Method for manufacturing a dual alloy cooled turbine wheel

A dual alloy cooled turbine is manufactured by casting a hollow cylinder of first nickel-base alloy material with high creep resistance to produce directionally oriented grain boundaries. A preform of a second nickel-base alloy material with high tensile strength and high low-cycle-fatigue strength is diffusion bonded into the bore of the hollow cylinder by subjecting the cylinder and preform to hot isostatic pressing. The resulting cylindrical block is cut into thin precisely flat wafers. A plurality of alignable holes for forming fluid cooling passages are photochemically etched into the individual wafers. The wafers then are laminated by vacuum diffusion bonding techniques, with the holes aligned to form fluid cooling passages. The resulting laminated block is machined to produce the turbine wheel with turbine blades through which the cooling passages extend.

BACKGROUND OF THE INVENTION 
The invention relates to dual alloy turbine wheels and, more particularly 
to dual alloy cooled turbine wheels and methods of manufacture thereof. 
Various dual alloy turbine wheels are used instead of single alloy turbine 
wheels in applications in which exceptionally high speed, high temperature 
operation is needed, since under these circumstances it is necessary to 
have high creep rupture strength at high temperatures in the blade or 
outer rim portion of a well designed turbine disk, and it is also 
necessary under high speed, high temperature conditions to have superior 
tensile strength and low-cycle-fatigue properties in the hub portion. 
Superalloy materials which have the former highly desirable 
characteristics in the blade and outer rim portions of a turbine wheel do 
not have the high tensile strength and low-cycle-fatigue resistance 
properties that are required in the hub, and vice-versa. In general, all 
the desirable qualities for turbine wheel hubs are associated with tough, 
fine-grained, nickel-base alloys, in contrast to the desired properties of 
the material of the blade, ring, or rim portions of a turbine disk, in 
which large-grained, nickel-base alloys with directional structures in the 
blades are used. The large grained, directional structure alloys possess 
high creep resistance, but inferior tensile properties. 
Where the performance compromises necessitated by use of a single alloy 
material in a turbine disk are unacceptable, dual alloy turbine wheels 
have been used for many years, for example, in connection with military 
engines which utilize AISI Type 4340 alloy steel hubs fusion welded to 
Timken 16-25-6 warm-worked stainless steel rims, the alloys of which could 
be fusion-welded to yield joints of adequate strength. More modern, 
stronger, more complex alloys, however, could not be fusion-welded in 
typical disk thicknesses without unacceptable cracking. Inertia-welding 
processes have been used in joining of axial-flow compressor disks into 
spools and in joining of dissimilar metal shafts to turbine wheels. 
However, the largest existing inertia welding machines are only capable of 
welding joints in nickel-based alloys which are a few square inches in 
cross section, so this process can be used only in the smallest turbine 
disks. 
The bonding of dissimilar metals by hot isostatic pressing (HIP) has been 
suggested for manufacture of dual alloy turbine wheels, since this process 
does not have the inherent joint size limitations of the inertia-welding 
process. Hot isostatic pressing is a process in which the pressure is 
applied equally in all directions through an inert argon gas in a high 
temperature pressure vessel or autoclave. Cross Pat. No. 4,096,615, Ewing 
et al., Pat. No. 4,152,816, and Catlin Pat. No. 3,940,268 are generally 
indicative of the state of the art for hot isostatic pressing as applied 
to manufacture of dual alloy turbine wheels. Kirby Pat. No. 3,927,952, 
assigned to the present assignee, is indicative of the state of the art in 
manufacture of cooled turbine disks and discloses photochemically etching 
recesses in thin single alloy disks to produce corresponding holes which 
are aligned when the disks are subsequently vacuum diffusion bonded 
together to create a laminated structure in which fluid cooling passages 
extend from a central bore of the hub to and through the turbine blades. 
Cooled turbine discs are necessary in small, high-temperature gas turbine 
components that are subjected to exceedingly high external gas 
temperatures, wherein the blade metal temperatures may reach the range of 
1700 to 1800 degrees Fahrenheit. The cooling passages are necessary to 
prevent the blades from exceeding this temperature range in order to 
prevent excessive creep of the blade material. 
The above mentioned dual alloy turbine wheels have become attractive 
because their optimum material properties in both the hub portion area and 
the ring and blade portion of turbine disks have allowed the minimization 
or elimination of cooling fluid requirements and have allowed lighter 
weight turbine disks to be utilized. However, there nevertheless remains a 
need for an ultra-high performance dual alloy turbine wheel that is 
capable of operating in conditions that would produce unacceptably high 
blade temperatures even in the best prior art uncooled dual alloy turbine 
wheels. 
Accordingly, it is object of this invention to provide an ultra-high 
performance turbine wheel and a practical method of manufacture thereof 
which has all of the advantages of prior dual alloy turbine wheels and 
further provides suitable fluid cooling passages to the blades of the 
disk. 
SUMMARY OF THE INVENTION 
Briefly described, and in accordance with one embodiment thereof, the 
invention provides a high performance, cooled, dual alloy turbine wheel 
and method of manufacture thereof, wherein a hollow cylinder of first 
superalloy material having high creep rupture strength up to approximately 
1800 degrees Fahrenheit is cast against a chill to produce a radial 
directional grain structure; the hollow cylinder then is filled with 
second superalloy material having the properties of high tensile and high 
low-cycle-fatigue strengths, after which deformable plates are bonded to 
the cylinder to tightly seal the second superalloy material therein and 
the assemblage then is subjected to hot isostatic pressing to achieve 
direct metallurgical diffusion bonding of the second superalloy material 
to the cast cylinder; the resulting dual alloy cylinder then is sliced 
into a plurality of thin, precisely flat dual alloy wafers or laminae, 
which are cut to produce cooling holes, and then are reassembled to 
produce a laminated cylinder from which the cooled dual alloy turbine 
wheel can be machined. In the described embodiment of the invention, the 
first superalloy material of which the cast cylinder is formed consists of 
MAR-M247 alloy and the second superalloy is in the form of a 
pre-consolidated preform composed of powder metal low carbon Astroloy 
material. After the hot isostatic pressing, the resulting dual alloy 
cylinder is machined to produce a precise cylinder. Slicing of the 
resulting dual alloy cylinder into wafers is accomplished by a process 
that results in precisely flat wafers. Photochemical etching or laser 
cutting techniques are used to cut cooling holes in locations at which the 
turbine blades will be formed later. The wafers are coated with elemental 
boron or a nickel-boron alloy, aligned so that their respective cooling 
holes form fluid cooling passages, and are subjected to hot axial pressing 
to vacuum diffusion bond the wafers together to produce the laminated 
structure. The laminated structure then is appropriately heat treated and 
inspected, and machined using conventional techniques to form the turbine 
blades and other features of the turbine wheel. Extremely high creep 
strength is achieved in the blade material. Extremely high tensile 
strength and high low-cycle-fatigue strength are achieved in the hub 
portion of the turbine wheel. These properties result in an extremely high 
performance turbine wheel that can withstand very high temperature, high 
speed operation.

DETAILED DESCRIPTION 
Referring now to the drawings, reference numeral 1 in FIG. 1 designates a 
cast hollow cylinder. Cylinder 1 is cast of a material having very high 
creep rupture strength. A suitable material would be a nickel-based 
superalloy material, such as MAR-M247 material. Preferably, the procedure 
of casting cylinder 1 would be to cast it against a chill (i.e., by 
providing a chilled copper outer mold wall against which the outer portion 
of the cast, molten alloy metal presses so that the outer portions of the 
molten metal rapidly freeze, producing radial, directional solidification. 
The radial lines shown in FIG. 1 on the top of cylinder 1 indicates the 
resulting radial grain structure. This results in maximum creep rupture 
strength. Note that this first step (of casting cylinder 1) is designated 
by reference numeral 35 in the process flow chart of FIG. 9. 
The next step in the process is to precisely machine the cylindrical hole 
1A in cylinder 1 so that a very close fit can be provided against the 
surface of a hub preform. The hub preform is designated by reference 
numeral 2 in FIG. 2. As mentioned above, the hub portion of the turbine 
wheel being manufactured needs to have maximum low-cycle-fatigue and high 
tensile strength properties. A suitable preform 2 having these properties 
can be composed of preconsolidated powder metal low carbon Astroloy, a 
fine grained superalloy material. 
The outer diameter face of preform 2 is machined to achieve a precise fit 
into the machined cylindrical hole 1A into cast cylinder 1. Subsequent to 
machining the outer diameter of preform 2, it is inserted into the center 
of the cast cylinder 1. This step is indicated in block 36 of FIG. 9. 
Normally, hub preform 2 would be manufactured by hot isostatic pressing 
techniques to make a cylindrical "log" from which the preforms 2 are 
machined. After the precise fit has been achieved, the two deformable end 
plates 3 and 4 are peripherally bonded to cast cylinder 1. The bonding can 
be achieved by the known technique of electron beam welding, which 
produces electron beam weld spikes 5 to affix and seal the deformable 
plates 2 and 4 to the cylinder 1. A secondary seal around the preformed 
hub 1 and deformable plates 3 and 4 is accomplished by brazing the outer 
circumference of deformable plates 3 and 4 to produce activated diffusion 
bonding that provides the additional seals designated by reference 
numerals 6 and 7. This step is recited in block 37 in the flow chart of 
FIG. 9. The electron beam welding techniques and peripheral brazing 
techniques are well known and can be easily provided by those skilled in 
the art. The deformable plates 3 and 4 can be composed of Inconel 625 
sheets, which are typically 0.040-0.080 inches thick. 
As indicated in block 38 of the flow chart of FIG. 9, the next step is to 
hot isostatically press the assemblage of FIG. 2 in order to achieve 
vacuum diffusion bonding of hub preform to cast cylinder 1. The hot 
isostatic pressing procedure would typically be performed for four (4) 
hours at 15,000 psi pressure and 2200.degree. F. temperature. Activated 
diffusion bonding is described in detail in the November 1970 welding 
research supplement of the Welding Journal of the American Welding Society 
at pages 505-S to 509-S by George Hoppin III, and T. F. Berry, also 
incorporated herein by reference. 
As indicated by block 39 in the flow chart of FIG. 9, the next step in the 
process for making the dual alloy cooled turbine wheel of the present 
invention is to machine the ends of the block illustrated in FIG. 2 and 
formed by the hot isostatic pressing procedure in order to remove the 
deformable end plates 3 and 4 and produce a machined cylindrical "log" 
designated by reference numeral 10 in FIG. 5 and having a rectilinear 
cross section. This rectilinear log is then suitable for the subsequent 
step which, as indicated in block 40 of FIG. 9, is to slice the dual alloy 
cylinder 10 into a large number of thin, extremely flat dual alloy wafers 
or laminae, generally designated by reference numeral 10A in FIG. 4. 
Typically, the thickness of each of the wafers 10A might be in the range 
from 0.020 to 0.040 inches. Reference numeral 1B in FIG. 4 designates the 
outer alloy portion of the wafers 10A, which has the desired high creep 
rupture strength needed in the turbine blades, while reference numeral 2A 
designates the hub portion having the desired fine grained alloy structure 
with high low-cycle-fatigue and high tensile strength properties. 
The degree of flatness required for the wafers 10A is quite high; a 
flatness of approximately plus or minus one percent of the wafer thickness 
is desirable. This is in contrast with aircraft engine industry normal 
standards for sheet thickness, where the tolerance is .+-.10%. Various 
techniques could be used for slicing the dual alloy block 10 of FIG. 3 
into the wafers 10A. The presently preferred technique is to use "wire 
EDM" (electrical discharge machining) devices which are widely used to 
obtain precise cutting of metals. 
As indicated in block 41 of the flow chart of FIG. 9, the next step in the 
manufacturing process of the present invention is to photochemically 
machine each of the dual alloy disks 10A to produce the fluid cooling 
passages that will be needed in the turbine blades of the turbine wheel 
ultimately produced by the process of the present invention. Reference 
numerals 11 in FIG. 5 generally designate a particular group of such 
cooling fluid holes that form a portion of one of such cooling passages 
which will ultimately extend through one of the subsequently formed 
turbine blades. Alternately, other machining techniques could be used, 
such as laser cutting to produce the fluid cooling holes 11. In FIGS. 5 
and 6, holes 11 are the air inlets for the respective blades of the 
turbine wheel being manufactured. Each air inlet hole 11 extends through a 
path, which may be quite complex, in a separate blade of the turbine 
wheel. 
Next, as indicated by block 42 in FIG. 9, it is necessary to align the 
corresponding fluid cooling passages 11 in all of the dual alloy disks 10A 
so that the fluid cooling passages of the turbine wheel are formed. The 
disks 10A are all laminated together to produce the reconstructed dual 
alloy block designated by reference numeral 10B in FIG. 6. As mentioned in 
the above referenced Kirby Pat. No. 3,927,952, (which is owned by the 
present assignee and is incorporated herein by reference) the laminated 
rectangular block 10B can be formed of the thin wafers 10A by coating them 
with a suitable braze or diffusion bonding alloy, which can be applied in 
various ways, such as by spraying, dusting, or placing a brazed alloy foil 
between the adjacent wafers. A preferred technique is to deposit elemental 
boron in carefully controlled amounts by chemical vapor deposition. The 
coated wafers then are stacked in a predetermined order, with the fluid 
cooling holes 11 properly aligned, and are subjected to a vacuum diffusion 
bonding process at a suitable elevated temperature, such as 2200.degree. 
Fahrenheit under a suitable axial pressing force (10-100 psi). 
After appropriately heat treating and inspecting the resulting "log" 10B of 
FIG. 6, the final step in the manufacturing process of the present 
invention is to utilize conventional machining techniques to produce a 
cooled, dual alloy turbine wheel, such as the radial flow turbine wheel, 
designated by reference numeral 10C in FIG. 7, wherein reference numeral 
13 generally designates the blades. Reference numeral 14 generally 
designates the ends of some of the fluid cooling passages in the blades of 
the final turbine wheel that are obtained by the above-mentioned 
photochemical machining of holes 11 in the dual alloy discs 10A and proper 
alignment thereof during the vacuum diffusion bonding procedure by which 
laminated cylinder 10B is formed. 
Although the above example leads to the construction of the cooled radial 
flow turbine wheel of FIG. 7, the same techniques can be applied to the 
manufacture of axial flow turbine wheels. FIGS. 8A and 8B show section 
views of blades of two such cooled axial flow turbine wheels. In FIG. 8A, 
reference numeral 2A designates high tensile strength, high 
low-cycle-fatigue strength material of the hub portion of an axial flow 
turbine wheel. Reference numeral 1B generally designates the high creep 
strength blade portion of the turbine wheel. Reference numeral 11 
designates the cooling air inlet of the blade, leading to a complex 
network of air passages 45 formed by properly aligned cooling holes in the 
various laminated disks. The arrows 46 indicate the general direction of 
cooling air flow in the passages 45. The cooling air is exhausted from 
outlets at the tip and the trailing edge of the blade and through 
"showerhead" holes in the leading edge of the blade (not shown in FIG. 
8A). FIG. 8B shows another section view of the blade of a simpler axial 
flow turbine wheel, wherein the cooling passages extend from the inlet 11 
to outlets only at the tip of the blade. 
Thus, the invention provides a dual alloy turbine wheel that has optimum 
materials and cooling circuits for a cooled integral turbine wheel. The 
method also provides a practical method of manufacture of the turbine 
wheel. The turbine wheel of the present invention should provide 
significant advantages for certain small, extremely high speed, high 
temperature turbine engines. 
While the invention has been described with reference to a particular 
embodiment thereof, those skilled in the art will be able to make various 
modifications to the described embodiment of the invention without 
departing from the true spirit and scope thereof. It is intended that 
elements and steps which are equivalent to those disclosed herein in that 
they perform substantially the same function in substantially the same way 
to achieve substantially the same result be encompassed within the 
invention. 
For example, it is not essential that the hub preform 2 be sliced along 
with the annular cast cylinder 1, since no cooling holes are needed in the 
hub. Therefore, the annular cast cylinder 1 as shown in FIG. 1 could be 
sliced to produce wafers or disks in which cooling passage holes are cut, 
as by photochemical etching. These etched disks can be laminated to 
reconstruct the annular cylinder 1, and the hub preform 2 then can be 
inserted into the hole (corresponding to 1A in FIG. 1) of the 
reconstructed annular cost cylinder and attached thereto by diffusion 
bonding.