Methods and arrangements for controlling a melt pool status

Abstract

The present invention relates to additive manufacturing arrangements and methods for additive manufacturing by selective fusion of layers of a three-dimensional product from a powder bed (240) comprising successively formed powder layers. An electron beam emitter (210) is configured to deliver an electron beam (220) towards a top surface (242) of the powder bed (240) and at least one detector (285) is configured to detect electron and / or photon radiation from a structure (291, 292) of a vacuum chamber (280) or inner surface (291, 292) of the vacuum chamber (280). An analysing unit (294) is configured to use the output signal from the at least one detector (285) to analyse the radiation distribution in the output signal to determine a resulting signal indicating a melt status of the melt pool and an electron beam controller (296) is configured to control the energy beam (220) in response to the resulting signal.

Description

[0001] METHODS AND ARRANGEMENTS FOR CONTROLLING A MELT POOL STATUS

[0002] TECHNICAL FIELD

[0003] The present disclosure relates generally to arrangements and methods for additive manufacturing for successively forming a three-dimensional product from a powder bed comprising at least one powder layer by means of powder bed fusion.

[0004] BACKGROUND

[0005] In Electron Beam Powder Bed Fusion (E-PBF), the melting properties of the powder bed are very dependent of build geometry. The local temperature and heat conduction of the material surrounding the melt pool will affect the size and shape of the melt pool. This is true both for continuous line melting and discrete spot melting. In continuous melting (also referred to as “raster melting”, “line melting” or “hatch melting”) , the electron beam is continuously scanned or moved over the top surface of the powder bed in a line pattern in different variants such as unidirectional line melting, line melting with different line orders, in consecutive order in alternating direction etc. They all have in common that the beam melts continuously with a melt pool moving over the powder bed. In discrete point melting or spot melting (also referred to as “point melting”), the electron beam is moved between discrete points or pixels in accordance with a heating pattern, which may be ordered or random. At each point, the electron beam dwells or stays a dwell time. A typical dwell time is of the order of microseconds to milliseconds. This means that 1000 points or more may be melted in one second. The dwell time is not a constant, it is often preferred to use different dwell times for different spots to compensate for different thermal conductivity in each spot. The goal is usually to have a consistent size of the melt pool in each spot, regardless of the thermal conditions in each spot. However, this is difficult to achieve in practice since it has been shown that it difficult to measure the melt pool during the fusion. In https: / / www.scienceclirect.com / science / article / pii / S026412752030700Q, continuous melting and spot melting are discussed in more detail.

[0006] In particular, in spot melting it is of high importance for quality and efficiency reasons, for example, that the electron beam is kept or dwelled at a point exactly as long as required for proper melting the powder at the point, i.e. creating the melt pool, and then directly jump to the next point in the heating pattern.

[0007] Hence, there is a need to measure and control the size of the melt pool in real-time, so that complex 3D geometries can be built with a consistent size of the melt pool. Patent EP3473358B1 describes a method where electrons emitted from the front surface of the powder bed as a result of irradiation of an electron beam are measured by a plurality of electron detectors. The plurality of detectors is positioned relative to the front surface such that they can detect electrons emitted directly from the front surface during the fusion process. A melting judging unit is arranged to detect based on a strength of a detection signal from the plurality of detectors that the powder layer is melted and generate a melting signal. The melting signal is output when a difference in strength between the detection signal from the plurality of detectors falls below a predetermined range. Based on the melting signal, the condition of irradiation of the electron beam can be determined.

[0008] However, there is still a need in the art for improved devices and methods for measuring and controlling the status and / or size of the melt pool in real-time, in particular, in spot melting technologies.

[0009] SUMMARY

[0010] The above-described problem is addressed by the claimed additive manufacturing arrangement for successively forming layers of a three-dimensional product from a powder bed comprising at least one powder layer by means of powder bed fusion.

[0011] In the improved arrangements and methods for additive manufacturing according to the present invention, the status of the melt pool, for example, shape and / or form and / or depth and / or temperature of melt pool, is detected and controlled to thereby improve the quality of the manufactured objects as well as enable manufacturing more complex 3-D objects.

[0012] The present invention is based on the insight that there is a clear and measurable difference between the backscattered radiation from un-melted powder compared to the melted powder in the melt pool, since the energy beam continuously jumps between new un-melted points on the surface of the powder bed. Thus, the signal that can be traced from the backscattered radiation is dynamic. Further, there is a clear and measurable relation between the back- scattered radiation and secondary radiation resulting from the back-scattered radiation colliding with structures within the vacuum chamber or inside walls of the vacuum chamber. Typically, the energy beam is maintained at each point or spot in a duration of microseconds to milliseconds. The time required to create a melt pool in a certain spot will differ depending on the thermal environment of that spot. One important goal in spot melting is often to keep the size of the melt pool (and depth) at a constant level. But in order to achieve this constant size in different spots, it will require that the dwell time at each spot varies in order to cope with changing thermal conditions. A further insight is that fusion process at the melt pool and the changing properties of the powder when transforming into fused or melted condition from a powder condition emits radiation in a broad spectrum, light in a number of different wavelengths including visible light, NIR- (Near-lnfra Red) light, IR- (Infra-Red) light and UV- (Ultra-Violet) light, as well as electrons and x-ray radiation. By detecting secondary radiation, e.g. secondary electrons, from structures within the vacuum chamber, such as for example components in the vacuum chamber or structures within the chamber, or inner surface of the vacuum chamber, such as walls or ceiling, it is possible to obtain an indirect radiation pattern from the melt pool indicating the status of the melt pool. It is also conceivable to arrange a specific structure, such as a plate or similar structure, within the vacuum chamber that emits secondary electron and / or photon radiation as a result of the radiation from the melt pool.

[0013] The status of the melt pool is according to embodiments of the invention used to determine when the electron beam can move or jump to the next point or spot in accordance with the heating pattern (i.e. the order in which the spots are heated, it may be a predetermined sequence or a randomized pattern). Hence, when the detected radiation indicates that the melt pool status of a spot has reached the desired status (for example melt pool diameter and / or melt pool circumference), based on the detected radiation, the electron beam jumps to the next spot according to the heating patterns. Thereby, it is possible to create a dynamic heating in that the electron beam only spends the required time at each spot and then is quickly moved on to the next spot. For example, this entails that the efficiency and speed of the process can be improved and optimized. The dwell time can be adapted to the specific thermal conditions or environment at each spot.

[0014] According to the new and inventive idea, secondary radiation, such as electrons, is detected and monitored so as to track, determine and control the size and status of the melt pool. The secondary electrons are emitted from a structure within the vacuum chamber or from inside walls of the chamber as a result of incident electrons from the top surface and melt pool, in turn, as result of the electron beam. This new and inventive idea that the electrons emitted directly from the top surface of the powder bed do not need to be detected in order to determine and control the size and status of the melt pool entails a number of advantages. For example, the radiation detector(s) can thereby be shielded from direct line-of-sight towards the powder bed surface, so that radiation directly from the powder bed cannot reach the radiation detector(s). Thereby, a dwell time and / or a jump speed of the electron beam can be controlled based on the received resulting signal indicating that melt status of the melt pool has reached a desired level. Hence, when the fusion is completed at a first spot, i.e. the melt status of the melt pool at that spot has reached a desired level, the electron beam can be controlled to move on to the next spot according to the heating pattern.

[0015] Thereby, the dwell time can be very accurate adapted to the specific thermal conditions at each spot, and a dynamic heating is obtained.

[0016] This has several advantages. For example, the robustness, stability and life span of the detectors can be increased. The environment in the vacuum chamber is challeging, with very high levels of metal vapor, thermal radiation, X-rays and electrons. This environment can be harsh to the detector and its wiring, resulting in sensitivity drift, a reduced life span and risk for failure and by shielding the detectors this hence entails radical improvement in life span and robustness. Furthermore, by shielding the detectors, more sensitive detectors can be used, for example semiconductor detectors similar to those commonly used for backscatter electron detection in scanning electron microscopes, or scintillation detectors for X-rays, or photodiodes or multi-wavelength spectrometers for light. There are many types of radiation detectors that cannot tolerate being coated by metal vapor or exposed to temperatures of 400 degrees C or more, conditions frequently encountered above the E-PBF powder bed.

[0017] Changes in the signal or signal pattern from the at least one radiation detector are used as an indication that a melt pool has been formed. The radiation can be electron radiation or X- ray radiation or light. In the case of electron radiation, the electrons can be backscattered electrons, secondary electrons, Auger electrons or thermionic electrons. In the case of X-ray radiation, the X-rays can be characteristic X-rays or bremsstrahlung X-rays. In the case of light, the light can be of different wavelengths in the visible, UV and IR wavelength regions. The radiation measured by the detector(s) may come from any structure, surface or component within the process chamber or the walls of the vacuum chamber.

[0018] According to an aspect of the present invention, there is provided an additive manufacturing arrangement comprising an additive manufacturing apparatus for additive manufacturing by selective fusion of layers of a three-dimensional product from a powder bed comprising successively formed powder layers. The apparatus comprises a powder distribution device configured to feed a powder layer. Further, an electron beam emitter is configured to deliver an electron beam towards a top surface of the powder bed, wherein the electron beam is used in the selective fusion of the three-dimensional product. At least one detector is configured to detect secondary electrons from structures within the vacuum chamber or the walls of the vacuum chamber resulting from back-scattered electrons or photons emitted from a melt pool at the powder layer. Hence, the detectors are arranged such that a field-of- view is directed to collect emission of electron and / or photon radiation from any structure, surface or component within the vacuum chamber or the walls of the vacuum chamber. In certain embodiments, the electrons or photons emitted directly from the top surface of the powder layer are not detected. The at least one detector is configured to provide an output signal relative to detected electron and / or photon radiation and an analysing unit is configured to use the output signal from said at least one detector to analyse the radiation distribution in said output signal to determine a resulting signal indicating a melt status of said melt pool. An electron beam controller is configured to control the electron beam in response to the resulting signal. For example, a dwell time and / or a jump speed of the electron beam can be controlled based on the received resulting signal indicating that melt status of the melt pool has reached a desired level, for example, a certain diameter, and / or a minimum circumference. Hence, when the fusion is completed at a first spot, i.e. the melt status of the melt pool at that spot has reached a desired level, the electron beam can be controlled to move on to the next spot according to the heating pattern. Thereby, the dwell time can be very accurately adapted to the specific thermal conditions at each spot, and a dynamic heating is obtained.

[0019] According to a second aspect of the present invention, there is provided a method for additive manufacturing by selective fusion of layers of a three-dimensional product from a powder bed comprising successively formed powder layers, comprising the steps of: exposing a top surface of the powder bed for the electron beam to melt the metal powder to form a melt pool; detecting secondary radiation, i.e. electrons, emitted from any component, structure or surfaces within the vacuum chamber or walls of the vacuum chamber resulting from back- scattered electrons emitted from a melt pool at the powder layer colliding with structures in the vacuum chamber, or walls, ceiling or similar of the vacuum chamber using at least one radiation detector. In certain embodiments, the radiation emitted directly from the top surface of the powder layer is not detected. The secondary radiation may be electrons, x-ray and / or NIR, UV or IR light; analysing a radiation distribution in output signals from the at least one detector resulting from the received secondary radiation to determine a resulting signal indicating a melt status of the melt pool; and controlling the electron beam from the electron beam emitter in response to the resulting signal. For example, a dwell time and / or a jump speed of the electron beam can be controlled based on the received resulting signal indicating that melt status of the melt pool has reached a desired level or status, such as a certain diameter, and / or a minimum circumference of the melt spot. Hence, when the melt status of the melt pool at that spot has reached the desired level, the electron beam can be controlled to move on to the next spot according to the heating pattern. Thereby, the dwell time can be very accurately adapted to the specific thermal conditions at each spot, and a dynamic heating is obtained. Furthermore, an intensity and / or shape and / or diameter of the electron beam may also be adapted or changes based on the resulting signal.

[0020] Hence, according to the invention, radiation emitted from structures, components or walls and / or the ceiling within the process chamber can be detected and measured by at least one radiation detector in order to detect or determine a status of the melt pool and used to determine the size of the melt pool in real-time and adapt dwell-time for example.

[0021] According to embodiments, the analysing unit is configured to determine a resulting signal reflecting a time point when the powder in the top surface melts. Thereby, it is possible, for example, to determine the elapsed time between the first time point when the electron beam first starts to expose a pixel or a point at the top surface of the powder bed and a second time point when the powder at the top surface starts to melt and create a melt pool. Further, it is also possible to determine the period between the second time point (i.e. when the powder at the top surface starts to melt) to a third time point when the melting is finished, and the electron beam moves to a subsequent pixel or point. This enables a dynamic adjustment of the period between the first time point and the third time point, i.e. the dwell time, based on the detection of the second time point.

[0022] According to embodiments of the present invention, the detector or detectors are arranged to detect secondary electrons emitted from structures within the build chamber, for example, the chamber surfaces, for example, the chamber ceiling or a chamber wall resulting from the back-scattered electrons from the top surface and / or melt pool colliding with the structures or chamber surfaces. Hence, indirect monitoring of the melt pool is possible. This inventive idea is based on the insight that it is not necessary to detect electrons emitted directly from the top surface of the powder bed in order to determine and control the size and status of the melt pool, which entails a number of advantages. For example, the sensing elements of the detector(s) can thereby be located, or arranged, or placed such that they are protected from direct line-of-sight towards the powder bed surface or top surface, so that back-scattered radiation emitted directly from the powder bed cannot reach the sensing elements of the detector(s). This results in an increased robustness, stability and life span of the detectors. The environment within the build chamber is demanding where there are very high levels of metal vapor, thermal radiation, X-rays and electrons. In particular, the back-scattered radiation is very harsh to the detector resulting in a reduced life span and risk for failure and by arranging the detectors such that the sensing elements avoid back-scattered electrons, this hence entails radical improvement in life span and robustness. Furthermore, by locating or arranging the detectors such that the sensing elements avoids back-scattered electrons from the melt pool or are located relative the melt pool such that back-scattered electrons (or radiation) is avoided, more sensitive detectors and sensing elements can be used, for example semiconductor detectors similar to those used for backscatter electron detection in scanning electron microscopes, or scintillation detectors for X-rays, or photodiodes or multiwavelength spectrometers for light. There are many types of radiation detectors that cannot tolerate being coated by metal vapor or exposed to temperatures of 400 degrees C or more, conditions frequently encountered above the E-PBF powder bed.

[0023] The detector(s) can have a narrow field of view, collecting radiation from specific surfaces only, or a broad field of view, collecting radiation from multiple structures within the process chamber or walls of the vacuum chamber. The field of view of a radiation detector can be adjusted by for example one or several slits or apertures placed in front of the detector. An electric potential may be applied to the slits or apertures, so that only electrons with predetermined kinetic energies may reach the detector. An optical detector can be provided with filters that are transparent only to selected wavelengths.

[0024] The detector(s) can identify when the powder layer melts and forms a continuous melt pool, because a powder layer and a melt pool emit backscattered electrons in different ways, in terms of intensity and spatial distribution. A powder layer and a melt pool also emit light in different ways, in terms of intensity and spatial distribution. Backscattered electrons emitted from the powder bed in turn collide with other surfaces in the process chamber, resulting in emissions of secondary electrons and X-rays from these surfaces. In a similar manner, light emitted from the powder bed is reflected against other surfaces in the process chamber, resulting in emission of light from these surfaces Thus the radiation measured from chamber surfaces is an indirect signal of the melt status. In embodiments, one or several radiation detectors may be used to obtain a “fingerprint” of the radiation distribution in the process chamber. This fingerprint will change as the un-melted powder transforms into a melt pool. If needed, advanced tools such as multivariate data analysis, machine learning or artificial intelligence may be used to correctly interpret the fingerprint. The actual position of the beam on the powder bed may also be needed to interpret the fingerprint correctly. The software of the additive manufacturing machine can be trained to recognize the fingerprint from different locations on the powder bed, depending on the melt status.

[0025] In embodiments of the present invention, at least one detector is configured to detect radiation emitted from the top surface resulting from the electron beam melting the powder and creating a melt pool at the powder layer, wherein said detected radiation includes electrons, and / or x-rays and / or visible light, NIR-, IR- and / or UV light. In embodiments of the present invention, the analysing unit comprises a processing unit configured to process and analyze the received output signal from the detectors using, for example, multivariate data analysis, machine learning or artificial intelligence (Al) modules. The analysing unit may be communicatively connected to a user input device and a display for displaying, for example, a melt pool status.

[0026] According to embodiments of the present invention, a radiation shield is arranged at the at least one detector such that a line-of-sight between the at least one detector and a top surface of the powder layer is blocked, wherein radiation emitted from the top surface is blocked by the radiation shield.

[0027] According to embodiments of the present invention, the at least one detector is arranged below a top surface of said powder layer in a direction seen from the electron beam.

[0028] According to embodiments of the present invention, the at least one detector is arranged with an aperture arrangement comprising at least one aperture to adjust a field of view of the at least one detector.

[0029] According to embodiments of the present invention, the detectors further includes at least one detector arranged or configured to detect emitted radiation from the top surface of the powder bed.

[0030] According to embodiments of the present invention, wherein the radiation shield is arranged with at least one aperture arrangement comprising at least one aperture to adjust a field-of- view of the at least one detector.

[0031] The scope of the invention is defined by the claims, which are incorporated into this section by reference. A more complete understanding of embodiments of the invention will be afforded to those skilled in the art, as well as a realization of additional advantages thereof, by consideration of the following detailed description of one or more embodiments. Reference will be made to the appended sheets of drawings that will first be described briefly.

[0032] BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Fig. 1 schematically shows an arrangement for electron beam powder bed fusion in which the present invention can be implemented. Fig. 2a schematically illustrates an additive manufacturing arrangement for successively forming a three-dimensional product from a powder bed comprising at least one powder layer by means of powder bed fusion, in accordance with one or more embodiments

[0034] Fig. 2b schematically illustrates another additive manufacturing arrangement for successively forming a three-dimensional product from a powder bed comprising at least one powder layer by means of powder bed fusion, in accordance with one or more embodiments described herein.

[0035] Fig. 3 schematically illustrates how the emission of backscattered electrons from exposure of the electron beam changes during formation of the melt pool.

[0036] Fig. 4a-d schematically illustrates aperture arrangements in accordance with the present invention.

[0037] Fig. 5 schematically illustrates an embodiment of a radiation shield including an aperture arrangement in accordance with the present invention.

[0038] Fig. 6 schematically illustrates an embodiment of an aperture arrangement in accordance with the present invention.

[0039] Fig. 7 illustrates steps of embodiments of a method in accordance with the present invention.

[0040] Fig. 8 schematically illustrates an embodiment of a detector arrangement in accordance with the present invention.

[0041] Embodiments of the present disclosure and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures.

[0042] DETAILED DESCRIPTION

[0043] Additive manufacturing and 3D-printing refer to the process of manufacturing objects from 3D model data by joining powder materials layer upon layer. Powder bed fusion means additive manufacturing or 3D-printing where objects are built up in a powder bed. Thin layers of powder are repeatedly spread by a powder distributing member over a powder bed and fused by a beam from an energy source to a predetermined geometry for each layer. The powder bed is preferably lowered one nominal layer thickness (e.g. 0,020-0,100 mm) before distribution of the next powder layer. The energy source can be, for example, a laser or an electron gun. Upon finishing a powder bed fusion process, the fused object will be embedded in powder. The powder is removed after completion of the build process.

[0044] The present disclosure relates generally to arrangements and methods for additive manufacturing for successively forming a three-dimensional product from a powder bed comprising at least one powder layer by means of powder bed fusion. Embodiments of the disclosed solution are presented in more detail in connection with the figures.

[0045] Figure 1 schematically illustrates an additive manufacturing arrangement 100 for successively forming layers of a three-dimensional product in which the present invention can be implemented. The arrangement 100 comprises an additive manufacturing apparatus 200, for additive manufacturing by selective fusion of a three-dimensional product from a powder bed comprising at least one powder bed 240 by means of powder bed fusion, in accordance with one or more embodiments described herein. Electron beam powder bed fusion normally takes place in vacuum, and the electron beam may operate in several process steps: it may preheat the powder layers to a semi-sintered state, fuse the powder by melting and solidifying the powder in the powder layers, and add additional heat to the powder bed to maintain a predetermined temperature of the powder bed throughout the build. These process steps are preferably carried out under computer control to achieve predetermined quality requirements of the manufactured objects. In an electron beam powder bed fusion process, such as an additive manufacturing process for metal parts, the powder bed is normally also preheated for semi-sintering of the powder to reduce the risk for later levitation of charged powder and to increase the electrical conduction in the powder bed for increased transportation of electrons from the powder bed.

[0046] The schematically illustrated additive manufacturing apparatus 200 comprises an energy beam source 210 and a powder bed 240, arranged in a vacuum chamber or a build chamber 280. The build chamber can be a vacuum chamber, typical of E-PBF. The powder bed 240 may e.g. be formed by metal powder being distributed from a powder container 230 using a recoater mechanism 290. The powder bed 240 may comprise powder of any kind, such as e.g. metal powder composed of pure metal, metal alloys, intermetallics, metal matrix composite, ceramic powder, a mix of metal powder and ceramic powder, glass, graphite, diamond, composites, polymers, nanomaterials, and / or ionic compounds or any powder mixture thereof. The powder can also be a mixture of metal-based powder and insulating or semiconducting powder. The powder in the powder bed 240 is exposed to an electron beam 220 from the beam source 210. During the exposure to the electron beam 220, the metal powder is melted to form a melt pool. In embodiments, the vacuum chamber 280 and the powder container 230 are separated with a partitioning wall 235. The partitioning wall 235 is provided with a shutter mechanism 238 arranged to swiftly open and close, for example, when the recoater mechanism 290 moves back and forth to deliver metal powder into the built chamber 280. Thereby, an efficient delivery mechanism of metal powder is enabled at the same time as the vacuum conditions in the vacuum chamber 280 can be preserved and maintained.

[0047] The electron beam source 210 may comprise a laser adapted to generate a laser beam to heat the back side of a charged particle emitter mounted in a cathode holder system in the vacuum chamber 280. The charged particle emitter, when radiated with the laser beam, emits an electron beam into a charged particle channel of an anode. When the electron beam source is used for electron beam powder bed fusion, the electron beam 220 is directed onto a powder bed 240. The electron beam source 210 schematically shown in figure 2 is thus adapted to direct an electron beam 220 generated by a back heated charged particle emitter of a cathode onto the powder bed 240 via an anode and thereby fuse a three-dimensional product by fusing layer by layer of the powder in the powder bed 240 using the electron beam 220. In embodiments, the laser is a CO2 laser. In operation, a high voltage in the range of, for example, 60 kV is applied over the cathode and the anode in a per se known manner.

[0048] The powder bed 240 has a top surface 242 comprised in a vacuum chamber 280. The top powder layer 242 is preferably formed by powder being distributed from a powder tank 220 using a recoater mechanism. The recoater mechanism may e.g. be in the form of a powder layer distributing member or recoater 290, which may e.g. be a linear actuator for distributing powder at the powder bed 240. The additive manufacturing arrangement 200 may also comprise a spillover bin 275, where spillover powder may be collected.

[0049] Turning now to Fig. 2a, an additive manufacturing arrangement 100 for successively forming layers of a three-dimensional product in accordance with embodiments of the present invention will be discussed. It should be appreciated that like reference numerals are used to identify like elements illustrated in the figures and thereby will not be described again. Further, in Fig. 2a only parts of the arrangement 100 are shown.

[0050] The arrangement 100 comprises an additive manufacturing apparatus 200, for additive manufacturing by selective fusion of a three-dimensional product from a powder bed. The electron beam source 210 emits an electron beam 220 that exposes and melts the metal powder in the powder bed 240. During the exposure to the electron beam 220, the metal powder is melted to form a melt pool 245. This is illustrated in Fig. 3. In the left part of Fig. 3, the electron beam has not delivered enough energy to front surface or top surface 242 of the powder bed to melt the powder and the radiation, e.g. back-scattered electrons and / or photons, has a certain intensity, spatial distribution, and energy distribution. During the fusion, the top surface 242 of the powder bed starts to melt and forms a melt pool entailing a changed intensity, spatial distribution and energy distribution of the back-scattered electrons and / or photons. The spots are melted in accordance with a heating pattern, which may be a predetermine sequence or a randomized pattern. In Fig. 2a, the radiation, i.e. the back- scattered electrons and photons are illustrated with arrows indicated with “a”. The backscattered electrons and photons in turn collide with other surfaces in the process chamber or built chamber 280, resulting in emission of electrons and X-rays from these surfaces, illustrated in Fig. 2 with arrows indicated with “b”. Further, light is emitted from the melt pool during the fusion, such as NIR light, UV light and IR light, illustrated with arrows indicated with “a”, and the light is reflected by other surfaces in the process chamber or built chamber 280, illustrated in Fig. 2 with arrows indicated with “b”. The emitted light changes properties when the powder is melted to a melt pool compared to the un-melted powder. Thus, the radiation, including electrons, x-ray radiation, as well as NIR, UV and IR light, reflected from chamber surfaces is an indirect signal of the melt status of the melt pool.

[0051] According to the present invention, at least one detector 285, in Fig. 2a two detectors are shown, is arranged and configured to detect radiation emitted from structures within the chamber, for example, components or the chamber ceiling 291 or a chamber wall 292. The detector(s) 285 detect or identify when the powder layer melts and forms a melt pool, because a powder layer and a melt pool emit backscattered electrons, x-rays and light in different ways, in terms of intensity, spatial distribution and energy distribution. Thereby, a “fingerprint” of the radiation distribution in the vacuum chamber 280 can be obtained. This fingerprint will look different for un-melted and melted powder, respectively. If needed, advanced tools such as multivariate data analysis, machine learning or artificial intelligence may be used to correctly interpret the fingerprint which may be implemented in an analyzing unit 294. In embodiments of the present invention, the detectors 285 communicate with the analyzing unit 294 configured to use the output signal from the detectors 285 to analyze the radiation distribution in the output signal to determine a resulting signal indicating a melt status of the melt pool.

[0052] In Fig. 2b, another embodiment of the present invention is shown. The embodiment of Fig. 2b is identical to the embodiment of Fig. 2a, with the exception that at least a further detector 285 is arranged in the embodiment of Fig. 2a and configured to detect radiation emitted from the top surface resulting from the electron beam melting the powder and creating a melt pool at the powder layer, wherein said detected radiation includes electrons, and / or x-rays and / or visible light, NIR-, IR- and / or UV light.

[0053] The analyzing unit 294 may comprise processing unit 297 configured to process and analyze the received output signal from the detectors using, for example, multivariate data analysis, machine learning or artificial intelligence (Al) modules. The analyzing unit 294 may be communicatively connected to a user input device 298 and a display 299 for displaying, for example, a melt pool status and a desired value of the melt pool status, which may be a predetermined diameter and / or circumference of the melt pool.

[0054] The analyzing unit 294 may be configured to determine a resulting signal reflecting a time point when the powder in the top surface melts. Thereby, it is possible, for example, to determine the elapsed time between a first time point to when the electron beam first starts to expose a pixel or a point at the top surface 242 of the powder bed and a second time point t1 when the powder at the top surface 242 starts to melt and create a melt pool. Further, it is also possible to determine the period between the second time point t1 (i.e. when the powder at the top surface starts to melt) to a third time point t2 when the melting is finished, and the electron beam moves to a subsequent pixel or point. This enables a dynamic adjustment of the period between the first time point and the third time point, i.e. the dwell time, based on the detection of the second time point. It is hence possible to very accurately determine second time point t1 when the powder at the top surface 242 starts to melt and create a melt pool based on the detected radiation emanating from structures within the vacuum chamber including the walls of the chamber as well as and / or directly from the top surface 242. Further, the time from t1 to t2, i.e. t2 being the time when the electron beam jumps away from the spot it is currently melting can be controlled, and that the time from t1 to t2 should be approximately the same for all melting points in the layer to create a better melting situation (avoiding porosity, over-melting, variations in microstructure, etc.).

[0055] According to embodiments, several detectors are used to detect radiation in different angles in relation to the top surface to capture direct radiation from the top surface as well as from different structures within the vacuum chamber. Thereby, it is possible to create a compiled picture or fingerprint of the emitted radiation. In other embodiments, only radiation emitted from the different structures within the vacuum chamber is used to detect t1.

[0056] Further, the user may select a desired melt pool status and monitor the melt pool status in real-time. The dwell time of the electron beam 21 may be dynamically adjusted based on the melt pool status to cope with different thermal conditions at different spots in order to achieve a uniform melt pool status at each spot. A user may monitor the dwell time in realtime on the display 299.

[0057] In embodiments, the fingerprint is determined not only by the melt status, but also on the melting location at the powder bed. The software of the additive manufacturing machine can be trained to recognize the fingerprint from different locations on the powder bed, depending on the melt status.

[0058] Furthermore, an electron beam controller 296, for example an electron deflector, may be configured to control the electron beam 220 in response to the resulting signal to move or jump to a next spot according to the heating pattern when it is determined that the melt pool of a particular spot has reached desired status. The dwell time at each spot can thus be optimized and the overall time required for a layer can be optimized or minimized. The efficiency of the process can be improved. Moreover, the status of the melt pool can be influenced, for example, shape and / or form and / or depth and / or temperature of melt pool.

[0059] The additive manufacturing apparatus 200 comprises an electron beam source 210 and a powder bed 240, arranged in a build chamber 280. The build chamber can be a vacuum chamber, typical of E-PBF. The powder bed 240 may e.g. be formed by metal powder being distributed from a powder container 230 using a recoater mechanism 290. The powder bed 240 may comprise powder of any kind, such as e.g. powder composed of pure metal, metal alloys, intermetallics, metal matrix composite, ceramic powder, a mix of metal powder and ceramic powder, glass, graphite, diamond, composites, polymers, nanomaterials, and / or ionic compounds or any powder mixture thereof. The powder can also be a mixture of metalbased powder and insulating or semiconducting powder. The metal powder in the powder bed 240 is exposed to an electron beam 220 from the beam source 210. During the exposure to the electron beam 220, the metal powder is melted to form a melt pool. In the improved methods for additive manufacturing according to the present invention, the status of the melt pool, for example, shape and / or form and / or depth and / or temperature of melt pool is detected and monitored. Thereby, for example, the dwell time may be adapted to the thermal conditions at a particular spot by moving the electron beam to the next spot as soon as a desired melt status is obtained, for example, a predetermined melt pool diameter and / or circumference. The quality of the manufactured objects can also be improved as well as manufacturing of more complex 3-D objects can be achieved.

[0060] Hence, the detectors 285 are arranged to detect secondary electrons emitted from the chamber surfaces, for example, the chamber ceiling 291 or a chamber wall 292 resulting from the back-scattered electrons from the top surface 242 and / or melt pool 245. Hence, it is not necessary to detect the electrons emitted directly from the top surface of the powder bed in order to determine and control the size and status of the melt pool, which entails a number of advantages. For example, the sensing elements 286 (see fig. 8) of the detector(s) 285 can thereby be arranged in a manner such that they are protected from direct line-of-sight towards the powder bed surface or top surface 242, so that back-scattered radiation “a” emitted directly from the powder bed 240 cannot reach the sensing parts of the detector(s) 285. This results in an increased robustness, stability and life span of the detectors. The environment in build chamber 280 is demanding where there are very high levels of metal vapor, thermal radiation, X-rays and electrons. In particular, the back-scattered radiation “a” is very harsh to the detector resulting in a reduced life span and risk for failure and by shielding the detectors this hence entails radical improvement in life span and robustness. Furthermore, by shielding the detectors, more sensitive detectors can be used, for example semiconductor detectors similar to those used for backscatter electron detection in scanning electron microscopes, or scintillation detectors for X-rays, or photodiodes or multiwavelength spectrometers for light. There are many types of radiation detectors that cannot tolerate being coated by metal vapor or exposed to temperatures of 400 degrees C or more, conditions frequently encountered above the E-PBF powder bed. In the embodiment shown in Fig. 2a, one detector is arranged below the top surface 242 of the powder bed 240 thereby avoiding the direct radiation “a” from the top surface 242. A second detector 285 is arranged behind a radiation shield 265. The detectors 285 are configured and arranged to detect radiation “b” emitted from the chamber surfaces, for example, the chamber ceiling 291 or a chamber wall 292. The detector(s) 285 can have a narrow field of view, collecting radiation from specific surfaces only, or a broad field of view, collecting radiation from multiple structures within the process chamber. The field of view of a radiation detector can be adjusted by for example one or several slits or apertures placed in front of the detector. In Fig. 4a - 4d, different arrangements 410 - 440 with one or more slit or aperture 450 in different sizes and shapes are shown. An electric potential may be applied to the slits or apertures, so that only electrons with predetermined kinetic energies may reach the detector. Also, for detectors sensitive to X-rays or light, slits or apertures can be useful to limit the field of view. Furthermore, optical detectors can be provided with filters that are transparent only to selected wavelengths.

[0061] In embodiments of the present invention, the radiation shield 265 may be arranged such that it comprises also slits or apertures for adjusting the field of view of the detector, see Fig. 5. Hence, the radiation shield 265 comprises an aperture arrangement 510 provided with an aperture 550 to capture secondary electrons and / or photons emitted from structures within the build tank 280, in the illustrated example from the ceiling 291.

[0062] In Fig. 6, a detector 285 arranged with an aperture arrangement 610 with two apertures 650 to capture secondary electrons and / or photons emitted from several structures within the vacuum chamber 280, in the example, different components or the ceiling 291 and a wall 292 resulting from the backscattered electrons emitted from the front surface or top surface 242 of the powder bed 240. Detector 285 is arranged below the front surface or top surface 242 of the powder bed 240.

[0063] With reference now to Fig. 7, embodiments of a method for additive manufacturing in accordance with the present invention will be discussed. The method is preferable used in an additive manufacturing apparatus 200 for additive manufacturing by selective fusion of a powder bed containing a metal powder which is exposed to an energy beam such as an electron beam as described above with reference to Fig. 2. The present invention is preferably used in spot heating where the electron beam is moved between discrete points or pixels in accordance with a heating pattern, which may be a predetermined sequence or a randomized pattern. At each point, the electron beam dwells or is held a dwell time. A typical dwell time is of the order of microseconds to milliseconds. This means that 1000 points or more may be melted in one second. According to the invention, the dwell times for different spots can be adjusted dynamically to compensate for different thermal conductivity in each spot. The goal is usually to have a consistent size of the melt pool in each spot, regardless of the thermal conditions in each spot. The beam jump length, i.e. the distance between adjacent points, is kept short enough to make the jump time short in comparison with the dwell time. Beam jump lengths between points are typically <10 mm. At the same time, the beam jump length should not be too short, because each spot (melt pool) should melt and solidify individually, without being merged (co-melted) with other spots.

[0064] The additive manufacturing apparatus 200 comprises an electron beam source 210 and a powder bed 240, arranged in a vacuum chamber 280. The powder bed 240 may e.g. be formed by metal powder being distributed from a powder container 230 using a recoater mechanism 290. The powder bed 240 may comprise powder of any kind, such as e.g. powder composed of pure metal, metal alloys, intermetallics, metal matrix composite, ceramic powder, a mix of metal powder and ceramic powder, glass, graphite, diamond, composites, polymers, nanomaterials, and / or ionic compounds or any powder mixture thereof. The powder can also be a mixture of metal-based powder and insulating or semiconducting powder. The metal powder in the powder bed 240 is exposed to an electron beam 220 from the beam source 210. During the exposure to the energy beam 220, the metal powder is melted to form a melt pool. In the improved methods for additive manufacturing according to the present invention, the status of the melt pool, for example, shape and / or form and / or depth and / or temperature of melt pool is detected and controlled to thereby improve the quality of the manufactured objects as well as enable manufacturing a more complex 3-D objects.

[0065] The method 700 may comprise:

[0066] Step 710: Initiating an additive manufacturing process to manufacture an object.

[0067] Step 720: forming the powder bed 240 by distributing the metal powder from the powder container 230 using a recoater mechanism 290.

[0068] Step 730: Exposing the front surface or top surface 242 of the powder bed 240 for the electron beam 220 to melt the metal powder to form a melt pool at a first spot. Preferably, spot heating is applied where the electron beam 220 moves or jumps from spot to spot in accordance with a heating pattern, which may be a predetermined sequence or a randomized pattern. Each spot should be melted in a uniform manner, i.e. the melt pool should have the same size at each spot in order to obtain a homogenous fusion.

[0069] Step 740: Detecting radiation, which may be electrons, x-rays or visible light, UV light, IR light or NIR light, emitted from the structures within the vacuum chamber 280 resulting from back-scattered electrons or photons emitted from a melt pool at the powder layer using at least one radiation detector 285. In embodiments of the present invention, electrons, x-rays or light emitted directly from the front surface or top surface 242 of the powder bed 240 are not detected.

[0070] Step 750: Analyzing a radiation distribution in output signals from the at least one detector 285 to determine a resulting signal indicating a melt status of the melt pool. The status of the melt pool may be the size of the melt pool such as diameter and / or circumference.

[0071] Step 760: Controlling the electron beam 220 from the electron beam emitter 210 in response to the resulting signal to deflect the electron beam 220 as quickly as possible to start melting at the next spot in the heating pattern. For example, when a desired or predetermined status of the melt pool is achieved at a first spot, e.g. a desired diameter of the melt pool, the electron beam 220 jumps to the next spot in the heating pattern. Further, the electron beam may also be controlled to change the intensity and / or shape and / or diameter of the electron beam 220.

[0072] Turning now to Fig. 8, an embodiment of the present invention where the detector or detectors 285 are arranged to detect secondary electrons emitted from structures within the build chamber 280, for example, the chamber surfaces, for example, the chamber ceiling 291 or a chamber wall 292 resulting from the back-scattered electrons from the top surface 242 and / or melt pool 245 colliding with structures. Hence, the detector or detectors 285 are arranged and located such that the back-scattered electrons from the melt pool are not detected. Hence, indirect monitoring of the melt pool 245 is possible. This inventive idea is based on the insight that it is not necessary to detect electrons emitted directly from the top surface of the powder bed in order to determine and control the size and status of the melt pool, which entails a number of advantages. For example, the sensing elements 286 of the detector(s) 285 can thereby be arranged such that they are protected from direct line-of-sight towards the powder bed surface or top surface 242, so that back-scattered radiation emitted directly from the powder bed 240 cannot reach the sensing elements 286 of the detector(s) 285. This results in an increased robustness, stability and life span of the detectors. The environment in build chamber 280 is demanding where there are very high levels of metal vapor, thermal radiation, X-rays and electrons. In particular, the back-scattered radiation is very harsh to the detector resulting in a reduced life span and risk for failure and by arranging the detectors 285 such that the sensing elements 286 avoid back-scattered electrons, this hence entails radical improvement in life span and robustness. Furthermore, by arranging the detectors 285 such that the sensing elements 286 avoids back-scattered electrons, more sensitive detectors and sensing elements can be used, for example semiconductor detectors similar to those used for backscatter electron detection in scanning electron microscopes, or scintillation detectors for X-rays, or photodiodes or multiwavelength spectrometers for light. There are many types of radiation detectors that cannot tolerate being coated by metal vapor or exposed to temperatures of 400 degrees C or more, conditions frequently encountered above the E-PBF powder bed.

[0073] The foregoing disclosure is not intended to limit the present invention to the precise forms or particular fields of use disclosed. It is contemplated that various alternate embodiments and / or modifications to the present invention, whether explicitly described or implied herein, are possible in light of the disclosure. Accordingly, the scope of the invention is defined only by the claims.

Claims

1. CLAIMS1. Additive manufacturing arrangement (100) comprising an additive manufacturing apparatus (200) for additive manufacturing by selective fusion of layers of a three- dimensional product from a powder bed (240) comprising successively formed powder layers, the apparatus (200) comprising: a powder distribution device (290) configured to form successive powder layers; an electron beam emitter (210) configured to deliver an electron beam (220) towards a top surface (242) of the powder bed (240), wherein the electron beam (220) is used in the selective fusion of the three-dimensional product; at least one detector (285) configured to detect secondary electrons emitted from a structure (291 , 292) within a vacuum chamber (280) or inner surface (291 , 292) of the vacuum chamber (280) resulting from back-scattered electrons emanating from the electron beam (220) melting the powder and creating a melt pool at the powder layer colliding with the structure (291, 292) within the vacuum chamber (280) or inner surface (291 , 292) of the vacuum chamber (280); wherein said at least one detector (285) is configured to provide an output signal relative to the detected secondary electrons; an analyzing unit (294) configured to use the output signal from said at least one detector (285) to analyze the electron distribution in said output signal to determine a resulting signal indicating a melt status of said melt pool; and an electron beam controller (296) configured to control said electron beam (220) in response to said resulting signal.

2. Additive manufacturing arrangement (100) according to claim 1, wherein said analyzing unit (294) is configured to determine a resulting signal reflecting a time point (t1) when the powder in the top surface (242) starts to melt.

3. Additive manufacturing arrangement (100) according to claim 1 or 2, wherein said electron beam controller (296) is configured to control a time between the time point (t1) and a time point (t2) when said electron beam (220) moves to a subsequent pixel or point.

4. Additive manufacturing arrangement (100) according to claim 1 , wherein said at least one detector is configured with a field-of-view limitation in order to collect emission of electrons from a structure within the vacuum chamber or an inner surface (291 , 292) of the vacuum chamber (280).

5. Additive manufacturing arrangement (100) according to claim 1 , further comprising at least one detector (285) configured to detect electrons emitted from the top surface (242) resulting from the electron beam (220) melting the powder and creating a melt pool at the powder layer.

6. Additive manufacturing arrangement (100) according to claim 1 , wherein a radiation shield (265) is arranged at said at least one detector (285) such that a line-of-sight between said at least one detector and the top surface (242) of said powder layer (240) is blocked, wherein electrons emitted from said top surface (242) is blocked by said radiation shield (265).

7. Additive manufacturing arrangement (100) according to claim 1 , wherein said at least one detector (285) is arranged below a top surface (242) of said powder bed (240) in a direction seen from the electron beam (210).

8. Additive manufacturing arrangement (100) according to claim 1 , wherein said at least one detector (285) is arranged with an aperture arrangement (410 - 440; 510; 610) comprising at least one aperture (450; 550; 650) to adjust a field of view of said at least one detector (285).

9. Additive manufacturing arrangement (100) according to claim 1 , wherein said radiation shield (265) is arranged with at least one aperture arrangement (510) comprising at least one aperture (550) to adjust a field-of-view of said at least one detector.

10. Additive manufacturing arrangement (100) according to claim 1 , wherein said at least one detector (285) is a semiconductor detector, scintillation detector, or photodiode or multiwavelength spectrometer.

11. Additive manufacturing arrangement (100) according to claim 1 , wherein said analyzing unit (294) comprises a processing unit (297) configured to process and analyze the receivedoutput signals from said at least one detector (285) using, for example, multivariate data analysis, machine learning or artificial intelligence (Al).

12. Additive manufacturing arrangement (100) according to claim 1 , wherein an electron beam controller (296) is configured to control a dwell time and / or a jump speed of the electron beam (220) based on the received resulting signal indicating that melt status of said melt pool has reached a desired level.

13. Method for additive manufacturing by selective fusion of layers of a three-dimensional product from a powder bed (240) comprising successively formed powder layers, comprising the steps of: exposing a top surface (242) of the powder bed (240) for the electron beam (220) to melt the metal powder to form a melt pool; detecting secondary electrons emitted from structures (291 , 292) within the vacuum chamber (280) or inner surface (291 , 292) of the vacuum chamber (280) resulting from back- scattered electrons emanating from the electron beam (220) melting the powder and creating a melt pool at the powder layer using at least one detector (285) colliding with structures (291 , 292) within the vacuum chamber (280) or inner surfaces (291 , 292) of the vacuum chamber (280); analysing an electron distribution in output signals from the at least one detector (285) to determine a resulting signal indicating a melt status of the melt pool; and controlling the electron beam (220) from the electron beam emitter (210) in response to the resulting signal.

14. The method for additive manufacturing according to claim 13, blocking backscattered electrons electrons, and / or x-rays and / or visible light, NIR-, IR-, and / or UV light emitted from said top surface (242) by said radiation shield (265) from reaching the at least one detector (285).

15. The method for additive manufacturing according to claim 13, wherein said analyzing comprises determining a resulting signal reflecting a time point (t1) when the powder in the top surface (242) starts to melt.

16. The method according to claim 13 or 15, further comprising controlling a time between the time point (t1) and a time point (t2) when said electron beam (220) moves to a subsequent pixel or point.

17. The method for additive manufacturing according to claim 13, further comprising detecting electrons emitted from the structures (291 , 292) of the vacuum chamber (280) or inner surface (291, 292) of the vacuum chamber (280) below said top surface (242) of said powder layer (240) in a direction seen from the electron beam (210).

18. The method for additive manufacturing according to claim 13, wherein said detecting includes detecting electrons emitted from the top surface (242) resulting from the electron beam (220) melting the powder and creating a melt pool at the powder layer.

19. The method for additive manufacturing according to claim 13 - 18, wherein said at least one detector (285) is a semiconductor detector, scintillation detectors, or photodiodes or multi-wavelength spectrometers.

20. The method for additive manufacturing according to any one of proceeding claims 13 -19, processing and analyzing received output signals from said at least one detector (285) using, for example, multivariate data analysis, machine learning or artificial intelligence (Al).

21. The method for additive manufacturing according to any one of proceeding claims 13 -20, further comprising controlling a dwell time and / or a jump speed of the electron beam (220) based on the received resulting signal indicating that melt status of said melt pool has reached a desired level.22

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