Diffusion bonded airfoil and method

An airfoil includes preformed first and second sides joinable together at a bond line extending between first and second opposite edges thereof. The first side includes a first extension having a proximal end forming a first end of the bond line, an intermediate section, and a distal end. The second side includes a second extension extending from the airfoil first edge which is fixedly joinable to the distal end of the first extension. The intermediate section of the first extension is spaced from the airfoil first edge to define therewith a working void so that pressure loading on the intermediate section is carried in part through the bond line first end for diffusion bonding thereof.

The present invention relates generally to diffusion bonding, and, more 
specifically, to diffusion bonding of large airfoils for gas turbine 
engines. 
BACKGROUND OF THE INVENTION 
Large aircraft turbofan gas turbine engines have correspondingly large fan 
blades which may be about 1.5 meters long for example. In order to reduce 
the weight of the fan blades, they are typically formed as hollow, thin 
skinned members having internal stiffening ribs which typically define 
several radially extending passages therein. 
Diffusion bonding is one conventional technique for forming the airfoils of 
the fan blades. In diffusion bonding, first and second airfoil preformed 
sections or halves are disposed together in abutting contact along a bond 
line or surface, with the leading and trailing edges of the airfoil being 
temporarily welded together so that the airfoil becomes pressure tight. 
The airfoil is then subjected to predetermined pressure-temperature-time 
conditions so that the airfoil sections may be metallurgically joined 
together at the bond line. Diffusion bonding does not rely on melting of 
the parent material as is required in conventional welding processes, but 
instead, the parent material undergoes plastic yielding and creeping at 
the elevated temperatures and pressures involved for forming an improved 
bond along the bond line. 
In most cases, the equipment used to form diffusion bonds is custom built 
for the specific parts being bonded, with the bonding usually occurring in 
a vacuum or in a suitable inert gas. Diffusion bonding may occur at 
various elevated temperatures and pressures with varying degrees of 
plastic deformation of the components. For example, the skin of the 
airfoil may actually collapse around the internal ribs during the 
diffusion process and must be returned to a suitable configuration 
typically accomplished by internally pressurizing the airfoil to inflate 
the skin to its original form. 
Diffusion bonding may occur at various pressures ranging from 1 to about 
1,000 atmospheres, but it is desirable to conduct diffusion bonding at 
relatively low pressures of about 1 to 6 atmospheres. However, it is 
extremely difficult to diffusion bond at moderate pressures gas turbine 
engine fan blades formed of titanium with large surface areas. This is 
particularly true for leading and trailing edges of blade airfoils which 
are relatively rigid and spaced relatively far from the adjacent internal 
passages formed in the airfoil by the radially extending ribs therein. The 
diffusion bond line typically extends substantially equidistantly between 
the two airfoil halves from the leading edge to the trailing edge and can 
have a relatively large camber length from each of the leading and 
trailing edges to the next adjacent internal passage. It is difficult to 
provide pressure loading along the leading and trailing edges for ensuring 
a suitable amount of mating force along the bond line thereat for 
obtaining effective diffusion bonding thereof. 
Furthermore, the mating surfaces of the airfoil halves along the leading 
and trailing edges, and specifically adjacent to the internal passage, may 
have irregularities therein which prevent the desired intimate contact 
therebetween. Such irregularities may result in incomplete bonding during 
the diffusion process, which may lead to rejection of the airfoil. 
In one conventional arrangement, an airfoil has a bonding line extending 
generally along the camber centerline thereof between the leading and 
trailing edges, and the leading and trailing edges each includes 
respective extensions thereof having a mating bond line therebetween. The 
extensions are welded along the edge of the bond line for making the 
airfoil pressure tight so that external pressure may be applied for 
diffusion bonding together the airfoil halves. The extensions are then 
conventionally machined away revealing the final, finish contour of the 
leading and trailing edges. However, the extensions merely form extensions 
of the bond line along the airfoil camber line and therefore increase the 
difficulty of achieving a suitable diffusion bond. 
SUMMARY OF THE INVENTION 
An airfoil includes preformed first and second sides joinable together at a 
bond line extending between first and second opposite edges thereof. The 
first side includes a first extension having a proximal end forming a 
first end of the bond line, an intermediate section, and a distal end. The 
second side includes a second extension extending from the airfoil first 
edge which is fixedly joinable to the distal end of the first extension. 
The intermediate section of the first extension is spaced from the airfoil 
first edge to define therewith a working void so that pressure loading on 
the intermediate section is carried in part through the bond line first 
end for diffusion bonding thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
Illustrated in FIG. 1 is an exemplary fan blade 10 for a turbofan aircraft 
gas turbine engine which includes an airfoil 12 formed in accordance with 
one embodiment of the present invention, and a conventional integral 
dovetail 14. The fan blade 10 may be relatively large, and for example is 
about 1.5 meters long. The airfoil 12 is preferably formed of titanium in 
this exemplary embodiment and is formed by diffusion bonding in accordance 
with the present invention. 
FIG. 2 illustrates in solid line the final, finished aerodynamic profile of 
the airfoil 12 at an exemplary radial section, with FIG. 3 illustrating a 
leading edge portion of the airfoil 12 prior to diffusion bonding thereof. 
As shown in FIGS. 2 and 3, the airfoil 12 includes a preformed first side 
or section 16 which is generally convex for forming the suction surface of 
the airfoil 12. The airfoil 12 also includes a complementary preformed 
second side or section 18 which is generally concave and forms the 
pressure side of the airfoil 12. The two halves or sides 16, 18 are 
joinable together at a bond line or surface 20 which extends generally 
between first and second opposite edges or end sections 22, 24, which in 
the embodiment illustrated in the Figures define the leading and trailing 
edges, respectively, of the airfoil 12 which are relative to the incoming 
air which flows over the airfoil 12 during operation. The bond line 20 
also extends through a plurality of internal ribs 26 which define 
chordally therebetween a plurality of radially or longitudinally extending 
internal cavities or passages 28. In the exemplary embodiment illustrated 
in FIG. 3, the bond line 20 bifurcates the ribs 26 in generally equal 
halves. 
As shown in FIG. 2, and in more particularity in FIG. 3, the first and 
second edges 22, 24 are relatively long along the airfoil chord between 
the adjacent passages 28 and the end-most portion thereof. In one 
conventional diffusion bonding process for joining together airfoil 
halves, the bond line would extend along the center camber line of the 
airfoil bifurcating the leading and trailing edges thereof, However, the 
bond line in these regions is relatively long and increases the difficulty 
of obtaining suitable bonding loads thereat and complete bonding thereof. 
In accordance with one embodiment of the present invention as illustrated 
in FIG. 3, the airfoil first side 16 includes a first integral extension 
30 disposed at the first edge 22 which extension 30 has a proximal end 30a 
forming a first end 20a of the bond line 20. The first extension 30 also 
includes an intermediate section 30b and a distal end or section 30c. The 
airfoil second side 18 correspondingly includes a second extension 32 
which extends generally coextensively from the airfoil first edge 22 and 
is fixedly joined to the distal end 30c of the first extension 30 by a 
conventional weld 34. FIG. 3 illustrates one radial section of the airfoil 
12, with the first and second extensions 30, 32 extending for the entire 
radial or longitudinal length of the airfoil 12. The weld 34 similarly 
extends the entire length of the airfoil and provides a seal to ensure 
that the airfoil 12 is pressure tight for undergoing diffusion bonding. 
As shown in FIG. 3, the intermediate section 30b of the first extension 30 
is preferably spaced laterally from the airfoil first edge 22 to define 
therewith a working void 36, with the first extension 30 being supported 
solely at its two opposite ends, i.e. the proximal end 30a and the distal 
end 30c, with the intermediate section 30b being unsupported over the void 
36. During diffusion bonding of the first and second sides 16, 18, the 
outer surfaces thereof, including the first and second extensions 30, 32, 
may be conventionally pressurized, preferably using a relatively low 
pressure force designated P of up to about 6 atmospheres. Pressurizing 
together the first and second sides 16, 18 is conventionally accomplished 
at a suitable elevated temperature and for predetermined time for 
diffusion bonding together the first and second sides 16, 18 along the 
bond line 20. After diffusion bonding of the airfoil halves 16, 18, the 
two extensions 30, 32 are removed as required by conventional machining 
for forming the airfoil first edge 22 to its final, aerodynamic shape. 
In the preferred embodiment illustrated in FIG. 3, the internal passages 28 
include an end-most passage 28a disposed directly adjacent to the airfoil 
first edge 22, and the bond line first end 20a extends through the first 
side 16 from the end-most passage 28a to the working void 36. The first 
extension 30 preferably extends substantially parallel to the airfoil 
first edge 22, and the working void 36 is chordally elongate between the 
proximal and distal ends 30a, 30c of the first extension 30. The bond line 
first end 20a preferably extends chordally obliquely from the end-most 
passage 28a, with both the first extension intermediate section 30b and 
the working void 36 extending completely over the airfoil first edge 22 
along the chord or camber lines of the airfoil 12. 
As shown in FIG. 3, the working void 36 has a first chordal length L.sub.1, 
and the finished airfoil first edge 22 has a second chordal length 
L.sub.2, with the first length L.sub.1 being preferably greater than the 
second length L.sub.2 for maximizing the pressure forces acting over the 
intermediate section 30b. The bond line first end 20a has a length L.sub.3 
which is substantially less than the void length L.sub.1. In this way, the 
pressure loading P over the intermediate section 30b is carried in part 
through the proximal end 30a, and through the bond line first end 20a to 
the mating portion of the airfoil first edge 22 for diffusing bonding 
thereof. Generally half of the pressure loading force P applied over the 
unsupported intermediate section 30b adds to the pressure forces acting 
over the proximal end 30a to increase the total bonding force along the 
bond line first end 20a for improving diffusing bonding thereat. The 
remaining half of the pressure load is carried by the distal end 30c. 
Since the bond line first end 20a extends obliquely from the end-most 
passage 28a and not along the camber centerline through the airfoil first 
edge 22, it has a shorter length than it otherwise would and further 
improves the effectiveness of diffusing bonding thereof. 
In the exemplary embodiment illustrated in FIG. 3, the second extension 32 
fully encases the airfoil first edge 22, shown in phantom line, along the 
entire extent of the working void 36, and after diffusing bonding of the 
airfoil sides 16, 18, the excess material of the second extension 32 is 
conventionally machined away for revealing the finished airfoil leading 
edge 22. The proximal end 30a of the first extension 30 is also 
conventionally machined away adjacent to the first end 20a of the bond 
line 20 for removing the first extension 30 and leaving the finished 
surface contour of the airfoil first side 16. 
If the specific diffusion bonding process results in plastic collapsing of 
the intermediate section 30b of the first extension 30 into the void 36 
and contact with the second extension 32, a layer of a conventional 
stop-off material 38 may be provided on the inner surface of the 
intermediate section 30b in the working void 36 for preventing diffusing 
bonding between the intermediate section 30b and the second extension 32 
containing the airfoil first edge 22. Any suitable stop-off material 38 
may be used such as Boron Nitride which is typically used to line the 
internal passages 28 to prevent diffusing bonding upon collapse of the 
opposing airfoil first and second sides 16, 18 during the diffusing 
bonding process. 
Illustrated in FIG. 4 is an alternate embodiment of the second extension 
designated 32A which extends solely from an end of the airfoil first edge 
22 and does not fully encase the first edge 22 as in the embodiment 
illustrated in FIG. 3. In this embodiment, the first edge 22 is preformed 
or finished to its final aerodynamic contour except for the small region 
thereof from which the second extension 32A extends. The second extension 
32 illustrated in FIG. 3 which encases the airfoil first edge 22 adds 
additional strength for accommodating the pressure forces experienced 
during diffusion bonding. If the strength of the finished airfoil first 
edge 22 is alone sufficient for withstanding the pressure forces during 
the diffusion bonding process, then the additional material thereon is not 
required, and the relatively simple second extension 32A shown in FIG. 4 
may be used instead. FIG. 4 also illustrates that additional parent 
material 40 may be added to the airfoil first side 16 at the corner of the 
end-most passage 28a adjacent to the first end 20a of the bond line 20 for 
ensuring a suitable minimum thickness of the airfoil first side 16 at that 
location after machining away of the proximal end 30a of the first 
extension 30. 
Although the invention has been described above with respect to the leading 
edge 22 of the airfoil 12, it may also be similarly applied to the 
trailing edge 24 as well, and is shown in phantom in FIG. 2. Corresponding 
configurations of the first and second extensions 30, 32 may be similarly 
used at the airfoil trailing edge 24 for providing an effective diffusion 
bond at a second end 20b of the bond line 20 at the trailing edge 24. 
The improved preformed airfoil sides 16, 18 having the first and second 
extensions 30, 32 provide minimum bond area at the critical leading and 
trailing edges 22, 24 of the airfoil 12. The diffusion bonding pressure at 
the respective ends 20a, b of the bond line 20 are increased due to the 
reduction in the bond area as well as the amplification of bonding 
pressure provided by the unsupported intermediate section 30b of the first 
extension 30. It is anticipated that better control of stress 
concentrations at the end-most internal passage 28a may be obtained since 
any surface irregularities in the bond line first end 20a may be 
eliminated by plastic creep under the improved pressure loading thereat. 
Furthermore the airfoil 12 is provided with near-finished airfoil surfaces 
in its preformed configuration for diffusion bonding, with minimum excess 
material at the leading and trailing edges 22, 24 being provided which 
must be removed by machining after the diffusion bonding process. And, the 
improved configuration of the airfoil 12 disclosed above may be used 
wherever beneficial in various conventional forms of diffusion bonding 
whether at low or high pressures as desired. 
While there have been described herein what are considered to be preferred 
and exemplary embodiments of the present invention, other modifications of 
the invention shall be apparent to those skilled in the art from the 
teachings herein, and it is, therefore, desired to be secured in the 
appended claims all such modifications as fall within the true spirit and 
scope of the invention. 
Accordingly, what is desired to be secured by Letters Patent of the United 
States is the invention as defined and differentiated in the following 
claims: