GB2180891A - Unitary composite turbine rotor - Google Patents

Abstract

A plurality of blades 12 having dovetails 14 at the radially inner ends thereof are arranged in a circumferentially spaced circular array. A metallic hub 24 is cast about the dovetails of said blades which are first coated with a plasma sprayed activated diffusion bonding alloy 16, the alloy acting to promote good bonding. Any voids formed about the blade root area during the process are displaced by electron beam welding. The claimed invention is directed to the product turbine rotor. <IMAGE>

Description

SPECIFICATION Integral bladed disk Background of the invention The power and fuel efficiency of a gas turbine engine is a function ofthetemperature of the combustion gases at the inlet to the turbine. The temperature is generally maximized consistent with turbine and nozzle structural integrity. The maximum turbine rotor inlet temperature allowed by current state-ofthe-art uncooled metal turbine rotors is app roximately 2000 F. increasing the turbine rotor inlet temperature beyond 2000"F requires the use of advanced super alloy blade materials which are generally not compatible with the mechanical properties ofthe rotor hub.

A solution to this incompatibility problem is to adopt a dual-property approach to thefabrication of the turbine rotor. In large gas turbines, where size and complexity constraints are not acute, this is accomplished by using discrete blades of a high rupture strength material mechanically attached to a high burst strength disk. However, the physical size, cost, and complexity associated with this dualproperty rotor concept has heretofore precluded its use in small, lightweight gas turbine engines.

Summary ofthe invention The turbine wheel ofthe instant invention is are latively simple, low cost multiple property integral turbine rotorfor use in small gas turbine engines.

The rotor has discrete, high rupture strength blades permanently bonded to a high burst strength alloy hub.

More specifically, individual turbine blades are fabricated, for example, from a single crystal alloy, directional solidification alloy including directional solidification eutectics, oxide dispersion strengthened alloy, rapid solidification rate alloy, mechanically alloyed material, etc. Therafter, the root and dovetail of each blade is coated with a conventional diffusion bonding material after which the blades are placed in an assembly fixture. The assemblyfixture comprises inner and outer rings, the annulusthere between being packed with resin sand or ceramic slurry as taught in U.S. Patent No. 4,494,287 and assigned to the assignee of this invention.After hardening, the annular core is stripped from thefixture, leaving a free standing sand orceramic corewith ex- posed blade dovetails. Alternatively, the uncoated blades may be assembled into the ceramic or sand core ring, and the exposed roots coated as an assembly. The core and blades are placed in a rotor hub mold and the rotor hub is cast aboutthe blade dovetails. The assembly is diffusion bonded incident to casting of the hub and subjected to a hot isostatic press cycleto complete the bond.

Briefdescription ofthe drawings Figure lisa plan view of a completed turbine wheel.

Figure2 is a view of a coated turbine wheel blade after casting thereof.

Figure 3 is a view of a mold for casting theturbine blades to the turbine wheel hub.

Figure4 is a perspective view of a core ring with the turbine blades positioned therein.

FigureS is a view taken in the direction of the arrow SofFigure4.

Figure 6is a view taken within the circle 6 of Figure 3.

Figure 7is a viewsimilarto Figure6 afterwelding.

Detailed description of the preferred embodiment of the invention As seen in Figure 1 of the d rawings, a turbine wheel 10 comprises a plurality of blades 12 which are fabricated from a high temperature material by known fabrication processes. Examples of such mat trials are single crystals of CMSX 2, MarM247, or NASAIR 100 in the form of directionally solidified eutectics, directionally solidified castings, or mechanicallystrengthened alloys.

As seen in Figure 2, a dovetail root portion 140f each blade 12 of Figure 1 is prepared by grit blasting and thereafter coated with a plasma sprayed activated diffusion bond alloy (ADB) 16. The material selec tedfortheADB coating 16, generally a Ni-Cr-B or Ni Cr-B-Si alloy, the coating thickness, and the method of coating are well known in the art. TheADB coating 16 is utilized to effect a metallurgical bond between components of the turbine wheel, as will be described.

As seen in Figure 3,aceramic blade ring 18, which may be fabricated in accordance with the teaching of application Serial No. ,filed ,and assigned to the assignee of this application, holds the blades 12 in a desired array, and in combination with a mold 20, defines a mold cavity 22 suitable for the bi-casting pro cess which results in the castturbine wheel 10.

A super heated melt is vacuum poured into the preheated mold 20 causing melting oftheADB alloy coating 16 on the dovetails 14therebytoform a met- allurgical bond between the blades 12 and a cast hub 24 as the entire mold slowly cools thereby to form the integraf,multiplealloyturbinewheel 10.

More specifically, the structural elements that coact in the bonding process are the blades 12,the superalloy hub 14, both of which have melting points of a pp roxi mateiy 2500"F, and the bond activator 16 on the dovetails 14 of the blades 12. The bond activator 16 has a melting point of approximately 2000 F.

When the aforesaid combination is heated above 2000"F in the mold 20 upon casting of the hub 24, the bond activator 16, for example, melts, dueto thefact that Boron has a relatively low melting point. As time progresses and the assembly cools, Boron migrates into the hub 24, and blades 12 in the solid state. Because Boron imparts a relatively low melting point to the bond activator coating 16, migration thereof raises the melting point of the bond activator 16 and lowers the melting point of the hub 24 and blades 1 2 until an equilibrium point of 2200"F is reached.When the entire assembly solidifies, the completed turbine wheel 10 its finish machined and conditioned for assembly with mating turbine engine components.

As seen in Figure 6, voids 30 sometimes occur due to incomplete fusion between the melt and the bla des 12 during solidification. This problem is typically solved by the use of hot isostatic pressing during the process cycle. However, in accordance with one feature ofthe instant invention, another solution is the use of a welding process comprising an electron beam that is passed through the base of the blade dovetail 14to produce a weld area 29. The electron beam weld eliminates any voids 30 at the critical radially inner boundary between the blade dovetails 14 and the hub 24 and results in columnar grains 32 extending axially of the turbine wheel 10. Any voids are driven to a non-critical iocation radially outwardly of the dovetails 14 of the blades 12 at which pointthey may even result in desirable dampening of blade vibrations.

Whilethe preferred embodimentofthe invention has been disclosed, it should be appreciated thatthe invention is susceptible of modification without departing from the scope fo the following claims.

Claims (3)

1. Aturbinerotorcomprising; a plurality of blades having dovetails atthe radially inner ends thereof; a metailic hub cast aboutthe dovetails of said blades; a Ni-Cr-B diffusion bonding material inter-posed between the dovetails of said blades and said hub; and an electron beam weld zone comprising columnar grains extending axially of said turbine rotor and radially limited so asto include onlythe radially inner end portions of the dovetails of said blades and the adjacent portions of said hub.

2. Aturbine rotor constructed and arranged substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

3. Agasturbine enginecomprising aturbine rotor as claimed in either preceding claim.