FR3127575A1 - Process for characterizing the microstructure of a green part produced by printing by binder projection on a powder bed - Google Patents

Abstract

Method for characterizing the microstructure of a green part produced by printing by projection of binder on a bed of powder Method comprising a) the acquisition of at least one and at most twenty radiographic images of a green part (12) produced by printing by binder projection on a powder bed, by emitting X-rays along an emission axis substantially parallel or substantially perpendicular to the manufacturing axis (W) of the green part, andb) determining the presence or the absence in the green part of irreducible defects by sintering (24), based solely on the radiographic image or images. Figure for abstract: 2

Description

Process for characterizing the microstructure of a green part produced by printing by binder projection on a powder bed

The present invention relates to the production of parts by additive manufacturing comprising printing by binder projection on a powder bed and sintering.

Binder jetting printing on a powder bed, also called "Binder Jetting" in English, is a common technique in the field of additive manufacturing. It is for example presented in the review of Engineering Techniques (https://www.techniques-ingenieur.fr/actualite/articles/le-metal-binder-jetting-une-technologie-qui-monte-69852/) . Binder projection printing on a powder bed consists of repeating a cycle comprising the deposition of a powder 1 to form a layer 2 by spreading the powder on a plate 4 by means of a spreading device 5 of axis U, generally a roller or a doctor blade, moved along an advancing axis V, the projection of a liquid binder 6 by means of printing nozzles 8 on predetermined zones 10 of the layer, and the displacement P of the plate along the manufacturing axis W, generally vertical. In this way, a preform 12 is printed in which the particles of the zone of a lower layer 14 covered with the binder are linked to the particles of a superimposed zone of the upper layer 15. Such a preform is called “green part”.

In an additive manufacturing process comprising printing by binder projection on a powder bed, the green part, which has low mechanical strength, is friable. It must then be debinded, sintered, then cooled, in order to be consolidated.

Sintering causes dimensional shrinkage and densification of the green part. In particular, it makes it possible to reduce in size, or even eliminate pores formed during the printing stages by binder projection. However, after sintering, large pores, which may exceed 100 μm, may remain, which result from the formation of defects during the process. For example, they result from poor spreading of the powder layer, for example due to punctual defects on the doctor blade or roller, poor binder projection or excessive layer height. Such sintered parts are defective because the large pores reduce their mechanical properties.

In order to avoid the sintering of a defective green part, that is to say which results after heat treatment in a defective sintered part, the measurement of the density and/or the observation of the microstructure of the green part s prove necessary.

It is for example known to carry out metallographic observations on a section of the green part. However, this technique only allows one section of the piece to be observed. It is also long to implement and is destructive.

It is also known to determine the density of the green piece by Archimedean thrust, pycnometry or dimensional analysis. However, these techniques do not make it possible to determine the type of existing defects, and in particular their potential link with the parameters of implementation of the process of printing by projection of binder on bed of powder.

X-ray tomography is the most suitable technique for characterizing the type and density of defects present in a green part. However, it requires specific and expensive equipment. In addition, it requires the acquisition of at least 500, and more generally between 900 and 1800 radiographic images for metallic green parts in order to build a 3D tomographic model representing the part with satisfactory resolution. The acquisition of such a number of images results in a high implementation time which makes tomography incompatible for the production of parts on an industrial scale, especially in large series.

There is therefore a need for a method for characterizing the microstructure of a green part produced by printing by spraying binder on a powder bed, which can be easily implemented in a printing and sintering cycle on an industrial scale. .

The invention relates to a method comprising
a) the acquisition of at least one and at most twenty radiographic images of a green part produced by printing by projection of binder on a bed of powder, by emitting X-rays along an emission axis substantially parallel or substantially perpendicular to the manufacturing axis of the green part, and
b) the determination of the presence or the absence in the green part of irreducible defects by sintering, from only the radiographic image or images.

As will appear in more detail subsequently, the invention makes it possible to determine simply and more quickly than with a tomographic acquisition and reconstruction method whether the green part will result in a defective sintered part, and if so, the invention makes it possible to avoid this sintering. In particular, the invention avoids the acquisition of a large number of images and the construction of a 3D model from these images as is implemented by an X-ray tomography process. feedback on the printing parameters to avoid the formation of irreducible defects in the green parts produced later.

A defect, for example a pore, "irreducible by sintering" does not disappear under the effect of sintering or has a size after sintering greater than a critical size. The critical size is dependent on the application in which the sintered part is consolidated. It may be greater than 50 μm, greater than 100 μm, or even greater than 200 μm.

The "manufacturing axis" of a green part obtained by printing by binder projection on a powder bed is the axis of movement, generally vertical, of the support on which the powder is spread to form layers to be linked together with the binder, following the projection of the binder.

The radiographic image or images are preferably digital, which facilitates their processing.

X-ray radiation consists of a set of rays forming a beam. The rays can be parallel to each other or form a cone whose vertex is the source of emission of the radiation.

The axis of emission of X-radiation "substantially parallel" to the manufacturing axis of the green part is such that each ray of the X-ray beam intersects a straight line parallel to said manufacturing axis and forms with said straight line an angle less than or equal to 10°, preferably less than or equal to 5°.

The axis of emission of X-radiation "substantially perpendicular" to the manufacturing axis of the green part is such that each ray of the X-ray beam intersects a straight line parallel to said manufacturing axis and forms with said straight line an angle between 80° and 100°, preferably between 85° and 95°, preferably equal to 90°.

Preferably, in step a), at most fifteen, or even at most ten, or even at most five, or even at most three radiographic images are acquired.

In particular, in step a), the method may comprise the acquisition of at least one, in particular a single, radiographic image of the green part by emitting X-rays along an emission axis substantially parallel to the manufacturing axis of the green part. Thus, in step b), it is possible to determine the presence of irreducible defects linked to poor spreading of the powder or to a defect on the spreading device. For example, such irreducible defects are encountered when the spreading device has a surface geometric defect. During the formation of the layer, the particles are thus distributed in a heterogeneous manner along the axis of advance V of the spreading device according to a given position along the axis U. This heterogeneous distribution is repeated layer after layer. Furthermore, according to this implementation variant, it is also possible to observe irreducible defects resulting from an imperfect design of the mechanical system governing the advance of the spreading device along the axis V, or from the heterogeneity the density of the deposited powder bed.

The method may include the acquisition of at least one radiographic image of the green part by emitting X-rays along an emission axis substantially perpendicular to the manufacturing axis of the green part. Thus, in step b), it is possible to determine the presence of irreducible defects linked to an unsuitable choice of the layer height parameter chosen to control the manufacture of the green part. When the thickness of the deposited layer is too great in relation to the depth of penetration of the binder into the layer, a band of irreducible defects can be formed, the height of which is equal to the difference between the thickness of the layer and the depth binder penetration. Such a tape can be reproduced periodically layer after layer.

In particular, to improve the detection of irreducible defects linked to an unsuitable choice of the layer height parameter, the method may comprise the acquisition of at least two, or even only two, radiographic images of the green part by emitting radiation X along two emission axes, preferably mutually perpendicular and substantially perpendicular to the manufacturing axis of the green part.

Preferably, the method comprises the acquisition of at least one, in particular a single, radiographic image of the green part by emitting X-rays along an emission axis substantially parallel to the manufacturing axis of the part green and/or the acquisition of at least one, or even a single, radiographic image of the green part by emitting X-rays along an emission axis substantially perpendicular to the manufacturing axis of the green part.

Preferably, the acquisition of a radiographic image comprises the detection of the X-radiation transmitted by the green part by means of a detector, the digital processing of the X-radiation transmitted and detected to form the radiographic image and the recording of the x-ray image on a digital recording medium. Alternatively, the acquisition may include the formation of the radiographic image on the detector which is a photographic film sensitive to X-rays.

Preferably, the process comprises, after step b), the sintering of the green part in order to obtain a consolidated part when the irreducible defects are absent.

The process may include, after step b), the recycling of the green part, in particular the separation of the binder and the particles bound by the binder, when the irreducible defects are present. The industrial yield of a production of sintered parts is thus improved. In particular, a new green part of identical shape can be produced from the recycled powder. The process can therefore include the production of a new green part from the recycled particles.

Furthermore, due to printing by binder projection, the green part undergoes significant shrinkage during its subsequent sintering in the rest of the additive manufacturing process. To avoid damage to the part during sintering under the effect of shrinkage, the green part preferably has a diameter, measured along the emission axis, of less than or equal to 20 cm, or even less than or equal to 10 cm, or even less than or equal to 5 cm. In addition, such a small diameter facilitates the transmission of X-rays and the obtaining of a radiographic image in which irreducible defects can be easily observed. The "diameter" of a coin is the diameter of the smallest sphere circumscribed to the coin.

The green part can comprise particles made of at least one metal, preferably of an alloy, or of a ceramic material or of a thermoplastic material. The particles have for example a median size of between 5 μm and 40 μm.

In step b), the determination of the presence or absence in the green part of irreducible defects by sintering can be carried out in different ways.

In particular, said presence or absence can be determined by visual observation of the radiographic image or images. A person skilled in the art knows how to easily distinguish such defects on a radiographic image, and in particular to differentiate them from variations in contrast linked to the geometry of the part.

The analysis of the radiographic image(s) may include a comparison of the radiographic image(s) with a standard radiographic image acquired on a green part of identical shape and free of irreducible defects. The comparison can be visual or can be done by subtracting the digital x-ray images.

The method may comprise, prior to step b) of determination, a digital processing comprising at least one processing operation chosen from among a change in the dynamic range of the image, for example a modification of the contrast, a noise reduction, a spatial filtering and their combinations. Such digital processing improves the detection, in particular visual, of irreducible defects.

In one mode of implementation, the analysis of the radiographic image may comprise the calculation of a digital theoretical image from a digital 3D model of the green part, and the comparison, for example by subtraction of the radiographic image with the theoretical image. Such a calculation can be made by calculating the attenuation of the intensity of X-rays passing through the 3D digital model of the green room, for example by means of the Beer-Lambert relationship. The 3D digital model of the green part is for example a CAD model, for example obtained by computer-aided design software such as SolidWorks®.

In one mode of implementation, the radiographic images are at least ten in number and distributed according to a linear or circular trajectory to construct, by means of a laminographic technique, focal planes parallel to the plane of the detector. These focus planes can be aligned with the (U,V), (V,W) or (U,W) axes to get a sharp representation of the defects present in that plane of the green part.

The invention also relates to an installation for implementing the method according to the invention, comprising
- a 3D printer for the production of a green part by printing by binder projection on a powder bed,
- an X-ray apparatus comprising an X-ray emission source for acquiring an X-ray image of a green part, and
- an oven to sinter the green part.

The invention will be better understood on reading the detailed description and the non-limiting examples which follow and the appended drawing in which:

schematically illustrates the production of a green part by printing by binder projection on a powder bed,

illustrates in a side view the acquisition of a radiographic image according to an example of implementation of the method according to the invention,

is a) a radiographic image of a green part acquired during an example of implementation of the method according to the invention with an axis of emission of the X-radiation substantially parallel to the axis W of manufacture, b) an enlargement a part of this radiographic image and, for comparison, c) a cross-sectional view of a tomographic reconstruction of the green part,

a) and b) are x-ray images of the green part of the acquired with an emission axis forming an angle of 30° and 45° respectively with the manufacturing axis of the green part,

illustrates in a side view the acquisition of a radiographic image according to another example of implementation of the method according to the invention,

is a) a radiographic image of a standard green part and b) a radiographic image of a green part comprising irreducible defects by sintering, the emission axes of the X-rays being in both cases substantially perpendicular to the axis W Manufacturing,

are a) and b) cross-sectional views of tomographic reconstructions of the green part of Figure 6a) and the green part of Figure 6b) respectively, the cross-section corresponding to a section parallel to the manufacturing axis, and

a) and b) are radiographic images of the green part illustrated in FIG. 6b) acquired with an emission axis forming an angle of 55° and 45° respectively with the manufacturing axis of the green part.

The invention requires the acquisition of a limited number of radiographic images by emitting X-rays along a specific emission axis.

The inventors have in fact observed that the presence or absence of irreducible defects can be easily observed on such radiographic images and not on radiographic images acquired according to other directions of observation.

As illustrated on the , the acquisition of a radiographic image requires the emission of X-rays, represented by the reference R, by means of a source 14. The X-rays, whose emitted intensity is known, passes through the green part 12 and reaches a detector 16. At each point of the detector, the intensity of the X-radiation, attenuated by its path through the green room, is processed and recorded in the form of a gray level to form a digital radiographic image.

According to an example of implementation of the method, the axis of emission of the X-radiation is substantially parallel to the manufacturing axis W of the green part 12. As shown in the , the source emits the X-radiation according to a cone 18. The X-radiation is formed of a plurality of rays forming a beam 20. Each ray forms, with a straight line D parallel to the manufacturing axis W, an angle α less than 10°, preferably less than or equal to 5°.

Figure 3a) is a radiographic image of a metallic green part obtained with the method according to the invention and an emission of X-rays as illustrated in the . The contrast of the radiographic image has been enhanced to better show variations in gray level. It is thus observed the presence of porous zones 24, which extend in short strips substantially parallel to each other in the direction of advance V of the spreading device and perpendicular to the manufacturing axis W of the green part, as can be seen in the enlargement of Figure 3b). By way of comparison, the observation in figure 3c) of a section of a tomographic reconstruction of the green part makes it possible to confirm that the clear zones observed on the radiographic image correspond to irreducible pores resulting from defects in the spreading of the layer during printing by binder projection on a powder bed. The analysis of the radiographic image obtained by the process thus made it possible to avoid sintering this green part. The method according to the invention has the advantage of a rapid and simple implementation with respect to the formation of a tomographic reconstruction. By way of illustration, the total duration of implementation to characterize by means of the method according to the invention the green part of the example illustrated in the , involving the acquisition and processing of a single radiographic image, took less than one minute. In comparison, 50 minutes were needed to acquire 900 projections with a resolution of 35 µm, then the digital construction of the 3D tomographic model from these projections took about ten minutes.

Figure 4a) and Figure 4b) are radiographic images of the same green part as that represented in Figures 3a) to 3c), except that the acquisition of said radiographic images was carried out with an emission of X-rays making an angle of 30° and 45° respectively with the manufacturing axis of the green part. The irreducible defects visible on the radiographic image of Figure 3b) are not detectable on the images of Figures 4a) and 4b).

According to another example of implementation of the method, the axis of emission of the X-radiation is substantially perpendicular to the axis of manufacture of the green part. As it appears on the , each ray of the X-ray beam can form, with a straight line parallel to the manufacturing axis, an angle α' of between 80° and 100°, preferably between 85° and 95°.

FIG. 6a) is a radiographic image of a metal standard green part obtained by the method according to the invention with an emission of X-rays as illustrated in the . The standard green part was obtained by applying a layer height parameter of 30 µm when printing by binder jetting on a powder bed. As can be seen, the radiographic image of the standard green part has a substantially homogeneous gray level, characteristic of a dense green part, free from irreducible defects. In comparison, figure 6b) represents a radiographic image of a green part made of the same particles and the same alloy as the standard green part and obtained under the same printing conditions, with the exception of the layer height parameter, which had been chosen equal to 110 μm. As observed in Figure 6b), light and dark areas are observed which extend along the horizontal direction H and form a regular pattern. These zones are characteristic of irreducible pores formed during the printing of the green part due to an excessively high layer height parameter leading to insufficient penetration of the binder into the bed of powders. Observation of the radiographic image thus made it possible to avoid sintering of this part.

The radiographic images shown in Figures 6a) and 6b) can be compared with cross-sectional images of a tomographic reconstruction of the corresponding green parts which are shown in Figures 7a) and 7b). As it appears, the presence or the absence of irreducible defects which could be observed by means of the method according to the invention is confirmed by tomography.

Figure 8a) and Figure 8b) are radiographic images of the same green part as that shown in Figure 6b), except that the acquisition of said radiographic images was carried out with an emission of X-rays making an angle of 55° and 45° respectively with the manufacturing axis of the green part. The irreducible defects visible on the radiographic image of Figure 6b) are not detectable on the images of Figures 8a) and 8b).

As appeared on reading the present description, and in particular the examples, the invention makes it possible, in particular by means of the specific orientation of the axis of emission of the X-rays with respect to the axis of manufacture of the green part, to determine simply and more quickly than with a process of acquisition and tomographic reconstruction if the sintering of the green part must be avoided.

Claims (12)

Process comprising
a) the acquisition of at least one and at most twenty radiographic images of a green part (12) produced by printing by projection of binder on a bed of powder, by emitting X-rays along an emission axis substantially parallel or substantially perpendicular to the manufacturing axis (W) of the green part, and
b) the determination of the presence or the absence in the green part of irreducible defects by sintering (24), from only the radiographic image or images. Method according to claim 1, comprising, after step b), the sintering of the green part in order to obtain a consolidated part when the irreducible defects are absent. Method according to claim 1, comprising, after step b), the recycling of the green part, in particular the separation of the binder and of the particles bound by the binder, when the irreducible defects are present. Process according to the preceding claim, comprising the production of a new green part from the recycled particles. Method according to any one of the preceding claims, according to which in step a), at most fifteen, even at most ten, even at most five, preferably at most three radiographic images are acquired. Method according to any one of the preceding claims, comprising the acquisition of at least one, in particular a single, radiographic image of the green part by emitting X-rays along an emission axis substantially parallel to the axis manufacturing (W) of the green part and the acquisition of at least one, or even a single, radiographic image of the green part by emitting X-rays along an emission axis substantially perpendicular to the manufacturing axis (W) of the green piece. Method according to any one of the preceding claims, comprising the acquisition of at least two, or even only two, radiographic images of the green part by emitting X-rays along two emission axes, preferably mutually perpendicular, and substantially perpendicular to the manufacturing axis of the green part. A method according to any preceding claim, said presence or absence of irreducible defects being determined by visual observation of the radiographic image(s). Method according to any one of the preceding claims, the analysis of the radiographic image(s) comprising a comparison of the radiographic image(s) with a standard radiographic image acquired on a green part of identical shape and free of irreducible defects. Method according to any one of the preceding claims, the analysis of the radiographic image comprising the digital calculation of a theoretical image from a digital 3D model of the green part, and the comparison, for example by subtraction of the x-ray image with the theoretical image. Method according to any one of the preceding claims, the green piece having a diameter, measured along the emission axis, less than or equal to 20 cm. Installation for implementing the method according to any one of the preceding claims, comprising
- a 3D printer for the production of a green part by printing by binder projection on a powder bed,
- an X-ray apparatus comprising an X-ray emission source for acquiring an X-ray image of a green part, and
- an oven to sinter the green part.