Method for producing a monobloc rotor with hollow blades and monobloc rotor with hollow blades obtained by said method

The present invention relates to a monobloc rotor having a disk and hollow blades which are secured to the disk. The disk includes a protuberance and a projection on a periphery of the disk such that the projection forms a block having a shape which approximately corresponds to a shape of an inside of a radially internal extremity of the blade. The present invention further relates to a monobloc rotor in which a block is welded to the protuberance with the block forming a projection having a shape which approximately corresponds to a shape of an inside of a radially internal extremity of the blade.

FIELD OF THE INVENTION 
The present invention concerns a method for producing a monobloc rotor with 
hollow blades, as well as said monobloc rotor with hollow blades obtained 
by said method and particular a low pressure compressor rotor. 
BACKGROUND OF THE INVENTION 
As part of new studies concerning the low pressure compressors of a 
turbojet engine, most research work has been directed towards lightening 
the weight of the rotors of these compressors so as to obtain a gain of 
weight. These rotors are mainly of the monobloc type and are known in 
English terminology under the name of "BLISK", namely a contraction of the 
term "bladed disk". These rotors are monobloc, that is the blades form an 
integral part of the hub or are linked to the latter, as opposed to 
"winged disk" type rotors whose blades are provided with feet engaged in 
machined alveoles at the periphery of the hub before being locked in said 
alveoles. 
So as to lighten these "blisk" type rotors, two technological solutions 
have been examined. One first solution consists of machining the turbine 
blade in the block, that is when the blades are embodied in situ at the 
periphery of the disk by machining the blade profiles. These profiles thus 
form an integral part of the hub. They are then recessed, also by 
machining. However, this technological solution is difficult to implement 
owing to the small thickness of the blades and complex twisted shape. This 
solution is far too costly to be produced on an industrial scale. 
A second solution consists of preparing the blades previously recessed by 
machining and twisted and which are next solely linked to the hub or boss 
by various techniques, such as welding or soldering. This solution, much 
easier to implement and less expensive industrially, has been retained. 
Various techniques for securing blades to a hub are already known. 
According to the patent application FR-A-2 602 26, there exists a method 
for producing a set of rotors for a gas turbo-engine. This method firstly 
consists of producing a disk fitted with two external annular flanges, and 
secondly a ring of blades, also provided with two annular flanges, and 
then of hot-assembling the ring of blades on the disk. Then the flanges 
are sealed under vacuum and the entire unit is subjected to hot isostatic 
compression. 
This method can only be applied to full blades or at least blades including 
a full base and two lateral flanges since it consists of crushing the two 
faces opposite the flanges of the blade and those of the disk so as to 
form a continuous link. This method is unable to correctly fix the hollow 
blades whose cavity opens at the base of said blade. 
According to the patent EP-A-O 458 630, there is also a method to secure 
individual blades to the disk of a "BLISK" type rotor. This method is 
applicable to full blades which are retained between the two jaws of a 
fixing device and linked on the disk by a linking operation by means of 
friction. This method is also applicable to the repair of blades. 
According to the pat. U.S. No. 4 034 182, there is also a method consisting 
of assembling via their lateral sides a series of slightly concave 
segments bearing on their outer surface a compressor blade so as to form a 
ring fitted with radial blades once these elements have been assembled. 
However, this method is difficult to apply for the embodiment of "BLISK" 
type rotors with hollow blades. 
Finally, according to the patent application FR-A-2 619 331, there exists a 
method for producing a rotor with full blades, especially for a gas 
turbine engine. This method concerns the securing of the full blades to 
the disk of a rotor. It includes stages consisting of firstly forming a 
cavity in a portion constituting the foot of a blade, and secondly a 
plurality of cubic projections on the periphery of the disk of the rotor. 
Then each blade is assembled by being nested on the disk. The actual 
fixing is effected by means of pressure and heating. This link is mainly 
effected between the upper face of the cubic projection and the bottom of 
the cavity provided in the blade. Subsequently, it is possible to carry 
out a machining so as to remove the portions of the blade which define the 
lateral portions of this cavity. This type of method cannot be applied to 
the entire height of hollow blades since the extremity surface of the 
cubic projection is then unable to abut against the bottom of the cavity 
of the blade. 
Finally, as shown on the attached FIG. 1, when it is desired to weld a 
hollow blade 1 to a rotor disk 3, the weld seam 5 exhibits a weld 
projection 7 inside the hollow blade and a projection 9 outside the 
latter. Now, it is impossible to mechanically machine the internal 
projection 7 owing to the extreme thinness of the blade 1 and its twisted 
geometry. As a result, the state of the internal surface of the blade is 
not compatible with non-destructive inspection tests (radiography, 
ultrasounds, etc) absolutely essential for observing aeronautic 
requirements. 
SUMMARY OF THE INVENTION 
The aim of the present invention is to resolve these drawbacks. 
To this effect, the invention concerns a method for producing a monobloc 
rotor with hollow blades. 
According to the characteristics of the invention, this method includes the 
following stages : 
providing on the periphery of the disk of the rotor and at the right of 
each blade means forming a projection whose shape corresponds 
approximately to the shape of the inside of the radially internal 
extremity of the hollow blades, these projection means being encompassed 
by an approximately annular flat joining surface, 
nesting the radially internal extremity of each hollow blade on said 
projection means, 
securing each blade to the disk of the rotor by welding means so that the 
approximately annular surface defined by the thickness of the walls of the 
hollow blade is welded to said flat joining surface of the disk. 
By virtue of the presence of the projection means, the weld seam is unable 
to open inside the blade and thus form an internal projection. In 
addition, it is impossible to suppress the extremities of the weld since 
the internal projection does not exist. 
Moreover, this method consisting of adding the blades to the extremity of 
the disk makes it possible to reduce production costs since the machining 
of the disk and blades is effected separately. 
Finally, this method is also applicable to the repair of a damaged blade 
which may then be simply replaced by a new blade. 
The invention also concerns a monobloc rotor with hollow blades of the type 
including a disk, several hollow blades being fixed at the periphery of 
said disk, wherein the disk exhibits on its periphery and at the right of 
each blade a protuberance and a projection cast with the disk, the 
projection having dimensions smaller than those of the protuberance so as 
to define with respect to the latter a shoulder forming a flat joining 
surface on which the surface defined by the thickness of the blade is 
welded. 
According to a second embodiment, the disk has on its periphery and at the 
right of each blade a protuberance, a block forming a projection whose 
shape approximately corresponds to the shape of the inside of the radially 
internal extremity being welded to the protuberance, said block being 
encompassed by a thin collar, the surface defined by the thickness of the 
blade being welded on said thin collar.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown on FIG. 2, the rotor of the invention is a monobloc rotor of the 
type with hollow blades ("BLISK"). This rotor includes a disk 11 seen as a 
partial section on FIG. 2, and a plurality of hollow blades 13 distributed 
uniformly at the periphery of said disk, only one blade being shown on the 
figure solely on one portion of its height. By way of illustration, a disk 
may include 36 blades distributed uniformly on its periphery. 
The hollow blade 13 has a slightly twisted conventional shape, its central 
cavity being given the reference 14, its leading edge 15 and its trailing 
edge 17. 
One first embodiment of the invention is now to be described with reference 
to the right half of FIG. 2. 
The disk 11 is provided on its two lateral faces and at the periphery of 
the latter with a continuous blade 19. At the time the disk 11 is 
produced, means forming a projection, generally denoted by the reference 
21, are provided on the periphery of said disk and at the right of each 
blade 13. These projection means 21 are composed of a protuberance 23 and 
an actual projection 25. This protuberance 23 and this projection 25 are 
cast on the disk. The projection 25 is a block whose shape corresponds 
approximately to that of the inside of the cavity 14 of the blade at the 
level of its radially internal extremity 26, that is the foot of the 
blade. This projection 25 thus has a slightly concave tapered shape. The 
dimensions of the projection 25 are slightly smaller than those of the 
protuberance 23 so as to define a shoulder with respect to said 
protuberance. This shoulder defines an approximately flat joining surface 
27. The meaning of the term "annular" is here to be interpreted as having 
the shape of a ring (continuous shape around the projection 25). Having 
regard to the relatively thin and tapered shape of this projection in the 
type of rotor shown here, the joining surface 27 has more the shape of a 
flattened ring. 
Advantageously, the radially internal extremity 26 of the hollow blade 13 
is externally surrounded by an annular flange 29. The intended purpose of 
this annular flange 29 is to reinforce the thickness of the base of the 
hollow blade which is secured to the disk. However, its full role shall be 
explained subsequently. 
At the time it is produced, each blade 13 is nested on a projection 25 of 
the disk 11 until the surface 31 defined by the thickness of the walls of 
the blade and the surface 33 defined by the thickness of the flange 29 are 
in contact with the flat joining surface 27 of the disk. 
Each blade 13 is then secured to the disk 11 by welding means. The term 
"weld" also includes soldering. These welding means may be welding by a 
beam of electrons, diffusion welding, soldering, diffusion soldering, 
spark machining and by means of a laser. 
As the walls of the hollow blade 13 are relatively thin (thickness E close 
to 10 mm), the welding bead 35 extends between the surfaces 31, 33 and 27, 
but the presence of the projection 25 prevents a weld projection forming 
inside the blade 13. Thus, it is not necessary to carry out any machining 
of this weld projection. On the other hand, the external projection 37 of 
the weld bead is then machined. More specifically, the flange 29 and the 
corresponding portion 39 of the protuberance 23 are also machined. Thus, a 
perfect surface continuity is obtained between the disk 11 and the blade 
13, this machined sur#ace being shown by the dotted line. 
In a simplified version where the blade 13 does not include a flange 29, 
only the external projection 37 is machined. 
A second embodiment of the production method of the invention is shown at 
the left portion of FIG. 2. According to this production method, the 
projection means 21 are composed of a protuberance 23, but the projection 
25 of the first embodiment is replaced by a block 41 independent of the 
disk 11. The shape of this block 41 approximately corresponds to that of 
the inside of the cavity 14 of the blade at the level of its radially 
internal extremity 26 so as to to be able to penetrate inside the latter. 
This block 41 is surrounded by an extremely thin flat small collar 43 
forming a flat annular joining surface similar to the joining surface 27. 
This appears more clearly in detail on FIG. 3. This collar 43 is provided 
at the level of the radially internal portion of the block 41. 
In addition, the block may preferably have on its face 45 opposite the 
cavity 14 a peripheral protruding portion 47 extending radially outwardly 
and over the entire periphery of this surface 45. 
At the time the disk 11 is produced, only the protuberance 23 is provided 
on the periphery of said disk. Then the projection means 21 are placed 
(that is, the block 41 fitted with the flange 43), and then each blade 13 
is nested on these means 21. Finally, the blade 14, the disk 11 and the 
projection means 21 are welded, for example by one of the previously 
mentioned welding methods. 
As the blade 14 is relatively thin (thickness E close to 10 mm), the weld 
or soldering bead traverses the internal walls of the blade and reaches 
the block 41. Also, welding (fixing) is firstly carried out between the 
surfaces 31 and 33 corresponding to the thickness of the walls of the 
blade 13 and the flange 29 and between the flange 43, and secondly between 
the flange 43 and the disk 11 (protuberance 23). Welding is effected also 
between the block 41 and the protuberance 23. This weld bead is given the 
reference 49 (see FIG. 2). 
The presence of the block 41 and in particular the protruding portion 47 
ensures that the weld bead does not penetrate inside the cavity 14 and 
that no internal weld projection forms. 
The external weld projection 51 is machined in the same way as for the 
projection 37.