FR3087270A1 - METHOD AND DEVICE FOR CONTROLLING THE QUALITY OF A BED OF POWDERS IN ADDITIVE MANUFACTURING PROCESSES - Google Patents

Abstract

The present invention relates to a device and a method for controlling the quality of a bed of powders in an additive manufacturing system. The method comprises steps consisting in: measuring an electrical signal representative of eddy current values, received in response to a time-varying electromagnetic excitation produced by an emitter over an area to be inspected in a layer of a bed of powders; - assess the temperature of the powder bed layer in the vicinity of the inspected area; correcting the values of the electrical signal representative of eddy current values represented by said measured electrical signal, as a function of the temperature values obtained by evaluating the temperature; - compare the corrected values with reference values; and - determine as a function of the result of the comparison, the presence or absence of a defect in said inspected area.

Description

METHOD AND DEVICE FOR CONTROLLING THE QUALITY OF A BED OF POWDERS IN ADDITIVE MANUFACTURING PROCESSES

Field of the invention

The invention lies in the field of Additive Manufacturing (AF), in particular the field of additive manufacturing based on the implementation of a bed of powders, and relates more particularly to a method and a device for quality control. a bed of powders spread by a layering device of an additive manufacturing system.

State of the art

The term additive manufacturing designates according to standard NF E 67001, "all of the procedures for manufacturing layer by layer by adding material to a physical object from a digital object". This term includes dozens of manufacturing technology designations, classified into seven process categories according to standard NF ISO 17296-2 June 2015.

Among these processes based on the implementation of a bed of powders, there are in particular those where the powder is melted and resolidified locally, designated as fusion on a bed of powders or by the English acronym (PBF) for “Powder Bed Fusion >>. The so-called PBF processes include the processes implementing a complete fusion of the powder grains (e.g. SLM ™ for “Selective Laser Melting” or EBM for “Electron Beam Melting”). By abuse of language, the acronym PBF is used for processes where the cohesion of the material is ensured by sintering the material (SLS processes for "Selective Laser Sintering"). Also within the framework of powder bed processes those based on the local injection of a binder on the bed, this is called Binder Jetting or binder jet process.

In these methods, it is essential to control the quality of the bed of powders, in particular to detect defects of the oxidation type of the powders constituting the bed or heterogeneities in said bed which may lead to defects on the parts produced.

For the analysis of the quality of the layering of a spreading system in PBF processes, optical techniques, in the visible wavelength range are generally used. To do this, one (or a plurality of) shots are taken, and the analysis of the signal makes it possible to detect possible shortages or excess of material. However, these optical techniques do not allow access to information on the morphology of the powder bed below the surface. They are also limited to detect the oxidation of powders.

Another monitoring technique commonly used in PBF processes is based on the analysis of a thermographic signal in the Infra Red range, as described in the article by B. Lane, S. Moylan, E. Whitenton, L. Ma , Rapid Prototyp J., 22 (2016) 778-787. This technique measures the flux radiated by the material and provides interesting information about the temperature field on the surface of the powder bed. However, here again this technique does not allow access to detailed information on the compactness and morphology of the powder bed below the surface and its capacities in terms of detection of oxidation type defects are limited.

The Eddy Currents (CF) technique, based on the detection of an electrical signal emitted by an electrically conductive material subjected to a time-varying electromagnetic field, is also a known technique for controlling. It is known to those skilled in the art that the depth probed by the excitation current depends on the frequency of the wave and the magnetic permeability and the electrical conductivity of the material probed. The eddy current technique has already been used in the field of additive manufacturing for the detection of defects on parts under development, such as cracks, lack of fusion, porosity, irregularities in shape. U.S. patent application 2016/0349215 A1 from E.l. Todorov describes the possibility of implementing a plurality of eddy current probes on a powder spreading device, using a frequency range between 1.5 MHz and 4 MHz. However, this approach does not provide information on the possibility of probing the powder bed itself.

There is therefore a need to overcome the drawbacks of known approaches and to propose a solution allowing the in situ analysis of the quality of a bed of powders subsequent to layering in the Additive Manufacturing processes of the PBF type. The present invention meets this need.

Summary of the invention

An object of the present invention relates to a method and a device for controlling the quality of the powder bed in additive manufacturing processes, and more particularly the family of powder bed fusion (PBF) methods. More specifically, the invention aims to detect drifts in the quality of the powders and in the quality of layering, drifts due for example to oxygen entering the manufacturing chamber or wear of the instrument (eg squeegee or roller) used for spreading and compacting powders or due to a lack of powder supply. The invention aims in particular to characterize the quality of layering in order to detect possible spatial heterogeneities of spreading or temporal drifts during the AF process.

A problem which the present invention aims to solve is that of the analysis of the quality of the powder beds in the additive manufacturing processes based on the use of a powder bed, using electrically conductive materials.

However, it is known to those skilled in the art that on a two-phase material such as a stack of powders, the signal CF obtained by an eddy current device in fact accounts for the electrical impedance of the medium. Furthermore, in a material such as a bed of powders, the imaginary components (inductance and capacity) of the impedance are generally greater than in a dense metallic material, where the impedance is dominated by the resistance (or in an equivalent manner conductance) of the material.

A first difficulty is that the electrical conductivity of a metallic material depends on the temperature, the latter varying during production in a PBF process. Also, the invention proposes to associate a temperature evaluation with that of the eddy currents (CF) in order to take into account in the interpretation of the signal CF, variations linked to the temperature of those associated with possible drifts linked to layering.

Although it was a priori not obvious to think that the powders were sufficiently conductive to allow analysis by CF, the inventors evaluated that this was possible for most powder beds of metallic materials. The materials concerned by the present invention are electrically conductive materials, more particularly metals.

To justify their assertion, the inventors base themselves at the first order on the law of Wiedemann and Franz which makes it possible to connect the thermal conductivity and the electrical conductivity of a metallic material and they use the fact observed experimentally that the thermal conductivities of metals vary enough little with the temperature. Thus at first order, the electrical conductivity of a metal can therefore be considered as varying in order of magnitude as the inverse of the temperature in Kelvin, which leads to significant variations.

Another very important factor that the inventors have taken into account for the analysis of the signals is the fact that the material is deposited in the form of powders which allow transfer of the charge carriers only at the points of contact between the grains of powders. . As a result, the conductivity of a bed of powders is therefore very significantly lower than that of a solid material, and as explained above, it may have a complex component, linked for example to capacitive effects.

In numerical simulation approaches to additive manufacturing processes, it is necessary in simulation models to indicate a value of thermal conductivity of the powder bed. Even if the literature reports differences in assessment of the relationship between the thermal conductivities of a material in the form of a powder bed and in massive form, this ratio is never taken in practice to be less than 0.01. At a given temperature, we can therefore think that the conductivity of the powder bed will not be less than a hundredth of that of the solid material.

Furthermore, the term "skin thickness" denotes the height of the area under the surface penetrated by the incident electromagnetic field. This height which is a function of the product of the electrical conductivity by the excitation frequency, is conventionally given by the following formula:

δ = (πμσί) ' ^1/2 where δ, μ, σ and f respectively represent the skin thickness, the magnetic permeability and the electrical conductivity of the material and the excitation frequency.

It is then necessary, depending on the nature of the material to be produced, the particle size and morphology of the powders, as well as their compaction rate, to use higher excitation frequencies than those usually encountered in the techniques. with eddy currents to compensate for the low electrical conductivity of the powders. According to different configurations, the excitation frequencies that the inventors have determined as operating for the implementation of the invention, can vary between 5 and 500 MHz, and preferably be in a range from 50 to 200 MHz.

In a preferred embodiment, the invention uses the variation of the conventional penetration depth or skin effect, with the excitation frequency to probe different depths at the subsurface, the probing scale being given by the thickness of skin defined above. In practice, the excitation signal for supplying the eddy current sensor (s) can advantageously include several frequencies, identical or not, which can be applied simultaneously or several frequencies applied successively.

Advantageously, the method of the invention is applicable to a wide range of pure metals and metal alloys, for example those based on aluminum (Al), chromium (Cr), iron (Fe), nickel ( Ni) and titanium (Ti).

The process of the invention is particularly well suited to materials having a high magnetic permeability, such as iron, nickel, which allows, at given electrical conductivity and excitation frequency, to reduce the thickness of the skin.

An additional difficulty that the inventors had to take into account is that linked to the roughness of the powder bed, which can recover on the surface of the ejectates, particles emitted in liquid form by the molten bath and which are redeposited in solid form at the surface of the powder bed. Also, preferably, a safety distance or air gap between an eddy current sensor and the powder bed must be kept, in order to take into account the presence of ejectates. Depending on the size (diameter) of the winding used for one or more of the eddy current probes, the device of the invention maintains an air gap which can range from 50 μm to 1 cm, and preferably from 50 μm to 2 mm. .

Thus, the general principle of the invention for the analysis of the quality of a bed of powders on an area to be inspected, consists in the use of a technique of in situ CF measurements of eddy currents (by an apparatus eddy current transmitter / receiver), then by combining the CF measurements obtained with an evaluation of the temperature of the powder bed made in the vicinity of the zone inspected. The expression 'in the vicinity' of the inspected area must be understood as indicating that the temperature assessment can be made for an area covering the whole of the inspected area or part of the inspected area or in the vicinity of the inspected area .

Advantageously, the temperature evaluation can be carried out on the surface and / or on the subsurface by any means known to those skilled in the art. It can be cited without limitation, the use of a camera sensitive to infrared radiation and / or the use of one or more pyrometers. In one embodiment, a plurality of infrared detectors is installed adjacent to a plurality of eddy current sensors on a mobile device capable of scanning the entire bed of powders.

In another embodiment, means of numerical simulation of heat transfers can be implemented as an alternative or complementary to the temperature measurement, to estimate the temperature in the upper part of the powder bed. The simulation can be based on data supplied by thermocouples located under the platform of the AF machine.

The invention will find advantageous applications in numerous technical fields such as the aeronautical, space, medical or automobile industries, to cite only these examples of industries which can make use of powder bed additive manufacturing processes.

Thus, the process of the invention is advantageously applied to additive manufacturing by the processes using a bed of powders, in particular the processes (SLM ™, EBM, SLS). The method of the invention also relates to binder jet processes known as "Binder Jetting" where a binder is sprayed to produce a raw piece which must subsequently be debonded and sintered.

To obtain the desired results, a method for controlling the quality of a bed of powders in an additive manufacturing system is proposed, the method comprising the steps of:

- measure an electrical signal representative of eddy current values, received in response to a time-varying electromagnetic excitation produced by a transmitter on an area to be inspected with a layer of a bed of powders;

- assess the temperature of the powder bed layer in the vicinity of the inspected area;

- correct the values of the electrical signal representative of eddy current values represented by said measured electrical signal, as a function of the temperature values obtained by evaluating the temperature;

- compare the corrected values with reference values; and

- determine according to the result of the comparison, the presence or absence of defect in said inspected area.

According to alternative or combined embodiments:

- the step of measuring an electrical signal consists in calculating amplitude and phase values of the received signal; and the comparison step consists of comparing the corrected amplitude and phase values with reference amplitude and phase values.

- the step of measuring an electrical signal consists of measuring an induced voltage at the terminals of a sensor, and calculating an impedance value of the powder bed below the inspected area; and the comparison step consists of comparing the corrected impedance value of the powder bed with a reference impedance value.

the method comprises an initial step of calibrating an eddy current transmitter / receiver device making it possible to carry out the measurement step, and which step of correcting the eddy current values represented by said measured electrical signal takes into account calibration data.

- the layer temperature evaluation step consists either of making a direct measurement of the temperature, or of estimating the temperature by simulation including in particular by applying a finite element method.

- the step of correcting the eddy current values represented by said measured electrical signal, consists in correcting said values with respect to reference charts giving the variation of the electrical conductivity as a function of the temperature for given powder beds.

- the step of determining the presence of at least one defect consists in identifying deviations in values which are representative of heterogeneities on the surface or of oxidation of the bed of powders.

- the electrical signal measurement step is carried out on a plurality of zones of the powder bed, at identical or different excitation frequencies.

- the step of correcting the eddy current values consists in correcting a mean value or a variance of the signal.

- the steps of measuring the electrical signal and evaluating the temperature are carried out sequentially.

the method sequentially comprises a first temperature evaluation step, a step of measuring the electrical signal, and a second temperature evaluation step in order to take into account any variation in temperature between the start and the end of the measurement of the electrical signal.

- the steps of measuring the electrical signal and evaluating the temperature are carried out simultaneously.

The invention also covers an additive manufacturing process on a powder bed which comprises steps for controlling the quality of the powder bed in accordance with the steps of the claimed process. The quality control steps of the powder bed can be carried out simultaneously either during a layering operation by a scraper, or following a melting operation.

The invention further covers a device for controlling the quality of a bed of powders for an additive manufacturing system, the device comprising at least one eddy current transmitter / receiver device, a temperature evaluation system, a acquisition and control system making it possible to collect and analyze data recorded by the eddy current apparatus and by the temperature evaluation system, the device for controlling the quality of the powder bed further comprising means for implement the quality control process for a claimed powder bed.

According to alternative or combined embodiments:

- the eddy current device comprises at least one coil for performing the excitation function and at least one sensor for performing the reception function.

- the receiving sensor is an inductive or magnetoresistive sensor.

- the eddy current device operates at one or a plurality of excitation frequencies in a range from 5 MHz to 500 MHz.

- the temperature evaluation system is an infrared camera or a pyrometer coupled to the eddy current device.

- the eddy current device is coupled to a powder dispenser of the additive manufacturing system.

The invention also covers an additive manufacturing system on a powder bed which comprises a device for controlling the quality of the powder bed as claimed and associated hardware and software means for implementing the method for controlling the quality of the bed. powders as claimed.

Description of the figures

Various aspects and advantages of the invention will appear in support of the description of a preferred mode of implementation of the invention but not limiting, with reference to the figures below:

FIG. 1 is a simplified representation of an additive manufacturing system making it possible to implement the device of the invention;

Figure 2 illustrates an embodiment of the device of the invention;

Figure 3 illustrates an alternative embodiment of the device of the invention;

Figure 4 illustrates another alternative embodiment of the device of the invention; and

FIG. 5 illustrates a sequence of steps of the method of the invention in an embodiment for controlling the quality of layering.

Detailed description of the invention

FIG. 1 shows an additive manufacturing system (100) making it possible to implement the device of the invention in a first embodiment. The AF system comprises a construction support plate (102) on which one or more parts (104) are produced according to an additive manufacturing process on a powder bed, a powder reservoir (106) and a powder dispenser or spreader ( 108), for example of the doctor blade or roller type, which makes it possible to bring and spread powder from the reservoir to the support plate to effect a layering in the bed of powders (110). Under the effect of a heat source (not shown), the melting of the powder operates in places intended to produce a layer of the part or parts to be manufactured. This process is repeated layer by layer until the final part (s) are obtained. The complete process is not described in more detail, and those skilled in the art can refer to the numerous literature on additive manufacturing processes and the variant embodiments based on this same principle.

To control the quality of layering, the device of the invention generally consists of an eddy current device (112), a temperature measurement system (114) and an acquisition system. and control (116) for collecting and analyzing the data recorded by the eddy current and temperature measurement systems.

The eddy current device (sensor or probe) consists of at least one coil to perform the excitation function and at least one sensor to perform the reception function. The receiving sensor can in particular be an inductive or magnetoresistive sensor. It is also possible to use Hall effect sensors, of the Giant MagnetoImpedance (GMI) or flux gate type. Those skilled in the art may refer to the literature on eddy current devices to consider implementation variants. In particular, it can be envisaged implementations where the two excitation and reception functions are separated or confused, in the latter case by the impedance response of the excitation coil.

Preferably, the CF sensor is chosen to be as sensitive as possible to variations in the conductivity. It can be optimized in this sense using known simulation tools. According to various implementations, a CF-based system can integrate a plurality of sensors, identical or not, working at identical or different frequencies.

According to a first embodiment illustrated in FIG. 1, the temperature measurement system (114) is an infrared (IR) camera which makes measurements over the entire layer of the powder bed.

In an alternative embodiment illustrated in FIG. 2, the temperature measurement system is a pyrometer type device (214) coupled to the CF sensor (112) which makes it possible to make local temperature measurements of the bed of powders.

In another alternative embodiment illustrated in FIG. 3, the eddy current system (112) is coupled to the powder diffuser (108). Preferably, the layering device (108) is integral with the CF sensor on the same axis, an offset along said axis being preferable. In such a configuration, the inspection of the layering by the CF probe (112) and by the temperature measurement system (114) is carried out simultaneously with the current operation of the layering by the scraper (108) .

In another variant not shown, a plurality of CF sensors can be arranged over all or part of the length of the axis of the powder diffuser.

In an implementation where the layering device is of the scraper or roller type, the CF sensors can for example be integrated into said scraper or roller.

FIG. 4 illustrates an alternative configuration of the device of the invention where the inspection of the layering by the CF probe (112) and by the temperature measurement system (114) is carried out during the manufacture of a part ( 104), ie during the fusion operation here illustrated by scanning a laser source (402). Various energy sources can be used, such as for example and without limitation, a laser beam (402), an electron beam or a plasma source.

The materials concerned by the present invention are electrically conductive materials, more particularly metals.

FIG. 5 illustrates a sequence of steps (500) of an implementation of the method of the invention for controlling the quality of a bed of powders. The process begins when a layer ‘n’ is spread. Those skilled in the art understand that the method which is based on a measurement of the eddy currents requires that there be a bed of powders of minimum thickness but sufficient to initiate a first measurement, in order to ensure that the measured currents are those induced in the powder bed, and not in the support tray. Thus, in the usual case of a tray with high electrical conductivity, it will be expected to deposit a height of powder of the order of a few millimeters before initiating the control process. Preferably, the process can be initiated when the height of the powder is greater than 1 cm, this value depending on the conductivity and the permeability of the powder. However, the pre-requisite height may be lower, in particular in the case where a preliminary calibration of the powder bed is carried out. The process is carried out to determine the quality of the powder bed in an area to be inspected.

The process continues with steps (502) and (504) which can be carried out either simultaneously as illustrated, or sequentially one after the other without an imposed order.

Step (502) consists of carrying out the eddy current measurements. Depending on the CF system implemented, but in a known manner for carrying out CF measurements, the excitation function creates above a virgin zone of the bed of powders to be inspected (ie an zone not subjected or not yet subjected to a flux of heat corresponding to a step of the additive manufacturing process), a variable electromagnetic field when the transmitter winding is traversed by a variable electric current. The powder bed opposite this field is conductive and becomes the seat of eddy currents. The receiving function of the CF sensor is then the seat of an electromotive force (voltage) of the same frequencies as on emission in the case of an inductive receiver or of a voltage variation (also of the same frequencies as those of 'emission) in the case of a magnetoresistive sensor. In one embodiment, the frequencies are in the range of 5 MHz and 500 MHz.

The amplitude and phase of these voltages depend on the values of the conductivity and the permeability of the powder bed. A demodulation module of the CF system makes it possible to extract the signal at the frequencies used, i.e. to measure the amplitudes and the phases of the electrical signal in reception for each of the frequencies (and for each of the sensors, possibly using a multiplexer). These signals are recorded for each elementary CF sensor and each position of these sensors. Step (502) makes it possible to generate a representation of this data for example in the form of eddy current maps in amplitude and / or in phase or, which is equivalent, in the form of maps CF in real and imaginary parts .

Step (504) consists in evaluating the temperature of the powder bed in the vicinity of the area inspected by CF. In one embodiment, the temperature T ° is evaluated everywhere above-mentioned, for example via an IR camera on the entire powder bed, or more locally by a temperature measurement device linked to the CF sensor. The temperature evaluation can be represented in the form of a temperature map. The measurements are transmitted to the acquisition and control system (116).

In an alternative embodiment of the temperature evaluation step, the temperature can be evaluated by simulation, for example by application of the finite element method.

In an alternative embodiment where the steps of measuring CF (502) and of evaluating temperature (504) are sequential, the method can chain a first step of measuring temperature, a step of measuring CF and a second step of measuring temperature after the CF measurement, to take into account any temperature variation between the start and the end of the CF measurement.

After the readings of T ° and of CF signals (possibly in the form of T ° and CF maps) are transmitted to the acquisition and control unit (116), the method continues to the next step (506 ) to correct the CF data from the temperature data. The data can optionally be pretreated with respect to calibration values, the temperature mapping can also be resampled, and the correction of the CF data or of the CF mapping from the temperature data is done on the pretreated data. .

The CF mapping is corrected by evaluating a curve giving the variation in conductivity as a function of a given temperature, either experimentally or theoretically for a given bed of powders. For a more precise quantitative analysis, as explained below, reference charts can be produced from experimental readings of CF signals obtained on samples of compacted powder representative of a plurality of powder beds, as a function of the temperature. . These allow an electromagnetic signature to be linked to the intrinsic properties of a powder - materials / composition, particle size, morphology, oxidation rate, humidity, etc. - and to the characteristics of the layering - compactness rate, density. . On all or part of these samples, the effect of temperature on the measured signals can also be quantified beforehand, possibly outside the machine, for example by using a temperature-controlled oven. These reference values are used to quantify the properties of the powder bed.

They are in particular necessary for obtaining a quantitative evaluation of certain parameters such as the density or the compactness of the powder bed.

In a following step (508), the method makes it possible to compare the corrected values with reference values in order to determine if there are defects or irregularities in the powder bed.

It is possible to cite without limitation, the defects or inhomogeneities linked to: the porosity or the lack of material, the presence of protuberances (ejecta, "balling", ...), the variation in compactness of the powder bed, the cracking of the powder bed, oxidation, etc.

If deviations from reference values are noted, signifying possible faults, the process allows (branch "yes") in a next step (512) feedback on the general AF process such as for example applying a corrective action such as triggering a new pass from the layering system, potentially with an increase in its engine torque, or then stopping the machine.

Advantageously, the method allows that the zones in which a part (s) during manufacture is / are subjected (s) to the heat flow according to the AF method (and their vicinity) is excluded (s) of the analysis, due to the disturbance brought about by the conductivity of the solid material of the part.

If there is no significant difference between the corrected values and the reference values ("no" branch), the process can loop back to the start for a new layering inspection (510).

Thus, the method of the invention makes it possible, from the estimation by Eddy Currents of the impedance of the bed of powders corrected by a measurement of the local temperature, the detection of faults, and in particular of heterogeneities, in a bed of powders during layering in an Additive Manufacturing device.

In one embodiment, the method may include an initial calibration step of the CF sensor and of the CF demodulation apparatus in order to compensate for thermal drifts and allow comparative measurements to be made. Such a calibration is known to a person skilled in the art as being commonly carried out during the detection of faults by the eddy current method. This step can be carried out before a measurement or a measurement cycle (assuming that, during scanning of the part, the temperature of the CF sensors and of the demodulation electronics will be stable), preferably before and after each measurement, in the aim of compensating for a change in the temperature of the sensor and possibly of the demodulation electronics.

In the case of a CF sensor network, all the elementary CF sensors should be calibrated. Optionally, this step can be used to balance the sensors, which is also a classic step in the CF method and which consists in bringing the CF signal back to the origin. This calibration step can be carried out according to one or a combination of the following approaches:

(a) Measurement of the CF signal on one or more standards with known properties arranged in the equipment, eg within the manufacturing chamber, at the edge of the powder bed, if possible having electrical conductivities close to that expected in the bed powders. These standards can include one or more samples of powder, of known and stable conductivity (for example the powder is enclosed in a container which does not interfere with the CF measurement). These standards can also be composed of low gap semiconductors (of the InSb, InAs type) suitably doped to be in the right range of conductivities, or of carbon or carbon-based composites, these examples not being of a limiting nature. Advantageously, this conductivity measurement is carried out before and after each measurement phase. This approach gives better results, without strong constraints.

(b) The calibration phase is performed upstream or even downstream of the FA manufacturing process, by using standards similar to those used for approach (a) but not integrated into the machine. These reference measurements are produced off-machine or by the temporary introduction of standards into the machine. This mode does not, however, overcome the drifts of the sensor and the demodulation electronics during manufacturing. This approach is simpler to implement.

The calibration sequence takes place at different stages of the powder bed inspection process. In one particular mode, the powder bed itself is used as a reference for the calibration of the process. A series of conductivity measurements on a representative surface of the bed is carried out. An average value - over the entire measurement surface, or only over regions with low variance (impedance, and / or real and / or imaginary component) - is calculated and used as a reference. A series of measurements carried out on the bed heated uniformly at different temperatures or on this same bed having controlled temperature gradients can also make it possible to obtain calibration points of the impedance 'Z' of the powder studied as a function of the temperature T °, namely'ôZ / ôT. The calibration data is transmitted to the acquisition and control unit (116) to be taken into account when processing the data (506). And during the defect determination step (508), the presence of bed irregularities is determined relative to this or these reference values.

In an alternative mode, no calibration phase is carried out, in particular in the case where the demodulation apparatus is robust with respect to thermal drifts.

The present description illustrates examples of implementation of the invention, but is not limiting, and must allow the person skilled in the art to make modifications and variant implementations while retaining the same principles, but taking into account that the inspection of the powder bed by CF requires choosing four main parameters:

- The air gap, i.e. the distance between the CF sensor and the powder bed. This is minimized in order to increase the sensitivity, while maintaining a sufficient distance to avoid contact between the sensor and the roughness of the powder bed. In one embodiment, the air gap can vary between 50 pm and 1 cm, preferably 50 pm and 2 mm. A system for detecting these protuberances can optionally be added, such as an optical profilometer or an image analysis system.

- The thickness of the bed of probing powders: the inspection thickness is typically from a few tens of μm to a few mm. In doing so, the present invention makes it possible to qualify the latest layering.

- The CF excitation frequency: as indicated above, depending on the nature of the material to be produced, the particle size and the morphology of the powders, as well as their compaction rate, the excitation frequencies can vary between 5 and 500 MHz, and preferably between 50 MHz and 200 MHz.

- The surface of the CF winding must first be chosen according to the maximum size of the defect sought in the powder bed. The active area of the reception system must preferably be less than or equal to the extent of the anomaly area. For magnetoresistive sensors, the size of the active area can vary between 50 µm and 2 mm. For inductive sensors, the size of the active area can vary between 0.7 mm and 5 cm, preferably 1 mm and 5 mm. It may be advantageous to use simulation tools to then evaluate the behavior of the sensor with respect to air gap noise and the depth at which eddy currents are generated.

Claims (21)

Claims 1. A method (500) of controlling the quality of a bed of powders in an additive manufacturing device, the method comprising the steps of: - measuring (502) an electrical signal representative of eddy current values, received in response to a time-varying electromagnetic excitation produced by a transmitter on an area to be inspected from a layer of a bed of powders; - evaluating (504) the temperature of the layer of the powder bed in the vicinity of the area inspected; - correct (506) the values of the electrical signal representative of eddy current values represented by said measured electrical signal, as a function of the temperature values obtained by evaluating the temperature; - comparing (508) the corrected values with reference values; and - Determine (510, 512) based on the result of the comparison, the presence or absence of a defect in said inspected area. 2. The method according to claim 1, in which: - the step (502) of measuring an electrical signal consists in calculating amplitude and phase values of the received signal; and - the comparison step (508) consists of comparing the corrected amplitude and phase values with reference amplitude and phase values. 3. The method according to claim 1, in which: - the step (502) of measuring an electrical signal consists in measuring an induced voltage at the terminals of a sensor, and in calculating an impedance value of the bed of powders below the inspected area; and - the comparison step (508) consists in comparing the corrected impedance value of the bed of powders with a reference impedance value. 4. The method according to any one of claims 1 to 3 comprising an initial step of calibrating an eddy current transmitter / receiver device making it possible to carry out the measurement step, and in which the step of correcting the values eddy currents represented by said measured electrical signal, takes into account calibration data. 5. The method according to any one of claims 1 to 4 wherein the step of evaluating the temperature of the layer consists either of making a direct measurement of the temperature, or of estimating the temperature by simulation including in particular by application of a finite element method. 6. The method according to any one of claims 1 to 5 wherein the step of correcting the eddy current values represented by said measured electrical signal, consists in correcting said values with respect to reference charts giving the variation of electrical conductivity as a function of temperature for given powder beds. 7. The method according to any one of claims 1 to 6 in which the step of determining the presence of at least one defect consists in identifying deviations in values which are representative of surface heterogeneities or of oxidation of the bed of powders. 8. The method according to any one of claims 1 to 7 wherein the electrical signal measurement step is carried out on a plurality of zones of the powder bed, at identical or different excitation frequencies. 9. The method according to claim 8 in which the step of correcting the eddy current values consists in correcting a mean value or a variance of the signal. 10. The method according to any one of claims 1 to 9 in which the steps of measuring the electrical signal and of evaluating temperature are carried out sequentially. 11. The method according to any one of claims 1 to 10 sequentially comprising a first temperature evaluation step, a step of measuring the electrical signal, and a second temperature evaluation step in order to take into account any variation in temperature between the start and the end of the measurement of the electrical signal. 12. The method according to any one of claims 1 to 9 in which the steps of measuring the electrical signal and of evaluating temperature are carried out simultaneously. 13. An additive manufacturing process on a bed of powders comprising steps of controlling the quality of the bed of powders in accordance with the process steps of any one of claims 1 to 12. 14. The method according to claim 13 wherein the steps of controlling the quality of the powder bed are carried out simultaneously either during a layering operation by a scraper, or following a melting operation. 15. A device for controlling the quality of a bed of powders for an additive manufacturing system, the device comprising at least one eddy current transmitter / receiver (112), a temperature evaluation system (114) , an acquisition and control system (116) making it possible to collect and analyze data recorded by the eddy current apparatus and by the temperature evaluation system, the device for controlling the quality of the powder bed comprising further means for implementing the method according to any one of claims 1 to 12. 16. The device according to claim 15 in which the eddy current apparatus (112) comprises at least one coil for performing the excitation function and at least one sensor for performing the reception function. 17. The device according to claim 16 in which the receiving sensor is an inductive or magnetoresistive sensor. 18. The device according to any one of claims 15 to 17 wherein the eddy current apparatus operates at one or a plurality of excitation frequencies in a range from 5 MHz to 500 MHz. 19. The device according to any one of claims 15 to 18 in which the temperature evaluation system is an infrared camera or a pyrometer coupled to the eddy current apparatus. 20. The device according to any one of claims 15 to 19 in which the eddy current apparatus is coupled to a powder dispenser of the additive manufacturing device. 10 21. An additive manufacturing system comprising a device for controlling the quality of the powder bed according to any one of claims 15 to 20.