METHOD FOR INSPECTING SURFACE DEFECTS ON A SINGLE-CRYSTAL METAL CASTING AND SYSTEM FOR IMPLEMENTING THEM

Abstract

One aspect of the invention relates to a method (100) for inspecting the surface finish of a cast part (20) made of single-crystal metal, the surface of the part potentially containing defects (G1, G2, G3, G4) resulting from an inhomogeneity of orientation of at least a crystal lattice of the single-crystal metal, said method comprising: - acquiring (110, 120), using an image-acquiring device (40), a series of images (130) of the cast part illuminated by means of a polarized and collimated illuminating device (30), then - analysing (140-180) the series of images (130) by means of an image-processing device (50), each image of the series of images (130) being taken at a different polarization angle. Another aspect of the invention relates to a system for implementing the inspecting method, comprising a polarized and collimated illuminating device (30), an image-acquiring device (40), and an image-processing device (50).

Description

DESCRIPTION Method for controlling surface defects of a single-crystal metal casting and system for implementing the same TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to a method for controlling the surface condition of a single-crystal metal casting, the surface of the part being likely to contain defects resulting from an inhomogeneity in the orientation of at least one crystal lattice of the single-crystal metal. The invention also relates to a system for implementing this control method.

[0002] The invention finds applications in the fields of casting of monocrystalline metal or alloy parts and, in particular, in the field of manufacturing of metal or metal alloy casting parts for aeronautics. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0003] Casting allows the production of complex metal parts, particularly aeronautical parts such as high-pressure engine turbine blades. Cast parts can be made from a single-crystal metal or alloy. These single-crystal metal or alloy parts must then be made of crystal lattices with a uniform orientation. In other words, the orientation of the crystal lattices must be identical for all, or at least a very large majority, of the crystal lattices in the part.

[0004] In aeronautics, the surface condition of each single-crystal metal or alloy casting is checked to ensure that the crystal orientation is uniform across the entire surface of the part. Indeed, when the crystal lattices of a part have different orientations, cracks or other defects can occur, particularly at the junction of the crystal lattices, and cause serious damage to the part. To avoid this damage, each aeronautical part is checked and the presence of grain boundaries, i.e. a boundary between two crystal lattices of different orientations, is detected. The level of non-homogeneity of the crystal orientation, i.e. the number of crystal lattices of different orientations, is determined. If this level exceeds a predefined acceptability threshold, the part is scrapped.

[0005] A reference technique in metallurgy for controlling the surface condition of parts in crystalline orientation uses an electron backscatter diffraction method (EBSD for Electron Backscatter Diffraction or BKD for Backscatter Kikuchi Diffraction). This method not only uses very expensive instruments, but also involves significant preparation of the measurement sample, which must be flat and polished. In addition, the surface to be analyzed by scanning is relatively small (of the order of mm 2 or cm 2 ) and electron beam scanning measurement is time-consuming and not compatible with rapid sorting of production parts.

[0006] Other optical methods have been implemented more recently, such as BRDF (Bidirectional Reflectance Distribution Function) testing. This BRDF testing is a method used by extending the empirical experience of checks that were previously carried out by operators with the naked eye. This method is based on the fact that after a chemical attack on the part, the presence of different grains visually produces a variation in light contrast on the surface of the part, thanks to a variation in the reflectance of the part. Previously, an operator would look for these variations in light contrast on the surface of the part with the naked eye. To do this, he would tilt the part at different angles in order to see differences in reflectivity on the surface of the part.Even for a trained and experienced operator, these inspection operations were delicate and required great concentration from the inspector, because any failure by the operator could lead to a critical decision on his part, with either the acceptance of a part whose condition did not reach the acceptability threshold, or the scrapping of an acceptable part. Compared to the empirical method, BRDF inspection makes the work of operators easier by implementing an optical inspection bench to measure the reflectance function of the part and to deduce the geometry of its crystalline orientation.A controlled angular displacement of the illumination and measuring instrument is necessary as described in the articles “Measuring crystal orientation from etched surfaces via directional reflectance microscopy” by Wang Xiaogang, in J Mater Sci (2020) 55:11669-11678 and “Optical characterization of grain orientation in crystalline materials” by Bernard Gaskey, in Acta Materialia 194 (2020) 558-564).

[0007] Surface condition control techniques of parts based on the electron backscattered diffraction method (such as EBSD) are difficult to can be integrated into a production line and within the framework of complex mechanical parts. Optical techniques have the advantage of offering more versatility for measurements in a complex environment and in non-destructive testing, but they are not well suited to the industrial framework.

[0008] There is therefore a need for an improved automated control process and device to assist operators in their task of controlling the surface condition of single-crystal metal or alloy parts. SUMMARY OF THE INVENTION

[0009] To address the problems mentioned above and to help operators control the surface condition of parts from foundries, the applicant proposes a method and a system for controlling the surface condition of parts made of monocrystalline metal or alloy, based on an analysis of a series of images produced for different orientations of the polarization of the light reflected by the surface of the part.

[0010] According to a first aspect, the invention relates to a method for controlling the surface condition of a single-crystal metal casting, the surface of the casting comprising possible defects resulting from an inhomogeneity of orientation of at least one crystal lattice of the single-crystal metal, said method comprising: the acquisition, by an image acquisition device, of a series of images of the casting illuminated by means of a polarized and collimated lighting device, then the analysis of the series of images by means of an image processing device, this analysis comprising the following operations: determination, for each pixel of the image acquisition device, of an intensity vector corresponding to a variation in intensity of the same pixel on each image of the series of images,determination of an average intensity of all the pixels of each image of the series of images and extraction of a general law of variation of the intensity for the series of images, determination of an individual law of variation of the intensity, specific to each crystal lattice, and determination of the pixels belonging to the same crystal lattice, each image in the image series being taken for a different polarization angle.

[0011] The polarized lighting used in this process enhances the reflectance contrast on the surface of the part. In addition, analyzing the series of images taken with different polarization angles allows for the generation of an image of the part highlighting the different grains. This process also has the advantage of being easily automated.

[0012] It will be understood that the expression "monocrystalline metal" used in the description and the claims includes all metals and alloys of monocrystalline type. The foundry parts to be controlled with the method and / or the system according to the invention are parts made of monocrystalline metal or monocrystalline alloy whose crystal lattices must have a homogeneous orientation.

[0013] The terms "grains" or "disoriented crystal meshes" or "crystal meshes of different orientations", used in the description and the claims have an identical meaning, a grain being a crystal mesh whose orientation does not conform to the expected orientation. A grain boundary is therefore a boundary between two crystal meshes of different orientations.

[0014] In addition to the characteristics which have just been mentioned in the preceding paragraph, the control method according to one aspect of the invention may have one or more complementary characteristics among the following, considered individually or according to all technically possible combinations: the polarization angle is modified by rotation of a polarization axis of the image acquisition device. the operation of determining the pixels belonging to the same crystal lattice is obtained by pairing the pixels having close intensity variations. the operation of determining an individual intensity variation law is obtained by normalizing the intensity of each pixel according to the general intensity variation law. the lighting device generates a beam of incoherent, linearly polarized light.The process involves a preliminary operation of chemical attack on the casting, bringing out the crystals on the surface of the casting.

[0015] A second aspect of the invention relates to a system for implementing the control method defined above, this system comprising: a polarized and collimated lighting device, an image acquisition device, and an image processing device.

[0016] The combination of a fixed lighting device and a fixed camera makes it possible to highlight the differences in reflectance on the surface of the part by simply rotating a polarizer in front of the camera, without relative movement of the part and the lighting and without problems of shadowing or depth of field for curved parts.

[0017] This control system according to a second aspect of the invention may have one or more complementary characteristics among the following, considered individually or according to all technically possible combinations: the lighting device comprises an incoherent light source, coupled to a first linear polarizer and to a collimation optic; the image acquisition device comprises a fixed camera, coupled to a second rotating mobile polarizer, each image of the series of images corresponding to a different setting of a polarization axis of the second polarizer. the image processing device comprises an image display device displaying at least one image of the foundry part with a location of the disoriented crystalline meshes. BRIEF DESCRIPTION OF THE FIGURES

[0018] Other advantages and characteristics of the invention will appear on reading the following description, illustrated by the figures in which:

[0019] Figure 1 schematically represents an example of a system for controlling the surface condition of a part according to the invention;

[0020] Figure 2 represents, in functional form, an example of the operations of the method for controlling the surface condition of a part, according to the invention;

[0021] Figure 3 shows examples of three images from a series of images taken for several different polarization angles;

[0022] Figure 4 represents an example of the average intensity of the pixels of an image of the series of images, determined in the method of Figure 2; and

[0023] Figure 5 represents an example of the individual intensity variation corresponding to three different grains, after analysis by the method of Figure 2. DETAILED DESCRIPTION

[0024] An exemplary embodiment of a method and system for controlling the surface condition of foundry parts, configured to automatically determine an inhomogeneity in the orientation of the crystal meshes, is described in detail below, with reference to the appended drawings. This example illustrates the characteristics and advantages of the invention. It is however recalled that the invention is not limited to this example.

[0025] In the figures, identical elements are identified by identical references. For reasons of readability of the figures, the size scales between the elements represented are not respected.

[0026] An example of a system 10 for controlling the surface condition of parts 20 made of monocrystalline metal from foundry is shown in FIG. 1. This system 10 comprises a lighting device 30, an image acquisition device 40 and an image processing device 50. The lighting device 30 is designed to emit incoherent (or non-coherent), polarized and collimated light. Indeed, the incident light beam 34 (also called incident beam) must be, on the one hand, linearly polarized (or electrically transverse) so that only the components of the incident beam parallel to the polarization axis are transmitted and, on the other hand, collimated so that the part 20 receives uniform illumination.Uniform illumination of the room 20 allows each area of ​​the surface of said room to receive the same light intensity, which makes it possible to detect any light deviations caused by disoriented crystal meshes, as explained below.

[0027] For this, in the example of Figure 1, the lighting device 30 comprises a non-coherent light source 31, that is to say one which does not produce interference. The lighting can be monochromatic or polychromatic because, as the part 20 is a metal part, its surface is reflective over a wide frequency band in the visible. The light source 31 can be a traditional light source, an LED type light source, a remote or non-remote fiber light source, or any other non-coherent light source.

[0028] The lighting device 30 also comprises a collimation optic 32, such as a collimation lens, and a linear polarizer 33, called the first polarizer, both coupled to the light source 31 and aligned along the lighting axis X. The order of positioning of the collimation optic 32 and the first polarizer 33, behind the light source 31, is of little importance since the incident beam 34 emitted by the lighting device 30 towards the part 20 is both polarized and collimated.

[0029] The first polarizer 33 is a linear transmission polarizer. Different technologies for constructing such a linear polarizer can be envisaged: for example a dichroic film, a microgrid or prolate nanoparticles. In the invention, the first polarizer 33 is preferably chosen to be weakly diffusing and with a maximum extinction rate at the wavelength considered. The polarization axis of the first polarizer 33, called the incident polarization axis, is fixed for the entire duration of the inspection of the part 20. The first polarizer 33 can therefore be fixed. According to a variant, the first polarizer 33 can be mounted on a rotating mount, which makes it possible to choose the incident polarization axis and, possibly, to modify it for the inspection of another part 20. The incident polarization axis is preferably chosen so that the incident light beam 34 is electrically transverse, that is to say perpendicular to the plane of incidence.In the invention, the reflecting surface being a metallic surface, it is considered that said surface is a perfect conductor. The reflection 44 of the incident beam 34 is therefore done without loss of energy and the angle of reflection obeys the laws of Snell Descartes.

[0030] In the embodiment shown in Figure 1, the light source 31 and the first polarizer 33 are two separate elements, aligned with each other along the illumination axis X. In another embodiment, the light source 31 is a polarized source which integrates the first polarizer. The polarized light source is then preferably rotatable so that the operator can position said light source along the chosen incident polarization axis.

[0031] The incident light beam 34 which illuminates the part 20 is linearly polarized so that the reflection coefficient of said part, called the Fresnel reflection coefficient, depends both on the angle of incidence and on the polarization state of the incident light. The optical response of the surface of the part 20, that is to say the reflected light beam 44 (also called the reflected beam), comprises two major components: a weakly dispersive RS specular reflection which comes from the average surface state of the part 20 (with random roughness). This reflection is predominant in intensity and keeps the same polarization axis orientation as the incident light. an RC reflection coming from the oriented faces of the crystal lattices. This reflection, produced by said oriented faces, can be seen as the common reflection of a multitude of micro-mirrors oriented in the same direction.

[0032] The optical response of the surface of the part 20 is detected by the image acquisition device 40 and analyzed by the image processing device 50. The image acquisition device 40 comprises a camera 41 equipped with a lens 42 and coupled to a polarizer 43, called a second polarizer, mounted in front of the lens 42 of the camera. The camera 41 may be, for example, a CCD or CMOS type camera in the visible range. The lens 42 of the camera is an optical lens whose characteristics are adapted, in field of vision and working distance, to the size of the area to be imaged of the part 20. This lens 42 may be, for example, of the telecentric type so as to promote a large depth of field. The second polarizer 43, aligned with the camera 41 and its lens 42 along the reflection axis Y, is of the same type as the first polarizer 33, i.e. of the linear type. It may even be identical to the first polarizer.

[0033] According to the invention, the second polarizer 43 is rotatable so that its polarization axis can be modified for each image acquisition so as to obtain a series of images each corresponding to a different polarization angle. For this, the second polarizer 43 can be fixed on a rotating mount, not shown in the figure, so that its polarization axis, called the reflected polarization axis, can be rotated. The rotating mount can be manipulated manually by an operator. It can, advantageously, be motorized so as to rotate automatically. The rotation of the second polarizer 43 makes it possible to acquire, by means of the camera 41, several images with different polarization angles. If the axis of the second polarizer 43 is oriented in the same direction as the axis of the reflected polarization, then the light intensity transmitted in the camera is maximum.Conversely, if the axis of the second polarizer 43 is rotated by 90° (so-called crossed polarizer configuration), the transmission of light intensity is minimal. Between these two angles, the transmission of light intensity follows Malus's law:. hrans hnc cos2 d, where Q is the angle formed between the axes of the two polarizers 33 and 43.

[0034] In one embodiment, the camera 41 is a polarimetric camera that directly integrates a polarizer. In such a polarimetric camera, the pixels of the camera sensor are equipped with polarizers and are therefore sensitive to the polarization axis and the intensity of the reflected beam 44 on the sensor. In one acquisition, it is then possible to obtain the same images as by rotating the polarization axis of the second polarizer 43, the difference being that the number of images is limited to the number of predefined polarization angles, for example four angles at 0°, 45°, 90° and 135°.

[0035] The image processing device 50 is a processing unit, such as for example a computer, adapted to carry out the image processing operations described below. It is connected by wired or wireless connection to the camera 41 in order to receive the series of images acquired by the camera 41. This image processing device 50 comprises at least one image display device, such as a screen or a printer, to display the result of the inspection of the part 20.

[0036] The control system 10 as just described makes it possible to implement cross-polarization between the illumination device 30 and the image acquisition device 40. With this cross-polarization, each grain corresponding to a particular crystalline orientation presents, after reflection on the surface of the part 20 and transmission in the second polarizer 43, a particular optical intensity response which depends on the chosen position of the incident polarization axis and the adjustment of the reflected polarization axis. Indeed, it is accepted that the facets of each crystalline mesh orient the polarization axis of the reflected light differently, as described in the article entitled "Correlation of Polarized Light Phenomena With the Orientation of Some Metal Crystals", by C.J. Newton and H.C. Vacher, in Journal of Research of the National Bureau of Standards Vol. 53, No. 1, July 1954.In other words, each crystal lattice disorientation propagates the light beam in the camera 41 with a reflection coefficient which is specific to it, because it depends on the polarization direction of the incident beam. This principle makes it possible to distinguish the different grains making up the surface of the part 20.

[0037] The optical response of the different grains of the surface of the part 20 is analyzed by means of steps 140 to 180 of the method for controlling the surface condition of the part 20. An example of this control method 100 is represented functionally in FIG. 2. As understood from the above, the first operations of the control method 100 consist of generating a series 130 of images of the surface of the part 20 or of the area of ​​the part 20 to be controlled. Indeed, depending on the dimensions of the part 20 to be controlled, the image acquisition device 40 can produce images of the entire surface of the part 20 or of an area of ​​this surface. For example, for a turbine blade, the dimensions of which are relatively large, the control of the surface condition of the blade can be carried out area by area, each area being the subject of a series of images analyzed as explained below, each series of images being analyzed one after the other. In the remainder of the description, the control method 100 will be described for images of the entire surface of the part, it being understood that the images can relate to only an area of ​​the surface of the part.

[0038] The series 130 of images of the surface of the part 20, more simply called images of the part, is produced by means of the image acquisition device 40 by producing an image of the part for each polarization angle and by modifying the polarization angle before each image capture. The term "polarization angle" refers to the angle between the polarization axis of the first polarizer 33 and the polarization axis of the second polarizer 43. In other words, before each image capture, the second polarizer 43 is set to a position different from its previous position in order to modify the analysis polarization axis and, consequently, the polarization angle. Thus, the control method 100 comprises a first operation 110 of adjusting the polarization angle, by rotating the polarization axis of the second polarizer 43. It then comprises an operation 120 of acquiring, by the camera 41, an image of the part 20 for the adjusted polarization angle.Several images are thus produced. By looping the image acquisition operation 120 and the polarization angle adjustment operation 110, a series 130 of images of the part is obtained. This series 130 of images of the part 20, consisting of several images of the same part 20 under several polarization angles, is then analyzed by means of the operations 140 to 180 of the method 100 in order to determine the presence of grains, that is to say of crystalline meshes having orientations different from that of most of the crystalline meshes of the surface of the part 20. The series of images 130 comprises several images, for example, one or several tens of images, the number of images having to be sufficient to allow the construction of a substantially sinusoidal curve, as explained below. In a practical example, the rotation of the polarization axis of the second polarizer 43 can be modified by one step. of 10° in an interval between 0° and 360°. It should be noted that the acquisition of images for polarization angles between 0° and 180° are in theory identical to those for angles between 180° and 360° due to the periodicity of rotation of the axis of the second polarizer 43; the acquisition of images for polarization angles between 0° and 180° may therefore be sufficient; however it may be chosen to also acquire images over the interval between 180° and 360°, for example to increase the signal-to-noise ratio of the data determined in the remainder of the method 100 and / or if the incident polarization axis is not perfectly oriented perpendicular to the plane of incidence.

[0039] To bring out the crystals on the surface of the part 20 and thus facilitate the analysis, the control method of the invention may include a preliminary operation, not shown in the figures, of chemical attack on the surface of the part 20. In fact, the chemical attack has the effect of slightly hollowing out the material on the surface of the part 20 in order to reveal the crystalline meshes flush with the surface. This chemical attack is carried out before the series of images is produced (operations 110 to 130).

[0040] An example of a series of three images is shown in Figure 3. These three images represent the same portion of turbine blade, after chemical etching, for three positions of the polarization axis of the second polarizer 43 and therefore three different polarization angles. Image A corresponds to the blade portion for a polarization angle of 0°; image B shows the same blade portion for a polarization angle of 45°; image C always shows the same blade portion for a polarization angle of 90°. It is clear from these three images A, B and C that there are several portions of the part 20 where the crystalline orientations differ. These portions of the part 20, referenced G1, G2, G3 and G4, correspond to surface defects of the part 20.

[0041] After the series of images 130 has been produced, the control method 100 comprises operations for analyzing this series of images. This analysis firstly comprises an operation 140 for determining an intensity vector, for each pixel of the sensor of the camera 41. Since all the images of the series of images 130 correspond to the same area of ​​the part 20 and the part 20 is not moved, the images of the series of images 130 can be superimposed on each other; it is then possible to look at the evolution, that is to say the variation, of the gray levels of the same pixel across the series of images. Indeed, the same pixel has a level of different gray on each image of the series of images 130. The variation of this gray level of the pixel across the different images of the series of images 130 forms an intensity vector of said pixel.

[0042] The method 100 then comprises an operation 150 of determining a normalized average intensity of all the pixels of each image of the series of images 130. This operation 150 consists of calculating the average of the light intensity of all the pixels of the same image and deducing therefrom, in operation 160, a general variation law for the series of images 130. This general variation law corresponds to the so-called “Malus” law which is the law relating to the transmission, through the second polarizer 43, of the light beam 44 reflected by the part 20 and corresponding to the average roughness of the surface of said part 20. The intensity from the oriented faces of the crystal lattices is the majority in the total signal because the part 20 is not a pure crystal lattice. Figure 4 represents, in the form of a generally sinusoidal curve, an example of the general variation law obtained at the end of operation 160.This curve in Figure 4 shows an example of normalized average intensity, transmitted by the second polarizer 43, for several polarization angles between 0° and 360°, the maximum intensity being obtained when the first and second polarizers are in phase, the minimum intensity being obtained when the first and second polarizers are crossed.

[0043] The method 100 then comprises an operation 170 for determining an individual variation law of the intensity, that is to say a variation law specific to each crystal lattice. For this, the operation 170 consists, for each image of the series of images, in normalizing the intensity of each pixel of the image between 0 and 1, by dividing this intensity by the value of the average intensity of the corresponding image. This amounts to dividing the intensity vector of each pixel of the series of images by the general variation law, or penalty law. This operation 170 makes it possible to eliminate the variation component of the specular reflection of the surface of the part 20 and to enhance the variations in intensity due to the reflections on the facets of the crystal lattice.Indeed, as explained previously, it is accepted that the facets of a crystal lattice orient the polarization axis of the reflected light differently so that each grain corresponding to a particular orientation of the crystal lattice has a specific intensity variation depending on the position of the rotation axis of the second polarizer 43. Thus, at the end of operation 170, the law. of variation specific to each crystal lattice (called the law of individual variation), is determined.

[0044] An example of the intensity variations of pixels belonging to three different grains is shown in Figure 5, these variations being determined by means of the different operations of the control method 100 described previously. In particular, curve C1 corresponds to the intensity variation of the pixels of a first grain, curve C2 corresponds to the intensity variation of the pixels of a second grain and curve C3 corresponds to the intensity variation of the pixels of a third grain. These three variation curves, which are very different from each other, show that the crystal meshes corresponding to these curves have different orientations. If the orientation of these crystal meshes were homogeneous, the three curves would be substantially parallel.It should be noted that these differences in intensity variations are notable due to the normalization of the pixel intensity; it is the normalization operation which makes the intensity variations caused by the disorientation of the crystal meshes perceptible.

[0045] The control method 100 then comprises an operation 180 of determining the pixels belonging to the same crystal lattice. This operation 180 consists of pairing the pixels having close intensity variations. Indeed, pixels having close intensity variations, that is to say similar or strongly correlated, belong to the same grain or have the same crystal orientation. The pairing of the pixels is carried out by scanning all the pixels of the series of images and by verifying, by a similarity method, whether the intensity vectors of two pixels have similarities. If the similarity measure between the two intensity vectors is greater than a predetermined threshold, then the two pixels are considered to be paired. On the contrary, if the similarity measure between the two intensity vectors is less than the predetermined threshold, then the two pixels are considered not to belong to the same crystal lattice.Several similarity methods can be used, such as correlation, the so-called "squared intensity differences" method, the so-called "absolute intensity differences" method, or the so-called "mean absolute difference" method. The segmentation of the different grains in the image is then deduced from this matching operation.

[0046] The control method 100 finally comprises an operation 190 of generating an image of the part 20 on which the disoriented crystalline meshes are identified, for example by an outline of the mesh, a box, etc. An example of such an image is shown in Figure 5 where the three grains, or disoriented crystal meshes, are surrounded by a rectangular frame.

[0047] The control method 100 can be coupled with a conventional technique for multidirectional reflectance measurement, as described in particular in the articles “Measuring crystal orientation from etched surfaces via directional reflectance microscopy” by Wang Xiaogang, in J Mater Sci (2020) 55:11669-11678 and “Optical characterization of grain orientation in crystalline materials” by Bernard Gaskey, in Acta Materialia 194 (2020) 558-564. Coupling the method of the invention with this conventional technique would make it possible to minimize the number of lighting angles and improve the signal-to-noise ratio on weak signals.

[0048] Although described through a certain number of examples, variants and embodiments, the method according to the invention for controlling the surface condition of a foundry part and the system for implementing this method include various variants, modifications and improvements which will be obvious to those skilled in the art, it being understood that these variants, modifications and improvements are part of the scope of the invention.

Claims

DEMANDS

1. A method (100) for inspecting the surface condition of a single-crystal metal casting (20), the surface of the casting having possible defects (G1, G2, G3, G4) resulting from an inhomogeneity of orientation of at least one crystal lattice of the single-crystal metal, said method comprising: - the acquisition (110, 120), by an image acquisition device (40), of a series of images (130) of the cast part illuminated by means of a polarized and collimated lighting device (30), then - the analysis (140-180) of the series of images (130) by means of an image processing device (50), this analysis comprising the following operations: o determination (140), for each pixel of the image acquisition device, of an intensity vector corresponding to a variation in intensity of the same pixel on each image of the series of images, o determination (150) of an average intensity of all the pixels of each image of the series of images and extraction (160) of a general variation law of intensity for the series of images, o determination (170) of an individual variation law of intensity, specific to each crystal lattice, and o determination (180) of the pixels belonging to the same crystal lattice, each image of the series of images (130) being made for a different polarization angle.

2. Control method according to claim 1, characterized in that the polarization angle is modified by rotation of a polarization axis of the image acquisition device (40).

3. Control method according to claim 1 or 2, characterized in that the operation (180) of determining the pixels belonging to the same crystal lattice is obtained by pairing pixels having similar intensity variations.

4. A control method according to any one of claims 1 to 3, characterized in that the operation (170) of determining an individual intensity variation law is obtained by normalizing the intensity of each pixel according to the general intensity variation law.

5. A control method according to any one of claims 1 to 4, characterized in that the lighting device (30) generates a beam of incoherent, linearly polarized light.

6. A testing method according to any one of claims 1 to 5, characterized in that it comprises a preliminary chemical etching operation on the casting, causing the crystals on the surface of the casting to be brought out.

7. A system for implementing the testing method according to any one of claims 1 to 6, characterized in that it comprises: - a polarized and collimated lighting device (30), - an image acquisition device (40), and - an image processing device (50).

8. A control system according to claim 7, characterized in that the lighting device (30) comprises an incoherent light source (31), coupled to a first linear polarizer (33) and a collimation optic (32).

9. A control system according to claim 7 or 8, characterized in that the image acquisition device (40) comprises a fixed camera (41, 42), coupled to a second rotating polarizer (43), each image in the image series (130) corresponding to a different setting of a polarization axis of the second polarizer (43).

10. Control system according to any one of claims 7 to 9, characterized in that the image processing device (50) comprises an image display device displaying at least one image of the casting part (20) with a tracking of the disoriented crystalline lattices.