Patent Description:
Additive manufacturing is a growingly important and capable method of manufacturing 3D workpieces. There are different variants of additive manufacturing, but herein we focus on methods and an apparatus for joining powder particles by selectively heating powder particles, e.g., on top of a bed of powder particles to adhere some of the particles to each other. The powder particles are adhered to each other by sintering, fusing and/or welding (hereinafter jointly "fused").

The heat for these processes is typically provided by focused radiation, for example by an electron beam or by a laser beam. These beams selectively heat portions of the top layer of the powder bed, thereby attaching particles of the top layer to particles of a preceding layer. This process is generally referred to as powder bed fusion process or simply powder-fusion process. Herein, we will not distinguish between different types of radiation and will simply refer to "beam" or "beams".

Modern apparatuses for powder-bed fusion have a housing with a process chamber. The process chamber has a support-opening for accommodating a movable support. Initially, a thin layer of powder is applied to the support. This is mostly accomplished by a recoater (see e.g. <CIT>, <CIT>, <CIT> and <CIT>, to name only a few. ) Once a layer has been subjected to the beam treatment a subsequent powder layer is applied, which then again is selectively fused. This process is iterated until additive manufacturing of the workpiece has been completed. Further treatment of the workpiece like e.g. grinding, cutting, milling etc. may still be required.

As taught, e.g. by <CIT>, the process chamber is advantageously filled with an inert gas. During the additive manufacturing process, the inert gas flows from a gas inlet over the bottom and thus over the top layer of powder on the support to a gas outlet. <CIT> aims for an essentially laminar flow of inert gas to thereby remove fumes, smoke or other side products of the fusing process (hereinafter jointly "smoke"). To this end, the inlet opening is made of a porous material to thereby release an essentially homogenous flow of the inert gas through the process chamber. The choice of the inert gas has been discussed in <CIT>, <CIT>, <CIT> or
<CIT> and specifically an Argon (Ar) or Nitrogen (N<NUM>) atmosphere with an oxygen concentration below 1000ppm (parts per million) have been suggested. The addition of Helium (He) to the inert gas atmosphere has been suggested as well to allow for higher laser scanning speeds.

<CIT> addresses the problem that very fine structures manufactured by selective laser melting are destroyed by a roller that controls the height of a levelling device for a powder bed by suggesting a process for producing a shaped body by selective laser melting by applying a powder layer using an applicator unit, fixing the applied powder layer to a layer below it using a focused laser beam and by applying the powder layer selectively by the applicator unit to regions of a surface of the shaped body that are located below or adjoin a plane that is defined by a layer thickness for a next powder layer that is to be processed.

<CIT> suggests a method of controlling an additive manufacturing process in energy beams are used to selectively fuse a powder contained in a build chamber having a gas flow therein in order to form a workpiece, in the presence of one or more plumes generated by interaction of the one or more energy beams with the powder. A trajectory of at least one of the plumes is controlled, so as to prevent the energy beams from intersecting the one or more plumes.

<CIT> suggest a method for generating a Ti-6Al-4V workpiece by melting a Ti-6Al-4V metal powder in a pure He-atmosphere with a built rate of more than <NUM>mm<NUM> s-<NUM>. Subsequently, the workpiece is subjected to Hot Isostatic Pressing to thereby obtain a high-density workpiece.

<CIT> relates to an additive fabrication device with a lower nozzle that blows out inert gas into a chamber in a horizontal direction through a lower opening portion formed in a lower part of a first side wall constituting the chamber. An upper nozzle that blows out the inert gas into the chamber through an upper opening portion formed in an upper part of the first side wall. The upper nozzle includes a window nozzle that blows out the inert gas along a window portion of a top board of the chamber, and an oblique nozzle that blows out the inert gas obliquely downward from the upper portion of the first side wall.

<CIT> relates to a cladding process by laser welding a wire in an inert gas atmosphere to a metal surface.

<CIT> suggest keeping a laser beam inlet window of an additive manufacturing process chamber clear of debris by providing a flow of an ionized gas over the surface of the beam inlet window.

<CIT> suggests a powder bed process in which multiple beams operate on a single pow-der bed, while a gas flow is provided over the powder bed. According to <CIT> it is to be avoided that a beam source is fusing a portion of the powder bed that is covered by a smoke plume that has been created by another beam source.

<CIT> suggests a selective powder melting process that uses argon or nitrogen as inert gas inside the process chamber.

<CIT> suggests an inert gas flow of argon, nitrogen or carbon dioxide inside process chamber of an additive manufacturing machine.

The initial powder bed fusion process suffered from being slow and there have been many attempts to decrease the manufacturing time, e.g. by using multiple beam sources simultaneously to thereby decrease the costs associated to a given additively manufactured workpiece. The difficulty of this approach is that a second beam may not fuse any portion of the powder bed while a smoke plume originating from the operation of a first beam is located in between of the second beam source and the corresponding portion of the powder bed without severely compromising the quality of the workpiece. These smoke plumes appear to distort, absorb and scatter the beams and accordingly many concepts have been developed to avoid fusing of portions of the powder bed being shadowed by smoke plumes originating from scanning the powder bed by other beams (see e.g., <CIT> or <CIT>).

<CIT> relates to an additive manufacturing apparatus and to additive manufacturing methods. <CIT> relates to a method and a device for feeding gas to an additive manufacturing space. <CIT> relates to a three-dimensional printer. <CIT> relates to an additive manufacturing process gas for lightweight metals.

The problem to be solved by the invention is to improve the powder bed process.

Solutions of the problem are described in the independent claims. The dependent claims relate to further improvements of the invention.

The process chamber housing comprises a process chamber with a bottom, a ceiling and side wall. The bottom, the ceiling and the side wall jointly enclose a volume of the process chamber. In a preferred embodiment, at least one inert gas inlet is in a front wall of the side walls and configured to provide an inert gas into the volume of the process chamber. At least a portion of the inert gas being provided to the volume can be removed via at least one inert gas outlet in a rear wall of the side walls, hence the inert gas outlet is configured to release the inert gas out of the process chamber. Alternatively or additionally, the gas inlet and/or the gas outlet may be provided in the ceiling, the bottom, in different or the same one of the side walls. The gas inlet and the gas outlet may be arranged preferably at opposite sides of the process chamber.

The bottom may have an opening. The opening may be delimited by opening walls. Preferably, a vertically movable support for supporting the powder bed and hence a three-dimensional (3D) object being manufactured by selectively fusing the powder bed may located in between of the opening walls. As already apparent, the support is preferably movably supported in the opening. For example, the support may be retracted further into the opening (i.e. lowered assuming a horizontal support surface) prior to adding a new layer of powder to the powder bed.

The opening walls may be and/or provide linear bearings restricting the movement to a direction at least essentially perpendicular to the edge being formed by the transition between the bottom and the opening walls. At least essentially perpendicular indicates that perpendicular is preferred, but small deviations, e.g. smaller <NUM>°, <NUM>°, <NUM>° and/or <NUM>° can be accepted. The opening walls may enclose a space for example a box or a circular cylinder. This space may accommodate already fused portions of the powder bed. The opening walls may be configured to be removed from the process chamber housing. This enables to simply replace the opening walls after a workpiece has been manufactured by an empty set of walls.

Preferably, the inert gas inlet and the inert gas outlet are positioned at opposite sides of the opening and face towards each other. This thus enables to provide a main inert gas flow in a main flow direction by injecting an inert gas via the (first) inert gas inlet while removing inert gas from the volume via the (first) inert gas outlet, or in other words by providing a pressure gradient from the inert gas inlet to the inert gas outlet. This main inert gas flow is preferably at least essentially parallel to an up-facing surface of the bottom and/or the optional support. The main inert gas flow may as well have an upward or downward component. In a preferred example, the direction of the main inert gas flow has a non-vanishing component being parallel to the bottom and/or the support. This non-vanishing component provides for a direction in which smoke plumes being produced when scanning the powder bed with the beam are inclined relative to the vertical.

In a preferred example, the gas inlet is connected to an inert gas source providing a He-comprising inert gas. The inert gas source is preferably configured to provide an inert gas comprising at least Helium (He) and one of another noble gas (i.e. at least one of Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), Radon (Rn)) and/or Nitrogen (N2). The inert gas source is configured to provide the He-comprising inert gas from the inert gas source into the volume of the process chamber. Particularly preferred, the inert gas source provides a gas comprising at least one of <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>% He and/or Ne, wherein the percentage relates to the amount mol He and/or Ne, respectively, relative to the total amount of gas in mol.

Unexpectedly it turned out that replacing Ar or N<NUM> by He and/or Ne enables to fuse portions of the powder bed with a second beam being emitted by a second beam source while smoke being produced by a first beam being emitted by first beam source is located in between of the beam outlet of a second beam source and the portion of the powder bed to be fused. Hence in an embodiment the invention comprises the step of fusing a powder bed using at least two beams, while at least one second beam is controlled to fuse a location of the powder bed being covered by a smoke plume being generated by fusing another portion of the powder bed with another beam (the at least one first beam), wherein the smoke plumes are removed from the process chamber by a main inert gas flow, wherein the main inert gas flow is established by an inert gas comprising at least <NUM>% of He and/or Ne, and/or having a density below at least one of <NUM>,<NUM>/m<NUM> <NUM>,<NUM>/m<NUM>, <NUM>/m<NUM>, <NUM>/m<NUM>, <NUM>/m<NUM> and <NUM>/m<NUM>. The density references preferably to normal conditions (<NUM>, 1013hPa) but may as well reference to the actual conditions in the process chamber. In other words, the at least one second beam source operates downstream of the beam spot of the another beam source, wherein downstream relates to the main inert gas flow direction over the powder bed. Being covered is intended to express that the smoke plume is in the beam path of the at least one second beam. As already mentioned, the beam sources are preferably laser beam sources, but not limited to these. The usage of the term "laser beam" or "laser beam source" herein is to be understood as a preferred example of for "beam" or "beam source". In operation the beam source may be pivoted to thereby scan locations to be fused on the powder bed. For example, a mirror being pivoted to project a laser beam onto said locations on the powder bed can be considered as a beam source, even if the beam is not generated by the mirror itself. In this application the location from which beam is emitted towards a location of the powder bed is relevant, not the type of beam or the beam generation device.

Different from all prior art teachings, operating in the smoke plume being produced by scanning a portion of the powder bed with the first beam is made possible by replacing at least a portion of the Ar and/or N<NUM> by He and/or Ne. Further investigations revealed that the smoke in these smoke plumes has a comparatively low impact on the quality of a fused area being fused by a penetrating a smoke plume of produced by another beam. These observations render it plausible that the heat induced density change in the smoke plumes are responsible laser beam distortions leading to a defocusing of an otherwise well focused beam and/or to a change of the beam profile and/or to change of the beam intensity distribution over the beam profile. The lower density of He and/or Ne combined with the increased heat conductivity of He and/or Ne, provides for lower density gradient between hotter and colder portions of the inert gas flow and hence to a reduced effect of defocusing. The positive effect of reducing laser beam distortion due to thermal inhomogeneities in the inert gas flow can be further enhanced by reducing the amount of gas in the volume, i.e. by operating the process chamber at pressures below ambient pressure. Lowering the pressure significantly may even allow to use Ar and/or N<NUM> as inert gas (, or to simply use air), i.e. to omit He and/or Ne. Another advantage of lowering the pressure (i.e. of reducing the amount of gas molecules in the volume) is that the flow speed of the inert gas flow can be increased without shifting (blowing away) powder particles previously deposited on the powder bed by the inert gas flow.

Particularly preferred, the inert gas has a thermal conductivity at or above at least one of <MAT> and <MAT> at normal conditions (<NUM>, 1013hPa) and/or a thermal conductivity at or above at least one of <MAT> and <MAT> at the conditions in the process chamber.

In a preferred example, the inert gas flow may have a flow speed in the main flow direction may and that the mean flow speed measured <NUM> over the opening is above <NUM>/s preferably above <NUM>/s and/or below <NUM>/s preferably below <NUM>/s even more preferred below <NUM>/s at 1013hPa. These boundaries may preferably be increased if the gas pressure in the volume of the process chamber is decreased and/or if the molar mass of the gas is decreased.

In a preferred example, the process chamber comprises at least one oxygen sensor and/or a gas density sensor and/or thermal conductivity sensor and/or thermal capacity sensor configured to determine a value being representative for the thermal conductivity and/or thermal capacity, respectively of the inert gas. At least one of these sensors is preferably located in the bottom portion of the volume. For example, at least one of these sensors may be located on the bottom and/or in a recess of the bottom and/or below a distance of <NUM> above the bottom and/or on the support and/or below the support, and/or at or within at least one of <NUM>, <NUM>, <NUM>, <NUM>, <NUM> from the edge encircling the opening. In another example, at least one of these sensors is preferably located in the (first) inert gas outlet, preferably at the bottom of the (first) inert gas outlet and/or within at least one of <NUM>, <NUM>, <NUM>, <NUM>, <NUM> from the edge encircling (first) inert gas outlet. Each of these example locations enable to measure the Oxygen concentration in the vicinity of the powder bed. Oxygen has a higher mass per molecule (16u) and will therefore accumulate in the bottom portion of the volume if He (4u) and/or Ne (10u) is used as inert gas (as usual, u is the unified atomic mass unit). Thus, potential leaks or impurities provided by the gas source can be detected quickly. Further the measurement is as representative as possible for the oxygen level right over the powder bed. In addition or alternatively, at least one of the sensors may be located in a duct connecting at least one inert gas outlet with at least one inert gas inlet.

The at least one Oxygen sensor and/or a gas density sensor and/or thermal conductivity sensor and/or thermal capacity sensor is preferably located in the inertgas flow through the chamber. Particularly preferred is the respective sensor oriented at least essentially parallel to the inert gas flow, wherein at least essentially parallel indicates that parallel is preferred but that deviations within a few degrees (e.g., within ±<NUM>°, ±<NUM>°, ±<NUM>°, ±<NUM>°, ±<NUM>°, ±<NUM>° or <NUM>°) can be accepted,.

The at least one Oxygen sensor and/or a gas density sensor is preferably coupled to a process chamber controlling device, i.e. to an electronic circuitry for controlling operation of the process chamber and/or an entire additive manufacturing apparatus with the process chamber (hereinafter simply "controller"). In particular if, e.g. the oxygen level is above a predefined threshold, the controller may increase the flow speed of the inert gas flow over the opening, e.g. by increasing the power provided to a vacuum pump being connected to the inert gas outlet and/or by opening a throttle valve upstream of the inert gas inlet. Further, prior to fusing the powder bed, the process chamber is preferably flooded with the inert gas. Once the oxygen level is below a predefined threshold the inert gas may be circulated from the inert gas outlet to the inert gas inlet using an inert gas pump to thereby establish the inert gas flow.

In a preferred example, the process chamber may comprise a gas component concentration sensor being configured to measure a value being representative for at least one of the concentrations of O<NUM>, N<NUM>, He, Ne, Ar, Kr, Xe, Rn and/or for a ratio of at least two of these gases in the inert gas. The gas component sensor may be located or have a gas inlet at the positions indicated above for the oxygen sensor. The above referenced sensors can be considered as examples for a gas component concentration sensor. In other words, a gas component concentration sensor may be or comprise at least one of an Oxygen sensor and/or a gas density sensor and/or a thermal conductivity sensor and/or thermal capacity sensor and/or a gas chromatograph and/or a spectrometer and/or gas-analyzer, in particular He-analyzer. Further, it is noted that partial pressure values of O<NUM>, N<NUM>, He, Ne, Ar, Kr, Xe, Rn may be considered as representative for at least one of the concentrations of these gases if the total pressure is known. A simple and yet efficient way to measure concentrations is thus to measure the partial pressure of at least one of these gases, e.g. via the diffusion rate through a semipermeable membrane. For example, if a semipermeable membrane is permeable only for He, the diffusion rate through the membrane at a given differential pressure between the spaces being separated by the membrane can be used to determine the partial pressure of He in the inert gas.

The gas component concentration sensor is preferably coupled to the process chamber controlling device by a data line. Hence, the values obtained by the gas component concentration sensor may be made available to the process chamber controlling device.

In any embodiment the method my comprise feeding at least a portion of the inert gas being removed through the inert gas outlet via the inert gas inlet to the process chamber. This is as well referred to a recycling or circling the inert gas.

A method for fusing at least a portion of the powder bed may thus comprise to control the composition of an inert gas flow being established above the powder bed.

The method may comprise determining the concentration and/or partial pressure of at least one of O<NUM>, N<NUM>, He, Ne, Ar, Kr and Xe in the inert gas flow established above the powder by determining the concentration and/or partial pressure of at least one of O<NUM>, N<NUM>, He, Ne, Ar, Kr and Xe in the inert gas in the process chamber and/or being removed from the process chamber via the at least one inert gas outlet and/or being provided via the inert gas inlet to the process chamber. The method may further comprise obtaining a measurement value being representative for the concentration and/or partial pressure of at least one of O<NUM>, N<NUM>, He, Ne, Ar, Kr and Xe in the inert gas and comparing this measurement value with a lower limit and/or with an upper limit for the concentration and/or the partial pressure of the respective at least one of O<NUM>, N<NUM>, He, Ne, Ar, Kr and Xe in the inert gas. It is noted that O<NUM> is not inert and hence should not be comprised in the inert gas. But monitoring the unintended O<NUM>-concentration may be used to increase the concentration(s) of inert components of the inert gas to thereby reduce the partial pressure and the concentration of the O<NUM>, what can be considered as effectively removing unintended O<NUM> from the process chamber and hence the fusing process.

In case said comparing provides that this measurement value is below a lower limit, the method may comprise adding the corresponding depleted component to the inert gas stream in the process chamber, e.g., by adding it to an inert gas stream circling from the inert gas outlet to the inert gas inlet and through the process chamber, while adding no or less of at least one other component of the inert gas mixture to the inert gas mixture. "less" in this context references to the amount of the depleted component, i.e. the amount added of the at least other component being added is less than the amount added of the depleted component.

Similarly, in case said comparing provides that this measurement value is above an upper limit, the method may comprise adding the at least one other component to the inert gas in the process chamber, e.g. by adding it to an inert gas stream fed from the inert gas outlet via duct to the inert gas inlet (thereby into the process chamber) than said component having a measurement value above the upper limit, while adding no or less of said component having a measurement value above the upper limit. Similarly to the above "less" references to the amount the at least one other component is added. In other words more of the at least one other component is added than of the component having a measurement value above the upper limit.

This method allows to partially and selectively replenish only those components of the inert gas that are underrepresented in a given composition as defined by the upper and lower boundaries for the respective components. This helps to keep operating costs low, while maintaining high workpiece quality. These method steps are based on the observation that for practical purposes the inert gas is often a mixture of essentially He and/or Ne with Ar and/or with N<NUM> wherein the concentrations of He, Ne, Ar, and N<NUM> in the mixture are well defined. However, He and Ne diffuse at a significantly higher rate through the duct wall and the other confining structures of the process chamber housing. The method thus allows to maintain the partial He and/or Ne-pressure and/or the He and/or Ne-concentration in the inert gas within given limits, while not replacing He and/or Ne-depleted inert gas from the process chamber by (costly) "fresh" inert gas. By measuring a value being representative for the, e.g., He and/or Ne-concentration (and/or partial pressure) and adding only He and/or Ne to the inert gas circling through the process chamber housing, if a He and/or Ne-depletion below the lower limit for He and/or Ne, respectively, has been observed. This approach can be used for any other of the inert gases mentioned above, i.e. curing a He and/or Ne-depletion is only a preferred example. Any other depletion can be resolved by adding the depleted component(s), preferably only.

These steps of controlling the composition of an inert gas flow being established above the powder bed may be executed by the process chamber controlling device, as well referred to as "controller". In other words, the process chamber controlling device may be configured to execute the any of the above method steps, be it directly or by controlling and or communicating with corresponding components, like ,e.g., a gas component concentration sensor.

The corresponding process chamber housing may thus comprise the process chamber controlling device. The process chamber controlling device may be connected to at least one gas component concentration sensor, e.g., via a data line and/or any other data transmission means. This gas component concentration sensor may be located in the process chamber. Alternatively or in addition, the (or another) gas component concentration sensor may be located at, attached to and/or integrated in a duct connecting the inert gas outlet with the inert gas inlet.

The process chamber may further comprise an inert gas component source comprising only a component or a limited number of components of the inert gas in the process chamber. For example, the inert gas component source may comprise only He and/or Ne but no Ar or N<NUM> if the inert gas is a mixture of all three gases. In practice it is sufficient if the concentration of the depleted inert gas component in the gas provided by the inert gas component source is greater than the preset or intended concentration of the depleted component in the inert gas, as in this case adding the gas mixture from the inert gas component source increases the concentration of component being depleted in the inert gas circling through the process chamber and the duct connecting the inert-gas outlet with the inert-gas inlet.

The process chamber may also comprise more than one inert gas component sources, comprising respectively one different inert gas component and/or comprising different inert gas mixtures.

As usual "limited number of components" means that at least one component of the (intended) inert gas mixture is not comprised or underrepresented in the inert gas component source. The inert gas component source may be fluidly connected via at least one inert gas component valve with the inert gas- inlet or with a separate gas inlet, e.g. via a branch of the duct. A fluid connection via the duct is preferred, as the concentrations of the components of the inert gas being provided to the process chamber homogenize. In other words, the inert gas in the process chamber has a more homogenous composition.

The process chamber controlling device may be connected via e.g. at least one control line and/or a contactless data connection to the inert gas component valve, thereby being enabled to open and close the inert gas component valve.

In another preferred example, the process chamber comprises a heater configured to heat the temperature of at least a portion of the inert gas flow through the process chamber to or above at least one of <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. When using higher temperatures, the process chamber housing is preferably thermally isolated. By this increase of the gas temperature, the temperature gradient in a smoke plume is decreased and thus beam distortion of the smoke plume is reduced if not neglectable small.

As already explained above, the process chamber preferably comprises a pressure controller, configured to maintain the pressure inside the process chamber below ambient pressure outside the process chamber and/or at or below at least one of 1000hPa, 900hPa, 800hPa, 700hPa, 600hPa, 500ha, 400hPa, 300hPa, 200hPa, 100hPa. The pressure controller may be integrated or form a part of a process chamber controller. In another preferred example, the pressure in the process chamber is above ambient pressure, thereby ensuring that no oxygen is sucked accidentally into the process chamber. In another example, the process chamber is enclosed in another housing, and the pressure in the process chamber is below ambient pressure, while the pressure in the volume being defined by the boundary of the process chamber and the another housing is above is above ambient pressure and in that the gas said volume is as well an inert gas, preferably the same as in the process chamber. This enables to ensure a lower pressure in the process chamber while at the same time reducing the risk that oxygen is accidentally sucked into the process chamber, because the volume between the boundary of the process chamber and the another housing is filled with an inert gas and has a pressure above ambient pressure.

For example, the inert gas outlet may be in fluid communication with a low pressure inlet of a gas pump (e.g. a vacuum pump inlet) e.g. via the above mentioned duct and/or that the inert gas inlet may be in fluid communication with an inert gas source (e.g. the higher pressure gas outlet of the gas pump), wherein a throttle valve may be located upstream of the inert gas inlet. The optional pressure controller may be configured to increase and/or decrease the power provided to the gas pump. It may further be configured to open and/or close the throttle valve, e.g. by powering an actuator. The optional pressure controller may hence as well control the flow speed of the inert gas flow. The optional pressure controller may be connected to at least one pressure sensor and/or flow speed sensor and may control the pressure and/or the flow speed in the process chamber in response to signals being provided by at least one of the pressure sensor and/or the flow speed sensor.

Changing the inert gas may have an impact of the signals being provided by flow speed and/or pressure sensors. This impact may require a recalibration of the sensors when changing the inert gas during manufacturing of a workpiece or between manufacturing of two workpieces. For example, when using an inert gas with an increased heat capacity and/or an increased thermal conductivity an anemometer for measuring the flow speed may require recalibration. For example, a hot wire anemometer experiences better cooling when increasing the thermal conductivity of the inert gas. Accordingly, the resistivity of the hot wire drops, which directly translates in presumably wrong flow speed readings, if the increased thermal conductivity is not considered. Similarly, a change of the (mean) molar mass and/or density of the inert gas may require a recalibration of the flow speed sensor, e.g., when using a vane anemometer or a cup anemometer.

In a preferred example, the process chamber further comprises at least one second gas outlet in at least one of the bottom, the support, the opening walls and the opening bottom. The second gas outlet may be used when replacing a gas(mixture) like air by the inert gas, e.g. by at least one of He and/or Ne and/or Ar and/or N<NUM>, which may then preferably be provided to the volume via at least one second inert gas inlet in the ceiling. The use of expensive He and/or Ne or another inert gas can be reduced by each of these measures.

The optional second gas outlet may be connected to a second gas outlet control valve, preferably to a check valve configured to disable gas flow into the process chamber via the second gas outlet. Further, the second gas outlet may be connected by a tube or the like to a gas inlet of a second outlet vacuum pump.

As already apparent, the process chamber preferably comprises at least one (laser) beam entry window, which may be located above the support. In a particularly preferred example, the process chamber further comprises at least one inert gas jet stream inlet nozzle. As will be explained blow in more detail the term "Jetstream" used only to indicate that it is a second as well linguistically distinguishable gas stream flowing above the inert gas stream. The inert gas jet stream inlet nozzle is preferably positioned in the upper portion of the process chamber and is preferably oriented to provide an inert gas jet stream between the window and the support. Particularly preferred, the inert gas jet stream attaches to window surface and/or is directed downwards. This can be obtained by orienting the inert gas jet stream inlet nozzle accordingly, e.g. by orienting is towards the window surface and/or by use of the Coanda effect.

Preferably opposite to the inert gas jet stream inlet nozzle may be at least one inert gas jet stream outlet nozzle, hence as well being positioned to provide an inert gas jet stream between the window and the support.

The inert gas jet stream compensates for the effect that the smoke in a reduced density atmosphere tends to rise higher. The inert gas jet stream protects the window from being polluted by condensed or sublimated smoke, which as well would deteriorate beam quality and hence workpiece quality. The suggested measure thus enables to keep the vertical dimension of the volume reasonable, which reduces operating costs as well as installation costs and last but not least workpiece quality, as an increase in the distance between laser source and the powder decreases workpiece quality, as imperfections in beam focusing become more apparent.

For example, the at least one inert gas jet stream inlet nozzle may have a nozzle outlet opening being oriented parallel within an angle αjs to the (first) inert gas inlet, wherein αjs ∈ A, and A = {<NUM>°, <NUM>°, <NUM>°, <NUM>°, <NUM>°, <NUM>°, <NUM>°, <NUM>°}. This measure reduces turbulences in the volume and hence increases efficient smoke removal from the volume.

Preferably, the flow speed of the inert gas jet stream relative to the window is at least <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. <NUM>, <NUM>, <NUM> and/or <NUM> times the flow speed of the inert gas flow <NUM> over the opening. This increased flow speed thus provides for a flow speed gradient from the bottom to the ceiling. This gradient enables to safely remove the smoke without blowing powder from the powder bed towards inert gas outlet.

In preferred example, the temperature of one the at least one optional inert gas jet stream is below the temperature of the main inert gas stream, at least measured at the corresponding nozzle openings. Residues in the smoke can thus condense prior to reaching the window and no or at least less residues in the smoke condense on the window.

In another example, the temperature of one the at least one optional inert gas jet stream is above the temperature of the main inert gas stream, at least measured at the corresponding nozzle openings. This allows to efficiently cool the powder bed by the main inert gas flow flowing preferably at least almost directly over the powder bed. This can be obtained by positioning the lower edged of the (first) inert gas inlet and/or the lower edge of the (first) inert gas outlet at the height of the edge of the support opening and/or at the height of the bottom or slightly above these heights. Slightly above means within at least one of <NUM>, <NUM>, <NUM>, <NUM> and/or <NUM> above the corresponding reference height. Further, the temperature of the main (first) inert gas flow from the (first) inert gas inlet to the (first) inert gas outlet is preferably below ambient temperature, e.g. at or below at least one of <NUM>, <NUM>, <NUM>, <NUM>, <NUM>°, -<NUM>, - <NUM>, -<NUM>. The lower the temperature, the better is the cooling, i.e. the heat transfer from the powder bed and/or the workpiece to the main inert gas stream. The temperature of the inert gas jet stream may be preferably at or above ambient temperature, e.g. at or above at least one of <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. This particularly preferred combination of having reduced temperature in the vicinity of the powder bed and an increased temperature above provides for both, a good cooling and low beam distortions.

Further, it is preferred, if the vertical thickness of the main inert gas stream or flow is significantly smaller than the vertical thickness of the inert gas jet stream above the main inert gas stream. The vertical thickness of the respective streams can be controlled, e.g. by the vertical dimension of the respective inlet opening. Thus, the vertical dimension d<NUM> of the inert gas jet stream inlet is preferably greater or equal to x-times the vertical dimension d<NUM> of the (first) inert gas inlet, wherein x ∈ {<NUM>, <NUM>, <NUM>, <NUM>, <NUM>,<NUM>}, i.e. x · d<NUM> ≤ d<NUM>.

As apparent, both the main inert gas flow from the inert gas inlet nozzle to the inert gas outlet nozzle and the inert gas jet stream are "inert gas streams" the terms main inert gas stream and inert gas jet stream are used only to enable to linguistically distinguish these. Alternatively, one could have used first gas stream and second gas stream, but we consider the initially suggested wording to be more vivid. As apparent, in a preferred example the second inert gas stream (i.e. the inert gas jet stream) has a higher flow speed across the volume than the first (main) inert gas stream. The flow rate of the second inert gas stream may be higher than the first (=main) inert gas stream. Hence "main" does not have any implications regarding the amount of gas flowing per unit of time compared to other inert gas streams.

An additive manufacturing apparatus according to the invention may of course comprise characterized in that it comprises the process chamber with at least one of the above explained features. In particular, the additive manufacturing apparatus may have one laser beam entry window and outside the process chamber, in front of the at least one window at least two (laser) beam sources each configured to emit at least one (laser) beam to a powder bed on top of the support. It is implicit that the window is at least essentially transparent for the beams being emitted by the (laser) beam sources through the at least one window towards the support. In a preferred example, the additive manufacturing apparatus is configured to scan a surface of a powder bed being below a smoke plume being produced by operation of a first (laser) beam source. This measure even allows to increase workpiece quality, e.g. by optimizing the thermal stress to the workpiece during manufacturing.

In the following, the invention will be described by way of example, without limitation of the general inventive concept, on examples of embodiment with reference to the drawing.

In <FIG> is a simplified sectional view of an example additive manufacturing apparatus <NUM> with a process chamber <NUM>. The process chamber <NUM> has a volume <NUM> being enclosed by side walls <NUM>, <NUM>, <NUM> (a fourth side wall is not visible), a ceiling <NUM> and a bottom <NUM>. The bottom <NUM> has an opening <NUM> with opening walls <NUM>. The opening walls <NUM> may provide a linear bearing for a movably supported support <NUM>, which can be lowered and raised. On top of the support <NUM> may be an optional powder bed <NUM> in which a partially manufactured workpiece <NUM> may be embedded. The powder bed <NUM> and the workpiece <NUM> are depicted only as an example, but the additive manufacturing apparatus <NUM> and/or the process chamber <NUM> are/is typically delivered without any powder bed or workpiece.

The ceiling <NUM> has windows <NUM>, <NUM>, being transparent for beams <NUM>, <NUM>, being emitted by beam sources <NUM>, <NUM>. Depicted are a first beam source <NUM> and a second beam source <NUM> emitting the first and the second beam <NUM>, <NUM>, respectively. Preferably, the process chamber <NUM> has more than two beam sources <NUM>, <NUM>. The window <NUM>, <NUM> can as well be unitary and hence there is at least one window above the opening <NUM> in the bottom.

An (first) inert gas inlet <NUM> in a front side wall <NUM> and an (first) inert gas outlet <NUM> in a rear side wall <NUM> enable to provide a main inert gas flow <NUM> across the opening <NUM> in the bottom <NUM>, i.e. in a main inert gas flow direction <NUM>. As can be seen, the main inert gas flow <NUM> may flow at least essentially parallel (i.e. ±αms (αms ∈ {<NUM>°, <NUM>°, <NUM>°, <NUM>°, <NUM>°, <NUM>°, <NUM>°, <NUM>°}) to the bottom <NUM> and hence at least essentially parallel to the powder bed surface and the powder bed supporting surface of the support <NUM>. In the depicted example, the main inert gas flow direction <NUM> has a small downward component. Preferably, the inert gas comprises at least <NUM>% Helium (He), and/or has a density below <NUM>/cm and/or a temperature above the dew point of the gas. In an example the pressure may be at or above ambient pressure. In another example the pressure may be at or below ambient pressure.

Above the main inert gas inlet <NUM> is at least one optional second inert gas inlet <NUM> and at least one optional third inert gas outlets <NUM>. Above the main inert gas outlet <NUM> is at least one optional second inert gas outlet <NUM>. These could as well be referred to an inert gas jet stream inlets or inert gas jet stream outlets, respectively.

In operation, a second inert gas stream may flow above the main (first) inert gas stream from the at least one optional second inert gas inlet <NUM> to the at least one optional second inert gas outlet <NUM>. As indicated by the arrows <NUM>, the volume per amount of time, i.e. the flow rate and/or the flow speed of the optional second inert gas flow <NUM> are/is preferably higher than the flow rate, and/or the flow speed, respectively of the main inert gas flow <NUM>. Further, the downward component of the flow direction of the second inert gas flow direction <NUM> is preferably greater than the downward component of main inert gas flow direction <NUM>. The temperature of the second inert gas flow <NUM> is preferably below the temperature of the main inert gas flow <NUM>.

The optional third inert gas inlet <NUM> is preferably located in the vicinity (within <NUM>, <NUM>, <NUM> and/or <NUM>) of the at least one window <NUM>, <NUM> in the ceiling <NUM> and located to attach a third inert gas stream to the surface of the at least one window <NUM>, <NUM>, to thereby contribute to keep the at least one window <NUM>, <NUM> clear of condensate. Preferably, the temperature of the inert gas exiting the third inert gas inlet is above the temperature of the second inert gas entering the volume <NUM> via the at least one second inert gas inlet <NUM>.

As indicated, each of the beams <NUM>, <NUM> is directed on a different location of the powder bed <NUM> and the fusing process produces first and a second smoke plumes <NUM>, <NUM>. As shown, the second beam <NUM> passes through the first smoke plume <NUM> originating from the interaction of the first beam <NUM> with the powder bed <NUM>.

The inert gas is removed by at least one pump <NUM>, i.e. the first and second inert gas outlets <NUM>, <NUM> are in fluid communication with the lower pressure inlet of the gas pump <NUM> which then feeds the inert gas to at least one of the inert gas inlets <NUM>, <NUM>, <NUM> via a duct <NUM>. The temperature of the different inert gas streams can be controlled preferably by optional indirect heat exchangers <NUM>, <NUM>, <NUM>.

A controller <NUM> may be connected by data and/or power lines to the beam sources <NUM>, <NUM>, sensors like <NUM>, the pump <NUM>, valves <NUM>, etc. Example connections are indicated by dashed or dotted arrows.

<FIG> shows another simplified sectional view of an example additive manufacturing apparatus <NUM> with a process chamber <NUM>. The description of <FIG> can as well be read on <FIG>. Only differences will be explained herein. Similar to <FIG>, at least one of the inert gas outlets <NUM> and <NUM> of the process chamber <NUM> may be connected via a duct <NUM> with at least one of the inert gas inlets <NUM>, <NUM>, <NUM>. A pump <NUM> may have a pump inlet being in fluid communication with at least one of the inert gas outlets <NUM>, <NUM> and a pump outlet may be in fluid communication with at least one of the inert gas inlets <NUM>, <NUM>, <NUM> via the duct <NUM>. The duct may comprise a gas component sensor <NUM>. The values measured by the gas component sensors <NUM>, regardless of its position, may be provided to the controller <NUM> by some data line or any other communication means. The controller may as well be referred to as process chamber controlling device <NUM>.

The process chamber housing preferably has at least one of these gas component sensors <NUM>. In <FIG>, two gas component sensors <NUM> are depicted at preferred positions for illustrative purposes. Other numbers of gas composition sensors can be used as well.

The process chamber housing may further comprise at least one inert gas component source <NUM>. In an example, the inert gas component source <NUM> may comprise a tank being filled or configured to be filled with, e.g., He and/or Ne or another inert gas or inert gas mixture.

The inert gas component source <NUM> is fluidly connected via an inert gas component valve <NUM> with at least one of the inert gas inlets <NUM>, <NUM>, <NUM>.

Claim 1:
An additive manufacturing apparatus comprising:
at least a first beam source (<NUM>) configured to emit at least a first beam (<NUM>),
at least a second beam source (<NUM>) configured to emit at least a second beam (<NUM>), and
a process chamber housing (<NUM>),
wherein the process chamber housing (<NUM>) comprises a process chamber (<NUM>),
wherein the process chamber (<NUM>) has at least:
- a bottom (<NUM>), a ceiling (<NUM>) and side walls (<NUM>, <NUM>, <NUM>), jointly enclosing a volume (<NUM>) of the process chamber (<NUM>),
- an inert gas inlet (<NUM>) configured to provide an inert gas into the process chamber (<NUM>),
- an inert gas outlet (<NUM>) configured to release the inert gas out of the process chamber (<NUM>),
wherein the bottom (<NUM>) has an opening (<NUM>), being delimited by opening walls (<NUM>) and a vertically movable support (<NUM>) for supporting a powder bed (<NUM>) and a three dimensional object (<NUM>) located in between of the opening walls (<NUM>), and
wherein the gas inlet (<NUM>) is configured to provide a light inert gas, having a density of less than <NUM>,<NUM>/m<NUM>, into the process chamber (<NUM>),
characterized in that
the additive manufacturing apparatus further comprises a process chamber controlling device (<NUM>) configured to control the additive manufacturing apparatus to execute the step of fusing at least
- a first location of the powder bed (<NUM>) using the first beam (<NUM>), thereby generating a first smoke plume (<NUM>), and
- a second location of the powder bed (<NUM>) using the second beam (<NUM>), thereby generating a second smoke plume (<NUM>),
wherein the first location is closer to the inert gas inlet (<NUM>) than the second location and the second location is closer to the inert gas outlet (<NUM>) than the first location,
while the at least one second beam (<NUM>) is controlled to fuse the second location of the powder bed (<NUM>) being covered by the first smoke plume (<NUM>) being generated by fusing the first location of the powder bed (<NUM>) with the at least one first beam (<NUM>), and in that
the inert gas inlet (<NUM>) and the inert gas outlet (<NUM>) are configured to remove the smoke plumes (<NUM>, <NUM>) from the process chamber (<NUM>) by a main inert gas flow, wherein the main inert gas flow is established by an inert gas comprising at least <NUM>% of He and/or Ne.