Article with material absorption cavities to reduce buckling during diffusion bonding

An airfoil and its sub components for a gas turbine engine have a convex wall and a concave wall that are connected at leading and trailing edges. Internal supports extending from the convex and concave walls define a series of airfoil supports that have at least one primary cavity between them. Internal supports in the proximity of the edges define material absorption cavities that have a smaller cross-section than the cross section of the primary cavities. Pressure applied to the walls during the diffusion bonding process forces material inside the airfoil, and most particularly the airfoil edges, to yield towards the center of the airfoil. The material absorption cavities absorb material that yields during the diffusion bonding process and prevent buckling of the walls.

TECHNICAL FIELD 
This invention relates to hollow articles that are manufactured using 
diffusion bonding techniques. 
BACKGROUND ART 
Hollow components such as airfoils (fan blades, compressor blades, and 
turbine blades) for gas turbine engines are used by engine manufacturers 
to reduce weight and to increase the operating engine efficiency. As 
engine manufacturers design engines that produce increased thrust, larger 
airfoils are needed. As engine size increases, reducing engine component 
weight becomes increasingly important. High bypass engines require very 
large diameter fans, and the larger fans make reducing the airfoil, or fan 
blade, weight while maintaining structural integrity critical. Hollow 
titanium fan blades help to decrease fan blade weight and provide a blade 
that can endure applied dynamic loads during engine operation. 
Titanium hollow fan blades are fabricated using diffusion bonding often in 
conjunction with superplastic forming processes. Diffusion bonding forms a 
metallurgical bond between parts by pressing the parts together at 
elevated temperatures and pressures. Bonding of the parts occurs by the 
diffusion of atoms between adjacent part faces. Diffusion bonding provides 
essentially parent metal joint strength. Localized flow at the mating 
faces is essential to get full contact and to permit diffusion. 
Superplastic forming relies on the property of certain metals, including 
titanium alloys, to exhibit high tensile or compressive material 
deformation with a minimal tendency toward local necking of the part, when 
the metal is exposed to time, temperature, and strain conditions within 
certain ranges. Both diffusion bonding and superplastic forming allow a 
variety of lightweight, high strength parts to be manufactured and are 
ideal for forming hollow titanium airfoils for gas turbine engines. 
Modern hollow airfoils, such as blades and vanes, have a convex wall (the 
suction wall), a concave wall (the pressure wall), and a series of 
internal support ribs or protrusions extending spanwise (radially) and/or 
chordwise between the concave and convex walls, that define at least one 
internal cavity to reduce the weight of the airfoils while maintaining 
structural requirements. The walls present a problem during the diffusion 
bonding process. The ribs are matched together at a bond line, and the 
surface area of the ribs (as a function of bond plane area) is usually 
less at the center of the airfoil than the surface area of the edges of 
the airfoil, at the bond plane. The loading required for diffusion bonding 
causes material to flow in the direction of least resistance, which is 
primarily in the net direction of the applied forces, or normal to the 
direction of applied forces if the forces are equal and opposite. The 
amount of material flow is partly a function of the amount of material at 
the bond line. During diffusion bonding, the airfoil walls are placed in 
compression near the airfoil edges, as a result of material flow, and tend 
to buckle. Buckling of the walls distorts the structural characteristics 
of the part and interferes with the airflow across the airfoil. Past 
solutions to prevent buckling have included increasing wall thickness to 
resist the compressive forces or shortening the span of the cavities. 
However, both of these solutions increase the weight of the airfoil. 
A hollow airfoil design that can be diffusion bonded without buckling and 
without increasing the weight of the airfoil is needed. 
DISCLOSURE OF THE INVENTION 
It is, therefore, an object of the present invention to provide an article 
geometry that can be diffusion bonded to form a hollow article without 
distortion and without adding features that increase the weight of the 
article. The present invention includes a sub component of the article, 
the article itself, and a method for making the article. The invention 
will be described in particular with reference to a hollow airfoil. 
However, the invention is not limited to airfoils and is applicable to 
other articles. 
According to the present invention, an airfoil, such as a fan blade, for a 
gas turbine engine has a convex wall and a concave wall that are connected 
at leading and trailing edges. Internal ribs or protrusions extending from 
the convex and concave walls meet to define a series of airfoil supports 
that define at least one primary cavity. Internal ribs in the proximity at 
least a portion of the edges off at least one primary cavity smaller 
material absorption cavities that are designed to absorb material flow 
during bonding. The wall thickness adjacent the material absorption 
cavities (prior to diffusion bonding) is thinner than the wall thickness 
adjacent to the primary cavities. The pressure applied to the walls by 
metal flow during the diffusion bonding process forces material inside the 
airfoil, particularly at the edges, to yield towards the center of the 
airfoil. The material absorption cavities absorb or accommodate the 
material that yields during the diffusion bonding process to prevent 
buckling of the walls and distortion of the airfoil outer surface and the 
primary cavities. The material absorption cavities do not add material to 
increase the wall thickness or shorten the span of the primary cavities. 
The airfoils may be designed to have a structurally required minimum wall 
thickness without regard for the wall's reaction to forces during the 
diffusion bonding process. 
The foregoing and other features and advantages of the present invention 
will become more apparent from the following description and accompanying 
drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
The term airfoil as used here may include rotating blades or stationary 
vanes and is also meant to encompass other hollow structures fabricated by 
diffusion bonding. 
According to preferred embodiment, a fan assembly 10 comprises a plurality 
of fan blades 12, or airfoils, supported by a fan disk 14, as shown in 
FIG. 1. Each airfoil 12 spans radially (span) from a root portion 16 to a 
tip 18, and in a chordwise (chord) direction from a leading edge 20 to a 
trailing edge 22 with a chordwise mid-portion 24. The chordwise 
mid-portion 24 is defined by a convex wall 26 and a concave wall 28 that 
are connected at the leading and trailing edges 20 and 22. 
As shown in FIG. 2, the chordwise mid-portion 24 is thicker than the 
leading and trailing edges 20 and 22. Wall ribs 34 protrde from a convex 
wall inside surface 30, and wall ribs 36 protrude from a concave wall 
inside surface 32. Ribs 34 and 36 are aligned and adjacent to each other 
at a bond line 38 to form a series of internal airfoil supports 40 which 
connect concave and convex walls 26, 28 after bonding. A pattern of local 
posts extending from the inside surfaces 30 and 32 could be used to create 
the internal supports in place of the ribs. The skilled designer will 
appreciate that a variety of rib and post patterns are possible. The area 
between the supports 40 defines at least one primary cavity 42. The 
material absorption cavities are located in areas where high metal flow is 
anticipated. The supports 40 in proximity to the leading and trailing 
edges 20 and 22 defines at least one material absorption cavity 44 that 
have a smaller cross section than that of the primary cavities 42. There 
may be a plurality of material absorption cavities 44 as required for that 
particular flow area of the airfoil 12 to absorb the amount of yielding 
material necessary to essentially eliminate buckling and distortion. 
The convex wall 26 and the concave wall 28 are diffusion bonded together to 
make the airfoil 12. As shown in FIG. 3, during diffusion bonding, forces 
50 that are normal to the convex and concave walls 26 and 28 are applied 
to the airfoil 12, and the leading and trailing edges 20 and 22 are free 
to expand. Applied forces from dies cause approximately a 10% upset, or 
crush, of the airfoil 12. A reduction in thickness of about 5% to 15% is 
required to assure complete diffusion bonding. Because of the conservation 
of mass, as material thickness is upset, the airfoil 12 lengthens along 
the chord. The material inside the airfoil 12 flows in the direction of 
least resistance, which is primarily in the net direction of the applied 
forces, represented by arrows 52. This inward displacement of material 
from the leading and trailing edges 20 and 22 places the airfoil 12 in 
compression along the chord. Friction from the dies reduces the movement 
of the material as the material gets progressively farther away from the 
edges 20 and 22. The wall areas closest to the leading and trailing edges 
experience the highest compressive stresses. As material yields from the 
leading and trailing edges 20 and 22, the walls (especially near edges 20, 
22) tend to yield by buckling or by thickening in the absence of a 
material absorption cavity according to the present invention. Some 
extrusion of metal, often called flash, will occur at the bond plane; for 
example, around the ribs at the bond plane, but this has not been shown on 
the drawings. 
As shown in FIG. 3, the material absorption cavity 44 has an original shape 
46 prior to diffusion bonding the walls 26 and 28 together. The original 
shape 46 (dotted lines) is designed to give the convex and concave walls a 
smaller wall thickness at that area prior to the diffusion bonding process 
relative to the wall thickness 48 adjacent to the primary cavities 42. 
This encourages accommodation or absorption of metal flow in the area of 
the absorption cavities in preference to causing buckling in the primary 
cavity. The small cross section of the absorption cavity prevents buckling 
of the wall adjacent the absorption cavity. The material absorption 
cavities 44 absorb or accommodate the yielding material that flows during 
diffusion bonding and prevents the walls from buckling. The original 
diameter 46 of the material absorption cavities are sized to a 
predetermined dimension to maintain essentially the same wall thickness 
after diffusion bonding as the adjacent primary cavity wall thickness 48. 
The material absorption cavities 44 allow the hollow airfoil 12 to have 
maximum structural strength with an aerodynamically non-distorted airfoil 
shape without adding material to the primary cavity wall thickness 48 or 
decreasing the span between airfoil supports 40. The wall thickness (after 
bonding) near the leading edge may be somewhat thicker than the wall 
thickness near the trailing edge to protect the airfoil from foreign 
object damage. 
As shown in FIG. 4, the ribs or protrusions 36 may extend radially spanwise 
from the root portion 16 to the tip 18, and chordwise to define the 
primary cavities 42. As previously mentioned, other patterns of ribs 
(continuous from one edge of the primary cavity to another edge or 
discontinuous) and/or pins are possible depending on the application. 
Multiple material absorption cavities 44 may be used at the leading and 
trailing edges 20 and 22, depending on the amount of material to be 
absorbed during the diffusion bonding process. The material absorption 
cavities 44 may have a series of radii and extend essentially non-linearly 
along the span in the airfoil 12, when viewed in the bond plane, to 
increase the airfoil shear strength. 
As shown in FIG. 5, the trailing edge 22 is adjacent to a trailing edge 
bond area 54. The bond area 54 is adjacent to a series of material 
absorption cavities 44, ribs 36 and one of the primary cavities 42. 
As shown in FIG. 6, in a separate embodiment, a first sub component 56 has 
first component material absorption cavities 58 that are offset along the 
bond plane 60 from a second component material absorption cavity 62 of a 
second sub component 64. As the sub components 56 and 64 are diffusion 
bonded together, material yields in the direction of 66. The material 
absorption cavities 58 and 62 absorb yielding material and have a final 
configuration of 68 and 70. Thus, there is considerable flexibility 
possible in the design of the absorption cavities. 
The material absorption cavities of this invention could have a number of 
configurations to achieve the final result of absorbing yielding material 
during diffusion bonding of sub components. In this embodiment, the first 
sub component 56 has two material absorption cavities 58. The second sub 
component 64 has only one material absorption cavity 62 to point out the 
variation of arrangements of material absorption cavities the present 
invention can have to achieve the results intended. The material 
absorption cavities may be a pattern of local material absorption cavities 
which may be located anywhere along the bond plane that will have high 
material flow during diffusion bonding. In general, more absorption cavity 
cross section will be required where more metal flow occurs. However, a 
cross section taken along the thickness of the airfoil at the material 
absorption cavities should have less area relative to a similar cross 
section of a primary cavity prior to diffusion bonding. 
FIG. 7 shows another embodiment of the present invention. An article 72 
that is generally flat in shape has a first sub component 74 and a second 
sub component 76 with walls 78. The sub components have an outside surface 
80, an inside surface 82, a first edge 84, and a second edge 86. A series 
of axially aligned ribs 88 extend from the inside surface 82 to a bond 
line 96 to define a series of supports 90, primary cavities 92, and 
material absorption cavities 94. When the sub components 74 and 76 are 
diffusion bonded together, the yielding material flows in the direction of 
98, the material absorption cavity 94 absorbs some of the yielding 
material, and the material absorption cavity 94 has a final bond shape 
100. The absorption of the yielding material prevents buckling of the 
walls 78 without adding weight to the article 72. As shown in this 
embodiment, the present invention may be used for flat articles that are 
to bonded together as well as for cambered, or arcuate shapes, like 
airfoils, as described in the best mode. 
Although this invention has been shown and described with respect to a 
detailed embodiment, those skilled in the art will understand that various 
changes in form and detail may be made without departing from the spirit 
and scope of the claimed invention. For example, in this preferred 
embodiment, a first stage fan blade is shown in FIG. 2; however, the 
present invention may be utilized on any two members that are diffusion 
bonded together. In addition, in this preferred embodiment, the airfoil 12 
is made of a titanium alloy and is fabricated with diffusion bonding; 
however, the airfoil 12 design taught herein could be applied with other 
alloys that may be diffusion bonded. The material absorption cavities 44 
taught herein will absorb yielding material whether they are used before 
or after the airfoil 12 has been formed with a twist and camber. 
Although this invention has been shown and described with respect to 
detailed embodiments thereof, it will be understood by those skilled in 
the art that various changes, omissions and additions in form and detail 
thereof may be made without departing from the spirit and scope of the 
claimed invention.