during the casting process of nickel-based single-crystal superalloys by adding trace / micro-elements of carbon. Background Technology

[0002] Nickel-based single-crystal superalloys are widely used in high-temperature hot-end components such as turbine blades for aero-engines and gas turbines due to their excellent high-temperature creep and endurance properties. Single-crystal superalloys significantly improve high-temperature load-bearing capacity by eliminating grain boundaries. However, during directional solidification, refractory elements such as Re, Mo, and Ta are prone to dendritic segregation, leading to the enrichment of residual melt components between dendrites, narrowing of feeding channels, and difficulty in localized feeding, thus forming casting defects such as shrinkage cavities / porosity. Porosity not only reduces the effective load-bearing cross-section but also becomes a crack initiation point, significantly weakening high-temperature fatigue, endurance, and fracture performance. Therefore, reducing porosity is crucial for the quality control of single-crystal blade casting.

[0003] In existing technologies, reducing porosity typically relies on optimizing directional solidification process parameters (temperature gradient, pulling rate, feeding channel design, etc.) or improving interdendritic permeability and feeding capacity by adjusting alloy element distribution. However, neither the optimization of directional solidification process parameters nor the adjustment of alloy element distribution strategies are universally applicable to single-crystal alloys of different systems. For example, the reactions and interdendritic product types at the end of solidification may differ for single-crystal alloys with different composition strategies, and the influencing factors of porosity defects also vary accordingly, leading to different composition optimization strategies for different single-crystal alloys.

[0004] Therefore, it is of great significance to provide a method for reducing micropores that is applicable to different single-crystal alloy systems. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a method for reducing micropores during the casting process of single crystal alloys. The method provided by the present invention can significantly reduce the porosity of single crystal alloys and is not limited to a specific composition system, but is applicable to various single crystal high-temperature alloys / single crystal alloy castings.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for reducing microporosity during the casting of single-crystal alloys by adding trace / micro-element carbon, comprising the following steps: Determine the target carbon content C of the single-crystal alloy to be prepared. target Optimized components were obtained; Prepare master alloy ingots with optimized composition; The master alloy ingot is melted and then directionally solidified to obtain a single crystal alloy; The target carbon content C of the single crystal alloy to be prepared is determined. target The process includes the following steps: inputting the main elemental composition of the single-crystal alloy to be prepared into thermodynamic calculation software, selecting a thermodynamic database that matches the single-crystal alloy to be prepared, and using solidification simulation to calculate the solid mass fraction f corresponding to the carbide phase precipitation initiation point under different carbon content conditions. s,start Iterative calculations are performed using carbon content as a variable, and the appropriate value is selected based on f. s,start The carbon content between 80% and 90% is used as the target carbon content C. target The carbide precipitation initiation point refers to the moment when the first carbide phase begins to precipitate from the residual liquid in the solidification simulation; when different carbide phases may precipitate, the carbide phase that precipitates first and has the greatest impact on the feeding channel is taken as the control object.

[0007] Preferably, the f s,start It is 85%~90%.

[0008] Preferably, the single-crystal alloy includes a nickel-based single-crystal superalloy.

[0009] Preferably, the nickel-based single-crystal superalloy includes Ni3Al-based single-crystal alloy or DD5 single-crystal alloy.

[0010] Preferably, the solidification simulation employs an equilibrium solidification model or a Scheil-Gulliver nonequilibrium solidification model.

[0011] Preferably, the thermodynamic software includes Thermo-Calc.

[0012] Preferably, the preparation of the master alloy ingot includes the following steps: according to the composition of the single crystal alloy, the corresponding raw materials are sequentially subjected to first melting, refining and second melting. After the carbon content is tested and found to meet the requirements, the resulting molten metal is cast to obtain the master alloy ingot.

[0013] Preferably, a carbon-containing intermediate alloy is added during the refining or second smelting stage to control the carbon content.

[0014] Preferably, the directional solidification is performed using the seed crystal method.

[0015] Preferably, the refining temperature is higher than the first melting temperature.

[0016] This invention provides a method for reducing microporosity during the casting of single-crystal alloys by adding trace / micro-element carbon. The method controls the carbide precipitation behavior at the end of solidification to reduce microporosity (shrinkage cavities / porosity) during casting. The method is not limited to a specific composition system and is applicable to various single-crystal high-temperature alloys / single-crystal alloy castings. The type of carbide precipitation at the end of solidification can vary with the alloy composition and can be different types such as MC, M6C, or combinations thereof. This invention determines the target carbon content for different alloy systems through thermodynamic-solidification simulation, causing carbides to precipitate when the solid phase mass fraction is 80%~90% during solidification. This promotes the feeding of residual melt between dendrites at the end of solidification, before the feeding channels are completely closed, thus significantly reducing the porosity of single-crystal castings.

[0017] This invention does not limit all alloys to a fixed content range, but determines the corresponding carbon content based on the solidification characteristics of each alloy system, thus making it applicable to single-crystal alloys of different systems. Attached Figure Description

[0018] Figure 1 Differential scanning calorimetry test results for four IC21 alloys are shown, including (a) differential scanning calorimetry cooling curves; and (b) liquidus temperature, solidus temperature, and the corresponding mushy region width. Figure 2 HAADF-STEM images and corresponding SAED spectra of interdendritic precipitates in four IC21 alloys are shown. (a) and (c) are representative HAADF-STEM images of bright blocky precipitates in alloys 1 and 2; (b) and (d) are corresponding SAED spectra collected along representative region axes; (e) is a HAADF-STEM image of the interdendritic composition of two phases in alloy 3; (f–h) are SAED spectra of the bright σ region in alloy 3 obtained along three different region axes; (i) is the SAED spectra of the gray region in alloy 3; and (j) is the SAED spectra of the skeleton-like precipitates in alloy 4. Figure 3 This is the reconstructed cavity morphology of alloy 1 in Example 1; Figure 4 This is the reconstructed cavity morphology of alloy 2 in Example 1; Figure 5 This is the reconstructed cavity morphology of alloy 3 in Example 1; Figure 6 This is the reconstructed cavity morphology of alloy 4 in Example 1; Figure 7 Diffraction pattern of MC-type carbides with different zone axes in DD5 single crystal alloy; Figure 8 The cavity reconstruction morphology of the DD5 single crystal alloy with a carbon content of 50ppm in Example 2; Figure 9 The cavity reconstruction morphology of the DD5 single crystal alloy with a carbon content of 150ppm in Example 2; Figure 10 The cavity reconstruction morphology of the DD5 single crystal alloy with a carbon content of 550ppm in Example 2 is shown. Detailed Implementation

[0019] This invention provides a method for reducing microporosity during the casting of single-crystal alloys by adding trace / micro-element carbon, comprising the following steps: Determine the target carbon content C of the single-crystal alloy to be prepared. target Optimized components were obtained; Prepare master alloy ingots with optimized composition; The master alloy ingot is melted and then directionally solidified to obtain a single crystal alloy.

[0020] This invention first determines the target carbon content (C) of the single-crystal alloy to be prepared. target , thus obtaining optimized components.

[0021] In this invention, the determination of the target carbon content C of the single-crystal alloy to be prepared is described. target The process includes the following steps: inputting the main elemental composition of the single-crystal alloy to be prepared into thermodynamic calculation software, selecting a thermodynamic database that matches the single-crystal alloy to be prepared, and using solidification simulation to calculate the solid mass fraction f corresponding to the carbide phase precipitation initiation point under different carbon content conditions. s,start Iterative calculations are performed using carbon content as a variable, and the appropriate value is selected based on f. s,start The carbon content between 80% and 90% is used as the target carbon content C. target The carbide precipitation initiation point refers to the moment when the first carbide phase begins to precipitate from the residual liquid in the solidification simulation; when different carbide phases may precipitate, the carbide phase that precipitates first and has the greatest impact on the feeding channel is taken as the control object.

[0022] In this invention, the single crystal alloy preferably includes a nickel-based single crystal high-temperature alloy; the nickel-based single crystal high-temperature alloy preferably includes a Ni3Al-based single crystal alloy or a DD5 single crystal alloy; this invention does not specifically limit the type of Ni3Al-based single crystal alloy, and any Ni3Al-based single crystal alloy well known in the art is acceptable, such as IC21, IC31, and IC32.

[0023] In this invention, the main elemental composition of the single-crystal alloy to be prepared is first input into the thermodynamic calculation software, and a matching thermodynamic database is selected.

[0024] In this invention, the thermodynamic calculation software can specifically be Thermo-Calc. In this invention, the main elements refer to elements other than trace and microelements; the main element composition refers to the types and amounts of main elements.

[0025] Then, this invention uses solidification simulation to calculate the solid mass fraction f corresponding to the carbide phase precipitation initiation point under different carbon content conditions. s,start Specifically, iterative calculations are performed using carbon content as the variable, selecting the value that makes f... s,start The carbon content between 80% and 90% is used as the target carbon content C. target .

[0026] In this invention, the solidification simulation preferably employs an equilibrium solidification model or a Scheil-Gulliver non-equilibrium solidification model. Specifically, the equilibrium solidification model or the Scheil-Gulliver non-equilibrium solidification model included in Thermo-Calc can be used. This invention preferably selects between an equilibrium solidification model and a non-equilibrium solidification model based on the actual solidification process. In this invention, the f s,start It is between 80% and 90%, specifically 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89% or 90%, preferably 85% to 90%.

[0027] The inventors discovered that the addition of carbon increases the tendency for carbide formation. At lower carbon content, carbides precipitate primarily from the interdendritic residue at the end of solidification. Their low-melting-point reaction lowers the solidus temperature and prolongs the feeding time in the pasty region, thus promoting feeding of the alloy melt at the end of solidification and reducing shrinkage porosity. However, when the carbon content is too high, the carbide precipitation initiation temperature increases, and carbide precipitation may occur at an earlier solidification stage. The carbide volume fraction increases, and it easily forms a skeletal / network-like interconnected structure, reducing interdendritic permeability and hindering downward feeding of the melt, which is detrimental to porosity control. Therefore, this invention controls the carbon content and synergistically controls the carbide content and its precipitation sequence, ensuring that carbide precipitation is mainly controlled at the end of solidification (after a solid mass fraction of 85%), thereby stably reducing porosity.

[0028] Furthermore, the addition of carbon to single-crystal alloys facilitates the formation of primary carbides (such as M6C / MC) during solidification, and these carbides can participate in melt purification during the smelting and refining stages: carbon reacts with dissolved oxygen in the melt to generate CO gas, which escapes, thereby reducing the oxygen content of the melt and decreasing oxide inclusions; simultaneously, under appropriate refining conditions, the presence of carbon helps reduce the oxygen potential of the melt and indirectly promotes the removal of harmful impurities such as sulfur and their inclusions, improving the cleanliness of the alloy melt. In addition, the carbon content can also affect the surface state and interfacial energy of the melt, thus altering the wettability of the alloy melt to the ceramic mold shell, and influencing the filling stability and final feeding conditions.

[0029] This invention determines the target carbon content C target Then, by keeping the matrix composition of the single-crystal alloy unchanged, the optimized composition can be obtained.

[0030] After obtaining the optimized composition, the present invention prepares a master alloy ingot with the optimized composition.

[0031] In this invention, the preparation of the master alloy ingot preferably includes the following steps: according to the composition of the single crystal alloy, the corresponding raw materials are sequentially subjected to a first melting, refining and a second melting. After the carbon content is tested and found to meet the requirements, the resulting molten metal is then cast to obtain the master alloy ingot.

[0032] The present invention does not impose any special limitation on the specific temperatures of the first melting, refining and second melting. Different single crystal alloys have different suitable melting temperatures, and melting conditions well known in the art can be used.

[0033] This invention melts the raw materials into molten metal through a first smelting process. In this invention, the refining temperature is preferably higher than the first smelting temperature. This invention further purifies the molten metal through refining. In this invention, the second smelting is preferably carried out under a protective atmosphere, specifically, such as argon. This invention removes oxidizing impurities from the molten metal through the second smelting process. This invention preferably incorporates a carbon-containing master alloy during the refining or second smelting stage to control the carbon content.

[0034] After obtaining the master alloy ingot, the present invention melts the master alloy ingot and then performs directional solidification to obtain a single crystal alloy.

[0035] In this invention, the directional solidification preferably employs the seed crystal method. This invention does not impose any particular limitation on the directional solidification process; any directional solidification process well-known in the art can be used. Specifically, for example, the directional solidification furnace can be evacuated to a vacuum of 0.8 × 10⁻⁶. -2Pa, the seed crystal is placed at the bottom of the mold shell, and the mother alloy ingot is placed in the single crystal growth furnace for melting. The molten metal flows from the alumina crucible into the mold shell. After holding at a certain temperature, the temperature gradient is controlled for pulling. After the crystal is pulled, it is cooled in the furnace for 30 min to obtain a cast single crystal alloy with

[001] orientation. In this invention, the pulling rate can be set according to the alloy system and furnace type, for example, 1~6 mm / min. This invention does not impose any special limitation on this.

[0036] As can be seen from the above, this invention can significantly reduce porosity by controlling the composition of trace amounts of carbon, without requiring complex modifications to directional solidification equipment or processes. The carbon content regulation can alter the type and morphology of interdendritic precipitates at the end of solidification, making the precipitates more conducive to the feeding process at the end of solidification, thereby reducing porosity. This invention is applicable to single-crystal alloy systems with different composition strategies and can provide guidance for the composition design and casting defect control of single-crystal alloys. Furthermore, this invention can utilize the deoxidizing and purifying effect of carbon during the melting stage, and, in conjunction with high-purity raw materials and atmosphere control, can reduce the content of harmful impurities such as oxygen and sulfur, as well as their inclusions. Simultaneously, controlled changes in carbon content can adjust the surface state of the melt, improving the wetting / impregnation conditions between the alloy melt and the mold shell, thus improving the casting quality from the source.

[0037] The following detailed description, in conjunction with embodiments, illustrates the method provided by the present invention for reducing micropores during the casting process of single-crystal alloys by adding trace / micro-element carbon. However, these descriptions should not be construed as limiting the scope of protection of the present invention.

[0038] Example 1: Effect of carbon content on porosity in Ni3Al-based single crystal alloys A second-generation Ni3Al-based single-crystal superalloy, IC21, with high Mo and Al content, was selected for verification. The contents of the main alloying elements, except for carbon, remained basically consistent. Taking this alloy system as an example, Scheil-Gulliver non-equilibrium solidification simulation was performed using Thermo-Calc software according to step (1) of this invention, which determined the target carbon content C that would initiate carbide precipitation at a solid phase mass fraction of 80%~90%. target Its typical value can fall within the range of 150~300 ppm. For comparative verification, this embodiment uses C target Different carbon content levels are set in the vicinity and on the outer side. It should be noted that the carbon content levels set in this embodiment are only for this alloy system to verify the idea of ​​the present invention and do not constitute a limitation on the carbon content of the present invention; for other single-crystal alloy systems, the corresponding target carbon content can be determined according to the solidification simulation method of the present invention.

[0039] Alloy preparation: Four groups of C contents were set at 0, 56 ppm, 180 ppm and 270 ppm (mass ppm; corresponding to mass fractions of approximately 0, 0.0056 wt.%, 0.018 wt.%, and 0.027 wt.%), and named Alloy 1-4 in sequence.

[0040] The master alloy was prepared using a vacuum induction melting method in a ZGJL-0.025 type vacuum induction furnace. The melting process consisted of three steps: first melting, refining, and second melting. The first melting temperature was set at 1550℃ for 25 minutes. The refining temperature was set at 1580℃ for 10 minutes. After refining, the melting equipment chamber was filled with a protective argon atmosphere. The argon purity should be greater than 99.9%, and the content of other impurities was limited to oxygen ≤10 ppm, nitrogen ≤50 ppm, and moisture ≤15 ppm. The second melting was carried out under an argon protective atmosphere at 1530℃ for 10 minutes.

[0041] Alloy raw materials must be weighed according to the designed proportions, accurate to 0.1g. Before melting, Ta, Re, Mo, and Ni are placed in an induction furnace. To prevent premature vaporization of Al, Al is added in a secondary feeder during the melting process. To reduce the impact of impurities on the alloy's microstructure and mechanical properties, the surface of the metal block is ground with a grinding wheel to remove 2mm of oxide scale, ultrasonically washed with acetone solution for 30 minutes, and then dried in an oven at 80℃ for 1 hour. After removing the oxide scale by grinding, the master alloy ingot is cut into ingots of approximately 4 kg for the production of single crystal test rods. In the second melting stage, a Ni-C master alloy is added to achieve trace carbon content control, and carbon content is sampled before and after tapping to verify the target values ​​for each group. After melting, the molten metal is electrically cast into master alloy ingots. Subsequently, single crystal preparation is carried out using the seed crystal method, employing a DZG-0025 directional solidification furnace: the single crystal growth furnace is first evacuated to 0.8×10⁻⁶. -2 Pa, place the seed crystal at the bottom of the mold shell; after the master alloy ingot melts in the furnace, let the molten metal flow from the alumina crucible into the mold shell, hold it at the temperature and control the temperature gradient to pull it, after the crystal pulling is completed, cool it with the furnace for 30 min to obtain the

[001] oriented cast alloy with a size of Φ15 mm×200 mm rod.

[0042] Example 1: Characterization and Testing (1) Solidification temperature range test: The liquidus and solidus temperatures were determined by differential scanning calorimetry (DSC) (cooling rate: 5℃ / min), and the results are as follows. Figure 1As shown, (a) is the cooling curve obtained by differential scanning calorimetry; (b) is the liquidus temperature, solidus temperature, and the corresponding width of the pasty region. The results show that as the carbon content increases from 0 to 270 ppm, the solidus temperature decreases from 1356.3℃ to 1350.4℃, while the liquidus temperature remains basically within the range of 1376.1~1375.5℃. This causes the width of the pasty region to increase from 19.8℃ to 25.1℃, indicating that the solidification temperature range is significantly widened and the solidification path is lengthened.

[0043] (2) Precipitation test at the end of solidification: The interdendritic precipitates of the four alloys were observed by HAADF-STEM and SAED, and the results are as follows. Figure 2 As shown. Among them, (a) and (c) are representative HAADF-STEM images of bright blocky precipitates in alloy 1 and alloy 2; (b) and (d) are the corresponding SAED spectra collected along the representative region axis and classified into the simple tetragonal σ phase (P42 / mnm) type; (e) is the HAADF-STEM image of the interdendritic composition of the two phases in alloy 3; (f–h) are the SAED spectra of the bright σ region in alloy 3 obtained along three different region axes; (i) is the SAED spectra of the gray region in alloy 3; (j) is the SAED spectra of the script-like precipitates in alloy 4, all of which are consistent with the fcc type M6C (Fd-3m).

[0044] Depend on Figure 2 It can be seen that when the carbon content is 56 ppm, no obvious carbide precipitation is observed between the dendrites, and the dendrites are mainly composed of σ-NiMoRe phase (simple tetragonal structure); when the carbon content increases to 180 ppm, the σ-NiMoRe phase coexists with the massive primary M6C carbide in the dendrites; when the carbon content further increases to 270 ppm, the dendrites are dominated by framework (network) Mo and Re enriched M6C carbide, and its crystal structure is face-centered cubic.

[0045] (3) Porosity test: The porosity of a 3×3×3 mm sample was measured using a nanoVoxel-2000 series X-ray computed tomography (XCT) scanner. 3 The samples underwent three-dimensional porosity characterization and statistical analysis. The test voltage was 150 kV, the test current was 30 μA, the exposure time was 0.8 s, the scanning resolution was 2 μm, and the scanning time was 2 h. The results are as follows: Figures 3-6 As shown, Figures 3-6 The shrinkage porosity reconstruction morphologies of alloys 1 to 4 are shown in sequence. The results show that as the carbon content increases from 0 to 270 ppm, the shrinkage porosity decreases significantly from 0.012% to 0.0005%, and the pore size and morphology change from irregular elongated shapes to fine, near-spherical shapes.

[0046] Based on the above solidification temperature range, the precipitated phase at the end of solidification and the porosity characterization results, it can be seen that when the carbon content is within the target range determined in this embodiment, an appropriate amount of carbides precipitate between dendrites at the end of solidification and prolong the final feeding time, which is beneficial to the feeding of residual melt and significantly reduces shrinkage porosity.

[0047] Example 2: Effect of carbon content on porosity in nickel-based single-crystal alloys DD5 nickel-based single-crystal superalloy was selected for verification. The contents of the main alloying elements, except for carbon, remained basically consistent. Following the method described in step (1) of this invention, Thermo-Calc solidification simulations were performed on the DD5 alloy to determine the target carbon content C that would allow the initial solid phase mass fraction of carbide precipitation to be between 80% and 90%. target The carbon content ranges from 120 to 260 ppm. To verify the regularity that "porosity can only be reduced when the carbon content falls within the target range," this example selects three carbon content levels for comparison: 50 ppm (below the target range), 150 ppm (near the target range), and 550 ppm (above the target range) (mass ppm). The alloy preparation and directional solidification process and equipment used are the same as in Example 1.

[0048] Alloy Preparation: The master alloy was prepared using a vacuum induction melting method in a ZGJL-0.025 type vacuum induction furnace. The melting process included three steps: first melting, refining, and second melting. The first melting temperature was set at 1550℃ for 25 minutes. The refining temperature was set at 1580℃ for 10 minutes. After refining, the melting equipment chamber was filled with a protective argon atmosphere. The argon purity should be greater than 99.9%, and the content of other impurities was limited to oxygen ≤10 ppm, nitrogen ≤50 ppm, and moisture ≤15 ppm. The second melting was carried out under an argon protective atmosphere at 1530℃ for 10 minutes.

[0049] Alloy raw materials must be weighed according to the designed proportions, accurate to 0.1g. Before melting, Ta, Re, Mo, and Ni are placed in an induction furnace. To prevent premature vaporization of Al, Al is placed in a secondary feeder and added during the melting process. To reduce the impact of impurities on the alloy's microstructure and mechanical properties, the surface of the metal block is ground with a grinding wheel to remove 2mm of oxide scale, ultrasonically washed with acetone solution for 30 minutes, and then dried in an oven at 80℃ for 1 hour. After removing the oxide scale by grinding, the master alloy ingot is cut into ingots of approximately 4 kg for the production of single crystal test rods. In the second stage, a Ni-C master alloy is added to achieve trace carbon content control, and carbon content is sampled before and after tapping to verify the target values ​​for each group. After melting, the molten metal is electrically cast into master alloy ingots. Subsequently, single crystal preparation is carried out using the seed crystal method, employing a DZG-0025 directional solidification furnace: the single crystal growth furnace is first evacuated to 0.8×10⁻⁶. -2 Pa, place the seed crystal at the bottom of the mold shell; after the master alloy ingot melts in the furnace, let the molten metal flow from the alumina crucible into the mold shell, hold it at the temperature and control the temperature gradient to pull it, after the crystal pulling is completed, cool it with the furnace for 30 min to obtain the

[001] oriented cast alloy with a size of Φ15 mm×200 mm rod.

[0050] Example 2 Characterization and Testing: (1) Test of precipitates at the end of solidification: The interdendritic microstructure and phase analysis of DD5 as-cast alloys with different carbon contents were observed. The results showed that no carbide precipitation was observed between the dendrites when the carbon content was 50 ppm; when the carbon content increased to 150 ppm and 550 ppm, the interdendritic precipitates were all primary MC-type carbides rich in Ta. As the carbon content further increased from 150 ppm to 550 ppm, the number and size of MC-type carbides increased significantly, and the interdendritic carbides were more likely to aggregate. TEM-SAED test was performed on the MC-type carbides, and the results are as follows: Figure 7 As shown, its crystal structure is a face-centered cubic structure.

[0051] (2) Porosity test: The porosity of a 3×3×3 mm sample was measured using a nanoVoxel-2000 series X-ray computed tomography (XCT) scanner. 3 The samples underwent three-dimensional porosity characterization and statistical analysis. The test voltage was 150 kV, the test current was 30 μA, the exposure time was 2 s, the scanning resolution was 2 μm, and the scanning time was 3 h. The test results are shown in […]. Figures 8-10 ,in, Figure 8 Corresponding to 50ppm, Figure 9 Corresponding to 150ppm, Figure 10The corresponding value is 550 ppm. The results show that the porosity is 0.010% when the carbon content is 50 ppm; the porosity decreases to 0.005% when the carbon content is 150 ppm; and the porosity increases to 0.012% when the carbon content increases to 550 ppm.

[0052] The results of Example 2 show that there is a suitable control range for carbon content in DD5 single-crystal alloys. When the carbon content is low (e.g., 50 ppm), it is difficult for carbide precipitates to form in the alloy that can participate in the feeding process at the end of solidification, and the decrease in porosity is not significant. When the carbon content is at a medium level (e.g., 150 ppm), the porosity reaches a low value. When the carbon content is too high (e.g., 550 ppm), the amount of primary MC-type carbide precipitates increases significantly and they tend to aggregate between dendrites, which is not conducive to the densification of the casting, and the porosity increases instead. Therefore, for different single-crystal alloy systems, the target carbon content should be determined according to the solidification simulation of this invention, and the carbon content should be precisely controlled through the melting process to make the precipitation of carbides more controlled in the later stage of solidification, so as to stably obtain a lower porosity.

[0053] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for reducing microporosity during the casting of single-crystal alloys by adding trace / micro-element carbon, characterized in that, Includes the following steps: Determine the target carbon content C of the single-crystal alloy to be prepared. target Optimized components were obtained; Prepare master alloy ingots with optimized composition; The master alloy ingot is melted and then directionally solidified to obtain a single crystal alloy; The target carbon content C of the single crystal alloy to be prepared is determined. target The process includes the following steps: inputting the main elemental composition of the single-crystal alloy to be prepared into thermodynamic calculation software, selecting a thermodynamic database that matches the single-crystal alloy to be prepared, and using solidification simulation to calculate the solid mass fraction f corresponding to the carbide phase precipitation initiation point under different carbon content conditions. s,start Iterative calculations are performed using carbon content as a variable, and f is selected to make f... s,start The carbon content between 80% and 90% is used as the target carbon content C. target The carbide precipitation initiation point refers to the moment when the first carbide phase begins to precipitate from the residual liquid in the solidification simulation; when different carbide phases may precipitate, the carbide phase that precipitates first and has the greatest impact on the feeding channel is taken as the control object.

2. The method according to claim 1, characterized in that, The f s,start It is 85%~90%.

3. The method according to claim 1, characterized in that, The single-crystal alloy includes nickel-based single-crystal superalloys.

4. The method according to claim 3, characterized in that, The nickel-based single-crystal superalloy includes Ni3Al-based single-crystal alloy or DD5 single-crystal alloy.

5. The method according to claim 1, characterized in that, The solidification simulation employs either an equilibrium solidification model or a Scheil-Gulliver nonequilibrium solidification model.

6. The method according to claim 1, characterized in that, The thermodynamic software includes Thermo-Calc.

7. The method according to claim 1, characterized in that, The preparation of the master alloy ingot includes the following steps: according to the composition of the single crystal alloy, the corresponding raw materials are sequentially subjected to first melting, refining and second melting. After the carbon content is tested and found to meet the requirements, the resulting molten metal is cast to obtain the master alloy ingot.

8. The method according to claim 7, characterized in that, A carbon-containing intermediate alloy is added during the refining or second smelting stage to control the carbon content.

9. The method according to claim 1, characterized in that, The directional solidification adopts the seed crystal method.

10. The method according to claim 6, characterized in that, The refining temperature is higher than the first melting temperature.

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