EP1444065B1 - Method for producing alloy ingots - Google Patents

Description

The invention relates to a novel melt metallurgical process for the cost-effective production of blocks of metallic and intermetallic alloys (ingots) with high chemical and structural homogeneity, in particular γ-TiAl ingots.

The intermetallic alloys based on γ-TiAl have made the leap from the development laboratory into industrial applications in aerospace and automotive racing in the year 2000. The advantageous high-temperature properties in combination with a low weight enable their use in the aerospace industry. The high temperature and corrosion resistance makes the material interesting for fast moving components in machines, eg for valves in internal combustion engines or for blades in gas turbines. The properties of this material depend on the chemical and structural homogeneity of a material previously unknown in structural materials. Consequently, the production of correspondingly high-quality ingots is technically very demanding and expensive. Homogeneous ingots are required for various process routes for the production of further semi-finished products or components made of TiAl as starting material (cf. H. Clemens and H. Kestler (2000), Advanced Engineering Materials 9, 551 ; Y.-W. Kim (1994), JOM 46 (7), 30 such as PA Bartolotta and DL Krause (1999) in Gamma Titanium Aluminides, ed. Y.-W. Kim, DM Dimiduk and MH Loretto, (TMS Warrendale, PA, USA 1999); 3-10 ).

The currently used technical alloys based on γ-TiAl have a multiphase structure and, in addition to the ordered tetragonal γ-TiAl as the main phase, contain the ordered hexagonal α ₂ -Ti ₃ Al, typically in an amount of 5-15 vol.%. Refractory metals as alloying elements can be used to form a metastable krz phase, which occurs either as β-phase (disordered) or as B2 phase (ordered). These alloying additives improve the oxidation resistance and creep resistance. Si, B and C serve in small amounts to increase the strength of the cast structure (cf. B. Inkson and H. Clemens (1999), MRS Symp. Proc. 552, KK3.12 ; S. Huang, E. Hall, D. Shuh (1991), ISIJ International 31 (10), 1100 and Y.-W. Kim and Dimiduk (1991), JOM 8, 40 ). Corresponding C contents can lead to precipitation hardening (cf. Güther, A. Otto, H. Kestler and H. Clemens, (1999) in Gamma Titanium Aluminides, ed. Y.-W. Kim, DM Dimiduk and MH Loretto, (TMS Warrendale, PA, USA 1999 ) 225-230 ). The alloying elements Cr, Mn and V increase the room temperature ductility of the otherwise very brittle TiAl. Depending on the application profile, alloy development has led to a number of different alloy variants, which are described in more detail below.

TiAl alloys are usually produced by repeated remelting in a vacuum arc furnace (see Figure 1) as ingots (VAR-Vacuum Arc Remelting). In this case, a pressed electrode containing all alloying components, melted by increasing their diameter. A fundamental problem arises from occurring inhomogeneities in the alloy composition of γ-TiAl ingots. A comparison of the Al content in double and triple remelted γ-TiAl ingot material shows that even in twice remelted γ-TiAl ingot local fluctuations of the Al content of ± 2 at.% Are observed (see Figure 2). To set sufficient alloy homogeneity, a triple remelting in the VAR system is necessary (cf. Güther, A. Otto, H. Kestler and H. Clemens, (1999) in Gamma Titanium Aluminides, ed. Y.-W. Kim, DM Dimiduk and MH Loretto, (TMS Warrendale, PA, USA 1999 ) 225-230 ; V. Güther, Properties, Processing and Applications of γ-TiAl, Proc. 9th Ti World Conference, 08-11 Jun. 1999, St. Petersburg and V. Güther, H. Kestler, H. Clemens and R. Gerling, Recent Improvements in γ-TiAl Ingot Metallurgy, Proc. Of the Aeromat 2000 Conference and Exhibition, (Seattle, WA, June 2000 ).

In contrast to titanium alloys (ingot diameter up to 1.5 m), the processable diameters of γ-TiAl are limited due to the limited formability significantly smaller values limited. At present, the market is mainly asking for ingots with a diameter of only approx. 200 mm.

Per melt, using the VAR technique, a diameter increase of about 40 mm takes place. This means for a final diameter of about 200 mm, that must be assumed from pressing electrodes with a maximum of about 60 mm diameter, the porosity is about 40%. The small diameter limits the strength of the pressing electrode and thus the possible usable length to about 1.5 m (corresponding to a total mass of about 18 kg). The smaller the diameter of the first pressing electrode, the higher the manufacturing cost, since less material can be melted per melting cycle. For a triple molten VAR ingot with a diameter of 180 mm and a length of 1000 mm - according to an industrial refinement- according to the prior art -in total 10 individual melts are required (6 primary melts, 3 secondary melts, 1 third molten salt), the high Incur costs. The material loss (voids, etc.) per ingot is currently 35%. In addition, the conventional manufacturing method offers no flexibility in the choice of ingot diameter.

Alternative fabrication options for titanium alloy ingots include electron beam melting in the cold hearth and plasma arc cold hearth melting (PACHM). While electron beam melting (see Figure 3 above) has found industrial application only for pure unalloyed titanium, the PACHM process (see Figure 3, below) is used for the production of titanium alloys and also γ-TiAl ingots. Here, the starting material is melted in the cold crucible through a plasma torch and the liquid melt fed via a torch system fired with plasma torches a likewise plasma-heated strand withdrawal. This process has hitherto led to inadequate alloy homogeneity, which can be attributed to the limits of the process (cf. W. Porter, Proceedings of 3rd Int. Symp. Structural Intermetallics, ed. KJ Hemker et al, TMS Warrendale 2001, p. 201 ). The additional attachment of an induction coil for better homogenization of the melt in the plasma-heated strand withdrawal did not lead to the desired success (see. M. Loretto, Titanium 95, Science and Technologies ; AL Dowson et. al., in Gamma Titanium Aluminides (1995), ed. YW Kim. R. Wagner and M. Yamaguchi (TMS Warrendale, PA, USA 1995), 46-474 ; M. Volas, Industrial Initiatives in Wrought Orthorhombic and Gamma TiAl mill products; Proc. of the Aeromat 2000 Conference and Exhibition, Seattle, WA, June 2000 ).

In addition, the production of γ-TiAl base alloys by means of chill casting from a cold wall induction or plasma furnace or by inert gas atomization from a cold wall crucible to γ-TiAl powder and powder metallurgy processing is technically realized. These alternatives have so far led to an insufficient microstructure (porosity in chill casting) or high costs (powder metallurgy).

Representing the state of VAR technology will be on the U.S. Patents 5,846,351 . 5,823,243 . 5,746,846 and 5 492 574 directed.

The object of the present invention is to provide a process for the reproducible production of γ-TiAl ingots of high chemical homogeneity and low porosity, which can be carried out in a simpler and cheaper manner than the VAR process described above, in which numerous melting steps are necessary to achieve the desired high homogeneity and low porosity. In addition, the method should provide the opportunity to adjust the dimensions of the alloy ingots, bypassing the limitations of the VAR method described above in a technically reasonable range.

1. (i) Herstellung von Elektroden durch übliches Vermischen und Verpressen der ausgewählten Ausgangsstoffe,
2. (ii) Mindestens einmaliges Umschmelzen der in Stufe (i) erhaltenen Elektroden durch ein übliches schmelzmetellurgisches Verfahren,
3. (iii) Induktives Abschmelzen der in Stufe (ii) erhaltenen Elektroden in einer Hochfrequenzspule,
4. (iv) Homogenisieren der in Stufe (iii) erhaltenen Schmelze in einem Kaltwand-Induktionstiegel, und
5. (v) Abziehen der Schmelze unter Kühlung aus dem Kaltwand-Induktionstiegel von Stufe (iv) in Form von erstarrten Blöcken mit frei einstellbaren Dimensionen.

This object is achieved by a method for the production of metallic and intermetallic alloy ingots according to the characterizing part of claim 1 by continuous and quasi-continuous strand withdrawal from a cold wall induction crucible by the alloy material in molten and pre-homogenized state continuously or quasi-continuously a cold wall Induction crucible is supplied (see Figure 4). The continuous casting process for producing high homogeneity and low porosity metallic and intermetallic alloy ingots is characterized in a first alternative according to the invention by the following chronologically listed steps:

1. (i) preparation of electrodes by customary mixing and compression of the selected starting materials,
2. (ii) at least one remelting of the electrodes obtained in step (i) by a standard melt-metallurgical process,
3. (iii) inductive melting of the electrodes obtained in step (ii) in a high-frequency coil,
4. (iv) homogenizing the melt obtained in step (iii) in a cold wall induction crucible, and
5. (v) withdrawing the melt under cooling from the cold wall induction crucible of step (iv) in the form of solidified blocks of freely adjustable dimensions.

1. (i) Herstellung von Elektroden durch übliches Vermischen und Verpressen der ausgewählten Ausgangsstoffe,
2. (ii) Mindestens einmaliges Umschmelzen der in Stufe (i) erhaltenen Elektroden durch ein übliches schmelzmetallurgisches Verfahren,
3. (iii) Erzeugung eines vorhomogenisierten, schmelzflüssigen Materials aus dem in Stufe (ii) erhaltenen Elektrodenmaterial durch Abschmelzen in einem Plasmaofen mit kaltem Tiegel,
4. (iv) Homogenisieren der in Stufe (iii) erhaltenen Schmelze in einem Kaltwandlnduktionstiegel, und
5. (v) Abziehen der unter Kühlung erstarrten Schmelze aus dem Kaltwand-Induktionstiegel von Stufe (iv) in Form von zylindrischen Blöcken mit frei einstellbaren Durchmessern und Längen.

Alternatively, the following sequence of the continuous casting process for the production of metallic and intermetallic alloy ingots of high homogeneity, low porosity is indicated in claim 1 (see FIG. 5):

1. (i) preparation of electrodes by customary mixing and compression of the selected starting materials,
2. (ii) at least one remelting of the electrodes obtained in step (i) by a conventional melt metallurgical process,
3. (iii) producing a pre-homogenized molten material from the electrode material obtained in step (ii) by melting in a cold crucible plasma furnace,
4. (iv) homogenizing the melt obtained in step (iii) in a cold wall crucible, and
5. (v) withdrawing the melt solidified under cooling from the cold wall induction crucible of step (iv) in the form of cylindrical blocks of freely adjustable diameters and lengths.

The process is preferably used for the production of γ-TiAl based intermetallic alloy ingots, which alloys can generally be described by the following empirical formula:

Ti _x Al _y (Cr, Mn, V) _u (Zr, Cu, Nb, Ta, Mo, W, Ni) _v (Si, B, C, Y) _w

• X = 100-y-u-v-w
• y = 40 bis 48, vorzugsweise 44 bis 48
• u = 0,5 bis 5
• v = 0,1 bis 10 und
• w = 0,05 bis 1.

The concentrations of the alloying components are usually within the following limits (expressed in at.%):

• X = 100-yuvw
• y = 40 to 48, preferably 44 to 48
• u = 0.5 to 5
• v = 0.1 to 10 and
• w = 0.05 to 1.

The inductive melting of the electrodes in step (iii) takes place in a high-frequency field with a frequency of preferably 70 to 300 kHz, in particular 70 to 200 kHz and preferably at temperatures of 1400 ° C to 1700 ° C, in particular 1400 ° C to 1600 ° C. to achieve uniform dripping, the electrode is rotated, with a speed of 4 rpm being preferred. The lowering speed of the electrode is continuously variable from 0 to 200 mm / min.

The method is preferably carried out quasi-continuously in the case of inductive melting by one or more electrodes are tracked quasi-continuously, while at the same time a block is withdrawn from the cold wall induction crucible.

The homogenization of the melt in the cold wall induction crucible in step (iv) is preferably carried out at an overheating of 10 to 100 K, preferably from 40 to 60 K. This corresponds to temperatures of 1400 ° C to 1750 ° C, preferably 1450 ° C to 1700 ° C, depending on the alloy composition. The frequency range of the coil is 4 to 20 kHz, preferably 4 to 12 kHz.

The cooling of the melt in the removal of the blocks in step (v) is preferably carried out with the aid of water-cooled copper segments, and the diameters of the blocks are preferably in a range of 40 to 350 mm, particularly preferably 140 to 220 mm.

The take-off speeds are adjustable between 5 to 10 mm / min. The deduction rate must be adjusted to the drip rate (stage iii). This can be around 50 kg / h.

1. (a) ein Verhältnis von Länge zu Durchmesser von >12,
2. (b) eine Homogenität, bezogen auf lokale Schwankungen des Aluminiums und Titans von < t 0,5 at.%: weitere metallische Legierungsbestandteile: ± 0,2 at.%; nichtmetallische Legierungszusätze (Bor, Kohlenstoff, Silizium) ± 0,05 at.%.

The present inventive method makes it possible to produce novel γ-TiAl-based intermetallic alloy ingots characterized by a new combination of dimensional dimensions on the one hand and homogeneity on the other, as characterized below:

1. (a) a length to diameter ratio of> 12,
2. (b) a homogeneity, based on local variations of aluminum and titanium of <t 0.5 at.%: other metallic alloy constituents: ± 0.2 at.%; non-metallic alloying additives (boron, carbon, silicon) ± 0.05 at.%.

The core of the process of the invention is the continuous or quasi-continuous supply of a pre-homogenized melt of the alloy material in a cold wall induction crucible (KIT). In the context of the present invention, it has surprisingly been found that substantial melting of the electrode material used to produce metallic and intermetallic alloy ingots results in substantial homogenization of the material, so that a single subsequent homogenization step in the cold wall induction crucible is sufficient to accomplish these two steps To achieve the greatest possible homogenization, as can be achieved comparatively only with very many remelting stages in the VAR process. The erfindunsgemäße method is thus much simpler and cheaper compared to the previously used VAR method.

Thus, the KIT loses its state of the art corresponding main function, namely the melting of material that is always charged in solid state in the KIT. It is a significant advantage of the method according to the invention that the always observed during the melting of solid, multi-phase alloys in the KIT Segregation phenomena do not occur as cause for inhomogeneities of the final material, since the material already arrives in the liquid state in the KIT.

Another advantage is that the frequency range of the induction coil which is advantageous for homogenizing the already molten alloy is higher than the frequency range which is advantageous for the melting of a solid alloy. Surprisingly, this significantly reduces the edge porosity of the block drawn off from the solidifying melt in the KIT and thus increases the block quality.

A particular advantage of the method according to the invention is that all the required dimensions of the alloy ingots can be realized by the dimensions of the cold wall induction crucibles that are freely selectable in a technically sensible framework, which is not guaranteed by the VAR technology.

The process is preferably carried out under vacuum or under inert gas, and non-contaminated production wastes can be recycled to the process. The material loss is according to a technical embodiment of the invention still 12% compared to 35% with the conventional VAR technology.

With the method according to the invention is a realization of local (macroscopic) variations of the main alloying elements aluminum and titanium of <± 0.5 at.%; other metallic alloy components: ± 0.2 at.%; Strength-increasing elements (boron, carbon, silicon): ± 0.05 at.%; possible over the entire ingot.

Novel combinations of prior art sub-processes which are known per se and which ensure a continuous or quasi-continuous supply of liquid, pre-homogenized material into a cold-wall induction crucible for the purpose of continuous or quasi-continuous strand withdrawal from the KIT are also considered as being inventive ,

In particular, this relates to the combination of an inductively heated ablation device for alloy rods or alloy electrodes (inductive dripping melt) a KIT with a strand extraction device and the combination of a plasma cold wall furnace with fired gutter system, designed as a skull overflow with said KIT and said strand extraction device. Both process combinations according to the invention are described in detail below on the basis of design examples.

Important sub-steps of these inventive combinations of processes, such as the inductive melting of electrodes, the PACHM process, the melting of alloys in the cold wall induction crucible and the block deduction of alloys of ceramic and cold wall induction crucibles are known under clearly different boundary conditions, targets and materials and already Use came.

The inductive melting of metals is for example in the U.S. Patents 4,923,508 . 5 003 551 and 5 014 769 described. In addition, the inductive melting of electrodes has also been described in connection with the production of titanium alloy powder by the so-called EIGA (Electrode Induction Melting Gas Atomization) method (see. DE-A-41 02 101 . DE-A-196 31 582 ). In this process, an alloy electrode immersed in a rollover with ceramic insulated RF coil. As in the present case, the electrode is completely melted by a top surface smearing process. The further processing of the melt takes place in a gas nozzle in which the drops are atomized. This process is used exclusively for powder production and not for the production of ingots. In the present description, the melt is subjected to further homogenization in the KIT before the block deduction (ingot production) takes place.

With regard to the state of the art for melting materials in the cold wall induction crucible, reference is made to the two US Pat. Nos. 5,892,790 and 6,144,690. However, both patents are not concerned with ingot production. The situation is different with the patents DE-A-198 52 747 and DE-A-196 50 856 , The crucial difference between the patents DE-A-198 52 747 such as DE-A-196 50 856 and the present invention is in the supply of material. While in the present case pre-homogenized, molten material is supplied to the KIT, the KIT in the specified patent is filled with solid. This means that in the present case the energy input in the KIT is used exclusively for further homogenization and liquid holding of the material, while in the cited patent the melting, the homogenization and the solidification process take place at the same location - the KIT-. This increases the likelihood of segregation occurring.

The block deduction is also known from the prior art, in particular from the ceramic crucible. The patents pertaining to this prior art relate predominantly to the block removal of non-ferrous metals (Cu, brass). The patents listed above DE-A-198 52 747 and DE-A-196 50 856 however, include the block vent from the cold wall induction crucible, however, the KIT from which the block vent takes place is supplied as a solid material and not as a pre-homogenized molten material. This situation may, as described above, lead to homogeneity differences in the material withdrawn as a block.

The electrodes are preferably produced by pressing and / or sintering powdery or granular alloy components (cf. DE-A-196 31 582 to -584, DE-A-198 52 747 ).

Fig. 1

den VAR-Prozeß für mehrfach umgeschmolzene γ-TiAl Ingots: (1) Elektrodenvorschub, (2) Ofenkammer, (3) luftgekühlte Stromversorgung, (4) Sammelschiene für Kabel, (5) Elektrodenführung, (6) Tiegel mit Wassermantel, (7) Teil der Vakuumeinrichtung, (8) XY_Anpassung, (9) Druckmeßdose,

Fig. 2

Abweichungen des Al-Gehaltes in Längsrichtung des Ingots nach zweifachem (schwarze Symbole) und dreifachem (graue Symbole) VAR-Umschmelzvorgang,

Fig. 3

Schematische Darstellung des Kalt-Wand-Elektronenstrahl-Schmelzens (oben) und des Kalt-Wand-Plasma-Schmelzens (unten)

Fig. 4

das erfindungsgemäße Verfahren (Beispiel 1) zur Fertigung chemisch homogener γ-TiAl Blöcke mit variablen Dimensionen: (1) rotierende Elektrode, (2) induktive HF-Spule, (3) Kaltwand-Induktionstiegel und (4) Kühlvorrichtung und Blockabzug,

Fig. 5

das erfindungsgemäße Verfahren (Beispiel 2) zur Fertigung chemisch homogener γ-TiAl Blöcke mit variablen Dimensionen: (1) Chargierrampe, (2) Plasmabrenner, (3) Kalter Herd, (4) Kaltwand-Induktionstiegel (KIT) und (5) Kühlvorrichtung und (6) Blockabzug.

The drawings show

Fig. 1

the VAR process for multiple remelted γ-TiAl ingots: (1) electrode feed, (2) furnace chamber, (3) air-cooled power supply, (4) cable busbar, (5) electrode guide, (6) water jacket crucible, (7) Part of the vacuum device, (8) XY_adjustment, (9) load cell,

Fig. 2

Deviations of the Al content in the longitudinal direction of the ingot after double (black symbols) and triple (gray symbols) VAR remelting process,

Fig. 3

Schematic of Cold Wall Electron Beam Melting (top) and Cold Wall Plasma Melting (bottom)

Fig. 4

the inventive method (Example 1) for producing chemically homogeneous γ-TiAl blocks with variable dimensions: (1) rotating electrode, (2) inductive RF coil, (3) cold wall induction crucible and (4) cooling device and block deduction,

Fig. 5

the inventive method (Example 2) for the production of chemically homogeneous γ-TiAl blocks with variable dimensions: (1) charging ramp, (2) plasma torch, (3) cold hearth, (4) cold wall induction crucible (KIT) and (5) cooling device and (6) block deduction.

• der Herstellung von vorhomogenisiertem, schmelzflüssigem Material mit Hilfe des induktiven Abschmelzens in einer HF-Spule oder dem PACHM-Verfahren. In beiden Fällen enthält das Ausgangsmaterial die Summe aller Legierungsbestandteile, die aber nur unzureichend homogen verteilt sind,
• der Zufuhr von schmelzflüssigem Material in einen Kaltwand-Induktionstiegel
• der weiteren Homogenisierung des flüssigen (abgeschmolzenen) Materials im Kaltwand-Induktions-Tiegels (KIT), und
• dem vorzugsweise kontinuierlichen Blockabzug aus dem KIT.

In summary, the method according to the invention is a melt-metallurgical technology for producing chemically and structurally homogeneous alloy ingots, in particular γ-TiAl blocks as ingot material for the forming route or for Remelter sticks for the casting route. The technology includes the combination of:

• the production of pre-homogenised, molten material by means of inductive melting in an RF coil or the PACHM process. In both cases, the starting material contains the sum of all alloy constituents, which, however, are only insufficiently homogeneously distributed,
• the supply of molten material into a cold wall induction crucible
• the further homogenization of the liquid (melted) material in the cold wall induction crucible (KIT), and
• the preferably continuous block discharge from the KIT.

The individual process steps will be described again in detail below.

First, the production of the electrodes. By means of a conventional melt metallurgical process, for example by means of the VAR technology, pressed electrodes containing all alloy constituents (Ti sponge, Al granules, master alloy granules) are melted down on diameter bars of 150 mm, for example, by enlarging the diameter. These are rods that have a low chemical homogeneity and a certain porosity. These serve as electrodes for the subsequent strand withdrawal.

The first technological step can be represented by two alternative ways - inductive melting or the PACHM process. Both methods have the goal of producing a pre-homogenized, molten material.
In inductive melting, the melted by a conventional method electrode using an RF coil (according to EIGA method, see DE-A-41 02 101 . DE-A-196 31 582 ) melted inductively into a KIT. The system Spulel Dripping material and the shape of the coil are in close interaction. According to the minimum requirements for Abschmelzraten and block diameter of the frequency range at the outer resonant circuit is 70 to 300 kHz. When using high-frequency induction fields, a pronounced skin effect can be expected in the ablation electrode. This effect, in combination with the relatively low thermal conductivity of the titanium aluminides, leads to local overheating in the surface layer and, as a consequence, to aluminum evaporations that can not be detected quantitatively. Since the pronounced current flow in the skin layer is an essential feature of the high-frequency alternating current fields and thus unavoidable, the only way to reduce the aluminum evaporation is to shorten the residence time of the material in the electromagnetic field. By uniformly pre-heating the Abtroefeldektrode, by means of inductive heating (center frequency about 500 Hz to 4 kHz) to temperatures below the melting point of the alloy, the energy required for melting in the field or power is reduced by the amount of energy already introduced. This shortens either the residence time in the AC field for a single volume element and in total for the entire Abtroefeldektrode As a result, the deposition rate increases, or it can be driven overall with lower power in the RF coil. From the stated requirements and conclusions it follows that the design or performance dimensioning of the outer resonant circuit and the RF frequency only makes sense in close interaction with the design of the electrode preheating. The feed rates for the electrodes should be controllable in such a range that for electrode diameters of 150 mm drip rates corresponding to the mass flow rates of at least 50 kg / h can be driven.

When using the PACHM process, the melting process is realized by plasma torches. The plasma torches perform two functions: melting the source material and maintaining constant ambient conditions during the block draw. Starting material, in the form of mechanically comminuted prealloyed compacts, is successively recharged via a hydraulic ramp into the melting chamber. In the water-cooled cold wall crucible made of copper, the material is finally melted using the plasma torch. The cold wall crucible ("cold hearth") serves as a "disposal tool" of undesirable high density (tub bottom) and low density inclusions (floating slag) of the melt and as a "reservoir" for supplying the system crucible block vent with molten material. The amperage of plasma torches above the cold hearth is between 275-550 A, but may vary depending on the type and number of plasma torches used.

In the next step, the melt is fed to the cold wall induction crucible. In the KIT equipped with a movable bottom, the stirring action of the electromagnetic field further improves the homogeneity of the melt in a larger, substantially constant molten volume. The residence time of the melt in the crucible is about 20 minutes to 45 minutes. Skull melting in cold wall induction crucibles (KIT) has been an industrially established technique for years. In this case, a field is generated by electromagnetic induction in a water-cooled copper crucible, which is used for heating or melting of the materials. At the same time, the Lorenz forces that occur express that Melting material partially from the crucible walls and establish a circulating flow in the melt, which leads in consequence to a good mixing of the melt phase. In the area of the bottom of the crucible and in the lower part of the crucible wall, due to the shape of the electromagnetic field, the formation of a species-specific, fixed marginal shell (skull) occurs. This skull, in combination with the free surface created by the Lorentz forces, prevents the direct contact of the melt material with the crucible, so that the risk of contamination throughout the melting phase and the plant safety are guaranteed.

In the case of inductive melting, the continuous feeding of the KIT with melted material is made possible by the connected electrode magazine, which can hold several electrodes at the same time, which are then melted one after the other. In the case of the PACHM process, the recharging of mechanically comminuted prealloyed material occurs via a hydraulic ramp.
The Bodenskull, which in its thickness and its habit depends directly on the shape of the induction field, provides the starting point for a possible production of semi-finished products. Namely, when the soil is lowered during the process, the system reacts in such a way that a new state of equilibrium is formed and thus a new layer grows on the old soil skip. The continuous lowering of the soil thus leads to a system of constantly adapting equilibrium states and consequently to an almost continuously growing soil layer. As the ground surface of the soil screed is fixed above the crucible bottom, the growth of new layers leads to the formation of a semi-finished product (block). However, the constant mass depletion from the KIT also causes the supply of new molten material.

The cooling of the melt during the removal of the blocks is preferably carried out with the aid of water-cooled Cu segments.

The block discharge from the KIT produces a chemically homogeneous and largely pore-free ingot. In this method, the diameter of the KIT is freely selectable in large areas, so that a variable choice exists in Ingot diameter. The take-off speeds may preferably be in a range of 0 to 50 mm / min.

The products produced according to the invention can be used for various purposes. First and foremost, semifinished products are produced from them in a first forming step (extrusion), which are used for further processing in the forming route (forging, rolling). Ingots of high structural and chemical quality are required for the production of γ-TiAl-based components via the forming route. The components are, for example, valves and turbine blades, which have an excellent property profile and must withstand the highest requirements.

Furthermore, the products of the invention can also serve as Remelter stocks for the production of cast blanks on the investment casting or centrifugal casting. Remelter sticks are needed as starting material for the investment casting and centrifugal casting route. The chemical and structural quality is not in the foreground, because the material - in contrast to the ingots - is melted again. Therefore, in the method according to the invention, the stage (ii) can be dispensed with and the pressed electrodes can be directly inductively melted or premixed compacts can be melted by the PACHM method. The investment casting route is used to produce components with a sophisticated design and complex requirement profiles. An example of this is the already commercialized turbocharger based on γ-TiAl. Centrifugal casting is an inexpensive process for the production of mass components (eg valves) with a simple design and requirement profiles. The production of Remelter stocks via the process according to the invention leads to products which are significantly more homogeneous than the corresponding products of the prior art, and can be produced by the block deduction in any cylindrical dimension, while in the previously used method on the dimensions the existing mold was instructed. By the method according to the invention, it is possible to freely choose the diameter and the length of the Remelter stocks and thus to be able to immediately consider each customer request in a simple manner.

The following examples for the specific embodiment of the invention serve to better explain.

Example 1 (see Fig. 4):

The example illustrates the preparation of a continuous ingot of a γ-TiAl alloy with the composition Ti -46.5Al -4 (Cr, Nb, Ta, B) (reported in at .-%) with a diameter of 180 mm and a length of 2,600 mm.

The first step is the production of 4 single VAR-melted electrodes with a diameter of 150 mm and a length of 1,000 mm from pressing electrodes, all alloying components in the form of Ti sponge, Al granules and suitable master alloys for Cr, Nb, Ta and B included. The not yet homogeneous rods serve as electrodes for the production of pre-homogenized, molten material via the inductive melting in an RF coil. The electrodes are cone-shaped at the base, wherein the angle of employment is about 45 °.

During inductive melting, an electrode is fed from the magazine, which holds all four electrodes, to the likewise cone-shaped HF melting-off coil and is inductively melted into a cold-wall induction crucible. The melt is formed on the entire surface of the cone and converges at the apex of the cone to a melt stream in which the material is pre-homogenized. The melt passes, by gravity, into the cold wall induction crucible located below the Abschmelzspule. The frequency at the outer ring of the Abschmelzspule is 80.6 kHz. By uniformly pre-heating the Abtroefeldektrode by means of inductive heating (center frequency about 500 Hz to 1 kHz) via an above the Abschmelzspule attached auxiliary coil to temperatures below the melting point of the alloy (about 1300 ° C), an increased melting capacity of over 50 kg / h is achieved. The electrode is rotated at a speed of 4 rpm; the lowering speed is approx. 12 mm / min.

The pre-homogenized molten material falls into a cold wall induction crucible with a bottom peel-off tray. The diameter of the crucible is 180 mm. The melt solidifies in the lower part of the crucible and is drawn off continuously downwards. The cooling of the melt when removing the blocks is done with water-cooled copper segments. The take-off speed is approx. 1 mm / min. The average residence time of the melt for homogenization in the cold wall induction crucible is about 20 minutes, which corresponds to a bath height of about 160 mm. The bath temperature is 1580 ° C and the frequency surrounding that of the crucible. Induction coil is applied, is 12 kHz.

After the first electrode has melted off, the second electrode is moved to the required position and heated to melting, whereby the strand withdrawal is interrupted during this time. Thereafter, the process continues as described until all 4 electrodes of the magazine have melted.

The process can be carried out both under vacuum and under protective gas.

The resulting block has a diameter of about 180 mm and a total length of 2,600 mm and is characterized by a very good chemical and structural homogeneity. The local variations for aluminum and titanium are less than ± 0.5 at.%, Those of Cr, Nb and Ta less than ± 0.2 at.% And those for B less than ± 0.05 at.%.

Example 2 (see Fig. 5):

Embodiment 2 differs in the manner of producing the molten material and the supply into the KIT of Embodiment 1. The process is performed under He shielding gas. An alternative to inductive melting offers the PACHM process (plasma arc cold hearth melting). In the present embodiment, the starting material in the form of simply VAR-melted electrodes according to Example 1 by means of a He plasma torch (150kW) in a water-cooled copper crucible melted and continued via a water-cooled channel also fired with a He plasma torch (150 kW). The current of the plasma torch above the cold hearth is about 500 A. The liquid alloy melt flows in the material's own skull to an overflow above the KIT, from where it flows continuously into the KIT. The starting material is continuously recharged via a hydraulically controlled ramp. The cold crucible performs two main functions: in addition to a reservoir for pre-homogenised, molten material, it also serves as a depository for unwanted high-density and ceramic inclusions.

The further process is analogous to the procedure described in Example 1.

The specified technical data in the examples are not intended to limit the invention in any way. In particular, the number, type and performance of the plasma torch, the material for the cold crucible, power and frequency ranges of the induction coil, diameter of the KIT, bath levels of the melts in the KIT and feed and withdrawal speeds can be varied within the scope of the prior art, without the invention is impaired.

Claims (11)

1. A method of producing metallic and intermetallic alloy ingots by continuous or quasi-continuous billet withdrawal from a cold wall induction crucible, in particular of producing metallic and intermetallic alloy ingots of high homogeneity and low porosity of any adjustable diameter, characterized in that it is based on the following sequence:

   (i) producing electrodes by customarily mixing and compressing the selected starting materials;

   (ii) at least once remelting the electrodes obtained in step (i) in a conventional fusion-metallurgical process;

   (iii) producing a pre-homogenized, molten material of the electrode material obtained in step (ii) by melting off in a cold crucible plasma furnace or by inductively melting off in a high frequency coil;

   (iv) homogenizing the pre-homogenized, molten material obtained in step (iii) in a cold wall induction crucible, by this alloy material being supplied in a molten and pre-homogenized state continuously or quasi-continuously to the cold wall induction crucible, and

   (v) withdrawing the melt, solidified by cooling, from the cold wall induction crucible of step (iv) in the form of solidified ingots of freely adjustable diameters and lengths.
2. A method according to claim 1, characterized in that inter-metallic γ-TiAl-based alloy ingots are produced.
3. A method according to claim 1 and 2, characterized in that the alloys are described by the following summation formula:
   
           Ti_xAl_y(Cr,Mn,V)_u(Zr,Cu,Nb,Ta,Mo,W,Ni)_v(Si,B,C,Y)_w
   
   with the concentrations of the alloying constituents being within the following ranges (in atomic percent):

   x = 100-y-u-v-w

   y = 40 to 48, preferably 44 to 48

   u = 0.5 to 5

   v = 0.1 to 10 and

   w = 0.05 to 1.
4. A method according to any of the claims 1 to 3, characterized in that the melting process for producing the pre-homogenized, molten material takes place in a high frequency field of a frequency in the range of 70 to 300 kHz.
5. A method according to claims 1 to 4, characterized in that the temperature of the pre-homogenized, molten material ranges between 1400 to 1600°C.
6. A method according to any of the claims 1 to 5, characterized in that the electrodes in step (iii) used for producing the molten, pre-homogenized material by means of an induction coil rotate preferably at a speed between 2 and 5 revolutions per minute.
7. A method according to any of the claims 1 to 6, characterized in that the method is executed quasi-continuously by one or several electrodes, in case of inductive melting, being quasi-continuously fed while an ingot is simultaneously withdrawn from the cold wall induction crucible.
8. A method according to any of the claims 1 to 7, characterized in that homogenization in the cold wall induction crucible in step (iv) takes place at a temperature of 1400 to 1700°C.
9. A method according to any of the claims 1 to 8, characterized in that homogenization in the cold wall induction crucible in step (iv) takes place in a range of frequency of 4 to 20 kHz.
10. A method according to any of the claims 1 to 9, characterized in that cooling the melt upon ingot withdrawal in step (v) takes place by the aid of water-cooled copper segments.
11. A method according to any of the claims 1 to 10, characterized in that the diameter of the ingots withdrawn in step (v) is in the range of 40 to 350 mm.

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