The present invention relates to a method for armoring TiAl vanes of turbomachines and a corresponding TiAl vane.
In the case of turbomachines, such as stationary gas turbines or aircraft engines for example, in some cases components are included that move at high speeds relative to other components and they touch at least during a run-in time resulting in sliding contact. For this reason, these types of components must be provided with armoring in order to protect against wear. This also applies for example to sealing ribs, so-called sealing fins, which are provided on rotating vanes of corresponding turbomachines.
However, the problem when applying a corresponding armoring is that it possibly requires process temperatures to apply the armoring which the base material of the component being armored cannot readily tolerate. This also applies for example to vanes of turbomachines which are fabricated of TiAl materials. These types of materials, which are made predominantly of titanium and aluminum to form intermetallic phase, such as gamma TiAl, can only be provided with corresponding armoring in a limited way because of limited temperature resistance.
Applying armoring with plasma spraying is known from the prior art, wherein the temperature stress of a TiAl material can be kept sufficiently low by means of a locally limited application. However, this requires correspondingly time-consuming masking devices in order to apply a spray coating only to a desired locally limited area. In addition, these types of plasma-sprayed hard-material layers have problems related to adhesive properties so that intermediate layers containing nickel must be provided, which further increases the cost and can also lead to the formation of brittle nickel-titanium phases.
Applying the armoring to sealing fins is difficult in particular because of the geometric conditions and dimensions.
A method for coating components is generally known from German Patent Document No. DE 10 2007 056 454 A1 in which a mixture of a braze material and a coating material is applied by means of kinetic cold gas spraying and then inductively heated. However, the time-consuming masking of areas that are not supposed to be coated is also required in conjunction with kinetic cold gas spraying even though it is possible to keep the temperature stress low due to the kinetic cold gas spraying.
Therefore, the object of the present invention is providing a method for armoring TiAl vanes and in particular sealing fins of TiAl vanes and correspondingly armored TiAl vanes, in which the problems of the prior art are eliminated or reduced, and, in particular, efficient production with low temperature stress is made possible for the base material so that the component properties of the TiAl vanes are not altered as much as possible.
The invention starts with the knowledge that precisely for TiAl vanes, a brazing application of hard materials by induction brazing is possible without the temperature stress being too high for the TiAl material, because only a very limited local area must be heated in induction brazing.
In particular, such a method can also be used for the sealing fins of TiAl vanes that are difficult to coat.
The present invention understands TiAl vanes or TiAl materials to be materials that are formed predominantly of titanium and aluminum or whose largest constituents are titanium and aluminum. This includes, in particular materials whose structure have a large proportion of intermetallic phases such as γ-TiAl or α2-Ti3Al. The corresponding materials can also be alloyed with other elements, such as niobium, manganese or the like for example.
The braze materials which can be used for the brazing application of hard materials include Ti-based braze materials, Ti60CuNi, Ti70CuNi, Ni-based braze materials, AMS47XX, ASM4775, ASM4776, ASM4777, ASM4778, ASM4779 and ASM4782.
Ti-based braze material is understood in this case to mean a braze material which has a predominant proportion of titanium or for which titanium constitutes the largest constituent, such as Ti60CuNi or Ti70CuNi for example. In the case of Ti60CuNi or Ti70CuNi, the proportion of titanium is 60 or 70% by weight.
In addition, Ni-based braze materials can be used in which nickel in turn constitutes the predominant proportion or the largest constituent. Known nickel braze materials are marketed with the designation AMS47XX, wherein XX is a placeholder for numbers between 1 and 9. Special examples that are well suited for the present invention are ASM4775 with 14% by weight chromium, 4.5% by weight iron, 4.5% by weight silicon, 3.1% by weight boron and 0.7% by weight carbon and the remainder nickel; ASM4776 with 14% by weight chromium, 4.5% by weight iron, 4.5% by weight silicon, 3.1% by weight boron and the remainder nickel; ASM4777 with 7% by weight chromium, 3% by weight iron, 4.1% by weight silicon, 3% by weight boron and the remainder nickel; ASM4778 with 4.5% by weight silicon, 2.9% by weight boron and the remainder nickel; ASM4779 with 3.5% by weight silicon, 1.9% by weight boron and the remainder nickel; and ASM4782 with 19% by weight chromium, 10% by weight silicon and the remainder nickel.
The hard materials, which are applied to the TiAl vane in a mixture of at least one hard material and at least one braze material, can comprise aluminum oxide, boron nitride, cubic boron nitride, silicon carbide and titanium carbide.
The mixture of hard material and braze material can be selected in a ratio of 1:1 to 9:1 hard material to braze material.
To apply the mixture of hard material and braze material, a brazing preform can be sintered of corresponding particles of hard material and braze material, which can then be arranged on the TiAl vane for example by means of an adhesive or by a welded joint, such as a spot weld for example, in particular a resistance spot weld, in order to be brazed subsequently by means of an inductive heating process. In addition to pre-sintered brazing preforms, corresponding brazing tapes can also be used, which can be configured as strip-shaped components and can have a binding agent instead of a sintered connection of the powder particles of hard material and braze material in order to keep the powder particles in tape form. This type of brazing tape can be arranged on the TiAl vane for example also via an adhesive in order to be brazed subsequently by means of the effect of temperature.
Furthermore, it is possible to apply the mixture of hard material and braze material in the form of a paste, which in addition to the particles of hard material and braze material includes an organic binder, which keeps the paste together in a flowable state so that the paste can be applied to the TiAl vane by brushing or imprinting. The organic binder of a brazing paste can have a proportion of 5 to 30% by weight, in particular 10 to 20% by weight, in the total weight of the brazing paste, wherein the proportions of braze material and hard material can be present as indicated above in a ratio of 1:1 to 9:1 of hard material to braze material.
The brazing preforms, brazing tapes or the brazing paste can be brazed on the TiAl vane at temperatures in the range of 900° C. to 1100° C., in particular 940° C. to 1060° C., preferably in the range of 960° C. to 1020° C., wherein the hold times of the corresponding temperatures can be selected to be in the range of 30 seconds to ten minutes, preferably in the range of one minute to two minutes.
After fusing the braze material, the brazed mixture can be cooled in a controlled manner with cooling rates in the range of 10 Kelvin per minute to 100 Kelvin per minute, preferably above the brittle-ductile transition. Uncontrolled cooling is also conceivable.