Patent Publication Number: US-2004045644-A1

Title: T-tial alloy-based component comprising areas having a graduated structure

Description:
[0001] 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.  
       [0002] 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.  
       [0003] 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 α 2 -Ti 3 Al 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-%)  
       [0004] 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/α 2 -Ti 3 Al 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.  
       [0005] 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.  
       [0006] 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.  
       [0007] 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 α 2  phase at grain boundaries and triple points), duplex microstructure (globular γ grains and lamellar α 2 /γ in almost equal proportions), nearly lamellar microstructure (grains of α 2 /γ lamellae and isolated globular γ grains) and fully lamellar microstructure (grains of α 2 /γ lamellae) can be set (see FIG. 2).  
       [0008] 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.  
       [0009] The number of alloy and microstructure designs of γ-TiAl already tested and the preparation processes leading to same is correspondingly large.  
       [0010] 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.  
       [0011] 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.  
       [0012] 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 10 4  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.  
       [0013] 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.  
       [0014] 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.  
       [0015] 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.  
       [0016] 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 α 2 /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.  
       [0017] The lamellar cast microstructure composed of α 2 /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.  
       [0018] 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 α 2 /γ-TiAl lamellae, and, in a part area or in part areas of the semi-finished product, the lamellar cast microstructure composed of α 2 /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.  
       [0019] 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.  
       [0020] 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.  
       [0021] 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.  
       [0022] 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. 
     
    
    
     [0023] In the drawings there are shown in:  
     [0024]FIG. 1 the lamellar cast microstructure of a VAR-TiAl ingot,  
     [0025]FIG. 2 a section from the TiAl phase diagram, the line running diagonally between α and α+γ being the α-transus which changes greatly with the Al content, and a heat treatment of a material dynamically recrystallized by metal-forming leading to a fully lamellar microstructure above the transus, and, depending on the temperature, to a nearly lamellar, duplex or globular near-γ microstructure below it,  
     [0026]FIG. 3 the diagram of the melting of homogeneous TiAl semi-finished product according to A. L. Dowson et al., Microstructure and Chemical Homogeneity of Plasma—Arc 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 &amp; Materials Society, 1995,  
     [0027]FIG. 4 a metallographic microstructure picture of the head area of a valve produced according to the invention, in which the picture shows the coarse-grained lamellar cast microstructure of α 2 /γ lamellae in the head, and where it can be seen that this microstructure in the conical part of the head changes continuously into a area with fine-grained near-γ microstructure which can no longer be resolved per se in the picture,  
     [0028]FIG. 5 a light-microscopic picture of the lamellar cast microstructure in the head centre, further enlarged, and  
     [0029]FIG. 6 a light-microscopic picture of the globular metal-formed microstructure in the stem area, further enlarged. 
    
    
     [0030] Firstly, the special casting process according to the invention, described in more detail below, itself already allows unforeseen advantageous material properties with a breadth of variation of property combinations which is relatively large and thus individually matched to the respective material requirement. Secondly, a dynamically recrystallized microstructure with thermomechanical, properties very different from the properties of the cast semi-finished product can be produced by massive metal-forming from a semi-finished product having a cast microstructure set in such a way. The properties of the dynamically recrystallized microstructure can likewise be varied by adapting the process parameters.  
     [0031] Both processes, the special melting and casting process and the subsequent metal-forming process, complement each other in an unforeseen manner. Overall, material properties and combinations of material properties can thus be achieved within a single component by means of a single-stage metal-forming process in a bandwidth which were not previously realizable even with multi-stage metal-forming processes. This finding relates to components which are locally subjected to very different stresses and to those industrial applications in which γ-TiAl suggests itself as a material in principle.  
     [0032] The material designation “intermetallic γ-TiAl alloy” covers a broad field of single alloys. A key alloy range is covered by the 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-%).  
     [0033] The microstructures which can be set according to the invention from the phases and basic microstructures described at the outset result from the process steps according to the invention according to which corresponding components are produced.  
     [0034] The previously described processes for the production of a γ-TiAl alloy from the melt or of a melt-cast blank result in inhomogeneously developed phases and microstructure structure within the blank which alone already necessitated homogenization through hot-isostatic pressing (HIP) and/or a high-temperature annealing or metal-forming. On the other hand, the continuous casting process according to the invention from a cold-wall crucible and ingot take-off has proved to be ideal for giving the component the required material properties for the applications in which high-temperature creep resistance and high fracture toughness, but to a lesser extent fatigue strength and elongation at break, are important. With the melt extraction via continuous casting, a property profile can largely be set such as is required for the finished component in the component area which is not metal-formed any further, e.g. the profile of the head part in a valve for combustion engines. The smaller the diameter of the continuously-cast semi-finished product can be selected, the smaller the lamellar colony sizes and inter-lamellar intervals with even greater fracture toughness and creep resistance can be produced.  
     [0035] The semi-finished product in the form of the cast blank is then massively metal-formed according to the invention in the temperature range between 900° C. and 1400° C. by extrusion or by means of an equivalent metal-forming process and fashioned into a shape which matches the dimensions of the end product. To achieve a graduated microstructure, the bars are extruded over only part of their overall length in an extrusion die of profile dimensions which at least approximately correspond to the final dimensions of the component in the metal-formed area, e.g. dimensions of a valve for combustion engines with conical transition between stem and head areas, i.e. the extrusion die has a conically tapering cross-section between the inlet area and the outlet area. The semi-finished product is metal-formed to an increasingly pronounced extent in the conically tapering die area and thus continuously converted from the microstructure condition of the cast microstructure to the recrystallized microstructure condition achieved by extrusion. The knowledge gained from previous experience makes it possible for a person skilled in the art to selectively change specific thermomechanical properties of the material within material-dependent limits by means of corresponding metal-forming parameters and optimize them to suit particular requirements.  
     [0036] Preferred components according to the invention are combustion engine valves. This applies in particular to future uses on the horizon. Whilst to date engine valves have usually been controlled via a camshaft and different types of steel have been used for this purpose as material, current development tends towards electromagnetic or pneumatic single-valve control. However, light valves are required for this which must have a sufficient strength and corrosion resistance at high temperatures, in extreme cases up to 850° C. in the head area.  
     [0037] Valves are subjected in the stem area to strong alternating stresses (fatigue) at more moderate temperatures. The material requirements in respect of strength and ductility are correspondingly high there. As has already been described above, these locally differing thermomechanical properties are achieved in outstanding manner with components of intermetallic γ-TiAl alloys according to the invention.  
     [0038] Further, particularly suitable components are gas turbine blades in which different thermomechanical properties are required at the base of the blade than in the circumferential area of the blade.  
     [0039] The invention is described in detail using the following example of combustion engine valves.  
     EXAMPLE  
     [0040] 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 &amp; Materials Society, 1995.  
     [0041] In contrast to this process described in the said literature in which the cold-wall induction coil serves only to produce a stirring effect in the melt, the coil is dimensioned in the present design according to the invention such that the energy is sufficient for the complete melting of the alloy located in the coil. The thus-obtained semi-finished product has a lamellar cast microstructure with colony sizes of the lamellar packets between 100 μm and 500 μm, but simultaneously an excellent material homogeneity. The individual bars thus obtained as semi-finished product are divided into cylindrical segments, raised under protective gas to a temperature of 1200° C. specified for metal-forming, and expressed by cold extrusion into a heated die having the shape of a valve. The metal-forming ratio in the stem area is approx. 15:1 and decreases continuously from the head projection in prolongation of the stem to the end of the head until there is a zero metal-forming. In the metal-formed area, a fine-grained near-γ microstructure is produced as a result of the dynamic recrystallization occurring during this process and the given process temperature, whilst the lamellar cast microstructure remains in the head area. The thus-expressed component is then cooled within 30 minutes to a temperature above the brittle-ductile transition temperature, left at this temperature for approx. 60 minutes and is then brought to room temperature by normal cooling.  
     [0042] The present invention is not limited to the embodiment detailed above, rather the invention also covers components for other applications, not named, in which a corresponding microstructure is required because of the application, or is of advantage. The material γ-base-TiAl alloy is not limited to the expressly named alloy compositions.