JP7515519B2 - Metal powder analysis method and device - Google Patents

Description

The present invention relates to methods and apparatus for analyzing metal powders, particularly but not exclusively, for analyzing metal powders used in additive manufacturing (AM) processes.

In a known additive manufacturing process, articles are produced from powdered metals or alloys using an additive manufacturing machine. In the machine, layers of powder are deposited on a build platform and selectively melted with a laser or electron beam to form one or more articles. The process is repeated to build the article layer by layer.

Once the build is complete, the unmelted powder can be reused for another build.

The composition and condition of the metal powder used in the building process can significantly affect the integrity of the article formed by the process.

For example, during the build process, unmelted powder can degrade. Metal powders can gradually oxidize and change their properties, which in turn changes the properties of the article produced from the powder. Typically, the tendency of powders to oxidize increases with temperature, and other powder properties can also be affected by exposure to high temperatures. As a result, the closer unmelted powder is to the article being built, i.e., the heat zone, the more likely it is to degrade.

Also, as the powder melts, the process can cause particles of the heated powder to fly out of the powder bed and into the surrounding area of the manufactured article, compromising the quality of the unmelted powder surrounding the article.

To ensure adequate build quality, it is known to analyze used powder and stop reusing powder that has degraded to a certain extent, and/or to mix unused powder with recycled powder so that the mixed powder has adequate bulk properties for continued use. Another approach places a certain upper limit on the number of times a batch of powder may be reused.

There are many problems with these methods.

Powder condition is typically analyzed by making a bulk oxygen content measurement. The measurement process includes testing a powder sample that cannot be reused. More importantly, it has been recognized by the inventors that when recycled powder is mixed with virgin powder to produce a mixed powder with an overall bulk oxygen content below a desired threshold, the bulk oxygen content (or other bulk) measurement may give erroneous results regarding the suitability of the powder for reuse. This is because even if the bulk oxygen content is below the desired threshold, it is not sensitive to the presence of highly oxidized or otherwise degraded particles that may have a significant adverse effect on the build.

Setting a rough limit on the number of times powder can be reused is a relatively crude approach and does not take into account the amount of powder degradation that may occur from a particular print. The nature and extent of degradation can vary significantly from print to print.

Existing testing methods for powders are destructive, so the results are not relevant to the powder that will actually be reused and are usually only an approximation of the average condition of the powder.

Other aspects of the powder composition and condition can also affect the quality of the print.

The presence of contaminating particles can have a similar effect to the presence of highly oxidized particles. The size and shape of the particles can also affect the build, as can the density with which the powder layer is formed in the 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 present invention to provide improved methods and apparatus for analyzing powder conditions in a non-destructive manner.

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 irradiating a region of the powder with electromagnetic radiation, including radiation in the invisible portion of the electromagnetic spectrum, without melting the powder; separately detecting the irradiated radiation, including radiation in the invisible portion of the electromagnetic spectrum, returned from separate portions of the irradiated region of the powder to generate an output based on the detected radiation; and processing the output to identify one or more properties of the powder.

According to a second aspect of the present invention, there is provided an apparatus for analysing a metal powder for use in an additive manufacturing process, the apparatus comprising: an illumination device for illuminating a region of the powder with electromagnetic radiation, including radiation in the invisible portion of the electromagnetic spectrum, without melting the powder; a detector for separately detecting the illumination radiation, including radiation in the invisible portion of the electromagnetic spectrum, returned from separate portions of the irradiated region of the powder to generate an output based on the detected radiation; and a processor for processing the output to identify one or more characteristics of the powder.

The method may be carried out in any suitable environment. The powder may be a sample of test powder placed in a test container. Alternatively, the analysis may be carried out in an additive manufacturing machine or a powder delivery device such as a pipe or conduit. Thus, the device may be included in an inspection device, a powder delivery device, or an additive manufacturing device.

The radiation is light, including some invisible light, such as ultraviolet and/or infrared light, and may also include visible light. The irradiating device may include any suitable irradiating device, such as lamps, including LEDs, incandescent and/or discharge lamps. It is also possible to irradiate the powder (in low power mode) using a laser, such as a laser forming section that operates to melt the powder in an additive manufacturing machine in a modeling process. The irradiating means may irradiate all areas of the powder simultaneously or parts of the areas separately. The irradiating means may scan a beam of radiation over the area of the powder.

When one or more irradiation devices are provided, they are preferably provided in an enclosure into which the metal powder is introduced or in an opening configured to irradiate the powder sample through a barrier and also receive return irradiation radiation. The enclosure can prevent or at least limit the ingress of electromagnetic radiation at the wavelength of interest to allow controlled irradiation of the metal powder at the wavelength of interest. The enclosure can be substantially light-tight or configured to be substantially light-tight. The enclosure can be included in an additive manufacturing machine or a powder transport, manufacturing, processing, or handling device.

The region of powder may be a surface of the powder, in particular a planar surface. The method may involve placing the powder in a suitable container, build platform or powder recess of an additive manufacturing machine, so that the powder has a substantially planar upper surface, which surface is irradiated. Alternatively, the method may involve irradiating the powder through a suitable transparent barrier forming part of a powder containment structure, such as a container for the manufacture, processing or transport of the powder. Such a barrier may be planar, but may also be curved, such as the wall or window of a pipe having a generally circular cross section.

The powder may be dense. The degree to which the powder is dense may be controlled by tapping the powder in a predetermined manner or number of times before taking the measurement and/or by passing the powder through a pretreatment funnel before the measurement. The powder preferably has a sufficient depth from the illuminated surface so that the powder is substantially opaque to the detector.

The apparatus may include a barrier, such as a window, transparent or at least partially transparent to the wavelength of radiation of interest to separate the powder being inspected from the illuminator and the detector. The apparatus is arranged such that, in use, the powder is supported or contained by the barrier such that a densely packed form of the powder is observable through the barrier. Thus, the illuminator is arranged to irradiate the window and the detector is arranged to receive radiation from the window, from a position below the window, or alongside the window if the window is substantially vertical.

If the barrier is horizontal or substantially horizontal, the powder is supported by the barrier in a compacted form. If the barrier is not horizontal or substantially horizontal, for example a side wall or window of a container or pipe, the arrangement is such that the powder is contained in a piled up position against the window. Here, in the case of a pipe or conduit, it may include a valve or flow restrictor that acts to pile the powder against the window in the downstream direction away from the window.

The powder is a sample of powder taken from a batch of powder. The powder may be virgin powder or recycled powder that has already been used in an additive manufacturing process. The powder may be analyzed at various stages of the process and may be analyzed at multiple points in the process. Thus, devices for analyzing the powder are located at various points in the additive manufacturing machine and in the powder transport and handling equipment. For example, the powder may be analyzed at one or more or any combination of the following stages of the process: during transport to the additive manufacturing machine, in the powder container of the additive manufacturing machine, on the additive manufacturing machine platform, in the powder bed in the additive manufacturing machine, in the build container in the additive manufacturing machine, during transport from the additive manufacturing machine, after a sieving step, and after mixing with another powder.

The detector may take a variety of forms and may include any suitable sensor. The detector may be arranged to detect radiation on the same side of the area of the powder, particularly the same side of the irradiation surface, as the means for irradiating the powder. The method includes detecting, the detector arranged to detect radiation reflected and/or scattered by the powder. The detector may be arranged to detect irradiating radiation returned from all areas of the powder simultaneously or separately from parts of the areas. The detector may be arranged to scan the area of the powder. Relative movement between the detector and the powder may be performed by moving the detector relative to a stationary powder or by moving the powder past the detector, as in the case of a powder conveying device.

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 to filter out and/or allow selected wavelengths of electromagnetic radiation.

The detector may include one or more focusing elements, such as lenses, to focus radiation returning from the powder onto one or more sensors. The one or more focusing elements may focus an image of at least a portion of the area of the powder onto an image plane where the one or more sensors are located. The detector may include an image capture device, such as a camera or microscope. The detector may include a hyperspectral camera.

These arrangements allow the device to separately detect the irradiated radiation returning from different parts of the irradiated area of the powder because the detector can spatially resolve the radiation source and/or because the detector detects light reflected from only a portion of the area at a time and/or because only a portion of the area is irradiated at a time.

The output can be processed to determine a variety of different properties of the powder, including: the state of the individual particles of the powder, the surface properties of the powder and/or the individual particles of the powder, properties of the powder or individual particles of the powder that affect their interaction with electromagnetic radiation, particularly their ability to reflect and/or scatter radiation of particular wavelengths, the shape of the individual particles of the powder, the surface texture of the powder or individual particles of the powder, the presence or absence of contamination, the compactness of the powder, and the spreadability of the powder.

The output includes a value responsive to the wavelength of radiation received or detected from each section of the region of the powder. The output also includes a value responsive to the intensity of radiation received from each section of the region of the powder. The output may include a value responsive to the intensity of radiation at different wavelengths. The output may include a value responsive to the intensity of radiation over a continuous range of wavelengths. And/or the value may depend on the intensity of radiation at one or more discrete wavelengths. The output may be a function of the detected wavelength. The output includes a value for each of a plurality of sections. The sections are substantially the same size. The sections may be adjacent or spatially separated. The sections together cover all or most of the region. Each section preferably has an area smaller than the area of the surface of the powder occupied by a single particle of the powder of average size, preferably much smaller, such as a quarter, a tenth, a hundredth, a five hundredth or a thousandth of that area. Thus, for typical particle sizes in metal powders for additive manufacturing, each zone may be less than 1000 μm ² or less than 100 μm ^2. In this way, at least one, and preferably several, values contained in the output are influenced by a single particle of the powder. Of course, the optimal size of the zones varies from powder type to powder type, since the average particle size varies from powder type to powder type. In this way, the state of the individual particles can be identified.

The values obtained by analyzing the areas of powder form a data set.

The steps of detecting radiation, generating an output, and processing the output can be performed at different locations.

The data set includes substantially an image of all or part of the irradiated area of the powder, but at least in part obtained by recording radiation of invisible wavelengths. As such, in practice, it is not possible to make all of the data perceptible to the human eye without applying artificial coloring to permit recognition of features that are only apparent when examining invisible wavelengths. However, for convenience, when reference is made herein to an image, the image includes one obtained, at least in part, by detecting invisible electromagnetic radiation, particularly invisible light.

Where the data set substantially comprises an image, the image may be a digital image. The digital image may be formed by or divided into a number of elements, such as pixels. Each element corresponds to an area of the powder from which the detector may separately receive radiation and generate an output. The elements are preferably of substantially the same size. Thus, the ratio of elements in the image to the number of particles in the imaged area or surface of the powder is preferably of the order of at least 1:1, and even more preferably higher, such as at least 4:1, 10:1, 100:1, 500:1 or 1000:1. The wavelength of detected radiation represented by that element will then be affected only by the properties of a single particle of the powder.

The data set for processing typically contains 2 to 6 million area values, typically representing about 5000 particles with sizes ranging from 10 to 110 μm or 40 to 50 μm. However, the size of the data set can vary significantly depending on the application.

If the image is produced in visible color, or a standard capable of representing the invisible electromagnetic radiation of interest is used, or the detected invisible wavelengths are represented in the dataset by artificial coloring, the dataset includes data representing each area or image element of the powder according to an established color standard, e.g., RGB or CIELAB.

Multiple regions of the same volume or sample of powder can be analyzed. The regions can be adjacent or separate. In embodiments, at least 2, 3, 4, 5, 10, 50, 100 or more regions of the same powder are analyzed to form multiple data sets of the powder. The number of data sets depends on the amount of data required for statistical significance. Analyzing multiple regions helps assess how well the powder is mixed.

The data in the set is processed by comparing the data values of one area of the powder region with the data values of one or more other areas and/or reference data.

Data in a dataset can be removed so that the data remaining represents a selected sub-region of the powder, similar to cropping an image. This is useful when the detector contains focusing elements that produce an image at the detector by allowing distortion to be eliminated. It can also make comparisons between different datasets, especially those relating to the same sample, easier and more reliable. As an example, reducing the size of the dataset can define a 2000 x 2000 contiguous area of powder, resulting in an image composed of 2000 x 2000 image elements.

Data sets can be processed to determine whether they are of sufficient quality for further processing and/or whether all the individually acquired data in a particular set are sufficiently similar. Data that does not meet a predetermined quality standard is filtered out and not further processed. This can include, for example, obtaining statistics from the data and determining whether those statistics are within a predetermined range or whether the data sets differ by more than a predetermined threshold. In one example, the average intensity of a particular detected wavelength, wavelength range, or color channel used to define the detected radiation is calculated as well as the deviation from that average for all areas of the powder where radiation was detected, or for all areas above a threshold intensity.

Data in the dataset that relates to spaces between particles of powder can be identified, allowing that data to be excluded from further processing. Such data can be identified by identifying data that defines areas of the powder region where the luminosity is below a predefined threshold. The spaces between particles of powder tend to appear darker than the particles, and therefore have a lower luminosity. Such areas of powder can be considered background areas, while the remaining lighter areas are foreground areas. If the dataset includes images, this step includes identifying darker image elements. The darker image elements are considered background elements, and the lighter elements are considered foreground elements.

The dataset can also be processed to identify, or at least estimate, the number of powder particles it represents. If the dataset contains images, this can be done by watershed segmentation. This step is preferably performed after removing background elements from the images, if present.

Alternatively, or in addition, the number of particles represented by the dataset may be estimated by another suitable method that is independent of the dataset, for example, by information such as the size of the area represented by the dataset, the average size of the powder particles, and/or the compactness of the powder.

The data representing the foreground area of the powder represents the properties of the powder particles, including surface properties, that affect how the particles interact with the applied radiation.

The number of powder particles represented by the data set that have surface properties that fall outside a predefined range can be determined. The percentage of powder that has measured surface properties that fall outside a selected range can be determined.

This can be done by identifying areas defined by the data where the wavelengths of the detected radiation fall outside a selected range, preferably foreground areas.

The selection of the area is based on a statistical analysis of the detected wavelengths. The selected area is an area that defines the detected wavelengths as outliers in the wavelength distribution across all foreground areas. For example, elements may be selected by identifying how their wavelengths deviate from the average wavelength of all areas. The selected area preferably represents the outer 5% or less, 1% or less, or 0.1% or less of the wavelength distribution of the foreground areas.

Once data reflecting a property within the range of interest has been identified, individual particles having that property can be identified. To do this, groups of connected areas exceeding a predetermined number are identified, the number being selected to be representative of the total area expected to be occupied by a single particle of powder. The identified group of areas can then be considered to represent at least one, but typically one, particle having a property that meets the selected criteria.

If the ratio of area to particle is close to 1:1, this step can be omitted and a single image element is considered to represent a single particle.

Data can then be stored for each identified particle, the data relating to the area or areas of the powder that the particle occupies. The number of areas in an identified group is indicative of the size of the particle that the group represents. The average wavelength of radiation detected from the areas in a group is indicative of the color or other characteristic of the particle that the group represents. This allows the number of particles identified, along with the particle characteristics, to be determined, and furthermore, by analysing this data, various information relating to the powder can be determined or inferred. The particles thus identified can be further classified by surface characteristics, such as color, to identify particles having surface characteristics that fall within a particular range. Other techniques can then be used to select particles of interest from the identified particles.

The ratio of background area to foreground area is an indication of the ratio between the powder particles and the space between them, and therefore of the compactness of the particles. Changes in compactness observed in images of a batch of powder taken over time can reveal changes that affect the flow properties of the powder.

Since oxidation of metal powders usually affects their interaction with radiation, determining the overall distribution of wavelengths received from a region of the powder or from all foreground areas of the powder indicates the chemical nature of the powder, and in particular the degree of oxidation of the powder. Information about how oxidation affects the color of a particular type of powder can be used to determine the bulk oxygen content of the powder.

The number of particles identified as returning wavelengths of radiation outside the selected range can reveal whether the powder contains highly oxidized particles or is contaminated, so that it may be desirable not to use or reuse the powder. That number, along with a calculated or estimated total number of particles imaged, can be used to calculate the percentage of particles having a measured property that falls outside the selected range.

This information can be used to inform or control further processing of the analyzed powder or the powder from which it was extracted.

The method may include indicating that the powder is not suitable for reuse if the number or percentage of particles identified as having properties outside a predefined range exceeds a predefined range. The predefined range and percentage will depend on the particular powder being analyzed and its intended use. However, since the aim is usually to identify the presence of significantly oxidized, degraded or contaminated particles, the range is preferably set to include values of properties indicative of powders that are considered to be normally degraded due to exposure to oxygen and low temperatures due to their use in the building process. Particles with measured surface properties outside this range are therefore outliers. Their measured surface properties reflect the fact that they have been abnormally degraded, usually as a result of exposure to high temperatures (but without melting to form part of the construct). If the population of such outliers exceeds a predefined percentage of the total population of particles, reuse of the powder may be considered to be risky.

The method may also include determining an average property of the percentage of powder whose measured property falls within a predetermined range. This measurement indicates the overall average degradation of the powder excluding outlier particles that are severely degraded. This measurement is therefore indicative of the level of degradation due to normal degradation.

The method may also include indicating that the inspected powder is not suitable for reuse if the average property of a proportion of the measured powder, i.e. the approximate proportion of particles whose measured surface properties are within a predetermined range, is greater (or less) than a predetermined threshold. Thus, it may be possible to indicate that the powder is no longer suitable for reuse, possibly as a result of cumulative normal deterioration effects.

The method may also include processing the data of all the foreground areas to determine the average properties of all the powders, and therefore of all the particles measured. Such a measurement will give an indication of the overall average degradation of all the particles, and it is possible to obtain a similar indication to the bulk oxygen measurement, except that the bulk oxygen measurement can measure not only the oxygen of the outer oxide layer that builds up and affects the surface properties of the particles, but also the "internal" oxygen of the particles, i.e. the oxygen present inside.

The method may also include indicating that the powder is not suitable for reuse if the average property of all measured particles is greater than or less than a predetermined threshold. The method may include controlling equipment based on the measured powder property, such as controlling an additive manufacturing machine or powder processing, handling or conveying equipment.

When a measured average property of a powder, i.e., the particles of interest, is required, this is an average and is obtained by identifying an average measured surface property represented by the area of the powder region in which the particles of interest reside.

The information about the powder analyzed by this method can be used to automatically control equipment, including additive manufacturing equipment and/or powder handling, transport and processing equipment. The equipment can, for example, cause the powder to be discarded and/or processed depending on the detected condition of the powder.

The processor may be a programmed computer and configured to cause the device to perform some or all of the method steps described above.

In all aspects of the invention, particles of powder that return radiation containing wavelengths within a particular range can be identified and this can be used to infer the degree of oxidation of the particles taking into account experimental data related to the powder type concerned.

Embodiments of aspects of the present invention provide a non-destructive method and apparatus for identifying powder condition and determining whether a powder sample is suitable for reuse in a particular build process. When identification is performed by examining the percentage of outlier particles, a novel and useful measurement of powder condition is provided that allows for improved decision-making and therefore powder usage over current measurements of bulk powder properties.

The method and apparatus are also useful for identifying the presence of contaminant particles that have surface characteristics that differ from similar characteristics of the target particle.

According to a third aspect of the present invention, there is provided a method for analysing a metal powder used in an additive manufacturing process, the method comprising the steps of providing a densely packed powder adjacent a barrier, irradiating an area of the powder through the barrier, separately detecting the irradiated radiation returned through the barrier from different portions of the irradiated area of the powder, generating an output according to the detected radiation, and processing the output to identify one or more properties of the powder.

According to a fourth aspect of the present invention, there is provided an apparatus for analysing a metal powder used in an additive manufacturing process, the apparatus comprising a container or conduit for the metal powder having a barrier for containing, in use, the powder to be analysed in a compacted state, an illumination device for illuminating an area of the powder with electromagnetic radiation through the barrier, a detector for separately detecting the illumination radiation returned through the barrier from different parts of the irradiated area of the powder and for generating an output in accordance with the detected radiation, and a processor configured to process the output to identify one or more properties of the powder.

The barrier may be a wall of a container or conduit. The barrier may be a window in the wall of the container or conduit, the window being at least partially transparent to the electromagnetic radiation of interest. The electromagnetic radiation may be light, including infrared, visible, and ultraviolet. The third and fourth aspects of the invention may include any or all of the features of the first and second aspects of the invention as desired or required, except that it is not necessary that the powder be irradiated with invisible electromagnetic radiation or that invisible radiation be detected.

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, in which:
FIG. 1 is a schematic diagram illustrating an embodiment of an apparatus for analyzing the condition of a powder. FIG. 1 is a schematic diagram illustrating an embodiment of an apparatus for analyzing the condition of a powder. FIG. 1 is a schematic diagram illustrating an embodiment of an apparatus for analyzing the condition of a powder. FIG. 1 is a schematic diagram illustrating an embodiment of a powder processing and delivery apparatus including an apparatus for analyzing the condition of the powder. FIG. 1 is a schematic diagram illustrating an embodiment of a powder processing and delivery apparatus including an apparatus for analyzing the condition of the powder. FIG. 1 is a schematic diagram illustrating an embodiment of a powder processing and delivery apparatus including an apparatus for analyzing the condition of the powder. FIG. 1 is a schematic side view of an additive manufacturing machine including various devices for analyzing the condition of the powder. FIG. 8 is a schematic plan view of the device of FIG. 7. 1 is a flow chart showing the steps associated with processing an image of a powder. 1 is a graph showing particle count versus wavelength.

Terms such as top, bottom, upper, and lower are used below to describe the device as depicted in the orientation shown in the figures, which orientation is intended for use but is not intended to be limited to that orientation. Like reference numbers are used to denote like components in all figures. The drawings are not to scale.

Referring to the drawings, FIG. 1 shows a first apparatus for analyzing metal powders. The apparatus includes a substantially light-tight housing 1 that can be opened and closed. The housing contains a container 2 for the powder 3, which can be a dish or slide or any other suitable form. The container is open at the top and has a generally square opening of about 10 mm on a side, with a cross-sectional area of about ¹⁰⁰ mm2. It also has a depth of at least 2 mm. The container shown is shallow, but it can be made deeper so that the container is longer. The container is removable from the housing. The housing further contains a lens (or microscope) 4 attached to (or contained within) a digital camera 5 that is sensitive to ultraviolet and infrared light in addition to visible light, and a lamp 6 that emits ultraviolet, infrared and visible light. The camera 4 includes a substantially square sensor, such as a CCD sensor having about 6 megapixels (12 megapixels in another embodiment), and is connected to a computer 7 that includes a keyboard and mouse or other user interface and is connected to a display 8 and/or other output device. The lamps are arranged to provide diffuse light. They are shown as dome or flat dome lamps. In another configuration (and in other embodiments), a ring light can 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 inside or outside the housing 1. The powder is introduced in an amount sufficient to provide a dense powder depth such that the bottom of the container 2 is completely obscured when viewed from above. The powder depth will therefore typically comprise 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 surface. If the powder was introduced into the container outside the housing, the container is placed under the microscope in the housing and the housing is closed.

Next, the lamp 6 is activated. The lamp may be controlled by the computer 7. The lamp is positioned to illuminate the top surface of the powder 3 in the container 2. By illuminating the powder with the lamp in a substantially light-tight enclosure, the powder can be analyzed under controllable and reproducible light conditions. Note that in this and other described embodiments, the lamp may be provided in different positions and placed in front of the camera and/or lens.

The camera 5 then takes a digital image of the illuminated powder surface in the container and transmits it to the computer 7. The digital image contains information relating to all wavelengths of light detected by the camera. The camera and microscope are positioned to take an image of approximately ¹² mm2 of the powder surface in the container. Multiple images of the powder surface are taken until substantially all of the powder surface has been imaged. In another embodiment, the camera and lens have different fields of view. Thus, the powder surface can be captured in a single image.

Metal powders used in additive manufacturing processes typically have an average diameter of the order of tens of microns. The number of particles visible on the surface of the powder imaged by the camera is then on the order of a few thousand, and thus about three orders of magnitude less than the number of pixels on the sensor. The camera can therefore generate a digital image of the powder surface in which there are about 1000 times as many pixels as there are particles of powder shown in the image.

The images captured by the camera are then sent to computer 7 for processing.

Figure 2 shows a second device for analyzing metal powders. In this case, a light-tight housing 1 is provided under a container 2 for containing powder 3. The housing contains a camera 5 and lens 4 similar to those of the embodiment of Figure 1 and a lamp 6 facing upwards. The base of the container is transparent or at least partially transparent to the wavelengths of light generated by the lamp and that the camera is sensitive to. The camera can therefore take an image of the surface of the powder 3 in the container 2 through the base of the container. The top surface of the container base on which the powder is supported is substantially flat, so that the camera is faced with the flat surface of the powder to be imaged. The bottom surface of the container base is substantially parallel to its top surface.

Figure 3 shows another apparatus for analysing a powder 3. This apparatus includes an elongated tube 2 closed at one end for containing the powder. The tube 2 has one flat side wall 2a. Conveniently, the tube has a square or rectangular cross section, although other cross sections are possible, such as a D-shaped cross section. An assembly of a camera 5 and a lens 4 is provided in a substantially light-tight housing 1 together with a lamp 6, similar to the apparatus of Figures 1 and 2. The camera is directed towards the flat side wall of the container, which is at least partially transparent to the wavelengths of light generated by the lamp and which the camera is sensitive to, and which has substantially parallel and opposed flat sides, similar to the base of the container 2 of Figure 2. The camera is arranged to image the densely packed powder contained in the container through the side wall. The container is mounted for movement relative to the housing and the camera, such that the camera can image different portions of the surface of a sample of the powder contained in the container.

4 shows a powder delivery apparatus including pipes 10 leading to and from the powder sieve and further to the powder mixer. Each pipe 10 includes a flat or substantially flat transparent window 11 on which is mounted a light-tight housing 1 housing a digital camera 5 fitted with a suitable lens 4 for taking an image of the powder in the pipe 10 through the window 11. A lamp 6 is provided in the housing around the lens 4 for illuminating the powder through the window 11. The camera outputs its image, including information relating to all wavelengths of light detected by the camera, to a computer 7 with an output device 8. Alternatively, or in addition, the output may be sent to a computer or processor controlling the powder delivery apparatus or its associated equipment. The pipe 10 downstream of the mixer is provided with a valve 12 (such as a butterfly valve) for allowing or stopping the flow of powder through the pipe. This allows the pipe to be filled with powder and the powder in a dense state is directed to the window 11 of the pipe, which is transparent to the wavelengths of light generated by the lamp and sensed by the camera.

Similar to the device shown in FIG. 1, the digital camera 5 has a sensor with a pixel count approximately 1000 times the expected number of powder particles visible in the area of the window 11 imaged by the camera when the pipe is filled with dense powder to be analyzed. Similarly, the lamp 6 emits light and the camera 4 senses a broad spectrum of light including ultraviolet, visible, and infrared light. The window 11 is substantially transparent to this broad spectrum of light.

Both pipes 10 leading to and leading from the sieve are provided with windows 11 and associated housings 1 with cameras 5, allowing the powder condition before and after the sieve to be analysed. The cameras also allow the powder condition entering both inlets to the powder mixer to be analysed.

The window can be provided in a powder delivery conduit or powder storage container in other types of equipment, such as an additive manufacturing machine.

5 shows a powder storage device with a hopper having a lower frusto-conical wall and a cylindrical side wall. An upright pipe 10 is connected to an inlet at the top of the hopper through a valve 12 for allowing and stopping the powder passing through the pipe 10 to the hopper. An outlet is provided at the bottom of the hopper, also fitted with a valve 12 for allowing and stopping the powder to flow out of the hopper. The hopper is for storing metal powder and may be used with or form part of an atomizer used to produce metal powder. The hopper has two windows 11 at its frusto-conical base, each fitted with a housing containing a lamp 6 and a camera 5 with a lens 4 (of the type shown in FIG. 4) for imaging the powder in the hopper against the windows. As with the device of FIG. 4, the windows are transparent to the wavelengths of light produced by the lamps and sensitive to the camera, and have substantially parallel, opposing planar faces.

The pipe 10 leading to the hopper 12a has five pairs of windows evenly spaced along the section of the pipe above the valve at the entrance to the hopper. Each window is fitted with a respective housing 1 housing a camera 5 with a lens 4 and a lamp 6, similar to the housing 1 at the bottom of the hopper 12a. These cameras allow images of the densely packed metal powder in the pipe to be taken at various positions along the length of the pipe. This is useful, for example, to be able to determine whether the powder fed into the pipe 10 has changed over time, for example during a manufacturing process.

Each 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 to equipment controlled by the output. Instead of providing multiple housings 1 and cameras 4 along the length of the pipe 10, a single camera (or a smaller number of cameras) could be movably mounted relative to the pipe and positioned to image the powder at different positions along the length of the pipe.

By closing the valve 12 at the entrance to the hopper, the powder to be imaged can be filled into the pipe in a dense state.

Figure 6 shows a part of a pipe for conveying powder. The pipe is fitted with a bypass conduit 10a, which is provided with a valve 12 that allows the flow of powder into and out of the bypass to be controlled. A further valve 12 is provided in the pipe between the connections with the bypass conduit. The bypass conduit, like the housings of figures 4 and 5, is provided with a substantially planar window 11 in which is fitted a housing 1 in which a camera with a lens and a lamp are housed. The output of the camera is connected to a computer or processor 7, a display or other output device 8, or a device controlled according to the output of the camera. The bypass conduit is formed by a pipe of smaller cross section than the pipe 10. The valve is controlled so that the powder flowing in the pipe 10 flows into the conduit, filling it and allowing the camera 4 in the housing 1 to image the powder in a compacted state. The bypass allows efficient collection of samples for analysis from the powder flowing in the conduit, so that the powder can be imaged in a compacted state even if it is not moving in a compacted state through the pipe.

7 and 8 show an additive manufacturing machine 13. The additive manufacturing machine comprises a housing 1 that is substantially light-tight or configured to be light-tight. The housing 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 on a movable support for transporting powder from the powder delivery container to the build container. The housing also houses output optics 18 of a laser for selectively melting powder on the build platform 16. All these features are common to known selective layer fusion additive manufacturing machines.

The housing 1 further houses two cameras 5 with suitable lenses 4 for taking images of the areas of the top surface of the powder in the powder delivery container and the modeling container, and lamps 6 arranged around each camera. Also, an image sensor 19 and lamps 20 are attached to the movable support of the wiper blade 17 and are arranged to scan images of the surface of the powder in the powder delivery container or modeling container as the wiper blade moves to and from the container. As in the other embodiments, the lamps 6, 20 emit light and the camera 5 and sensor 19 sense light in a broad spectrum including ultraviolet, visible, and infrared.

A camera 5 with a lens 4 and associated lamp 6 are also attached to the housing 1, which is located behind the window through the side wall of the powder container and the modeling container, and can image the powder in its dense state inside the container through the window.

Additionally, a camera 5 with a lens 4 and associated lamp 6 is also attached to the housing 1, positioned below a window formed in the platform extending between the powder container and the build container, to image the powder on the platform.

All three windows are transparent to the wavelengths of light produced by the lamp and that the camera can sense. The platform windows have substantially parallel, opposing planar surfaces. The windows to the powder container and build container may also have such surfaces or, if they are not flat, may be shaped to match the shape of the container's inner walls.

The camera 5 and sensor 19 are arranged to output the image to a connected computer or processor 7 with an output device 8 or to control the additive manufacturing machine in response to the output. As with the apparatus shown in Figures 1 and 2, the digital camera 5 and sensor 19 are arranged to generate an image of the powder having a pixel count of about 1000 times the number of particles shown in the imaged area of the powder.

It will be appreciated that the additive manufacturing machine shown in Figures 7 and 8 need not include all of the imaging devices shown. However, multiple devices can be used to image the powder at various stages of its use.

In use, each embodiment of the device generates a digital image of the surface of the powder within the device. The digital image includes a set of data defining characteristics of the image elements, including information related to all wavelengths of light detected by the camera, i.e., the wavelengths of light received by the camera from each area of the powder surface that corresponds to an element of the image. The ratio of image elements to the number of particles of powder shown in the image is approximately 1000. The image data is transmitted to a computer and stored in the computer in a manner that defines the detected wavelengths represented by the wavelengths or combinations of wavelengths of light, or the luminous intensity of each element of the image, such as in the CIELAB color space with the variables L, a, and b.

The computer is configured to process the image data to determine information about the condition of the powder shown in the image by performing at least some of the steps shown in FIG. 9.

As an optional first step 21, the image is cropped to a predetermined size, removing elements outside the original image boundary (or other selected region). This optional step allows for the elimination of distorted areas of the image, or for images captured by different cameras or sensors to be merged to represent the same area or have the same number of pixels.

The remaining image data is then inspected at 22 to ensure it is of sufficient quality for further processing. If not, it is rejected at 23 and a new image is acquired.

If the image data indicates that it is of sufficient quality, the computer then identifies elements whose brightness is below a predetermined threshold and removes these from the image data at 24 with the aim of removing elements that represent spaces between powder particles (or other background material) in the image. The actual threshold will vary depending on the characteristics of the particular apparatus being used and the type of powder being inspected. Once the elements of the image that are outside the threshold have been removed, the remaining image elements represent powder particles that are in the foreground of the image.

The data for the remaining image elements can 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 device, the number of particles expected to be visible in an area of the powder surface corresponding to the number represented by the image data can be calculated using information about the expected powder particle size and the expected powder compactness.

The remaining image element data is then statistically analyzed at 26 to determine how the wavelengths represented or detected by each image element are distributed around the corresponding mean value of all remaining elements, and to detect outlier elements that represent wavelengths or detected wavelengths outside a threshold percentage of the total element population. This can be performed using a chi-square test for outlier detection. Other techniques can also be used, including the use of machine learning. A suitable percentage of the total population can be selected depending on the type of powder being analyzed, but a typical percentage is 0.1% of the elements of interest, i.e., the outlier population represents 0.1% of the total element population.

A visualization of this step is shown in Figure 10, with the number of image elements on the vertical axis versus the measured wavelength (or color) detected on the horizontal axis. A generally bell-shaped distribution curve is shown, centered around a mean value at 27, with low and high thresholds at 28 and 29, with the area outside both thresholds and below the curve representing the 0.1% of the total population that are outliers.

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

At this stage, the computer has generated a set of image data that defines the size and surface properties of individual particles that affect their interaction with the various wavelengths of light shone on them and reveal statistical outliers within the powder. The surface properties include color as far as visible light is concerned, but are broader in that they include properties that change their interaction with ultraviolet and infrared light. The inclusion of these non-visible wavelengths increases the range of particle properties that can be detected by the instrument, especially as it relates to particle composition.

This data is then analyzed at 31 to extract useful data related to the condition of the powder analyzed.

The data includes:
the number and percentage of particles having surface properties outside of a predetermined range;
the average wavelength of light received from particles within a given range; and
and the average wavelength received from all image elements.

It has been found that the way metal particles behave in relation to light changes as the particles age, in particular if they are oxidized or exposed to heat. The more the particle is oxidized or the higher the temperature it is exposed to, the more the behavior changes. The amount of change is therefore related to the degree of degradation the particle has undergone, and therefore to its suitability for reuse. For some types of particles, this change in properties is visible to an observer as a change in the color of the particle, but by examining the particles using more invisible wavelengths, it is more likely that changes in a wider range of particle types can be detected.

Furthermore, it has been found that the presence of highly degraded particles can identify a powder batch as unsuitable for reuse, even if the batch is in normal condition, as the presence of even a single highly degraded particle in a built object can significantly affect the properties of the built object. Highly degraded particles, when incorporated into an article, can render the article unsafe, especially if the particles are incorporated in locations of the article where stress concentrations occur during use.

The first measurement above represents the percentage of severely degraded particles in the entire sample and the entire batch of powder examined, assuming that the batch of powder from which the sample is taken is well mixed or that the imaged area of the powder represents a representative composition of the powder as a whole. Accuracy can be improved by taking multiple samples from a particular batch and analyzing them separately, or by performing multiple tests on a batch of powder, such as taking multiple images of the powder surface, and/or by analyzing, mixing, and then reanalyzing a particular sample. For a particular powder and build, a suitable wavelength range and a minimum percentage outside that threshold can be determined, and if the percentage of particles outside the threshold exceeds a selected limit, the batch of powder can be considered unsuitable for reuse, at least for the build of interest.

This measurement therefore allows the powder state to be identified regardless of the bulk amount.

The second measurement gives an indication of the average degradation of the remaining powder, without taking into account particles that are outside the threshold. Such a measurement resembles the results of a conventional bulk oxygen measurement, but is obtained in a more convenient and non-destructive manner, except that it excludes the influence of significantly degraded particles (or internal oxygen). The powder is considered unsuitable for reuse when the average wavelength received from the remaining particles, without taking into account particles that are outside the predetermined range, is outside another predetermined range.

The third measurement is similar to the second, but takes into account particles that have deteriorated significantly. If the average wavelength of light received from the particles falls outside another predetermined range, the powder is deemed unsuitable for reuse.

The decision to reuse the powder can be based on one or more of the three measurements above. Typically, if it is determined based on any of the measurements that the powder should not be reused, the powder is not reused. In one embodiment, the first and second measurements are calculated and if either measurement indicates that the powder is not suitable for reuse, the powder is deemed unsuitable for reuse.

It is useful to analyze new powder before it is used and then after its use in the build process and any subsequent build processes. Analysis of the new powder can provide useful control data for comparison with data from subsequent analyses.

Other information about the powder can be determined by detecting the light returned by the powder. Non-powder artifacts may also be detected in the powder. Abnormal powder particles, for example particles made of a different material than intended, may also be detected and the abnormal particles may be identified by observable characteristics. An estimate of the total incident energy received by the powder may also be made. If multiple images of a sample of powder or a batch of powder are made and analyzed, comparing the results between images can determine how well the powder is mixed. Images of powder processed in separate machines, such as additive manufacturing machines, can be used to compare machine performance or determine machine condition. Images taken at separate time periods and/or separate locations within the machine are useful for tracking powder as it passes through the machine.

If the device is integrated into a powder processing system or additive manufacturing machine, the device can automatically perform certain functions based on the output of the test results. For example, the powder can be deemed unfit for continued use or it can be refreshed by combining it with other powder and made available for continued use. The additive manufacturing machine can also be shut down or wiper blades can be used to remove a layer of powder and replace it before it melts.

Data relating to the analysis of the powder can be stored to verify the build using the powder. In particular, an analysis of at least a portion of the surface of a layer of powder deposited during the build process can be stored to provide consistency or other facts about the powder used throughout the build process. Additionally, time-stamped data can be compared from multiple images of the powder taken at various points throughout the powder delivery system to assess the performance of the system, for example, how efficiently oxidized or contaminated powder moves through the system.

In each example, the computer is equipped with appropriate software which causes the camera to take images, processes the images to determine colour distribution between image elements, allows a user to input ranges, percentages or other values, calculates one or more of the three measurements, determines whether a particular sample can be reused given the user specified ranges and percentages, and outputs this result to a user via a display 8 or the like.

The above embodiments have been described by way of example only. Many variations are possible without departing from the scope of the invention as defined in the appended claims.

Claims (21)

1. A method for analyzing metal powders used in an additive manufacturing process, comprising:
irradiating a region of the powder with electromagnetic radiation, including radiation in the non-visible portion of the electromagnetic spectrum, without melting the powder;
separately detecting the illumination radiation, including radiation in the non-visible portion of the electromagnetic spectrum, returned from separate portions of the illuminated area of the powder and generating an output based on the detected radiation;
processing the output to determine a property of individual particles of the powder and thereafter infer the degree of umbrellaing of the particles, the property being the ability of the particle to reflect and/or scatter radiation of a particular wavelength ;
A method for providing the same. The powder region is irradiated with light including ultraviolet and/or infrared light, and at least the returned ultraviolet and/or infrared light is detected.
The method of claim 1. The powder is in a dense state, and the surface of the powder is irradiated.
3. The method according to claim 1 or 2. the output includes a value based on one or more wavelengths of radiation received or detected from each area of the powder region.
4. The method according to any one of claims 1 to 3 . the areas being substantially the same size,
The method according to claim 4 . each region having an area smaller than the area of the surface of the powder occupied by a single particle of the powder of average size;
6. The method according to claim 4 or 5 . the area of each region being less than one tenth, one five-hundredth, or one thousandth of the area of the surface of the powder occupied by a single particle of the powder of average size;
The method according to claim 6 . storing said values as a data set.
8. The method according to any one of claims 4 to 7 . 9. The method of claim 8, wherein the data set is processed to identify values associated with areas of powder regions having a light intensity below a predetermined threshold, thereby identifying values associated with spaces between particles. and processing values other than the identified values to identify values representative of particles having properties outside a predetermined range.
The method according to claim 9 . The processing identifies values that represent areas that define the detected wavelengths as outliers in the wavelength distribution of all processed values.
The method of claim 10 . By identifying a group having more than a predetermined number of identified areas connected, a particle or group of particles of interest is identified.
The method of claim 11 . 1. An apparatus for analyzing metal powders used in an additive manufacturing process, comprising:
means for irradiating a region of said powder with electromagnetic radiation, including radiation in the non-visible portion of the electromagnetic spectrum, without melting said powder;
a detector that separately detects the illumination radiation, including radiation in the non-visible portion of the electromagnetic spectrum, returned from separate portions of the illuminated area of the powder and generates an output based on the detected radiation;
a processor for processing the output to determine a property of individual particles of the powder and thereafter infer the degree of umbrellaing of the particles, the property being the ability of the particle to reflect and/or scatter radiation of a particular wavelength ;
An apparatus comprising: 14. The apparatus according to claim 13 , provided in a powder delivery apparatus or an additive manufacturing apparatus. Further comprising an irradiation device configured to irradiate a surface of the powder region with light including ultraviolet and/or infrared light;
The detector is arranged so as to detect radiation on the same side as the irradiation surface of the irradiation device.
15. Apparatus according to any of claims 13 or 14 . characterised in that the illumination device and the detector are housed in an enclosure into which the metal powder can be introduced, the enclosure being capable of preventing or at least limiting the ingress of electromagnetic radiation at a wavelength of interest in order to allow controlled illumination of the metal powder at that wavelength.
16. The apparatus of claim 15 . the detector comprises a focusing element for focusing an image of at least a portion of the area of powder onto an image plane in which one or more sensors are disposed,
17. Apparatus according to any one of claims 13 to 16 . the detector being capable of spatially resolving the position of radiation returned from the region of powder;
18. Apparatus according to any one of claims 13 to 17 . the detector detects radiation returned from only a portion of the powder area at a time.
18. Apparatus according to any one of claims 13 to 17 . and storing the output as a data set including values responsive to wavelengths of radiation received or detected from each area of the powder region.
18. Apparatus according to any one of claims 13 to 17 . The processor is configured to cause the device to carry out a method according to any one of claims 1 to 12 .
21. Apparatus according to any one of claims 13 to 20 .

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