Technical Field of the Invention

The present invention relates to a method and apparatus for analysing metal powder, particularly but not exclusively a metal powder used in an additive manufacturing (AM) process.

Background to the Invention

In a known AM process an AM 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.

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.

For example, during a build operation unfused powder is subject to degradation. A metal powder may gradually oxidise which alters its properties and thus those of an article produced from the powder. The tendency of a powder to oxidise typically increases with temperature, and exposure to temperature may also affect other powder properties. Consequently, the nearer unfused powder is to an article being built, or a heat zone, the more likely it is to suffer degradation.

Also, when powder is fused the process may cause some heated particles of powder to be scattered from the powder bed around the manufactured article, degrading the quality of the unfused powder around the article.

To ensure adequate build quality of an article it is known to analyse used powder and stop recycling the powder when it has been degraded to a certain extent and/or to blend virgin powder with recycled powder so that the blended powder has an adequate bulk property for continued use. In an alternative approach a fixed upper limit is imposed on the number of times a batch of powder is recycled.

There are a number of problems with these approaches. Powder condition is typically analysed by making a bulk oxygen content measurement. The measurement process involves testing a powder sample, which cannot then be re-used. More significantly, it has now been realised by the inventors that bulk oxygen content (or other bulk) measurement can give a false impression as to suitability of a powder for re-use, especially where recycled powder is blended with virgin powder to produce a blend with an overall bulk oxygen content below a desired threshold. This is because it is not sensitive to the presence of highly oxidised or otherwise degraded particles which may have a significant deleterious effect on a build even though the bulk oxygen content is below a desired threshold.

Applying a general limit to the number of times a powder is recycled is a relatively crude approach and does not take account of the likely amount of powder degradation caused by a specific build. The nature and extent of degradation can vary considerably between builds.

Because existing testing methods for powders are destructive test results never relate to powder that is actually re-used and so are always an approximation of the average condition of that powder.

Other aspects of powder composition and condition can also effect build quality.

The presence of contaminant particles can have a similar effect to the presence of highly oxidised particles. Particle size and shape can also effect a build, as can packing density when a layer of powder is formed in an AM machine.

It is an object of embodiments of the present invention to address some or all of these problems. In particular it is an object of embodiments of the invention to provide improved methods and apparatus for analysing powder condition in a non-destructive way.

Summary of the Invention

According to a first aspect of the present invention there is provided a method for analysing a metal powder for use in an additive manufacturing process, the method comprising the steps of:

providing the powder in a close packed state adjacent a barrier; illuminating a region of the powder through the barrier; separately detecting the illuminating radiation received back from different parts of the illuminated region of the powder through the barrier, to produce an output which depends on the detected radiation; and processing the output to determine one or more properties of the powder.

According to a second aspect of the present invention there is provided apparatus for analysing a metal powder for use in an additive manufacturing process, the apparatus comprising: a container or conduit for metal powder comprising a barrier which, in use, contains powder to be analysed in a close packed state; an illumination device for illuminating a region of powder with electromagnetic radiation though the barrier; a detector for separately detecting the illuminating radiation received back from different parts of the illuminated region of the powder through the barrier and producing an output which depends on the detected radiation; and a processor arranged to process the output thereby to determine one or more properties of the powder.

The method may be performed in any suitable environment. The powder may be a sample of powder for testing placed in a test container comprising the barrier. Alternatively, analysis may take place in an additive manufacturing machine or powder transport apparatus, such as a pipe or conduit. As such the apparatus may be comprised in testing apparatus, powder transport apparatus or additive manufacturing apparatus.

The barrier may be comprised in an additive manufacturing machine, powder transport apparatus or powder manufacturing apparatus. The barrier may be comprised in a powder or build container of additive manufacturing apparatus and/or comprised in a platform of additive manufacturing apparatus which extends between powder and build containers of the apparatus.

The barrier is preferably formed of a material which is at least partially transparent to electromagnetic radiation of interest. Where visible light is of interest the barrier may be formed from glass or a transparent plastics material. The barrier may comprise a window in a container or conduit. The barrier may be generally planar or comprise a substantially planar surface adjacent which powder is, in use, contained in a close packed state, but could be curved such as a wall of or window into a pipe with a generally circular cross-section.

Where the barrier is horizontal or substantially horizontal powder can be supported on the barrier in a close packed form. Where a barrier is not horizontal or substantially horizontal, for example a sidewall of, or window into, a container or conduit an arrangement may be provided to contain powder so that it will stack against the window. This may comprise a device, such as a valve, operable to selectively prevent powder from flowing out of the container or conduit which may be closed in use to thereby cause powder admitted to the container or conduit to be formed into a close packed state adjacent the barrier.

The illumination device and detector are preferably housed in an enclosure which surrounds at least part of the barrier, the enclosure preventing or at least restricting ingress of electromagnetic radiation at wavelengths of interest, in order to allow controlled illumination of the metal powder at those wavelengths. The enclosure may be substantially, or capable of being made, substantially light tight.

The radiation may be light and may include visible light and/or ultra violet light and/or infrared light. The illumination device may comprise any suitable illumination device, such as a lamp including an LED, incandescent and/or discharge lamp. A laser, such as a laser forming part of an additive manufacturing machine operable to fuse powder in the machine during a build process, could also be used (in a low power mode) to illuminate the powder. The means for illumination may illuminate all of the region of powder simultaneously, or parts of the region separately. The means for illuminating may scan a beam of radiation over the region of powder.

The degree to which the powder is close packed may be controlled, such as by tapping the powder in a predetermined way or number of times prior to making a measurement and/or by including a passing the powder through a pre-conditioning funnel prior to measurement. The powder preferably has sufficient depth away from the illuminated surface so that the powder is substantially opaque to the detector. The powder may be a sample of powder taken from a batch of powder. The powder may be virgin powder, or recycled powder which has already been used in an additive manufacturing process. Powder may be analysed at different stages in a process, and may be analysed at multiple points in a process. As such apparatus for analysing powder may be provided at different points on additive manufacturing and powder transport and processing apparatus. For example powder could be analysed at one or more or any combination of the following stages of a process: as it is transferred into an additive manufacturing machine, in a powder container in the machine, on a platform in the machine, in a powder bed in the machine, in a build container in the machine, as it is transferred out of the machine, after a sieving operation, and after blending with another powder.

The detector make take a variety of forms, and may comprise any suitable sensor or sensors. The method may involve detecting and the detector may be arranged to detect radiation reflected and/or scattered by the powder. The detector may be arranged to detect illuminating radiation received back from all of the region of powder simultaneously, or from parts of the region separately. The detector may be arranged to scan the region of powder. Relative movement between the detector and powder may be achieved either by moving the detector relative to stationary powder or moving powder past the detector, such as in the case of powder transport apparatus.

The detector may include a one or two dimensional array of sensing elements, such as a CCD or CMOS sensor. The detector may include one or more, or an array of photodiodes, spectrometers or spectrophotometers. The detector may include one or more filters for excluding and/or admitting selected wavelengths of electromagnetic radiation.

The detector may include one or more focussing elements, such as a lens, for focussing radiation received back from the powder onto one or more sensors. The one or more focussing elements may focus an image of at least part of the region of powder into an image plane, in which may lie one or more sensors. The detector may comprise an image capture device, such as a camera or microscope. The detector may comprise a hyperspectral camera. With these arrangements the apparatus is able to separately detect illuminating radiation received back from different parts of the illuminated region of powder, either because the detector is able to spatially resolve the source of radiation and/or because the detector only detects light reflected from a part of the region at one time and/or because only part of the region is illuminated at one time.

The output may be processed to determine a variety of different properties of powder, including: condition of individual particles of powder; a surface property of the powder and/or individual particles of the powder, such as colour of the powder and/or individual particles of the powder; a property of the powder or individual particles of powder which affects their interaction with electromagnetic radiation, in particular their ability to reflect and/or scatter particular wavelengths of radiation; the shape of individual particles of powder; surface texture of the powder or individual particles of powder; the presence of contamination; packing density of the powder; and spreadability of powder.

The output may comprise values depending on the wavelength or wavelengths or perceived wavelength of radiation, or the colour of light received from respective areas of the region of powder. The output may also comprise values depending on the intensity of radiation received back from respective areas of the region of powder. The output may comprise values depending on the intensity of radiation at different wavelengths. The output may comprise values depending on the intensity of radiation over a range of wavelengths, which may be continuous. And/or the values may depend on the intensity of radiation at one or more discrete wavelengths. The output may be a function of the wavelengths detected. The output may comprise a value or values for each of plurality of areas. The areas may be substantially the same size. The areas may be contiguous, or they may be spatially separated. The areas may, together, encompass all or most of the region. It is preferred that each area has an area less than the area of the surface of the powder occupied by a single particle of the powder of average size, and it may be significantly smaller such as ¼, 1/10 ^th , l/100 ^th l/500 ^th or 1/1000 ^th of that area. As such, for particle sizes common in metal powders for additive manufacturing each area may be less than 1000 pm ² or less than 100 pm ² . In this way, at least one and preferably multiple values included in the output are influenced by a single particle of powder. Of course as average particle sizes vary between types of powder the optimum size of the area will vary with powder type. This enables the condition of individual particles to be determined.

Values obtained from analysing a region of powder may form a data set and/or comprise an image or image file.

The steps of detecting radiation, producing an output and of processing the output may take place at different locations.

The data set may effectively comprise an image of all or part of the illuminated region of powder which may comprise, at least in part, information obtained from recording non-visible wavelengths of radiation.

Where the data set effectively comprises an image the image may be a digital image. It may be formed by, or divided into, a plurality of elements such as pixels. The elements may each correspond to an area of the powder from which the detector has separately received radiation, and produced an output. The elements are preferably of substantially the same size. As such, it is preferred that the ratio of elements in the image to the number of particles in the imaged region or surface of the powder is at least of the order of 1 : 1, but preferably higher such as at least 4: 1 or 10: 1 or 100: 1 or 500: 1 or 1000: 1. That way the wavelength or wavelengths of detected radiation represented by that element is likely to be influenced only by properties of a single particle of powder.

A data set for processing might typically comprise a value or values for each of 2 to 6 million areas and represent around 5000 particles of size generally in the range 10-110pm or 40-50pm, but the size of a data set may vary significantly depending on application.

A data set may comprise data which describes each area of powder, or image element, according to an established colour standard, for example RGB or CIELAB, insofar as the image is formed of visible colours or the standard is capable of describing relevant non-visible electromagnetic radiation of interest, or non-visible wavelengths detected have been represented in a dataset by a false colour. Multiple regions of the same volume or sample of powder may be analysed. The regions may be adjacent or spaced apart. In embodiments at least 2, 3, 4, 5, 10, 50, 100 or more regions of the same powder are analysed to form multiple data sets for the powder. The number of data sets will depend on the volume of data required for statistical significance. Analysing multiple regions can help in assessing how well blended the powder is.

Data in a set may be processed by comparing a data value or values for one area of the region of powder with those for one or more other areas and/or with reference data.

Data in a data set may be removed so that the remaining data represents a chosen sub region of powder, equivalent to cropping of an image. This is useful where the detector includes a focussing element which produces an image in the detector by enabling distortion to be excluded. It also allows for an easier and more reliable comparison between different data sets especially those relating to the same sample. In an example a data set is reduced in size to define 2000 x 2000 contiguous areas of the powder, and so may define an image consisting of 2000 x 2000 image elements.

A data set may be processed to determine if its quality is sufficient for further processing and/or to determine if all separately acquired data in a particular set is sufficiently similar. Data that does not meet specified quality criteria may be rejected and not processed further. This may for example involve determining statistics from data and determining if those statistics do, or do not, fall within predetermined ranges, or differ from data set to data set by more than a predetermined threshold. In an example the mean intensity of a particular detected wavelength or range of wavelengths or colour channel used to define detected radiation may be calculated for all areas of the powder from which radiation has been detected, or all areas having greater than a threshold luminance, as well as a deviation from that mean.

Data in a data set which relates to space between particles of powder may be identified. This enables that data to be excluded from further processing. Such data may be identified by identifying data which defines areas of the region of powder with a luminous intensity below a predetermined threshold. Space between particles of powder will tend to appear darker, and thus have a lower luminous intensity, than the particles. Such areas of the powder may be regarded as background areas, with the remaining, more luminous, areas being foreground areas. Where a data set comprises an image this step comprises identifying darker image elements. Darker image elements may be regarded as background elements and lighter elements foreground elements.

A data set may also be processed to determine, or at least estimate, the number of particles of powder it represents. Where a data set comprises an image this may be achieved by watershed segmentation. This step, where present, is preferably performed after removal of background elements from the image.

As an alternative, or in addition, the number of particles represented by a data set may be estimated by another suitable method not relying on the data set, for example by a knowledge of the size of the area the data set represents, mean size of powder particles and/or packing density of the powder.

Data representing foreground areas of the powder represents properties of particles of the powder which affect how the particles interact with the illuminated radiation, including surface properties.

The number of particles of powder represented by a data set with a surface property which falls outside a predetermined range may be determined. The proportion of powder with a measured surface property which falls outside a chosen range may be determined.

This may be achieved by identifying areas, preferably foreground areas defined by the data where the wavelength or wavelengths of radiation detected lies outside a chosen range.

Area selection may be based on a statistical analysis of detected wavelengths. Selected areas may be those for which the detected wavelength(s) defines them as outliers in the wavelength distribution across all the foreground areas. For example, elements may be selected by determining how their wavelength(s) deviate(s) from the mean wavelength(s) of all areas. Preferably the selected areas represent the outlying 5% or less 1% or less, or 0.1% or less of the wavelength distribution of foreground areas. Having identified the data that reflects a property falling within a range of interest, individual particles that have that property can then be identified. To do so groups of connected areas exceeding a predetermined number are identified. The number is chosen to be that which represents the combined area expected to be occupied by a single particle of powder. The identified groups of areas may thus be assumed to represent at least one, but typically one, particle with a property which meets the chosen criteria.

Where the ratio of areas to particles is close to 1 : 1 this step may be omitted and it assumed that a single image element represents a single particle.

Data can then be stored for each identified particle, being the data relating to the area or areas of the powder occupied by the particle. The number of areas in an identified group is indicative of the size of the particle the group represents. The average wavelength(s) of radiation detected from the areas in the group is representative of the colour or other properties of the particle the group represents. This enables the number of identified particles to be determined, as well as properties of the particles and analysis of this data enables various information relating to the powder to be determined or inferred. Particles identified in this way may be further classified by surface property, such as colour, to identify particles having a surface property which falls into a particular range. Other techniques can then be used to select particles of interest from the identified particles.

The ratio of background to foreground areas gives an indication of the ratio between particles of powder and space between those particles and thus an indication of the packing density of the particles. Changes in packing density observed in images of a batch of powder taken over time may reveal changes which affect powder flow properties.

Determining the overall distribution of wavelengths received from a region of powder, or from all foreground areas of the powder, is indicative of chemical properties of the powder and in particular the degree of oxidation of the powder, since oxidation of metal powders typically affects their interaction with radiation. A knowledge of how oxidation affects the colour of a particular powder type may be used to determine a bulk oxygen content for the powder. The number of particles identified which have returned a wavelength or wavelengths of radiation which falls outside a chosen range can reveal if a powder contains highly oxidised particles or is contaminated, such that it may be desirable that the powder is not used or re-used. The number, together with the calculated or estimated total number of imaged particles may be used to calculate the proportion of particles with a measured property falling outside the chosen range.

This information may be used to inform or control subsequent processing of the analysed powder or powder from which the analysed powder was taken.

The method may involve the step of indicating that a powder is not suitable for re-use when the number or proportion of particles identified as having a property outside a pre-determined range exceeds a predetermined range. The pre-determined range and the proportion may be established depending on the particular powder being analysed and its intended use. Typically, though, as the intention is to identify the presence of significantly oxidised or degraded or contaminant particles the range is preferably set to encompass values for the measured property indicative of powder that has suffered what may be regarded as normal degradation as a result of being used in a build process, as might typically be caused by exposure to oxygen and low temperatures. Thus, those particles having a measured surface property outside of this range are outliers. Their measured surface property reflects the fact that they have been exposed to abnormal degradation, typically as result of being exposed to a high temperature (but without becoming fused to form part of a constructed article). It is thought that when the population of such outliers exceeds a certain proportion of the overall population of particles re-use of the powder carries an increased risk.

The method may also involve determining the average measured property of the proportion of measured powder whose measured property falls within the predetermined range. This measure is indicative of the overall average degradation of the powder excluding the outlying significantly degraded particles. This measure therefore gives an indication of the level of degradation resulting from normal degradation.

The method may also involve indicating that the tested powder is not suitable for re-use when average measured property of the proportion of powder, and thus approximate proportion of measured particles whose measured surface property falls within the predetermined range is greater (or less) than a predetermined threshold. Thus, powder can be indicated as no longer suitable for re-use as a result of the, possibly cumulative, effect of normal degradation.

The method may also involve determining the average measured property of all of the powder, and so all measured particles, by processing data for all foreground areas. Such a measure is indicative of the overall average degradation of all the particles, and so may give a similar indication to a bulk oxygen measurement, save that a bulk oxygen measurement will also measure“internal” oxygen of a particle, that being oxygen present inside a particle, as well as oxygen of any oxide outer layer the build-up of which affects a surface property of the particle.

The method may also involve indicating that the powder is not suitable for re use when an average measured property of all measured particles, is greater or less than a predetermined threshold. The method may involve controlling apparatus based on a measured property of a powder, such as controlling an AM machine or powder processing, handling or transport apparatus.

Where an average measured property of a powder, and thus particles of interest, is required this may be a mean, and may be obtained by determining the average measured surface property represented by those areas of the region of powder where there are particles of interest.

Information relating to a powder analysed by the method may be used to automatically control apparatus including additive manufacturing apparatus and/or powder handling, transport and processing apparatus. Apparatus may for example be caused to discard and/or process powder depending on detected condition of the powder.

The processor may be a programmed computer, and may be arranged to cause the apparatus to perform some or all of the method steps discussed above.

In all aspects of the invention particles of powder returning radiation comprising a wavelength or wavelengths falling within a particular range may be identified, and this may be used to infer the degree of oxidation of the particles having regard to experimental data relating to the powder type concerned.

Embodiments of aspects of the invention provide a non-destructive method and apparatus for determining powder condition and deciding whether or not a powder sample is suitable for re-use in a particular build operation. Where the determination is made by looking at the proportion of outlier particles this provides a new and useful measure of powder condition which enables improved decision making, and therefore powder use, over current measurements of bulk powder properties.

The method and apparatus is also useful for identifying the presence of contaminant particles where those particles a surface property which differs to the same property of particles of interest.

According to a third aspect of the present invention there is provided a method for analysing a metal powder for use in an additive manufacturing process, the method comprising the steps of: illuminating a region of the powder, without melting the powder, with electromagnetic radiation comprising radiation in the non-visible part of the electromagnetic spectrum; separately detecting the illuminating radiation, comprising radiation in the non- visible part of the electromagnetic spectrum, received back from different parts of the illuminated region of the powder, to produce an output which depends on the detected radiation; and processing the output to determine one or more properties of the powder.

According to a fourth aspect of the present invention there is provided apparatus for analysing a metal powder for use in an additive manufacturing process, the apparatus comprising: an illumination device for illuminating a region of powder, without melting the powder, with electromagnetic radiation comprising radiation in the non-visible part of the electromagnetic spectrum; a detector for separately detecting the illuminating radiation, comprising radiation in the non-visible part of the electromagnetic spectrum, received back from different parts of the illuminated region of the powder and producing an output which depends on the detected radiation; and a processor arranged to process the output thereby to determine one or more properties of the powder.

The third and fourth aspects of the invention may include any of the features of the first and second aspects of the invention as desired or as appropriate.

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

Figures 1 to 3 are schematic views of embodiments of apparatus for analysing powder condition; Figure 4 to 6 are schematic views of embodiments of powder processing and transport apparatus including apparatus for analysing powder condition;

Figure 7 is a schematic side view of an additive manufacturing machine including various apparatus for analysing powder condition;

Figure 8 is a schematic plan view of the apparatus of figure 7; Figure 9 is a flowchart showing steps involved in processing an image of powder;

and

Figure 10 is a graph showing number of particles against wavelength.

In what follows the terms up, down, top, bottom and like terms are used to describe the illustrated apparatus in the orientation in which it is shown in the drawings, which is the orientation in which it is intended to be used, but should not be taken as otherwise limiting. Like reference numerals are used throughout the drawings to denote like components. The drawings are not to scale. The drawings are also schematic and so do not necessarily represent the optimum positioning of components, as will be appreciated by a person of ordinary skill in the art. Referring to the drawings, figure 1 shows a first apparatus for analysing metal powder (not falling within the scope of the claims). It comprises an openable substantially light tight enclosure 1. The enclosure houses a container 2 for powder 3 which may take the form of a dish or slide, or any other suitable form. The container is open to the top and has a substantially square opening with a side of about 10mm, giving it a cross-sectional area of about 100mm ² . It has a depth of at least 2mm. The illustrated container is shallow, but it could be significantly deeper so that the container is elongate. The container may be removed from the enclosure. The enclosure also houses a lens (or microscope) 4 which is mounted to (or comprised in) a digital camera 5 sensitive to visible, and optionally also to ultraviolet and/or infra-red light, and lamps 6 emitting visible and optionally ultraviolet, infra-red and visible light. The camera 4 comprises a substantially square sensor, such as a CCD sensor, with approximately six mega pixels (twelve mega pixels in another embodiment) and is connected to a computer 7 which comprises a keyboard and mouse or other user interface and is connect to a display 8 and/or other output device. The lamps are arranged to provide a diffuse light. They are shown as dome or flat dome lamps. In an alternative arrangement (and in other embodiments) a ring light could be used.

In use, a sample of powder 3 taken from a batch of powder to be analysed is introduced into the container 2, either with the container in or out of the enclosure 1. The powder is introduced in sufficient quantity to form a close packed depth of powder which entirely obscures the bottom of the container 2 when viewed from above. So the depth of powder typically comprises at least two, and preferably more than two, layers of particles. The powder is levelled in the container, such as by tapping the container, so that it has a substantially flat upper, planar surface. If powder has been introduced into the container whilst outside the enclosure the container is then positioned in the enclosure beneath the microscope and the enclosure closed.

The lamps 6 are then activated. The lamps may be controlled by the computer 7. The lamps are arranged to illuminate the upper surface of the powder 3 in the container 2. Illuminating the powder with lamps in a substantially light tight enclosure enables powder to be analysed in controllable and repeatable light conditions. The camera 5 is then caused to take a digital image of the illuminated surface of the powder in the container and to transmit it to the computer 7. The digital image comprises information relating to all wavelengths of light detected by the camera. The camera and microscope are arranged to take an image of about 12mm ² of the surface of the powder in the container. Multiple images of the surface of the powder are taken until substantially all of the surface of the powder has been imaged. In alternative embodiments the camera and lens may have a different field of view. The surface of the powder could thus be captured in a single image.

Metal powders used in AM processes typically have an average diameter of the order of tens of microns. As such, the number of particles visible to the surface of the powder imaged by the camera will be of the order of thousands and so about three orders of magnitude less than the number of pixels of the sensor. The camera is thus able to produce a digital image of the surface of powder in which there about 1000 times as many pixels as the number of particles of powder shown in the image.

The image taken by the camera is then transmitted to the computer 7 for processing.

Figure 2 shows a second apparatus for analysing metal powder. In this case a light tight enclosure 1 is provided underneath a container 2 for containing powder 3. The enclosure houses an upwardly directed camera 5 and lens 4 and lamps 6, similar to those of the embodiment of figure 1. Note that in this and other described embodiments the lamps may be positioned differently and may be positioned in front of the camera and/or lens. The base of the container is transparent or at least partially transparent to the wavelengths of light produces by the lamps and to which the camera is sensitive. The camera is thus able to take an image or images of the surface of close packed powder 3 in the container 2 through the base of the container. The top surface of the base of the container, on which powder is supported, is substantially planar, so that camera is inherently presented with a flat surface of powder to image. The bottom surface of the base of the container is substantially parallel to its top surface. In use powder is introduced into the container to sufficient depth so that it is substantially opaque when viewed through the base of the container, and optionally the container tapped so that the powder is settled into a close packed state. Figure 3 shows another apparatus for analysing powder 3. This comprises an elongate tube 2, with one closed end, for containing powder. The tube 2 has one flat side wall 2a. Conveniently the tube is of square or rectangular cross-section, both other cross-sections are possible, such as a D-shaped cross section. A camera 5 and lens 4 assembly is provided in a substantially light enclosure 1 together with lamps 6 in the manner of the apparatus of figures 1 and 2. The camera is directed towards the flat side wall of the container. This side wall is at least partially transparent to the wavelengths of light produced by the lamps and to which the camera is sensitive, and, similar to the base of the container 2 of figure 2 it has two substantially parallel, opposite, planar sides. The camera is arranged to image close packed powder contained in the container through the side wall. The container is mounted for movement relative to the enclosure and camera allowing the camera to image different parts of the surface of a sample of powder contained in the container.

Figure 4 shows powder transport apparatus comprising pipes 10 leading into and out of a powder sieve, and into a powder blending device. Each pipe 10 includes a planar or substantially planar transparent window 11 over which is fitted a light tight enclosure 1 which houses a digital camera 5 fitted with an appropriate lens 4 for taking an image of powder in the pipe 10 through the window 11. A lamp or lamps 6 is/are provided in the enclosure around the lens 4 to illuminate the powder through the window 11. The camera outputs its image, comprising information relating to all wavelengths of light detected by the camera to a computer 7 with output device 8. Alternatively or additionally the output may be sent to a computer or processor controlling the powder transport apparatus or associated equipment. A valve 12 (such as a butterfly valve) is provided in the pipe 10 downstream of the blending device for permitting and preventing the flow of powder through the pipe. In use the valve is closed to enable the pipes to be filled with powder so that close packed powder is presented to the windows 11 in the pipe, which are transparent to the wavelengths of light produced by the lamps and to which the cameras are sensitive.

As with the apparatus shown in figure 1 the digital camera 5 has a sensor with about 1000 times the number pixels than the expected number of powder particles visible in the area of the window 11 imaged by the camera when the pipe is full of close packed powder to be analysed. Similarly the lamp or lamps 6 emit and the camera 4 is sensitive to a broad spectrum of light including ultra violet, visible and infra-red light. The window 11 is substantially transparent to this broad spectrum of light.

A window 11 and associated enclosure 12 with camera 5 is provided in both of the pipes 10 leading to and from the sieve enabling the condition of powder to be analysed before and after sieving. Cameras also enable the condition of powder entering both inlets to the powder blending device to be analysed.

Windows could be provided into powder transport conduits or powder storage containers of other types of apparatus, such as for example an additive manufacturing machine.

Figure 5 shows a powder storage apparatus comprising a hopper with a lower frustro conical wall and a cylindrical sidewall. An upright pipe 10 is connected to an inlet at the top of the hopper, via a valve 12 for permitting and preventing the flow of powder through the pipe 10 into the hopper. An outlet is provided at the bottom of the hopper, also fitted with a valve 12 for permitting and preventing the flow of powder out of the hopper. The hopper is for storing metal powder, and could be used together with or form a part of an atomiser used to produce metal powder. Two windows 11 are provided in the frustro conical base of the hopper, each of which is fitted an enclosure housing a lamp 6 and camera 5 with lens 4 (of the type shown in figure 4) for imaging powder in the hopper which is close packed against the window. As with the apparatus of figure 4 the window is transparent to the wavelengths of light produces by the lamps and to which the camera is sensitive, and has opposed substantially parallel planer surfaces.

The pipe 10 leading into the hopper 12a is provided with five pairs of windows evenly spaced along a section of the pipe above the valve at the inlet to the hopper. Each window is fitted with a respective enclosure 1 housing a camera 5 with lens 4 and lamps 6 in the manner of the enclosures 1 under the hopper 12a. These cameras enable close packed metal powder in the pipe to be imaged at various positions along the length of the pipe. This is useful, for example, in being able to determine if a powder being received into the pipe 10 has changed over time, for example during a production run. Each of the camera provides an output to a computer or processor 7, which in turn provides an output to a user via a suitable user interface 8 or equipment controlled by the output. As an alternative to providing a plurality of enclosures 1 and cameras 4 along the length of the pipe 10 a single camera (or a smaller number of cameras) may be moveably mounted relative to the pipe and arranged to image powder at different positions along the length of the pipe.

The valve 12 at the inlet to the hopper can be closed to cause powder to fill the pipe in a close packed fashion for imaging.

Figure 6 shows a part of a pipe for transporting powder. The pipe is fitted with a bypass conduit 10a, provided with valves 12 which enable the flow of powder to be controlled into and out of the bypass. A further valve 12 is disposed in the pipe between the connections to the conduit. The bypass conduit comprises a substantially planar window 11 to which is fitted an enclosure 1 housing a camera with lens and lamp in the manner of the enclosures of figures 4 and 5. The output of the camera is connected to a computer or processor 7 and a display or other output or apparatus to be controlled depending on the output of the camera 8. The bypass conduit is formed by a pipe with a smaller cross-section than pipe 10. The valves may be controlled to allow powder travelling in the pipe 10 to flow into and fill the conduit, enabling the camera 4 in the enclosure 1 to image the powder in a close packed fashion. Effectively the bypass enables a sample to be taken from powder flowing in the conduit for analysis, enabling close packed powder to be imaged even if powder is not moving through the pipe in a closed packed state.

Figures 7 and 8 show an additive manufacturing machine 13. The machine comprises an enclosure 1 which is or can be made substantially light tight. The enclosure houses a powder delivery container 14 with a powder delivery piston 15, a build container 14a with a build platform 16, and a wiper blade 17 mounted to a moveable support for transferring powder from the powder delivery container to the build container. The enclosure also houses output optics 18 of a laser for selectively melting powder on the build platform 16. These features are all common to known selective layer melting additive manufacturing machines. The enclosure 1 additionally houses two cameras 5 with appropriate lenses 4 for taking an image of an area of the top surface of powder in the powder delivery and build containers, and lamps 6 disposed around each camera. An imaging sensor 19 and lamp 20 is also mounted to the moveable support for the wiper blade 17 and arranged to scan an image of the surface of powder in the powder supply or build containers as the wiper blade travels to and fro across the containers. As with the other embodiments the lamps 6, 20 emit, and the cameras 5 and sensor 19 may be sensitive to a broad spectrum of light including one or more of ultra violet, visible and infra-red light.

A camera 5 with lens 4 and associated lamps 6 is also mounted in enclosures 1 positioned behind windows through the sidewall of the powder and build containers 14, 14a, through which they are able to image close packed powder in those containers.

A further camera 5 with lens 5 and associated lamps 6 is mounted in an enclosure 1 positioned beneath a window formed in the platform extending between the powder and build containers, and thus able to image powder on that platform.

The three windows are all transparent to the wavelengths of light produces by the lamps and to which the camera is sensitive. The window in the platform has opposed substantially parallel opposed planar surfaces. The windows into the powder and build containers may also have such surfaces, or be shaped to conform to the shape of the internal wall of the container where this is not flat.

The cameras 5 and sensor 19 are arranged to output an image to a connected computer or processor 7 with an output device 8, or arranged to control the additive manufacturing machine in dependence on the output. As with the apparatus shown in figures 1 and 2 the digital camera 5 and sensor 19 are arranged to produce an image of powder with about 1000 times the number pixels than the number of particles shown in the imaged area of powder.

It will be appreciated that the machine shown in figures 7 and 8 need not have all of the illustrated imaging devices. More than one device may be employed, though, to image powder a different stages in its use.

In use each embodiment of the apparatus produces a digital image of the surface of powder in the apparatus. The digital image comprises a set of data defining properties of image elements comprising information relating to all wavelengths of light detected by the camera, and thus the wavelength or wavelengths of light received by the camera from each area of the surface of the powder corresponding to an element of the image. The ratio of image elements to the number of particles of powder shown in the image is about 1000. The image data is transmitted to the computer where it is stored in a manner where the wavelength or combination of wavelengths of light, or perceived wavelength represented by and the luminous intensity of each element of the image is defined - such as in the CIELAB colour space by variables L, a and b.

The computer is arranged to process the image data in order to determine information relating to the condition of the powder shown in the image by performing at least some of the steps shown by figure 4.

As a first optional step 21 the image may be cropped to a predetermined size, excluding elements outside a boundary (or some other chosen region) of the original image. This optional step allows distorted areas of an image to be excluded as well as enabling images taken by different cameras or sensors to be reduced to represent the same area and/or to have the same number of pixels.

The remaining image data, or remaining image data, may then be tested 22 to ensure that it is of sufficient quality for further processing. If not, it is rejected at 23 and a new image is obtained.

If the image data is of sufficient quality, the computer then identifies elements with a luminance below a predetermined threshold and removes these from the image data at 24, with the aim of removing elements which represent space between particles of powder (or other background material) in the image. The actual threshold will depend upon characteristics of the particular apparatus being used and type of powder being tested. With the elements of the image removed which lie outside the threshold the remaining image elements are taken to represent particles of powder in the foreground of the image.

The data for the remaining image elements may then be processed at 25 to estimate the number of particles they represent using a suitable technique, such as watershed segmentation. The total number of particles represented can also be estimated in other ways. For a given powder and apparatus the number of particles expected to be visible in an area of the surface of the powder corresponding to that represented by the image data can be calculated with a knowledge of the expected particle size and expected packing density of the powder.

The data for the remaining image elements is then statistically analysed at 26 to determine how the wavelength(s) or perceived wavelength or colour represented by each image element is distributed about the corresponding mean value for all remaining elements to detect outlier elements representing a wavelength(s) or perceived wavelength that places them outside a threshold proportion of the entire population of elements. This may be performed using a Chi-squared test for outlier detection. Other approaches may be used, including the use of machine learning. The relevant proportion of the population may be selected according to the type of powder being analysed, but a typical proportion is 0.1%, that is to say that the elements of interest, the outlier population, make up 0.1% of the entire population of elements.

Visual representations of this step are shown in figure 5 which plots the number of image elements on the vertical axis against a measure of perceived wavelength (or colour) on the horizontal axis. This shows a generally bell-shaped curve of distribution about a mean value at 27, and lower 28 and upper 29 thresholds which identify the outlying 0.1% of the population represented by the area under the curve outside the thresholds.

The outlier elements are then subjected to a connected component filter at 30 to determine if they are spatially connected in the image they define. Any group of connected image elements which exceeds a predetermined number of elements is considered to represent a single particle. The data representing each such identified group is associated with a unique particle identifier with the first identifier identifying the largest group of connected elements, the second identifier identifying the next largest group of connected elements, and so on.

At this stage the computer has produced sets of image data which define the size and surface property of individual particles affecting how they interact with the different wavelengths of light with which they have been illuminated that causes them to represent statistical outliers within the powder. That surface property includes colour insofar as visible light is concerned but is broader in that it includes properties that alter interaction with ultraviolet and infrared light. Inclusion of these non-visible wavelengths increases the range of particle properties, especially relating to particle composition, which can be detected by the apparatus.

This data is then analysed at 31 to extract useful data relating to the condition of the analysed powder, including:

• The number and thus proportion of particles with a surface property which lies outside a predetermined range.

• The mean wavelength of light received from particles lying within the predetermined range.

• The mean wavelength received from of all image elements.

It has been found that the way in which metal particles interacts with light changes as the particles degrade. In particular it changes as particles oxidise and/or are exposed to heat. The more a particle is oxidised or the higher temperature a particle is exposed to the more its interaction changes. So, the amount of change is related to the degree of degradation a particle has suffered and thus also its suitability for re-use. This change in property, for some particles types, is visible to an observer as a change in the colour of the particle, but additionally interrogating particles using non-visible wavelengths increases prospects of detecting change in a wider range of particle types.

It has further been found that, notwithstanding the average condition of a batch of powder, the presence of highly degraded particles can render the batch unsuitable for re-use. This is because inclusion of even a single highly degraded particle in a build can significantly affect properties of the build. Where a highly degraded particle or particles become(s) incorporated into an article this could render the article unsafe, especially if the particle(s) is/are incorporated into the article at a location where there will be a stress concentration in use.

The first measure above will, assuming that the batch of powder from which the sample is taken is well mixed or the imaged area of a powder is representative of the constitution of the powder as a whole, generally mirror the proportion of significantly degraded particles throughout the sample and throughout the, or batch of, powder tested. Multiple samples may be taken from a given batch and analysed separately, or multiple tests performed on a batch of powder in order to improve accuracy such as by taking multiple images of a surface of the powder. And/or a particular sample could be analysed, mixed, and then reanalysed. An appropriate wavelength range and threshold minimum proportion outside that range can be determined for a given powder and build and where the proportion of particles outside the threshold exceeds the chosen limit the batch of powder is deemed unsuitable for re-use, at least for the build in question.

Thus, this measure enables powder condition to be determined independent of a bulk quantity.

The second measure provides an indication of the average degradation of the remaining powder when the particles lying outside the threshold have been discounted. Such a measure is more akin to the result of a conventional bulk oxygen measurement, but obtained in a more convenient and non-destructive way, save that it excludes the influence of significantly degraded particles (or any internal oxygen). Powder may be deemed unsuitable for re-use where the average wavelength received from the remaining particles, when the particles lying outside the predetermined range have been discounted, lies outside another predetermined range.

The third measure is similar to the second measure, but takes account of the significantly degraded particles. Powder may be deemed unsuitable for re-use where the average wavelength of light received from particles lies outside another predetermined range.

A decision whether or not to re-use powder can be based on one or more of the three measures described above. Typically a powder would not be re-used if any measure determines that the powder should not be re-used. In one embodiment the first and second measures are calculated and powder deemed unsuitable for re-use if either measure indicates this.

It is useful to analyse new powder before it is used and to subsequently analyse it after it has been used in a build process and any further build processes. Analysis of the new powder provides useful control data with which to compare that of subsequent analysis. Other information about a powder may be determined by detecting light returned by the powder. Non-powder artefacts may be detected in the powder. Anomalous powder particles may also be detected, for example particles made of different material to that intended, where the anomalous particles may be identified by an observable property. An estimate of total incident energy received by the powder may be made. Where multiple images of a sample or batch of powder are made and analysed it is possible to determined how well blended the powder is by comparing results between images. Images taken form powder processed by different machines, such as AM machines, may be used to compare machine performance and/or determine machine health. Images taken a different time periods and/or different positions in apparatus can help to track transit of powder through the apparatus.

Where apparatus is incorporated into powder handling apparatus or an additive manufacturing machine the output of test results may automatically cause the apparatus to perform a certain function. For example, powder may be rejected for further use, or combined with other powder to refresh it before further use. An additive manufacturing machine may be stopped or a wiper blade caused to remove a layer of powder and replace it before any powder is fused.

Data relating to analysis of powder may be stored so as to validate a build using the powder. In particular, analysis of at least part of the surface of layers of powder deposited during a build process may be stored to provide evidence of the consistency or otherwise of the powder used throughout a build process. Also, time stamped data can be compared from multiple images of powder taken at different points throughout a powder transport system to audit the performance of that system, e.g to show how effectively oxidised or contaminated powder moves through it.

In each example, the computer is provided with suitable software to cause the camera to take an image, to process the image to determine colour distribution amongst image elements, to enable a user to input ranges, proportions or other values, to calculate one or more of the three measures, to determine if a particular sample may or may not be re-used having regard to the range(s) and proportion specified by a user and to output this result to a user via the display 8 or otherwise. The above embodiments are described by way of example only. Many variations are possible without departing from the scope of the invention as defined in the appended claims.