USE OF SPARK PLASMA SINTERING FOR MANUFACTURING SUPERALLOY COMPOUND COMPONENTS

[0001] The invention provides a method of manufacturing a superalloy compound component.

[0002] Due to their superior properties, high temperature Nickel-based superalloys are commonly used for a wide range of components that have to operate under high mechanical stress while withstanding harsh operating conditions. For this reason Nickel-based superalloys are a preferred material for hot gas path components of gas turbines such as discs, casings, vane segments and turbine blades. Some of these superalloys, particularly the highly alloyed materials with a high content of Al and Ti are very difficult to weld. The same is true for refractory metals which are also very difficult to weld. In the case of the Nickel-based superalloys the high content of Al and Ti will cause a precipitation of hardened Gamma-prime phase and an increased crack susceptibility of the parts during the welding process. The three main types of cracking and defects are solidification cracking, grain boundary liquation cracking and strain age cracking Hence, welding of highly strengthened Nickel-based superalloys like PWA 1483, Rene 80, CM 247, IN 738 and IN 939 shows many quality problems. In view of the aforegoing it is an object of the invention to provide a method of manufacturing a superalloy compound component.

[0003] In order to solve the abovementioned object, the invention provides a method of manufacturing a superalloy compound component having a first component portion primarily consisting of a first superalloy and a second component portion primarily consisting of a second superalloy or of a refractory metal. According to the invention the method includes using Spark Plasma Sintering for forming the superalloy compound component.

[0004] Spark Plasma Sintering is a sintering process that is also known as Field Assisted Sintering Technique or Pulsed Electric Current Sintering. In Spark Plasma Sintering a pulsed or continuous current is led through compacted metal powder contained within a mould. The heat produced by the current causes sintering of the metal powder achieving densification close to theoretical maximum density but at lower sintering temperatures compared to conventional sintering processes. Spark Plasma Sintering has an advantage that the heat is generated uniformly within the compacted metal powder which allows for very high heating and cooling rates. For this reason Spark Plasma Sintering is a very fast sintering process.

[0005] The invention is based on the idea that Spark Plasma Sintering may be used for joining components of the same or different Nickel-based superalloys or refractory metals which may not be joined by welding due to the abovementioned difficulties. Thus, the invention proposes using a sintering process for a different object, i.e. joining two components to produce a superalloy compound component rather than producing a component from metal powder as is known in the art.

[0006] This idea encloses two main approaches the first of which will be referred to herein as powder metallurgy. According to this approach the method of the invention further comprises providing a mould having first and second mould portions each defining an outer shape of the first component portion and the second component portion, respectively. Then a first metal powder primarily consisting of the first superalloy is arranged in the first mould portion and a second metal powder primarily consisting of the second superalloy or of the refractory metal is arranged in the second mould portion. Finally the superalloy compound component is integrally formed by Spark Plasma Sintering the first and second metal powders in the mould. This is especially useful for producing superalloy compound components where different segments of the superalloy compound component are formed of different superalloy materials or refractory metals which may not be joined by welding techniques. This may be useful for manufacturing specific components such as turbine blades of a gas turbine using different superalloys for different parts of the turbine blade in accordance with the expected mechanical and chemical stress to be experienced by the respective part of the turbine blade in operation of the gas turbine.

[0007] The second approach is referred to herein as diffusion bonding. According to this approach the method of the invention may further comprise providing a first component primarily consisting of the first superalloy and a second component primarily consisting of the second superalloy or of the refractory metal. The first and second components are arranged as to contact each other before using Spark Plasma Sintering for forming the superalloy compound component. In this way the first component forms the first component portion and the second component forms the second component portion of the superalloy compound component. Thus, the diffusion bonding of the invention allows for firmly joining a first component to a second component which would normally be achieved by welding if it were not for the materials that cannot be welded. Contrary to the first inventive approach the first and second components are provided as macroscopic components and not in the form of powders which then are solidified. Thus, the Spark Plasma Sintering process known in the art may be used for an entirely different object, i.e. for joining two construction components to each other.

[0008] To support diffusion bonding of the two components, the method of the invention may further comprise providing a layer of a superalloy powder in a contact region where the first component contacs the second component. This superalloy powder will solidify during the Spark Plasma Sintering thereby supporting the diffusion bonding of the two components. The superalloy powder may primarily consists of either one of the first superalloy, the second superalloy or a mixture of the first superalloy and the second superalloy.

[0009] Preferably the Spark Plasma Sintering is carried out under vacuum. Furthermore, the Spark Plasma Sintering may include a step of pressing the first component portion and the second component portion. Suitable pressures may be in the range of one to fourty MPa (Megapascal). The Spark Plasma Sintering may include heating the first component portion and the second component portion. The components may be heated to a temperature of 1000 to 1200 degrees Celsius. Suitable heating and cooling rates may be in the range of 20 to 200 Kelvin per minute. The time required for bonding will be typically in the range of 3 to 60 minutes resulting in a total time for

[0010] Spark Plasma Sintering of approximately 2 to 3 hours including heating, bonding and cooling. The current used for the Spark Plasma Sintering may be provided in a pulsed or continuous mode.

[0011] Preferably the superalloy compound component is a gas turbine component. At least one of the first and the second superalloys may be a Ni-based superalloy. This Ni-based superalloy may comprise at least one of Al and Ti. The inventive method is especially useful for joining components consisting of different superalloys. Thus, the second superalloy may be different from the first superalloy.

[0013] FIG. 1 shows a cross sectional view of an example superalloy compound component formed using the method of the invention. In this example two buttons made from Nickel-based superalloys were joined using Spark Plasma Sintering in accordance with the invention. One button was made from CM 247, the other from Rene 80. The two buttons were joined firmly to each other illustrating the superior bonding of two usually unweldable parts. In the FIGURE a bond line between the two materials having a horizontal orientation is indicated. This bond line can hardly be seen thus underlining the quality of the joining of the two buttons. The method of the invention can be used for joining components of the same or different superalloys or of refractory metals. The method of the invention therefore allows for production of superalloy compound components from two or more components thereby reducing the cost of production and repair of components that otherwise have to be produced monolithically. This makes the invention especially useful in the field of gas turbine production and repair.

[0014] Although the invention has been shown and described with respect to exemplary embodiments thereof, various other changes, omissions, and additions in form and detail thereof may be made therein without departing from the spirit and scope of the invention.

[0015] While the invention has been described in connection with a preferred embodiment, it is not intended to limit the scope of the invention to the particular form set forth, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the scope of the invention as defined by the appended claims.