EP1207013A1 - Method for increasing the service life of rotor blade locking means - Google Patents

Abstract

The rotor comprises a rim (10) on the periphery (12) of which several removable blades are fastened. The blade fasteners comprise a dovetailed groove (14) and a foot fitted in the groove. The groove is shot blasted by projecting balls (1) in order to create a pre-stressed compression in its surface. The shot blasting is carried out by ultrasonics, the balls being projected by the percussion of a vibrating sonotrode (30). The balls are contained in an enclosure (40) in which they form a mist.

Description

Technical field of the invention

The invention relates to the blade attachments on a rotor and more particularly to a process for increasing its lifespan, this process using a form particular of the shot blasting technique called "ultrasonic".

State of the art and problem posed

In an aircraft turbojet, the rotor blades are traditionally made a rim at the periphery of which are mounted a plurality of removable blades. The mounting device is called in this patent application "blade attachment". This device comprises a dovetail groove machined in the rim and a foot also in dovetail machined at the base of the blade, the assembly being carried out by the nesting of the foot in the groove. In a more elaborate form called "at the foot of fir ", the dovetail has several" bulbs "of decreasing size, typically three bulbs, each bulb performing the tail function separately dove. In what follows, these two forms of attachment will be said interchangeably "in dovetail ". The blade roots are slid into the grooves with limited play, the feet then being immobilized without play by various means of locking. It is understood that the grooves and the feet of blades are the seat of strong stress concentrations and that their realization is therefore particularly neat. Turbojet rotors are usually made of steel, titanium alloy or a superalloy based on nickel or chromium.

Usually, the rotors are the subject of a shot peening by projection of small balls in hard material using one or more compressed air nozzles. This shot peening has for effect of creating a compression preload on the surface of the rotor over a depth by a few tenths of a millimeter, this prestress delaying the appearance of cracks resulting from high loads and thus increasing the life of the rotor. In if necessary, the shot blasting is preceded by a heat treatment of the part to be treated to release the residual stresses remaining in this room. In the event that some parts of the workpiece must not be blasted, they are usually protected by coating with a material, such as an elastomer, of sufficient hardness to withstand impacts of the balls.

A strong shot of Almen intensity of the order of F15A to F17A and making it possible to create the rotor surface a compression preload of the order of 900 to 1100 MPa (mega pascals) is desirable, these rotors usually being made of steel, alloy titanium or superalloys based on chromium or nickel. Unfortunately, such shot peening greatly increases the roughness of the treated surfaces and thus reduces the resistance to wear by vibratory friction of the surfaces of the grooves and feet blades in mutual contact.

• Brevet EP 0 922 532 paragraphe [0005] colonne 1 lignes 33 à 38. L'une des solutions préconisée est de réduire l'intensité et le taux de recouvrement du grenaillage paragraphe [0006] lignes 39-40. Ce même brevet indique ligne 41 qu'il peut en résulter une réduction de la durée de vie de la pièce.
• Dans le magazine "Souder" n°5 de septembre 1998, l'étude "Le principe du choc laser et ses applications au traitement des matériaux" effectue une comparaison du grenaillage dit "par choc laser" avec le grenaillage conventionnel et indique page 13 avant dernier paragraphe que le grenaillage conventionnel crée des microcratères résultant des impacts des billes et augmentant la rugosité. Selon les exemples donnés par le premier tableau page 14, la rugosité (Ra) d'une surface usinée peut augmenter de 2,3 µm à 5,5 µm après un grenaillage fort.

This increase in the roughness of a surface undergoing heavy blasting by the projection of small balls is attested by various documents:

• Patent EP 0 922 532 paragraph [0005] column 1 lines 33 to 38. One of the recommended solutions is to reduce the intensity and the recovery rate of shot peening paragraph [0006] lines 39-40. This same patent indicates line 41 that it can result in a reduction in the service life of the part.
• In the magazine "Souder" n ° 5 of September 1998, the study "The principle of laser shock and its applications to the treatment of materials" makes a comparison of shot peening said "by laser shock" with conventional shot peening and indicates page 13 before last paragraph that conventional shot blasting creates microcraters resulting from the impact of the balls and increasing the roughness. According to the examples given in the first table on page 14, the roughness (Ra) of a machined surface can increase from 2.3 µm to 5.5 µm after heavy blasting.

The article "Shot peening of prestressing" published in 1992 by CETIM page 105-123 reports on a national conference held on September 25-26, 1991 at Senlis in France. He indicates on page 108 before the last paragraph that the blasting of a machined surface leads to an increase in the value of the roughness. This same article specifies in the last paragraph of this same page that we can reduce the roughness by blasting in several decreasing intensity passes. We can understand that the strong blasting at the start increases the roughness and that the lighter and lighter shots which follow reduce roughness by leveling the blasted surface. This solution however has the disadvantage of being long because it several shot blasting is necessary, the first ensuring the prestressing of the surface shot blasting and the following shot blasting gradually ensuring the reduction of the roughness that appeared during the first shot blasting.

The problem to be solved is to simultaneously increase the fatigue strength and the resistance to vibration friction of the rotors at the blade attachments, this increase should not result in a significant increase in the time and cost of manufacture of rotors.

Shot-peening shot blasting is currently experiencing a new form of setting in so-called "ultrasonic" work in which the balls are no longer projected by a nozzle with compressed air but by percussion on these balls from the surface of a sonotrode vibrating at frequencies of the order of 20 to 60 KHertz, the balls being kept at the inside of an enclosure, the part to be shot blasted being, according to its dimensions, immersed in inside the enclosure or presented in front of an opening of this enclosure.

We know from patent FR 2 743 742 an application of ultrasonic shot peening to cookware to reduce the microcavities created prior to the surface of the utensil to favor the attachment of a coating on part of the utensil. This patent indicates on page 5 line 32 that the cookware is made of aluminum. he it is known that this material is soft, and its prestressing does not exceed 150 to 200 MPa. It is much lower than the desired prestressing from 900 to 1100 PMa. This patent also indicates on page 1 lines 31 that the surface obtained is smooth, but he specifies on page 5 line 14 that shot peening or "billing" lasts from 0.5 to 5 seconds. Even for a soft material, it is therefore only a very light peening of surfacing unrelated to strong shot peening of prestressing compression applied to aeronautical parts, the surfaces of these parts in front be exposed to shot blasting for a period of at least four to ten minutes. This patent therefore does not provide a solution to the problem to be solved.

Statement of the invention

The invention provides a method for increasing the life of the blade attachments on a rotor, the rotor comprising a rim at the periphery of which are attached a plurality of removable blades, the blade attachments comprising two components, the one of the components being a dovetail groove disposed at the periphery on the rotor, the other component being a foot disposed on the blade, the foot being of a shape complementary to the groove, the foot being capable of being fitted into the groove in order to make the attachment of the blade to the rotor, at least one component being shot blasted by projection of balls in order to create a compression preload on its surface.
Such a process is remarkable in that the shot peening is carried out by the so-called "ultrasonic" process, the balls being projected by the percussion of a vibrating sonotrode, the balls being contained in an enclosure and forming a fog at inside the enclosure, the shot blasting component being brought into contact with the ball mist,
and in that the shot blasting is carried out at an Almen index at least equal to F8A.

The inventors have observed that a strong blasting by ultrasound increases only slightly roughness of the treated part, unlike conventional shot blasting using a compressed air jet nozzle. The invention thus takes advantage of this unexpected property to increase the fatigue strength of blade attachments while while maintaining good resistance to wear by vibration friction.

An advantage of the invention is also to increase the resistance to wear by vibratory friction of the blade attachments, because the prestressing of high compression of the surfaces of the blade attachment components causes the hardening by work hardening.

Advantageously, balls having a diameter at least equal to 0.8 mm will be used in order to to improve the efficiency of shot peening and to stabilize or even reduce the roughness of parts processed.

Advantageously, the compression preload will be at least equal to 500 MPa.

In a particular embodiment of the method, the invention proposes the application to the so-called "axial" grooves on the rims of the vane rotors.

In another particular embodiment of the method, the invention proposes application to the so-called "annular" grooves on the rims of the vane rotors.

In another particular embodiment of the method, the invention proposes application at the feet of the blades.

Description of the figures

The invention will be better understood and the advantages it provides will appear more clearly in view of a detailed description of the properties of ultrasonic shot peening, of three examples of application to blade attachments and of the appended figures.

FIG. 1 illustrates the deformation of the material under the effect of the impacts of the balls in movement.

Figure 2 illustrates the rim of a rotor and shows the so-called "axial" grooves at its periphery.

FIG. 3 illustrates the mounting of a blade on a rim with axial grooves.

Figures 4 and 5 illustrate by a front view and a side view the method of ultrasonic shot blasting of the axial grooves, Figure 4 being a sectional view along B in FIG. 5, this FIG. 5 itself being a sectional view along A in FIG. 4.

FIG. 6 illustrates the rim of a rotor with “annular” grooves.

FIG. 7 illustrates the mounting of a blade on a rim with annular grooves.

Figures 8 and 9 illustrate by a front view and a side view the method of ultrasonic shot blasting of the annular grooves. On the model of the previous example, Figure 8 is a section on D in Figure 9, this figure 9 itself being a view in section along C in FIG. 8 ..

Figures 10 and 11 illustrate by a front view and a top view the method of ultrasonic peening of the blade roots, FIG. 10 being a sectional view along E in Figure 11.

detailed description

• le première ligne indique les rugosités (Ra) mesurées avant le grenaillage,
• la seconde ligne indique les rugosités (Ra) mesurées sur ces mêmes surfaces après un grenaillage conventionnel fort d'indice Almen égal à F17A et une mise en précontrainte sous la surface grenaillée atteignant 1000 MPa,
• la troisième ligne indique les rugosités (Ra) mesurées sur ces mêmes surfaces après un grenaillage par ultrasons équivalent au précédent d'indice Almen égal à F17A avec une mise en précontrainte sous la surfaces atteignant 1000 MPa, Surface n°1 (fraisée) Surface n°2 (tournée) Rugosité (Ra) avant le grenaillage 0,27 µm 0,90 µm Rugosité (Ra) après un grenaillage conventionnel, indice Almen = F17A, billes  = 0,315 mm. 1,53 µm 1,94 µm Rugosité après un grenaillage US, indice Almen = F17A, billes  = 1,5mm 0,47 µm 0,93 µm

Tests have shown that shot blasting known as "ultrasonic" of high intensity does not significantly increase the roughness of the treated surfaces, unlike conventional shot blasting by spraying balls using a compressed air jet nozzle. The table below shows some comparative results made on a part in nickel-chromium base refractory superalloy:

• the first line indicates the roughness (Ra) measured before shot peening,
• the second line indicates the roughness (Ra) measured on these same surfaces after a strong conventional shot peening with an Almen index equal to F17A and a prestressing under the shot peened surface reaching 1000 MPa,
• the third line indicates the roughness (Ra) measured on these same surfaces after an ultrasonic shot peening equivalent to the previous index Almen equal to F17A with a prestressing under the surface reaching 1000 MPa, Surface n ° 1 (milled) Surface n ° 2 (tour) Roughness (Ra) before shot peening 0.27 µm 0.90 µm Roughness (Ra) after conventional shot blasting, Almen index = F17A, balls  = 0.315 mm. 1.53 µm 1.94 µm Roughness after US shot blasting, Almen index = F17A, balls  = 1.5mm 0.47 µm 0.93 µm

In a first example illustrated in column 1, surface n ° 1 machined by milling with a roughness Ra = 0.27 µm sees its roughness increase by 1.67 µm after shot blasting conventional while its roughness only increases by 0.2 µm with shot blasting ultrasound, this surface No. 1 being arranged parallel to the sonotrode.

In a second example illustrated in column 2, surface n ° 2 machined by turning with Ra = 0.90 µm sees its roughness increase by 0.63 µm after shot blasting conventional, while this roughness remains substantially unchanged after a ultrasonic shot blasting, this surface No. 2 being this time arranged perpendicular to the sonotrode.

• Les billes 1a impactant la surface à grenailler avec un angle α proche de la perpendiculaire à cette surface seront efficaces pour créer aux points d'impacts une précontrainte de compression importante, ces billes laissant cependant sur la surface 3 impactée des cratères 4 entourés de bourrelets 5 formant aspérités.
• A l'inverse, les billes 1b impactant la surface 3 avec un angle β faible, c'est à dire rasant plutôt la surface 3, tendront à niveler les bourrelets 5 et à résorber au moins partiellement les cratères 4, ces billes 1b étant bien évidemment peu ou pas du tout efficaces pour créer une mise en précontrainte de compression suffisante. Ce rôle des billes rasantes est confirmé par le tableau précédent. En effet, la surface n°2 reçoit préférentiellement des billes rasantes car elle est grenaillée par ultrasons dans une position perpendiculaire à la sonotrode, ce qui explique que la rugosité n'augmente sensiblement pas.

This remarkable property of ultrasonic shot blasting, which is to allow strong shot blasting without significantly increasing the roughness of the treated surface, could be explained in the light of FIG. 1: The balls 1 supplied with kinetic energy by the percussions of the sonotrode in vibration rebound on the sonotrode itself, on the walls of the enclosure and on the surface 3 of the part 2 subjected to shot peening, these balls 1 then rebound on this surface with an angle α, β of random incidence according to a substantially uniform distribution between 0 ° and 90 ° and in any direction. In these conditions :

• The balls 1a impacting the surface to be shot-blasted with an angle α close to the perpendicular to this surface will be effective in creating at the points of impact a significant compression preload, these balls however leaving on the impacted surface 3 craters 4 surrounded by beads 5 forming asperities.
• Conversely, the balls 1b impacting the surface 3 with a small angle β, that is to say rather leveling the surface 3, will tend to level the beads 5 and at least partially resorb the craters 4, these balls 1b being good obviously little or not at all effective in creating a sufficient compression preload. This role of grazing balls is confirmed by the preceding table. In fact, surface No. 2 preferably receives grazing balls because it is shot blasted by ultrasound in a position perpendicular to the sonotrode, which explains why the roughness does not increase appreciably.

We can therefore say that a strong shot peening does not significantly increase the roughness of the shot blasted surface 3 must be able to combine on the treated surface impacts of balls 1a with angles of incidence α close to the perpendicular to this surface 3 and impacts of balls 1b grazing this surface. By an angle of incidence α close to the perpendicular, we mean an angle α at least equal to 45 °, the effectiveness of the impact being all the better as this angle α approaches 90 °. By an angle of incidence β grazing means an angle β less than 45 ° and preferably between 15 ° and 30 °.

It will be noted that the strong shot peening according to the invention also implies a rate called "of recovery "ranging from 120% to 300%, that is to say that the blasting is carried out for a duration T equal to 120% to 300% of the duration T1 necessary to obtain a 98% normal recovery rate, the normal recovery rate being the ratio between the impacted surface and the total surface exposed to shot blasting.

In the case of conventional shot blasting, on the contrary, the balls strike the surface shot blasted with an angle of incidence and a preferred direction, this angle of incidence should be sufficient to pre-stress the blasted surface. Therefore, the bulges forming around the craters are not leveled and tend, on the contrary, to gather in waves clearly visible under the microscope with a magnification x50 to x100, the craters tending to gather more or less in furrows perpendicular to the waves.

• ces billes sont parfaitement sphériques et donnent donc un meilleur état de surface,
• ces billes très dures ne se cassent pas et ne produisent donc pas d'arrêtes vives susceptibles d'altérer l'état de surface de la pièce grenaillée.

A significant advantage of ultrasonic shot peening is that its implementation requires only a small quantity of balls. It is therefore possible in this case to use high quality balls comparable to balls for rolling bearings of steel or ceramic. Unlike the classic shot:

• these balls are perfectly spherical and therefore give a better surface condition,
• these very hard balls do not break and therefore do not produce sharp edges capable of altering the surface finish of the blasted part.

We will now refer to FIG. 2. The vane rotor has a rim 10 having a general form of revolution around a geometric axis 11, this rim 10 being bounded radially outwards by a peripheral surface 12 and laterally by two sidewalls 13. The rim 10 has at its periphery 12 a plurality of grooves 14 substantially rectilinear called "axial" each comprising a mouth 15 laterally extended by two lateral openings 16, the mouth 15 opening on the periphery 12, the lateral openings 16 opening onto the sides 13. The grooves 14 have a substantially trapezoidal profile called "dovetail" with a mouth 15 narrower. These grooves 14 may be parallel to the geometric axis 11 or oblique. They can be straight or in an arc.

We will now refer to FIG. 3. A blade 20 successively comprises top to bottom in this figure a thin blade 21, a platform 22 extending laterally on each side of the blade 20 and a foot 23 of substantially shaped trapezoidal and complementary to that of the groove 14. The blade 20 is fitted by its foot 23 in the groove 14 with limited play, the foot 23 then being immobilized in the groove 14 by various locking means not shown. Foot 23 arrives at contact of the groove 14 along two contact lines 24 located at the rear of the mouth 15 and set back from this mouth 15. The attachment 26 of the blade 20 has the groove 14 and the foot 23.

• disposer la jante 10 au dessus de la sonotrode 30 dans une position appropriée pour amener chaque rainure 14 au dessus de la sonotrode 30 par rotation de la jante 10 selon son axe géométrique 11,
• disposer une dose de billes 1 sur la sonotrode 30, la sonotrode étant de préférence rentrée dans le fourreau 32 afin de constituer au dessus de sa surface vibrante 31 un récipient susceptible de contenir les billes 1,
• faire tourner la jante 10 pour amener successivement chaque rainure 14 au dessus de la sonotrode 30, chaque rainure 14 faisant alors l'objet des opérations suivantes :
• recouvrir les ouvertures 16 avec les oreilles 33 et amener la sonotrode 30 dans l'embouchure 15 de la rainure 14, cette opération se faisant de préférence en remontant simultanément la sonotrode 30 et le fourreau 32 jusqu'à ce que les oreilles 33 recouvrent les ouvertures 16, la sonotrode 30 étant ensuite remontée seule dans l'embouchure 15, ce qui a pour effet simultanément d'amener les billes 1 dans la rainure 14 et de mettre la sonotrode 30 en position de travail,
• effectuer le grenaillage de la rainure 14 par la mise en vibration de la sonotrode 30,
• retirer la sonotrode 30.

Reference will now be made simultaneously to FIGS. 4 and 5. The sonotrode 30 has a vibrating surface 31 capable of being introduced into the mouth 15 of the groove 14. The sonotrode 30 slides in a sheath 32 with a clearance E less than the diameter of the balls 1. Sealing of the sonotrode 30 with respect to the mouthpiece 15 can be ensured by the sheath 32. In a preferred embodiment, however, this sealing is more simply ensured by giving the sonotrode 30 a complementary shape. to that of the mouth 15, for example rectangular in the case of straight grooves, with a clearance E less than the diameter of the balls 1. The sheath 32 supports two shutters or ears 33 on either side of the sonotrode 30, these ears being capable of covering the openings 16 of the groove 14 with a clearance E less than the diameter of the balls 1. It is understood that the sonotrode 30 and the ears 33 cooperate to close the rain ure 14 and contain the balls in this groove 14 during the peening.
With such a device, the method of ultrasonic peening of the axial grooves 14 comprises the following operations:

• placing the rim 10 above the sonotrode 30 in a suitable position for bringing each groove 14 above the sonotrode 30 by rotation of the rim 10 along its geometric axis 11,
• placing a dose of balls 1 on the sonotrode 30, the sonotrode preferably being returned to the sheath 32 so as to constitute, above its vibrating surface 31, a container capable of containing the balls 1,
• rotate the rim 10 to successively bring each groove 14 above the sonotrode 30, each groove 14 then undergoing the following operations:
• cover the openings 16 with the ears 33 and bring the sonotrode 30 into the mouth 15 of the groove 14, this operation preferably being carried out by simultaneously raising the sonotrode 30 and the sheath 32 until the ears 33 cover the openings 16, the sonotrode 30 then being raised alone in the mouth 15, which simultaneously has the effect of bringing the balls 1 into the groove 14 and of putting the sonotrode 30 in the working position,
• blasting the groove 14 by vibrating the sonotrode 30,
• remove the horn 30.

• d'améliorer l'homogénéité et l'isotropie du brouillard de bille 1 produit à l'intérieur de la rainure 14, afin de mieux combiner une forte mise en précontrainte et une faible rugosité notamment sur les lignes de contact 24 à l'arrière et en retrait de l'embouchure 15,
• de protéger du grenaillage l'embouchure 15 elle-même, cette embouchure 15 formant aspérités et étant de ce fait susceptible d'être écrasée par les impacts des billes 1.

It is advantageous to bring the vibrating surface 31 of the sonotrode 30 into the mouth 15 itself, the vibrating surface 31 being substantially at the level of the narrowest section of this mouth. This has the effect of:

• improve the homogeneity and isotropy of the ball mist 1 produced inside the groove 14, in order to better combine a high prestressing and a low roughness, in particular on the contact lines 24 at the rear and set back from the mouth 15,
• to protect the mouthpiece 15 itself from shot-blasting, this mouthpiece 15 forming asperities and therefore being susceptible to being crushed by the impacts of the balls 1.

We will now refer to Figure 6. The blade rotor has in this second example a rim 10 having a general shape of revolution around an axis geometric 11, this rim 10 being limited radially outward by a surface peripheral 12 annular. The rim 10 comprises in this example at the periphery 12 three grooves 14 annular whose descriptions are identical: Each groove annular 14 has a mouth 15 also annular and opening onto the periphery 12. Each groove also has a local opening 18 opening also on the periphery 12, this so-called "local" opening 18. Each groove annular 14 has a substantially trapezoidal profile called "dovetail" with a mouth 15 narrower.

We will now refer to FIG. 7. A blade 20 successively comprises top to bottom in this figure a thin blade 21, a platform 22 extending laterally on each side of the blade 20 and a foot 23 of substantially shaped trapezoidal and complementary to that of the groove 14, the foot 23 being in this example arranged transversely to the blade 21. This figure also shows for information section 21a of the blade 21. The blade 20 is fitted by its foot 23 in the annular groove 14 with limited play and blocked by non-locking means represented. Each annular groove 14 thus receives a plurality of blades 20, the foot 23 is introduced through the local opening, referenced 18 in FIG. 6, and brought into position by sliding in the annular groove 14. The foot 23 comes into contact with the annular groove 14 along two contact lines 24 also annular and located at the back of the mouth 15. By the term attachments 26 of the blades 20, is meant also the annular groove 14 and the feet 23.

Reference will now be made simultaneously to FIGS. 8 and 9. The sonotrode 30 comprises a vibrating surface 31 capable of being introduced into the mouth 15 of the groove 14. The sonotrode 30 slides in a sheath 32 with a clearance E less than the diameter balls 1. The sealing of the sonotrode 30 balls relative to the mouth 15 may be secured by the scabbard or by any other means. In a preferred mode of However, this sealing is more simply ensured by giving the sonotrode 30 a rectangular section with a width equal to that of the mouth 15 subtracted from a clearance E less than the diameter of the balls 1. The sheath 32 supports two ears 33 on either side of the sonotrode 30, these ears being capable of slide in the annular groove 14 with a clearance E less than the diameter of the balls 1. It is understood that the sonotrode 30 and the ears 33 cooperate to contain the balls 1 to inside a portion of the annular groove 14 and against the walls of this groove 14.

• disposer la jante 10 au dessus de la sonotrode 30 dans une position appropriée pour faire défiler la rainure annulaire 14 au dessus de la sonotrode 30 par rotation de la jante 10 selon son axe géométrique 11,
• disposer une dose de billes 1 sur la sonotrode 30, la sonotrode étant de préférence rentrée dans le fourreau 32 afin de constituer au dessus de sa surface vibrante 31 un récipient susceptible de contenir les billes 1,
• présenter l'ouverture locale 18 au dessus de la sonotrode 30,
• amener la sonotrode 30 et les deux oreilles 33 dans l'ouverture locale 18, la sonotrode 30 étant au niveau de l'embouchure 15 et dans l'alignement de cette embouchure 15, les deux oreilles 33 étant de part et d'autre de la sonotrode 30 et dans l'alignement de la rainure 14,
• faire tourner la jante 10 et mettre la sonotrode 30 en vibration quand les deux oreilles 33 et la sonotrode 30 sont dans la rainure annulaire 14,
• arrêter la sonotrode 30 dès qu'une oreille 33 ressort dans l'ouverture locale 18,
• arrêter la rotation de la jante 10 quand les deux oreilles 33 et la sonotrode 30 sont dans l'ouverture locale 18.

With such a device, the ultrasonic shot peening process of an annular groove 14 comprises the following operations:

• placing the rim 10 above the sonotrode 30 in a suitable position for scrolling the annular groove 14 above the sonotrode 30 by rotation of the rim 10 along its geometric axis 11,
• placing a dose of balls 1 on the sonotrode 30, the sonotrode preferably being returned to the sheath 32 so as to constitute, above its vibrating surface 31, a container capable of containing the balls 1,
• present the local opening 18 above the sonotrode 30,
• bring the sonotrode 30 and the two ears 33 into the local opening 18, the sonotrode 30 being at the mouth 15 and in alignment with this mouth 15, the two ears 33 being on either side of the sonotrode 30 and in alignment with the groove 14,
• rotate the rim 10 and put the sonotrode 30 in vibration when the two ears 33 and the sonotrode 30 are in the annular groove 14,
• stop the horn 30 as soon as one ear 33 comes out in the local opening 18,
• stop the rotation of the rim 10 when the two lugs 33 and the sonotrode 30 are in the local opening 18.

• introduire une dose de billes 1 dans l'enceinte 40,
• introduire les pieds 23 dans les ouvertures 46 du couvercle 45 et immobiliser les aubes 20 sur le couvercle 45,
• mettre la sonotrode 30 en vibration pour effectuer le grenaillage.

Reference will now be made simultaneously to FIGS. 10 and 11. To blast the feet 23 of the blades 20, an enclosure 40 is used, the bottom 41 of which has an opening 42 through which a sonotrode 30 passes with a clearance E less than the diameter of the balls 1, the enclosure 40 being covered by a preferably thin cover 45, the cover 45 comprising a plurality of openings 46 of shape complementary to the feet 23 to be treated, the distance between the cover 45 and the sonotrode 30 being at least equal to the height feet 23 so that the bases 23a of feet 23 do not touch the sonotrode 30.
With such a device, the ultrasonic peening process of the feet 23 of the blades 20 comprises the following operations:

• introduce a dose of beads 1 into the enclosure 40,
• insert the feet 23 into the openings 46 of the cover 45 and immobilize the vanes 20 on the cover 45,
• put the sonotrode 30 in vibration to perform the blasting.

An advantage of the process is to avoid coating with a protective coating of parts of the blade that are not to be shot blasted, i.e. platform 22 and blade 21, this protection provided by the cover, the platform 22 and the blade 21 remaining behind the cover 45 outside the enclosure 40.

Advantageously, the feet 23 are positioned above the vibrating surface 31 of the sonotrode 30 in order to ensure a homogeneous peening of all the feet 23.

Advantageously also, the vanes 20 comprising a cooling cavity emerging at the base 23a of the foot 23, this base 23a is positioned at a distance of the sonotrode 30 smaller than the diameter of the balls 1 in order to prevent the balls 1 from enter the cooling cavity.

Claims (8)

Method for increasing the service life of blade attachments on a rotor, the rotor comprising a rim (10) at the periphery (12) of which are attached a plurality of removable blades (20), the blade attachments being referenced (26) and comprising two components (14,23), one of the components (14) being a dovetail groove disposed at the periphery (12) on the rim (10), the other component (23 ) being a foot disposed on the blade (20), the foot (23) being of complementary shape to the groove (14), the foot (23) being capable of being fitted into the groove (14) in order to achieve the 'attachment of the blade (26) on the rim (10), at least one component (14,23) being blasted by projection of balls (1) in order to create on its surface a compression preload,
characterized in that : a) the peening is carried out by the so-called "ultrasonic" process, the balls (1) being projected by the percussion of a sonotrode (30) set in vibration, the balls (1) being contained in an enclosure (40) and forming a mist inside the enclosure (40), the component (14, 23) to be shot blasted being brought into contact with the mist of balls (1), b) the shot peening is carried out at an Almen index at least equal to F8A. Method according to claim 1, characterized in that the balls (1) have a diameter at least equal to 0.8 mm. Method according to claim 1 or 2, the part being made of material from the steel group, titanium alloy, chromium-based superalloy, nickel-based superalloy, characterized in that the prestressing is at least 500 MPa. Method according to any one of claims 1 to 3 applied to the shot blasting of the axial grooves (14), these axial grooves (14) being substantially rectilinear and open at each end, the lateral openings thus formed being referenced (16),
characterized in that a sonotrode capable of being introduced into the mouth (15) of the grooves (14) is used with means for sealing the sonotrode (30) with the mouth (15), as well as two ears (33 ) capable of covering the lateral openings (16) and of concealing them with a clearance E less than the diameter of the balls (1),
and in that it comprises the following operations: placing the rim (10) above the sonotrode (30) in a suitable position to bring each axial groove (14) above the sonotrode (30) by rotation of the rim (10) along its geometric axis (11), place a dose of beads (1) on the sonotrode (30), rotate the rim (10) to successively bring each axial groove (14) above the sonotrode (30), each axial groove (14) then being subject to the following operations: cover the lateral openings (16) with the ears (33) and bring the sonotrode (30) into the mouth (15) of the axial groove (14), blasting the axial groove (14) by vibrating the sonotrode (30), remove the horn (30). Method according to any one of claims 1 to 3 applied to the shot peening of an annular groove (14), this annular groove (14) having a local opening (18) allowing the feet (23) of the blades (20) to pass ,
characterized in that use is made of a sonotrode (30) capable of being introduced into the mouth (15) of the annular grooves (14) with means for sealing the sonotrode (30) with the mouth (15), as well as two ears (33) capable of passing through the annular groove (14) with a clearance E less than the diameter of the balls (1),
and in that it comprises the following operations: place the rim (10) above the sonotrode (30) in a suitable position to slide the annular groove (14) above the sonotrode (30) by rotation of the rim (10) along its geometric axis (11) , place a dose of beads (1) on the sonotrode (30), present the local opening (18) above the sonotrode (30), bring the sonotrode (30) and the two ears (33) into the local opening (18), the sonotrode (30) being at the mouth (15) and in alignment with this mouth (15), the two ears (33) being on either side of the sonotrode (30) and in alignment with the annular groove (14), rotate the rim (10) and put the sonotrode (30) in vibration when the two ears (33) and the sonotrode (30) are in the annular groove (14), stop the horn (30) as soon as one ear (33) comes out in the local opening (18), stop the rotation when the two ears (33) and the horn (30) are in the local opening 18). Process according to any one of Claims 1 to 3, applied to the blasting of the blade roots,
characterized in that an enclosure (40) is used, the bottom (41) of which has an opening (42) through which a sonotrode (30) passes with a clearance E less than the diameter of the balls (1), the enclosure (40 ) being covered by a thin cover (45), the cover (45) comprising a plurality of openings (46) of complementary shape to the feet (23), the distance between the cover (45) and the sonotrode (30) being at less equal to the height of the feet (23),
and in that the method comprises the following operations: introduce a dose of beads (1) into the enclosure (40), insert the feet (23) into the openings (46) of the cover (45) and immobilize the blades (1), put the sonotrode (30) in vibration to perform the blasting. Method according to Claim 7, characterized in that all the blades (20) are positioned above the vibrating surface (31) of the sonotrode (30). Method according to claim 7, the blades (20) comprising a cooling cavity opening at the base (23a) of the foot (23), characterized in that the base (23a) is positioned at a distance from the sonotrode (30) lower the diameter of the balls (1).

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