Method and apparatus for determining the density of metal powder

Abstract

The present invention relates to an apparatus for determining the density of a metal powder for use in an additive manufacturing process, the apparatus comprising an inductor associated with a body for receiving metal powder to be analysed, wherein the inductor is comprised in an electrical circuit for driving an alternating current in the inductor and measuring an output which depends on the inductance of the inductor; and a processor arranged to determine the density of the powder based on the output.

Description

[0001] Method and Apparatus for Determining the Density of Metal Powder

[0002] Technical Field of the Invention

[0003] The present invention relates to an apparatus for determining the density of a metal powder for use in an additive manufacturing process and to a method for determining the density of a metal powder in a component of an additive manufacturing system.

[0004] Background to the Invention

[0005] In a known additive manufacturing process an additive manufacturing machine produces articles from a powdered metal or alloy. The machine deposits a layer of powder on a build platform and the powder is subsequently selectively fused with a laser or electron beam, to form an article or articles. The process is repeated so that articles are formed layer by layer. On completion of a build, unfused powder may be re-used in another build.

[0006] The composition and condition of metal powder used in a build process can have a significant effect of the integrity of an article formed by the process. Powder density represents a key property that influences the performance and quality of articles produced by additive manufacturing. Factors which affect powder density include particle size, particle shape, distribution and flowability, as well as environmental factors such as moisture levels.

[0007] Multiple methods for measuring powder density are available. For example, powder density can be determined by passing a known quantity of powder through a sieve into a graduated cylinder. The powder is then levelled (without compacting) and the bulk density (g / mL) is then calculated based on the measured volume of powder in the cylinder using the formula m / Vo in which m is the mass of the powder and Vo measured volume.

[0008] Powder density can also be determined using a volumeter where sieved powder passes through a funnel into a baffle box comprising four angled glass plates. The powder then passes through a further funnel into a receiving cup having a known mass and volume. Excess powder is then scraped from the top of the cup using the blade of a spatula until a smooth surface is obtained. The mass of the powder is then determined by subtracting the mass of the empty cup from the weight of the powder filled cup. The bulk density of the powder is then calculated in g / mL by the formula m / Vo in which Vo is the volume of the cup.

[0009] While both methods enable the density of a powder to be determined they suffer from the disadvantages that they cannot be easily integrated into a component of an additive manufacturing system. These methods also typically require powder to be removed from the environment in which it is being stored or processed, meaning there is an increased risk of water vapour from the ambient air condensing on the powder and changing its properties. Therefore, density data obtained by these methods may not be representative of the actual density of the powder in the component of an additive manufacturing system.

[0010] It is an object of embodiments of the invention to provide an improved apparatus and method for determining the density of powder for use in an additive manufacturing process. In particular, it is an object of embodiments of the invention to provide an improved apparatus and method for determining the density of powder in a non- destructive manner while the powder remains within a component of an additive manufacturing system.

[0011] Summary of the Invention

[0012] According to a first aspect of the invention there is provided apparatus for determining the density of a metal powder for use in an additive manufacturing process, the apparatus comprising: an inductor associated with a body for receiving metal powder to be analysed, wherein the inductor is comprised in an electrical circuit for driving an alternating current in the inductor and measuring an output which depends on the inductance of the inductor; and a processor arranged to determine the density of the powder based on the output.

[0013] According to a second aspect of the invention there is provided a method of determining the density of a metal powder for use in an additive manufacturing process, the method comprising the steps of: placing an inductor in proximity to the metal powder; driving an alternating current in the inductor to generate an alternating magnetic field; and measuring an output which depends on the inductance of the inductor thereby to determine the density of the metal powder.

[0014] The apparatus and method enable the density of metal powder to be determined quickly and in a non-contact matter. Advantageously, the apparatus and method are deployable to determine the density of powder both in static storage and when moving within a component of an additive manufacturing system such as a pipeline. Moreover, because the apparatus may be comprised in or attached to a component of an additive manufacturing system, and the method may be deployed in such as system, density measurements can be obtained in-situ which avoids exposing the powder to the external environment. By monitoring the density of the powder, a user is able to determine and identify any changes in the condition of the powder which may render it unsuitable for use in an additive manufacturing process.

[0015] The electrical circuit may be a resonant LC circuit comprising a capacitor in addition to the inductor. The capacitor may be fixed or variable. The output may be the frequency of oscillation of current in the circuit.

[0016] Density of powder may be inferred by computing the change in frequency of oscillation which occurs between when the inductor is driven in free space (or with no metal powder in the apparatus) and when it is driven in proximity to metal powder (or when metal powder is introduced into the body. The inductor may be a coil. The coil may be formed from a conductor such as wire, e.g. copper wire. The inductor may comprise a core, typically an air core or a plastics material such as fibre glass. Or the core may comprise a magnetic material such as iron or ferrite, e.g. if an encapsulated inductor is used.

[0017] The inductor may be on or at least partially embedded within a substrate. In some embodiments the substrate comprises a printed circuit board. In other embodiments the inductor may be a discrete element, such as an encapsulated coil.

[0018] The inductor may comprise a conductor arranged in a line pattern on or in a substrate. The line pattern may comprise a plurality of rectangular loops. The line pattern may comprise a folded structure. In other embodiments the conductor pattern may comprise a spiral. The shape of the spiral is not limited to a circular pattern and may comprise a square or rectangular pattern for example.

[0019] The inductor may be disposed adjacent and may be in contact with a wall of the body for receiving metal powder to be analysed.

[0020] The body for receiving metal powder to be analysed may include an aperture. A barrier may be disposed in the aperture. The inductor may be disposed on, adjacent to, in contact with or at least partially embedded in the barrier.

[0021] The barrier may be planar, but in some embodiments the barrier may be curved, if, for example, powder is to be analysed within a conduit having a generally circular cross-section. The barrier may comprise a window at least partially transparent to a selected wavelength or wavelengths of electromagnetic radiation. Advantageously this enables optical analysis of powder to be made via the window in addition to sensing density. Such a window may comprise or be formed from glass or crystal, such as sapphire crystal or magnesium fluoride crystal. In some embodiments the barrier may comprise a non-magnetic material such as plastic. Suitably, the plastic may comprise acrylonitrile butadiene styrene (ABS), polycarbonate (PC), PC-ABS, nylon or polyacetal.

[0022] The inductor may be arranged so that, in use, the alternating magnetic field induces an electrical current in the powder to be analysed.

[0023] The processor may be configured to determine, and the method may involve determining, the absence or presence of metal powder in the body based on the measured inductance. The processor may be configured to determine, and the method may involve determining, the absence or presence of metal powder on or in front of the barrier. Advantageously, this enables an operative to determine whether the barrier has become contaminated in use. This is useful where the barrier is an optical window used for other purposes.

[0024] The processor may be configured to process the output to determine, and the method may involve determining, the base alloy of the metal powder e.g. by matching a metal or alloy type to a measured density. Accordingly, the processor can distinguish between different metal powders which helps to ensure that the intended metal powder is used in an additive manufacturing process.

[0025] The body may be a container for storing and / or transporting metal powder. The body may be comprised in an additive manufacturing machine. The body may be a conduit for conveying metal powder.

[0026] The inductor may be associated with a flat internal surface of the body.

[0027] The body may be a sampling body. It may be configured to control, e.g. limit, the rate at which powder flows across a barrier comprised in the body. The sampling body may be in the form of a conduit or pipe, which may be elongate, cylindrical or have a D-shaped cross-section. The sampling body may comprise an inlet, an outlet and a passage extending between the inlet and outlet. A portion of the passage may have a cross-sectional area that is smaller than that at the entrance or exit. In this way, the flow of powder through the passage can be controlled as it passes across the barrier, resulting in improved accuracy when determining the density of the powder. The apparatus may be comprised in a container for storing and / or transporting metal powder, in an additive manufacturing machine, or in a conduit such as a pipeline. The apparatus may be attached to or integrated within the container, additive manufacturing machine or conduit. The apparatus may be arranged so that the barrier provides a sloping surface in use.

[0028] The apparatus may comprise a device which is operable to selectively prevent the flow of powder. The device may comprise a valve. The device may be located downstream of the apparatus, so that when flow of powder is prevented it accumulates in front of the apparatus, for example in front of a barrier of the apparatus, so that the density of static (motionless) metal powder can be determined. On the other hand, the flow of powder across the barrier can be controlled or prevented by partially or fully closing a valve located upstream of the apparatus.

[0029] The method may be performed on metal powder in a component of an additive manufacturing system. In this way, the density of the powder can be determined in-situ which in turn minimises the risk of the metal powder becoming contaminated following its removal from the component. The component may comprise a container for transporting and / or storing metal powder, an additive manufacturing machine or a conduit. The conduit may comprise a pipeline, which in use, conveys metal powder between different components of an additive manufacturing system, e.g. between the container and the additive manufacturing machine. The conduit may have a generally circular or D-shaped cross-section.

[0030] The method may comprise the step of determining the density of the metal powder while the metal powder is flowing through or out of the component of the additive manufacturing system. For example, the density of the powder may be determined as it flows through a conduit arranged between a powder storage / transport container and an additive manufacturing machine. Alternatively, the density of the metal powder may be determined within the powder storage / transport container itself while metal powder exits the container via an outlet.

[0031] The method may comprise the step of controlling the flow of powder through the component. In some embodiments the method comprises the step of preventing metal powder from flowing out of the component of the additive manufacturing system and thereafter determining the density of the metal powder. In some embodiments an outlet of a powder storage / transport container may be closed to prevent the flow of powder out of the container. In other embodiments, the method may comprise the step of closing a valve to prevent the flow of powder out of or through the component. The valve may be located downstream of the apparatus for determining the density of the metal powder.

[0032] The apparatus of the first aspect of the invention may be configured to carry out the method of the second aspect of the invention. In particular, the processor may be configured to carry out the method of the second aspect of the invention.

[0033] The apparatus of first aspect of the invention may be comprised in a device comprising one or more other apparatuses for determining the condition of and / or mechanical properties of metal powder. In this respect the device may comprise apparatuses for determining one or more of the extent of degradation / oxidation of metal powder, powder flowability and the temperature of powder. Advantageously, by determining the density, the extent of degradation / oxidation, flowability and the temperature of metal powder a user can make a better-informed decision on whether the powder is suitable for use (or re-use) in an additive manufacturing process.

[0034] The apparatus may be operatively connected to a communication network, for example a local area network. For instance, there may be a data connection between the apparatus and a local powder processing apparatus such as an additive manufacturing machine, blender, sieve or conveyance system. This may be achieved by using an industry standard protocol such as Modbus or I / O-Link. In this way, status information and point measurements can be communicated when polled.

[0035] The apparatus may be operatively connected to a wide area network. For instance, the apparatus may be wirelessly connected to a cloud server so that the degradation measurements (and any other measurements that can be used to provide an indication of a powder’s health) can be logged. This also enables the apparatus’s firmware to be updated when needed and allows calibration information to be downloaded.

[0036] The apparatus may comprise a power source, e.g. a rechargeable or non- rechargeable electric battery e.g. a non-rechargeable lithium primary cell which powers the apparatus during transit when connected to a powder storage and / or transport container.

[0037] The apparatus may comprise a power connection to an external power source, e.g. a DC electrical connection to an external power source, such as a local powder processing apparatus, e.g. additive manufacturing machine, blender, sieve or conveyance system.

[0038] Detailed Description of the Invention In order that the invention may be more clearly understood one or more embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:

[0039] Figure 1 is a perspective view of apparatus for determining the condition of metal powder, with a cover shown separated from the apparatus;

[0040] Figure 2 is a schematic block diagram showing components of the apparatus of figure 1

[0041] Figure 3 is a longitudinal cross-section taken along the line A-A of figure 1;

[0042] Figure 4 is an end view of the apparatus of figure 1 ;

[0043] Figure 5 is a transverse cross-section taken along the line B-B of figure

[0044] 3, with the cover removed, with an enlarged region;

[0045] Figure 6 is an enlarged view of part of Figure 3 ;

[0046] Figure 7 is a schematic cross-sectional view of the enlarged region of figure 5;

[0047] Figure 8 is a graph showing reflectance against oxide thickness;

[0048] Figure 9 is a schematic view of apparatus for determining the flowability of a metal powder comprised in the part shown in figure 3;

[0049] Figure 10 is a schematic view of an embodiment of apparatus for determining the temperature of a metal powder comprised in the part shown in figure 3; Figure 11 is a schematic view of an embodiment of apparatus for determining the density of a metal powder comprised in the part shown in figure 3;

[0050] Figure 12 is a schematic view of a conduit comprising the apparatus shown in Figure 1.

[0051] Figure 13 is a partially transparent view of the apparatus shown in figure 1 as depicted in figure 12;

[0052] Figures 14 to 16 are schematic views of a conduit arrangement comprising the apparatus shown in Figure 12 in different states; and Figures 17 is a side view of a metal powder container comprising apparatus for determining the condition of a metal powder expanded into an enlargement.

[0053] Referring to the drawings, figures 1 to 11 show an apparatus 1 for determining the condition of a metal powder.

[0054] The apparatus comprises a sampling body 2 which defines a conduit 2a for conveying metal powder in use. The conduit 2a comprises an inlet 3 and an outlet 4. Each defines a circular opening. The internal surfaces of the circular inlet out outlet openings transition smoothly to the internal surface of the conduit extending between the openings, which has a substantially D-shaped cross-section. That is to say. there is a flat, planar internal surface 6 to the conduit 2a, which transitions through smooth 90° curves at opposite longitudinal edges to form two opposed side walls which are straight and parallel to each other adjacent the planar surface and then transition into a generally semi-circular shape to join up. The conduit is formed by a machined metal sampling body 2 with a generally part-circular external cross-section with a single flat face 23 which extends parallel and adjacent to the internal flat surface 6 of the conduit 2a. In use the apparatus is typically arranged so that the flat planar internal surface 6 is lowermost, to form the bottom of the conduit 2a. The inlet 3 and outlet 4 comprise flanged connections enabling the apparatus to be connected in-line with another conduit or other apparatus for conveying metal powder.

[0055] The part of the conduit 2a with a D-shaped cross-section has an overall cross- sectional area smaller than that of the inlet and outlet.

[0056] A housing 7 with a lid 8 partially surrounds the sampling body 2 and, as best shown in figure 1, the housing 7 encloses a circuit board 9. The circuit board mounts components for analysing metal powder in the sampling body and associated electronics 10 including a processor 11 with memory. The circuit board also mounts an electric battery 12 and super capacitor 13 forming part of a power supply, global positioning system electronics (not shown), communication electronics 14, local communications connector 15, sim card 16, cellular modem 17, antenna 18, status LEDs 20 which are visible through corresponding openings 22 on the lid when fitted, and drivers and sensing circuit apparatus associated with various metal powder conditions sensing apparatus described further below.

[0057] The circuit board extends parallel and adjacent to the flat external surface 23 of the sampling body 2.

[0058] A first substantially circular aperture 24 is formed through flat side of the sample body at a position approximately mid-way along its length with a counter-bore 25 in the outside surface of the sampling body. A magnesium fluoride crystal window 26 is fitted in the counter-bore along with an O-ring 35 to form a seal between the window 26 and the sampling body. Other suitable materials can be used, e.g. Saphire crystal, depending on the specific application of the apparatus.

[0059] A spacer 27 is fitted over the flat external surface of the sampling body, configured to hold the window 26 and O-ring 35 in place. The circuit board 9 is fitted over and abuts the spacer 27.

[0060] Coaxial circular apertures are formed through the printed circuit board 9 and spacer 27 centred at the centre of the window, with a diameter of about one twentieth of that of the window. A detector 29 is mounted on the side of the printed circuit board 9 facing away from the window over the aperture through the circuit board, so that it faces the window 26. The detector includes a focussing element 28, such as a lens, which is extends into the aperture, towards the window, and serves to focus received radiation onto the detector.

[0061] In the illustrated embodiment the detector is a point sensor e.g. a photodiode. In other embodiments the detector is an image sensor such as a multi pixel sensor e.g. a CCD or CMOS sensor.

[0062] An inner shroud 30 extends around the aperture on the side of the printed circuit board 9 facing the window 26, sitting in the corresponding aperture in the spacer 27, and extends from the circuit board to contact the window. The inner shroud 30 is annular in cross-section and has a substantially cylindrical inner surface. The wall of the illustrated inner shroud 30 tapers in thickness as it extends from the printed circuit board so that it defines a slightly diverging passage from the circuit board to the window with an inside diameter the same as that of the aperture through the circuit board where it meets the circuit board. The shroud has a frustro-conical external shape that tapers toward the window. This shape facilitates moulding the inner shroud 30 from plastics. Functionally the inner shroud 30 need only be a thin-walled cylindrical tube.

[0063] The spacer 27 forms an outer shroud which extends around and concentrically with the inner shroud, extending parallel to but spaced just inside the outer edge of the window 26, and extending from the circuit board 9 to the window 26. The inside diameter of the outer shroud is about three times that of the maximum outside diameter of the inner shroud 30.

[0064] There is therefore an annular region of circuit board 9 between the two shrouds. In this region three LEDs 31 are mounted on the circuit board, adjacent and equally spaced around the outer shroud. The LEDs 31 are arranged to direct light towards the centre of the opposite side of the window to the printed circuit board, and the apparatus is sized and configured so that light is directed by the LEDs is refracted to form a substantially parallel beam as it enters the window 26, the beam extending at an acute angle 32 to the surface of the window of about 41°.

[0065] In the illustrated embodiment the LEDs have a beam spread of about 28° and are positioned so that the edge of the beam defines an outer edge of the area of the surface of the window adjacent to the LEDs, and the inner shroud 30 defines the inner edge of that region. This, in turn, ensures that a desired limited area of interest on the opposite side of the window is illuminated. In use, metal powder 33 to be analysed is introduced into the sampling body so that it contacts (and ideally is close packed against) the window 26. The powder may be static, or moving.

[0066] The LEDs 31 are activated and emit a single wavelength (850 nm) of infrared radiation. The resulting beam illuminates particles of metal powder 33 lying adjacent the outside of the window 26 in the area of interest opposite and within the viewing area of the detector 29. Radiation reflected by the particles is received by the detector 29.

[0067] The chosen wavelength and angle of illumination in this embodiment is suited to analysis of a titanium alloy. Other wavelengths and illumination angles may be chosen for analysis of other metal powder compositions, e.g. a nickel alloy is conveniently illuminated with a radiation having a wavelength of 850 nm at an illumination angle of about 56°.

[0068] The inner shroud 30 ensures that light from the LEDs is not directly received by the sensor, and the outer shroud ensures that no ambient light is received by the sensor, thus creating dark field illumination of the metal powder. In use when the apparatus is connected in line with powder handling / processing apparatus no ambient light will enter the sampling body via the inlet or outlet. The sensor measures the intensity of the radiation it receives. The lens 28 associated with the sensor is configured to focus light reflected by the metal powder 33 in the area of interest on to the sensor 29. The radiation received by the sensor 29 is the sum of the radiation reflected by any oxide surface of the individual particles of metal powder and radiation reflected by the underlying metal / alloy surface of the individual particles of the metal powder. Where an oxide layer has a thickness of one quarter (or less) of the wavelength of the illuminating radiation, destructive interference will occur at the oxide surface between radiation reflected by the oxide surface and radiation which has passed through the oxide surface and reflected back from the underlying metal / alloy surface, leading to a sharp reduction in the intensity of reflected light received by the sensor. Where the thickness of the oxide layer is less than a quarter of the wavelength of illuminating radiation, as the oxide layer becomes thicker the intensity of reflected light decreases, and will reach a minimum as the oxide layer thickness approaches one quarter of the wavelength.

[0069] So, by choosing a relevant wavelength for a particular metal powder type and oxide layer thickness, there is a strong correlation between the intensity of reflected radiation received by the sensor and the thickness of the oxide layer, and thus the degree of oxidation of the metal powder. Figure 8 shows reflectance on the y axis against oxide thickness on the x axis for various metal powders. In each case there is a generally linear, or at least a straightforward relationship, over the range of oxide thickness of interest allowing oxide thickness to be reliably determined by measuring the intensity of reflected radiation.

[0070] As the detector 29 is a point sensor, the measured intensity effectively represents an average oxide layer thickness for all of the illuminated particles within the area of interest within the view of the sensor that reflect light back towards the sensor. Point sensors are suited to taking measurements when metal powder is both static and moving past the window. In the latter case the processor may cause the sensor to take periodic measurements which are averaged over time, so, for example, an intensity measurement may be taken every 0.2 seconds and a mean of five values calculated every second. Mean values obtained over 15 seconds may then be used to calculate measurement statistics.

[0071] Where a multi-pixel sensor is used this can record a dark field image of the metal powder being analysed. In an example a sensor having an array of 320 x 320 pixels is used to take an image every 0.2s. The image may then be analysed to determine the intensity of radiation reflected by individual particles enabling statistical information relating to oxide layer thickness to be calculated. The dark field image also enables other characteristics of particles to be determined through image analysis, e.g. particle size and shape distributions.

[0072] For both single point and image sensors illumination with and / or detection of a single or narrow band of radiation in a dark field arrangement enables oxide thickness to be determined by measuring only the intensity of reflected radiation. This requires relatively simple processing owing to a strong and in most cases approximately linear relationship between intensity and oxide layer thickness, e.g. oxide thickness may be determined from a look-up table.

[0073] The apparatus also includes apparatus 40 for analysing the flow characteristics of a metal powder, positioned between two of the LEDs 31. This apparatus is shown schematically in figure 9 and comprises a vertical-cavity surface-emitting laser diode (VCSEL) 41 and a speckle image sensor such as a CCD or CMOS sensor 42 mounted alongside each other on the circuit board 9 behind the window 26. The sensor 42 is a “bare” sensor, in that it does not have any associated focussing optics. It will therefore capture a speckle pattern caused by light reflected by the metal powder, rather than an image of the powder. The spacer 27 forms a shroud extending around the laser diode 41 and sensor 42, and from the circuit board 9 to the window 26. The shroud forms a generally square cavity which accommodates the laser diode and sensor, the sides of which diverge away from the circuit board towards the window.

[0074] In use the laser diode 41 emits a well collimated narrow beam of near infrared radiation 43 towards and through the window 26 to illuminate moving metal powder adjacent the opposite face of the window. The beam illuminates a spot of diameter about 2mm on the opposite face of the window. The radiation 43 is reflected and / or scattered back through the window 26 by the metal powder where it is detected by the speckle image sensor 42. The area of illumination and the relative position of the image sensor are chosen so that substantially the whole area of the image sensor is illuminated by radiation reflected back onto the sensor by the power. This improves the signal to noise ratio detected by the sensor.

[0075] Speckle images are obtained as the powder 33 flows past the window 26. These are stored on the speckle image sensor where an internal state-machine computes the difference between the speckle patterns of the frames and thus the rate of movement of powder particles across the window 26 enabling flow speed of the powder to be calculated, e.g. in mm / s.

[0076] In the described embodiment the processor 11 polls the sensor every second. The sensor takes five images per poll. These are stored on the sensor and the internal state machine computes the difference between sequential images to determine the rate of movement of powder particles, which is transmitted to the processor 11. If the measured rate of movement exceeds a threshold (e.g. lOmm / s but other thresholds could be chosen) the powder is determined to be moving the rate that images are captured is increased. The poll rate may be increased . The poll rate can be increased as desired but may typically be increased so that readings are taken from 4 to 25 times every second. And / or the rate of image capture on the sensor may be increased. For example, the sensor may capture an image at least every 10ms, and accumulated images are polled by the processor at least every 100ms. The rate of movement can then be calculated. If the calculated rate of movement falls below the threshold the powder is treated as no longer moving and image capture rate / poll rate is decreased.

[0077] Based on the measured flow speed and the (known) cross-section of the sampling body where the sensor 42 is located, an indication of the metal powder’s flowability can be inferred, and a volumetric flow rate (e.g. in mm3 / s) of metal powder through the apparatus calculated.

[0078] In other embodiments speckle images, or optical images of the powder, may be transmitted to the processor 11 for analysis instead.

[0079] The laser diode 41 only illuminates the metal powder when not illuminated by the LEDs 31 used to measure oxide layer thickness, so that one light source does not adversely affect measurement made by using the other light source.

[0080] By monitoring the flow rate of the metal powder 33 it is possible to detect inline blockages, or poorly flowing material. The latter is a key indicator of a metal powder’s ability to consistently spread on a metal powder bed of an additive manufacturing machine. This helps to minimise or prevent damage to an additive manufacturing machine or to a component of an additive manufacturing system. It also helps to ensure that additively manufactured products of sufficient quality are produced. Flow rate of the metal powder will be affected by the angle of slope of the apparatus. The angle of slope may be processed together with measured flow rate to provide a more accurate determination of the metal powder’s flowability.

[0081] In some installations the slope of the apparatus will be fixed and known. For situations where the slope is not known an accelerometer 49, forming part of an inertial measurement unit, is provided on the circuit board 9. This enables the angle of the apparatus with respect to gravity to be measured.

[0082] The apparatus also includes apparatus for determining the temperature of metal powder 33 within the sampling body, shown schematically in figure 10. The apparatus comprises a thermal IR sensor 44. This is mounted to the printed circuit board 9 behind the window 26 between two of the LEDs 31. The spacer 27 forms a shroud around the thermal IR sensor 44 defining a generally square cavity in which the thermal IR sensor 44 sits. The shroud extends from the circuit board 9 to the window 26 its side walls diverge from the circuit board to the window. The thermal IR sensor 44 faces the window 26 and is configured to detect thermal electromagnetic radiation 45 emitted by metal powder particles 33 adjacent to the window. Specifically, the thermal sensor is configured to detect radiation 45 in the mid-infrared region of the electromagnetic spectrum. The apparatus further comprises an internal temperature sensor (not shown) which is configured to directly measure the temperature within the housing. The output of the thermal sensor 44 is processed to compensate for leakage current of the IR sensor 44, owing to its ambient temperature.

[0083] In use, the thermal sensor 44 detects the intensity of mid-infrared radiation emitted by the metal powder 33. This information is processed, together with the temperature measured inside the housing, to determine the temperature of the metal powder 33 adjacent the window 26, whether moving or static.

[0084] The apparatus also includes apparatus for determining the apparent density of a metal powder, shown schematically in figure 11.

[0085] The apparatus comprises an inductor 46 formed by a conductive coil positioned adjacent and behind a second window 47 fitted to a second aperture formed through the flat wall of the sampling body. The second window can be formed from any suitable non-electrically conductive and non-magnetic material such as a plastics material. The inductor is embedded in the spacer 27 behind the window. It could however be embedded in the window, or the circuit board 9 or provided as an encapsulated inductor connected to the printed circuit board.

[0086] The coil 46 is electrically connected with a resonance capacitor 48 to form a resonant LC tank circuit. The coil 44 is provided in a folded line pattern that comprises two rectangular loops. To improve the sensitivity the line pattern can comprise more than two loops.

[0087] The apparatus is operable to determine the apparent density of static and moving metal powder 33. In use, the inductor is driven with an alternating current by the LC circuit. This produces an alternating magnetic field which induces eddy currents 46 within the metal powder 33 proximal to the window 47 opposite the inductor. The concentration / density of the eddy currents induced into the metal powder 33 affects the amount of magnetic energy that the coil 44 can store, altering its inductance. Since the concentration of eddy currents 46 within the metal powder 33 is associated with / indicative of its apparent density, measuring the inductance of the coil 46 allows the apparent density of different types of metal powders to be determined or inferred in a non-contact manner.

[0088] As the inductor is comprised in an LC circuit the frequency of oscillation of current in the circuit is dependent on its inductance, so a change in inductance caused by the presence of metal powder, and the density of that powder, alters the frequency of oscillation. Therefore, an indication of a metal powder’s density may be inferred or determined by measuring the frequency of oscillation of current in the circuit and, in particular, by determining the difference between the measured frequency of oscillation and the natural frequency of oscillation for the circuit when no powder is present.

[0089] Apparent density is the density of the powder in the state it is in, in the sample body, in contrast to a bulk or “tap” density measurement is consolidated (e.g. by tapping a vessel containing the power) before density is measured.

[0090] The apparatus may be calibrated so that a frequency or frequency difference from natural frequency of oscillation may be mapped to an apparent density value, for example by way of a look up table. Frequency and / or density values may also be mapped to one or more possible powder types so that a frequency measurement may be used to infer what type of powder is or is not present in the sample body.

[0091] Multiple density measurements may be taken over time and stored by the processor 11 , and a mean value calculated. In the described example measurements are taken every second for 15 seconds and a mean value calculated from the 15 measurements.

[0092] Each of the apparatuses described above may be continuously sampled multiple times per second or polled at regular intervals as desired. The outputs from the apparatuses may be logged and stored by the processor 11 or an associated memory and / or on a separate computer. If one or more of the outputs fall outside of acceptable ranges for degradation, flowability, temperature and density, a user may be alerted. The apparatus may comprise one or more indicators and a user may be provided with a visual indication, an audible indication or an audio-visual indication if the outputs fall outside of one or more of the acceptable ranges. The visual indication could be a light source in the form of a light emitting diode (LED) 20. As an example, an LED may be configured to illuminate red if an output falls outside of an acceptable range. Alternatively, or additionally, a user may be provided with a visual indication on a display screen associated with a computer. This may be accompanied by an audible indication.

[0093] In other embodiments some or all of the apparatus for measuring flow rate, angle of slope, temperature and density may be omitted, in any combination.

[0094] Figure 12 shows the apparatus 1 connected in line to a conduit such as a pipeline 50 by way of the flanged connections of the inlet 3 and outlet 4. Such pipelines are used for conveying metal powder within an additive manufacturing machine and / or for conveying metal powder into and out of sieve and blending apparatuses, for example. Valves may be provided upstream and downstream of the apparatus 1 operable to selectively prevent or allow the flow of metal powder through the pipeline 50.

[0095] The pipeline and apparatus are configured so that the flat internal surface of the conduit of the apparatus forms the bottom of the conduit and has a slope of about 45° to the horizontal along its long axis, and its horizontal along its short axis. Accordingly, when the upstream and downstream valves are both open metal powder is allowed to flow through the pipeline 50 it will flow along the flat surface of the conduit through the apparatus 1 and thus over the windows 26 and 47 in that surface. Information relating to the extent of degradation / oxidation, flowability, temperature and density of the moving metal powder can be obtained by the apparatus. This information can then be assessed independently or in combination to determine the condition or ‘health’ of the metal powder and whether it is suitable for use in additive manufacturing.

[0096] It is also possible to determine the condition and / or mechanical properties of static metal powder in the pipeline 50. In this respect the valve located downstream of the apparatus 1 is closed which causes metal powder flowing through the pipeline 50 to accumulate in the sampling body of the apparatus 1 as shown in figure 12 whereupon measurements of the properties of the static metal powder can be made.

[0097] Figures 14 to 16 show the apparatus 1 fitted into a pipeline, again with the flat surface of the conduit forming the bottom of the conduit with its long axis extending at an angle of about 45° to the horizontal. In this arrangement the pipeline above the conduit, connected to the inlet 3 of the apparatus, branches into two. One branch 52 connects to a source of powder, and the other 53 to a pressurised gas cleaning system. Each branch is fitted with a valve 54. Likewise, the pipeline connected to the outlet 4 of the apparatus also branches into two. A lower branch 55 connects to further metal powder processing apparatus and a higher branch 56 connects to the pressurised gas cleaning system. Again, each branch is fitted with a valve 54. In use, as powder flows through the apparatus it may contaminate the window 26 through which electromagnetic radiation is transmitted and received to measure properties of the metal powder. Contamination of the window restricts passage of radiation and the ability to make accurate measurements of properties of the metal powder. The processor is configured to determine when the window 26 has become contaminated during operation based on changes in the measured reflectance of the powder and / or measured reflectance when the apparatus is notionally empty of powder. Information from the apparatuses used to measure flowability, temperature and density characteristics of metal powder, can be used to supplement the reflectance data to improve the accuracy of detection.

[0098] When the processor determines that the window 26 is contaminated a cleaning cycle can be initiated.

[0099] Figure 14 shows the inline system in its normal operating state where metal powder flows through and open valve in branch 52, though the apparatus 1, and out through an open valve in branch 55. The other two valves are closed.

[0100] When a cleaning cycle is initiated the valve in branch 52 is closed. When the apparatus detects that powder is no longer flowing (or after a pre-determined period of time) the valve in branch 55 is closed, as shown in figure 15.

[0101] Then the valves in branches 53 and 56 connecting to the pressurised gas cleaning system are opened, allowing a pressurised gas (typically an inert gas such as argon) to flow into the apparatus 1 via branch 56 and blow any contaminants or trace amounts of metal powder from the apparatus and the surface of the window 26 out through branch 53. When the cleaning operation is complete measurement may be made by the sensors on the apparatus (with no powder present) to detect if the window has been decontaminated. If not, the cleaning process can be repeated. When the window is decontaminated, the valves are returned to their original state, shown in figure 14, and processing of metal powder resumed.

[0102] Figure 17 shows a metal powder storage and / or transport container in the form of a hopper 60. The hopper 60 comprises container with a frustro-conical lower portion leading to an outlet 61 which may be fitted with a valve to control flow of metal powder out of the container.

[0103] Apertures 62, 63 are formed in the frustro-conical wall of the container 60. One aperture 62 is fitted with an IR transparent window and the other 63 with a non-ferrous window. Apparatus for determining powder condition 64 is mounted over the windows. This apparatus is functionally the same as that shown and described in relation to figures 1 to 11, save that the windows through which measurements of powder properties are measured is / are into the container rather than into a conduit. This enables measurements of properties of powder in the container to be made. The powder may be static when the container outlet is closed or moving if the outlet is open to allow powder to flow out of the container. Measurements may be made whilst the container is in transit.

[0104] In addition to measuring powder properties, the apparatus is also able to determine the location of the container by virtue of its global positioning system (where present) and therefore a user may determine the location of the container 60 during transit and confirm whether it has arrived at its intended destination. The accelerometer of the apparatus can be used to determine the tilt, vibration, shock, or free fall of the container 60, including during transit.

[0105] Information regarding powder condition before, during and after transit, as well as movement and handling of the container 60 enables a user to determine the condition of metal powder in the container more accurately and whether it can be used in an additive manufacturing process.

[0106] In addition, use of the accelerometer allows the apparatus 62, 63 to infer the consistency and thoroughness of a blending process when blending occurs by rotating the container 60. This allows a fully closed-loop blending process, with feedback, without any operator exposure to metal powder, and no metal powder exposure to the environment.

[0107] The one or more embodiments are described above by way of example only. Many variations are possible without departing from the scope of protection afforded by the appended claims.

Claims

CLAIMS1. Apparatus for determining the density of a metal powder for use in an additive manufacturing process, the apparatus comprising: an inductor associated with a body for receiving metal powder to be analysed, wherein the inductor is comprised in an electrical circuit for driving an alternating current in the inductor and measuring an output which depends on the inductance of the inductor; and a processor arranged to determine the density of the powder based on the output.

2. Apparatus according to claim 1, wherein the electrical circuit is a resonant LC circuit comprising a capacitor in addition to the inductor.

3. Apparatus according to either claim 1 or claim 2 wherein the output is the frequency of oscillation of current in the circuit.

4. Apparatus according to any of claims 1 to 2, wherein the inductor is a coil.

5. Apparatus according to any preceding claim wherein the inductor is disposed adjacent to a wall of the body for receiving metal powder to be analysed.

6. Apparatus according to any preceding claim wherein the body for receiving metal powder to be analysed includes an aperture, a barrier is disposed in the aperture and the inductor is disposed on, adjacent to or at least partially embedded in the barrier.

7. Apparatus according to claim 6 wherein the barrier is formed from a non-magnetic plastic material, glass or crystal.

8. Apparatus according to any preceding claim, wherein the inductor is arranged so that, in use, the alternating magnetic field induces an electrical current in the powder to be analysed.

9. Apparatus according to any preceding claim, wherein the processor is configured to determine the absence or presence of metal powder in the body.

10. Apparatus according to any preceding claim wherein the processor is configured to process the output to determine the base alloy of the metal powder.

11. Apparatus according to any preceding claim wherein the body is a container for storing and / or transporting metal powder, comprised in an additive manufacturing machine, or is a conduit for conveying metal powder.

12. Apparatus according to claim 11 wherein the inductor is associated with a flat internal surface of the body.

13. A method of determining the density of a metal powder for use in an additive manufacturing process, the method comprising the steps of: placing an inductor in proximity to the metal powder; driving an alternating current in the inductor to generate an alternating magnetic field; and measuring an output which depends on the inductance of the inductor thereby to determine the density of the metal powder.

14. A method according to claim 13 wherein the inductor is comprised in a resonant LC circuit comprising a capacitor in addition to the inductor and the output is the frequency of oscillation of current in the circuit.

15. A method according to either claim 13 or 14 comprising providing a barrier which separates the inductor from the powder.

16. A method according to any of claim 13 to 15, wherein the method is performed on metal powder in a component of an additive manufacturing system.

17. A method according to any of claims 13 to 16, wherein the method comprises the step of determining the density of the metal powder while the metal powder is flowing through the component.

18. A method according to claim 17, wherein the method comprises the step of controlling the flow of the powder through the component.

19. A method according to either claim 17 or 18 wherein the method comprises the step of preventing the metal powder from flowing through the component of the additive manufacturing process and thereafter determining the density of the metal powder.

20. A method according to any of claims 13 to 19, wherein the method is performed on metal powder in an additive manufacturing machine, in a container for storing and / or transporting the metal powder or in a conduit for conveying metal powder.

21. A method according to any of claims 13 to 20, wherein the method comprises the step of determining the base alloy of the metal powder based on the measured inductance.

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