Technical Field

The invention relates to the field of physical metallurgy and relates to a method of controlled alloying of intermetallic g-TiAl alloys by carbon directly during their induction melting in graphite crucibles. The aim of the invention is to achieve the required and reproducible carbon content in the final castings by controlling the technological parameters of their induction melting

Background of the Invention

Intermetallic g-TiAl alloys are characterized by low density, high specific strength and good oxidation resistance. The biggest problem for the wider use of these alloys in practice is the problematic production of components, their high cost and low strength at temperatures above 800 °C. Precision casting is the most cost-effective method for producing components from g-TiAI alloy. Usually, vacuum induction furnaces equipped with either a ceramic crucible or a cold crucible (ISM - induction scull melting) are used for melting and casting of g-TIAI alloys.

The ISM process allows the preparation of g-TiAl alloys with an oxygen content of less than 0.02 weight percent (hereinafter referred to as wt.%). However, it is a very costly technology, especially for the high purchase price of the melting furnace and its uneconomical operation, as a substantial part of the energy required for melting is converted into waste heat into cooling water. In addition, the cooled walls of the cold crucibles are thermally inefficient and prevent achieving required superheat temperature of the melt to fill the complex shaped moulds. Due to the low superheating of the melt, the rejection rate increases, which significantly contributes to the increase in the price of components from g-TiAl alloys.

The melting and casting in oxide crucibles (AI2O3, Y2O3, Z1O2, CaO) leads to an increase in oxygen content of the alloys, which has an adverse effect on the mechanical properties. From the point of view of the thermodynamic stability, Y2O3 appears to be the most suitable oxide ceramic. However, this ceramic is expensive, increases the total cost of casting production and moreover, it is not completely resistant to g-TίAI melts. Graphite crucibles, which are relatively cheap, are not recommended for melting of g-TiAl alloys due to the high alloy contamination by carbon and the formation of large primary carbide particles that cause a significant drop in mechanical properties. However, alloying with carbon in the range of 0.06 to 0.2 wt.% is currently used to increase the high temperature strength of g-TiAl alloys. Carbon is added to the alloys during their metallurgical preparation. The melting of these alloys is then accomplished by either ISM or induction melting in ceramic crucibles followed by casting into ceramic or permanent moulds.

A process route using a vacuum induction furnace and melting in graphite crucibles was reported in the literature for the preparation of castings from Ti-47A1 (at.%) alloy. The melting was carried out under argon. Although the dependence of content of oxygen, carbon, and carbides on different holding time of the melt ranging from 30 to 90 sec was measured in the final castings, this process cannot lead to reproducible processing because the key processing parameters affecting the final content of carbon in the alloy were not controlled. The present invention allows achieving the required carbon content in g-TiAl castings by a controlled and reproducible manner, which is a fundamentally different from the stochastic process described by "Cegan T. et aL: Effect of TbAlC particles on the microstructure and elevated temperature deformation" properties of g-TiAl alloys, MTAEC9, vol. 48 (6), pp. 831-835, 2014, UDK 669.04: 548.4 See section 2, 3.1, Table 1.

The document by "Szurman I., et aL: Preparation of alloys based on intermetaliic compounds by VIM with centrifugal casting". METAL 2015: 24 ^th International Conference on Metallurgy and Materials: Conference Proceedings: June 2015, Brno, Czech Republic, EU, Tanger, 2015, pp. 1700-1705, ISBN 978-80-87294-62-8 "describes the preparation of alloys based intermetaliic compounds by vacuum induction melting (VIM) in graphite crucibles followed with centrifugal casting. According to this document, Ti-Al, Ti-Ni, Fe-Al, Ni-Al, and other alloy systems require specific conditions during melting in a high-frequency induction vacuum furnace, particularly crucible material and charge arrangements. The most common casting technology is the gravity casting. Another option, often used in practice, is centrifugal casting. Combining VIM and centrifugal casting, it is possible to produce precise casting in a relatively compact system, also using protective gas or vacuum. The document is focused on the method of melting and centrifugal casting of Ti-Ni-X and Ti-Al-X alloys using graphite crucibles.

Another document by "Cegan T., et al.: Preparation of TiAl-based alloys by induction melting in graphite crucibles, Kovove Materialy-Meta!lic Materials 53 (2), 2015, p.69-78, DOI: 10.4149/km_2015j2j69" refers to the preparation of TiAl-based alloys by induction melting in graphite crucibles and casting into graphite molds. The abstract indicates that the melting in graphite crucibles leads to an increase in the carbon content from 0.046 wt.% to 0.102 wt.% and the oxygen content is between 0,033 wt.% and 0.078 wt.%. It is stated in the conclusion of the cited document that the induction melting in graphite crucibles and centrifugal casting into graphite moulds can be considered to be a suitable method for the preparation of TiAl-based alloys. It is dear from this and the previous document that the authors failed to control the carbon content during vacuum induction melting in graphite crucibles and casting of y-TiAl alloys into graphite molds. The low carbon content of ingots published in these works is only the result of a random and umeprodudble process.

The document by "Jovanovid M.T., et al .: Precision cast Ti-based alloys - microstructure and mechanical properties. Metallurgical & Materials Engineering, Vol 15 (1) 2009, pp. 53-69” describes the preparation of a classical titanium Ti-6A1-4V and also y-TiAl alloys by the induction melting in a graphite crucible and centrifugal casting into a ceramic mold. Although the melting in a graphite cmtible is used, the exact chemical composition and properties of the crucible (type of graphite, density, open porosity, mean grain size) are not characterized at all. Moreover, the surface of the graphite crudble is covered with Y2O3 layer prepared by plasma spraying. In this document, the resulting carbon contents are not reported and it is not dear whether the protective Y2O3 layer has prevented the studied alloys from the carbon contamination. It is clear from the document that the authors did not use the reaction between the graphite crudble and the melt to control the increase in carbon content of the studied alloys but tried to prevent this type of reaction by protecting graphite crudble walls with plasma sprayed Y2O3 and using ceramic moulds for centrifugal casting. European patent no. 1939566 A1 solves the problem of using graphite crucibles in the melting of high-reactive alloys such as TiAl. In one aspect, it is stated that the use of ceramic crucibles results in contamination of the TiAl alloy with oxygen alloy and contamination of TiAl alloy with carbon occurs when graphite crucibles are used. A reduction in carbon contamination can be achieved by using a protective layer in a graphite crucible. It Is noted that in general, the melting point of TiAl-based alloys ranges from 1370 °C to 1700 °C, This document addresses the problem of forming protective layers on the surface of graphite crucibles used to melt reactive alloys, including TiAl-based alloys. The aim of this document is to prevent contact between the melt and the graphite or at least to minimize the melt interaction with the graphite in order to eliminate or minimize the contamination of the alloys with carbon. The solution proposed in this patent is fundamentally different. The aim of the present invention is to guarantee the contact of the melt with the surface of the graphite crucible, to control the enrichment of the melt with the carbon to the required level by controlling the melting and casting parameters and to prepare castings with the required carbon content, which will improve their high temperature mechanical properties.

The present solution of controlled alloying of highly reactive g-TiAl alloys with carbon during their induction melting and centrifugal casting is original and cannot be achieved on the base of the previously died documents or any combinations of the information contained in these documents. The first documents describe the stochastic processes of contamination of TiAl-based castings with carbon during their induction melting and casting without reporting adequate thermodynamic relationships of this process and without interconnection with all key metallurgical parameters. In addition, the latter document has a completely different focus because it is intended to prevent contamination of reactive alloys with carbon or at least minimize it by forming protective layers on the surface of graphite crucibles. The procedures described in the last document essentially prevent alloying of g-TiAl alloys with carbon during their induction melting and casting.

Chinese patent application "CN 1676658 A (HARBIN POLYTECHNIK UNIV, HARBIN INSTITUTE OF TECHNOLOGY) 2005-10-05", solves the surface treatment of TiAl based alloy in which the alloy is heat treated at temperatures between 800 and 1500 °C and pressures ranging from 1 to 300 MPa in a graphite vessel or in a vessel containing graphite material. The melting time of the alloy is 1 min. up to 20 hrs. The aim of this method is to improve the surface properties of the alloy.

In another Chinese patent application "CN 104264012 A (NORTHWEST INSTITUTE FOR NON-FERROUS METAL RESEARCH) 2015-01-07", a process for preparing an alloy containing Al, Nb, Mo and Ti alloy is described in a vacuum induction furnace under an inert argon atmosphere, wherein the ingot is processed in a crucible, which can be made from graphite.

Detailed description of tike Invention

The basis of the present invention is the process of the controlled alloying of intermetallic g-TiAl alloys with carbon in a range from 0.09 wt. % to 0.29 wt. %. The g-TiAl intermetallic alloy with die chemical composition Ti-28.6Al-9.1Nb-2.3Mo (wt.%) and oxygen content of 0.04 wt. % is melted in 100 cm ³ crucible prepared from isostatically pressed graphite with a density of 1.8 g/cm ³ , open porosity lower than 2% and an average grain size of graphite grains of less than 40 mpi. The melting of g-TiAl alloys is carried out in a vacuum induction furnace using medium frequency induction heating with a medium frequency inductor with an output power from 20 to 30 KW and a frequency ranging from 20 to 30 kHz using an argon protective atmosphere with a purity at least of 99.995%. The vacuum chamber of the induction furnace is only partially filled with argon to maintain a vacuum pressure ranging from 1 to 10 kPa.

Furthermore, the invention is based on the fact that heating of the g-TiAl alloy to the melting temperature is ensured by a gradual increase of the inductor output power while maintaining a heating rate of 90 to 100 °C/min.

The maximum total time from the start of the melting (first hint of the melt) to the selected melt temperature is 60 s, which corresponds to the alloy heating rate ranging from 150 to 200 °C/min and depends on the selected superheating temperature of the melt.

According to another aspect of the invention, the superheating temperature of the melt is in the range from 1650 to 1700 °C

According to even another aspect of the invention, the holding time of the melt at the superheating temperature ranges from 20 to 90 s depending on the melt temperature and the required carbon content of the final casting

From a theoretical point of view, the present invention is based on a thermochemical reaction that takes place between the g-TiAl melt and the graphite crucible during induction melting. From a practical point of view, the present invention is based on the development of a technological process for the induction melting of g-TiAl alloys and the definition of the parameters of this process, which influence the thermochemical reaction between the melt and the graphite crucible.

The technological parameters that influence the carbon content in Ti-28.6Al-9.lNb- 2.3Mo alloy (wt.%) in the course of induction melting are as follows: (i) type of graphite crucible; (ii) type of induction heating (tii) the alloy heating rate to the superheating temperature, (iv) the melt superheating temperature, and (v) the holding time of the melt at the superheating temperature. The proposed technological procedure specifies the following technological parameters as follows:

(i) The crucible for induction melting need to be made of isostatically pressed graphite with a minimum density of 1.8 g/cm ³ , a low open porosity (<2%) and an average graphite grain size of less than 40 pm.

(ii) A vacuum induction furnace equipped with a medium-frequency inductor having an output power of 20 to 30 kW (for a charge volume of 100 cm ³ ) and a frequency of about 20-30 kHz is required for melting The initial oxygen content in the g-TiAl alloy must not exceed 0.05 wt.

%.

(iii) The alloy must be heated at a rate of 90-100 °C/min to the melting temperature by gradually increasing the inductor power. The melting takes place in an argon protective atmosphere with a minimum purity of 99.995%. (ίn) The vacuum pressure in the vacuum chamber must not be less than 1 kPa to prevent evaporation of aluminium during melting. Optimal vacuum pressure ranges from 1 kPa to 10 kPa.

(v) The maximum total time from the start of melting (first hint of the melt) to the selected melt temperature is 60 s, which corresponds to the alloy heating rate ranging from 150 to 200 °C/min and depends on the selected superheating temperature of the melt.

(vi) The melt superheating temperature must be between 1650 and 1700 °C.

(vii) The holding time of the melt at the superheating temperature ranges from 20 to 90 s depending on the melt temperature and the required carbon content of the final casting.

(viii) The holding of the melt at the superheating temperature must be followed by centrifugal casting of the melt into a cold graphite mould with a cavity diameter of 20 mm and length of 220 mm at a rotation speed of 250 rpm.

Best Modes for Carrying out the Invention

Example 1

A rod-shaped casting with a diameter of 20 mm and a length of 220 mm was prepared by induction melting of the charge with the chemical composition Ti-28.6Al-9.1Nb-2.3Mo (wt. %) and an oxygen content of 0.04 wt. % in a commercially available graphite crucible made of isostatically pressed graphite with a density of 1.8 g/cm ³ , open porosity <2% and graphite grain size less than 40 pm. Melting was performed in a Supercast-titan vacuum induction furnace (Linn High Therm) with a regulated inductor output power up to 20 kW and a frequency of 20 kHz under an argon atmosphere with a purity of 99.995% and at a vacuum pressure of 1 kPa. The batch was first heated at a rate of 90 °C/min to the melting temperature by gradually increasing the inductor power. Upon reaching the melting point, the heating rate was increased to 150 ^e C/min and after reaching 1650 °C the melt was held at this temperature for 20 s. The holding period was followed by centrifugal casting into a cold graphite mould at a rotation speed of 250 rpm. In this way, the casting without defects with a homogeneous chemical composition, an oxygen content of 0.06 wt. % and a carbon content of (0.09 ± 0.02) wt. % was produced.

Example 2

A rod-shaped casting with a diameter of 20 mm and a length of 220 mm was prepared by induction melting of the charge with the chemical composition Ti-28.6AI-9.1Nb-2.3Mo (wt. %) and an oxygen content of 0.04 wt. % in a commercially available graphite crucible made of isostatically pressed graphite with a density of 1.8 g/cm ³ , open porosity <2% and graphite grain size less than 40 pm. Melting was performed in a Supercast-titan vacuum induction furnace (Linn High Therm) with a regulated inductor output power up to 30 kW and a frequency of 30 kHz under an argon atmosphere with a purity of 99.995% and at a vacuum pressure of 10 KPa. The batch was first heated at a rate of 100 °C/min to the melting temperature by gradually increasing the inductor power. Upon reaching the melting point, the heating rate was increased to 200 °C/min and after reaching 1650 °C the melt was held at this temperature for 20 s. The holding period was followed by centrifugal casting into a cold graphite mould at a rotation speed of 250 rpm. In this way, the casting without defects with a homogeneous chemical composition, an oxygen content of 0.06 wt. % and a carbon content of (0.10 ± 0.02) wt. % was produced.

Example 3

A rod-shaped casting with a diameter of 20 mm and a length of 220 mm was prepared by induction melting of the charge with the chemical composition Ti-28.6Al-9.1Nb-2.3Mo (wt. %) and an oxygen content of 0.04 wt. % in a commercially available graphite crucible made of isostatically pressed graphite with a density of 1.8 g/cro ³ , open porosity <2% and graphite grain size less than 40 pm. Melting was performed in a Supercast-titan vacuum induction furnace (Linn High Therm) with a regulated inductor output power up to 20 kW and a frequency of 20 kHz under an argon atmosphere with a purity of 99.995% and at a vacuum pressure of 1 kPa. The batch was first heated at a rate of 90 ^e CZmin to the melting temperature by gradually increasing the inductor power. Upon reaching the melting point, the heating rate was increased to 150 ^e C/min and after reaching 1650 °C the melt was held at this temperature for 50 s. The holding period was followed by centrifugal casting into a cold graphite mould at a rotation speed of 250 rpm. In this way, the casting without defects with a homogeneous chemical composition, an oxygen content of 0.06 wt. % and a carbon content of (0,19 ± 0.02) wt. % was produced.

Example 4

A rod-shaped casting with a diameter of 20 mm and a length of 220 mm was prepared by induction melting of the charge with the chemical composition Ti-28.6Al-9.lNb-2.3Mo (wt. %) and an oxygen content of 0.04 wt. % in a commercially available graphite crucible made of isostatically pressed graphite with a density of 1.8 gZcm ³ , open porosity <2% and graphite grain size less than 40 pm. Melting was performed in a Supercast-titan vacuum induction furnace (linn High Therm) with a regulated inductor output power up to 30 kW and a frequency of 30 kHz under an argon atmosphere with a purity of 99.995% and at a vacuum pressure of 10 kPa. The batch was first heated at a rate of 100 °C/min to the melting temperature by gradually increasing the inductor power. Upon reaching the melting point, the heating rate was increased to 200 °C/min and after reaching 1650 °C the melt was held at this temperature for 50 s. The holding period was followed by centrifugal casting into a cold graphite mould at a rotation speed of 250 rpm. In this way, the casting without defects with a homogeneous chemical composition, an oxygen content of 0.06 wt. % and a carbon content of (0.18 ± 0.02) wt. % was produced.

Example 5

A rod-shaped casting with a diameter of 20 mm and a length of 220 mm was prepared by induction melting of the charge with the chemical composition Ti-28.6Al-9.1Nb-2.3Mo (wt. %) and an oxygen content of 0.04 wt. % in a commercially available graphite crucible made of isostatically pressed graphite with a density of 1.8 gZcm ³ , open porosity <2% and graphite grain size less than 40 pm. Melting was performed in a Supercast-titan vacuum induction furnace (Linn High Therm) with a regulated inductor output power up to 20 kW and a frequency of 20 kHz under an argon atmosphere with a purity of 99.995% and at a vacuum pressure of 1 kPa. The batch was first heated at a rate of 90 ⁰ C/min to the melting temperature by gradually increasing the inductor power. Upon reaching the melting point, the heating rate was increased to 150 °C/min and after reaching 1650 °C the melt was held at this temperature for 90 s. The holding period was followed by centrifugal casting into a cold graphite mould at a rotation speed of 250 ipm. In this way, the casting without defects with a homogeneous chemical composition, an oxygen content of 0.06 wt. % and a carbon content of (0.23 ± 0.02) wt. % was produced.

Example 6

A rod-shaped casting with a diameter of 20 mm and a length of 220 mm was prepared by induction melting of the charge with the chemical composition Ti-28.6Al-9.1Nfc-2.3Mo (wt. %) and an oxygen content of 0.04 wt. % in a commercially available graphite crucible made of isostatically pressed graphite with a density of 1.8 g/cm ³ , open porosity <2% and graphite grain size less than 40 pm. Melting was performed in a Supercast-titan vacuum induction furnace (Linn High Therm) with a regulated inductor output power up to 30 kW and a frequency of 30 kHz under an argon atmosphere with a purity of 99.995% and at a vacuum pressure of 10 kPa. The batch was first heated at a rate of 100 °C/min to the melting temperature by gradually increasing the inductor power. Upon reaching the melting point, the heating rate was increased to 200“C/min and after reaching 1650 °C the melt was held at this temperature for 90 s. The holding period was followed by centrifugal casting into a cold graphite mould at a rotation speed of 250 rpm. In this way, the casting without defects with a homogeneous chemical composition, an oxygen content of 0.06 wt. % and a carbon content of (0.22 ± 0.02) wt. % was produced.

Example 7

A rod-shaped casting with a diameter of 20 mm and a length of 220 mm was prepared by induction melting of the charge with the chemical composition Ti-28.6Al-9.1Nb-2.3Mo (wt. %) and an oxygen content of 0.04 wt. % in a commercially available graphite crucible made of isostatically pressed graphite with a density of 1.8 gfcm ³ , open porosity <2% and graphite grain size less than 40 pm. Melting was performed in a Supercast-titan vacuum induction furnace (Linn High Therm) with a regulated inductor output power up to 20 kW and a frequency of 20 kHz under an argon atmosphere with a purity of 99.995% and at a vacuum pressure of 1 kPa. The batch was first heated at a rate of 90 °C/min to the melting temperature by gradually increasing the inductor power. Upon reaching the melting point, the heating rate was increased to 150 °C/min and after reaching 1700 °C the melt was held at this temperature for 20 s. The holding period was followed by centrifugal casting into a cold graphite mould at a rotation speed of 250 rpm. In this way, the casting without defects with a homogeneous chemical composition, an oxygen content of 0.06 wt. % and a carbon content of (0.12 ± 0.02) wt % was produced.

Example 8

A rod-shaped casting with a diameter of 20 mm and a length of 220 mm was prepared by induction melting of the charge with the chemical composition Ti-28.6AI-9.1Nb-2.3Mo (wt %) and an oxygen content of 0.04 wt. % in a commercially available graphite crucible made of isostatically pressed graphite with a density of 1.8 g/cm ³ , open porosity <2% and graphite grain size less than 40 pm. Melting was performed in a Supercast-titan vacuum induction furnace (Linn High Therm) with a regulated inductor output power up to 30 kW and a frequency of 30 kHz under an argon atmosphere with a purity of 99.995% and at a vacuum pressure of 10 kPa. The batch was first heated at a rate of 100 °C/min to the melting temperature by gradually increasing the inductor power. Upon reaching the melting point, the heating rate was increased to 200 °C/min and after reaching 1700 °C the melt was held at this temperature for 20 s. The holding period was followed by centrifugal casting into a cold graphite mould at a rotation speed of 250 rpm. In this way, the casting without defects with a homogeneous chemical composition, an oxygen content of 0.06 wt. % and a carbon content of (0.11 ± 0.02) wt. % was produced.

Example 9

A rod-shaped casting with a diameter of 20 mm and a length of 220 mm was prepared by induction melting of the charge with the chemical composition Ti-28.6Al-9.1Nb-2.3Mo (wt. %) and an oxygen content of 0.04 wt. % in a commercially available graphite crucible made of isostatically pressed graphite with a density of 1.8 g/cm ³ , open porosity <2% and graphite grain size less than 40 pm. Melting was performed in a Supercast-titan vacuum induction furnace (Linn High Therm) with a regulated inductor output power up to 20 kW and a frequency of 20 kHz under an argon atmosphere with a purity of 99.995% and at a vacuum pressure of 1 kPa. The batch was first heated at a rate of 90 °C/min to the melting temperature by gradually increasing the inductor power. Upon reaching the melting point, the heating rate was increased to 150 ^e C/min and after reaching 1700 °C the melt was held at this temperature for 50 s. The holding period was followed by centrifugal casting into a cold graphite mould at a rotation speed of 250 rpm. In this way, the casting without defects with a homogeneous chemical composition, an oxygen content of 0.06 wt. % and a carbon content of (0.22 ± 0.02) wt % was produced.

Example 10

A rod-shaped casting with a diameter of 20 nun and a length of 220 mm was prepared by induction melting of the charge with the chemical composition Ti-28.6Al-9,lNb-2.3Mo (wt. %) and an oxygen content of 0.04 wt. % in a commercially available graphite crucible made of isostatically pressed graphite with a density of 1.8 g/cm ³ , open porosity <2% and graphite grain size less than 40 pm. Melting was performed in a Supercast-titan vacuum induction furnace (Linn High Therm) with a regulated inductor output power up to 30 kW and a frequency of 30 kHz under an argon atmosphere with a purity of 99.995% and at a vacuum pressure of 10 kPa. The batch was first heated at a rate of 100 °C/min to the melting temperature by gradually increasing the inductor power. Upon reaching the melting point, the heating rate was increased to 200 °C/min and after reaching 1700 °C the melt was held at this temperature for 50 s. The holding period was followed by centrifugal casting into a cold graphite mould at a rotation speed of 250 rpm. In this way, the casting without defects with a homogeneous chemical composition, an oxygen content of 0.06 wt. % and a carbon content of (0.22 ± 0.02) wt. % was produced.

Example 11

A rod-shaped casting with a diameter of 20 mm and a length of 220 mm was prepared by induction melting of the charge with the chemical composition Ti-28.6Al-9.lNb-2.3Mo (wt. %) and an oxygen content of 0.04 wt. % in a commercially available graphite crucible made of isostatically pressed graphite with a density of 1.8 g/cm ³ , open porosity <2% and graphite grain size less than 40 pm. Melting was performed in a Supercast-titan vacuum induction furnace (Linn High Thenn) with a regulated inductor output power up to 20 kW and a frequency of 20 kHz under an argon atmosphere with a purity of 99.995% and at a vacuum pressure of 1 kPa. The batch was first heated at a rate of 90 °C/min to the melting temperature by gradually increasing the inductor power. Upon reaching the melting point, the heating rate was increased to 150 ^e C/min and after reaching 1700 °C the melt was held at this temperature for 90 s. The holding period was followed by centrifugal casting into a cold graphite mould at a rotation speed of 250 rpm. In this way, the casting without defects with a homogeneous chemical composition, an oxygen content of 0.06 wt. % and a carbon content of (0.29 ± 0.02) wt. % was produced.

Example 12

A rod-shaped casting with a diameter of 20 mm and a length of 220 mm was prepared by induction melting of the charge with the chemical composition Ti-28.6Al-9.1Nb-2.3Mo (wt. %) and an oxygen content of 0.04 wt. % in a commercially available graphite crucible made of isostatically pressed graphite with a density of 1.8 g/cm ³ , open porosity <2% and graphite grain size less than 40 pm. Melting was performed in a Supercast-titan vacuum induction furnace (Linn High Therm) with a regulated inductor output power up to 30 kW and a frequency of 30 kHz under an argon atmosphere with a purity of 99.995% and at a vacuum pressure of 10 kPa. The batch was first heated at a rate of 100 °C/min to the melting temperature by gradually increasing the inductor power. Upon reaching the melting point, the heating rate was increased to 200 °C/min and after reaching 1700 °C the melt was held at this temperature for 90 s. The holding period was followed by centrifugal casting into a cold graphite mould at a rotation speed of 250 rpm. In this way, the casting without defects with a homogeneous chemical composition, an oxygen content of 0.06 wt, % and a carbon content of (0.28 ± 0.02) wt. % was produced.

As shown by the examples of the invention application (Example 1 to 12), the experimentally measured carbon contents in the cast cylindrical samples varied from 0.09 to 0.29 wt. % depending on the melt superheating temperature and the holding time of the melt at the superheating temperature. Based on the experimental measurements, the dependence of carbon content in the castings ac (in wt. %) on absolute superheating temperature T (in Kelvin) and holding time of the melt at superheating temperature t (in seconds) can be expressed in the form

The measured carbon content satisfies the kinetic equation describing the carbon content dependence on the superheating temperature and the holding time of the melt at the - superheating temperature, which allows alloying of the g-TiAl alloys with carbon in a reproducible manner during the induction melting in graphite crucibles based on the designed and experimentally verified technological process.

Industrial Utility

The inventions represents a new technological method of alloying g-TiAl alloys during their induction melting in graphite crucibles. The use of graphite crucibles for melting will significantly reduce production costs and ensure controlled and reproducible alloying of g-TiAl carbon alloys in the range from 0.09 to 0.29 wt. %. This caibon content ensures solid solution and precipitation strengthening of g-TiAl alloys, thereby improving their high-temperature mechanical properties. The proposed technology of induction melting combined with centrifugal casting can also be used to produce precise castings, e.g. turbocharger wheels for combustion engines, turbine blades or exhaust valves for combustion engines.