WO2001088214A1 - Η-tial alloy-based component comprising areas having a graduated structure - Google Patents

Abstract

The invention relates to a single-piece gamma -TiAl-based intermetallic alloy component having a graduated structural transition between spatially adjacent areas, each area being provided with a different structure. Said component has a lamellar cast structure composed of alpha 2/ gamma lamellae in at least one area; a near- gamma structure, a duplex structure or a fine lamellar structure in at least one other area; and a transition zone with a graduated structure between these areas. The lamellar cast structure gradually changes in said transition zone into the other named structure. The invention also relates to a method for producing said component.

Description

 Component based on γ-TiAl alloys with areas with a graded structure

description

The invention relates to components based on intermetallic γ-TiAl alloys with a graded microstructure transition between spatially separated areas, each with a different microstructure, and a method for their production.

Intermetallic γ-TiAl alloys have received great attention in recent years due to their combination of unique material properties. Their advantageous mechanical and thermophysical properties with low specific weight recommend their use in the aerospace industry. The high temperature and corrosion resistance makes the material interesting for fast moving components in machines, e.g. for valves in internal combustion engines or for blades in gas turbines. The technical alloys currently used, based on γ-TiAl, have a multi-phase structure and, in addition to the ordered tetragonal γ-TiAl, contain the ordered hexagonal α ₂ -Ti ₃ Al as the main phase, typically with 5-15% by volume. Refractory metals as alloying elements can lead to the formation of a metastable, cubic, body-centered phase, which occurs either as the β phase (unordered) or as the B2 phase (ordered). These alloy additives improve oxidation resistance and creep resistance. Si, B and C are used in small quantities to fine-tune the cast structure.

Appropriate C contents can lead to precipitation hardening. The alloying elements Cr, Mn and V increase the room temperature ductility of the otherwise very brittle TiAl. Depending on the application profile, the development of alloys has led to a number of different alloy variants, which can generally be described by the following empirical formula:

Ti Al _{(44. 48)} (Cr, MnN) _{0.5. 5} (Zr, Cu _I Nb, Ta, Mo _I W ₎ Ni) _0ι1 . ₁₀ (Si, B, C _I Y) _0.05 . ₁ (in atomic%)

TiAl alloys are usually produced by multiple melting in a vacuum arc furnace as ingots (VAR - Vacuum Are Remelting). Alternatively, the production of alloys based on γ-TiAl by means of permanent mold casting from a cold-wall induction or plasma furnace or by means of

Inert gas atomization from a cold wall crucible to γ-TiAl powder and powder metallurgical processing technically implemented. The γ-TiAl melted via the Ingot route usually has a coarse-grained structure, the grains being essentially composed of γ-TiAl / α ₂ -Ti ₃ Al lamellae (see FIG. 1). Depending on the melting process used, the alloy composition and the type and speed of solidification of the melt to form a solid base alloy and the subsequent cooling, a wide range of more or less homogeneous small and / or large grain diameters can be found in the casting structure, but also fine or achieve a coarse lamellar structure within a grain of the alloy. US Pat. Nos. 5,846,351, 5,823,243, 5,746,846 and 5,492,574 are representative of this prior art.

Depending on the phases and structures actually created in the material, very different combinations of mechanical properties can be achieved in the material - e.g. with regard to ductility, fatigue strength (corresponding to the elongation at break and tensile strength), creep resistance at high temperatures and fracture toughness.

The range of structural mechanical properties of a γ-TiAl alloy is known to be significantly expanded over that of cast structures by means of massive forming at temperatures in the range between 900 ° C and 1400 ° C. Massive forming creates a dynamically recrystallized fine-grained structure. By choosing the forming temperature and / or subsequent heat treatments above or below the so-called α-transus temperature, the 4 basic structure types near-γ structure (globular γ grains with α ₂ phase at grain boundaries and triple points), duplex Structure (globular γ grains and lamellar α ₂ / γ at approximately equal proportions), nearly lamellar structures (grains made from oi ₂ / γ lamella and isolated globular γ grains) and fully lamellar structures (grains made from c ₂ / γ- Slats) (see Fig. 2).

Fine-grained near-γ and duplex structures have good room temperature ductility, high elongation at break and high tensile strength and thus high fatigue strength, but at the same time low creep resistance and low fracture toughness. In contrast, structures with comparatively coarser grains and with a pronounced lamellar structure show a significantly better creep resistance and a higher fracture toughness, but on the other hand also a lower fatigue strength and elongation at break.

The number of tried and tested alloy and structural configurations of γ-TiAl and there is correspondingly large leading manufacturing process. On the one hand, it is about achieving the best possible compromise between individual thermomechanical properties in the material that often change in opposite directions with the treatment steps, and on the other hand, to optimize costs when determining the individual treatment steps that are indispensable to be used.

For the generation of defined phase and structure structures by means of material post-treatment, γ-base TiAl alloys solidified from the melt are generally used. According to the prior art, the aftertreatments consist either in special heat treatment cycles (see D. Zhang, P. Kobold, V. Güther and H. Clemens: Influence of Heat Treatments on Colony Size and Lamellar Spacing in a Ti-46A1- 2Cr-2Mo- 0.25Si-0.3B Alloy, Zeitschrift für Metallkunde, 91 (2000) 3, see page 205) or in various forming steps.

DE-C-43 18 424 C2 describes a process for the production of moldings from γ-TiAl alloys, for example also in the form of valves and valve disks for engines. For this purpose, a cast blank is first deformed in the temperature range from 1050 ° C to 1300 ° C under quasi-isothermal conditions with a high degree of deformation, the part is then cooled and finally at

Temperatures from 900 ° C to 1100 'C at a low forming speed of 10 ^"4 to 10 ^~ Vs are superplastically formed into a molded part close to the final dimension. The process is multi-step and therefore technically complex.

Components are often required, and this includes, for example, valves for internal combustion engines and rotor blades for gas turbines, for which different, in some cases very different, material properties are required in individual component areas, in particular also with regard to their thermomechanical properties. So far, this has generally been met by assembling a component from areas of different materials, eg by means of non-positive and / or material joining. Valves for internal combustion engines are today made, for example, from different types of steel for the ¹ stem and for the plate area, the parts being connected to one another by friction welding.

According to AT-U-381/98, poppet valves for internal combustion engines made of γ-based TiAl alloys are described, which consist of a one-piece, e.g. a melted blank or manufactured by hot isostatic pressing of alloy powders. The raw part is uniformly brought to thermomechanical material properties by means of a first forming process which correspond to the later requirements for the plate area of the valve. In a second forming process by means of extrusion and simultaneous shaping to the desired component mass, the semi-finished product, which has already been formed, is shaped further into the shaft in an appropriately equipped extrusion mold and in application of process parameters adapted to the material requirements. The thermomechanical material properties required for a valve stem are formed in this section. The extrusion process for the part is "broken off" in a press mold with a conical transition between the inlet and outlet areas at the point in time that a finished valve with twice formed, slim

Shank area with a once formed, thick plate area and with a conical transition zone is created. The structure, in particular grain shape and size, between the plate and shaft area change in a graded manner, which is determined by the forming parameters of the two forming steps. This method also comprises several forming steps and is therefore complex and expensive.

The object of the present invention is for components made of alloys based on γ-TiAl, which in the final state have local areas with different thermomechanical requirement profiles and should have a transition zone with regard to the material properties, one versus the other State of the art to create a more economical manufacturing process and a comparatively inexpensive component produced by this process. The aim here is to utilize the entire possible range of structure-determined property profiles by setting different basic structures in one component. Correspondingly, for components with very different temperature and strength stresses in individual areas, structures that are as well adapted to the requirements as possible are to be generated and thermomechanical properties are to be generated that are qualitatively superior or at least not inferior to those of components obtained by known processes with multi-stage forming, the components being inferior but should be cheaper to manufacture.

This object is achieved by a one-piece component made of an intermetallic alloy based on γ-TiAl with a graded microstructure transition between spatially adjacent areas of different microstructures, which has at least in one area a lamellar structure consisting of c ₂ / γ-lamellas and has a near-γ structure, duplex structure or fine lamellar structure in at least one further area, a transition zone with a graded structure being present between these areas, in which the lamellar cast structure gradually merges into the other structure mentioned.

The lamellar casting structure consisting of cü ₂ / γ-lamellae was preferably produced by directional solidification of a molten alloy. The near-γ structure, duplex structure or fine-lamellar structure has preferably been produced in the at least one further area by solid forming and, if appropriate, by post-treatment from the cast structure.

The object is further achieved by a method for producing components of this type, a suitable TiAl melt being produced in a customary manner in a first step, and the TiAl melt in a second step using solidification is converted into a semi-finished product which has a lamellar cast structure consisting of α ₂ / γ-TiAl lamellae, and - in a third step in a partial area or in partial areas of the semi-finished product, the lamellar, made of cü ₂ / γ- Existing cast aluminum TiAl lamellas are transformed into a near-γ-structure, duplex structure or fine-lamellar structure by massive forming in a temperature range from 900 ° C to 1400 ° C.

In a preferred embodiment, a pore-free, cylindrical semifinished product is produced from the TiAl melt by means of continuous casting, which is then massively deformed by extruding a rod area.

In a further preferred embodiment, the TiAl melt becomes a cylindrical one by means of centrifugal casting

Semi-finished product made without blow holes, which is then mass-formed by extrusion of a rod area.

With the invention, areas of high tensile strength, ductility and fatigue strength with areas of high fracture toughness and high creep resistance can be realized in one and the same component.

A major advantage of. Components produced according to the invention consist in the fact that the selection of the manufacturing steps in comparison to the prior art enables considerable savings in manufacturing costs to be achieved. The economic advantage results from the technical knowledge that multiple reshaping of the semi-finished product with a cast structure can be dispensed with in such components.

Show in the drawings

1 shows the lamellar cast structure of a VAR-TiAl ingot,

2 shows an excerpt from the phase diagram TiAl, the oblique line between a and α + γ being the α-transus, which strongly depends on the Al content. and a heat treatment of a material dynamically recrystallized by forming leads to a fully lamellar structure above the transus, and to a nearly lamellar, duplex or globular near-γ structure depending on the temperature,

Fig. 3 shows the scheme of melting homogeneous semi-finished TiAl according to A. L. Dowson et al. , Microstructure and Chemical Homogeneity of Plasma - Are Cold-Hearth

Melted Ti-48Al-2Mn-2Nb Gamma Titanium Aluminide, Gamma Titanium Aluminides, ed. Y.-W. Kim, R. Wagner and M. Yamaguchi, The Minerals, Metals & Materials Society, 1995,

4 shows a metallographic micrograph of the plate area of a valve produced according to the invention, the picture in the plate showing the coarse-grained lamellar cast structure made of o! ₂ / γ-lamella shows and it can be seen that this structure in the conical part of the

Tellers continuously merges into an area with a fine-grained, near-γ structure that cannot be resolved as such,

Fig. 5 is a light micrograph of the lamellar

Cast structure in the plate center at a higher magnification, and

Fig. 6 is a light micrograph of the globular deformed structure in the shaft area in higher

Enlargement.

On the one hand, the special casting method according to the invention described in more detail below allows even unforeseen advantageous material properties with a comparatively large and thus individually tailored to the respective material requirements variation range of property combinations. On the other hand, it can be made from a semi-finished product to achieve a dynamically recrystallized structure with thermomechanical properties that deviate greatly from the properties of the cast semifinished product. The properties of the dynamically crystallized structure can also be varied by adapting the process parameters.

Both processes, the special melting and casting process as well as the subsequent forming process, complement each other in an unforeseen manner. In total, material properties and combinations of material properties can be achieved in a bandwidth within a single component using a single-stage forming process that previously could not be achieved even with multi-stage forming processes. This finding relates to components with different local loads and technical applications in which γ-TiAl is a suitable material.

The material designation "intermetallic γ-TiAl alloy" encompasses a wide range of individual alloys. An essential alloy range is due to the molecular formula

Ti AI _{(44.48 )} (Cr, Mn, V) _0.5 . ₅ (Zr.Cu.Nb.Ta.Mo.W.NiJo ^ o (Si.B.CN i (data in atomic%)

covered.

This group of materials also includes orthorhombic titanium aluminide base alloys, for example with a typical alloy composition T1-25A1-20 Nb (atomic%). Their comparatively higher specific weight makes this group less interesting for those applications in which components are exposed to fast and oscillating movements, as is the case with valves in internal combustion engines, for example. The structures which can be set according to the invention from the phases and basic structures described at the outset result as a result of the method steps according to the invention, according to which corresponding components are produced.

The previously described methods for producing a γ-TiAl alloy from the melt or a melt-cast blank result in inhomogeneously formed phases and structural structures within the blank, which alone are a homogenization by hot isostatic pressing (HIP) and / or high-temperature annealing or Forming required. In contrast, the continuous casting process according to the invention from a cold-wall crucible and block extraction of the suitable semi-finished product has proven itself extremely well in order to give the component the required material properties for the applications in which high-temperature creep resistance and high fracture toughness are less important, but less fatigue strength and elongation at break. With the melt application via continuous casting, a property profile can be set to a large extent, as is required for the finished component in the component area not further formed, e.g. the profile of the plate part in a valve for internal combustion engines. The smaller the diameter of the continuously cast semifinished product can be, the smaller lamellar colony sizes and lamella spacings with even higher fracture toughness and creep resistance can be produced.

According to the invention, the semifinished product in the form of the cast blank is then subjected to massive shaping in the temperature range between 900 ° C. and 1400 ° C. by extrusion or by means of an equivalent shaping process and is brought into a shape which is matched to the dimensions of the end product. To achieve a graded structure, the rods are extruded only over part of their total length in an extrusion die of profile dimensions that at least approximately correspond to the final dimensions of the component in the formed area, for example dimensions of a valve for internal combustion engines with a conical transition between the stem and plate area, ie the extrusion mold has a tapered cross-section between the inlet area and the outlet area. The semifinished product is increasingly deformed in the conically tapering die area and thus continuously transferred from the structural state of the cast structure to the recrystallized structural state achieved by extrusion. The experience already available makes it possible for the person skilled in the art to change certain thermomechanical properties of the material in a targeted manner using appropriate forming parameters within material-related limits and to optimize them to meet special requirements.

Preferred components according to the invention are valves for internal combustion engines. This applies in particular to emerging future applications. While one so far

Controlling engine valves usually via a camshaft and using different types of steel as a material, the ongoing development is towards electromagnetic or pneumatic individual valve control. For this, however, light valves are required, which must have sufficient strength and corrosion resistance at high temperatures, in extreme cases up to 850 ° C in the plate area.

Valves are stressed in the stem area at rather moderate temperatures by strong alternating loads (fatigue). The demands on the material in terms of strength and ductility are correspondingly high. As has already been described above, these components of intermetallic γ-TiAl alloys according to the invention achieve these locally different thermomechanical properties in an outstanding manner.

Additional, particularly suitable components are blades of gas turbines, which require different thermomechanical properties at the base of the blade than in the peripheral area of the blade. The invention is described in detail using the following example for valves for internal combustion engines.

example

A TiAl starting alloy with the composition Ti-46Al-8.5Nb- (1-3) (Ta, Si, B, C, Y) (data in atomic%) is melt-metallurgically converted into a rod material with a diameter of 40 mm manufactured, which corresponds approximately to the diameter of a valve plate. The alloy is produced by mixing titanium sponge, Al granules and a multi-component master alloy AINbTaSiBYC, in which the atomic ratios between the alloy elements Nb, Ta, Si, B, C and Y correspond to those in the final TiAl alloy. A stable rod is pressed from the material mixture, which is used as a melting electrode in a vacuum arc furnace and remelted into a primary ingot. The primary ingot has an inhomogeneous alloy composition and is therefore melted and homogenized again in a plasma furnace (cold hearth) in a skull made of the same material, which is in a water-cooled copper crucible. The melt flows through a channel heated by a plasma torch into a strand extraction device, at the upper end of which a third homogenization takes place in the molten phase by means of a cold wall induction crucible. The molten TiAl alloy is pulled down as a block or rod, whereby the material solidifies in a non-porous manner. The process is shown schematically in Fig. 3 and is by A.L. Dowson et al. in Microstructure and Chemical Homogeneity of Plasma - Are Cold-Hearth Melted Ti-48Al-2Mn-2Nb Gamma Titanium Aluminide, Gamma Titanium Aluminides, ed. Y.-W. Kim, R. Wagner and M. Yamaguchi, The Minerals, Metals & Materials Society, 1995.

In contrast to the method described in the cited literature reference, in which the cold-wall induction coil is only intended to provide a stirring effect in the melt, In the present embodiment according to the invention, the coil is dimensioned such that the energy is sufficient for the complete melting of the alloy located in the coil. The semi-finished product obtained in this way has a lamellar cast structure with colony sizes of the lamella packs between 100 μm and 500 μm, but at the same time has excellent material homogeneity. The individual rods thus obtained as semi-finished products are divided into cylindrical segments, brought to a temperature of 1200 ° C. under protective gas and pressed in protective gas by extrusion into a heated die with a valve shape. The forming ratio in the shaft area is approx. 15: 1 and decreases continuously from the base of the plate in the extension of the shaft to the end of the plate up to zero forming. In the deformed area, the dynamic recrystallization and the given process temperature create a fine-grained near-γ structure, while the lamellar cast structure is retained in the plate area. The component pressed out in this way is then cooled to a temperature above the brittle-ductile transition temperature within 30 minutes, left at this temperature for about 60 minutes and then brought to room temperature by normal cooling.

The present invention is not limited to the example set out above, rather the invention also encompasses components for other, not mentioned applications in which a corresponding structure is required or advantageous for the application. The material γ-base TiAl alloy is not limited to the alloy compositions explicitly mentioned.

Claims

claims 1. One-piece component made of an intermetallic alloy based on γ-TiAl with a graded microstructure transition between spatially adjacent areas of different microstructures, characterized in that there is at least in one area a lamellar casting structure consisting of α ₂ / γ-lamellas ( has, and in at least one further area a 1 near-γ structure, duplex structure or fine lamellar structure and between these areas there is a transition zone with a graded structure, in which the lamellar cast structure gradually merges into the other structure mentioned. 2. Component according to claim 1, characterized in that the lamellar casting structure consisting of α ₂ / γ-lamellae has been produced by directional solidification of a molten alloy. 3. Component according to claims 1 or 2, characterized in that the near-γ structure, duplex structure or fine-lamellar structure has been produced in the at least one further area by solid forming and, if appropriate, an aftertreatment from the cast structure. 4. Component according to claims 1 to 3, characterized in that it is a cylindrical, by means of continuous casting in rod form pore-free obtained from the melt, which is then massively formed by extrusion of a rod area. 5. Component according to claims 1 to 3, characterized in that it is a cylindrical semifinished product obtained by blow-molding from the melt without blowholes, which is then massively formed by extrusion of a rod region. 6. Component according to at least one of claims 1 to 5, characterized in that the alloy of the empirical formula Ti AI _(M8) (Cr, Mn, v) _0ι5 . ₅ (Zr, Cu ₎ Nb ₎ Ta, Mo ₎ W ₎ Ni) _0ι1 . ₁₀ (Si ₎ B, CN) ₀₀₅ . ₁ corresponds to expressed in atomic%. 7. Component according to at least one of claims 1 to 6, characterized in that it is a valve for internal combustion engines. 8. The method for producing components according to claim 1, characterized in that in a first step a suitable TiAl melt is produced in a conventional manner, in a second step the TiAl melt is converted into a semi-finished product by directional solidification, which is a lamellar , has a casting structure consisting of c ₂ / γ-TiAl lamellas, and in a third step in a partial area or in partial areas of the semi-finished product the lamellar, made of α? ₂ / γ-TiAl lamellas are converted into a near-γ-structure, duplex structure or fine-lamellar structure by massive forming in a temperature range from 900 ° C to 1400 ° C. 9. The method according to claim 8, characterized in that a pore-free, cylindrical semifinished product is produced from the TiAl melt by means of continuous casting, which is then massively formed by extrusion of a rod region. 10. The method according to claim 8, characterized in that a cylindrical semi-finished product is produced without blowholes from the TiAl melt by centrifugal casting, which is then massively formed by extrusion of a rod region. 11. The method according to at least one of claims 8 to 10, characterized in that the TiAl alloy of the empirical formula s Ti Al _{(44. 48)} (Cr, Mn.V) _{0ιM (Zr.Cu.Nb.Ta.Mo.W.NiJo,.} ^ (i Si.BCY corresponds to expressed in atomic%. 12th Method according to at least one of claims 8 to 11, 0, characterized in that a valve for internal combustion engines is manufactured.