Method for improving as-cast solidification quality of inconel 718 high-temperature alloy and alloy ingot thereof

Abstract

The present application belongs to the technical field of high-temperature alloy smelting production, and particularly relates to a method for improving the as-cast solidification quality of Inconel718 high-temperature alloy and an alloy ingot thereof. The method comprises: short-time ultrahigh-temperature treatment of completely melted alloy at 1550-1600 DEG C and falling back to pouring temperature to realize melt purification and activation; then, a nickel-magnesium-molybdenum intermediate alloy with specific components is put in, and the magnesium content is accurately controlled in the range of 0.0040%-0.080%; finally, stepwise treatment of rapid cooling and isothermal holding is carried out in the critical temperature range of 1350-1380 DEG C. Through the synergistic effect of physical purification, chemical alloying and solidification control, the primary dendrite arm spacing is significantly refined, the Laves phase and MC carbide are in fine granular and dispersed distribution, and the as-cast structure uniformity is fundamentally improved, thereby providing a reliable solution for obtaining high-performance Inconel718 alloy ingot.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of high-temperature alloy smelting production, and particularly relates to a method for improving the as-cast solidification quality of Inconel718 high-temperature alloy and an alloy ingot thereof. BACKGROUND

[0002] Inconel718 alloy is a nickel-based wrought high-temperature alloy taking γ'' (Ni3Nb) phase as the main strengthening phase. Because it exhibits excellent high-temperature strength, creep resistance, fatigue resistance and good corrosion resistance in a high-temperature environment, it has become the core material of key components in the hot end of high-end equipment such as aircraft engines and gas turbines.

[0003] However, due to the complex multi-component alloy design of the alloy, serious dendritic segregation occurs in the solidification process from the liquid state to the solid state. This segregation leads to a large amount of enrichment of solute elements, especially niobium (Nb), molybdenum (Mo) and titanium (Ti), in the interdendritic region. This micro-uniformity creates conditions for the subsequent precipitation of harmful phases. The most typical and most harmful are the topologically close-packed Laves phase and the coarse MC primary carbide.

[0004] The Laves phase is a hard and brittle intermetallic compound, which is distributed in the interdendritic region in a continuous or semi-continuous network in the as-cast structure. Not only does it seriously cut the continuity of the matrix, becoming the source of stress concentration and crack initiation, but it also consumes a large amount of valuable solid solution strengthening elements Nb and Mo in the alloy, leading to insufficient precipitation of the main strengthening phase γ'' in subsequent heat treatment, which seriously deteriorates the high-temperature plasticity, endurance life and fatigue performance of the alloy. At the same time, coarse and unevenly distributed blocky or strip-shaped MC carbides (mainly NbC and TiC) are also prone to become the source of micro-cracks under stress, which directly threatens the toughness and reliability of the material.

[0005] To address the above problems, the prior art mainly adopts two approaches: one is to apply a very slow cooling rate or to perform high-temperature homogenization diffusion annealing. This method is energy-consuming, has a long production cycle, and for large-size ingots, the center segregation problem is often difficult to eliminate, and the economy and effect are both unsatisfactory. The second is to attempt to modify the solidification process by adding trace elements. However, the choice of elements and their introduction method is crucial. For example, some technicians consider adding pure magnesium to the melt, but magnesium, as an extremely active and high-vapor-pressure element, is severely oxidized and volatilized at a smelting temperature as high as 1500℃, resulting in extremely low and fluctuating recovery rates, making it impossible to accurately control the magnesium content in the alloy, and even possibly further contaminating the melt with oxide inclusions. If other magnesium-containing intermediate alloys such as nickel-magnesium alloys are used, although the burning loss of magnesium can be partially alleviated, their low density makes them easy to float on the surface of the melt, and they also face the problems of unstable recovery rate and poor control accuracy of composition, and may introduce new uncertainties due to the introduction of unintended impurity elements.

[0006] Therefore, there has been a long-standing lack of a smelting method that can efficiently, accurately and stably regulate the as-cast solidification structure of Inconel718 alloy, effectively suppress and refine Laves phase and MC carbide, and is suitable for industrial production. This has become a bottleneck restricting the further improvement of the as-cast quality of the alloy and the reduction of its production cost. SUMMARY

[0007] The purpose of the present application is to provide a method for improving the as-cast solidification quality of Inconel718 high-temperature alloy and an alloy ingot thereof, to solve the problems of coarse Laves phase, uneven distribution of MC carbide and difficulty in accurately controlling the addition of trace elements in the prior art.

[0008] To achieve the above-mentioned purpose, the present application provides the following technical solutions:

[0009] A method for improving the as-cast solidification quality of Inconel718 high-temperature alloy, comprising the following steps:

[0010] (1) Place the Inconel718 alloy sample in a MgO crucible, and nest it in a graphite crucible, and heat it to 400-500℃;

[0011] (2) Argon is introduced, and the temperature is further increased to 1450-1500℃, and the alloy sample is completely melted; then, the temperature of the melt is further increased to 1550-1600℃, and the temperature is maintained at this temperature, and then the temperature is reduced to 1450-1500℃, to obtain a molten alloy;

[0012] (3) Put a nickel-magnesium-molybdenum alloy into the molten alloy and maintain the temperature;

[0013] (4) After the holding in step (3) is finished, firstly, the temperature of the melt is cooled from the holding temperature to 1350-1380 DEG C, and then, the heating power is turned off, and the alloy melt is cooled naturally with the furnace;

[0014] (5) When the temperature is reduced to 850-950 DEG C, the argon is turned off, and the Inconel718 alloy ingot is obtained.

[0015] Further, the holding time after the temperature is raised to 1450-1500 DEG C in step (2) is 8-15 min.

[0016] Further, the holding time after the temperature of the melt is further raised to 1550-1600 DEG C in step (2) is 1-3 min.

[0017] In this step, the completely melted alloy is briefly raised to the high temperature interval of 1550-1600 DEG C and held for 1-3 min, and the core role is to purify the melt at high temperature and optimize the structure. This short-time treatment higher than the conventional melting temperature can effectively dissolve the trace high-melting-point heterogeneous nucleation cores remaining in the alloy melt, and at the same time, the inherent atomic short-range ordered structure of the melt is destroyed by high-temperature disturbance. Then, the temperature is lowered to the pouring temperature, so that the melt is in a metastable activated state. This creates a very clean and more active energy state melt environment for the subsequent uniform distribution and efficient action of magnesium elements, which is an important prerequisite for precise control of trace elements.

[0018] Further, the composition of the nickel-magnesium-molybdenum alloy in step (3) is as follows in terms of percentage by mass: nickel 80%-90%, magnesium 5%-10%, and molybdenum 5%-10%.

[0019] Further, after the nickel-magnesium-molybdenum alloy is put in step (3), the magnesium content in the alloy is controlled to be in the range of 0.0040%-0.080%.

[0020] Further, the holding time in step (3) is 3-8 min.

[0021] The present application uses a nickel-magnesium-molybdenum ternary alloy as a carrier of magnesium, which is a key design to realize high yield and precise control of composition. Among them, nickel and magnesium are mutually soluble, which can effectively wrap and stabilize the active magnesium element, and significantly inhibit the oxidation and volatilization of magnesium during feeding and melting; and the addition of molybdenum, on the one hand, increases the overall density of the intermediate alloy, so that it can quickly sink through the slag layer into the metal melt inside, further reducing the burn loss, on the other hand, molybdenum itself is a base element of Inconel718, and will not introduce new impurities. This composition design enables magnesium to be introduced into the melt in a controllable and efficient manner, so that the magnesium content in the final alloy can be stabilized in the best range of 0.0040%-0.080% verified.

[0022] Further, the step (4) is kept at the temperature interval of 1350-1380℃ for 3-8 minutes.

[0023] Further, the cooling rate of the step (4) is 20-50℃ / min when the melt temperature is cooled from the holding temperature to 1350-1380℃.

[0024] The step (4) is first rapidly cooled to 1350-1380℃ at a rate not less than 20℃ / min, aiming to increase the melt supercooling degree and significantly increase the nucleation rate of the Laves phase and MC carbide. The subsequent isothermal holding process provides a key time window for the segregation of magnesium element at the solidification interface and the interface regulation role of magnesium. As a surface active element, magnesium can reduce the interface energy and hinder the diffusion of solute elements by enriching at the precipitate / matrix interface, thereby effectively inhibiting the Ostwald ripening of the precipitate, and finally locking the large number of crystal nuclei formed by rapid cooling into fine and dispersed stable precipitates.

[0025] The second aspect of the present application provides an Inconel 718 alloy ingot obtained by the above method for improving the as-cast solidification quality of Inconel 718 high-temperature alloy.

[0026] Compared with the prior art, the present application has the following advantages and beneficial effects:

[0027] 1. By using a specific component of a nickel-magnesium-molybdenum ternary intermediate alloy as an additive, the mutual solubility of nickel and magnesium is used to effectively encapsulate and stabilize the active magnesium element, and the molybdenum element is used to increase the alloy density, ensuring that it can quickly sink after being put into the melt, greatly inhibiting the oxidation and volatilization loss of magnesium. This design makes the magnesium recovery rate reach up to 46.25%, and can stably and accurately control the magnesium content in the alloy within the optimal range of 0.0040%-0.080%, providing a reliable solution to the industry problem of difficult control of trace element addition.

[0028] 2. The present application introduces the physical regulation steps of short-time high-temperature purification and step cooling in the key temperature interval. The ultra-high temperature treatment effectively dissolves the heterogeneous nucleation core in the melt, creating clean and activated initial conditions for subsequent solidification. The step cooling is implemented by rapid cooling + isothermal holding in the key interval of large-scale precipitation of Laves phase and MC carbide, which greatly increases the nucleation rate and inhibits the coarsening of the precipitate. The synergistic effect of the two makes the primary dendrite arm spacing (PDAS) significantly refined, and promotes the harmful Laves phase and MC carbide to change from coarse and discontinuous network distribution to fine and dispersed granular distribution, so that a uniform and refined high-quality solidification structure is obtained in the as-cast state.

[0029] 3. Because this invention achieves homogenization and refinement of the microstructure during the as-cast stage, it eliminates the need for the high-temperature, long-duration homogenization and diffusion annealing process required in traditional methods to eliminate segregation. This not only significantly shortens the production cycle but also substantially reduces energy consumption and production costs. Furthermore, the method has clear steps and well-defined parameters, making it easy to implement on existing smelting equipment and possessing excellent prospects for industrial application. It provides an efficient, economical, and reliable new method for preparing high-performance Inconel 718 alloy ingots. Attached Figure Description

[0030] Figure 1 The images show the as-cast microstructure of the Inconel 718 alloy ingots obtained in Comparative Example 1 and Examples 1-4 of this invention; wherein, (a) is the as-cast microstructure of Comparative Example 1, (b) is the as-cast microstructure of Example 1, (c) is the as-cast microstructure of Example 2, (d) is the as-cast microstructure of Example 3, and (e) is the as-cast microstructure of Example 4.

[0031] Figure 2 These are metallographic photographs of the as-cast microstructure of the Inconel 718 alloy ingots obtained in Comparative Examples 2-7 of the present invention; wherein, (a) is a metallographic photograph of the as-cast microstructure of Comparative Example 2, (b) is a metallographic photograph of the as-cast microstructure of Comparative Example 3, (c) is a metallographic photograph of the as-cast microstructure of Comparative Example 4, (d) is a metallographic photograph of the as-cast microstructure of Comparative Example 5, (e) is a metallographic photograph of the as-cast microstructure of Comparative Example 6, and (f) is a metallographic photograph of the as-cast microstructure of Comparative Example 7.

[0032] Figure 3 This is a schematic diagram showing the statistical results of the primary dendrite arm spacing of the Inconel 718 alloy ingots obtained in Examples 1-4 and Comparative Examples 1-7 of the present invention. Detailed Implementation

[0033] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] Example 1

[0035] This embodiment provides a method for improving the solidification quality of Inconel 718 high-temperature alloy in the as-cast state, including the following steps:

[0036] (1) Take the vacuum induction melted Inconel718 alloy ingot, wire cut into Φ50x35mm cylindrical sample, weight about 560 g, and polish the sample surface to remove the oxide skin. Then, the treated sample is placed in a high-purity MgO crucible, and the MgO crucible is nested in a larger graphite crucible, and finally the whole crucible assembly is placed in the heating zone of the MoSi2 resistance furnace, the furnace body is closed, and then the power is turned on to start heating, and the temperature in the furnace is raised to 450℃.

[0037] (2) Continue to pass high-purity argon gas as a protective atmosphere into the furnace, and control the gas flow to be 3 L / min. Under the protection of argon, continue to heat to 1500℃, and keep the temperature at this temperature for 10 minutes to completely melt the alloy sample. Then, the melt temperature is further increased by 80℃ based on 1500℃, reaching 1580℃, and kept at this temperature for 2 minutes. Then, the melt temperature is lowered to 1500℃ and stabilized to obtain clean and activated molten alloy.

[0038] (3) Put 1.74 g of nickel-magnesium-molybdenum intermediate alloy into the molten alloy obtained in step (2) through a dedicated feeding pipe. The specific mass percentage composition of the intermediate alloy is: nickel 85%, magnesium 8%, molybdenum 7%. After pouring, continue to keep the temperature at 1500℃ for 5 minutes.

[0039] (4) After magnesium alloying, multi-stage step cooling treatment is carried out. First, the melt temperature is rapidly cooled from 1500℃ to 1360℃ at a cooling rate of 30℃ / min, then kept at this temperature of 1360℃ for 5 minutes; after the end of the heat preservation, the heating power of the resistance furnace is turned off, and the alloy melt is naturally cooled with the furnace under argon protection.

[0040] (5) When the temperature in the furnace drops to 900℃, the high-purity argon is turned off, and the furnace is taken out to obtain the Inconel718 alloy ingot of this embodiment.

[0041] Example 2

[0042] This embodiment provides a method for improving the as-cast solidification quality of Inconel718 high-temperature alloy, which is different from example 1 in that the mass of nickel-magnesium-molybdenum intermediate alloy poured into the molten alloy in step (3) is 2.93 g.

[0043] Example 3

[0044] This embodiment provides a method for improving the as-cast solidification quality of Inconel718 high-temperature alloy, which is different from example 1 in that the mass of nickel-magnesium-molybdenum intermediate alloy poured into the molten alloy in step (3) is 5.82 g.

[0045] Example 4

[0046] The embodiment provides a method for improving the solidification quality of a cast Inconel 718 high-temperature alloy, and the difference from the embodiment 1 is that the mass of the nickel-magnesium-molybdenum intermediate alloy put into the molten alloy in step (3) is 11.65 g.

[0047] Comparative example 1

[0048] The difference between the comparative example and the embodiment 1 is that step (3) is omitted, that is, no nickel-magnesium-molybdenum intermediate alloy is added to the molten alloy.

[0049] Comparative example 2

[0050] The difference between the comparative example and the embodiment 1 is that the process of heating to 1580 DEG C in step (2) is omitted. That is, after the alloy is completely melted at 1500 DEG C and kept for 10 minutes, step (3) is directly performed.

[0051] Comparative example 3

[0052] The difference between the comparative example and the embodiment 1 is that the holding time at 1580 DEG C in step (2) is extended to 10 minutes.

[0053] Comparative example 4

[0054] The difference between the comparative example and the embodiment 1 is that the nickel-magnesium-molybdenum intermediate alloy in step (3) is replaced by a nickel-magnesium binary alloy, and the specific mass percentage composition is: nickel 92%, magnesium 8%.

[0055] Comparative example 5

[0056] The difference between the comparative example and the embodiment 1 is that the nickel-magnesium-molybdenum intermediate alloy in step (3) is replaced by a nickel-magnesium-chromium alloy, and the specific mass percentage composition is: nickel 85%, magnesium 8%, chromium 7%.

[0057] Comparative example 6

[0058] The difference between the comparative example and the embodiment 1 is that the specific mass percentage composition of the nickel-magnesium-molybdenum intermediate alloy in step (3) is: nickel 85%, magnesium 12%, molybdenum 3%.

[0059] Comparative example 7

[0060] The difference between the comparative example and the embodiment 1 is that after the magnesium alloying is completed in step (4), the heating power of the resistance furnace is directly turned off, and the alloy melt is naturally cooled in the furnace under the protection of argon.

[0061] Performance test

[0062] Test sample: Inconel 718 alloy ingots obtained from the embodiments 1-4 and the comparative examples 1-7.

[0063] Test method:

[0064] 1. Magnesium content detection: Inductively coupled plasma atomic emission spectrometry (ICP-AES) was used to quantitatively analyze the magnesium element in the alloy ingot. The sample was taken from the center of the ingot, and the average value was obtained by repeating the measurement three times.

[0065] 2. Chemical composition analysis: Spark direct-reading spectroscopy was used to detect the full element composition of the alloy ingot, including C, Si, Mn, P, S, Al, Ti, Cr, Mo, Nb, Ni, Mg, Fe, etc. After polishing, the sample was measured at multiple points to obtain the average value.

[0066] 3. Quantitative characterization of precipitated phase composition: Scanning electron microscopy (SEM) equipped with an energy dispersive spectrometer (EDS) was used to analyze the MC carbide and Laves phase in the ingot sample. Ten points were analyzed for each phase, and the average mass percentage of elements was calculated.

[0067] 4. Observation of precipitated phase distribution morphology: Metallographic microscope and scanning electron microscope (SEM) were used to observe the as-cast structure of the ingot sample, focusing on the size, shape and distribution of Laves phase and MC carbide.

[0068] 5. Primary dendrite arm spacing measurement: On the metallographic photograph, at least 50 primary dendrite arm spacings were measured using image analysis software, and the average value and standard deviation were calculated to evaluate the refinement degree of the solidification structure.

[0069] The test results are as follows:

[0070] 1. Magnesium content detection results

[0071] The magnesium content (mass fraction) and magnesium yield calculation results are shown in Table 1. The yield is calculated according to formula (1):

[0072] (1)

[0073] Table 1 Magnesium content (mass fraction / %) and yield / % in alloy ingot

[0074]

[0075] Comparative Example 2 omitted the high temperature process, due to insufficient melt purification, more heterogeneous cores, and reduced magnesium yield; Comparative Example 3 had high temperature and long time holding, resulting in magnesium loss and crucible erosion due to excessive heat exposure, further reducing the yield; Comparative Examples 4 and 5 used alternative alloys, and the magnesium yield was significantly reduced due to the poor physical and chemical properties of the intermediate alloy; Comparative Example 6 used an unstable alloy, and the magnesium was severely oxidized, resulting in extremely low yield.

[0076] 2. Chemical composition analysis results

[0077] The chemical composition of the alloy ingot (mass fraction / %) is shown in Table 2.

[0078] Table 2 Chemical composition of the alloy ingot (mass fraction / %)

[0079]

[0080] 3. Quantitative characterization results of precipitate phase composition

[0081] The element composition (mass fraction / %) of the precipitate phase (MC carbide and Laves phase) is shown in Table 3.

[0082] Table 3 Quantitative characterization of composition change of precipitate phase (mass fraction / %)

[0083]

[0084] The analysis of the data in Table 3 shows that the present application effectively controls the key element composition of MC carbide and Laves phase through trace magnesium alloying treatment. With the optimization of magnesium content, the Ti content in the MC phase is significantly improved, and the Ni content in the Laves phase is moderately enriched, which helps to improve the phase stability and distribution. In contrast, the composition of the precipitate phase of each comparative example is either retrograded to the untreated state or presents unhelpful fluctuations due to defects in the process, and fails to achieve similar optimization effect.

[0085] 4. Observation results of precipitate phase distribution morphology

[0086] Figures 1-2 as shown. Figure 1 (a)-(e) are metallographic photos of the as-cast microstructure of the Inconel718 alloy ingots obtained in Comparative Example 1 and Examples 1-4 (the scale is 100 μm). It can be seen that Comparative Example 1 (a): the precipitate phase is discrete, sparse, and irregular in shape, and is scattered in the matrix. Example 1 (b): the number of precipitate phases increases, and begins to show a local aggregation trend, with higher distribution density. Example 2 (c): the aggregation of precipitate phase is more obvious, and larger size agglomerates appear. Example 3 (d): the distribution of precipitate phase tends to be uniform, and the agglomeration phenomenon is alleviated, with fine particles or short strips uniformly dispersed. Example 4 (e): the precipitate phase is uniformly distributed, fine and continuous, with fine size and good dispersion.

[0087] Figure 2(a)-(f) are metallographic photos of the as-cast microstructure of Inconel 718 alloy ingots obtained in Comparative Examples 2-7 (the scale is 100 μm). It can be seen that, in Comparative Example 2 (a), the precipitated phases are unevenly distributed, and there are local coarse agglomerations, and the degree of refinement is lower compared with Example 1. In Comparative Example 3 (b), the precipitated phases are coarse and unevenly distributed, and the phases are possibly coarsened due to excessive holding. In Comparative Example 4 (c), the distribution of the precipitated phases is only slightly improved, and the phase morphology is irregular and the aggregation phenomenon is obvious. In Comparative Example 5 (d), the precipitated phases are unevenly distributed, and the phase size is large, and the refinement effect is poor compared with Example 1. In Comparative Example 6 (e), the precipitated phases are sparse and discrete, and are similar to Comparative Example 1, and the control effect is weak due to the extremely low magnesium yield. In Comparative Example 7 (f), the precipitated phases are coarse and unevenly distributed, and the interdendritic segregation is serious, and the refinement effect of the step cooling is lacking.

[0088] 5. Primary dendrite arm spacing statistical results

[0089] The primary dendrite arm spacing measurement results are shown in Table 5. Figure 3 . Figure 3 It is shown that the Mg element has a significant refinement effect on the solidification dendritic structure of the Inconel 718 alloy, and the refinement effect is significant in the range of 0%-0.02% Mg content, and there is a refinement saturation interval.

[0090] The above describes the preferred embodiments of the present application, and it should be noted that, for those skilled in the art, without departing from the principles of the present application, a number of improvements and refinements can be made, and these improvements and refinements should also be considered as the protection scope of the present application.

Claims

1. A method for improving the solidification quality of Inconel 718 superalloy in the as-cast state, comprising the following steps: (1) Place the Inconel 718 alloy sample in an MgO crucible and nest it in a graphite crucible, and heat it to 400-500℃; (2) Introduce argon gas and continue to heat to 1450℃-1500℃, hold the temperature, and let the alloy sample melt completely; then, further increase the melt temperature to 1550-1600℃, hold the temperature at this temperature for 1-3 minutes, and then lower the temperature to 1450℃-1500℃ to obtain the molten alloy. (3) Add nickel-magnesium-molybdenum alloy to the molten alloy and keep it warm; the composition of the nickel-magnesium-molybdenum alloy by mass percentage is: nickel 80%-90%, magnesium 5%-10%, molybdenum 5%-10%; after adding the nickel-magnesium-molybdenum alloy, control the magnesium content in the alloy to be within the range of 0.0040%-0.080%; (4) After the heat preservation in step (3) is completed, the temperature of the melt is first cooled from the heat preservation temperature to 1350℃-1380℃ at a cooling rate of 20-50℃ / min. After heat preservation, the heating power is turned off and the alloy melt is allowed to cool naturally with the furnace. (5) When the temperature drops to 850-950℃, turn off the argon gas and take out the Inconel718 alloy ingot.

2. The method for improving the as-cast solidification quality of Inconel 718 high-temperature alloy according to claim 1, characterized in that, Step (2) After heating to 1450℃-1500℃, the holding time is 8-15 minutes.

3. The method for improving the as-cast solidification quality of Inconel 718 high-temperature alloy according to claim 1, characterized in that, The composition of the nickel-magnesium-molybdenum alloy in step (3) by mass percentage is: nickel 85%-88%, magnesium 7%-9%, and molybdenum 5%-8%.

4. The method for improving the as-cast solidification quality of Inconel 718 high-temperature alloy according to claim 1, characterized in that, The heat preservation time in step (3) is 3-8 minutes.

5. The method for improving the as-cast solidification quality of Inconel 718 high-temperature alloy according to claim 1, characterized in that, Step (4) Keep warm for 3-8 minutes in the temperature range of 1350℃-1380℃.

6. An Inconel 718 alloy ingot, characterized in that, Obtained by the method described in any one of claims 1-5.

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