Method for non-destructively testing the quality of a workpiece made of a nickel-based superalloy

Abstract

The invention relates to a method for testing a workpiece (10), said method being implemented in an optical system (1) configured to analyse workpieces of birefringent majority phase, the method comprising: - emitting an incident beam (101) towards a surface (100) of the workpiece (10), - linearly polarizing the incident beam (101) along a first polarization axis and transmitting a polarized incident beam (102) to the surface (100), - linearly polarizing the reflected beam (103), resulting from reflection of the polarized incident light beam (102) by the surface (100), along a second polarization axis perpendicular to the first axis and transmitting a polarized reflected beam (104), - obtaining an image representative of reflections from the surface, - determining whether at least one area, called the light area, is lighter than a predefined threshold, - detecting a defect when the light area meets a predefined geometric criterion.

Description

[0001] NON-DESTRUCTIVE PROCESS FOR TESTING THE QUALITY OF A PART

[0002] MADE OF A NICKEL-BASED SUPERALLOY

[0003] TECHNICAL FIELD

[0004] The present invention relates to the field of non-destructive testing and more particularly relates to a method for testing the surface of a part comprising a non-birefringent majority phase implemented in an optical system.

[0005] STATE OF PRIOR ART

[0006] Parts composed of a material comprising a non-birefringent majority phase, such as a nickel-based superalloy, are frequently used in the aeronautics sector for their thermomechanical properties. However, such nickel-based superalloy parts are generally manufactured using forging and / or machining techniques that can generate the formation of microstructural defects. Such defects comprise undesirable phases, of a chemical composition different from the composition of the majority phases of the parts, due to chemical segregation. The defects thus generated weaken the microstructure of the parts and lead to a reduction in the service life of the parts.

[0007] It is then necessary to detect the presence of such defects on the surface of nickel-based superalloy parts. A commonly used method for defect detection is macrography, which requires a preliminary chemical etching step on the surface to reveal the microstructure of the part. The defects then appear as black or white spots and can be detected by an operator. However, macrography is slow and complex due to the need for the preliminary chemical treatment step and also uses harmful chemicals.

[0008] Electron backscatter diffraction (EBSD) can be used to analyze the crystallographic orientation of a microstructure and to index different phases present in the microstructure. However, such a method must use a scanning electron microscope, which requires measurements to be carried out in a vacuum and involves analyzing samples of a few cm 2 at most. In addition, the samples must be polished and the measurements are particularly slow since it takes several hours to analyze a surface of the order of cm 2 . Local chemical analysis methods such as energy-dispersive X-ray spectroscopy (EDS) have similar drawbacks. Such methods are therefore not suitable for performing defect testing on an industrial scale, quickly and easily.

[0009] It is then desirable to overcome these disadvantages of the state of the art.

[0010] In particular, it is desirable to provide a solution for non-destructive testing of the surface of a part composed of a material comprising a non-birefringent majority phase such as, for example, a nickel-based superalloy in order to detect the presence of microstructural defects comprising undesirable phases. It is also desirable to provide a testing solution that does not require a prior treatment or preparation step. Finally, it is desirable to provide a solution that is rapid and applicable to large surfaces.

[0011] STATEMENT OF THE INVENTION

[0012] A method for controlling the quality of a part to be controlled is proposed here, the control method being implementable in an optical control system configured to analyze parts having a birefringent majority phase. The method is intended to determine whether the material of the part comprises at least one birefringent minority phase representative of a defect, and comprises: emitting an incident light beam towards a surface of the part to be controlled; linearly polarizing the incident light beam along a first polarization axis so as to transmit a polarized incident light beam onto the surface of the part to be controlled;linearly polarizing the reflected light beam along a second polarization axis oriented perpendicular to the first polarization axis so as to transmit a polarized reflected light beam, the reflected light beam (103) resulting from the reflection of the polarized incident light beam (102) by the surface (100) of the part (10); obtaining an image representative of reflections on the surface of the part; determining whether at least one area of ​​the image has a light intensity greater than a predefined threshold, said area of ​​the image being called a light area;and detect a defect when at least one light area of ​​the image meets a predefined geometric criterion. Thus, it is easy to detect the presence of birefringent minority phases representative of defects and thus to detect the presence of defects, non-destructively and without any prior processing step. In addition, the contrast obtained between the areas of the image representative of the majority phase and the areas of the image representative of the birefringent minority phases makes it possible to simplify and improve the reliability of defect detection by optical observation. It is thus easy to detect defects in a part made of a nickel-based superalloy, without a prior chemical treatment step.;

[0013] According to a particular embodiment, the predefined geometric criterion comprises that the clear zone has at least one extent greater than a predefined value.

[0014] According to a particular embodiment, the predefined value is equal to 100 pm.

[0015] Thus, it is easy to detect freckle-type defects in a part made of nickel-based superalloy while considering the “ô” and “y” phases as not representative of defects.

[0016] According to a particular embodiment, the predefined geometric criterion comprises that the clear zone has a surface area greater than a predefined surface area.

[0017] According to a particular embodiment, the control method further comprises recording the image of the surface of the part obtained, and transmitting said image of the surface to a processing device comprising electronic circuitry configured to implement the steps of determining whether at least one area of ​​the image has a light intensity greater than a predefined threshold and of detecting a defect when at least one light area of ​​the image meets a predefined geometric criterion.

[0018] This makes it possible to automatically detect defects on the surface of the part.

[0019] An optical inspection system is also provided for inspecting the quality of a part made of a material comprising a non-birefringent majority phase, the optical inspection system being configured to analyze parts having a birefringent majority phase, the optical inspection system being intended to determine whether the material comprises at least one birefringent minority phase representative of a defect. The optical inspection system comprises: a light source configured to emit an incident light beam towards a surface of the part to be inspected; a polarizer configured to linearly polarize the incident light beam along a first polarization axis so as to transmit a polarized incident light beam onto the surface of the part to be inspected;a second polarizer, called an analyzer, configured to linearly polarize a reflected light beam along a second polarization axis oriented perpendicular to the first polarization axis so as to transmit a polarized reflected light beam, the reflected light beam originating from the reflection of the incident light beam polarized by the surface of the part; an optical device for forming an image representative of reflections on the surface of the part; means for determining whether at least one area of ​​the image has a light intensity greater than a predefined threshold, called a light area; and means for detecting a defect when at least one light area of ​​the image satisfies a predefined geometric criterion.;

[0020] According to a particular embodiment, the defect is a freckle type defect.

[0021] According to a particular embodiment, the minority phase representative of a defect is a q phase, a p phase, a G phase, a Laves phase and / or a boride.

[0022] BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The above-mentioned features of the invention, as well as others, will appear more clearly on reading the following description of at least one exemplary embodiment, said description being made in relation to the attached drawings, among which: [Fig. 1] schematically illustrates an optical control system in which a method for controlling the quality of a part is implemented; and [Fig. 2] schematically illustrates the method for controlling the quality of the part.

[0024] DETAILED PRESENTATION OF IMPLEMENTATION METHODS

[0025] Fig. 1 thus schematically illustrates an optical control system 1 in which a method for controlling the quality of a part 10 is implemented. The part 10 to be controlled is made of a material comprising a majority crystalline phase which is non-birefringent, in other words a majority crystalline phase which is optically isotropic and has an identical refractive index in all directions of the crystal. In the material constituting the part 10, a crystalline phase is distinguished from another crystalline phase by its crystalline structure and / or by its composition, in other words by the proportions of chemical elements which compose it. The majority crystalline phase of the part 10 is a phase present on more than 80% of a surface 100 of the part 10.

[0026] Preferably, the material constituting the part 10 to be inspected is a nickel-based superalloy which comprises a y-phase matrix, the y-phase being the majority phase of the part 10. The y-phase matrix has a crystalline structure belonging to the cubic-type crystalline system and is non-birefringent. According to one example, the nickel-based superalloy further comprises the y' phase which is not representative of defects and is non-birefringent and the set of phases y and y' is then present on more than 80% of a surface 100 of the part 10 and is considered to be the majority phase of the part 10. Alternatively, the nickel-based superalloy further comprises a δ phase and a y” phase which are birefringent and not representative of defects. In this case, the y phase is present on more than 80% of a surface 100 of the part 10 made of nickel-based superalloy.

[0027] In addition, the nickel-based superalloy may comprise birefringent minority phases including precipitates such as borides, a q phase, a p phase, a G phase and / or a Laves phase. The birefringent phases are optically anisotropic. In other words, the refractive index of said birefringent phases differs along the crystallographic axis of the material. The presence of said birefringent minority phases on a surface 100 of the part 10 is representative of defects on said surface 100. The defects are, for example, undesirable phases whose presence alters the thermomechanical properties of the part 10. The defects may furthermore be freckle-type defects which are segregation defects appearing during the solidification of the part 10 and which comprise q-phase grains.Freckle-type defects can have a significantly elongated shape with a length of several tens of micrometers, for example 100 micrometers.

[0028] The part 10 is considered to have a defect if a minority phase has a predefined geometric characteristic, for example a predefined size and / or a predefined shape. Preferably, a minority phase is considered to be representative of a defect when said minority phase comprises at least one extent greater than a first predefined value, the first predefined value relating to the surface 100 of the part 10. The first predefined value is for example equal to 100 μm. An extent is a one-dimensional measurement such as the length of the largest side of a portion of the surface 100 of the part 10 formed by a minority phase or such as the largest segment crossing said portion of the surface 100 of the part 10 formed by a minority phase.

[0029] When the nickel-based superalloy also contains the ô and y” phases, which are birefringent but not representative of defects, the ô and y” phases, however, have extents less than the first predefined value, for example less than 100 pm.

[0030] The optical control system 1 is configured to analyze parts having a majority birefringent phase, in other words parts made of a material whose phase present on more than 80% of a surface is birefringent.

[0031] Preferably, the optical control system 1 comprises a light source 11, a polarizer 12, the part 10, a second polarizer, called analyzer 13, and an optical image formation device 14.

[0032] The light source 11 emits an incident light beam 101 towards a surface 100 to be analyzed of the part 10. Said surface 100 is oriented facing the polarized incident light beam 101.

[0033] The polarizer 12 linearly polarizes the incident light beam 101, along a first polarization axis, so as to transmit a polarized incident light beam 102 onto the surface 100 of the part 10. The first polarization axis is perpendicular to the direction of propagation of the incident light beam 101 emitted by the light source 11. The polarizer 12 is placed on the optical path of the incident light beam 101, between the light source 11 and the surface 100 of the part 10. The incident light beam 101 is an unpolarized light beam. Alternatively, the incident light beam 101 is a polarized beam which comprises at least one non-zero linear polarization component in the direction of the first polarization axis, so that the polarized incident light beam 102 has a non-zero luminous intensity.

[0034] The polarized incident light beam 102 is reflected on the surface 100 into a reflected light beam 103. In other words, the surface 100 of the part 10 is capable of reflecting the polarized incident light beam 102 so as to return a reflected light beam 103.

[0035] The analyzer 13 linearly polarizes the reflected light beam 103, along a second polarization axis oriented perpendicular to the first polarization axis, so as to transmit a polarized reflected light beam 104. The second polarization axis is furthermore perpendicular to the direction of propagation of the reflected light beam 103. The analyzer 13 is placed on the optical path of the reflected light beam 103, between the surface 100 of the part 10 and the optical image-forming device 14. The analyzer 13 allows at least partial passage of the components of the reflected light beam 103 which are reflected by the minority phases and does not allow passage of the components of the reflected light beam 103 which are reflected by the non-birefringent majority phases.

[0036] The optical image-forming device 14 makes it possible to obtain an image representative of reflections on the surface 100 of the part 10. Said image is formed from the polarized reflected light beam 104.

[0037] The image representative of reflections on the surface 100 has areas of zero light intensity, called dark areas, which represent non-birefringent areas of the surface 100. The linear polarization of the polarized incident light beam 102 received on said non-birefringent areas is not modified by reflection. The parts of the reflected light beam 103 which are reflected by the non-birefringent areas of the surface 100 are therefore linearly polarized in the direction of the first polarization axis of the polarizer 12 and are blocked by the analyzer 13 since they do not comprise any polarization component in the direction of the second polarization axis. No light signal is therefore detected by the optical image-forming device 14 from the non-birefringent areas.The majority phase of the material constituting the part 10 being non-birefringent, the dark areas cover a portion of the image representing more than 80% of the surface 100.

[0038] Thus, for a part 10 made of a nickel-based superalloy, the dark areas of the image represent areas of the surface 100 comprising the majority crystalline phase y, and, where appropriate, the phase y'. The dark areas of the image may also represent carbides which are not representative, in themselves, of the defects sought.

[0039] The image representative of reflections on the surface 100 may furthermore have areas of light intensity greater than a predefined threshold, called bright areas, which represent areas of the surface 100 exhibiting birefringence. The predefined threshold is strictly greater than zero, so that the bright areas necessarily have a non-zero light intensity. Unlike the non-birefringent areas, the birefringent areas of the surface 100 modify the direction of polarization during the reflection of the polarized incident light beam 102. Thus, the parts of the reflected light beam 103 which are reflected by the birefringent areas of the surface 100 comprise a non-zero polarization component oriented in the direction of the second polarization axis and which is transmitted by the analyzer 13.The light intensity of the portions of the reflected light beam 103 which are reflected by the birefringent zones is potentially attenuated by the analyzer 13 but not canceled.

[0040] Since the majority crystalline phase of the material is non-birefringent, the light areas therefore represent minority phases of the material and cover a portion of the image representing less than 20% of the surface 100. It is then possible to easily detect the presence and location of minority phases on the surface 100.

[0041] The surface 100 of the part 10 is considered to have a defect when at least one minority phase has a predefined geometric characteristic. A defect is then detected on the surface 100 when at least one light area meets a predefined geometric criterion, a size or shape criterion for example. It is thus easy to detect at least one defect on the surface when the image has at least one light area meeting the predefined geometric criterion.

[0042] According to a preferred embodiment, a light zone is considered to be representative of a defect when said light zone is located and has at least one dimension greater than a second predefined dimension, the extent being a first predefined dimension, the second predefined dimension being representative of the first predefined dimension on the surface 100. Said second predefined dimension, relating to the image, can be calculated as a function of the first predefined dimension and an optical magnification of the optical image-forming device 14. Freckle-type defects can thus be easily detected.

[0043] Furthermore, the contrast between the light areas of the image, which are areas of light intensity greater than the predefined threshold and therefore of non-zero light intensity, and the dark areas of the image, which are areas of zero light intensity, makes it possible to simplify and improve the reliability of defect detection by optical observation. According to one embodiment, the light areas can be easily distinguished from the dark areas by an image processing method of the binarization type by thresholding from a histogram of the image, such as the Otsu method. Defect detection is thus carried out non-destructively and furthermore does not require a prior chemical treatment step. Defect detection is simplified and accelerated compared to techniques using such prior treatment steps. No use of harmful chemicals is also necessary.

[0044] For a part 10 made of a nickel-based superalloy, the light areas of the image represent areas of the surface 100 comprising minority phases including borides, q, p, G and / or Laves phases which are representative of defects.

[0045] The light areas of the image represent areas of the surface 100 which may also comprise δ and γ phases. However, the δ and γ phases have dimensions smaller than the dimensions of the minority phases representative of defects, for example less than 100 μm. Thus, a light area having at least one dimension greater than the second predefined dimension cannot be representative of a δ or γ phase.

[0046] According to an exemplary embodiment, the optical image-forming device 14 is configured so that the image representative of reflections on the surface 100 obtained is enlarged relative to the surface 100 of the part 10. The optical image-forming device 14 is for example an optical microscope. According to an exemplary embodiment, the optical image-forming device 14 further comprises a resolution limit equal to or less than 100 micrometers, preferably equal to or less than 10 micrometers, and more preferably equal to or less than 1 micrometer. Thus, the image obtained makes it possible to observe microstructural defects present on the surface 100 having minimum dimensions equal to the resolution limit. Freckle-type defects having a length of 100 micrometers are then easily observable.According to an exemplary embodiment, the optical image-forming device 14 is configured to obtain an image representing an area greater than or equal to 1 cm. 2 of the surface 100 of the part 10. The same image obtained thus makes it possible to quickly detect defects over an area of ​​at least 1 cm 2 .

[0047] According to a particular embodiment, the optical control system 1 comprises a beam splitter 15 configured to separate the polarized incident light beam 102 from the reflected light beam 103 so that only the reflected light beam 103 is sent to the analyzer 13 then to the optical image-forming device 14.

[0048] The optical control system 1 further comprises an acquisition device 16. The acquisition device 16 is configured to receive the image representative of reflections on the surface 100, obtained by the optical image-forming device 14, and to record said image. The acquisition device is for example an optical camera.

[0049] The optical control system 1 further comprises a processing device 17. The acquisition device 16 is configured to transmit the received and / or recorded image to the processing device 17. The processing device 17 is configured to receive the image from the acquisition device 16 and apply image processing algorithms to the received image in order to detect defects. For example, the processing device 17 applies, for example, a thresholding binarization method to the image to distinguish dark areas from light areas and determine whether at least one area of ​​the image has a light intensity greater than the predefined intensity threshold.

[0050] The processing device 17 implements a comparison of the detected light areas with the predefined geometric criterion in order to determine whether at least one light area satisfies the predefined geometric criterion. Preferably, the processing device 17 measures the largest dimension of each detected light area and then compares said dimension with the second predefined dimension. The processing device 17 detects a defect for each detected light area whose largest dimension is greater than the second predefined dimension. According to one embodiment, the processing device 17 receives information representative of the magnification of the optical image-forming device 14 and calculates the second predefined dimension as a function of the first predefined dimension and said magnification.

[0051] The processing device 17 comprises a processor capable of executing instructions loaded into a memory, said instructions forming a computer program causing the implementation of the image processing steps described here. All or part of the image processing steps described here can thus be implemented in software form by executing a set of instructions by a programmable machine, for example a DSP (Digital Signal Processor) type processor or a microcontroller, or be implemented in hardware form by a machine or a dedicated electronic component (chip) or a dedicated set of electronic components (chipset), for example an FPGA (Field Programmable Gate Array) or ASIC (Application Specific Integrated Circuit) component.Generally, the processing device 17 comprises electronic circuitry adapted and configured to implement the image processing steps described herein.

[0052] Fig. 2 schematically illustrates the method for controlling the quality of the part 10. The method is implemented in an optical control system 1 configured to analyze parts having a birefringent majority phase. In other words, at least one step of the method is executed by said optical control system 1.

[0053] In a step 201, the optical control system 1 emits the incident light beam 101 towards a surface 100 of the part 10 to be controlled.

[0054] In a following step 202, the optical control system 1 linearly polarizes the incident light beam 101 along the first polarization axis, so as to transmit the polarized incident light beam 102 onto the surface 100 of the part 10.

[0055] In a following step 203, the optical control system 1 obtains the reflected light beam 103, said reflected light beam 103 coming from the reflection of the polarized incident light beam 102 by the surface 100 of the part 10.

[0056] In a following step 204, the optical control system 1 linearly polarizes the reflected light beam along the second polarization axis oriented perpendicular to the first polarization axis, so as to transmit a polarized reflected light beam 104.

[0057] In a following step 205, the optical control system 1 obtains an image representative of reflections on the surface 100 of the part 10. The image thus obtained is formed from the polarized reflected light beam 104.

[0058] In a following step 206, the method comprises determining whether at least one light area is present in the image, in other words whether at least one area of ​​the image has a light intensity greater than the predefined threshold. According to one embodiment, the method comprises detecting each light area present in the image.

[0059] In a following step 207, the method comprises detecting a defect when at least one light area of ​​the image satisfies the predefined geometric criterion. Preferably, a defect is detected in each light area comprising at least one dimension greater than the second predefined dimension, in other words representative of a dimension of the surface 100 greater than the first predefined dimension.

Claims

CLAIMS 1. Method for controlling the quality of a part (10) to be controlled, the method being intended to determine whether the material of the part comprises at least one birefringent minority phase representative of a defect, the method comprising: emitting (201) an incident light beam towards a surface of the part to be controlled, linearly polarizing (202) the incident light beam along a first polarization axis so as to transmit a polarized incident light beam onto the surface of the part to be controlled, linearly polarizing (204) the reflected light beam along a second polarization axis oriented perpendicular to the first polarization axis so as to transmit a polarized reflected light beam, the reflected light beam (103) coming from the reflection of the polarized incident light beam (102) by the surface (100) of the part (10), and obtaining (205) an image representative of reflections on the surface of the part,determining (206) whether at least one area of ​​the image has a light intensity greater than a predefined threshold, said area of ​​the image being called a light area, detecting (207) a defect when at least one light area of ​​the image meets a predefined geometric criterion., 2. Control method according to claim 1, in which the predefined geometric criterion comprises that the clear zone has an extent greater than a predefined value.

3. Control method according to claim 2, wherein the predefined value is equal to 100 pm.

4. Control method according to claim 1, wherein the predefined geometric criterion comprises that the clear zone has a surface area greater than a predefined surface area.

5. Control method according to one of claims 1 to 4, further comprising recording the image of the surface of the part obtained, and transmitting said image of the surface to a processing device (17) comprising electronic circuitry configured to implement the steps of determining (206) whether at least one area of ​​the image has a light intensity greater than a predefined threshold and of detecting (207) a defect when at least one light area of ​​the image meets a predefined geometric criterion.

6. Optical control system (1) intended to control the quality of a part (10) made of a material comprising a non-birefringent majority phase, the optical control system (1) being configured to analyze parts having a birefringent majority phase, the optical control system (1) being intended to determine whether the material comprises at least one birefringent minority phase representative of a defect, the optical control system (1) comprising: a light source (11) configured to emit an incident light beam (101) towards a surface (100) of the part (10) to be controlled, a polarizer (12) configured to linearly polarize the incident light beam (101) along a first polarization axis so as to transmit a polarized incident light beam (102) onto the surface (100) of the part (10) to be controlled, a second polarizer, called analyzer (13),configured to linearly polarize a reflected light beam (103) along a second polarization axis oriented perpendicular to the first polarization axis so as to transmit a polarized reflected light beam (104), the reflected light beam (103) originating from the reflection of the polarized incident light beam (102) by the surface (100) of the part (10), an optical device (14) for forming an image representative of reflections on the surface (100) of the part (10), means (17) for determining whether at least one area of ​​the image has a light intensity greater than a predefined threshold, called a light area, and means (17) for detecting a defect when at least one light area of ​​the image satisfies a predefined geometric criterion., 7. An optical control system according to claim 6, wherein the defect is a freckle type defect.

8. Optical control system according to one of claims 6 and 7, in which the minority phase representative of a defect is a q phase, a p phase, a G phase, a Laves phase and / or a boride.

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