WO2017014675A1

WO2017014675A1 - A method for obtaining electrodes from alloys based on nickel aluminide

Info

Publication number: WO2017014675A1
Application number: PCT/RU2016/000450
Authority: WO
Inventors: Evgeny Aleksandrovich Levashov; Yury Sergeevich POGOZHEV; Zhanna Aleksandrovna SENTYURINA; Aleksandr Anatolevich ZAITSEV; Vladimir Nikolaevich SANIN; Vladimir Isaakovich YUKHVID; Dmitry Evgenevich ANDREEV; Denis Mihajlovich IKORNIKOV
Original assignee: National University Of Science And Technology "Misis"
Priority date: 2015-07-23
Filing date: 2016-07-19
Publication date: 2017-01-26
Also published as: RU2607857C1; EA201800040A1; EA035488B1; CN107848034B; CN107848034A

Abstract

The invention relates to obtaining cast electrodes from high alloys based on nickel aluminide and may be used in additive 3D technologies in order to obtain geometrically complicated articles. The method includes obtainment of the semi-finished product through the centrifugal SHS casting with using the reaction mixture containing, wt%: Nickel oxide 47.0-49.1, Aluminum 28.6-32.4, Alloying additive 13.1-17.9, Functional additive 6.5-7.0; a two-stage remelt of the semi-finished with obtaining the refined degassed ingot during the first stage and the electrode from the nanomodified alloy during the second stage, wherein during the said second stage some amount of the master alloy that consists of pressed aluminum and nanopowder mixture with a specific surface area of 5÷30 m2/g and lump aluminum is introduced into the melt before pouring the latter into the crystallizer to ensure the nanopowder content of 0.5-7 vol % in the melt; with its further cooling down to the room temperature and extraction from the crystallizer. The invention is directed to the development of the integrated technology of electrodes made of alloys based on nickel aluminide.

Description

A Method for Obtaining Electrodes from Alloys Based on Nickel

Aluminide

Field of the Invention. The invention relates to the area of special metallurgy, in particular, to obtaining cast electrodes from high alloys based on nickel aluminides and may be used for centrifugal atomization of the electrode material and obtaining the granules for application in additive 3D technologies in order to obtain geometrically complicated articles from heat-resistant metallic materials.

Prior Art. A method is known (RU 2032496, published on April 10, 1995) for obtaining aluminides of transition metals, preferably nickel, tantalum, titanium, niobium, and iron, that includes preparation of the exothermic mixture of the transition metal and aluminum powders, briquetting the mixture, heating the briquettes to initiate the self-propagating high-temperature synthesis (SHS) reaction and further hot deformation of the synthesis products.

The high energy consumption, high cost of the initial reagent metal powders, highly required purity of the initial powders in terms of contaminants: oxygen, nitrogen, carbon, and others, which are often difficult to implement, are disadvantages of the method.

A method is known (RU 2523049, published on July 20, 2014) for obtaining cast alloy based on gamma titanium aluminide intended for obtaining shaped castings, that includes obtainment of the mixture from pure metal powders that comprise titanium, aluminum, and niobium; production of the briquette with a relative density of 50-85 %; thermovacuum processing of the briquette at a temperature of 550-650 °C for 10-40 min., a heating rate of 5-40 °C/min., and a pressure of 10^_1-10^"3 Pa, and the SHS is carried out at an initial temperature of 560- 650 °C.

A method is known for obtainment of heat-resistant alloys (RU 2534325, published on November 27, 2014), which includes preparing the reaction mixture of the initial component powders that comprises oxides of nickel, cobalt, chrome III, molybdenum, titanium, pure aluminum, as well as carbon, boron, and zirconium; placing the reaction mixture into a refractory mold, placing the mold onto a centrifuge, igniting the mixture and then conducting the synthesis in combustion mode at a centrifugal acceleration of 200-3 OOg, with further separating the as-cast alloy based on nickel aluminides, and the mixture is prepared with the following proportion of the components (wt%): nickel oxide (40.0-43.7), cobalt oxide (12.0-13.2), chrome oxide (2.9-4.3), molybdenum oxide (3.1-3.9), titanium oxide (1.3-2.4), carbon, boron, and zirconium.

But the disadvantage is that it does not allow obtaining long electrodes with required geometry from the nanomodified alloy.

The analogous solution closest to what is elaborated herein is the method (CN 100497700 C, published on June 10, 2009) for obtaining electrodes from alloys based on nickel aluminides that includes a multi-stage remelting of the alloy components (Ni, Al, Cr, Mo, Ta) prone to liquation, the degassed ingot is obtained during the first, refining stage, and the electrode homogenized in terms of its chemical composition is produced at the following ones. The said remelt is carried out in protective inert atmosphere or vacuum.

High energy consumption associated with the multiple remelt stages (the substances are remelted from 3 to 6 times) and heightened requirements to chemical purity of the initial components (the required purity of the initial metals is approximately 99.999 %) in terms of contaminants, which make the process and the product significantly more expensive, while there is no possibility to obtain the electrodes with a nanomodified structure, are disadvantages of this method.

Disclosure of the Invention. The technical result of the announced invention consists in the reduction of energy consumption and the cost reduction due to decreasing the number of remelts and applying only two of them and by using less expensive oxide raw stock, while simultaneously ensuring chemical purity of the resultant electrode in terms of the contaminant content, namely: oxygen less than 0.2 %, nitrogen less than 0.01 %, carbon less than 0.1 %. Furthermore, the technical result consists in increasing the obtained electrode's thermal stability due to the smaller grain size of the main NiAl phase of the electrode material obtained through nanomodification of the material.

The technical result of the invention announced herein is achieved as follows.

The method for obtaining the electrodes from alloys based on nickel aluminide includes obtainment of the semi-finished product through the centrifugal SHS casting with using the reaction mixture that contains the following proportion of the components, wt%:

Nickel oxide 47.0 - 49.1

Aluminum 28.6 - 32.4

Alloying additive 13.1 - 17.9

Functional additive 6.5 - 7.0.

Then a two-stage remelt of the semi-finished product is carried out; the refined degassed ingot is produced during the first stage. The electrode from the nanomodified alloy is obtained on the second stage. During the said second stage, some amount of master alloy that consists of pressed aluminum and nanopowder mixture with a specific surface area of 5÷30 m /g and lump aluminum is introduced into the melt 2-3 minutes before pouring the latter into the crystallizer, to provide the nanopowder content of 0.5-7 vol % in the melt. After that the melt cools down to the room temperature and extracted from the crystallizer.

The centrifugal SHS casting is carried out by placing the reaction mixture into a refractory mold covered inside with a functional protective layer made of refractory inorganic compound, then placing the mold onto a centrifuge, igniting the mixture, conducting the SHS process at a centrifugal acceleration of 60 (±10) g, and separating the synthesized as-cast intermetallic alloy from the slag.

M0O₃, Cr₂0₃, Co₃0₄, Hf and B components are used as the alloying additive to the reaction mixture. AI2O3 and Na₃AlF₆As are used as the functional additive to the reaction mixture.

The semi-finished product is remelted in two stages in protective inert atmosphere or vacuum.

WC or TaC or NbC or Zr0₂ or Y₂0₃ or A1₂0₃ powder is used as the nanopowder.

Embodiments of the Invention. The invention is realized as follows.

The stage of synthesizing the cast semi-finished product through the centrifugal SHS casting methods is carried out by preparing the reaction mixture of aluminum, nickel oxide together with the alloying and functional additives. The mixture is loaded into the refractory mold covered inside with the functional protective layer made of refractory inorganic compound; the mold is placed onto the centrifuge rotor, the mixture is ignited, and the synthesis is carried out in the combustion mode at a centrifugal acceleration of 60 (±10) g. The reaction mixture is prepared with the following proportion of the components, wt%: nickel oxide - 47.0-49.1 ; aluminum - 28.6-32.4; alloying additive - 13.1-17.9; functional additive - 6.5-7.0. At least 4 components from the series: Cr₂0₃ and Hf and B and Co₃0₄ and/or M0O₃ are used as the alloying additive.

Furthermore, the functional A1₂0₃ and Na₃AlF₆ additives with a total content of not more than 7.0 wt% are introduced into the composition of the initial exothermic mixture.

The further stage of processing the semi-finished product includes a two- stage induction remelting in protective inert environment or vacuum. During the first stage, the ingot is refined and degassed. During the second stage, the alloy is nanomodified by introducing some amount of master alloy into the melt (the master alloy consists of pressed aluminum and nanopowder mixture with a specific surface area of 5÷30 m²/g and of lump aluminum) that ensures 0.5-7 vol % of the nanopowder in the melt 2-3 minutes before pouring the melt and casting it into the crystallizer of a preset geometry. After that the melt is cooled to the room temperature and extracted from the crystallizer.

Reduction of the cost, decrease in the number of the remelts down to two, and use of the less expensive oxide raw stock with simultaneously ensuring the chemical purity of the obtained electrode in terms of the contaminants content, namely: oxygen less than 0.2 %, nitrogen less than 0.01 %, carbon less than 0.1 %, is achieved due to using the energy-saving SHS casting technology, which allows synthesizing, in the combustion mode, the highly pure semi-finished product in terms of the contaminants, without liquation, from which it is possible to further obtain the electrode with a required structure and purity during a single remelt. Thermal stability of the obtained electrode is increased by introducing the optimal amount of WC, Zr0₂, and Y₂0₃ nanoparticles, which pulverize the main NiAl phase grain significantly (2-3 times).

The technical problem is solved by selecting the initial mixture that includes a high content of Al, nickel oxide, and alloying additives (Cr₂0₃ and Hf and B and C03O4 and/or M0O₃), introducing the functional A1₂0₃ H Na₃AlF₆ additives into the mixture in order to regulate the slag phase viscosity, setting a centrifugal acceleration of 60 (±10) g, which allows obtaining high heat-resistant alloys based on nickel aluminides without liquation. The further two-stage re-melt makes it possible to reduce the content of the gaseous contaminants down to values not exceeding 0.4 %, to nanomodify the melt by introducing the master alloy with nanosized particles into the melt and to form the long electrodes by pouring the melt into the crystallizer with a preset geometry.

Introduction of the alloying additive makes it possible to ensure:

1) a solid-solution (Co, Mo) reinforcement of the matrix material (NiAl- based);

2) a composite reinforcement of the matrix by the precipitations based on the complex boride Ni₂₀Al₃B₆ compound and CrB with partial substitution of Ni and Cr by Mo; 3) that Hf and B components are the structural modifiers and positively affect formation of the fine-grained and non-liquated structure of the devised compositions.

If the content of the mixture components falls within the specified range and the g- force acceleration value is 60 (±10) g, the non-porous ingots are formed with their structural components evenly distributed within the volume.

The choice of a centrifugal acceleration of 60 (±10) g is conditioned by optimization of the synthesis process aimed at increasing the mass of the synthesized ingot as much as possible. The range of the g-force acceleration values is conditioned by the cumulative effect aimed at reaching the maximal possible combustion volume (taking into account the centrifugal device's characteristics) and the maximal output (phase separation depth) of the target phase (metal) to the ingot.

If there is a deviation from the preset component content ranges: NiO (47.0- 49.1), Al (28.6-32.4), the alloying additive (13.1-17.9), the functional additive (6.5-7.0), and the g-force acceleration effect is lower (50g), discontinuity forms in the synthesized ingots, which is observed both on macro and micro level.

If the total content of the alloying additive exceeds 17.9 % (Example 7, Table 2), multiple plate-like precipitations of complex Mo(V, Cr)B monoborides form within the alloy structure, as well as corundum-based ceramic inclusions, which reduces plasticity and scale resistance of the obtained materials. If the alloying additive content is less than 13.1 % (Example 6, Table 2), a highly brittle alloy forms, which may not be used in the two-stage re-melt of the electrodes for centrifugal atomization of the granules.

The complex effect of the alloying and functional additives, as well as the optimal choice of the centrifugal effect value range (60 ± lOg) for the synthesis process ensures the maximum output of the target product (alloy) into the ingot and formation of the structure without liquation. If the composition and g-force acceleration effect are chosen non-optimally (Examples 6, 7, Table 3), an abrupt reduction of the phase separation depth is observed (down to 86-82 %), which significantly impairs the efficiency of the semi-finished product obtainment method announced herein.

When introducing the master alloy that consists of pressed aluminum and nanopowder mixture, less than 2 minutes before pouring the melt into the crystallizer, the nanoparticles do not have enough time to distribute evenly within the melt volume, which leads to the lack of the ingot's structural homogeneity and high variation of the grain size values. If the nanoparticles stay in the melt for more than 3 minutes before pouring it into the crystallizer, the WC, TaC, and NbC carbide nanoparticles dissolve and the Zr0₂, Y2O3, Al₂0₃ ones agglomerate together, which does not result in the required melt structure modification either.

If the nanopowder concentration is less than 0.5 vol % of the melt, no marked modification of the ingot structure takes place; and it is not reasonable to increase the nanopowder concentration so that it exceeds 7 vol %, because this increases the melt viscosity, impairs its fluidity, but the structural component grains are not further pulverized.

Also, the range of the nanopowder's specific surface area values of 5-30 m /g is justified experimentally. If the specific surface area is less than 5 m /g, the submicron-sized powders have no significant modifying effect on the ingot structure. The selection of the nanodispersed powder with a specific surface area of more than 30 m /g results in disadvantages for a number of reasons: the oxide compound nanoparticles agglomerate together and the carbide compound particles dissolve in the melt.

The two-stage remelt of the semi-finished product is carried out in protective inert atmosphere or vacuum in order to prevent oxidation of the melt and increase the furnace assemblies' service life.

Examples to justify the centrifugal SHS casting and remelt modes are aggregated in Tables 1 to 9. E x a m p l e 1 (Table 2. Example 1).

In order to obtain the cast semi-finished product, the following components are taken: powders, nickel oxide, molybdenum oxide, chrome oxide, cobalt oxide, aluminum oxide, aluminum, hafnium, and boron. The main characteristics of the reagents are provided by Table 1.

Table 1. Characteristics of the initial substances and functional additives

The reaction mixture is prepared with the following proportion of the components, wt%: nickel oxide - 47.5, aluminum - 32.4, alloying additive - 13.1, functional additive - 7.0. The following powders are used as the alloying additive, wt%: M0O₃ - 0.6, Cr₂0₃ - 5.4, C03O4 - 5.7, Hf - 1.3, B - 0.1, A1₂0₃ (6.4 wt%) and Na₃AlF₆ (0.6 wt%) powders are used as the functional additive.

The ready-to-use mixture in placed into the graphite mold covered inside with the protective fireproof layer made of refractory corundum-based inorganic compound. The mold is placed onto the centrifuge rotor, the mixture is ignited locally with the tungsten coil, and the synthesis is carried out in the combustion mode at a centrifugal acceleration of 70g. After the SHS process ends, the product cools down and is extracted from the mold. The product is a double-layer ingot: the upper layer is corundum-based oxide solution (slag), the bottom layer (target product) is the heat-resistant alloy based on nickel aluminides. The output of the target product (the alloy based on nickel aluminides) is 98 % of the calculated value. The main NiAl phase grain size amounts to 10-20 μπι. The synthesized alloy comprises, wt%: nickel - 58.8, aluminum - 27.0, molybdenum - 0.7, chrome - 5.8, cobalt - 6.6, boron - 0.1 , hafnium - 1.0. The gaseous contaminants content is, wt%: oxygen - 0.110, nitrogen - 0.0012, carbon - 0.078. The main NiAl phase grain size amounts to 10-20 μιιι.

The semi-finished product is processed by re-melting it in two stages within protective inert atmosphere. During the first stage of the remelt, the semi-finished product is remelted for refinement in an induction furnace by melting it in the periclase crucible at a temperature of 1680-1700 °C within high-purity grade argon atmosphere (99.995 % Ar) that fills the induction furnace chamber after pumping out down to the diffusion vacuum (10^~5 Pa), at a pressure of 0.95 x lO⁵ Pa. The induction heating rate is 150 ± 30 °C/min. In order to remove the gaseous contaminants, the obtained melt is kept at a temperature of 1680-1700 °C for 3 minutes. The inductor being switched on, the obtained melt is poured into the graphite crucible with a diameter of 50-100 mm, which has been installed in the furnace chamber in advance and where the ingot crystallizes. When the pouring is finished, the inductor is switched off. The obtained ingot from the NiAl-based heat-resistant alloy cools down in the induction furnace chamber within the argon atmosphere for 3-5 hours.

During the second remelting stage, the homogenizing induction remelt of the obtained ingot is carried out, with an additional alloying by lump A99 grade aluminum (in order to compensate the aluminum evaporated during the refining remelt), and master aluminum-based powder alloys with nanosized WC particles. The master alloys are added to the melt through a vacuum gate in the furnace chamber, and their amount ensures 1 vol % of the nanopowder and 26.3 ± 0.5 % of aluminum in the alloy composition. The mixtures for making the master alloy are obtained in a planetary ball mill with a gravity factor of at least 90g by mixing ΠΑ-4 grade aluminum powder with the nanosized particles keeping the 3: 1 weight ratio, the diameter of the milling agents is 3-5 mm, the balls-to-material weight proportion is 10: 1, the processing time is 5 minutes. The compact master alloy powder is obtained through cold pressing in a steel press mold with a diameter of 20-50 mm at a load of 3-5 t/cm², which ensures the relative density at a level of 0.7-0.9.

The remelt is carried out under the following conditions: the Ar pressure is 0.95 x 10⁵ Pa, the temperature is 1680-1700 °C and the heating rate is 150 ± 30 °C/min. For the obtained melt to be homogenized, it is kept at a temperature of 1680-1700 °C for 2 minutes, which ensures the even distribution of the nanomodifier within the alloy volume. The inductor being switched on, the obtained melt is poured into the graphite crucible with a diameter of 50-100 mm that has its thermally insulated sinkhead as high as 15-25 % of the electrode height. The obtained electrode cools down in the induction furnace chamber within the argon atmosphere for 3-5 hours. After the cooling, the electrode is extracted from the container, the surface is cleaned in order to remove the casting mold residues, and the sinkhead is cut off.

The obtained electrode comprises, wt% (Table 4): nickel - 57.0; aluminum - 26.5; molybdenum - 0.7; chrome - 5.6; cobalt - 6.4; boron - 0.1 ; hafnium - 1 ; WC nanophase - 2.66.

The obtained electrode is tested for heat resistance through the following methodology. The electrode is placed into a muffle furnace preheated up to a temperature of 1000 °C, kept inside for 20 minutes, then retrieved from the furnace onto a chamotte-lined surface, where it cools down in the air down to the room temperature. After that, the electrode is kept in the furnace at 1000 °C for 20 minutes once more; then the electrode is extracted f om the furnace and cools in the air down to the room temperature. The thermal stability tests through the heating-and-cooling cycle are carried out until slip cracks are detected. More than 10 cycles are considered satisfactory, which makes it possible to prognosticate a sufficient stability against the thermal shock during plasma centrifugal atomization.

The contaminants content is: oxygen - 0.132 %, nitrogen - 0.006 %, carbon - 0.082 %. The main NiAl phase grain size is 40-50 μπι, the residual porosity is 0.5 %, the thermal stability equaled 22 cycles, no blebs and slip cracks have been detected.

E x a m p 1 e 2 (Table 2, Example 3).

In order to synthesize the cast semi-finished product (similarly to Example 1), the reaction mixture is prepared with the following component proportion, wt%: nickel oxide - 49.1 ; aluminum - 30.5; alloying additive - 13.6; functional additive - 6.8. The following powders are used as the alloying additive, wt%: Mo0₃ - 5.8, Cr₂0₃ - 3.6, Co₃0₄ - 2.7, Hf - 1.2, B - 0.3, A1₂0₃ (5.0 wt%) and Na₃AlF₆ (1.8 wt%) powders are used as the functional additive.

The ready-to-use mixture in placed into the graphite mold covered inside with the functional protective layer made of refractory corundum-based inorganic compound. The mold is placed onto the centrifuge rotor, the mixture is ignited locally with the tungsten coil, and the synthesis is carried out in the combustion mode at a centrifugal acceleration of 60g.

After the combustion process ends, the synthesis product cools down and is extracted from the mold. The combustion product is a double-layer ingot: the upper layer is corundum-based oxide solution (slag), the bottom layer (target product) is the heat-resistant alloy based on nickel aluminides. The output of the target product (the alloy based on nickel aluminides) is 95 % of the calculated value. The synthesized alloy comprises, wt%: nickel - 62.0; aluminum - 23.3; molybdenum - 6.2; chrome - 3.9; cobalt - 3.2; boron - 0.4; hafnium - 1.0. The gaseous contaminants content is, wt%: oxygen - 0.130, nitrogen - 0.0013, carbon - 0.085. The main NiAl phase grain size amounts to 30-40 μηι. The semi-finished product is processed (similarly to Example 1) by remelting it in two stages within protective inert atmosphere. During the first stage, the semi-finished product is remelted for refinement in the induction furnace by melting it in the periclase crucible at a temperature of 1680-1700 °C within high- purity grade argon atmosphere (99.995 % Ar) that fills the induction furnace chamber after pumping out down to the diffusion vacuum (10^"5 Pa), at a pressure of 0.95x 10^s Pa. The induction heating rate is 150 ± 30 °C/min. In order to remove the gaseous contaminants, the obtained melt is kept at a temperature of 1680-1700 °C for 3 minutes. The inductor being switched on, the obtained melt is poured into the graphite crucible with a diameter of 50-100 mm, which has been installed in the furnace chamber in advance and where the ingot crystallizes. When the pouring is finished, the inductor is switched off. The obtained ingot from the NiAl-based heat-resistant alloy cools down in the induction furnace chamber within the argon atmosphere for 3-5 hours.

During the second stage, the homogenizing induction re-melt of the obtained ingot is carried out, with an additional alloying by lump A99 grade aluminum (in order to compensate the aluminum evaporated during the refining remelt), and aluminum-based master alloy powders with nanosized Zr0₂ particles. The master alloys are added to the melt through the vacuum gate in the furnace chamber, and their amount ensures 3 v/v % of the nanopowder and 26.1 ± 0.5 % of aluminum in the alloy composition. The mixtures for making the master alloy are obtained in the planetary ball mill with a gravity factor of at least 90g by mixing ΠΑ-4 grade aluminum powder with the nanosized particles keeping the 3:1 weight ratio, the diameter of the milling agents is 3-5 mm, the balls-to-material weight proportion is 10: 1, the processing time is 5 minutes. The compact master alloy powder is obtained through cold pressing in a steel press mold with a diameter of 20-50 mm at a load of 3-5 t/cm², which ensures the relative density at a level of 0.7-0.9. The remelt is carried out under the following conditions: the Ar pressure is 0.95 ^χ 10⁵ Pa, the temperature is 1680-1700 °C and the heating rate is 150 ± 30 °C/min. For the obtained melt to be homogenized, it is kept at a temperature of 1680- 1700 °C for 3 minutes, which ensures the even distribution of the nanomodifier within the alloy volume. The inductor being switched on, the obtained melt is poured into the graphite crucible with a diameter of 50-100 mm that has its thermally insulated sinkhead as high as 15-25 % of the electrode height. The obtained electrode cools down in the induction furnace chamber within the argon atmosphere for 3-5 hours. After the cooling, the electrode is extracted from the container, the surface is cleaned in order to remove the casting mold residues, and the sinkhead is cut off.

The obtained electrode comprises, wt%: nickel - 56.9; aluminum - 25.7; molybdenum - 6.1 ; chrome - 3.8; cobalt - 3.1 ; boron - 0.4; hafnium - 1 ; nanophase - 2.9.

The contaminants content is: oxygen - 0.987 %, nitrogen - 0.09 %, carbon - 0.121 %. The main NiAl phase grain size is 10-20 μπι, the residual porosity is 0.5 %, the thermal stability equals 18 cycles, no blebs and slip cracks have been detected.

E x a m p 1 e 3 (Table 2, Example 5 .

In order to synthesize the cast semi-finished product (similarly to Example 1), the reaction mixture is prepared with the following component proportion, wt%: nickel oxide - 47.0; aluminum - 28.6; alloying additive - 17.9; functional additive - 6.5. The following powders are used as the alloying additive, wt%: Mo0₃ - 12.4, Cr₂0₃ - 2.9, Co₃0₄ - 0.3, Hf - 1.1, B - 1.2, A1₂0₃ (3.5 wt%) and Na₃AlF₆ (3.0 wt%) powders are used as the functional additive.

The ready-to-use mixture in placed into the graphite mold covered inside with the functional protective layer made of refractory corundum-based inorganic compound. The mold is placed onto the centrifuge rotor, the mixture is ignited locally with the tungsten coil, and the synthesis is carried out in the combustion mode at a centrifugal acceleration of 50g. After the combustion process ends, the synthesis product is cooled and extracted from the mold. The combustion product is a double-layer ingot: the upper layer is corundum-based oxide solution (slag), the bottom layer (target product) is the heat-resistant alloy based on nickel aluminides. The output of the target product (the alloy based on nickel aluminides) is 94.0 % of the calculated value. The synthesized alloy comprises (Table 3), wt%: nickel - 61.4; aluminum - 16.6; molybdenum - 15.8; chrome - 3.2; cobalt - 0.3; boron - 1.7; hafnium - 1.0. The gaseous contaminant content is, wt%: oxygen - 0.17, nitrogen - 0.0017, carbon - 0.098. The main NiAl phase grain size amounts to 40-50 μηι.

The semi-finished product is processed (similarly to Example 1) by remelting it in two stages within protective inert atmosphere. During the first stage, the semi-finished product is remelted for refinement in the induction furnace by melting it in the periclase crucible at a temperature of 1680-1700 °C within high- purity grade argon atmosphere (99.995 % Ar) that fills the induction furnace chamber after pumping out down to the diffusion vacuum (10^"5 Pa), at a pressure of 0.95 x lO⁵ Pa. The induction heating rate is 150 ± 30 °C/min. In order to remove the gaseous contaminants, the obtained melt is kept at a temperature of 1680- 1700 °C for 3 minutes. The inductor being switched on, the obtained melt is poured into the graphite crucible with a diameter of 50-100 mm, which has been installed in the furnace chamber in advance and where the ingot crystallizes. When the pouring is finished, the inductor is switched off. The obtained ingot from the NiAl-based heat-resistant alloy cools down in the induction furnace chamber within the argon atmosphere for 3-5 hours.

During the second stage, the homogenizing induction remelt of the obtained ingot is carried out, with an additional alloying by lump A99 grade aluminum (in order to compensate the aluminum evaporated during the refining remelt) and aluminum-based master alloy powders with nanosized Y₂0₃ particles . The master alloys are added to the melt through the vacuum gate in the furnace chamber, and their amount ensures 5 vol % of the nanopowder and 25.4 ± 0.5 % of aluminum in the alloy composition. The mixtures for making the master alloy are obtained in the planetary ball mill with a gravity factor of at least 90g by mixing ΠΑ-4 grade aluminum powder with the nanosized particles keeping the 3 : 1 weight ratio, the diameter of the milling agents is 3-5 mm, the balls-to-material weight proportion is 10: 1, the processing time is 5 minutes. The compact master alloy powder is obtained through cold pressing in a steel press mold with a diameter of 20-50 mm at a load of 3-5 t/cm², which ensures the relative density at a level of 0.7-0.9. The remelt is carried out under the following conditions: the Ar pressure is 0.95 x 10⁵ Pa, the temperature is 1680-1700 °C and the heating rate is 150 ± 30 °C/min.

For the obtained melt to be homogenized, it is kept at a temperature of 1680- 1700 °C for 2,5 minutes, which ensures the even distribution of the nanomodifier within the alloy volume. The inductor being switched on, the obtained melt is poured into the graphite crucible with a diameter of 50-100 mm that has its thermally insulated sinkhead as high as 15-25 % of the electrode height. The obtained electrode cools down in the induction furnace chamber within the argon atmosphere for 3-5 hours. After the cooling, the electrode is extracted from the container, the surface is cleaned in order to remove the casting mold residues, and the sinkhead is cut off.

The obtained electrode comprises, wt%: nickel - 49.4; aluminum - 25.7; molybdenum - 14.6; chrome - 3.1 ; cobalt - 0.3; boron - 1.6; hafnium - 1 ; nanophase (Υ₂0₃) - 4.3.

The contaminants content is: oxygen - 0.974, nitrogen - 0.022, carbon - 0.096. The main NiAl phase grain size is 10-20 μιη, the residual porosity is 1.2 %, the thermal stability equals 14 cycles, no blebs and slip cracks have been detected.

The compositions and properties of the synthesized alloys for the examples presented by Table 2.

Table 4 gives the compositions and properties of the electrodes obtained through the two-stage remelt of the semi-finished SHS product for Example 1 of Table 3, when using the WC nanopowder with a specific surface area of 16 m²/g, the time before the pouring was 2.5 min. Table 4.

Examples within the limits of the WC nanomodifier's specified concentrations

Exa

The main

mple Content of

Composition of the modified electrode, % Thermal

NiAl main No. stability,

phase contaminants,

WC, number of

grain size, wt%

Ni Al Cr Mo Co Hf B v/v cycles

μιη

%

0₂ - 0.121

1 57.6 26.8 5.7 0.8 6.7 1.0 0.1 0.5 50-60 17 N₂ - 0.004

C - 0.079

0₂ - 0.132

2 57.0 26.5 5.6 0.7 6.4 1.0 0.1 1.0 40-50 22 N₂ - 0.006

C - 0.082

0₂ - 0.146

3 55.7 25.6 5.5 0.7 6.3 0.9 0.1 2.0 30-40 28 N₂ - 0.007

C - 0.087

0₂ - 0.152

4 54.5 24.8 5.4 0.6 6.1 0.8 0.1 3.0 20-30 35 N₂ - 0.012

C - 0.090

0₂ - 0.164

5 51.3 23.9 5.2 0.5 5.8 0.8 0.1 5.0 20-30 46 N₂ - 0.019

C - 0.092

0₂ - 0.178

6 48.7 22.4 4.9 0.6 5.7 0.8 0.1 7.0 20-30 59 N₂ - 0.021

C - 0.096

Examples beyonc the limits of the WC nanomodifier's specified concentrations

0₂ - 0.252

7 58.3 27.0 5.9 0.7 6.5 1.0 0.1 0.2 120-150 3 N₂ - 0.004

C - 0.080

0₂ - 0.610

8 46.1 21.4 4.7 0.6 5.3 0.8 0.1 9.0 20-30 10 N₂ - 0.051

C - 0.131

Table 5 gives the compositions and properties of the electrodes obtained through the two-stage remelt of the semi-finished SHS product for Example 1 of Table 3, the ZrC>2 nanopowder's specific surface area is 18 m²/g, the time before the pouring was 2.5 min.

Table 6 gives the compositions and properties of the electrodes obtained through two-stage remelt of the semi-finished SHS product for Example 1 of Table 3, the Y2O3 nanopowder's specific surface area is 21 m /g, the time before the pouring was 2.5 min.

Tables 7-8 give the compositions and properties of the electrodes obtained through two-stage remelt of the semi-finished SHS product for Example 1 of Table 3, where the specific surface area of the WC nanoadditive is 16 m²/g (Table 7) and the specific surface area of the Zr0₂ nanoadditive is 28 m²/g (Table 8). Table 7.

Examples within the limits of the specified time before pouring the melt into the crystallizer, with the Al+WCnano master alloy

Exa

mple Composition of the modified electrode, % Time Thermal Spread of main No. before stability, NiAl phase

WC, pouring, number of grains in the

Ni Al Cr Mo Co Hf B v/v min cycles ingot, μη

%

1 55.7 25.6 5.5 0.7 6.3 0.9 0.1 2.0 2.0 21 30-50

2 55.8 25.5 5.5 0.7 6.3 0.9 0.1 2.0 2.5 25 30-40

3 55.9 25.4 5.5 0.7 6.3 0.9 0.1 2.0 3.0 19 30-50

Examples beyond the limits o f the specified time before pouring the melt into the crystallizer, with (Al+WCnano] master alloy

7 55.7 25.6 5.5 0.7 6.3 0.9 0.1 2.0 1.0 5 30-150

8 56.0 25.3 5.5 0.7 6.3 0.9 0.1 2.0 5.0 2 120-150

Tables 9-10 give the compositions and properties of the electrodes obtained through two-stage remelt of the semi-finished SHS product for Example 1 of Table 3, when the time before pouring the melt into the crystallizer was 2.5 min. Table 9

Examples within the limits of the specified ranges of the WC nanopowder's specific surface area values

Exa

Specific

mple Composition of the modified electrode, %

surface Thermal

No. The main NiAl area of stability,

WC, phase grain

WC, number of

% size, μηι

Ni Al Cr Mo Co Hf B m²/g cycles

volum

e

1 51.3 23.9 5.2 0.5 5.8 0.8 0.1 5 5 34 10-30

2 51.4 23.8 5.2 0.5 5.8 0.8 0.1 5 16 46 10-20

3 51.7 23.5 5.2 0.5 5.8 0.8 0.1 5 30 37 20-30

Examples beyond the limits of t e specified ranges of the WC nanopowder's specific surface area values

7 51.4 23.8 5.2 0.5 5.8 0.8 0.1 5 2 4 80-100

8 51.5 23.7 5.2 0.5 5.8 0.8 0.1 5 58 3 90-120

Table 10.

Examples within the limits of the specified ranges of the Y₂0₃ nanopowder's specific surface

values

Thus, the claimed features make it possible to obtain the as-cast electrodes from the high nanomodified alloys based on nickel aluminides, which may be used for the centrifugal plasma atomization of the granules and their further application in additive 3D technologies in order to obtain geometrically complicated articles from heat-resistant metallic materials.

An example of obtaining the electrode through the known method according to the prototype (CN 100497700). The charge mixture is prepared from highly pure components in the form of melted rods and ingots with the main component content at least 99.999 % with the following quantities, wt%: nickel - 58.8; aluminum - 27.0; molybdenum - 0.7; chrome - 5.8; cobalt - 6.6; boron - 0.1 ; hafnium - 1.0.

The processing is carried out as the three-stage remelt in protective inert atmosphere. During the first stage, the charge mixture is remelted for refinement in an induction furnace by melting it in the periclase crucible at a temperature of 1680-1700 °C within high-purity grade argon atmosphere (99.995 % Ar) that fills the induction furnace chamber after pumping out down to the diffusion vacuum (10^"5 Pa), at a pressure of 0.95 x lO⁵ Pa. The induction heating rate is 150 ± 30 °C/min. In order to remove the gaseous contaminants, the obtained melt is kept at a temperature of 1680-1700 °C for 3 minutes. The inductor being switched on, the obtained melt is poured into the graphite crucible with a diameter of 50- 100 mm, which has been installed in the furnace chamber in advance and where the ingot crystallizes. When the pouring is finished, the inductor is switched off. The obtained ingot cools down in the induction furnace chamber within the argon atmosphere for 3-5 hours.

During the second stage, the first homogenizing induction remelt of the obtained ingot is carried out with an additional alloying by lump A99 grade aluminum (in order to compensate the aluminum evaporated during the refining remelt). The remelt is carried out at Ar pressure of 0.95 ^χ 10⁵ Pa, a temperature of 1680-1700 °C, and a heating rate of 150 ± 30 °C/min. For the obtained melt to be homogenized, it is kept at a temperature of 1680-1700 °C for 2 minutes. The inductor being switched on, the obtained melt is poured into the graphite crucible with a diameter of 50-100 mm. The obtained ingot cools down in the induction furnace chamber within the argon atmosphere for 3-5 hours. After the cooling, the ingot is extracted from the container, the surface is cleaned in order to remove the casting mold residues. During the third stage, the second homogenizing induction remelt of the obtained ingot is carried out at Ar pressure of 0.95 χ 10⁵ Pa, a temperature of 1680-1700 °C and a heating rate of 150 ± 30 °C/min. The melt is kept at the achieved temperature for 2 minutes. The inductor being switched on, the obtained melt is poured into the graphite crucible with a diameter of 50-100 mm that has its thermally insulated sinkhead as high as 15-25 % of the electrode height. The obtained ingot cools down in the induction furnace chamber within the argon atmosphere for 3-5 hours. After the cooling, the electrode is extracted from the container, the surface is cleaned in order to remove the casting mold residues, and the sinkhead is cut off. The appearance of the electrode is similar to what is shown in Figure 5. The obtained fused electrode comprises the preset quantity of the alloying components, wt%: nickel - 58.8; aluminum - 27.0; molybdenum - 0.7; chrome - 5.8; cobalt - 6.6; boron - 0.1 ; hafnium - 1.0. The contaminants content is, wt%: oxygen - 0.105, nitrogen - 0.008, carbon - 0.063, the residual porosity is 0.4%, the thermal stability equals 5 cycles, blebs and slip cracks have been detected. The prototype method ensures high chemical purity and homogeneity of the electrode, though the main NiAl phase grain size achieves 250-280 μιη.

In comparison with the proposed method, the three remelting stages lead to a 1.4-fold rise in the energy consumption, and since highly pure components are charged, the electrode costs half as much again in total. And the large-grain alloy electrode demonstrates weaker thermal stability, and it can probably be destroyed during the centrifugal atomization.

Thus, the features claimed allow obtaining the as-cast electrodes from the high nanomodified alloys based on nickel aluminides, which may be used for the centrifugal atomization and further application in additive 3D technologies in order to obtain geometrically complicated articles from heat-resistant metallic materials.

Claims

What is claimed

1. A method for obtaining electrodes from alloys based on nickel aluminides, comprising obtaining the semi-finished product through the centrifugal SHS casting with using the reaction mixture at the following component proportion, % weight:

Nickel oxide 47.0-49.1

Aluminum 28.6-32.4

Alloying additive 13.1-17.9

Functional additive 6.5-7.0

and the further two-stage remelt of the semi-finished product with obtaining the refined degassed ingot during the first stage and the electrode from the nanomodified alloy during the second stage; wherein during the said second stage some amount of the master alloy is introduced into the melt before pouring the latter into the crystallizer to ensure the nanopowder content of 0.5-7 vol % in the melt; the master alloy consists of pressed aluminum and nanopowder mixture with a specific surface area of 5÷30 m²/g and lump aluminum; with its further cooling down to the room temperature and extraction from the crystallizer.

2. The method as per claim 1 wherein the centrifugal SHS casting is carried out by placing the reaction mixture into a refractory mold covered inside with a functional protective layer made of refractory non-organic compound, placing the mold onto a centrifuge, igniting the mixture and conducting the SHS process at a centrifugal acceleration of 60 (±10) g, and separating the synthesized intermetallic as-cast alloy from the slag.

3. The method as per claim 1 wherein not less than 4 components out of the following series are used as the alloying additive to the reaction mixture: Cr₂0₃ and Hf and B and C03O4 and/or M0O₃.

4. The method as per claim 1 wherein A1₂0₃ H Na₃AlF₆ are used as the functional additive to the reaction mixture.

5. The method as per claim 1 wherein the two-stage remelt of the semifinished product is carried out in protective inert atmosphere or vacuum.

6. The method as per claim 1 wherein WC or TaC or NbC or Zr0₂ or Y₂0₃ or AI2O3 powder is used as the nanopowder.