Method of manufacturing an integral bladed turbine disk

The disclosure relates to a turbine rotor and method of manufacture thereof. A plurality of blades having dovetails at the radially inner ends thereof are arranged in a circumferentially spaced circular array. A metallic hub is cast about the dovetails of said blades which are metallurgically bonded by the operation. Alternatively, the blades may be joined to the hub thereafter by an electron beam weld that extends axially to the rotor.

BACKGROUND OF THE INVENTION 
The power and fuel efficiency of a gas turbine engine is a function of the 
temperature 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-of-the-art uncooled metal turbine rotors is approximately 
2000.degree. F. Increasing the turbine rotor inlet temperature beyond 
2000.degree. F. requires the use of advanced super alloy blade materials 
which are generally not compatible with the mechanical properties of the 
rotor hub. 
A solution to this incompatibility problem is to adopt a dual-property 
approach to the fabrication 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 dual-property rotor concept has 
heretofore precluded its use in small, lightweight gas turbine engines. 
SUMMARY OF THE INVENTION 
The turbine wheel of the instant invention is a relatively simple, low cost 
multiple property integral turbine rotor for 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 materials, etc. 
Thereafter, 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 assembly fixture comprises inner and outer 
rings, the annulus therebetween being packed with resin sand or ceramic 
slurry as taught in application Ser. No. 466,166, filed 2-14-83, now U.S. 
Pat. No. 4,494,287, issued 1-22-85, and assigned to the assignee of this 
invention. After hardening, the annular core is stripped from the fixture, 
leaving a free standing sand or ceramic core with exposed 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 about the 
blade dovetails. The assembly is diffusion bonded incident to casting of 
the hub and subjected to a hot isostatic press cycle to complete the bond.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION 
As seen in FIG. 1 of the drawings, 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 materials are single 
crystals of CMSX 2, MarM 247, or NASAIR 100 in the form of directionally 
solidified eutectics, directionally solidified castings, or mechanically 
strengthened alloys. 
As seen in FIG. 2, a dovetail root portion 14 of each blade 12 of FIG. 1 is 
prepared by grit blasting and thereafter coated with a plasma sprayed 
activated diffusion bond alloy (ADB) 16. The material selected for the ADB 
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. The ADB 
coating 16 is utilized to effect a metallurgical bond between components 
of the turbine wheel, as will be described. 
As seen in FIG. 3, a ceramic blade ring 18, which may be fabricated in 
accordance with the teaching of application Ser. No. 06/466,166, filed 
Feb. 14, 1983, 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 process which results in the 
cast turbine wheel 10. 
A super heated melt is vacuum poured into the preheated mold 20 causing 
melting of the ADB alloy coating 16 on the dovetails 14 thereby to form a 
metallurgical bond between the blades 12 and a cast hub 24 as the entire 
mold slowly cools thereby to form the integral, multiple alloy turbine 
wheel 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 approximately 2500.degree. 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.degree. F. When the aforesaid combination is 
heated above 2000.degree. F. in the mold 20 upon casting of the hub 24, 
the bond activator 16, for example, melts, due to the fact 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 12 until an 
equilibrium point of 2200.degree. F. is reached. When the entire assembly 
solidifies, the completed turbine wheel 10 is finish machined and 
conditioned for assembly with mating turbine engine components. 
As seen in FIG. 6, voids 30 sometimes occur due to incomplete fusion 
between the melt and the blades 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 of the 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 14 to 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 location radially 
outwardly of the dovetails 14 of the blades 12 at which point they may 
even result in desirable dampening of blade vibrations. 
While the preferred embodiment of the invention has been disclosed, it 
should be appreciated that the invention is susceptible of modification 
without departing from the scope of the following claims.