FR2889092A1 - Mechanical testing of a test bar by cutting and filling with a repair metal prior to further cutting and fatigue testing, especially for characterizing repairs to the blades of monobloc turbine rotors - Google Patents

Abstract

Method for mechanical characterization of metallic material in comparison with the metal of a part that is to be repaired and for validation of a repair installation in which the part to be repaired is refilled with the repairing metal, e.g. using a TIG welding process. The method has the following steps: machining of a cavity in a bar (50) of the said metal; refilling of the cavity by the installation; cutting a test piece from the bar such that it has a central region made up entirely of the filler material and; subjection of the test piece to vibration fatigue testing. The invention relates particularly to turbine blades made from the titanium alloy Ti17.

Description

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  The present invention relates to the field of turbomachines, including aeronautics, and aims at repairing parts such as bladed mobile discs.

  In order to meet the increased engine performance requirements, bladed wheels or bladed discs, designated DAM, made of titanium alloy for gas turbine engine compressors are now manufactured. In a conventional rotor, the blades are held by their feet which is mounted in a housing formed on the rim of the disc. The disks and the blades are therefore manufactured separately before being assembled into a bladed rotor. In a DAM, the vanes and disk are machined directly into a forged blank; they are only one piece. This technique allows significant gains on the total mass of the engine but also on manufacturing costs.

  However, this type of rotor has the disadvantage of being difficult to repair. In operation, the compressor blades may be damaged by impacts caused by the engine being absorbed by foreign bodies or by erosion caused by dust and other particles entrained by the air passing through. the engine and coming into contact with the blade surface. These wear or damage, if they can not be repaired according to the criteria of the manufacturer's documentation, result in the replacement of one or more blades. In the case of one-piece bladed parts, the blades are integral parts of a solid part and, unlike conventional assemblies, they can not be replaced or even dismantled to be repaired individually. It must be possible to repair the part directly on the disc. The repair must then take into account the entire room with its dimensions, its weight, in the case of large rooms, and the accessibility of the areas to be repaired.

  Thus, for a DAM, the zones generally concerned by the repair are, for each blade, the top, the blade corner on the leading edge, the blade edge on the trailing edge, the leading edge and the trailing edge. .

  The repair techniques that have been developed consist in eliminating the damaged area on the damaged blades, then replacing the part eliminated by a piece of appropriate shape or by reloading material. These techniques generally implement a conventional machining for the removal of the damaged part, the contactless inspection of the repaired part, ultrasonic shot blasting and a specific machining for the recovery of the repaired area.

  The present invention relates to the repair of parts by reloading o material.

  The repair is particularly difficult to achieve for some alloys used whose welding causes the formation of defects in the mass. These include titanium alloy Ti17. This alloy is reported for example in the patent application EP 1340832 of the Applicant on a product, such as a blade, made in this matter. When it comes to reloading material, the TIG or microplasma type techniques traditionally and commonly used in the aeronautics industry only make it possible to treat Ti17 titanium for applications limited to zones. weakly solicited.

  These conventional reloads lead to the formation of defects. Thus TIG reloading, using a high energy compared to the small thickness involved, is a generator of deformations, and causes 2s formation of a high level of porosity, such as micro porosities or micro-blasts, as well as an extended heat affected area (ZAT). These micro porosities, which are very difficult to detect, generate a reduction of up to 80% in mechanical strength. This type of soldering by reloading therefore finds application to areas of low stress, only. Microplasma reloading results in the formation of a lower but still relatively high HAZ. In addition, the method requires special attention and periodic control of the equipment and products used so that no operating parameter of the machine drifts and changes the expected results.

  US Patent 6568077 discloses a method of repairing a blade on a DAM disk in which the damaged part of the blade is machined and then, according to a first procedure, the missing part is recharged by the deposition of metal by means of a welding machine with arc and tungsten electrode, TIG technique. According to a second operating mode, an insert is welded by means of an electron beam welding machine. The profile of the blade is then restored by appropriate machining. This method, however, does not mention the problem encountered with the welding of certain titanium alloys.

  In particular, the reloading by laser beam is a technique to avoid defects in the welding zone.

  The reloading by laser beam is already known and used, for example, in applications where it is necessary to generate metal contours, in particular from CAD data. The walls have a thickness of between 0.05 and 3 mm and the layers are 0.05 to 1 mm high. It makes it possible to obtain an excellent metallurgical bond with the substrate.

  The technique of recharging by means of a laser beam has the following advantages: the supply of heat is constant over time. The heat does not have time to store in the mass and diffuse; this results in a slight degassing in the case of titanium and a limit of the reduction in resistance. In addition, the repeatability and reliability of this technique are good once the machine parameters are set; his driving is easy.

  The laser techniques currently used implement simultaneous actions of material supply and laser beam irradiation of the substrate.

  The material is generally deposited on the work area in the form of a powder or wire. According to other embodiments, it is projected in the form of powder jets to the work area using a suitable nozzle. o o

  However, the implementation of such a method is difficult.

  On the one hand, it is necessary to ensure that the metal material for refilling is suitable for repair without any detrimental reduction of the mechanical properties of the repaired area.

  On the other hand, it is also necessary that the installation concerned is able to provide a repair without undue abatement nor the properties of the material.

  The subject of the invention is therefore a method enabling the mechanical characterization of a metallic material with respect to a metal constituting a part to be repaired and the validation of an installation for repairing said metal part by reloading with said metallic material, characterized by the fact that it consists: - machining a cavity in a bar made of the metal of the part to be repaired - reloading the cavity by means of said installation with said metal material - cutting a specimen in said bar so that it presents a central zone consisting solely of the recharged metal material - subjecting the test piece to an axial vibration fatigue test to determine the reduction of the mechanical properties with respect to the metal constituting the part.

  As far as parts repair is concerned, the manufacturer or the user of the machines is obliged to use subcontractors of any origin, using alloys which are not identical to those constituting the parts, it is important to have a simple way to verify that parts can be repaired satisfactorily. The method of the invention thus meets this objective. It is sufficient for the manufacturer or the user to supply the subcontractor with a series of specimens mentioned above and the latter to return them to him after reloading according to the present method. The analysis performed on the samples after rupture resulting from the tests will give an accurate image of the ability to produce a satisfactory repair in terms of mechanical strength.

  The method is aimed at a preferentially type laser charging installation, however it remains applicable to any type of recharging.

  The process is particularly aimed at a titanium alloy metal material, in particular Ti 17 or TA 6 V for a piece also made of titanium alloy.

  Advantageously, the bar has a parallelepipedal shape, and the cavity machined in the bar has a shape corresponding to that formed in the part to be repaired. In particular, the cavity is cylindrical with a transverse axis with respect to the bar.

  The invention will now be described in greater detail with reference to the drawings in which: FIG. 1 represents a partial view of a blisk, FIG. 2 is a diagrammatic sectional view of a reloading nozzle, FIGS. 3 to 6 show a test specimen of mechanical characterization with laser refilling according to the invention.

  FIG. 7 shows the vibratory fatigue test step of a refilled test specimen.

  - Figure 8 shows a macrophotography of the fracture surface.

  FIG. 9 shows an analysis graph of the test results.

  FIG. 1 shows part of a blisk 1. The blades 3 are radial and distributed around the periphery of a disc 5. The assembly is a monobloc in that it has been manufactured either by machining of a single blank, or by welding at least part of its components. The blades in particular are not linked to the disc by removable mechanical means. Areas susceptible to damage are the leading edges 31, the trailing edges 32, the leading edge corners 33, the trailing edge corners 34 and the provided blade top line 35. a thinned portion forming a sealing lip as is known.

  The damage that we see depends on the position of the area. At the leading edge, trailing edge or blade corner for example it may be a loss of material caused by the impact of a foreign object or a crack. At the top of the blade, it is most often a wear due to friction with the motor housing.

  Depending on the damaged area, a quantity of material is removed such that the geometry, dimensions and dimensions of the area to be repaired are determined. This shaping is done by mechanical machining, especially by milling on a suitable tool, according to a range ensuring a surface condition compatible with the quality of reloading sought.

  The welding surface is then cleaned to receive the filler metal, both mechanically and chemically. This cleaning is adapted to the material of the substrate. This is important for the Ti17 titanium alloy in particular or the TA6V alloy.

  FIG. 2 shows a nozzle 30 for recharging by laser beam. This nozzle comprises powder metal supply channels to be deposited on the area to be repaired along the axis of propagation of the laser beam. The beam is directed towards the workpiece and the metal powder M is driven by a flow of gas G on the zone heated by the beam.

  The nozzle moves along the area to be repaired in a back and forth motion gradually building a stack of layers of material deposited and melted by the laser beam. Reloading is done at constant speed and intensity even if the thickness on the part varies.

  The parameters are adapted to limit in particular the internal deformations and the rework, as well as the extent of the heat affected zone (zat). The parameters to be taken into account for recharging are: - the height of the focal point of the laser beam (preferably a YAG laser) with respect to the surface, - the speed of advance of the head 30, - the applied energy by the beam.

  the powder used (Ti17 or TA6V) which is not necessarily of the same metal as the substrate, its particle size, preferably between 30 and 100 m; its focal point, - the nature of the entrainment and confinement gas, preferably helium or argon.

  The type of nozzle to be used has been defined beforehand. Speed and energy are related to the type of machine used.

  In particular, in the case of titanium Ti17, to avoid the appearance of porosities 1 in the mass, it was found that the variation of the parameters should not exceed + or - 5%.

  The invention relates to the validation of a laser beam welding installation for the implementation of the reloading repair method. Indeed, before putting a machine in service and dedicate it to the repair of DAM by reloading, it is necessary to check if the repaired parts will not suffer from an abatement prejudicial to their use.

  This validation is carried out by performing tests on specimens called characterization and validation. These specimens 50 shown in FIGS. 3 to 6 make it possible to visually check the existence of non-oxidation and to measure the geometry of the refilling, to evaluate the metallurgical quality of the refilling after machining, without and with heat treatment, by controls. non-destructive and destructive, such as bleeding and micrographic sections, - characterizing the Ti17 material, laser-charged, after machining and heat treatment, in mechanical strength, ie by carrying out cyclic fatigue tests (HCF) .

  In the particular case of DAM repair, a bar 50 originating from a DAM forged blank is preferably used, since it then has a fiber drawing direction of the same nature as the DAMs which will be repaired with such an installation. To perform these tests the bar is parallelepiped with, for example, the following dimensions: 100mm x 19mm x 8mm.

  As can be seen in FIG. 4, a cavity 52 is machined whose geometry of the profile corresponds to a cavity that is hollowed in a damaged zone of the leading edge or the trailing edge of a blade to form a zone. to fix. Here this cavity has a cylindrical shape whose axis is transverse to that of the bar.

  The bar 50 is wider than a blade. This cavity 52 is reloaded, by means of the installation that it is desired to validate. The cavity has a sufficient depth, for example a maximum depth of 5 mm, so that it is necessary to proceed by stacking several layers. Moreover, because of the width of the bar, reloading is performed by crossing the different layers.

  When the reload is finished, as shown in Figure 5, with possibly some overflows considered inconsequential, is cut a slice 56 of the bar. This slice 56, shown hatched in FIG. 5, includes the refilled portion 54. As can be seen in the figure, the wafer is parallel and slightly recessed, for example by 1 mm, with respect to the surface on which it has been made. reloading. For example, for a bar 8 mm thick, a 2.5 mm thick slice is extracted. This slice then comprises three distinct parts with a central part consisting only of the metal recharged between two elements o of the original bar.

  Figure 6 shows the wafer 56 which has been machined to obtain a central portion 56a forming a bar incorporating the recharged area. In its middle part, the entire thickness of the bar 56a consists of recharged material.

  On either side of the bar 56a wider tabs 56b form gripping tabs for the jaws of the apparatus with which the cyclic fatigue tests are performed.

  These tests, shown diagrammatically in FIG. 7, consist in applying alternating axial forces of compression and traction. The frequency, the amplitude of the vibrations, the number of cycles and the temperature, in particular, are determined.

  In Figure 8, there is shown a macro photography of the surface of the broken test piece. The test piece is broken in the recharged area. The examination of this surface makes it possible to check the quality of the reloading and to observe the nature of the defects present. For different test specimens, a logarithmic graph is plotted on the abscissa, the level of alternating stresses in MPa and from which number of cycles, the rupture occurred. For example, in this graph comprising a sample of several test specimens, the occurrence of the failure of the different specimens caused by a defect leading to A or defects at heart BA is postponed from the analysis of the results, thus determining a material abatement rate for the intended installation. This rate is the ratio between the mechanical strength of the material after refilling and the mechanical strength of this material on a new part.

  When the tests on the test pieces are satisfactory and the rate exceeds a minimum threshold value, determined experimentally, the installation is validated.

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Claims (5)

io-CLAIMS   1. A method for the mechanical characterization of a metallic material with respect to a metal constituting a part to be repaired and the validation of a repair installation of said part by reloading with said metallic material, characterized in that it consists : machining a cavity (52) in a bar (50) of said metal -recharging (54) the cavity by means of said installation - cutting a specimen (56) in said bar so that it has a central zone ( 56a) consisting solely of refilled metal - subject the test specimen to an axial vibration fatigue test 2. Method according to claim 1, the installation of which is laser-reloading type.   3. Method according to claim 1 or 2, the metal material is a titanium alloy, including Ti 17 or TA6V.   4. The method of claim 1 wherein the bar (50) has a parallelepiped shape, and the cavity (52) machined in the bar has a shape corresponding to that formed in the part to be repaired.   5. Method according to the preceding claim, the cavity is cylindrical axis transverse to the bar.