EP2342365B1 - Process for manufacturing a beta-gamma tial-based alloy - Google Patents

Abstract

A method for the production of a γ-TiAl base alloy by vacuum arc remelting, which γ-TiAl base alloy solidifies via the (&bgr;-phase (&bgr;-γ-TiAl base alloy), comprises the following method steps: forming a basic melting electrode by melting, in at least one vacuum arc remelting step, of a conventional γ-TiAl primary alloy containing a lack of titanium and/or of at least one (&bgr;-stabilising element compared to the (&bgr;-γ-TiAl base alloy to be produced; allocating an amount of titanium and/or (&bgr;-stabilising element to the basic melting electrode, which amount corresponds to the reduced amount of titanium and/or (&bgr;-stabilising element, in an even distribution across the length and periphery of the basic melting electrode; adding the allocated amount of titanium and/or (&bgr;-stabilising element to the basic melting electrode so as to form the homogeneous (&bgr;-γ-TiAl base alloy in a final vacuum arc remelting step.

Description

The invention relates to a process for the production of γ-TiAl base alloys by means of vacuum arc melting (VAR), which solidify completely or at least partially primarily via the β-phase. Such target alloys will hereinafter be referred to as β-γ-TiAl base alloy.

The technical field of the present invention is the melt metallurgical production of β-γ-TiAl alloys by means of vacuum arc melting (VAR). Originally, starting from the raw materials titanium sponge, aluminum and alloying elements and master alloys compact bodies are pressed in which the desired alloying constituents are present in the stoichiometrically appropriate form. Optionally, this evaporation losses caused by the subsequent melting are kept available. The compacts are either melted directly by means of plasma melts into so-called ingots (PAM) or assembled to self-consuming electrodes and melted down to ingots (VAR). In both cases, materials are created whose chemical and structural homogeneity is unsuitable for technical use and which consequently must be remelted at least once (see p. V. Guether: "Microstructures and Defects in γ-TiAl based Vacuum Arc Remelted Ingot Materials", 3rd Int. Symp. On Structural Intermetallics, September 2001, Jackson Hole WY, USA ).

1. (i) Herstellung von Elektroden durch übliches Vennischen und Verpressen der ausgewählten Ausgangsstoffe,
2. (ii) mindestens einmaliges Umschmelzen der in Stufe (i) erhaltenen Elektroden durch ein übliches schmelzmetallurgisches Verfahren,
3. (iii) induktives Abschmelzen der in Stufe (i) oder (ii) erhaltenen Elektroden in einer Hochfrequenz-Spule,
4. (iv) Homogenisieren der in Stufe (iii) erhaltenen Schmelze in einem Kaltwandinduktionstiegel und
5. (v) Abziehen der Schmelze unter Kühlung aus dem Kaltwandinduktionstiegel von Stufe (iv) in Form von Blöcken mit frei einstellbarem Durchmesser.

From the DE 101 56 336 A1 a method for the production of alloy ingots is known, which comprises the following steps:

1. (i) preparation of electrodes by conventional Vennisch 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) inductive melting of the electrodes obtained in step (i) or (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 blocks of freely adjustable diameter.

The DE 195 81 384 T1 describes intermetallic TiAl compounds and processes for their preparation, wherein the alloy by heat treatment of an alloy having a Ti concentration of 42 to 48 atom%, an Al concentration of 44 to 47 atom%, a Nb concentration of 6 to 10 at.% And a Cr concentration of 1 to 3 at.% At a temperature in the range of 1300 to 1400 ° C.

The DE 196 31 583 A1 discloses a method for producing an alloy TiAl-Nb product in which an alloy electrode is first prepared from the alloy components. The formation of the alloy electrode is carried out by pressing and / or sintering the alloy components to the electrode. The latter is melted off by an induction coil.

From the JP 02277736 A For example, a heat-resistant TiAl based alloy is known in which specific amounts of V and Cr are incorporated into a Ti-Al intermetallic compound for the purpose of improving heat resistance and ductility.

The DE 1 179 006 A Finally, ternary or higher titanium-aluminum alloys with such elements stabilize the α- and β-phase of the titanium.

The usual process for remelting is self-consuming vacuum-arc melting, since the plasma-melting plants are generally not designed for feeding compact ingots as starting material. In the case of conventional, biphasic in the form of lamellar colonies of the α ₂ -Ti ₃ Al phase and the γ-TiAl phase constructed γ-TiAγ base alloys remelting takes place in the vacuum arc furnace (VAR furnace) easily and leads to the desired result (s. V. Guether: "Status and Prospects of γ-TiAl Ingot Production", Int. Symp. On Gamma Titanium Aluminides 2003, ed. H. Clemens, Y.-W. Kim and AH Rosenberger, San Diego, TMS 2004 ).

A new generation of γ-TiAl high performance materials, such as Applicant's designated ^TNM® alloys, has a structural design different from conventional TiAl alloys. In particular, due to the lowering of the aluminum content to usually 40 at .-% to 45.5 atom%, but also due to the addition of β-stabilizing elements such as Cr, Cu, Hf, Mn, Mo, Nb, V, Ta and Zr is set a primary solidification path over the β-Ti phase. This results in very fine microstructure, in addition to lamellar α ₂ / γ colonies also contain globular β grains and globular γ grains, sometimes including globular α ₂ grains. Materials with such structures have significant advantages in terms of thermo-mechanical properties and processability by forming technologies (s. H. Clemens: "Design of Novel β-Solidifying TiAl Alloys with Adjustable β / B2 Phase Fraction and Excellent Hot Workability", Advanced Engineering Materials 2008, 10, No.8, p. 707-713 ). Such alloys are - as already stated - hereinafter referred to as β-γ-TiAl base alloys.

The disadvantage is that cracking occurs again during the remelting of electrodes from this material in the VAR furnace, the result of which is frequently the flaking off of constituents of the self-consumable alloy electrode from the primary melting zone. These chipped parts fall into the molten bath and are no longer completely remelted therein. This causes structural defects in the ingot, making the ingot material unusable. Remelting in the VAR furnace is no longer technically reproducible under these conditions.

As a cause for the disturbing chipping behavior massive phase transformations in the temperature range between the eutectoid temperature and the phase boundary temperature to the β-phase phase are considered. Due to the different linear expansion coefficients of the various phase components, in particular during phase transformations, sudden changes in the integral linear thermal expansion coefficient of the alloy and, as a consequence thereof, internal stresses which exceed the strength of the material in the given temperature range occur.

Corresponding dilatometer measurements on a TNM ^® -B1 alloy (Ti - 43.5A1 - 4.0Nb - 1.0Mo - 0.1B at .-%) show that the linear expansion coefficient of a corresponding alloy sample in the temperature range between 1000 ° C and 1.200 ° C more than quadrupled from 9 x 10 ^-6 to 40 x 10 ^-6 K ^-1 . This behavior is in the attached Fig. 4 shown, in which the curve A represents the linear expansion coefficient of this alloy. The curve R represents the heating rate of the sample.

During VAR melting, based on the length of the consumable electrode, a temperature field of melting temperature (about 1570 ° C.) at the bottom of the electrode extends to near room temperature at the electrode suspension through the material. Not far from the melt front, the critical temperature interval between 1000 and 1200 ° C is reached. The relatively poor ductility of the intermetallic material then leads in this zone to the fact that the stresses formed there discharge in the form of cracks, which in turn lead to the described chipping of unmelted pieces from the electrode.

Based on this described problem of the prior art, the present invention seeks to provide a method for producing a β-phase solidified γ-TiAl base alloy - hereinafter referred to as β-γ-TiAl base alloy - specify that Avoiding the cracking problem leads to a reliable production of such a target alloy.

• Erschmelzen einer Basisschmelzelektrode einer herkömmlichen γ-TiAl-Primärlegierung mit einem defizitären Gehalt an Titan und/oder an mindestens einem β-stabilisierenden Element gegenüber der herzustellenden β-γ-TiAl-Basislegierung in mindestens einem ersten Vakuum-Lichtbogen-Umschmelzschritt,
• Zuordnen einer dem defizitären Gehalt des Titans und/oder β-stabilisierenden Elements entsprechenden Menge an Titan und/oder β-stabilisierendem Element zur Basisschmelzelektrode in gleichmäßiger Verteilung über deren Länge und Umfang, und
• Zulegieren der zugeordneten Menge des Titans und/oder β-stabilisierenden Elements in die Basisschmelzelektrode zur Bildung der homogenen β-γ-TiAl-Basislegierung in einem letzten Vakuum-Lichtbogen-Schmelzschritt.

This object is achieved by the method steps indicated in claim 1 as follows:

• Melting a base melt electrode of a conventional γ-TiAl primary alloy with a deficient content of titanium and / or on at least one β-stabilizing element in relation to the β-γ-TiAl base alloy to be produced in at least one first vacuum arc remelting step,
• Assigning to the deficient content of the titanium and / or β-stabilizing element corresponding amount of titanium and / or β-stabilizing element to the base melt electrode in a uniform distribution over the length and circumference, and
• Alloying the associated amount of the titanium and / or β stabilizing element in the base melt electrode to form the homogeneous β-γ-TiAl base alloy in a final vacuum arc melting step.

The successive remelting steps during the vacuum arc melting are thus subdivided into the melting of a primary alloy in the first remelting steps, wherein a base melted electrode is produced from a conventional γ-TiAl primary alloy, and the melting of the target alloy in the form of the desired β- γ-TiAl-based alloy in the last remelting step. The primary alloy has a deficiency of titanium and / or a deficiency of β-stabilizing elements such as Nb, Mo, Cr, Mn, V, and Ta. In this case, the alloy is a defined amount of titanium and / or β in the preparation of the pressed base melt electrode deprived of stabilizing elements, so that an aluminum content of the primary alloy preferably between 45 at .-% (particularly preferably 45.5 at .-%) and 50 at .-% sets. The contents of aluminum and β-stabilizing elements are chosen so that the solidification path of the primary alloy is at least partially via the peritectic conversion. It is thus set a structure analogous to conventional TiAl alloys, which can be processed easily in the VAR oven.

In the last melting step, the target alloy is readjusted by the addition of the materials originally removed from the press electrode. Preferably, these materials are welded as cladding to form a composite electrode firmly on the outer surface of the Abschmelzelektrode to safely exclude a solid state drop into the molten bath. It is also possible to accomplish this by a sheath insert of the deficient alloy content on the inside of the Umschmelzkokille the VAR furnace.

Surprisingly, it has been found that, with a suitable selection and suitably evenly distributed attachment of the deficient alloy constituents on the electrode jacket surface, there are no negative consequences for the local chemical homogeneity of the resulting ingot of the produced β-γ-TiAl base alloy as target alloy.

Fig.1

eine Prinzipskizze eines Vakuum-Lichtbogen-Schmelzofens,

Fig. 2

eine perspektivische Ansicht einer Komposit-Elektrode in einer ersten Ausführungsform,

Fig. 3

eine perspektivische Ansicht einer Komposit-Elektrode in einer zweiten Ausführungsform und

Fig. 4

ein Diagramm des linearen Ausdehnungskoeffizienten als Funktion der Temperatur einer TNM®-B1-Legierung.

Further preferred embodiments of the manufacturing method according to the invention are specified in further subclaims, whose details and features will become apparent from the following description of embodiments with reference to the accompanying drawings. Show it:

Fig.1

a schematic diagram of a vacuum arc melting furnace,

Fig. 2

a perspective view of a composite electrode in a first embodiment,

Fig. 3

a perspective view of a composite electrode in a second embodiment and

Fig. 4

a graph of the linear expansion coefficient as a function of the temperature of a TNM®-B1 alloy.

Based on Fig. 1 In principle, a vacuum arc melting furnace 1 and the method for remelting a corresponding electrode 2 to an ingot 3 will be explained. Thus, the VAR furnace 1 has a copper crucible 4 with a bottom plate 5. Around this copper crucible 4 around a water jacket 6 with water inlet 7 and 8 water outlet is arranged. The copper crucible 4 is also closed at the top of a vacuum bell 9, passes through the top of a lifting bar 10 vertically displaceable. At this lifting bar 10 sits the holder 11, on which the actual electrode 2 is suspended.

Via a DC power supply 12, a DC voltage is applied between the copper crucible 4 and the lifting rod 10, due to which a high-current arc is ignited and maintained between the electrode 2 electrically connected to the lifting rod 10 and the copper crucible 4. This leads to the melting of the electrode 2, wherein the molten alloy material collects in the copper crucible 4 and solidifies there. In a continuous process in which, between the self-consuming electrode 2 via the electrode arc gap 13, the arc to the molten reservoir 14 at the top of the ingot 3 runs, the electrode 2 is successively remelted to ingot 3 under homogenization of the alloy components.

This process can be repeated several times with larger diameter crucibles 4, wherein the ingot of a remelting step to the electrode of the next Umschmelzschrittes. Thus, the degree of homogenization of the ingots to be produced is improved with each remelting step.

Various embodiments for producing a β-γ-TiAl base alloy will now be described below:

Embodiment 1

The target composition of the β-γ-TiAl alloy is Ti-43.5Al-4.0Nb-1.0Mo-0.1B (at.%) Or Ti-A128.6-Nb9.1-Mo2.3. B0.03 (m-%). The composition of the primary alloy for the base melt electrode is determined by a reduction of the titanium content to Ti - 45.93Al - 4.22Nb - 1.06Mo - 0.11B (at .-%). First, conventionally, a ingot 3 of the primary alloy of 200 mm in diameter and 1.4 m in length is prepared from a press electrode 2 by 2-fold VAR melting as described above, without cracking problem. As starting materials for the production of the pressing electrode 2 titanium sponge, pure aluminum and master alloys are used.

In order to raise the reduced titanium content in the base melt electrode to the desired value of the β-γ-TiAl alloy in the target alloy, the entire surface area of the ingot 3 becomes of the primary alloy Pure titanium sheet 15 with a thickness of 3 mm (mass 12 kg) wound and partially welded to the outer surface 16 of the ingot 3, as shown in Fig. 2 is shown. In this case, the upper edge 17 of the titanium sheet 15 is completely welded over the circumference of the ingot 3 with this. Furthermore, welding point 18 are set distributed over the lateral surface 16. The self-consumable electrode thus assembled is remelted as a composite electrode 19 in a final melting step in the VAR furnace 1 to a ingot 3 having a diameter of 280 mm and the composition of the target alloy.

Embodiment 2

The target composition, the feeds used and the composition of the primary alloy correspond to Embodiment 1. From the primary alloy, an ingot 3 having a diameter of 140 mm and a length of 1.8 m is manufactured by simply VAR-melting press electrodes 2. The mass of the ingot is 115 kg. In the mold formed by the copper crucible 4 of the VAR furnace 1 is before the last melt of the base melt electrode 2, a sheet of pure titanium with the dimensions circumference 628 mm x height 880 mm x thickness 3 mm (mass 7.6 kg) in the inserted inner surface. In sum, the composition of the primary alloy ingot forming the base melt electrode 2 and the titanium sheet thus provide the target composition. The remelting takes place in the lined with the titanium sheet copper crucible 4 to an intermediate electrode such that the outer skin of the titanium sheet is not completely melted with and remains as a stable shell. Although in the subsequent last VAR remelting step of the intermediate electrode, cracking may occur, but due to the mechanical stabilization by the ductile outer shell, this does not lead to cracking Drop down of electrode material in the melt reservoir 14 lead.

Embodiment 3

The target composition, the feeds used and the composition of the primary alloy correspond to the embodiment 1, also the production of the composite electrode 19. In contrast to Example 1, the last remelting takes place in a so-called VAR skull melter, ie a vacuum arc melting device with a water-cooled, tiltable copper crucible. The target material's molten alloy material is poured into permanent molds made of stainless steel, which are attached to a rotating casting wheel. The casting bodies produced by centrifugal casting are used as starting material for the production of components from the target alloy.

Embodiment 4

A β-γ-TiAl alloy according to U.S. Patent 6,669,791 has a composition (target alloy) of Ti - 43.0Al - 6.0V (at .-%) and Ti - A129.7 - V7.8 (m%). The composition of the primary alloy is determined by the complete reduction of the strongly β-stabilizing element vanadium to Ti - 45.75A1 (at .-%) or Ti - A132.2 (m -%). The starting materials used are titanium sponge, aluminum and vanadium. First of all, a base melt electrode 2 is conventionally produced as an ingot of the binary TiAl primary alloy with a diameter of 200 mm and a length of 1 m by double VAR melting (mass 126 kg). As Fig. 3 shows, along the entire surface 16 of the base melt electrode 2 along axialaxialparallel eight vanadium rods 20 with a diameter of 16.7 mm and a length of 1 m (total mass 10.7 kg) each offset by 45 ° to each other and thus uniformly over the circumference of Electrode 2 distributed welded. The resulting composite electrode 19 'of the binary primary alloy and the welded vanadium rods 20 is remelted in the final third melting process to an ingot of target alloy with a diameter of 300 mm in the VAR furnace 1.

Embodiment 5

The target composition of the γ-TiAl alloy corresponds to that of Embodiment 1 (Ti - 43.5A1 - 4.0Nb - 1.0Mo - 0.1 B at .-%). The composition of the primary alloy is determined by a complete reduction of the molybdenum content and a partial reduction of the titanium content to Ti - 49.63A1 - 4.57Nb - 0.11 B (at .-%). From the primary alloy, a base melt electrode 2 having a diameter of 200 mm and a length of 1 m is produced by double VAR melting. The ingot mass is 126 kg. On the lateral surface 16 of the electrode 2, eight rods made of the commercial TiMo15 alloy are welded on, parallel to the longitudinal direction parallel to Embodiment 4. The diameter of the rods is 26 mm, the length of the rods corresponds to the ingot length. The total mass of TiMo15 rods is 19.6 kg. The resulting composite electrode consisting of one ingot of the primary alloy and eight TiMo15 rods is remelted in the final third melting process to an ingot of the target alloy with a diameter of 300 mm in the VAR furnace 1.

Claims (10)

1. Method for the production of a γ-TiAl base alloy by vacuum arc remelting which γ-TiAl base alloy solidifies via the β-phase (β-γ-TiAl base alloy),
   characterized by the following method steps:

   - forming a basic melting electrode by melting, in at least one vacuum arc remelting step, of a conventional γ-TiAl primary alloy containing a lack of titanium and/or of at least one β-stabilising element compared to the β-γ-TiAl base alloy to be produced;

   - allocating an amount of titanium and/or β-stabilising element to the basic melting electrode, which amount corresponds to the reduced amount of titanium and/or β-stabilising element, in an even distribution across the length and periphery of the basic melting electrode;

   - adding the allocated amount of titanium and/or β-stabilising element to the basic melting electrode so as to form the homogeneous β-γ-TiAl base alloy in a final vacuum arc remelting step.
2. Method for the production of a β-γ-TiAl base alloy according to claim 1, characterized in that the basic melting electrode (2) of the conventional γ-TiAl base alloy has an aluminium content of 45 at. % to 50 at. %.
3. Method for the production of a β-γ-TiAl base alloy according to claim 1 or 2, characterized in that the basic melting electrode (2) has a lack of titanium and/or at least one element from the group of B, Cr, Cu, Hf, Mn, Mo, Nb, Si, Ta, V and Zr which have a β-stabilizing effect in TiAl alloys.
4. Method for the production of a β-γ-TiAl base alloy according to one of the preceding claims, characterized in that the basic melting electrode (2) is produced by single or multiple remelting of a compacted electrode comprising the alloy components of the basic melting electrode (2) in a homogeneous distribution.
5. Method for the production of a β-γ-TiAl base alloy according to one of the preceding claims, characterized in that in order to allocate the amount of titanium and/or γ-stabilizing element corresponding to the lacking amount of titanium and/or γ-stabilizing element to the basic melting electrode, a composite electrode (19, 19') is produced which consists of the basic melting electrode (2) and a layer (15) of a corresponding thickness of titanium and/or the β-stabilizing element which is constant across the periphery and length thereof.
6. Method for the production of a β-γ-TiAl base alloy according to claim 5, characterized in that the layer consists of a coat (15) of titanium sheet which extends along the length of the basic melting electrode (2).
7. Method for the production of a β-γ-TiAl base alloy according to claim 6, characterized in that the coat (15) of titanium sheet is secured to the basic melting electrode by means of welding spots (18) which are evenly distributed across the outer peripheral surface (16) thereof and/or by means of a weld seam which runs along the upper edge of the welding electrode (2) across the entire periphery thereof.
8. Method for the production of a β-γ-TiAl base alloy according to claim 6, characterized in that the coat (15) of titanium sheet is formed by a coat lining on the inside of the remelting die (4) of the vacuum arc melting furnace (1), with the coat (15) of titanium sheet being fused to the basic melting electrode (2) in an intermediate remelting step so as to form an intermediate electrode which is then remolten to form the homogeneous β-γ-TiAl base alloy in a final vacuum arc melting step.
9. Method for the production of a β-γ-TiAl base alloy according to one of claims 1 to 4, characterized in that in order to allocate the amount of titanium and/or β-stabilizing element corresponding to the lacking amount of titanium and/or β-stabilizing element to the basic melting electrode, a composite electrode (19') is formed which consists of the basic melting electrode (2) and several rods (20) of corresponding thickness consisting of titanium and/or the β-stabilizing element which are arranged parallel to the longitudinal axis of the basic melting electrode (2) and are distributed evenly across the periphery of the basic melting electrode (2).
10. Method for the production of a β-γ-TiAl base alloy according to one of the preceding claims, characterized in that the final vacuum arc melting step for forming the homogeneous β-γ-TiAl base alloy is performed in a vacuum arc skull melting device after which the molten material of the β-γ-TiAl base alloy is cast to form cast bodies of the β-γ-TiAl base alloy in a lost-wax or die casting process.

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