Additive Manufacturing using Powder Bed Fusion

Abstract

A powder bed fusion apparatus arranged to irradiate a powder bed using a charged particle beam is provided, the apparatus comprising a neutralizing particle source operable to provide neutralizing particles having an opposite charge to the charged particle beam, and a neutralizing particle focusing system arranged to weakly focus a neutralizing particle beam from the neutralizing particle source onto the powder bed near an irradiation location of the charged particle beam on the powder so that the neutralizing particles neutralize the charging of the powder bed by the charged particle beam. A method of additive manufacturing using the powder bed fusion apparatus is also provided, wherein a part is formed as a series of layers, each layer being formed by scanning a charged particle beam across a layer of the powder bed to melt the powder according to a predetermined pattern.

Description

Technical Field

[0001] The present invention relates to the use of powder bed fusion apparatus in additive manufacturing and in particular, but not exclusively, to charge control when irradiating metal powders during electron beam additive layer manufacturing. Background Art

[0002] One of the best-known techniques employed for additive manufacturing is powder bed fusion, in which a thin layer of powder (usually metal or plastic) is selectively melted by an energy source such as a laser or electron beam. The melted areas of the powder layer form a cross-sectional portion of the object to be built. After the layer has been selectively melted, a new layer of powder is deposited and then also selectively melted, thereby building the complete object layer by layer.

[0003] Such metal powders are typically metal alloys, which have the disadvantage of being easily oxidized and becoming insulating or semi-insulating. When in this insulating or semi-insulating state, irradiation with a charged particle beam (e.g., a high-energy electron beam) during a powder bed fusion process can cause the metal powder particles themselves to become charged and retain the charge or a small portion thereof. As the charge accumulation increases, the metal powder particles experience increased Coulomb repulsion, which can cause the metal powder to overcome the gravity and friction acting on the lower powder layer or molten material. The charged powder layer can then become movable and can even be expelled from the powder bed, immediately disrupting the layer-by-layer additive process and potentially damaging the device. For example, the powder may contaminate and melt into components of the device. High-voltage arcs may also form, and the moving powder may scatter the electron beam. Summary of the invention

[0004] According to one aspect of the present invention, a method for additive manufacturing using a powder bed melting device is provided, the method comprising forming a component in the form of a series of layers, each layer being formed by the following steps: (a) irradiating the powder bed with first charged particles to form a region of first charged particles on and adjacent to the powder bed; (b) after forming the region of first charged particles, irradiating the powder bed with a second charged particle beam having an opposite charge to the first charged particles, the method comprising scanning the second charged particle beam across the powder bed to melt the powder in a predetermined pattern; and (c) deactivating the second charged particle beam and forming a new powder layer of the powder bed; wherein the first charged particles reduce the charging of the powder bed by the second charged particle beam.

[0005] The method may also include controlling the kinetic energy and current of the first charged particles so that when the second particle beam is scanned across the powder bed, an equilibrium potential below a threshold required to induce powder movement is maintained on the powder bed due to interaction between the first charged particles and the second charged particle beam.

[0006] The method may further include weakly focusing the first charged particles onto the powder bed near an irradiation position of the second charged particle beam so that the first charged particles mitigate charging of the powder bed by the second charged particle beam.

[0007] According to another aspect of the present invention, a powder bed melting device for additive manufacturing is provided, the device comprising: a first source of first charged particles, operable to provide first charged particles; a second source of second charged particles, operable to provide a second charged particle beam having an opposite charge to the first charged particles; a powder bed for receiving the first charged particles and the second charged particle beam; and a controller, configured to control the operation of the first source and the second source to form a component in the form of a series of layers, each layer being formed by the following steps: (a) irradiating the powder bed with first charged particles to form an area of ​​first charged particles on and adjacent to the powder bed; (b) after forming the area of ​​first charged particles, irradiating the powder bed with a second charged particle beam having an opposite charge to the first charged particles, wherein the second charged particle beam is scanned across the powder bed to melt the powder in a predetermined pattern; and (c) deactivating the second charged particle beam and forming a new powder layer of the powder bed; wherein the first charged particles reduce the charging of the powder bed by the second charged particle beam.

[0008] The second charged particle beam may be a negatively charged electron beam, wherein the first charged particles are positively charged ions.

[0009] The first source of first charged particles may be an ion source configured to generate positive ions of a noble gas.

[0010] The powder bed fusion apparatus may include a plasma source as a first source of first charged particles to generate positive ions.

[0011] The plasma source may be a thermionic emission plasma flood source, a radio frequency plasma source, a hollow cathode plasma source, or a dual plasma tube.

[0012] The plasma source can be a hollow cathode plasma source or a dual plasma tube, and the device may also include a neutralizing particle focusing system, which is arranged to weakly focus the beam of first charged particles onto the powder bed near the irradiation position of the second charged particle beam on the powder, so that the first charged particles reduce the charging of the powder bed by the second charged particle beam.

[0013] The powder bed fusion apparatus may further comprise a negative extractor electrode or a grid extractor at the output of the plasma source for extracting positively charged ions from the plasma to form a first charged particle beam, wherein the current of the first charged particle beam is not affected by a bias potential applied to the plasma source.

[0014] The neutralized particle focusing system may include a series of electrostatic cylindrical or aperture lens elements arranged along the first charged particle beam between the plasma source and the powder bed and spaced apart to support an electric field therebetween, wherein each lens element is referenced to a respective different electric potential.

[0015] The electrostatic lens element closest to the powder bed may be configured to be substantially at ground potential such that the energy of the first charged particles is substantially defined by a bias potential applied to the plasma source.

[0016] The neutralized particle focusing system may include a series of electromagnetic lens elements arranged along the first charged particle beam between the plasma source and the powder bed, wherein each lens element conducts a respective different current.

[0017] The powder bed fusion apparatus may further comprise one or more ferrites surrounding one or more of the electromagnetic lens elements such that substantially no magnetic field is emanated outside of the neutralized particle focusing system.

[0018] The powder bed fusion apparatus may further comprise a scanning system arranged to control the first charged particle beam such that the first charged particle beam may be scanned across the powder bed in pseudo-synchronization with the second charged particle beam, wherein the scanning system may comprise an electrostatic or electromagnetic deflector.

[0019] Powder bed fusion may also include a wire mesh configured to be at ground potential and located at the output of the electrostatic lens element closest to the powder bed so that substantially no electric field penetrates into the powder bed from the neutralizing particle focusing system or electrostatic deflector.

[0020] The focusing system may be curved so that there is no direct line of sight between the plasma source and the powder bed through the focusing system.

[0021] The powder bed fusion apparatus may further comprise a heat shield positioned about the powder bed, wherein the heat shield may be configured to be at ground potential such that substantially no electric field exists between the heat shield and a portion of the focusing system closest to the powder bed.

[0022] The powder bed fusion apparatus may further include a buffer circuit configured for use in series with a bias potential applied to the neutralized particle source.

[0023] The first source of first charged particles may be contained in an auxiliary chamber coupled to a main build chamber of the powder bed fusion apparatus.

[0024] Based on the above, embodiments of the present invention provide a method and apparatus for additive manufacturing, in which the excess charge accumulation on the powder bed caused by a beam for melting the powder can be effectively mitigated.

[0025] During the build process, particles with an opposite charge to the particles used to irradiate the powder bed during additive manufacturing neutralize the charge on the metal particles on the powder bed caused by irradiation with the charged particle beam used for melting. A reduction in charging of the powder bed is thereby achieved. As a result, instances of charge-induced motion of metal powder particles can be substantially reduced, or even eliminated, avoiding the associated adverse effects.

[0026] The techniques disclosed in this application are able to optimize this mechanism via a "priming" process that controls the availability of the first charged particles for charge mitigation.

[0027] During the pre-energization phase, the powder bed is irradiated with first charged particles having an opposite charge to the second charged particles used to irradiate the powder bed during additive manufacturing. When the build process begins, the mitigation of charge accumulation on the powder bed occurs in a self-regulating manner. Since the first charged particles are available in the vicinity of the powder bed before the second charged particle beam irradiates the powder bed, once the second charged particle beam is activated, neutralization of the powder particle charge can occur, and in a manner that mitigates excess build-ending charge accumulation on the powder bed that would otherwise be caused by the second charged particle beam. Including the pre-energization phase before forming each layer using the second charged particle beam ensures that the charge generated on the powder bed is below the threshold required to induce powder mobility, avoiding an increase in the above-mentioned adverse effects.

[0028] The technology disclosed in the present application also utilizes a neutralizing particle focusing system that is arranged to provide a focused beam of neutralizing first charged particles that can be scanned over a powder bed. This approach enables efficient operation of the neutralizing particle source, which may also result in extending the life of the neutralizing first charged particle source and enabling a uniform distribution of the neutralizing particles over the build area. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Embodiments of the invention will be described by way of example only and with reference to the accompanying drawings, in which:

[0030] Figure 1 An additive layer manufacturing apparatus according to an embodiment of the present invention is shown;

[0031] Figure 2a shows a passive buffer circuit used in an additive layer manufacturing apparatus according to an embodiment of the present invention;

[0032] Figure 2b An active buffer circuit used in an additive layer manufacturing apparatus according to an embodiment of the present invention is shown;

[0033] Figure 3 An additive layer manufacturing apparatus including a neutralized particle focusing system according to an embodiment of the present invention is shown; and

[0034] Figure 4 A method of operating an additive manufacturing apparatus according to an embodiment of the present invention is shown.

[0035] It will be appreciated that for ease of illustration, some elements in the drawings are not shown to scale. DETAILED DESCRIPTION

[0036] Figure 1 A powder bed fusion apparatus 1 according to an embodiment of the present invention is shown. Figure 1 The device 1 shown is configured for additive manufacturing using an electron beam 17 to melt metal powder layer by layer to form a component 3 .

[0037] The powder bed fusion device 1 comprises an electron optical assembly 21 for forming, adjusting and manipulating the electron beam 17. The electron optical assembly 21 comprises an electron source 7 arranged to emit electrons. The electron optical assembly 21 also comprises an electron extraction and focusing element 8 for forming an electron beam 17 focused at the powder bed from the emitted electrons, which is along Figure 1 The electron optical assembly 21 further comprises an electron deflection system 9 for scanning the electron beam 17 across the bed 2 of metal powder to melt the powder into the desired additively manufactured component 3. The electron deflection system 9 comprises an electromagnetic deflector arranged around the electron beam 17.

[0038] The operation of the electron-optical assembly 21 is controlled via signals originating from a build controller (not shown), such as one or more appropriately programmed computers or processors, in accordance with a scan file for the desired part 3, as is known in the art.

[0039] The apparatus 1 further comprises at least one hopper 4 operable to dispense powder via a dispensing mechanism (not shown), and a stage 20 supporting a build pot 19 positioned to receive the dispensed powder in a volume defining the powder bed 2. The stage 20 is movable in the z-direction via a piston, and the hopper 4 and the piston are controlled in conjunction with signals originating from a build controller (not shown).

[0040] The apparatus 1 further comprises an ion source 11 for generating and emitting ions for use in a charge mitigation mechanism used in an additive manufacturing method according to an embodiment of the invention, as will be described in more detail below.

[0041] exist Figure 1 In the configuration shown, the ion source 11 is a plasma source. The direction of travel of the ion stream 16 emitted from the plasma source 11 forms an angle with respect to the direction of travel of the electron beam 17. Figure 1In the illustrated embodiment, the electron beam 17 is parallel to the z-axis and the direction of the ion flow 16 is angled with the z-axis, but in alternative embodiments, the direction of the ion flow 16 can be aligned with the z-axis and the electron beam 17 is angled with the z-axis. Operation of the plasma source 11 is controlled via signals originating from a build controller (not shown).

[0042] In an embodiment of the present invention, additive manufacturing is performed under vacuum conditions. Therefore, the apparatus 1 further comprises a build vacuum chamber 5 through which the focused electron beam 17 and ion stream 16 pass before being incident on the powder bed 2. Coupled to the build vacuum chamber 5 is a first auxiliary vacuum chamber 6 containing an electron optical assembly 21, and a second auxiliary vacuum chamber 10 containing a plasma source 11. As is known in the art of powder fusion systems, the 1x 10 -3 mbar to 1x 10 -7 Vacuum conditions are maintained by vacuum pressures in the millibar range.

[0043] The hopper distributes the powder so that a measured amount of powder is deposited on the surface of the powder bed 2. A mechanism such as a scraper or blade (not shown) is used to evenly disperse the powder on the movable stage 20. The electron optical assembly 21 forms and manipulates the electron beam 17 so that the electron beam 17 is scanned across the powder bed 2 to heat and melt the powder and form a solid layer of the component 3. After each layer of the component 3 is formed, the stage 20 is lowered in the z direction to accommodate the increased height of the component 3 and allow the next layer to spread.

[0044] Charge reduction

[0045] As described above, the interaction of the negatively charged electron beam 17 with the powder particles may cause the unmelted powder particles to become negatively charged due to the insulating or semi-insulating oxide layer on the metal powder particles.

[0046] In the absence of the charge mitigation methods used in embodiments of the present invention, this can lead to the accumulation of negative charge on the powder, adversely affecting the build process, including the end-of-build event described above, where powder particles are displaced from the powder bed 2 and travel throughout the build chamber 5 due to Coulomb repulsion imparted by other charged particles of like charge polarity in the powder bed 2.

[0047] In the "pre-excitation" stage before the electron beam 17 is activated at the beginning of the build process, the powder bed 2 is irradiated by ions from the plasma source 11 so that the powder bed becomes charged to a positive potential. For a static powder bed 2 (i.e., a powder area without an electron beam incident), the ion flow 16 incident on the powder will continue to deposit charge on the powder until the induced potential (in V) on the powder bed 2 is equal to the magnitude of the kinetic energy of the ions (in eV). At this point, the potential of the charged powder bed 2 is sufficient to form a barrier to further entering ions, which do not have the necessary kinetic energy to overcome the potential barrier to reach the powder bed 2. The potential barrier reduces the kinetic energy of further entering positive ions until their kinetic energy when they reach the charged powder bed 2 is close to zero, and a "cloud" of free positive ions is formed above the charged powder bed 2. These free positive ions with kinetic energy close to zero can be used to mitigate excessive accumulation of negative charge during the build process.

[0048] The pre-excitation phase is configured so that the powder bed 2 is not charged to such an extent as to induce movement of the powder. This is achieved by appropriately limiting the energy of the first charged particle beam.

[0049] After the pre-excitation phase, the electron beam 17 is activated and directed to the powder bed 2. The electron source cathode is maintained at a high voltage, such as approximately -60 to -80 kilovolts, relative to the electron source anode referenced to ground to accelerate the electrons in the electron beam 17 away from the electron source 7 and toward the powder bed 2. This corresponds to an electron beam energy of approximately 60-80 keV with pseudo-monochromatic energy characteristics. The plasma source 11 is biased at a relatively low positive voltage, such as +80 to +300 V, such as +200 V, which is selected so as to optimize the ion energy to maximize the deposition of positive charges on the powder particles without initiating charging-induced powder motion that would interfere with the melting process of the powder by the electron beam 17.

[0050] The positive charge accumulated on the powder bed 2 during the static state is removed by the negative charge accumulated on the powder bed 2 due to the electron beam 17. The electron beam 17 is able to induce a negative potential much greater than the positive potential induced by the ion flow 16, so that the irradiated area of ​​the powder bed 2 becomes negatively charged. This negative charge attracts positive ions from the cloud of free positive ions above the powder bed 2, in some cases via the conduit of the electron beam 17 itself, which acts to counteract the negative charge generated by the electrons.

[0051] Thus, as the electron beam 17 is scanned over the powder bed 2, an equilibrium potential is established over the region of the powder being melted due to the interaction of the ion flow 16 and the electrons from the electron beam 17. The equilibrium potential is below a threshold that would initiate powder mobility. As the electron beam 17 and ion flow 16 continue to be used, the cloud of free ions above the powder bed 2 is continually replenished. Thus, throughout the build process, ions are readily available for the required charge neutralization (i.e., depending on the amount of negative charge accumulated on the powder bed 2 due to, for example, the use of a particular material or electron beam scanning strategy). In this way, a self-regulating charge mitigation process is achieved.

[0052] As described above, this self-regulating process prevents the powder from accumulating excessive negative charge, thereby avoiding several possible build-ending events caused by excess charge on the powder bed 2.

[0053] Some embodiments of the present invention include a user interface connected to a build controller to enable a user to customize the charge neutralization parameters according to a specific build environment (e.g., materials, feature resolution), thereby affecting the number of ions available for charge neutralization, the time scale of the charge neutralization process, and the maximum number of ions that can be deposited on the powder bed 2. The electron beam parameters used during the build (e.g., electron beam current, beam spot size, scan speed, scan path, and hatch geometry) all determine how much negative charge is deposited on a given area of ​​the powder bed during a given time. Therefore, the charge neutralization process is highly dependent on the exact build parameters used.

[0054] Build parameters can vary significantly from layer to layer, so in some embodiments, charge neutralization parameters can be adjusted between layers based on such variations.

[0055] exist Figure 1 In the embodiment illustrated in FIG. 1 , a plasma source 11 is shown, which is embodied as a plasma flood source. The plasma source 11 generates low energy positive ions by applying an atomic ionization process to a gas, for example one of the inert gases, such as argon, helium or xenon, which is selected so as not to cause interstitial contamination of the metal lattice of the final metal part 3 formed on the build surface. Helium is the lowest mass and the most mobile of the inert gases, and its use can help the efficiency of the neutralization process. The atomic ionization process can be based on thermionic emission from a current-carrying tungsten filament to ionize the gas in a discharge chamber that is maintained at a positive bias potential relative to ground. The plasma generated in this way is caused to leave the discharge chamber via an aperture in the plasma source 11.

[0056] like Figure 1 As shown in FIG. 1 , the plasma flood source 11 is such that it is contained in a separate vacuum chamber 10 attached to the build vacuum chamber 5 .

[0057] Cascade ionization and suppression

[0058] It may also be necessary to mitigate the effects on the build process of electron impact ionization of particles present in the build region.

[0059] During the build process, electrons from the incident electron beam 17 may be elastically backscattered from the surface of the powder bed 2. Electrons from the incident electron beam 17 may also generate secondary electrons by ionizing material at the build surface, and these secondary electrons may be ejected from the surface of the powder bed 2. Both the backscattered electrons and the secondary electrons may lead to further ionization of ions and atoms (atoms and / or ions emitted from the plasma flood source, or atoms evaporated from the molten pool 18) present in the build region, generating more electrons, which in turn may lead to further ionization events. Such a sequence of ionization events is referred to herein as cascade ionization.

[0060] An electric field exists near the build surface due to the proximity of the positively biased discharge chamber of the plasma flood source 11 and the powder bed 2, and due to the positively charged ion flow 16 around the negatively charged conduit of the electron beam 17. The magnitude of this combined electric field is sufficient to impart additional energy to secondary electrons in the build region, resulting in increased electron-atom interactions, thereby playing an important role in the occurrence of cascade ionization.

[0061] As a result of the above process, large electron and ion currents are generated within the build volume. The large electron currents generated in the build area can interfere with the operation of the plasma flood source 11 and the power supply attached to the plasma flood source 11. If the interference with the operation of the plasma flood source causes the electric field generated by the ion flow 16 around the electron beam 17 to change due to changes in the ion current density around the electron beam 17 and / or the molten pool 18, this may cause the position of the electron beam to move.

[0062] The above-mentioned field effects have the potential to cause a number of problems with the porosity, surface finish and geometric accuracy of additively manufactured components. The charge mitigation method and apparatus described below provide a solution to such problems.

[0063] In the above described embodiments, substantial removal of charge induced effects associated with charged powder particles leaving the powder bed 2 is achieved by introducing a pre-excitation phase into the build process. Where it is desired to mitigate cascade ionization effects, a buffer circuit may be arranged in series with the bias potential applied to the plasma flooding source 11. Where cascade ionization occurs at the powder bed 2, the buffer circuit enables adjustment of the bias potential.

[0064] As described above, the plasma flood source 11 is biased with a positive voltage. In some embodiments, the plasma source bias power supply unit (PSU) sets the voltage external to the plasma flood source 11 to approximately +80 to +300 V. During normal operation, this bias voltage acts to extract electrons from the plasma generated by the plasma flood source 11, resulting in a dominant ion flow 16 leaving the plasma flood source 11 and impinging on the powder bed 2.

[0065] However, when cascade ionization occurs at the powder bed 2, a large electron current is generated that bombards the outer surface of the plasma flood source 11 and travels to ground via the PSU. This exceeds the maximum current of the PSU (typically ~2.5A) and causes the PSU to reduce its set voltage via self-regulation, and thus reduces the bias potential of the plasma flood source 11. The reduction in voltage has the effect of reducing the electric field at the molten pool 18. This is because fewer electrons are extracted from the plasma, resulting in a reduction in the overall ion current leaving the plasma flood source 11; thus, the size of the interaction between the electron beam 17 and the ion flow 16 is reduced, thereby reducing the electric field. A reduction in voltage external to the plasma flood source 11 may also be a contributing factor.

[0066] The reduction in the electric field suppresses the cascade ionization process in the build area, thereby reducing the electron current incident on the outer surface of the plasma flood source 11 traveling to the ground via the PSU. When the electron current through the PSU drops below the maximum current of the PSU, the voltage of the PSU returns to a set level.

[0067] Once the voltage of the PSU is restored, the electric field is re-established as before (due to the proximity of the plasma flooding source 11 and the presence of positive ions near the powder bed 2), and the cascade ionization process is thereby restarted.

[0068] In this way, the above process repeats itself continuously, and the effects of cascade ionization are repeatedly initiated and suppressed. As a result, cascade ionization introduces instability to the ion current reaching the powder bed 2, resulting in inconsistent availability of ions for charge neutralization near the molten pool 18. This instability in the ion current density induced around the molten pool 18 at the powder bed 2 and along the entire length of the electron beam changes the physical position of the electron beam 17, compromising the build process.

[0069] The primary goal of the buffer circuit is to stabilize the ion current density. Figure 2aIn the illustrated embodiment, the buffer circuit 101 includes a high power resistor 102 connected in series on the positive line of the PSU between the PSU 103 and the plasma flood source discharge chamber 104. This arrangement is referred to herein as a "passive buffer". For example, the resistor 102 can be a high power (200W) 22Ω resistor. In other embodiments, the passive buffer 101 can be a capacitive or inductive design.

[0070] When cascade ionization occurs, a large electron current reaches ground via resistor 102. This induces a voltage drop across resistor 102 that is proportional to the product of the current and the resistance. This voltage drop reduces the effective bias voltage of the plasma flood source 104, which, as described above, reduces the electric field caused by the plasma flood source 104 and the electric field caused by ion-electron interactions. The overall reduction in the electric field suppresses the intensity of the cascade ionization effect, or may eliminate the cascade ionization effect entirely.

[0071] The buffer 101 can operate in the manner described above faster than the PSU 103 can adjust its own supply voltage to compensate for the high current. The buffer 101 can therefore assist the self-regulation of the PSU 104 and result in reduced deflections in the electron beam position both in terms of frequency of occurrence and magnitude.

[0072] In other embodiments, the buffer circuit utilizes an active feedback system to control the ion current density at the powder bed 2. Such an arrangement is referred to herein as an "active buffer". Figure 2b In the illustrated embodiment, active buffer 201 includes a variable resistor 202, such as a MOSFET or variable resistor, which supports an effective bias voltage to the plasma flooding source body 203. Feedback controller 204 actively controls the effective bias voltage based on the bias current measured by one or more current sensors 205a, 205b.

[0073] When cascade ionization occurs and the resulting bias current spike is detected, feedback controller 204 adjusts variable resistor 202 to control the effective bias voltage provided so as to stabilize the bias current. Feedback controller 204 is capable of operating to suppress cascade ionization, but faster than a passive buffer, where the required time dynamics justify providing an active feedback system.

[0074] Focus Mechanism

[0075] As described above, many of the different charge induced effects can be substantially removed by pre-exciting the powder bed 2 with a positive charge prior to building to account for the negative charge induced by the electron beam 17, and optionally, by employing a buffer to adjust the bias potential of the plasma flood source 11 as the cascade ionization process occurs in the build area.

[0076] This charge neutralization technique involves neutralizing ions which are distributed over an area of ​​the powder bed 2 which is significantly larger than the area of ​​the molten pool 18. The intensity of the broad ion flux 16 incident on the powder bed 2 decreases rapidly with increasing distance from its central axis.

[0077] As the required build size increases, it will be appreciated that the required coverage of the ion current 16 increases, and in the case of using a plasma flood source 11, this increase in coverage area introduces non-uniformities in the neutralization distribution of the ions at the powder bed 2. Furthermore, a larger build area will result in a longer build time, which increases the time that the plasma flood source 11 will be operated, thereby increasing the use time of its consumable filaments.

[0078] Embodiments of the present invention aim to provide a solution to these problems by providing a focused ion beam that can be scanned across a powder bed. This approach relaxes the requirement to flood the build area with neutralizing ions and enables the plasma source to operate more efficiently.

[0079] By focusing the ions from the plasma source, the current density of the ions at the molten pool can be increased while significantly reducing the total ion output from the plasma source, thereby extending the life of the plasma source, while the scanning of the focused ion beam also enables the neutralized ions to be evenly distributed over the build area. For example, if a wide area flood system uses a current of 1A, a comparable focused ion column system will use a current of 10-100mA.

[0080] exist Figure 3 In the illustrated configuration, the powder bed melting apparatus 10 also includes an ion focusing system 220. Ions are provided to the focusing system 220 using a point source or near-point source of positive ions. The ion beam can have a wider beam width and / or divergence than the electron beam 170, and the term "beam" is not used herein to require the use of any particular beam width. The plasma source 11 can be, for example, a dual plasma tube or a hollow cathode plasma source. In the following description, embodiments are described in the context of a dual plasma tube 110.

[0081] The focusing system 220 is arranged to condition and manipulate the ion beam 160 produced by the dual plasma tube 110. By focusing the ion beam 160, it is possible to ensure that the ions are concentrated in a specific area of ​​the powder bed 20. By manipulating the ion beam 160, it is possible for its position to track the position of the electron beam 170, thereby ensuring that ions are only provided to areas of the powder bed 20 that require charge neutralization. This combination achieves a high ion current density at the molten pool 180, thereby allowing the absolute ion output of the dual plasma tube 110 to be reduced without compromising the charge neutralization process. The ability to scan the ion beam 160 across the powder bed 20 further achieves uniform charge neutralization over a large build area.

[0082] refer to Figure 3 , the ion focusing system 220 is shown within a column arranged at an angle relative to the direction of travel of the electron beam 170. Figure 3 In the illustrated embodiment, the electron beam 170 is parallel to the z-axis and the ion focusing system 220 is angled with the z-axis, but in alternative embodiments, the column can be aligned with the z-axis and the electron beam 170 is angled with the z-axis. The column is also referred to herein as an ion column. In more detail, the ion focusing system 220 includes a plurality of ion focusing lens elements 130 for forming a weakly focused ion beam 160 from unfocused ions exiting the dual plasma tube 110, the ion beam traveling along the longitudinal axis of the ion focusing system 220. The operation of the ion focusing system 220 is controlled via signals originating from a build controller (not shown).

[0083] The apparatus 10 also includes a high voltage negative extractor element 120 located at the output aperture of the dual plasma tube 110 to block electrons and extract ions in the plasma produced by the dual plasma tube 110. The extractor element 120 is referenced to a large negative potential, such as -4 to -10 kV, and can be an electrode, a grid, or a series of grids.

[0084] The apparatus 10 is arranged so that the dual plasma tubes 110, extractor electrodes 120, and ion focusing system 220 are contained in a separate vacuum chamber 100 shaped to match the ion column, which is attached to the build vacuum chamber 50. The high electric field between the dual plasma tubes 110 and the extractor electrodes 120 acts as a focusing element, attracting and effectively directing positive ions exiting the dual plasma tubes 110 toward the input elements of the ion focusing system 220. In this way, only the ion component of the plasma enters the ion focusing system 220 to be focused in the manner described below.

[0085] The length of the ion column allows the dual plasma tubes 110 to be located farther from the powder bed 20 while still maintaining sufficient ion current density to enable effective charge neutralization through the use of the ion focusing system 220. The increased distance between the dual plasma tubes 110 and the powder bed 20 reduces the incorporation of tungsten evaporated from the thermionic filament into the build material.

[0086] As described above, the use of the ion focusing system 220 increases the ion current density at the molten pool 180, thereby allowing a reduction in the rate at which gas flows into the dual plasma tube 110. This in turn reduces the gas pressure at the build surface, minimizing the risk of cascade ionization processes of atoms and ions emitted from the dual plasma tube 110, thereby significantly reducing charge-induced effects that can adversely affect the build.

[0087] In some embodiments, the ion focusing system 220 is curved. This arrangement further reduces contamination of the build area by the dual plasma tube 110 or contamination of the dual plasma tube 110 by metal atoms evaporated from the powder bed 20 by eliminating the direct line of sight between the dual plasma tube 110 and the powder bed 20 through the ion focusing system 220.

[0088] As previously described, the ion focusing system 220 includes a series of lens elements arranged along the ion beam 160 between the dual plasma tube 110 and the powder bed 20. The series of lens elements includes the ion focusing lens element 130 to form a weakly focused ion beam 160. The ion beam 160 is weakly focused on an area of ​​50-60 mm and is scanned pseudo-synchronously with the irradiation position of the electron beam 170 so that the powder area around the irradiation position of the electron beam 170 accumulates positive charge before being irradiated to its position via the scanning electron beam 170 during the building process.

[0089] In some embodiments of the present invention, the ion focusing system 220 includes a series of electrostatic lens elements arranged along the longitudinal axis of the ion focusing system 220, each lens element biased to a different potential and longitudinally spaced apart to support an electric field therebetween. The ion focusing system 220 can be composed of a series of multi-element cylindrical or aperture lenses. In some embodiments, the lens elements 130 are cylindrical lenses of diameter D, separated by gaps G, where the ratio G / D is about 0.1. The lens elements can be formed from stainless steel, aluminum, or the like. The focusing characteristics of such an electrostatic lens system are a function of the lens voltage, the cylinder or aperture diameter, the gaps between the lens elements, and the energy of the ions passing through the lens system.

[0090] The configuration of an embodiment of the present invention provides a higher ion current density and a lower ion source current with a focused ion beam than would be the case without the ion focusing system 220. As described above, this allows the dual plasma tube 110 to be operated using a lower thermionic filament current and a lower gas throughput because a lower total ion source current can be used to maintain a high current density at the build surface. This extends the life of the dual plasma tube 110 and reduces downtime by extending the time period between user services. Thus, increased build times required for larger builds covering wider areas of the powder bed 20 and including more layers can be achieved.

[0091] The lens element 140 closest to the powder bed 20 is configured to be substantially at ground potential so that the ion energy is defined by the potential difference between the bias potential applied to the dual plasma tube 110 and the lens element 140 closest to the powder bed 20, in addition to the starting energy of the emitted positive ions (on the order of 10 eV). The grounding of the final lens element 140 avoids the introduction of electric fields due to the proximity of electrodes at a certain potential to a region of the powder bed 20, while the energy of the ion beam 160 can be easily controlled via the dual plasma tube bias potential. "Substantially" grounding is to be interpreted as including electrical tolerances or deviations from ground that can be accommodated within a particular build process.

[0092] In an alternative embodiment, the ion focusing system 220 includes a series of electromagnetic coil lens elements, each carrying a respective different current and being longitudinally spaced apart.

[0093] The powder bed fusion apparatus 10 also includes a scanning system, referred to herein as an ion deflection system 150, which is arranged to control a weakly focused ion beam 160 so that the ion beam 160 can be scanned across the powder bed 20 in pseudo-synchronization with the focused electron beam 170. The scanning process is controlled based on a scan file of the build process in conjunction with instructions provided from a build controller. The scan file specifies several parameters, such as beam energy, beam current, and beam spot size, as well as a series of beam positions and dwell times that define the path of the beam through the powder bed 20 for each layer of the component 30, as is known in the art.

[0094] This pseudo-synchronization enables the ion beam 160 to completely surround the region of the irradiation position of the electron beam 170. Figure 3 170 is shown as corresponding to the molten pool 180 so that at least some of the positive ions, particularly those closest to the irradiation location of the electron beam 170, are attracted to the negatively charged powder particles, where they help to offset some or all of the negative charge. As described above, this helps to mitigate the adverse effects of negative charge caused by the electron beam 170 and keeps the powder particles below the potential threshold at which Coulomb interactions will cause them to leave the powder bed. Some of the positive ions farther away from the irradiation location of the electron beam 170 cause areas of the powder bed 20 that have not yet been irradiated by the electron beam 170 to acquire positive charge, but because the Coulomb force drops rapidly with increasing distance according to the inverse square law, they themselves are not attracted to the negatively charged powder particles. The mechanism by which the powder bed 20 that has not yet been irradiated by the electron beam 170 acquires positive charge is similar to the pre-excitation process described above, and results in replenishment of the ion cloud for neutralization during the build process.

[0095] The ion deflection system 150 includes an electrostatic deflector, which in some embodiments is a series of plates arranged around the longitudinal axis of the ion column so that the normal to the surface of each plate is perpendicular to the longitudinal axis of the ion column. The plates are paired, with each member of the pair located on opposite sides of the longitudinal axis of the ion column, and each is biased at a potential so that the electric field is generated perpendicular to the direction of the ion beam 160. In some embodiments, the xy deflection system includes two pairs of plates, each pair of plates arranged at 90 degrees relative to each other around the longitudinal axis of the ion column.

[0096] In some embodiments, the ion deflection system 150 is located in the grounded final lens element 140 so that penetration of the deflecting transverse electric field from the lens element 140 onto the powder bed 20 is kept to a minimum. Preferred materials for the deflector are non-magnetic materials such as stainless steel, aluminum or titanium.

[0097] The electrostatic deflector 150 is electronically coupled to the electromagnetic deflector 90 for the electron beam 170. The coupling is controlled by the build controller in a manner that ensures that the weakly focused ion beam 160 and the incident electron beam 170 loosely follow each other in a pseudo-synchronous manner when scanned. Specifically, pseudo-synchronization enables the electron beam 170 to move quickly within the envelope of the ion beam 160. The use of this combination of electrostatic and electromagnetic deflectors will make the best use of the advantages of each of them in terms of deflection angle and response rate. In an alternative embodiment, the ion deflection system 150 uses an electromagnetic deflector. The distance between the deflector and the powder bed 20 is taken into account in the properties of the deflection mechanism used and the required deflection angle. In some embodiments, the distance between the deflector and the powder bed 20 is in the region of 200-500mm.

[0098] In some embodiments, the apparatus 10 further includes a wire mesh configured to be at substantially ground potential and located at the output of the final electrostatic lens element 140 such that substantially no electric fields generated from the electrostatic deflector 150 or the ion focusing system 220 penetrate into the build region.

[0099] In the case where the ion focusing system 220 includes an electromagnetic lens and / or the ion deflection system 150 includes an electromagnetic deflector, the powder bed fusion apparatus 10 may further include one or more high magnetic permeability ferrites so that there is substantially no magnetic field outside the ion focusing system 220 or the ion deflection system 150 .

[0100] In some embodiments, the device 10 also includes an electrically isolated metal plate positioned around the powder bed 20 that acts as a heat shield, reflecting infrared radiation from the powder bed 20 away from other components of the device 10 and back to the powder bed 20. In this case, the heat shield is configured to be substantially at ground potential so that there is substantially no electric field between the heat shield and the lens element 140 closest to the powder bed 20.

[0101] In some embodiments, the plasma source is a hollow cathode plasma source. By avoiding resistively heated thermionic filaments, the life of the plasma source is significantly extended to thousands of hours. In addition, the hollow cathode plasma source has an advantage in its ability to emit high ion currents, making more ions available for neutralization. This enables a higher ion current density at the molten pool 18, and, in some cases, the ion beam generated by the hollow cathode plasma source can provide sufficient ion current density at the molten pool 18 without using an ion focusing system 220.

[0102] Ion beam control

[0103] Due to the use of the extractor electrode 120, as described above, the ion current from the plasma is not affected by the relatively small bias potential used at the dual plasma tube 110, but is instead determined by parameters that affect the rate of positive ion generation, such as the current through the tungsten filament of the dual plasma tube 110, the gas pressure in the dual plasma tube 110, and the size of the output aperture of the gas release chamber.

[0104] Therefore, ion energy and ion current are affected by different parameters and can be controlled independently of each other. Adjustment of the ion current can be used to control the number of ions available for charge neutralization, as well as the time scale of the charge neutralization process. Adjustment of the ion energy can be used to control the maximum number of ions that can be deposited on the powder bed.

[0105] As previously described, the optimal parameters for charge mitigation can take into account the specific electron beam parameters used during a particular portion of a build, based on the scanning strategy applied to the electron beam 17. For example, during a build using a high electron beam current at a slow electron beam scan speed, the electron beam 170 will deposit a greater amount of charge per unit time and per unit area on the powder bed 20, thereby requiring more ions to achieve effective charge neutralization.

[0106] In addition, for different materials used in the build and different electron beam scanning strategies adopted, independent variation of ion current and energy is required to fully optimize the charge neutralization parameters. The optimal parameters for charge neutralization of a particular metal powder may be affected by factors such as the density of the material, the size and bulk density of the powder particles, the capacitance of the metal oxide layer on the powder particles, the resistance to ground, and whether the powder is static or moving during irradiation with the electron beam 170. If the powder particles are static, the static friction coefficient between the powder layer and the underlying material (i.e., the supporting metal plate, a previously melted powder layer, sintered powder, or loose unsintered powder) will play a role. If the powder is moving, such as due to excessive charging, the dynamic friction coefficient will play a role in determining the ease with which the powder can be further moved by the incident electron beam 170.

[0107] The combination of build parameters and charge mitigation parameters used during the build will affect the properties of the manufactured part 30. For example, the material properties of the finished part 30 depend on the interaction between the build material layer and the electron beam 170 scanning across the layer (i.e., the formation of a melt pool, the movement of particles, and large thermal gradients).

[0108] Additive Manufacturing Methods

[0109] According to an embodiment of the present invention, an additive manufacturing method using the powder bed fusion device 10 is also provided, as shown in FIG. Figure 4 As shown in the figure, and in conjunction with reference Figure 3 The powder melting device 10 is described.

[0110] The build controller obtains an instruction file for the part 30 to be manufactured at step S10. The instruction file contains computer executable instructions that the controller is to follow to form the part 30, such as electron beam build parameters (e.g., beam energy, current, scan speed, beam spot size) and an address sequence for positioning the electron beam 170 on the powder bed 20 to form each layer of the part 30.

[0111] The pre-excitation phase is performed at step S20. The build controller controls the ion source 110 based on the received specifications of the charge mitigation parameters (e.g., specific ion beam current and energy to optimize a specific build process). The powder bed 20 is irradiated with the ion beam 160 to charge the powder bed 20 to a low positive potential of the same magnitude as the energy of the ion beam 160 (e.g., positive ions leaving the ion source 110 with a user-specified kinetic energy of 200 eV will induce a +200 V potential on the powder bed 20). As described above, pre-excitation of the powder bed 20 before the build process begins, and before the build of each new layer begins, results in the accumulation of a free ion cloud above the powder bed 20, which can then be used for charge neutralization throughout the build process.

[0112] The pre-energization stage may involve positioning the ion beam 160 on the powder bed 20 at a location corresponding to the first address of the electron beam 170 so that the area of ​​the powder bed 20 where the build begins is pre-energized in advance. In alternative embodiments, the ion focusing system 220 may be utilized to defocus the ion beam 160 so that a larger area of ​​the powder bed 20 is pre-energized before the build begins, or the ion deflection system 150 may be utilized to scan the ion beam 160 across the entire area of ​​the powder bed 20, depending on the requirements of the build.

[0113] In step S30, the build controller starts the electron source 70 according to the specifications of the build parameters and positions the electron beam 170 at the first address retrieved from the instruction file. Embodiments of the present invention are compatible with any particular scanning strategy. When the electron beam 170 is incident on the powder bed 20, it begins to melt the powder. Before melting the powder, in some embodiments, step S30 may also include a preheating stage, in which the area to be melted is heated before melting to assist the melting process. The positive charge accumulated on the powder bed 20 during the pre-excitation stage is immediately removed by the negative charge accumulated on the powder bed 20 due to the electron beam 170. The positive ions from the free ion cloud above the powder bed 20 are thereby attracted to the negative potential accumulated on the powder bed 20, where they offset the negative charge caused by the electron beam 170, establishing a balanced potential on the area of ​​the powder being melted.

[0114] At step S40, the build controller retrieves the next address from the instruction file and moves the electron beam 170 to the specified address on the powder bed 20. As the electron beam 170 moves over the powder bed 20, the electron beam 170 melts the powder to form the desired additively manufactured part 30. The weakly focused ion beam 160 loosely follows the position of the electron beam 170 so that the electron beam 170 and the ion beam 160 are scanned across the powder bed 20 in a pseudo-synchronous manner. In some embodiments, the ion beam 160 does not move continuously and changes position only when the position of the electron beam 170 moves outside the area of ​​the powder bed 20 covered by the ion beam 160.

[0115] At step S50, it is determined by the build controller whether there are more addresses in the instruction file at which the electron beam 170 will be positioned within the layer of the component 30 being produced. If there are more locations, the method returns (S50-Y) to step S40 and moves the electron beam 170 to the next location in the address sequence in the instruction file. If there are no more locations within the layer (S50-N), the method continues to step S60.

[0116] In addition to the initial pre-energization stage described above, continuous pre-energization of the powder occurs throughout the build process. Since the ion beam 160 is weakly focused on a larger powder area than the electron beam 170 and is scanned pseudo-synchronously with the electron beam 170, the powder area around the irradiation position of the electron beam 170 is pre-energized before it is irradiated by the electron beam 170 as the electron beam 170 moves through the address sequence on the powder bed 20.

[0117] In step S60, the electron beam 170 is turned off. In step S70, the build controller determines whether there are more layers to be processed in the instruction file. If there are no more layers to be processed (S70-N), the method ends. However, if all layers have not been processed, the method returns (S70-Y) to step S20 via step S80. In step S80, the build parameters for the next layer of the electron beam 170 are retrieved from the instruction file, and the stage 200 is lowered and new powder is spread to form a powder bed 20 for the next layer of the component 30. After returning to step S20, pre-excitation of the newly spread powder is performed.

[0118] In this way, for each layer of the part 30, the electron beam 170 can be scanned across all addresses specified in the instruction file, and the ion beam 160 can be scanned pseudo-synchronously with the electron beam 170 so that the part 30 is formed by additive layer manufacturing, and the ion beam 160 is used to pre-excite the powder bed 20 before each layer is processed.

[0119] The ion source is a plasma flood source 11 (i.e., Figure 1 In an alternative embodiment of the configuration shown), the build process is substantially as described above, but includes using the ion flow 16 to neutralize the powder bed 2, rather than pseudo-scanning the weakly focused ion beam 160 across the powder bed 20.

[0120] In the above embodiments, the ion source is described as providing positive ions having an opposite charge to the electrons of the irradiating electron beam. However, in alternative embodiments, the ion source may provide electrons or negatively charged ions to mitigate the charge from the positively charged high energy beam. The same operating principles as described above apply.

[0121] It will be appreciated that the powder bed fusion apparatus may be configured in a variety of different ways depending on the user's requirements for a particular build process, and that compatible features of different embodiments may be readily combined, such as the properties of the neutralizing particle source, the buffer system, and the focusing system for the neutralizing particles. As discussed above, independent control of ion energy and ion current is one such aspect that may be varied.

Claims

1. A method of additive manufacturing using a powder bed fusion apparatus, the method comprising forming a part in a series of layers, each layer being formed by: (a) irradiating a powder bed with first charged particles to form a region of first charged particles on and adjacent to the powder bed, wherein: The first charged particles are positively charged ions, and the powder bed is irradiated such that the powder bed becomes charged to a positive potential; (b) after forming the region of the first charged particles, irradiating the powder bed with a second charged particle beam having an opposite charge to the first charged particles, wherein the second charged particle beam is a negatively charged electron beam, and scanning the second charged particle beam across the powder bed to melt the powder in a predetermined pattern; and (c) deactivating the second charged particle beam and forming a new powder layer of the powder bed; Wherein, when the powder bed is irradiated with the second charged particle beam, the source of the first charged particles is activated so that the region of the first charged particles adjacent to the powder bed is maintained and serves to reduce the charging of the powder bed by the second charged particle beam.

2. The method according to claim 1, wherein: The source of the first charged particles is an ion source configured to generate positive ions of a passive gas.

3. The method according to claim 2, wherein: The inert gas is one of the noble gases.

4. The method according to any one of claims 1 to 3, comprising: A plasma source is used as a source of the first charged particles to generate positive ions.

5. The method according to claim 4, wherein: The plasma source is a thermal ion emission plasma flood source, a radio frequency plasma source or a hollow cathode plasma source.

6. The method according to claim 5, further comprising: Positively charged ions are extracted from the plasma to form a positively charged ion beam using a negative extractor electrode or a grid extractor positioned at the output aperture of the plasma source.

7. The method according to any one of claims 1 to 3, comprising: The kinetic energy and current of the first charged particles are controlled so that when the second charged particle beam is scanned over the powder bed, an equilibrium potential below a threshold required to induce powder movement is maintained on the powder bed due to interaction between the first charged particle region adjacent to the powder bed and the second charged particle beam.

8. A powder bed fusion device for additive manufacturing, the device comprising: a first source of first charged particles operable to provide first charged particles, wherein the first charged particles are positively charged ions; a second source of second charged particles operable to provide a second charged particle beam having an opposite charge to the first charged particles, wherein the second charged particle beam is a negatively charged electron beam; a powder bed for receiving the first charged particle beam and the second charged particle beam; and A controller configured to control operation of the first source and the second source to form a component in a series of layers, each layer being formed by the following steps: (a) irradiating the powder bed with the first charged particles to form a region of first charged particles on and adjacent to the powder bed, wherein the powder bed is irradiated such that the powder bed becomes charged to a positive potential; (b) after forming the region of the first charged particles, irradiating the powder bed with a second charged particle beam having an opposite charge to the first charged particles, wherein the second charged particle beam is scanned across the powder bed to melt the powder in a predetermined pattern; and (c) deactivating the second charged particle beam and forming a new powder layer of the powder bed; Wherein, the first source of the first charged particles is configured to be activated when the powder bed is irradiated with the second charged particle beam so that a region of the first charged particles adjacent to the powder bed is maintained and serves to reduce charging of the powder bed by the second charged particle beam.

9. The powder bed fusion device according to claim 8, wherein: The source of the first charged particles is an ion source configured to generate positive ions of a passive gas.

10. The powder bed fusion device according to claim 9, wherein: The inert gas is an inert gas.

11. The powder bed fusion device according to any one of claims 8 to 10, comprising: A plasma source is used as a source of the first charged particles to generate positive ions.

12. The powder bed fusion device according to claim 11, wherein: The plasma source is a thermal ion emission plasma flood source, a radio frequency plasma source or a hollow cathode plasma source.

13. The powder bed fusion device according to claim 12, further comprising: A negative extractor electrode, or grid extractor, is positioned at the output aperture of the plasma source to extract positively charged ions from the plasma to form a positively charged ion beam.

14. The powder bed fusion device according to any one of claims 8 to 10, wherein: The source of the first charged particles is contained in an auxiliary chamber coupled to a main build chamber of the powder bed fusion apparatus.

15. The powder bed fusion device according to any one of claims 8 to 10, wherein: The kinetic energy and current of the first charged particles are controlled so that when the second charged particle beam is scanned across the powder bed, an equilibrium potential below a threshold required to induce powder movement is maintained on the powder bed due to interaction between the first charged particles and the second charged particle beam.

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