A prediction method for eliminating defects in nickel-based superalloy castings by hot isostatic pressing

Abstract

The present invention provides a prediction method for eliminating defects in nickel-based superalloy castings by hot isostatic pressing, which relates to the field of alloy metal forming prediction. The present invention includes the following steps: Step 1, investment casting treatment, preparing K424 alloy test bars according to a weight ratio; Step 2, hot isostatic pressing treatment, treating the obtained test bars at different hot isostatic pressing temperatures; Step 3, observing the γ′ precipitate phase with the corroded specimens. The present invention can promote the closure of internal pores by increasing the hot isostatic pressing temperature. When the hot isostatic pressing temperature reaches 1200 °C, the pores can be efficiently closed. The pore closure during hot isostatic pressing mainly includes two stages: plastic flow and creep. The decrease in yield strength caused by the increase in hot isostatic pressing temperature is the main factor promoting pore closure in the plastic flow stage, and the stress gradient and composition gradient are the main driving forces for pore healing in the creep stage.

Description

Technical Field

[0001] The present invention relates to a prediction method for nickel alloys, specifically a prediction method for eliminating defects in nickel-based superalloy castings by hot isostatic pressing, and belongs to the technical field of alloy metal forming prediction. Background Technique

[0002] Alloy K424 is a precipitation-hardening nickel-based superalloy, with γ′-Ni3(Ti,Al) type precipitates as the main strengthening phase. It still has good strength and plasticity above 800 °C and can be used for a long time within the range of 850 °C - 1000 °C. It is often used as the raw material for high-temperature turbine components of aeroengines. The main constituent phases in as-cast Alloy K424 are γ, γ′, (γ / γ′) eutectic, and MC phase. This material has a high content of Ti and Al, and the mass ratio of Ti and Al elements is as high as 9 - 10%, and Ti / Al is close to 0.9. The high Ti content and the Ti / Al content ratio will cause the (γ + γ′) eutectic structure to precipitate earlier at the end of solidification, hindering the liquid-phase flow, resulting in the material being prone to form porosity or cracks during the casting process, which is harmful to the mechanical properties of the material.

[0003] At present, the hot isostatic pressing process has been widely verified in eliminating internal pores of materials and repairing creep damage of materials. Hot isostatic pressing can be used as an effective method for eliminating internal pores of various materials. However, the research on the densification mechanism of superalloy materials during hot isostatic pressing is relatively less. Currently, the pore healing mechanism is divided into plastic flow, diffusion creep, and dislocation creep. Among them, plastic flow mainly depends on the yield strength of the material, while the rates of diffusion creep and dislocation creep depend on the rates of self-diffusion and grain boundary diffusion of the alloy. Some scholars have also established a pore healing model based on thermodynamics theory, clarifying the mutual influence relationship among the hot isostatic pressing temperature, pressure, and healing time. It is found that the pore healing of superalloys during hot isostatic pressing is affected by the formation of γ′ rafting zone and the directional migration of γ / γ′ phase-forming elements. The pore healing of materials during the creep stage depends on element diffusion. Plastic flow and element diffusion are affected by the yield strength of the material and the element diffusion coefficient. Therefore, different alloy compositions will cause differences in the dominant pore healing mechanism. However, there is still relatively little research on the pore healing mechanism of Alloy K424 under high Ti and Al contents. It is still necessary to further understand the pore healing mechanism of Alloy K424 at different hot isostatic pressing temperatures to obtain process parameters for efficiently eliminating internal pores of the material. Summary of the Invention

[0004] The purpose of the present invention is to explore the above problems and achieve them through the following technical solutions: A prediction method for eliminating defects in nickel-based superalloy castings by hot isostatic pressing, including the following steps:

[0005] Step 1. Investment casting treatment: Prepare K424 alloy test bars according to the weight ratio, where Al is 5.05 Wt.%, Co is 12.8 Wt.%, Cr is 9.48 Wt.%, Fe is 0.12 Wt.%, Mo is 3.19 Wt.%, Nb is 0.68 Wt.%, Ti is 4.3 Wt.%, V is 0.82 Wt.%, W is 1.4 Wt.%, C is 0.18 Wt.%, and Ni is Bal;

[0006] Mix the elements in the above mass percentages and fill them into the test bar mold shell, fill the sand for molding, control the temperature, bake it at 1020 °C for 3 h and then transfer it to the melting and pouring furnace, and the pouring temperature is 1500 °C to obtain as-cast K424 test bars;

[0007] Step 2. Hot isostatic pressing treatment: Treat the obtained test bars at different hot isostatic pressing temperatures;

[0008] The first hot isostatic pressing process is: temperature 1160 °C, pressure 140 MPa, time 2 h; denoted as HIP1;

[0009] The second hot isostatic pressing process is: temperature 1180 °C, pressure 140 MPa, time 2 h; denoted as HIP2;

[0010] The third hot isostatic pressing process is: temperature 1200 °C, pressure 140 MPa, time 2 h; denoted as HIP2;

[0011] Use a metallographic microscope to observe the grain size of the material and the distribution of precipitation phases such as carbides, and characterize the internal defects and precipitation phases of the material through a field emission scanning electron microscope (QuantaFEG);

[0012] Step 3. Observe the γ′ precipitation phase with the corroded specimen.

[0013] Preferably, Step 2 includes: polishing the specimen successively with metallographic sandpapers from low to high mesh numbers, and the highest mesh number is up to 2000;

[0014] Use 0.5 μm diamond for rough polishing;

[0015] Use OPS polishing fluid for fine polishing;

[0016] Vibratory polishing for at least 1 hour.

[0017] Preferably, Step 3 includes the requirements for the ratio of the etching solution. The ratio of the etching solution is: 20 mL HCl + 20 mL C2H5OH + 1 g CuCl2. Etch the specimen, and the etching time is not less than 2 min, and then ultrasonically clean the specimen.

[0018] Preferably, the minimum stress that can cause plastic flow during the hot isostatic pressing process is:

[0019]

[0020] Among them, σ y is the yield strength of the material at the hot isostatic pressing temperature, and ρ is the porosity of the material. It can be seen from Equation (1) that under a given external pressure, there is a critical porosity in the internal pores of the material, that is:

[0021]

[0022] Among them, σ is the stress applied by hot isostatic pressing. Therefore, as the hot isostatic pressing pressure σ increases, the yield strength σ y of the material decreases, the critical porosity of the material decreases, and smaller pore sizes can be obtained in the plastic flow stage, while the yield strength σ y of the material decreases with increasing temperature;

[0023] The yield strength of K424 alloy at different temperatures can be obtained by software calculation;

[0024] The calculated yield strengths of K424 alloy at 1160 °C, 1180 °C, and 1200 °C are 120 MPa, 109 MPa, and 66 MPa respectively;

[0025] Substituting the hot isostatic pressing pressure and the yield strength at each temperature into Equation (2), the critical porosity at each temperature can be obtained, which are ρ 1160 = 0.07%, ρ 1180 = 0.06% and ρ 1200 = 0.01% respectively.

[0026] Preferably, based on the detection standard of the metallographic micro-porosity of superalloys, the minimum size of the loose pores in the metallographic atlas corresponding to each porosity can be obtained.

[0027] Preferably, for a certain point near the pore, the stress field at this point can be decomposed into a radial stress component σ x pointing to the center of the pore and a circumferential stress component σ y perpendicular to the diameter direction. The radial stress distribution state at the pore can be calculated by the Lame solution:

[0028]

[0029] Among them, R i is the pore radius, R e is the radius of the γ′ phase rafting region, and r is the distance from a point outside the pore to the center of the pore.

[0030] Preferably, using the pore healing model:

[0031]

[0032] wherein, Ω A is the atomic volume, k is the Boltzmann constant, γ s is the pore surface energy, D gb is the grain boundary diffusion coefficient, δ is the grain boundary width, T is the hot isostatic pressing temperature. Considering the grain boundary diffusion of elements at the pores, based on this formula, further considering the volume diffusion under high temperature conditions during the hot isostatic pressing process, the derivation calculation model after adding volume diffusion is as follows:

[0033]

[0034] wherein, D v is the volume diffusion coefficient;

[0035] The pore healing time at each temperature in the creep stage is calculated using the above model.

[0036] The present invention provides a prediction method for eliminating defects in nickel-based superalloy castings by hot isostatic pressing, and the beneficial effects thereof are as follows:

[0037] According to the research method of the present invention, it can be known that the hot isostatic pressing process can close the internal loose pores of K424 alloy. Temperature plays a major role in the pore closing efficiency. Increasing the hot isostatic pressing temperature can shorten the pore healing time;

[0038] When the isostatic pressing temperature is increased to 1200 °C, the yield strength of K424 alloy can be reduced to 65 MPa, the porosity of the material in this stage can be reduced, and the pore healing time in the creep stage can be accelerated;

[0039] The stress gradient and composition gradient become the main driving forces for the directional diffusion of elements. Elements diffuse towards the pores to form a γ-phase transition zone. Pore healing mainly occurs through element diffusion and the formation of new γ-phase. At the same time, a large amount of Ti elements are enriched in the transition zone, promoting the formation of fine γ' in this region. Description of the Drawings

[0040] Figure 1 is the microstructure of K424 alloy of the present invention: (a) grain morphology after corrosion; (b) carbide distribution before corrosion; (c) loose pores and eutectic structure; (d) γ' and σ precipitation phases;

[0041] Figure 2 is the morphology and element distribution at the loose pores of K424 alloy of the present invention: (a) morphology of loose pores; (b-f) distributions of Ni, Cr, Ti, Al, and Mo elements in Figure (a);

[0042] Figure 3Microstructure of K424 alloy of the present invention after hot isostatic pressing at 1160 °C / 140 MPa / 2 h: (a) Morphology of compressed pores; (b) Transition zone at the pore edge; (c) γ'-phase rafted structure at the pores; (d) Element line scan distribution at the horizontal line in Fig. (b);

[0043] Figure 4 Microstructure of K424 alloy of the present invention after hot isostatic pressing at 1180 °C / 140 MPa / 2 h: (a) Morphology of loose pores and transition zone; (b) Fine γ'-phase in the transition zone; (c) Morphology of closed pores; (d) Carbides in the closed pores;

[0044] Figure 5 Microstructure of K424 alloy of the present invention after hot isostatic pressing at 1200 °C / 140 MPa / 2 h; (a) Microstructure after complete pore healing; (b) Two scales of γ'-phase at the eutectic structure;

[0045] Figure 6 γ'-phase formation of K424 alloy of the present invention at different hot isostatic pressing temperatures: (a) HIP1; (b) HIP2; (c) HIP3;

[0046] Figure 7 Calculated yield strength values of K424 alloy of the present invention at different temperatures;

[0047] Figure 8 Stress state at the pores of the present invention: (a) Stress distribution near the pores; (b) γ'-rafted region at the pores;

[0048] Figure 9 Calculated values of pore healing time in the creep stage of the present invention: (a) Influence of temperature on pore healing time; (b) Influence of initial pore size on healing time at different hot isostatic pressing temperatures;

[0049] Figure 10 Influence of hot isostatic pressing temperature and pore size on healing time of the present invention. Detailed implementation manners

[0050] An embodiment of the present invention provides a prediction method for eliminating defects in investment cast nickel-based superalloy castings by hot isostatic pressing, including the following steps: Step 1, investment casting treatment, preparing K424 alloy test bars according to the weight ratio, where Al is 5.05 Wt.%, Co is 12.8 Wt.%, Cr is 9.48 Wt.%, Fe is 0.12 Wt.%, Mo is 3.19 Wt.%, Nb is 0.68 Wt.%, Ti is 4.3 Wt.%, V is 0.82 Wt.%, W is 1.4 Wt.%, C is 0.18 Wt.%, and Ni is Bal;

[0051] Mix the elements with the above mass percentages and fill them into the test bar mold shell. Fill the sand to form a mold, control the temperature, and bake it at 1020 °C for 3 h, then transfer it to a melting and pouring furnace. The pouring temperature is 1500 °C to obtain an as-cast K424 test bar. The alloy composition is shown in Table 1:

[0052] Table 1 K424 alloy composition

[0053]

[0054] Step 2: Hot isostatic pressing treatment. Treat the obtained test bars with different hot isostatic pressing temperatures;

[0055] The first hot isostatic pressing process is: temperature 1160 °C, pressure 140 MPa, time 2 h; denoted as HIP1;

[0056] The second hot isostatic pressing process is: temperature 1180 °C, pressure 140 MPa, time 2 h; denoted as HIP2;

[0057] The third hot isostatic pressing process is: temperature 1200 °C, pressure 140 MPa, time 2 h; denoted as HIP2;

[0058] Use a metallographic microscope to observe the grain size of the material and the distribution of precipitation phases such as carbides, and characterize the internal defects and precipitation phases of the material through a field emission scanning electron microscope (QuantaFEG);

[0059] The hot isostatic pressing equipment can be a QIH-9 type hot isostatic press, and the effective loading working area of the equipment: The maximum temperature is 1250 °C, and the maximum pressure is 155 MPa. Keep the hot isostatic pressing pressure and time consistent, and treat the as-cast K424 alloy test bars with different hot isostatic pressing temperatures respectively. The hot isostatic pressing process parameters are shown in Table 2:

[0060] Table 2 Hot isostatic pressing process parameters of K424 alloy test bars

[0061]

[0062] Use a metallographic microscope (Leica DFC450) to observe the grain size of the material and the distribution of precipitation phases such as carbides, and characterize the internal defects and precipitation phases of the material through a field emission scanning electron microscope (QuantaFEG). The samples are polished successively with metallographic sandpaper from low to high mesh numbers, up to 2000 mesh, then roughly polished with 0.5 μm diamond and finely polished with OPS polishing liquid, and finally vibratory polished for 1 hour. The γ′ precipitation phase is observed with the corroded samples. The corrosion solution ratio is: 20 mL HCl + 20 mL C2H5OH + 1 g CuCl2. After corroding the polished surface for 2 min, then perform ultrasonic cleaning;

[0063] Step 3: Observe the γ′ precipitation phase using the corroded sample.

[0064] Please refer to Figure 1 , Figure 1 This is the microstructure of the cast K424 alloy. From the figure, we can observe γ, γ′, (γ+γ′) eutectic, carbide, σ phase, etc. According to metallographic statistics, the area ratio of precipitated phases such as carbide and σ phase in the matrix is ​​about 3.19%, the size of γ′ is about 0.35±0.11μm, and the area ratio is about 25.30%. At the same time, it can be observed that there are many loose pores in the cast alloy, and the loose pores are mainly attached to the eutectic structure, indicating that the formation of loose pores is closely related to the formation of eutectic structure during solidification.

[0065] like Figure 2 As shown in the figure, the element distribution in the loose pores and eutectic structure is further analyzed. The eutectic structure is rich in Ti and Al elements and poor in Cr and Mo elements. Because the intergranular segregation coefficient of Ti and Al elements is high, a large amount of Ti and Al elements are excluded into the liquid phase during the solidification process, resulting in an increase in the Ti and Al content in the liquid phase. At the same time, the Ti content in the K424 alloy is relatively high, and the mass fraction of Al and Ti elements accounts for about 10%. High Ti and Al content will increase the (γ+γ′) eutectic precipitation temperature. The Ti and Al elements segregated in the liquid phase make the liquid phase components at the end of solidification easier to approach the eutectic formation point, thereby precipitating a large amount of eutectic structure. In addition, the Al / Ti ratio of the K424 alloy is relatively low. It is generally believed that the lower the Al / Ti ratio, the easier it is to form a eutectic structure in the material, and the greater the thermal cracking tendency of the material.

[0066] Therefore, high Ti and Al contents and low Al / Ti ratio cause the eutectic structure to precipitate prematurely during the solidification process of the alloy, which shortens the feeding time of the alloy during the solidification process. The eutectic structure between dendrites hinders the feeding channel, resulting in the formation of loose pores mainly concentrated near the eutectic structure.

[0067] Please refer to the attached Figure 3 The microstructure of the defect after hot isostatic pressing (HIP1) at 1160℃ / 140MPa / 2h is shown in the figure. The loose pores have undergone obvious compression deformation after hot isostatic pressing, but they are still not completely closed. There is a thin transition zone along the edge of the pores. EDS line scanning analysis ( Figure 3 -d) It was found that a large amount of Ti element was enriched in the transition region, indicating that the element migration would be directional migration to the pores during the hot isostatic pressing process. In addition, the γ′ phase rafting phenomenon can be observed near the deformed pores. The rafted γ′ phase takes the pore as the center, and the rafting direction is perpendicular to the diameter direction, forming a nearly circular rafted area. The γ′ phase rafting is closely related to the applied stress and local composition gradient in the alloy. The higher the external stress and the larger the composition gradient, the easier it is for the γ′ phase rafting to form.

[0068] Furthermore, please refer to Figure 4 , after the hot isostatic pressing temperature is increased to 1180 °C (HIP2), there are still many unclosed pores in the material, and the width of the transition zone at the pore edge is relatively wider. Figure 4 -b is Figure 4 -a, a magnified view of the boxed area. It can be found that there are extremely fine γ′ precipitate phases in the transition zone, with a size of about 0.2 - 0.35 μm. During the hot isostatic pressing process, the diffusion of elements into the pores and the formation of new phases are considered important mechanisms for pore healing. As the degree of pore healing increases, the range of the transition zone expands, and the Ti element enriched in the transition zone promotes the formation of a large number of fine Ni3(Al,Ti)-type γ′ phases in this area. At the same time, in Figure 4 -d, it can be found that there are a small number of carbide particles in the pores about to close, which may be formed due to the increased degree of element diffusion during the pore closing process.

[0069] Furthermore, please refer to the appendix Figure 5 . The figure shows the microstructure of the alloy after hot isostatic pressing at 1200 °C / 140 MPa / 2 h (HIP3). At this temperature, the internal pores of the material have been basically eliminated, and a denser microstructure is obtained. The γ′ solvus temperature in the K424 alloy is about 1210 °C. When the hot isostatic pressing temperature approaches the γ′ phase solvus temperature in the superalloy, the dissolution of the γ′ phase will greatly promote the pore healing rate. Therefore, hot isostatic pressing at 1200 °C can efficiently eliminate the internal pores of the K424 alloy. At the same time, two γ′ phase aggregation regions with different sizes and morphologies can be observed near the eutectic structure. The fine γ′ phases are mostly square-shaped, and there are carbides in the middle of the region. The coarse γ′ phases are mainly irregular, and a small amount of rafted γ′ phases can still be observed, which may be the rafted γ′ phases formed due to the existence of stress gradients in this area.

[0070] Please refer to Figure 6 , Figure 6The microstructures of K424 alloy after hot isostatic pressing at different temperatures are shown below. When the hot isostatic pressing temperature is 1160 °C, the γ' phase coarsens compared to the as-cast state, the size increases to about 1.14 μm, the shape changes from square to irregular, and the area of the γ' phase increases to 45.12%. When the hot isostatic pressing temperature is increased to 1180 °C, partial dissolution and reprecipitation of fine secondary γ' phase can be observed. The size of the primary γ' phase is about 1.07 μm, the shape is mainly square, and the area ratio is 36.16%. The size of the secondary γ' phase is about 0.06 μm, and the shape is also square. Since 1180 °C is relatively close to the solution temperature (1210 °C) of the γ' phase in K424 alloy, partial dissolution occurs while the γ' phase coarsens, which also makes the primary γ' phase still maintain a square structure. After hot isostatic pressing at 1200 °C / 140 MPa / 2 h, the size of the γ' phase is about 0.762 μm, and the area ratio increases to 48.23%. The dissolution trend of the γ' phase increases when approaching the solution temperature, so the coarsening degree of the γ' phase is smaller at this temperature. For further convenience of understanding, Table 3 shows the size and distribution of the γ' phase at different hot isostatic pressing temperatures:

[0071] Table 3 Size and Distribution of γ' Phase in K424 Alloy at Different Hot Isostatic Pressing Temperatures

[0072]

[0073] As shown above, please refer to Figures 1 - 10 , at the initial stage of hot isostatic pressing, the porosity inside the material is relatively high. Under the action of high temperature and high pressure, plastic flow occurs first at the pores due to stress, making the pores shrink. When the pores shrink to a certain critical porosity, it is difficult for the pores to close through plastic flow anymore. At this time, the pore healing mechanism mainly depends on creep. Plastic flow is closely related to the yield strength of the alloy. The minimum stress that can cause plastic flow during hot isostatic pressing is:

[0074]

[0075] where σ y is the yield strength of the material at the hot isostatic pressing temperature, and ρ is the porosity of the material. It can be seen from Equation (1) that under a given external pressure, there is a critical porosity in the internal pores of the material, that is:

[0076]

[0077] where σ is the stress applied during hot isostatic pressing. Therefore, as the hot isostatic pressing stress σ increases, the yield strength σ y of the material decreases, the critical porosity of the material decreases, and smaller pore sizes can be obtained in the plastic flow stage. And the yield strength σ yIt decreases with the increase of temperature. The yield strength of K424 alloy at different temperatures can be calculated by using JMatPro software, as Figure 7 shown.

[0078] The calculated yield strengths of K424 alloy at 1160 °C, 1180 °C, and 1200 °C are 120 MPa, 109 MPa, and 66 MPa respectively. Substituting the hot isostatic pressing pressure and the yield strength at each temperature into Equation (2), the critical porosity at each temperature can be obtained, which are ρ 1160 = 0.07%, ρ 1180 = 0.06%, and ρ 1200 = 0.01% respectively. Assuming that the loose pores are evenly distributed in the material and are mainly isolated-type loosening formed at the eutectic structure, based on the metallographic microscopic loosening detection standard of superalloys, the minimum size of the loose pores in the metallographic atlas corresponding to each porosity can be obtained. Therefore, for isolated-type loosening, the minimum pore size that can be achieved by plastic flow at each hot isostatic pressing temperature is approximately l 1160 = 39 μm, l 1180 = 35 μm, and l 1200 = 13 μm. It can be seen that when the hot isostatic pressing temperature is 1160 °C and 1180 °C, due to the basic equivalence of the material yield strength and the hot isostatic pressing pressure, the pore size is still relatively large during the plastic flow stage. When the hot isostatic pressing temperature is increased to 1200 °C, since it is close to the γ′ phase solution temperature, the content of the γ′ phase decreases, and the strengthening effect on the material weakens, resulting in a significant reduction in the yield strength. Therefore, during the plastic flow stage at this temperature, the pores can be compressed to a relatively small size, which is beneficial to the pore healing in the subsequent creep stage.

[0079] There are two main driving forces for the elimination of pores during the creep stage in the hot isostatic pressing process: the stress gradient formed due to the existence of pores and the composition gradient formed by element diffusion. Under the action of the isotropic pressure in the hot isostatic pressing, a complex stress field will be formed near the pores. For a certain point near the pores, the stress field at this point can be decomposed into a radial stress component σ x pointing to the center of the pores and a circumferential stress component σ y perpendicular to the diameter direction. The radial stress distribution state at the pores can be calculated by the Lame solution:

[0080]

[0081] where R i is the pore radius, R e is the radius of the γ′ phase rafting region, and r is the distance from a point outside the pores to the center of the pores. It can be seen from Equation (3) that a stress distribution decreasing from the pores to the matrix is formed at the pores, as Figure 8As shown in Fig. 8-a. The stress gradient promotes the directional diffusion of γ'-phase forming elements, resulting in the formation of γ'-phase rafting regions at the pores after hot isostatic pressing. Figure 8 -b shows the γ'-phase rafting region. The rafting region usually forms a circular area centered around the pore. The γ'-phase rafting direction is along the circumferential direction of the circular area, which is a typical N-type rafting. Outside the rafting region, the γ'-phase is mostly irregular. In addition, the transition region formed at the pore edge is mainly due to the directional diffusion of elements towards the pore. The formation of the new phase promotes pore healing. At the same time, the diffusion of γ'-phase forming elements (Al, Ti) is enriched in the transition region, resulting in the formation of a large number of fine γ'-phases in this region.

[0082] Based on the diffusion and stress effects at the pores during the hot isostatic pressing of superalloys, a pore healing model was established:

[0083]

[0084] where Ω A is the atomic volume, k is the Boltzmann constant, γ s is the pore surface energy, D gb is the grain boundary diffusion coefficient, δ is the grain boundary width, and T is the hot isostatic pressing temperature. This model mainly considers the grain boundary diffusion of elements at the pores. However, in the actual hot isostatic pressing process, volume diffusion under high-temperature conditions also needs to be considered. After adding volume diffusion, the model becomes:

[0085]

[0086] where D v is the volume diffusion coefficient. Using this model, the pore healing time at each temperature during the creep stage was calculated, as shown in Figure 9 Fig. 8-a. When the hot isostatic pressing temperature increases, the pore healing time is correspondingly shortened. Since the minimum size that the pore plastic flow can reach is different at different hot isostatic pressing temperatures, the healing state of large pores at each hot isostatic pressing temperature was further analyzed, as shown in Figure 9 Fig. 8-b. It can be seen from the figure that as the hot isostatic pressing temperature increases, the pore size decreases during the plastic flow stage, and the initial pore size during the creep stage decreases, thus greatly shortening the time required for pore healing.

[0087] Please refer to the appendix Figure 10, when the pore size is comparable, increasing the hot isostatic pressing temperature only slightly reduces the healing time. However, as the pore size increases, the pore healing time in the creep stage increases significantly, indicating that the initial pore size in the creep stage has the most important influence on the pore healing time. Therefore, it is more beneficial to improve the pore healing efficiency by increasing the degree of deformation in the pore plastic flow stage during hot isostatic pressing to obtain a smaller pore size. It can be found that the decrease in the yield strength of the material at high hot isostatic pressing temperatures promotes pore plastic flow, resulting in a smaller initial pore size in the creep stage. Therefore, the degree of pore healing is significantly increased at high hot isostatic pressing temperatures.

[0088] In summary: Through the above prediction method, it can be known that the hot isostatic pressing process can close the internal loose pores of K424 alloy. Temperature plays a major role in the pore closing efficiency. Increasing the hot isostatic pressing temperature can shorten the pore healing time. After hot isostatic pressing at 1200 °C / 140 MPa / 2 h, the internal loose pores of the material can be significantly eliminated;

[0089] The pore closing during hot isostatic pressing mainly includes two stages: plastic flow and creep. The plastic flow stage is mainly affected by the yield strength of the material. Increasing the hot isostatic pressing temperature to 1200 °C can reduce the yield strength of K424 alloy to 65 MPa, reduce the porosity of the material in this stage, and accelerate the pore healing time in the creep stage;

[0090] The pore elimination in the creep stage mainly depends on element diffusion. The stress gradient and composition gradient become the main driving forces for the directional diffusion of elements. Elements diffuse towards the pores to form a γ-phase transition zone. The pore healing mainly occurs through element diffusion and the formation of new γ-phase. At the same time, a large amount of Ti elements are enriched in the transition zone, promoting the formation of fine γ′ in this area.

[0091] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. The above embodiments and the descriptions in the specification only illustrate the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed. The scope of protection claimed by the present invention is defined by the appended claims and their equivalents.

Claims

1. A prediction method for eliminating defects in nickel-based superalloy castings by hot isostatic pressing, characterized in that, It includes the following steps: Step 1, investment casting treatment. Prepare K424 alloy test bars according to the weight ratio, where Al is 5.05Wt.%, Co is 12.8Wt.%, Cr is 9.48Wt.%, Fe is 0.12Wt.%, Mo is 3.19Wt.%, Nb is 0.68Wt.%, Ti is 4.3Wt.%, V is 0.82Wt.%, W is 1.4Wt.%, C is 0.18Wt.%, and Ni is Bal; Mix the elements with the above mass percentages and fill them into the test bar mold shell, fill the sand for molding, control the temperature, roast it at 1020°C for 3h and then transfer it to the melting and pouring furnace, and the pouring temperature is 1500°C to obtain as-cast K424 test bars; Step 2, hot isostatic pressing treatment. Treat the obtained test bars at different hot isostatic pressing temperatures; The first hot isostatic pressing process is: temperature 1160°C, pressure 140MPa, time 2h; denoted as HIP1; The second hot isostatic pressing process is: temperature 1180°C, pressure 140MPa, time 2h; denoted as HIP2; The third hot isostatic pressing process is: temperature 1200°C, pressure 140MPa, time 2h; denoted as HIP3; Use a metallographic microscope to observe the grain size of the material and the distribution of carbide precipitation phases, and characterize the internal defects and precipitation phases of the material through a field emission scanning electron microscope; Step 3, observe the γ′ precipitation phase with the corroded specimen; Utilize the pore healing model: where, Ω A is the atomic volume, k is the Boltzmann constant, γ s is the pore surface energy, D gb is the grain boundary diffusion coefficient, δ is the grain boundary width, T is the hot isostatic pressing temperature, R i is the pore radius, R e is the radius of the γ′ phase rafting region. Considering the grain boundary diffusion of elements at the pores, based on this formula, further considering the volume diffusion in the high-temperature condition during the hot isostatic pressing process, the calculation model is derived after adding the volume diffusion as follows: Among them, D v is the volume diffusion coefficient; Use the above-derived calculation model after adding volume diffusion to calculate the pore healing time at each temperature during the creep stage.

2. The prediction method for eliminating defects in nickel-based superalloy castings by hot isostatic pressing according to claim 1, characterized in that Step 2 includes: successively polish the specimen with metallographic sandpaper from low to high mesh numbers, and the highest mesh number is up to 2000 mesh; Use 0.5μm diamond for rough polishing; Use OPS polishing liquid for fine polishing; Vibratory polishing for at least 1 hour.

3. A prediction method for eliminating defects in nickel-based superalloy castings by hot isostatic pressing according to claim 1, characterized in that, Step 3 includes the requirements for the ratio of the etching solution. The ratio of the etching solution is: 20mL HCl + 20mL C2H5OH + 1g CuCl2. Etch the specimen, and the etching duration is not less than 2min, and then ultrasonically clean the specimen.

4. A prediction method for eliminating defects in nickel-based superalloy castings by hot isostatic pressing according to claim 1, characterized in that: The minimum stress at which plastic flow can occur during hot isostatic pressing is: where σ y is the yield strength of the material at the HIP temperature, and ρ is the porosity of the material; it can be seen from Equation (1) that under a given external pressure, there is a critical porosity in the internal pores of the material, which is: Among them, σ is the stress applied by hot isostatic pressing; therefore, as the hot isostatic pressing pressure σ increases, the yield strength σ of the material y decreases, the critical porosity of the material decreases, and a smaller pore size is obtained in the plastic flow stage, while the yield strength σ of the material y decreases with the increase of temperature; Use software to calculate the yield strength of K424 alloy at different temperatures; The calculated yield strengths of K424 alloy at 1160°C, 1180°C, and 1200°C are 120MPa, 109MPa, and 66MPa respectively; Substitute the hot isostatic pressing pressure and the yield strength at each temperature into Equation (2) to obtain the critical porosity at each temperature, which are ρ 1160 = 0.07%, ρ 1180 = 0.06% and ρ 1200 = 0.01%, respectively.

5. The prediction method for eliminating defects in nickel-based superalloy castings by hot isostatic pressing according to claim 3, characterized in that: Based on the high-temperature alloy metallographic microscopic porosity detection standard, obtain the minimum size of the loose pores in the metallographic atlas corresponding to each porosity.

6. A prediction method for eliminating defects in nickel-based superalloy castings by hot isostatic pressing according to claim 1, characterized in that: For a certain point near the pore, the stress field at this point can be decomposed into a radial stress component σ pointing towards the center of the pore x and a circumferential stress component σ perpendicular to the diameter direction y ; The radial stress distribution state at the pore can be calculated by the Lame solution: Among them, R i is the pore radius, R e is the radius of the γ′ phase rafting region, and r is the distance from a point outside the pore to the pore center.

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