The invention relates to components based on intermetallic γ-TiAl alloys with a graduated microstructure transition between spatially separated areas each with a different microstructure structure and also to a process for their production.
Intermetallic γ-TiAl alloys have received much attention in recent years for their combination of unique material properties. Their advantageous mechanical and thermophysical properties with low specific weight recommend their use in aviation and space travel. The high temperature and corrosion resistance makes the material useful for rapidly-moving components in machines, e.g. for valves in combustion engines or for blades in gas turbines.
The industrial γ-TiAl-based alloys currently used have a multiphase structure and contain, in addition to the ordered tetraginal γ-TiAl as main phase, the ordered hexagonal α2-Ti3Al typically with 5-15 vol.-% proportion. Refractive metals as alloy elements can lead to the formation of a metastable cubically body-centred phase which occurs either as β phase (random) or as B2 phase (ordered). These alloy additives improve the oxidation resistance and creep resistance. Si, B and C serve in small quantities to refine the grain of the cast microstructure. Corresponding C contents can lead to precipitation hardenings. The alloy elements Cr, Mn and V increase the room-temperature ductility of the otherwise very brittle TiAl. Alloy development has led, depending on the use profile, to a range of different alloy variants which can be described in general by the following empirical formula:
Ti Al(44-48)(Cr, Mn, V)0.5-5(Zr, Cu, Nb, Ta, Mo, W, Ni)0.1-10(Si, B, C, Y)0.05-1(figures in atom-%)
TiAl alloys are usually prepared as ingots by repeated melting in a vacuum arc furnace (VAR—Vacuum Arc Remelting). Alternatively, industrial γ-TiAl-based alloys can be prepared by means of chill casting from a cold-wall induction or plasma furnace or by means of inert-gas atomizing from a cold-wall crucible to give γ-TiAl powder and powder-metallurgical further processing. The γ-TiAl melted via the ingot route usually has a coarse-grained microstructure, the grains being essentially constructed from γ-TiAl/α2-Ti3Al lamellae (see FIG. 1). Depending on the melting process used, the alloy composition, and depending on the type and speed of the solidification of the melt to solid base alloy, and the subsequent cooling, a wide bandwidth of more or less homogeneous small and/or large grain diameters, but also fine or coarse lamellar structure, can be achieved within a grain of the alloy in the cast microstructure.
There may be mentioned as representative of this state of the art the U.S. Pat. No. specifications 5,846,351, 5,823,243, 5,746,846 and 5,492,574.
According to the phases and microstructures actually produced in the material, very different combinations of mechanical properties can be achieved in the material—e.g. in respect of ductility, fatigue strength (according to the elongation at break and tensile strength), creep resistance at high temperatures and fracture toughness.
It is known that the bandwidth of microstructure-dependent mechanical properties of a γ-TiAl alloy is substantially broadened, compared with that of cast microstructures, by massive metal-forming at temperatures in the range between 900° C. and 1400° C. During massive metal-forming, a dynamically recrystallized fine-grained microstructure forms. By selecting the metal-forming temperature and/or by downstream heat treatments above or below the so-called α-transus temperature, the 4 basic microstructure types near-γ microstructure (globular γ grains with α2 phase at grain boundaries and triple points), duplex microstructure (globular γ grains and lamellar α2/γ in almost equal proportions), nearly lamellar microstructure (grains of α2/γ lamellae and isolated globular γ grains) and fully lamellar microstructure (grains of α2/γ lamellae) can be set (see FIG. 2).
Fine-grained near-γ and duplex microstructures have a good room-temperature ductility, a high elongation at break and a high tensile strength and thus a high fatigue strength, simultaneously however a low creep resistance and a low fracture toughness. On the other hand, microstructures with relatively coarser grains and with strongly pronounced lamellar structure have a clearly better creep resistance and a higher fracture toughness, on the other hand however also a lower fatigue strength and elongation at break.
The number of alloy and microstructure designs of γ-TiAl already tested and the preparation processes leading to same is correspondingly large.
These involve on the one hand the achievement of the best possible compromise between individual thermomechanical properties in the material which are frequently undergoing contrary changes vis-à-vis each other with/the treatment steps, and on the other hand an optimization of costs when setting the individual indispensable successive treatment steps to be applied.
In principle, γ-TiAl-based alloys solidified from the melt are used to produce defined phase and microstructure structures by means of material post-treatments. The post-treatments consist according to the state of the art either of special heat treatment cycles (see D. Zhang, P. Kobold, V. Gülther and H. Clemens: Influence of Heat Treatments on Colony Size and Lamellar Spacing in a Ti46Al-2Cr-2Mo-0, 25Si-0, 3B alloy, Zeitschrift für Metallkunde, 91 (2000) 3, see page 205) or of metal-forming steps of various kinds.
DE-C-43 18 424 C2 describes a process for the preparation of shaped bodies from γ-TiAl alloys, for example also in the form of valves and valve heads 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 metal-forming, the item is then cooled and finally superplastically metal-formed at temperatures of 900° C. to 1100° C. at a low metal-forming speed of 104 to 10−1/s to give the pre-shaped part close to the final measurements. The process has several steps and is thus costly in technical terms.
Components are often required, and these also include for example valves for combustion engines and rotor blades for gas turbines for which, in individual component areas different, sometimes very different, material properties are required, in particular also in respect of their thermomechanical properties. This requirement is met as a rule in that a component is composed of areas of different materials, e.g. by means of force- and/or material-locking jointing. Today, valves for combustion engines are for example manufactured from different types of steel for the stem and for the head area, the parts being joined together by friction welding.
According to EP 0965 412 A1, head valves for combustion engines made from γ-TiAl-based alloys are described which are produced from a one-piece blank, e.g. one that is molten or has been prepared by hot-isostatic pressing of alloy powders. The unfinished part is uniformly endowed by means of a first metal-forming procedure with thermomechanical material properties which satisfy the later requirements for the head area of the valve. In a second metal-forming process by means of extrusion and simultaneous metal-forming to component target dimensions, the semi-finished product, already metal-formed once, is metal-formed further in a part section in a suitably equipped extrusion mould using process parameters adapted to the material requirements, to produce the stem. The thermomechanical material properties required for a valve stem are developed in this part section. The extrusion procedure for the part is “interrupted” in a compression mould with conical transition between inlet and outlet areas at the moment when a finished valve with a double-metal-formed, narrow stem area with a single-metal-formed, thick head area and with a conical transition zone forms. The microstructures, in particular grain microstructure and size, between head and stem area change in graduated manner as determined by the metal-forming parameters of the two metal-forming steps. This process likewise comprises several metal-forming steps and is therefore time-consuming and expensive.
The object of the present invention is to create, for components of γ-TiAl-based alloys which, in the final state, have local areas with different mechanical requirement profiles and are to display a transition zone in respect of the material properties, a production process which is more economical compared with the state of the art, and a relatively more cost-favourable component produced according to this process. The aim is to exploit the whole of the possible bandwidth of microstructure-dependent property profiles by specifying different basic microstructures in one component. Accordingly, for components subjected to markedly different temperature and strength stresses in individual areas, a microstructure is to be produced which is suited to the requirements as well as possible, and thermomechanical properties generated which in terms of quality are superior, or at least not inferior, to those of components obtained according to known processes with multi-stage metal-forming, in which however the components can be produced more cheaply.
This object is achieved by a component produced in one piece from an intermetallic γ-TiAl-based alloy with graduated microstructure transition between spatially adjacent areas, each having a different microstructure structure, which has a lamellar microstructure composed of α2/γ lamellae in at least one area, and a near-γ microstructure, duplex microstructure or fine-lamellar microstructure in at least one other area, a transition zone with graduated microstructure, in which the lamellar cast microstructure gradually changes into the other named microstructure, being present between these areas.
The lamellar cast microstructure composed of α2/γ lamellae has preferably been produced by oriented solidification of a molten alloy. The near-γ microstructure, duplex microstructure or fine-lamellar microstructure has preferably been produced from the cast microstructure in the at least one other area by massive metal-forming and optionally by a post-treatment.
The object is furthermore achieved by a process for the production of such components, a suitable TiAl melt being produced in customary manner in a first step, the TiAl melt converted by oriented solidification in a second step to a semi-finished product which has a lamellar cast microstructure composed of α2/γ-TiAl lamellae, and, in a part area or in part areas of the semi-finished product, the lamellar cast microstructure composed of α2/γ-TiAl lamellae being converted by massive metal-forming in a third step in a temperature range of 900° to 1400° to a near-γ microstructure, duplex microstructure or fine-lamellar microstructure.
In a preferred version, a pore-free, cylindrical semi-finished product is produced from the TiAl melt by means of continuous casting, and is then massively metal-formed by extrusion of a bar area.
In a further preferred version, a cylindrical semi-finished product is produced cavity-free from the TiAl melt by means of centrifugal casting, and is then massively metal-formed by extrusion of a bar 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 the components produced according to the invention is that through selection of production steps a considerable saving in production costs can be achieved compared with the state of the art. The economic advantage results from the technical finding that, with such components, a repeated metal-forming of the semi-finished product with cast microstructure can be dispensed with.
A TiAl starting alloy of the composition Ti46Al-8,5Nb-(1-3) (Ta, Si, B, C, Y) (figures in atom-%) is converted by melting-metallurgy route to bars with a diameter of 40 mm, which approximately corresponds to the diameter of a valve head. The alloy is produced by, mixing titanium sponge, Al granulated metal and a polynary master alloy AlNbTaSiBYC in which the atomic ratios between the alloy elements Nb, Ta, Si, B, C and Y correspond to those in the TiAl final alloy. A stable bar is pressed from the material mixture, and this is used as fusible arc-welding electrode in a vacuum arc furnace and remelted into a primary ingot. The primary ingot has an inhomogeneous alloy composition and is therefore remelted and homogenized in a plasma furnace (cold hearth) in a skull made of material of like kind which is located in a water-cooled copper crucible. The melting material flows via a channel heated with a plasma furnace into a bar-offtake apparatus at the top end of which a third homogenization takes places in the molten phase by means of a cold-wall induction crucible. The molten TiAl alloy is drawn off underneath as a block or bar, the material solidifying in pore-free manner. The process is shown schematically in FIG. 3 and has been described by A. L. Dowson et al. in Microstructure and Chemical Homogeneity of Plasma—Arc Cold-Hearth Melted Ti48Al-2Mn-2Nb Gamma Titanium Aluminide, Gamma Titanium Aluminides, ed. Y. -W. Kim, R. Wagner and M. Yamaguchi, The Minerals, Metals & Materials Society, 1995.