EP2342365A1 - Method for producing a ss- -tial base alloy - Google Patents

Abstract

The invention relates to a method for producing a ?-TiAl base alloy solidifying over the ß-phase (ß-?-TiAl base alloy) by means of vacuum arc remelting, having the following method steps: smelting a base smelting electrode (2) of a conventional ?-TiAl primary alloy having a deficit content of titanium and/or at least one ß-stabilizing element in respect of the ß-?-TiAl base alloy to be produced in at least one first vacuum arc remelting step, allocating a quantity of titanium and/or ß-stabilizing element corresponding to the deficit content of titanium and/or ß-stabilizing element to the base smelting electrode (2) in uniform distribution over the length and circumference thereof, and alloying the allocated quantity of titanium and/or ß-stabilizing element in the base smelting electrode to form the homogenous ß-?-TiAl base alloy in a final vacuum arc remelting step.

Description

 Process for producing a β-γ-TiAl base alloy

The invention relates to a process for the production of γ-TiAl base alloys by means of vacuum arc melting (VA) 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. If necessary, evaporation losses caused by the later melting are stored here. 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 formed whose chemical and structural homogeneity is unsuitable for industrial use and which consequently must be remelted at least once (see V. Guether: Microstructures and Defects in γ-TiAl ba- sed Vacuum Are Remelted Ingot Materials ", 3 ^nd International Symp on Structural Intermetallics, September 2001, Jackson Hole WY, USA). From DE 101 56 336 A1 a process for the production of alloy ingots is known which comprises the following steps:

 (i) preparation of electrodes by customary mixing and pressing of the selected starting materials,

(ii) at least one remelting of the electrodes obtained in step (i) by a conventional melt metallurgical process,

 (iii) inductive melting of the electrodes obtained in step (i) or (ii) in a high-frequency coil,

 (iv) homogenizing the melt obtained in step (iii) in a cold wall induction crucible and

 (v) withdrawing the melt under cooling from the cold wall induction crucible of step (iv) in the form of blocks with freely adjustable

 Diameter. DE 195 81 384 TI 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 atomic%, a Nb concentration of 6 to 10 atom% and a Cr concentration of 1 to 3 atom% at a temperature in the range of 1300 to 1400 ° C is prepared.

DE 196 31 583 A1 discloses a method for producing a TiAl-Nb product from an alloy, in which first an alloy electrode is produced 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. JP 02277736 A discloses a heat-resistant TiAl-based alloy in which specific amounts of V and Cr are introduced into an intermetallic Ti-Al compound to improve heat resistance and ductility.

Finally, DE 1 179 006 A discloses ternary or higher titanium-aluminum alloys with such elements which stabilize the α- and β-phase of the titanium. The usual process for remelting is vacuum-arc melting with self-consuming electrode, since the plasma melting plants are generally not designed for the supply of compact ingots as starting material. In the case of conventional, biphasic in the form of lamellar colonies of the a2-Ti ₃ Al phase and the γ-TiAl phase constructed γ-TiAl base alloys remelting takes place in the vacuum arc furnace (VAR furnace) easily and leads to the desired result (see V. Guether: "Status and Prospects of γ-TiAl Intote Production", International 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 ß-stabilizing elements such as Cr, Cu, Hf, Mn, Mo, Nb, V , Ta and Zr, a primary solidification path is established over the β-Ti phase. This results in very fine microstructures, in addition to lamellar α ₂ / γ- Colonies also include globular β grains and globular γ grains, sometimes including globular 012 grains. Materials with such structures have significant advantages in terms of thermo-mechanical properties and processability by means of forming technologies (see H. Clemens: "Design of Novel ß-Solidifying TiAl Alloys with Adjustable ß / B2 Phase Fraction and Excellent Hot Workability", Advanced Engineering Materials 2008, 10, No.8, pp. 707-713) Such alloys are referred to below as β-γ-TiAl base alloys, as already stated above.

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. The cause of the disturbing chipping behavior is considered to be massive phase transformations in the temperature range between the eutectoid temperature and the phase boundary temperature to the β-phase region. 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, 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 - Ι, ΟΜο - Ο, ΙΒ at .-%) show that the linear expansion coefficient of a corresponding alloy sample in the temperature range between 1000 ° C and 1200 ° C of 9 x 10 ^"6 to 40 x ^{10" 6} K ^"1 more than quadrupled. this behavior is illustrated in the accompanying Fig. 4, in which the curve A represents the linear expansion coefficient of this alloy. the curve represents the During VAR melting, based on the length of the self-consumable electrode, a temperature field extends from the melting temperature (about 1570 ° C.) at the bottom of the electrode to near room temperature at the electrode suspension between 1000 and 1200 ° C. The relatively poor ductility of the intermetallic material then leads in this zone to the formation of the S Tear discharges in the form of cracks, which in turn lead to the described chipping of unmelted pieces of the electrode. Based on this described problem of the prior art, the present invention seeks to provide a method for producing a solidifying over the β-phase 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.

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 a deficient content of the titanium and / or ß-stabilizing element corresponding amount of titanium and / or ß-stabilizing element to the base melt electrode in a uniform distribution over the length and circumference, and

 - Adding the associated amount of the titanium and / or ß-stabilizing element in the base melt electrode to form the homogeneous β-γ-TiAl-based alloy in a last 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 melt 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 base alloy in the last remelting step. The primary alloy has a deficit of titanium and / or a deficiency of ß-stabilizing elements such as Nb, Mo, Cr, Mn, V, and Ta. In the production of the pressed base melt electrode, the alloy is a defined amount of titanium and / or ß 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 ß-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 VA 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-ιιι1 base alloy as target alloy.

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:

1 is a schematic diagram of a vacuum arc melting furnace, 2 is a perspective view of a composite electrode in a first embodiment,

Fig. 3 is a perspective view of a composite electrode in a second embodiment and

Fig. 4 is a diagram of the linear expansion coefficient as

Function of the temperature of a TNM®-B 1 alloy. 1, a vacuum arc melting furnace 1 and the method for remelting a corresponding electrode 2 to form an ingot 3 will be explained in principle. 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 1 1, where 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 Umschmelz- step becomes the electrode of the next Umschmelzschrittes. This improves the degree of homogenization of the ingots to be produced with each remelting step. Various embodiments for producing a β-γ-TiAl base alloy will now be described below:

Exemplary Embodiment 1 The target composition of the β-γ-TiAl alloy is Ti - 43.5A1 - 4.0Nb - Ι, ΟΜο - 0.1B (at .-%) or Ti - A128.6 - Nb9, l - 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 - 45.93A1 - 4.22Nb - l, 06Mo - 0.1 1B (at .-%). First, conventionally, a ingot 3 of the primary alloy of 200 mm in diameter and a length of 1.4 m by 2-fold VAR melting as described above is produced from a pressing electrode 2, without the occurrence of a 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 lateral surface 16 of the ingot 3, as shown in Fig. 2. 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, weld spots 18 are set distributed over the lateral surface 16. The thus assembled self-consumable electrode is remelted as a composite electrode 19 in a final melting step in the VAR furnace 1 to a ingot 3 with 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 the exemplary embodiment 1. From the primary alloy, an ingot 3 having a diameter of 140 mm and a length of 1.8 m is produced by simple VAR melting of press electrodes 2. The mass of the ingot is 1 15 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 copper plate 4 lined with the titanium sheet to form an intermediate electrode in such a way that the outer skin of the titanium sheet is not completely melted with it and remains as a stable shell. In the following 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 result in cracking. Drop down of electrode material in the melt reservoir 14 lead.

Embodiment 3

The target composition, the starting materials used and the composition of the primary alloy correspond to the embodiment 1, likewise the production of the composite electrode 19. In contrast to embodiment 1, the last remelting takes place in what is known as a VAR skull melter, ie a vacuum arc - Melting device with a water-cooled, tiltable crucible made of copper. 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 thus produced by centrifugal casting are used as starting material for the production of components from the target alloy.

Exemplary Embodiment 4: A β-γ-TiAl alloy according to US Pat. No. 6,669,791 has a composition (target alloy) Ti-43,0A1-6,0V (at.%) Or 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-45J5A1 (at .-%) or Ti-A132.2 (m-%). As insert materials titanium sponge, aluminum and vanadium are used. First, conventionally, a base melt electrode 2 is prepared as an ingot of the binary TiAl primary alloy having a diameter of 200 mm and a length of 1 m by double VAR melting (mass 126 kg). As shown in FIG. 3, eight vanadium rods 20 with a diameter of 16.7 mm and a length of 1 m (total mass 10.7 kg) are each offset by 45 ° relative to one another along the entire lateral surface 16 of the base melt electrode 2, and thus uniformly welded over the circumference of the electrode 2 welded. The resulting composite electrode 19 'from the binary primary alloy and the welded vanadium rods 20 is in the final third

Melting process to an ingot of target alloy with a diameter of 300 mm in the VAR furnace 1 remelted.

Embodiment 5

The target composition of the γ-TiAl alloy corresponds to that of Embodiment 1 (Ti - 43.5A1 - 4.0Nb - Ι, ΟΜο - 0.1B 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 - 0.1 1B (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 TiMol5 alloy are welded on, parallel to the longitudinal axis parallel to exemplary embodiment 4. The diameter of the rods is 26 mm, the length of the rods corresponds to the ingot length. The total mass of the TiMol5 rods is 19.6 kg. The resulting composite electrode consisting of one ingot of the primary alloy and eight TiMol5 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

claims 1. A process for producing a β-phase solidifying γ-TiAl base alloy (β-γ-TiAl base alloy) by vacuum arc melting,  characterized by the following process steps:  Melting a base melt electrode (2) 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 a deficient content of the titanium and / or ß-stabilizing element corresponding amount of titanium and / or ß-stabilizing element to the base melt electrode (2) in a uniform distribution over the length and circumference, and - Adding the associated amount of the titanium and / or ß-stabilizing element in the base melt electrode to form the homogeneous β-γ-TiAl-based alloy in a last vacuum arc melting step. 2. A process for producing a β-γ-TiAl base alloy according to claim 1, characterized in that the base melt electrode (2) of the conventional γ-TiAl primary alloy has an aluminum content of 45 at .-% to 50 at .-%. 3. A process for producing a β-γ-TiAl base alloy according to claim 1 or 2, characterized in that the base melt electrode (2) has a deficiency of titanium and / or at least one in TiAl Alloys ß-stabilizing element from the group of B, Cr, Cu, Hf, Mn, Mo, Nb, Si, Ta, V and Zr has. 4. A process for producing a β-γ-TiAl base alloy according to any one of the preceding claims, characterized in that the base melt electrode (2) by one or more remelting of the alloy components of the base melt electrode (2) is produced in a homogeneous distribution pressing electrode. 5. A process for producing a β-γ-TiAl base alloy according to any one of the preceding claims, characterized in that for the allocation of the deficient content of the titanium and / or the ß-stabilizing element corresponding amount of titanium and / or ß-stabilizing Element to the base melt electrode a composite electrode (19, 19 ') is produced, which consists of the base melt electrode (2) and a uniform over its circumference and length layer (15) of titanium and / or the ß-stabilizing element. 6. A method for producing a β-γ-TiAl base alloy according to claim 5, characterized in that the layer consists of a over the length of the base melt electrode (2) extending titanium sheet jacket (15). 7. A method for producing a β-γ-TiAl base alloy according to claim 6, characterized in that the titanium sheet jacket (15) with evenly over its lateral surface (16) distributed weld points (18) and / or one at the upper edge ( 17) of the welding electrode (2) is attached to the base welding electrode over its entire circumference. 8. A process for producing a β-γ-TiAl base alloy according to claim 6, characterized in that the titanium sheet jacket (15) by a jacket lining on the inside of the Umschmelzkokille (4) of the vacuum arc melting furnace (1) in an intermediate reflow step, the titanium sheet jacket (15) is fused to the base melt electrode (2) to form an intermediate electrode, and then the intermediate electrode in a final vacuum arc melting step to form the homogeneous β-γ-TiAl base alloy is remelted. 9. A process for producing a β-γ-TiAl-based alloy according to one of claims 1 to 4, characterized in that for the allocation of the deficient content of titanium and / or ß-stabilizing element corresponding amount of titanium and / or ß - stabilizing element to the base melt electrode, a composite electrode (19 ') is produced, consisting of the base melt electrode (2) and a plurality of axially parallel arranged parallel, distributed uniformly over the circumference of the base melt electrode rods (20) corresponding thickness of titanium and / or the ß-stabilizing element , 10. A method for producing a β-γ-TiAl base alloy according to any one of the preceding claims, characterized in that the last A vacuum arc melting step is carried out to form the homogeneous β-γ-TiAl base alloy in a vacuum arc-skull melting apparatus, followed by melting Material of the β-γ-TiAl base alloy is poured into casting by precision casting or chill casting from the β-γ-TiAl base alloy.