Abstract:
A part to be shot peened includes at least one thin wall ( 12 ″) defining two opposite main faces, the square root of the area of each face being greater than the mean distance between the two faces by a factor of at least five, and preferably by a factor of at least ten. According to the method, the part is caused to rotate at least intermittently relative to one or more vibrating surfaces, with at least one of the main faces being exposed to projectiles ( 6 ) set into motion by one or more of the vibrating surfaces, treatment taking place progressively on the face(s) so as to impart compression stresses thereto, with a portion only of the part being treated at any one time and with regions of the part preferably being exposed on several occasions to the projectiles, with relative rotation taking place between the exposures.

Description:
This application claims the benefit of U.S. Provisional Patent Application No. 60/246,095, filed Nov. 7, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a method of shot peening metal parts, and also to a machine enabling such a method to be implemented. 
     2. Discussion of Related Art 
     The technique of shot peening metal parts is well known and consists in imparting compression stresses to the surface of the part by subjecting it to bombardment from projectiles constituted by balls or small shot. 
     The permanent stress induced on the part by such bombardment has the effect of opposing the appearance and the propagation of cracks, thereby improving the fatigue resistance of the treated part. 
     It is known that a part can be shot peened by projecting the projectiles towards the surface for treatment by means of a nozzle that is fed with compressed gas. 
     That technique poses at least two problems when applied to thin walls such as the blades of aeroengine turbines. 
     Firstly, blades must be treated simultaneously on both opposite faces in order to avoid any deformation that would lead to any perceptible modification of their shape. 
     Secondly, it is difficult or even impossible to engage the nozzle between the blades when they are close together. 
     Furthermore, it is difficult with that technique to control the intensity of shot peening with accuracy. 
     SUMMARY OF THE INVENTION 
     If a part is not sufficiently shot peened, then it is not given the desired fatigue resistance, whereas if it is excessively shot peened, then the part suffers irreversible damage and its resistance diminishes, and the part may need to be rejected. 
     Shot peening is made more difficult to control when the wall exposed to the projectiles is thin and easily deformable, as is the case in particular for most aeroengine turbine blades. 
     The invention seeks to make it possible to shot peen at least one thin wall with precision. 
     The invention provides a method of shot peening one part comprising at least one thin wall, in which method said part is caused to rotate at least intermittently relative to one or more vibrating surfaces, the or each thin wall defining two main opposite faces, at least one of said main faces being exposed to projectiles such as balls or small shot set into motion by means of one or more of said vibrating surfaces, the treatment being performed progressively on the or each of said faces to impart compression stresses, while treating only a portion of the part at a time and exposing preferably regions of the part several times over to the projectiles with relative rotation between said exposures. 
     In the present application, a “thin wall” is a wall defining two main opposite faces, with the square root of the area of each face being definitely greater than the mean distance between said two faces, e.g. greater by a factor of at least five, preferably by a factor of more than ten, and preferably by a factor of at least thirty. Mention can be made of various triplets (mean height; mean width; mean thickness) corresponding to various types of aeroengine turbine blade constituting thin walls in the meaning of the present invention, for example: ( 130 ;  210 ;  4 ), ( 50 ;  63 ;  1 . 3 ), ( 33 ;  40 ;  1 ), ( 170 ;  410 ;  4 ), where these dimensions are given in millimeters (mm). For these triplets, the ratio of the square root of area over thickness then takes on substantially the following values:  41 ,  43 ,  36 , and  66 , all of which are clearly greater than thirty, and some of which are greater than forty or even greater than sixty. By way of example, the mean thickness of a thin wall can lie in the range 0.1 mm to 10 mm when said wall is constituted by an aeroengine turbine blade. 
     The method of the invention is advantageously implemented to treat a part comprising a plurality of thin walls that are angularly spaced around a support intended to be driven in rotation, and in particular to treat a one-piece vaned wheel or any other rotor possessing vanes. 
     The use of one or more vibrating surfaces in the invention makes it possible to avoid using nozzles and makes it possible to shot peen the blades suitably, even when the space between the blades is small. 
     The vibrating surface(s) is/are advantageously constituted by one or more sonotrodes. 
     In an embodiment of the invention, protection is provided for an edge of the part to be treated that is liable to be damaged by impacts from the projectiles. 
     Such protection can be provided by means of at least one protection element fitted to the part or placed on the machine. 
     The projection element can extend directly in contact with the edge to be protected, or it can be spaced apart therefrom. 
     In the first circumstance, the protection element can comprise an endpiece fixed removably to the blade. 
     In the second circumstance, the protection element acts like a deflector and is preferably placed on the projectile path between the vibrating surface or sonotrode and the edge to be protected. 
     Said edge to be protected can be a sharp edge, e.g. a trailing edge, or it can be some other edge, e.g. the flat present at the end of each blade remote from the support to which the blade is connected. 
     A part can be treated while it is caused to rotate about an axis of rotation and while exposing each of its thin walls in succession to the projectiles in a treatment chamber through which said thin wall passes. 
     Rotation can be driven continuously or sequentially. 
     The method can be implemented by means of one or more treatment chambers. 
     Advantageously, the part is rotated in such a manner that each of its thin walls performs a plurality of passes through a given treatment chamber or successive passes through different treatment chambers, preferably at least five passes. 
     When the thin wall presents front and rear faces that are exposed in succession to impacts during rotation of the part, the fact of performing a plurality of revolutions or passes through the treatment chambers enables the shot peening to be made more progressive and makes it possible to use projectiles having lower energy levels. 
     This reduces the risk of excessively deforming the thin wall(s), while nevertheless ensuring that satisfactory shot peening is obtained after a plurality of revolutions or passes. 
     Advantageously, thin walls of the part to be treated are used to prevent the projectiles from leaving the treatment chamber(s). 
     Under such circumstances, the part to be treated can be rotated sequentially, with treatment being interrupted while the part is rotating so as to avoid any projectiles escaping from the treatment chamber(s). 
     To prevent or to contribute to preventing projectiles from leaving the treatment chamber(s), it is also possible to use one or more jets of compressed gas directed to return the projectiles into the corresponding treatment chamber(s) and also preferably directed in such a manner as to accelerate return of the projectiles towards a vibrating surface. 
     The use of such jets of compressed gas makes it possible to simplify recovery of the projectiles and, where appropriate, to omit having one or more passive recovery enclosures. 
     Advantageously, when the vibrating surfaces are defined by sonotrodes and when the part is rotated in sequential manner, power supply to the sonotrodes is interrupted while the part is rotating. 
     By performing rotation in sequential manner in combination with interrupting power supply to the sonotrodes while the part to be treated is rotating, it is possible to avoid using passive chambers for recovering projectiles, in particular downstream from the treatment chamber. 
     Sonotrode excitation is advantageously controlled in such a manner as to increase shot peening energy during treatment. 
     This makes it possible to increase shot peening intensity as the number of passes of the thin wall(s) through the treatment chamber(s) increases. 
     This thus takes account of the fact that the more a part has been exposed to the impacts of projectiles, the harder its surface becomes and the greater the amount of energy required to give rise to new compression stresses. 
     The part to be treated can be rotated about an axis of rotation that is vertical or horizontal or otherwise, and in particular that is substantially parallel to one of the edges of the surface to be treated. 
     An advantage of rotating about a vertical axis is to reduce the effect of the projectiles being entrained by the thin walls to outside the treatment chamber, since the projectiles are less likely to be caught between two thin walls when the surface of the support to which they are connected is substantially vertical and the space between the thin walls in the vicinity of the support is directly open in a downward direction. 
     Two vibrating surfaces are advantageously located respectively on either side of the path followed by the treated walls, so as to obtain treatment that is more uniform. 
     The part that includes the thin wall(s) can also be placed in a single enclosure, thus making it possible to avoid problems associated with losing projectiles. 
     Preferably, the treated part is rotated relative to the vibrating surface(s). In a variant, use is made of one or more vibrating surfaces moved in rotation relative to the thin wall(s), which can then be stationary. 
     It is possible to take advantage of the shot peening to mark the part by interposing a mask between a face of the part and the projectiles, the mask having apertures that correspond to the marking that is to be made. 
     Such marking presents the advantage of withstanding any subsequent chemical or heat treatment to which the part might be subjected. 
     The shot peening can be performed by using a mixture of balls or small shot having different diameters and/or made of different materials, depending on the desired result. 
     Preferably, both faces of the thin wall are treated simultaneously or almost simultaneously, thus compensating for the effects of projectiles striking one of the faces by means of the effects of the projectiles striking the other face. 
     It is also possible to treat only one of the two faces of the thin wall at a time. 
     Under such circumstances, the thin wall is preferably turned over at the end of treating one of its two faces so as to enable its other face to be treated. 
     In a particular implementation of the invention, the thin wall for treatment is inserted between two moving shutters, each of the shutters being capable of being displaced between a retracted position and a closed position, the shutter situated on the side of the thin wall that is remote from the vibrating surface being in the closed position to close the treatment chamber while the shutter situated between the thin wall and the vibrating surface is in the retracted position to enable the projectiles to bounce on the vibrating surface. 
     After one face of the thin wall has been treated, the part is turned over together with the shutters. The vibrating surface remains stationary and the shutter which was previously in the closed position is retracted, and vice versa. 
     In an embodiment of the invention, treatment is applied to a part of annular shape that comprises a plurality of blades constituting thin walls. This part is rotated about an axis which is preferably vertical. The part is preferably treated by means of two sonotrodes having axes that are substantially parallel and preferably vertical. The axes of the two sonotrodes are offset angularly about the axis of rotation of the treated part and the vibrating surfaces defined by these two sonotrodes lie in register respectively with two opposite ends of an inter-blade space. 
     The part is preferably rotated at a speed which is selected in such a manner that the difference in treatment between a face coming into the treatment zone and a face going out from the treatment zone remains negligible in the treatment as a whole. 
     The part can be rotated sequentially, with sonotrode excitation being interrupted during rotation of the part. 
     The invention also provides a machine for shot peening one part comprising at least one thin wall defining two opposite main faces, the square root of the area of each face being greater than the mean distance between the two faces by a factor of at least five, or preferably by a factor of at least ten, said machine comprising excitation members enabling at least one vibrating surface to be set into vibration, and drive members enabling relative rotation to be imparted, at least intermittently, between said part and the vibrating surface(s), the machine further comprising at least one chamber enabling the thin wall(s) to be received for treatment, at least one vibrating surface looking into said chamber, the vibrating surface(s) being suitable for creating a cloud of projectiles in the or each chamber, the machine being arranged to treat a portion only of the part at any one time, such that the treatment is performed progressively on said face(s) in order to introduce compression stresses therein. 
     Advantageously, the machine comprises elements enabling jets of compressed gas to be generated that are directed onto the projectiles so as to prevent them from leaving the treatment chamber and/or so as to accelerate their return towards the vibrating surface. 
     Preferably, the machine comprises one or more sonotrodes defining the vibrating surface(s). 
     Advantageously, the machine includes control means enabling the excitation energy of the sonotrodes to be increased progressively. 
     For some parts to be treated, in particular a radial collar at the periphery of a part, the machine can include a casing defining a treatment chamber that is capable of being turned relative to a vibrating surface so as to treat the two faces of a thin wall in succession. 
     Advantageously, the above-mentioned casing houses two moving shutters with the thin wall being inserted between them, each shutter being capable of being displaced between a retracted position enabling the projectiles to bounce on the vibrating surface and a closed position in which it closes the treatment chamber. 
     The invention also provides a machine for treating a part of annular shape and comprising a plurality of blades, in particular a one-piece vaned wheel that rotates about an axis of rotation that is preferably vertical. Such a machine can have two sonotrodes with substantially parallel axes that are preferably vertical and that are angularly offset about the axis of rotation of the part to be treated in such a manner as to be situated respectively between the bottom edges and the top edges of two blades defining an inter-blade space. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood on reading the following detailed description of non-limiting embodiments, and on examining the accompanying drawing, in which: 
         FIG. 1  is a diagrammatic fragmentary side view of a shot peening machine constituting a first embodiment of the invention; 
         FIG. 2  is a view analogous to  FIG. 1  showing a variant embodiment of the invention; 
         FIG. 3  is a diagrammatic perspective view of a machine constituting a variant embodiment of the invention; 
         FIG. 4  is a view analogous to  FIG. 3  showing another variant embodiment of the invention; 
         FIG. 5  shows how a part can be marked and also shows an edge protection piece; 
         FIG. 5A  is a diagrammatic section showing another way of protecting the edge of a blade; 
         FIG. 6  is a fragmentary diagrammatic perspective view of a shot peening machine constituting another embodiment of the invention; 
         FIG. 7  is a fragmentary diagrammatic perspective view of a shot peening machine constituting another embodiment of the invention, this machine including shutters; 
         FIG. 8  is a fragmentary diagrammatic view on section line VIII—VIII of  FIG. 7 , showing the shutters in a first relative configuration; 
         FIG. 9  is a view analogous to  FIG. 8 , showing the shutters in a second relative configuration; and 
         FIG. 10  is a diagrammatic and fragmentary side view of a machine constituting another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1  shows a first embodiment of shot peening apparatus  1  enabling the method of the invention to be implemented. 
     This apparatus  1  comprises a treatment chamber  2  formed between a top wall  3  and bottom walls  4  and  5 , in which chamber a cloud of projectiles  6  is generated by means of a vibrating surface  7  which in this case corresponds to the top end of a sonotrode  9 . 
     The wall  5  is situated upstream from the treatment chamber  2  while the wall  4  is situated downstream. 
     The part to be treated is constituted in this case by a vaned wheel  10  rotated about a vertical axis X, the wheel comprising a generally annular support  11  provided on its outer periphery with blades  12 , e.g. blades that are formed integrally with the support  11 . 
     It would not go beyond the ambit of the present invention for the blades to be made separately and assembled on the support prior to treatment. 
     The blades  12  are relatively thin compared with their height, as measured in the direction of the axis X, and relative to their radial dimension. The blades  12  constitute thin wall in the meaning of the present invention. 
     Each blade  12  presents a top edge  13  corresponding to its trailing edge and a bottom edge  14  that is broader, corresponding to its leading edge. 
     The cloud of projectiles  6  generated in the cavity is sustained by a sonotrode  9  which is controlled by a generator  15 . 
     The projectiles  6  bounce against the walls defining the treatment chamber  2  and on the sonotrode  9  where they acquire kinetic energy. 
     In the embodiment described, the distance between the bottom walls  4  and  5  situated respectively upstream and downstream from the sonotrode  9  and the top wall  3  is selected in such a manner as to ensure that the clearance left between said walls and the edges  13  or  14  is small enough to prevent projectiles  6  escaping. 
     The wall  4  situated downstream presents a ramp extending towards the wheel  10  on going away from the sonotrode  9 . 
     A duct  16  opens out at the top of the ramp to blow in compressed air so that projectiles which have been entrained towards the outside of the apparatus by rotation of the wheel  10  are blown back towards the treatment chamber  2 , where rotation of the wheel in this case takes place continuously but could in a variant take place discontinuously. 
       FIG. 2  shows apparatus  1 ′ constituting a variant embodiment of the invention and comprising a treatment chamber  2 ′. 
     This apparatus  1 ′ differs from the above-described apparatus  1  mainly by the fact that in addition to the sonotrode  9 , it further comprises a second sonotrode  9 ′ defining a vibrating surface  7 ′ parallel to the vibrating surface  7  and placed facing it, above the wheel  10 . 
     The top wall  3  of the preceding embodiment is replaced by a top wall  3 ′ provided with a passage for the sonotrode  9 ′, which sonotrode is connected to the generator  15 . 
     Otherwise, the treatment chamber  2 ′ is identical to the treatment chamber  2 . 
     The presence of the second sonotrode  9 ′ makes it possible for the treatment to be made more uniform and for its duration to be shortened by enabling the projectiles  6  to acquire kinetic energy from the vibrating surface  7 ′ without needing to fall back onto the surface  7 . 
     In the embodiment described, the wheel  10  is rotated about the axis X on a continuous basis, through a number of revolutions that is preferably greater than or equal to five, however rotation could also be performed in sequential manner. 
     The energy of the projectiles  6  is selected so that a single pass through the treatment chamber  2  or  2 ′ is insufficient to shot peen the blades adequately. 
     By increasing the number of revolutions, it is possible to accumulate the effects of treatment until a satisfactory level of shot peening is reached, while at no time subjecting the blades to bombardment by projectiles having too high a level of kinetic energy. 
     The blades are bombarded simultaneously on both of their main faces while they are substantially in the center of the treatment chamber  2  or  2 ′. 
     When a blade enters the treatment chamber, only its leading face relative to the direction of rotation of the wheel is bombarded, whereas after it has passed through the treatment chamber, it is bombarded on its trailing face only. 
     The fact of exposing only one face of each blade bombardment from projectiles on entering the chamber or leaving the chamber is not harmful since the projectiles  6  are given energy at a level which is selected to remain low enough to avoid excessively deforming the blades. 
     Performing a relatively large number of revolutions presents the advantage whereby a small amount of overlap concerning portions treated at the beginning and at the end of the treatment of the part as a whole does not lead to excessive shot peening because the energy of the projectiles  6  remains relatively low. 
     In the embodiment described, the amplitude through which the sonotrodes  9  and/or  9 ′ are excited can be increased on each revolution of the wheel  10  so as to take account of the fact that the surface hardness of the treated part increases during treatment, which means that a higher level of energy is required to impart new compression stresses. 
       FIG. 3  shows apparatus  30  constituting another embodiment of the invention. 
     This apparatus  30  differs from the apparatuses  1  and  1 ′ described above mainly by the fact that the part to be treated, in this case a wheel  10  provided with blades  12 , is no longer rotated about a vertical axis X, but is rotated about a horizontal axis. 
     The blades pass through a treatment chamber which is provided with a single vibrating surface on one side or with two vibrating surfaces on two opposite sides. 
     The openings in the treatment chamber through which the blades enter and leave are preferably of section that corresponds substantially to the section of the blades. 
     In the embodiment described, a nozzle  31  is used to return projectiles towards the treatment chamber during rotation of the wheel  10 , thus making it possible to avoid using passive enclosures for recovering the projectiles. 
       FIG. 4  shows a variant of the  FIG. 3  apparatus  30  in which the nozzle  31  is replaced by an internal channel  32  passing through a wall defining the treatment chamber  33 , with compressed air exiting this channel  32  serving firstly to prevent the projectiles from leaving the treatment chamber  33  and secondly to accelerate return of the projectiles back to the vibrating surface  34 . 
     In this figure, the front wall  35  of the treatment chamber  33  is shown in part only, so as to reveal the channel  32  and the vibrating surface  34 . 
     Advantageously, advantage is taken of the shot peening to mark a part that is being treated, as described below with reference to  FIG. 5 . 
     This figure shows the support  11  partially covered by a mask  40  that includes apertures  41  corresponding to the marking that is to be made. 
     During shot peening, the region of the support  11  that is covered by the mask  40  is not subjected to the effects of the shot peening, with the exception of the apertures  41 . 
     As a result, when the mask  40  is removed from the support  11 , visible marking remains that corresponds to the apertures  41 . 
     By way of example, the marking can correspond to a serial number or a batch number, and such marking turns out to be particularly good at withstanding the conventional treatments to which the part is subjected subsequently. 
     The trailing edges  13  of the blades  12  can be protected by means of a protection element in the form of an endpiece  42  which is fitted on each blade during shot peening, as shown in  FIG. 5 . 
     Instead of using protection endpieces fitted to the part that is to be treated, it is also possible to place one or more deflectors in front of the edge(s) to be protected, which deflectors are situated at a relatively short distance from the edge(s) to be protected. This prevent projectiles from striking the edge(s) in question head-on. 
     Each deflector can be placed at a few millimeters, for example, from the edge to be protected, said edge possibly being a sharp edge  13  or a flat  17  (visible in  FIG. 5 ) present at the free end of each blade, at its end remote from the support  11 . 
     Each deflector can be removably secured to the part if the part is rotated continuously, or it can be fixed to the shot peening machine if the part is rotated intermittently. 
     By way of example,  FIG. 5A  shows a blade whose edge  18  is protected from the impacts of the projectiles  6  by means of a deflector  45 . 
     In the example shown, the deflector  45  is placed on the part of the projectiles between the sonotrode and the edge  18  to be protected. As shown, the deflector  45  can be constituted by a bar which is substantially parallel to the edge to be protected and of a diameter corresponding substantially to the mean thickness of the blade in the vicinity of the edge in question. 
     When the part is rotated sequentially, the nozzles  16 ,  31 , or the channel  32  as described above can be omitted since it suffices to interrupt sonotrode excitation while the part is being rotated to ensure that the projectiles drop back into the bottom of the treatment chamber and are not entrained out from the chamber by the blades. 
       FIG. 6  shows another example of a shot peening machine  50  enabling the method of the invention to be implemented. 
     This machine  50  comprises a structure  51  supporting a one-piece vaned wheel  60 . The machine  50  also comprises a casing  63  defining a treatment enclosure  65  having a sonotrode (not shown) located in the bottom thereof. 
     A horizontal shaft  55  rotates in bearings  54  formed at the top of the structure  51 . 
     The shaft  55  is secured at one end to a drive wheel  57  and at its other end to a mandrel  58 . 
     The vaned wheel  60  is mounted on the mandrel  58 . 
     The wheel  57  is rotated by a motor  65  via a belt  66 . 
     The structure  51  has uprights  52  enabling the casing  63  and the associated sonotrode to be moved vertically from a low position which is remote from the wheel  60  to a high position in which the blades can be treated. 
     The blades then become engaged in succession in the treatment enclosure  65  while the wheel  60  is rotating. 
     In a direction parallel to the axis of rotation of the wheel  60 , the enclosure  65  is defined by walls  64  that match the diameter of the cylindrical surface of the wheel  60  to which the blades are connected. 
     The side walls of the casing  63  comprise uprights  69  covering a height which is sufficient to prevent projectiles that are present in the bottom of the enclosure from leaving it, with the blades that are received in the uprights  69  opposing upward travel of the projectiles. 
     The machine  50  also has a control panel  70  enabling rotation of the wheel  60  and operation of the sonotrode to be controlled, inter alia. 
       FIG. 7  shows a portion of a shot peening machine  71  for treating two opposite faces  72   a  and  72   b  of an annular collar  72  which projects radially from the base of a generally frustoconical part  73 . 
     The part  73  is rotated about its axis of symmetry which is vertical in this case. 
     The shot peening machine  71  comprises a casing  74  defining a treatment chamber  75  which is seen in  FIGS. 8 and 9 . 
     The machine  71  has a sonotrode  76  defining a vibrating surface  77  and constituting the bottom of the treatment chamber  75 . 
     The casing  74  has a passage  78  for passing the collar  72 , and a side opening of this passage  78  can be seen in  FIG. 7 . 
     The casing  74  has a chamfered edge  80  extending along a circular arc over the conical portion of the part  73 , and co-operating therewith to leave clearance that is small, smaller than the diameter of the projectiles used. 
     The casing  74  is supported by a structure (not shown) enabling it to be turned over by turning about an axis perpendicular to the axis of rotation of the part  73  to be treated. 
     The casing  74  houses two shutters  82  and  83  that can be moved in translation along an axis X. 
     In the example described, the shutters  82  and  83  include racks (not shown) and a drive mechanism including a pinion meshing with each of the racks, rotation of the pinion causing both shutters to move simultaneously, one towards the part  73  to be treated, while the other moves away therefrom, and vice versa. 
     The casing  74  has a passage  85  which is a through passage when the shutters are absent. 
     The sonotrode  76  closes the bottom end of this passage  85 . 
     The top end of the passage  85  is closed by one of the shutters  82  and  83 , depending on whether or not the casing  74  has been turned over. 
     In the configuration shown in  FIG. 8 , it can be seen that it is the shutter  82  which closes the top portion of the treatment chamber  75 , whereas in  FIG. 9  it can be seen that it is the shutter  83  which performs this function, the other shutter being in a retracted position enabling the sonotrode  76  to close the bottom of the treatment chamber  75 . 
     When the casing  74  is in the configuration shown in  FIG. 8 , it is the face  72   a  of the collar  72  which is shot peened. 
     When the casing  74  is in the configuration of  FIG. 9 , it is the other face  72   b  which is shot peened. 
     It is possible to use two sonotrodes and to cause the part to be treated to pass between them, for example by rotating it about an axis of rotation, which axis can be vertical for example, or otherwise. 
       FIG. 10  shows an example in which two blades  12 ″ define an inter-blade space I between each other within which a cloud of moving projectiles  6  is generated by means of two sonotrodes  9 ″. 
     The axes of the sonotrodes  9 ″ are substantially parallel and they are angularly offset about the axis of rotation of the part so as to accommodate the twisting of the blades  12 ″. 
     The axes of the sonotrodes  9 ″ are thus disposed substantially in the centers of the top and bottom regions of the inter-blade space I, respectively. 
     Pairs of opposite walls  90  &amp;  91  and  92  &amp;  93  are placed respectively beneath and above the blades  12 ″ so as to prevent the projectiles  6  from escaping and so as to facilitate return of the projectiles towards the vibrating surfaces. 
     After the facing surfaces of the blades  12 ″ have been treated, the excitation of the sonotrodes is interrupted by control means  95 , and the projectiles  6  drop back between the walls  90  and  91 . 
     The part is then turned so as to bring the next inter-blade space I between the sonotrodes  9 ″, and treatment is restarted. 
     The part can also be treated continuously, in which case it is rotated continuously while the sonotrodes  9 ″ are being excited. 
     A recovery enclosure (not shown) can serve to recover the projectiles  6  which escape from the inter-blade space I during treatment and means are advantageously provided to feed the inter-blade space with projectiles  6  so as to compensate for the loss of projectiles leaving the treatment zone. 
     When the thickness of the layer which is put into compression is large relative to the thickness of the blades, the speed of rotation is preferably selected to be high enough to ensure that the difference in treatment between the face entering the treatment zone and the face leaving the treatment zone is negligible for the treatment overall. 
     During rotation of the part, its axis of rotation is not necessarily vertical. In particular, it could be horizontal or it could be at an acute angle relative to the vertical, for example it could be substantially parallel to the flat that is present at the free end of a blade. 
     Naturally, the invention is not limited to the embodiments described above. 
     In particular, it is possible to use apparatuses as described above for treating parts other than aeroengine parts, in particular parts for use on land or at sea. 
     The sonotrodes can be replaced by other elements capable of producing vibrations enabling projectiles such as balls or small shot to be projected in comparable manner against a part to be treated. 
     The acoustic elements can be removable and portable so as to be suitable for use in other applications, in particular for maintenance.