Additive Manufacturing using Powder Bed Fusion

Abstract

Additive Manufacturing Using Powder Bed Fusion There is provided a powder bed fusion apparatus arranged to irradiate a powder bed with a charged particle beam, the apparatus comprising: a neutralizing particle source operable to provide neutralizing particles of opposite charge to the charged particle beam; and a neutralizing particle focusing system arranged to weakly focus the beam of neutralizing particles from the neutralizing particle source to the powder bed proximate the location of irradiation of the charged particle beam on the powder, whereby the neutralizing particles neutralize charging of the powder bed by the charged particle beam. There is also provided a method of additive manufacturing using a powder bed fusion apparatus, where a part is formed in a series of layers, each layer being formed by scanning a charged particle beam over 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, particularly but not exclusively to charge control when irradiating metal powders during electron beam additive layer manufacture. [Background technology]

[0002] One of the best-known techniques used in 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 article to be built. After the layer is selectively melted, a new layer of powder is deposited and then also selectively melted, thereby building up the complete article layer by layer.

[0003] Metal powders are typically metal alloys that suffer from the disadvantage of tending to oxidize and become insulating or semi-insulating. When in this insulating or semi-insulating state, irradiating them with a charged particle beam, such as a high-energy electron beam, in a powder bed fusion process causes the metal powder particles to charge themselves and retain their charge, or a portion of it. As the charge builds up, the metal powder particles are subject to increasing Coulomb repulsion, which can result in the metal powder overcoming the gravitational and frictional forces acting on the underlying powder layer or molten material. The charged powder layer can then become mobile and be ejected from the powder bed, instantly disrupting the layer-by-layer addition process and potentially even damaging the device. For example, the powder can contaminate and melt-bond to device components. High-voltage electric arcs can also form, and the mobile powder can scatter the electron beam. Summary of the Invention

[0004] According to an aspect of the present invention, there is provided a method of additive manufacturing using a powder bed fusion apparatus, the method comprising forming a part as a series of layers, each layer being formed by: (a) irradiating a powder bed with first charged particles to form regions of first charged particles in and adjacent to the powder bed; (b) after the regions of first charged particles are formed, irradiating the powder bed with a beam of second charged particles of opposite charge to the first charged particles, the method comprising scanning the second beam of charged particles over the powder bed to melt the powder according to a predetermined pattern; and (c) terminating the second beam of charged particles and forming a new layer of powder in the powder bed, wherein the first charged particles relieve charging of the powder bed by the beam of second charged particles.

[0005] The method may further comprise controlling the kinetic energy and current of the first charged particles as the second particle beam is scanned over the powder bed such that an equilibrium potential is maintained over the powder bed due to interaction of the first charged particles and the second charged particle beam that is below a threshold required to induce powder motion.

[0006] The method may further comprise weakly focusing the first charged particles onto the powder bed near a location of irradiation of the second beam of charged particles, whereby the first charged particles relieve charging of the powder bed by the second beam of charged particles.

[0007] According to another aspect of the invention, there is provided a powder bed fusion apparatus for use in additive manufacturing, the apparatus 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 beam of second charged particles of opposite charge to the first charged particles; a powder bed for receiving the first charged particles and the beam of second charged particles; and a controller configured to control operation of the first source and the second source to form a part in a series of layers, each layer comprising: (a) irradiating the powder bed with the first charged particles to form a part of a preform; (b) forming a region of first charged particles in and adjacent to the powder bed; (b) after the region of first charged particles is formed, irradiating the powder bed with a beam of second charged particles of opposite charge to the first charged particles, wherein the beam of second charged particles is scanned over the powder bed to melt the powder according to a predetermined pattern; and (c) terminating the beam of second charged particles and forming a new layer of powder in the powder bed, wherein the first charged particles relieve charging of the powder bed by the beam of second charged particles.

[0008] The second beam of charged particles may be a beam of negatively charged electrons and 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 the first charged particles for generating the 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 duoplasmatron.

[0012] The plasma source may be a hollow cathode plasma source or a duoplasmatron, and the apparatus may further comprise a neutralizing particle focusing system arranged to weakly focus the first beam of charged particles onto the powder bed proximate to the location of irradiation of the powder by the second beam of charged particles, whereby the first charged particles mitigate charging of the powder bed by the second beam of charged particles.

[0013] The powder bed fusion apparatus may further comprise a negative extractor electrode or grid extractor positioned at the output of the plasma source to extract positively charged ions from the plasma to form the first beam of charged particles, the current of the first beam of charged particles being unaffected by a bias potential applied to the plasma source.

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

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

[0016] The neutralizing particle focusing system may include a series of electromagnetic lens elements aligned along the beam of the first charged particles between the plasma source and the powder bed, each lens element in the series of electromagnetic lens elements passing a different respective 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 there is substantially no magnetic field emanating outside of the neutralizing particle focusing system.

[0018] The powder bed fusion apparatus may further comprise a scanning system arranged to control the first beam of charged particles so that the first beam of charged particles can be scanned over the powder bed in quasi-synchronous with the second beam of charged particles, and the scanning system may comprise an electrostatic or electromagnetic deflector.

[0019] The powder bed fusion apparatus may further comprise a wire mesh configured to be at ground potential and positioned at the output of the electrostatic lens element closest to the powder bed, such that there is substantially no electric field transmission from the neutralizing particle focusing system or electrostatic deflector to the powder bed.

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

[0021] The powder bed fusion apparatus may further include a heat shield positioned around the powder bed, and 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 comprise a snubber circuit configured for use in series with a bias potential applied to the neutralizing particle source.

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

[0024] Based on the above, embodiments of the present invention provide methods and apparatus for additive manufacturing that can effectively mitigate excessive charge buildup in a powder bed caused by the beam used to melt the powder.

[0025] During the build process, particles of opposite charge to those used to irradiate the powder bed during additive manufacturing act to neutralize the charge of the metal particles on the powder bed resulting from irradiation by the charged particle beam used for melting. Relaxation of the powder bed charge is thus achieved. Therefore, instances of charge-induced metal powder particle motion can be substantially reduced or even eliminated, avoiding associated adverse effects.

[0026] The techniques disclosed herein allow for the optimization of this mechanism through a "priming" process that controls the availability of the first charged particles for charge relaxation.

[0027] In the priming stage, the powder bed is irradiated with first charged particles of opposite charge to the second charged particles used to irradiate the powder bed during additive manufacturing. Once the build process begins, mitigation of charge buildup on the powder bed occurs in a self-regulating manner. Because the first charged particles are available in the vicinity of the powder bed prior to irradiating the powder bed with the beam of second charged particles, neutralization of powder particle charge can occur as soon as the beam of second charged particles is activated and to mitigate excess charge buildup that would otherwise be caused by the beam of second charged particles, terminating the build on the powder bed. Including a priming stage prior to forming each layer using the beam of second charged particles ensures that the resulting charge on the powder bed is below the threshold required to induce powder mobility, avoiding amplification of the adverse effects described above.

[0028] The disclosed technology also utilizes a neutralizing particle focusing system arranged to provide a focused beam of neutralizing first charged particles that can be scanned over the powder bed. This approach allows for efficient operation of the neutralizing particle source, which may also lead to an increased lifetime of the neutralizing first charged particle source, as well as allowing for a uniform distribution of the neutralizing particles over the build area. [Brief explanation of the drawings]

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

[0030] [Figure 1] 1 illustrates an additive manufacturing apparatus according to an embodiment of the present invention. [Figure 2a] 1 illustrates a passive snubber circuit for use in an additive manufacturing apparatus according to an embodiment of the present invention. [Figure 2b] 1 illustrates an active snubber circuit for use in an additive manufacturing apparatus according to an embodiment of the present invention. [Figure 3] 1 illustrates an additive manufacturing apparatus including a neutralizing particle focusing system, according to an embodiment of the present invention. [Figure 4] 1 illustrates a method of operating an additive manufacturing apparatus according to an embodiment of the present invention.

[0031] It should be understood that for illustrative purposes, some elements in the drawings have not been drawn to scale. DETAILED DESCRIPTION OF THE INVENTION

[0032] Figure 1 illustrates a powder bed fusion apparatus 1 according to an embodiment of the present invention. The apparatus 1 illustrated in Figure 1 is configured for additive manufacturing using an electron beam 17 to melt metal powder to form a part 3 layer by layer.

[0033] Powder bed fusion apparatus 1 includes an electron optics assembly 21 for forming, conditioning, and directing electron beam 17. Electron optics assembly 21 includes an electron source 7 positioned to emit electrons. Electron optics assembly 21 further includes electron extraction and focusing elements 8 for forming an electron beam 17 from the emitted electrons that is focused on the powder bed, with electron beam 17 traveling along what is shown in FIG. 1 as the z-axis of apparatus 1. Electron optics assembly 21 further includes an electron deflection system 9 for scanning electron beam 17 over metal powder bed 2 to direct the fused powder into the desired additively manufactured part 3. Electron deflection system 9 includes electromagnetic deflectors positioned around electron beam 17.

[0034] The operation of the electro-optical assembly 21 is controlled according to the scan file for the desired portion 3 by signals derived from a build controller (not shown), such as one or more appropriately programmed computers or processors, as is known in the art.

[0035] The apparatus 1 further comprises at least one hopper 4 operable to dispense powder via a dispensing mechanism (not shown), and a stage 20 for supporting a build tank 19 positioned to receive the dispensed powder within a volume defining the powder bed 2. The stage 20 is movable in the z direction by a piston, the hopper 4 and piston being controlled in conjunction with signals derived from a build controller (not shown).

[0036] The apparatus 1 further comprises an ion source 11 for generating and emitting ions to be used in the charge relaxation mechanisms used in additive manufacturing methods according to embodiments of the present invention, which are described in more detail below.

[0037] In the configuration shown in Figure 1, the ion source 11 is a plasma source. The direction of travel of the ion flux 16 emanating from the plasma source 11 is at an angle to the direction of travel of the electron beam 17. In the embodiment shown in Figure 1, the electron beam 17 is parallel to the z-axis and the direction of the ion flux 16 is oblique thereto, although in alternative embodiments the direction of the ion flux 16 may be aligned with the z-axis and the electron beam 17 may be oblique thereto. Operation of the plasma source 11 is controlled by signals derived from a build controller (not shown).

[0038] In an embodiment of the present invention, additive manufacturing is carried out under vacuum conditions. Accordingly, the apparatus 1 further comprises a build vacuum chamber 5 through which the focused electron beam 17 and ion flux 16 pass before being incident on the powder bed 2. Connected to the build vacuum chamber 5 are a first auxiliary vacuum chamber 6 containing an electron optics assembly 21 and a second auxiliary vacuum chamber 10 containing a plasma source 11. The vacuum conditions are, as known in the art of powder fusion systems, on the order of 1×10 -3 mbar (1 × 10 -1 Pa) ~ 1 × 10 -7 mbar (1 × 10 -5 The vacuum pressure is maintained at the order of 100 Pa.

[0039] Hopper 4 dispenses powder so that a measured amount of powder is deposited on the surface of powder bed 2. A mechanism (not shown), such as a scraper or blade, is used to evenly distribute the powder on movable stage 20. Electron optics assembly 21 shapes and directs electron beam 17 as it scans over powder bed 2, heating and melting the powder to form solid layers of portion 3. After each layer of portion 3 is formed, stage 20 is lowered in the z-direction to accommodate the increasing height of portion 3 and allow the next layer to spread.

[0040] charge relaxation As mentioned above, the interaction of the negatively charged electron beam 17 with the powder particles can cause the unmelted powder particles to become negatively charged due to the insulating or semi-insulating oxide layer on the metal powder particles.

[0041] Without the charge mitigation methods used in embodiments of the present invention, this can result in a buildup of negative charge on the powder that can adversely affect the build process, including the build termination event described above, where powder particles are expelled from powder bed 2 and travel through build chamber 5 due to Coulomb repulsion exerted by other charged particles of the same charge polarity in powder bed 2.

[0042] During the "priming" stage prior to activation of the electron beam 17 at the start of the build process, the powder bed 2 is bombarded with ions from the plasma source 11 so that it is charged to a positive potential. With the powder bed 2 in a quiescent state (i.e., areas of the powder not impinged by the electron beam), the ion flux 16 impinging on the powder continues to deposit charge on the powder until the induced potential (in V) on the powder bed 2 equals the magnitude of the ion's kinetic energy (in eV). At this point, the potential of the charged powder bed 2 is sufficient to form a barrier to further incoming ions, which do not have the kinetic energy necessary to overcome the potential barrier and reach the powder bed 2. The potential barrier reduces the kinetic energy of further incoming positive ions until they have near-zero kinetic energy when they reach the charged powder bed 2, forming a "cloud" of free positive ions above the charged powder bed 2. These free positive ions with near-zero kinetic energy are available to mitigate excess negative charge buildup during the build process.

[0043] The priming stage is configured so that the powder bed 2 is not charged enough to induce powder motion, which is achieved by appropriately limiting the energy of the first charged particle beam.

[0044] After the priming stage, the electron beam 17 is activated and directed toward the powder bed 2. The cathode of the electron source is held at a high voltage, such as approximately -60 to -80 kV, relative to the anode of the electron source 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 to 80 keV, characterized by quasi-monochromatic energy. The plasma source 11 is biased at a lower positive voltage, such as +80 to +300 V, e.g., +200 V, selected to optimize the ion energy to maximize the deposition of positive charge on the powder particles while not initiating charge-induced powder motion that would interfere with the powder melting process by the electron beam 17.

[0045] The positive charge that accumulated on the powder bed 2 during the quiescent state is removed by the negative charge that accumulates on the powder bed 2 due to the electron beam 17. The electron beam 17 can induce a negative potential that is much larger than the positive potential induced by the ion flux 16, thus causing the irradiated area of ​​the powder bed 2 to become negatively charged. This negative charge attracts positive ions, possibly via the route of the electron beam 17 itself, from the cloud of free positive ions above the powder bed 2, which serves to counteract the negative charge caused by the electrons.

[0046] Then, as the electron beam 17 is scanned over the powder bed 2, an equilibrium potential is established over the area where the powder is being melted due to the interaction of the ion flux 16 and the electrons from the electron beam 17. The equilibrium potential is below the threshold at which powder mobility is initiated. As the electron beam 17 and ion flux 16 continue to be used, the cloud of free ions above the powder bed 2 is continually replenished. Thus, ions are readily available for charge neutralization throughout the build as needed (i.e., depending on the amount of negative charge that accumulates in the powder bed 2 due to, for example, the use of a particular material or the scanning strategy of the electron beam). In this way, a self-regulating charge relaxation process is possible.

[0047] This process of self-regulation prevents the powder from accumulating too much negative charge, thus avoiding many of the potential build termination events caused by excessive charge on the powder bed 2 as described above.

[0048] Some embodiments of the invention include a user interface connected to the build controller that allows the user to tailor the charge neutralization parameters to the particular build environment (e.g., material, feature resolution) to affect the quantity of ions available for charge neutralization, the timescale of the charge neutralization process, and the maximum number of ions that can be deposited on powder bed 2. The electron beam parameters used during the build (e.g., electron beam current, spot size, scan speed, scan path, and hatch shape) all determine the amount of negative charge that will be deposited on a given area of ​​powder bed 2 in a given amount of time. Therefore, the charge neutralization process is highly dependent on the exact build parameters used.

[0049] Build parameters can vary significantly from layer to layer, thereby allowing in some embodiments the charge neutralization parameters to be adjusted from layer to layer accordingly.

[0050] In the embodiment shown 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, such as one of the noble gases argon, helium, or xenon, selected so as not to cause interstitial contamination of the metal lattice of the resulting metal portion 3 formed on the build surface. The use of helium, which has the smallest mass and highest mobility among the noble gases, may improve the efficiency of the neutralization process. The atomic ionization process may be based on thermionic emission from a current-carrying tungsten filament for ionizing the gas in a discharge chamber held at a positive bias potential relative to ground. The plasma thus generated leaves the discharge chamber through an opening in the plasma source 11.

[0051] As shown in FIG. 1, the plasma flood source 11 is housed in a separate vacuum chamber 10 attached to the build vacuum chamber 5 .

[0052] Cascade Ionization and Suppression It may be necessary to mitigate the impact of electron impact ionization of particles present in the build area on the build process.

[0053] During the build process, electrons from the incident electron beam 17 may elastically backscatter 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, which may be repelled from the surface of the powder bed 2. Both the backscattered and secondary electrons may cause further ionization of ions and atoms present in the build area (atoms and / or ions emitted from the plasma flood source 11 or atoms evaporated from the molten pool 18) to produce additional electrons, which may then cause further ionization events. Such successive ionization events are referred to herein as cascade ionization.

[0054] An electric field exists near the build surface due to the proximity of the positively biased plasma flood source 11 discharge chamber and powder bed 2, and due to the positively charged ion flux 16 surrounding the negatively charged root of the electron beam 17. This combined electric field is large enough to add additional energy to secondary electrons in the build area, resulting in increased electron-atom interactions and thus playing a key role in the generation of cascade ionization.

[0055] The above-described process results in the generation of large electron and ion currents within the build volume. The large electron currents generated within the build area can disrupt the operation of the plasma flood source 11 and the power supplies attached to the plasma flood source 11. If disrupting the operation of the plasma flood source changes the electric field generated by the ion flux 16 surrounding the electron beam 17 due to changes in the ion current density around the electron beam 17 and / or molten pool 18, this can result in a shift in the position of the electron beam.

[0056] The electric field effects described above can create a number of problems with respect to porosity, surface finish, and shape accuracy of additively manufactured components, and the charge relaxation methods and apparatus described below provide solutions to such problems.

[0057] In the above-described embodiment, substantial elimination of charge-induced effects associated with charged powder particles leaving powder bed 2 is achieved by introducing a priming stage into the build process. If it is necessary to mitigate the effects of cascade ionization, a snubber circuit may be placed in series with the bias potential applied to plasma flood source 11. The snubber circuit allows for adjustment of the bias potential in the event that cascade ionization occurs in powder bed 2.

[0058] As described above, the plasma flood source 11 is biased with a positive voltage. In some embodiments, a 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, causing a primary ion flux 16 to leave the plasma flood source 11 and enter the powder bed 2.

[0059] However, when cascade ionization occurs in the powder bed 2, the large electron current generated impacts the exterior surface of the plasma flood source 11 and travels through the PSU to ground. This exceeds the PSU's maximum current (typically about 2.5 A), causing the PSU to self-regulate and lower its set voltage, thereby lowering the bias potential of the plasma flood source 11. The reduced voltage has the effect of reducing the electric field in the molten pool 18. This is because fewer electrons are extracted from the plasma, resulting in a reduced overall ion current exiting the plasma flood source 11; thus, the magnitude of the interaction between the electron beam 17 and the ion flux 16 is reduced, thereby reducing the electric field. A reduced voltage external to the plasma flood source 11 may also be a contributing factor.

[0060] The reduction in the electric field suppresses the cascade ionization process in the build area, thereby reducing the electron flow incident on the exterior surface of the plasma flood source 11 and passing through the PSU to ground. When the electron flow through the PSU drops below the maximum current of the PSU, the voltage of the PSU recovers to the set level.

[0061] When the PSU voltage is restored, the electric field is re-established as before (due to the proximity of the plasma flood source 11 to the powder bed 2 and the presence of positive ions in the vicinity of the powder bed 2), and thus the cascade ionization process resumes.

[0062] Thus, the process described above continually repeats itself, with the effects of cascade ionization being repeatedly initiated and suppressed. Cascade ionization therefore introduces instabilities in the ion current reaching powder bed 2, causing variations in the ions available for charge neutralization in the vicinity of molten pool 18. Such instabilities in ion current density induced around molten pool 18 in powder bed 2 and along the length of the electron beam change the physical position of electron beam 17, disrupting the build process.

[0063] The primary purpose of a snubber circuit is to stabilize the ion current density. In the embodiment shown in FIG. 2a, snubber circuit 101 comprises a high-power resistor 102 in series with the positive wire of PSU 103 between PSU 103 and plasma flood source discharge chamber 104. This arrangement is referred to herein as a "passive snubber." For example, resistor 102 may be a high-power (200 W) 22 Ω resistor. In other embodiments, passive snubber 101 may have a capacitive or inductive design.

[0064] When cascade ionization occurs, a large current of electrons passes through resistor 102 to ground. This induces a voltage drop across resistor 102 proportional to the product of the current and the resistance. This voltage drop reduces the effective bias voltage of plasma flood source 104, which, as discussed above, reduces both the electric field due to plasma flood source 104 and the electric field due to ion-electron interactions. The overall reduction in the electric field reduces the strength of the cascade ionization effect or may eliminate the cascade ionization effect altogether.

[0065] Although PSU 103 can adjust its supply voltage to compensate for high currents, snubber 101 can operate more quickly in the manner described above. Thus, snubber 101 can assist PSU 104 in self-regulating and reduce electron beam position deflections, both in terms of frequency of occurrence and magnitude.

[0066] In other embodiments, the snubber circuit utilizes an active feedback system to control the ion current density in the powder bed 2. Such an arrangement is referred to herein as an "active snubber." In the embodiment shown in FIG. 2b, the active snubber 201 comprises a variable resistor 202, e.g., a MOSFET or variable resistor, that supports an effective bias voltage for the plasma flood source body 203. A feedback controller 204 actively controls the effective bias voltage based on the bias current measured by one or more current sensors 205a, 205b.

[0067] When cascade ionization occurs and a resulting spike in bias current is detected, feedback controller 204 adjusts variable resistor 202 to control the effective bias voltage provided to stabilize the bias current. Feedback controller 204 can operate to suppress cascade ionization more quickly than a passive snubber, and the required time dynamics justify providing an active feedback system.

[0068] focusing mechanism As noted above, by priming the powder bed 2 with a positive charge before the build to counteract the negative charge induced by the electron beam 17, and optionally by utilizing a snubber to adjust the bias potential of the plasma flood source 11 as the cascade ionization process occurs in the build area, many different charge-induced effects can be substantially eliminated.

[0069] Such charge neutralization techniques involve neutralizing ions that are distributed over an area of ​​the powder bed 2 that is significantly larger than the area of ​​the molten pool 18. The intensity of the broad area ion flux 16 incident on the powder bed 2 falls off rapidly with increasing distance from its central axis.

[0070] It will be appreciated that as the required build size increases, the required coverage of the ion flux 16 increases, and when using a plasma flood source 11, such an increase in coverage area introduces non-uniformity into the neutralization distribution of ions in the powder bed 2. Furthermore, a larger build area results in a longer build time, which increases the length of time that the plasma flood source 11 must operate, thereby consuming more time for its consumable filament.

[0071] Embodiments of the present invention aim to provide a solution to these problems by providing a focused beam of ions that can be scanned over the powder bed, which relaxes the requirement to flood the build area with neutralizing ions and allows for more efficient operation of the plasma source.

[0072] By focusing the ions from the plasma source, it is possible to increase the ion current density in the molten pool while significantly reducing the total ion output from the plasma source, thereby extending the plasma source's lifetime, while scanning the focused ion beam also allows for a uniform distribution of neutralizing ions over the build area. For example, where a large area flood system utilizes a current of 1 A, a comparable focused ion column system would utilize a current of 10-100 mA.

[0073] In the configuration shown in FIG. 3, the powder bed fusion apparatus 10 further includes an ion focusing system 220. A point or near-point source of positive ions is used to provide ions to the focusing system 220. The beam of ions may have a wider beam width and / or divergence than the electron beam 170, and the term "beam" is used herein without requiring the use of any particular beam width. The plasma source 11 may be, for example, a duoplasmatron or a hollow cathode plasma source. In the following description, embodiments are described in the context of a duoplasmatron 110.

[0074] The focusing system 220 is positioned to condition and guide the ion beam 160 produced by the duoplasmatron 110. Focusing the ion beam 160 ensures that the ions are concentrated in a specific region of the powder bed 20. Steering the ion beam 160 allows its position to track the position of the electron beam 170, thereby ensuring that ions are provided only to regions of the powder bed 20 that require charge neutralization. This combination allows for high ion current densities in the molten pool 180, which in turn allows for a reduction in absolute ion output from the duoplasmatron 110 without compromising the charge neutralization process. The ability to scan the ion beam 160 over the powder bed 20 further enables uniform charge neutralization over a large build area.

[0075] Referring to FIG. 3, the ion focusing system 220 is shown in a column positioned at an angle relative to the direction of travel of the electron beam 170. In the embodiment shown in FIG. 3, the electron beam 170 is parallel to the z-axis and the ion focusing system 220 is tilted relative to it, although in alternative embodiments the column may be aligned with the z-axis and the electron beam 170 may be tilted relative to it. The column is also referred to herein as an ion column. More specifically, the ion focusing system 220 includes a plurality of ion focusing lens elements 130 for forming a weakly focused ion beam 160 traveling along the longitudinal axis of the ion focusing system 220 from unfocused ions exiting the duoplasmatron 110. The operation of the ion focusing system 220 is controlled by signals derived from a build controller (not shown).

[0076] The apparatus 10 further comprises a high voltage negative extractor element 120 positioned at the output aperture of the duoplasmatron 110 for retarding electrons and extracting ions in the plasma generated by the duoplasmatron 110. The extractor element 120 is referenced to a large negative potential, such as −4 to −10 kV, and may be an electrode, a grid, or a series of grids.

[0077] The apparatus 10 is arranged so that the duoplasmatron 110, extractor electrode 120, and ion focusing system 220 are housed in a separate vacuum chamber 100 shaped to match the ion column mounted in the build vacuum chamber 50. The high electric field between the duoplasmatron 110 and extractor electrode 120 acts as a focusing element, attracting positive ions leaving the duoplasmatron 110 and efficiently channeling them toward the input element of the ion focusing system 220. In this way, only the ionic component of the plasma enters the ion focusing system 220 and is focused as described below.

[0078] The length of the ion column allows the duoplasmatron 110 to be located far from the powder bed 20 while still maintaining a sufficient ion current density to allow effective charge neutralization through the use of the ion focusing system 220. Increasing the distance between the duoplasmatron 110 and the powder bed 20 reduces the entrapment of evaporated tungsten from the thermionic filament in the build material.

[0079] As mentioned above, the use of the ion focusing system 220 increases the ion current density in the molten pool 180, thereby reducing the gas flow rate entering the duoplasmatron 110. This in turn reduces the gas pressure at the build surface, minimizing the risk of cascade ionization processes of atoms and ions emanating from the duoplasmatron 110, thus substantially reducing charge-induced effects that adversely affect the build.

[0080] In some embodiments, the ion focusing system 220 utilizes a bend. This arrangement eliminates a direct line of sight through the ion focusing system 220 between the duoplasmatron 110 and the powder bed 20, thereby further reducing contamination of the build area by the duoplasmatron 110 or contamination of the duoplasmatron 110 by evaporated metal atoms from the powder bed 20.

[0081] As outlined above, the ion focusing system 220 comprises a series of lens elements aligned along the ion beam 160 between the duoplasmatron 110 and the powder bed 20. The series of lens elements comprises ion focusing lens elements 130 for forming a weakly focused ion beam 160. The ion beam 160 is weakly focused over an area of ​​50-60 mm and scanned quasi-synchronously with the location of the electron beam 170, allowing an area of ​​powder surrounding the location of the electron beam 170 to accumulate a positive charge before being irradiated by the electron beam 170 scanning towards that location during the build process.

[0082] In some embodiments of the present invention, the ion focusing system 220 comprises a series of electrostatic lens elements aligned along the longitudinal axis of the ion focusing system 220, each biased to a different potential and longitudinally spaced to support an electric field therebetween. The ion focusing system 220 may be comprised of a series of multi-element cylinder or aperture lenses. In some embodiments, the lens elements 130 are cylinder lenses of diameter D separated by a gap G, with a G / D ratio of approximately 0.1. The lens elements may be formed from stainless steel, aluminum, or the like. The focusing characteristics of such an electrostatic lens system vary depending on the lens voltage, the diameter of the cylinder or aperture, the gap between the lens elements, and the energy of the ions traveling through the lens system.

[0083] The configuration of an embodiment of the present invention provides a higher ion current density due to a focused ion beam and a lower ion source current than would be possible without the ion focusing system 220. As discussed above, the ability to maintain a high current density at the build surface using a lower total ion source current allows the duoplasmatron 110 to operate at a lower gas throughput using a lower thermionic filament current. This extends the life of the duoplasmatron 110 and reduces downtime by extending the period between user service. This allows for coverage of a larger area of ​​the powder bed 20, enabling the increased build time required for larger builds with more layers.

[0084] The lens element 140 closest to the powder bed 20 is configured to be at substantially ground potential so that the ion energy is defined by the potential difference between the bias potential applied to the duoplasmatron 110 and the lens element 140 closest to the powder bed 20, in addition to the initiation energy of the emitted positive ions (on the order of 10 eV). Grounding the last lens element 140 avoids the introduction of electric fields due to the proximity of an electrode at a potential to the region of the powder bed 20, while the energy of the ion beam 160 can be easily controlled by the bias potential of the duoplasmatron. "Substantially" ground should be interpreted to include any electrical tolerances or deviations from ground that can be accommodated within a particular build process.

[0085] In an alternative embodiment, the ion-focusing system 220 comprises a series of electromagnetic coil lens elements, each carrying a different respective current and spaced apart longitudinally.

[0086] Powder bed fusion apparatus 10 further includes a scanning system, referred to herein as ion deflection system 150, arranged to control weakly focused ion beam 160 so that it can be scanned over powder bed 20 in quasi-synchronous fashion with focused electron beam 170. The scanning process is controlled in conjunction with instructions provided by a build controller based on a scan file for the build process. The scan file, as known in the art, specifies a number of parameters for each layer of part 30, such as beam energy, beam current, and beam spot size, as well as a series of beam positions and dwell times that define the beam's path across powder bed 20.

[0087] This pseudo-synchronization allows the ion beam 160 to completely encompass the area of ​​the electron beam 170's irradiated location, shown as corresponding to the molten pool 180 in FIG. 3 , so that at least some of the positive ions, particularly those closest to the electron beam 170's irradiated location, are attracted to the negatively charged powder particles, where they help to counteract some or all of the negative charge. This, as discussed above, helps mitigate the adverse effects of the negative charge caused by the electron beam 170 and keeps the powder particles below the potential threshold at which they separate from the powder bed 20 due to Coulomb interactions. Some positive ions far from the electron beam 170's irradiated location cause areas of the powder bed 20 not yet irradiated by the electron beam 170 to acquire a positive charge, but they themselves are not attracted to the negatively charged powder particles because the Coulomb force decreases rapidly with increasing distance due to the inverse square law. The mechanism by which the powder bed 20 not yet irradiated by the electron beam 170 acquires a positive charge is similar to the priming process discussed above, leading to the replenishment of the ion cloud for neutralization during the build process.

[0088] The ion deflection system 150 comprises electrostatic deflectors, which in some embodiments are a series of plates aligned about the longitudinal axis of the ion column such that the normal to the surface of each plate is perpendicular to the longitudinal axis of the ion column. The plates are arranged in pairs, with each member of the pair on either side of the longitudinal axis of the ion column and biased with an electric potential to generate an electric field perpendicular to the direction of the ion beam 160. In some embodiments, the xy deflection system comprises two pairs of plates, each pair positioned 90 degrees from each other about the longitudinal axis of the ion column.

[0089] In some embodiments, the ion deflection system 150 is mounted on the last lens element 140, which is grounded, thereby minimizing transmission of deflection across the electric field from the lens element 140 to the powder bed 20. Preferred materials for the deflector are non-magnetic materials such as stainless steel, aluminum, or titanium.

[0090] The electrostatic deflector 150 is electrically coupled to an electromagnetic deflector 90 for the electron beam 170. This coupling is controlled by the build controller to ensure that the weakly focused ion beam 160 and the incident electron beam 170 roughly track each other in quasi-synchronization as they are scanned. In particular, quasi-synchronization allows the electron beam 170 to move rapidly within the envelope of the ion beam 160. Using such a combination of electrostatic and electromagnetic deflectors allows optimal use of the advantages of each in terms of deflection angle and response speed. In an alternative embodiment, the ion deflection system 150 uses an electromagnetic deflector. The nature of the deflection mechanism used and the required deflection angle take into account the distance between the deflector and the powder bed 20. In some embodiments, the distance between the deflector and the powder bed 20 is in the range of 200-500 mm.

[0091] In some embodiments, the apparatus 10 further comprises a wire mesh configured to be at substantially ground potential and positioned at the output of the last electrostatic lens element 140, such that there is substantially no electric field penetration into the build area from the electric fields generated by the electrostatic deflector 150 or the ion focusing system 220.

[0092] If 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-permeability ferrites so that there is substantially no magnetic field outside the ion focusing system 220 or the ion deflection system 150.

[0093] In some embodiments, the apparatus 10 further comprises an electrically insulating metal sheet positioned to surround the powder bed 20, which acts as a heat shield to reflect infrared radiation from the powder bed 20 away from other components of the apparatus 10 and back toward the powder bed 20. In such cases, the heat shield is configured to be at substantially ground potential such that substantially no electric field exists between the heat shield and the lens element 140 closest to the powder bed 20.

[0094] In some embodiments, the plasma source is a hollow cathode plasma source. By avoiding resistive heating of the thermionic filament, the lifetime of the plasma source is significantly extended to thousands of hours. Furthermore, hollow cathode plasma sources are advantageous for their ability to emit large ion currents, making more ions available for neutralization. This allows for higher ion current densities in the molten pool 18, and in some cases, the beam of ions produced by a hollow cathode plasma source can provide sufficient ion current densities in the molten pool 18 without the use of an ion focusing system 220.

[0095] Ion Beam Control As mentioned above, due to the use of the extractor electrode 120, the ion current from the plasma is not affected by the relatively small bias potential used in the duoplasmatron 110, but instead is determined by parameters that affect the rate of generation of positive ions, such as the current through the tungsten filament of the duoplasmatron 110, the gas pressure within the duoplasmatron 110, and the size of the output aperture of the gas discharge chamber.

[0096] As a result, ion energy and ion current are affected by different parameters and can be controlled independently of each other. Adjusting the ion current can be used to control the amount of ions available for charge neutralization as well as the timescale of the charge neutralization process. Adjusting the ion energy can be used to control the maximum number of ions that can be deposited on the powder bed 20.

[0097] As outlined above, the optimal parameters for charge relaxation may be based on the scanning strategy to be applied to the electron beam 17, and may take into account the particular electron beam parameters used during a particular portion of the build. For example, during a build using a high electron beam current with a slow electron beam scanning speed, the electron beam 170 will deposit a larger amount of charge onto the powder bed 20 per unit area per unit time, and thus a larger number of ions will be required for effective charge neutralization.

[0098] Furthermore, independent variation of ion current and energy is necessary to fully optimize charge neutralization parameters for different materials used in builds and different electron beam scanning strategies employed. The optimal parameters for charge neutralization of a particular metal powder can be influenced by factors such as the density of the material, the size and packing density of the powder particles, the capacitance of the metal oxide layer on the powder particles, its resistance to ground, and whether the powder is stationary or moving during irradiation by the electron beam 170. If the powder particles are stationary, the coefficient of static friction between the powder layer and the underlying material (i.e., the supporting metal plate, the previously melted powder layer, the sintered powder, or the unbonded green powder) plays a role. If the powder is mobile, for example due to excess charging, the coefficient of kinetic friction plays a role in determining how easily the powder can be further moved by the incident electron beam 170.

[0099] The combination of build and charge relaxation parameters used during the build affects the properties of the fabricated part 30. For example, the material properties of the finished part 30 depend on the interaction of the layer of build material with the electron beam 170 scanning over the layer (i.e., molten pool formation, particle motion, and large temperature gradients).

[0100] Additive Manufacturing Methods A method of additive manufacturing using a powder bed fusion apparatus 10 according to an embodiment of the present invention is also provided, which is illustrated with reference to FIG. 4 and described in relation to the powder bed fusion apparatus 10 described with reference to FIG. 3.

[0101] The build controller, in step S10, retrieves an instruction file for the part 30 to be manufactured. The instruction file stores computer-executable instructions for the controller to follow to form the part 30, such as electron beam build parameters (e.g., beam energy, current, scan speed, spot size), and the sequence of addresses on the powder bed 20 where the electron beam 170 should be positioned to form each layer of the part 30.

[0102] In step S20, a priming stage is performed. The build controller controls the ion source 110 according to the received charge relaxation parameter specifications (e.g., a specific ion beam current and energy to optimize a particular build process). The powder bed 20 is irradiated with the ion beam 160, charging 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 induce a potential of +200 V in the powder bed 20). As described above, priming the powder bed 20 prior to the start of the build process and prior to the start of the build of each new layer results in the accumulation of a cloud of free ions on the powder bed 20, which are then available for charge neutralization throughout the build process.

[0103] The priming 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, thereby pre-priming the area of ​​the powder bed 20 where the build will begin. In alternative embodiments, depending on the requirements of the build, 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 primed prior to the start of the build, or the ion deflection system 150 may be utilized to scan the ion beam 160 across as much as the entire area of ​​the powder bed 20.

[0104] In step S30, the build controller initiates the electron source 70 according to the build parameter specifications and positions the electron beam 170 at the first address obtained from the instruction file. Embodiments of the present invention are compatible with any particular scanning strategy. As the electron beam 170 impinges on the powder bed 20, it begins to melt the powder. Prior to melting the powder, step S30, in some embodiments, may further include a preheating stage in which the area to be melted is heated prior to melting to aid in the melting process. The positive charge accumulated on the powder bed 20 during the priming stage is quickly removed by the negative charge that accumulates on the powder bed 20 due to the electron beam 170. Positive ions from the cloud of free ions on the powder bed 20 are thus attracted to the negative potential that accumulates on the powder bed 20, where they counteract the negative charge caused by the electron beam 170, establishing an equilibrium potential in the area of ​​the powder to be melted.

[0105] In 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, it melts the powder to form the desired additively manufactured part 30. The weakly focused ion beam 160 roughly follows the position of the electron beam 170, such that the electron beam 170 and the ion beam 160 are scanned quasi-synchronously across the powder bed 20. In some embodiments, the ion beam 160 does not move continuously, but only changes position when the position of the electron beam 170 moves outside the area of ​​the powder bed 20 covered by the ion beam 160.

[0106] In step S50, a determination is made by the build controller as to whether there are any more addresses in the instruction file at which to position the electron beam 170 within the layer of the part 30 being created. If there are more locations (YES in S50), the method returns to step S40 to move the electron beam 170 to the next location in the sequence of addresses in the instruction file. If there are no more locations within the layer (NO in S50), the method proceeds to step S60.

[0107] In addition to the initial priming stage described above, continuous priming of the powder occurs throughout the build. Because the ion beam 160 is weakly focused on a larger area of ​​powder than the electron beam 170 and is scanned quasi-synchronously with the electron beam 170, the area of ​​powder surrounding the location of the electron beam 170 is primed before being irradiated by the electron beam 170 as it moves through the sequence of addresses in the powder bed 20.

[0108] In step S60, the electron beam 170 is turned off. In step S70, the build controller determines whether there are more layers in the instruction file to process. If there are no more layers to process (No in S70), the method ends. However, if processing of all layers has not been completed (Yes in S70), the method returns to step S20 via step S80. In step S80, build parameters for the next layer for the electron beam 170 are obtained from the instruction file, the stage 200 is lowered, and new powder is spread to form the powder bed 20 for the next layer of the part 30. Returning to step S20, priming of the newly spread powder is performed.

[0109] In this manner, the electron beam 170 may be scanned through all addresses specified in the instruction file to form the part 30 layer by layer through additive manufacturing, the ion beam 160 may be scanned quasi-synchronously with the electron beam 170, and the ion beam 160 may be used to prime the powder bed 20 prior to each layer being processed.

[0110] In an alternative embodiment where the ion source is a plasma flood source 11 (i.e., the configuration shown in FIG. 1 ), the build process is substantially as described above, but includes using an ion flux 16 to neutralize the powder bed 20 rather than quasi-scanning a weakly focused ion beam 160 across the powder bed 20.

[0111] In the above-described embodiments, the ion source is described as providing positive ions of opposite charge to the electrons of the illuminating electron beam. However, in alternative embodiments, the ion source may provide electrons or negatively charged ions to relieve the charge from a positively charged high energy beam. The same principles of operation as described above apply.

[0112] It will be appreciated that the powder bed fusion apparatus can be configured in a number of different ways depending on the user's requirements for a particular build process, and that compatible features of different embodiments, such as the nature of the neutralizing particle source, the snubber system, and the focusing system for the neutralizing particles, may be readily combined. As noted above, independent control of ion energy and ion current is one such aspect that can be varied. 。 [Item 1] 1. A method of additive manufacturing using a powder bed fusion apparatus, the method comprising forming a part as a series of layers, each layer comprising: (a) irradiating a powder bed with first charged particles to form a region of first charged particles in and adjacent to the powder bed; (b) irradiating the powder bed with a beam of second charged particles of opposite charge to the first charged particles after the region of the first charged particles is formed, the method comprising scanning the beam of second charged particles over the powder bed to melt the powder according to a predetermined pattern; and (c) stopping the beam of the second charged particles and forming a new layer of powder in the powder bed; is formed by The method of claim 1, wherein the first charged particles relieve charging of the powder bed by the beam of the second charged particles. [Item 2] Item 10. The method of item 1, wherein the beam of second charged particles is a negatively charged electron beam and the first charged particles are positively charged ions. [Item 3] 3. The method of claim 1, comprising controlling the kinetic energy and current of the first charged particles such that, as the beam of the second particles is scanned over the powder bed, an equilibrium potential is maintained on the powder bed due to interaction of the first charged particles and the beam of the second charged particles that is below the threshold required to induce powder motion. [Item 4] 4. The method of claim 1, further comprising: weakly focusing the first charged particles onto the powder bed in the vicinity of an irradiation position of the beam of second charged particles, whereby the first charged particles relieve charging of the powder bed by the beam of second charged particles. [Item 5] 1. A powder bed fusion apparatus for use in additive manufacturing, said apparatus comprising: a first source of first charged particles operable to provide the first charged particles; a second source of second charged particles operable to provide a beam of second charged particles of opposite charge to said first charged particles; a powder bed for receiving the beam of the first charged particles and the beam of the second charged particles; and a controller configured to control operation of the first source and the second source to form the portion as a series of layers. Each layer comprises: (a) irradiating the powder bed with the first charged particles to form a region of first charged particles in and adjacent to the powder bed; (b) irradiating the powder bed with the beam of second charged particles of opposite charge to the first charged particles after the region of first charged particles is formed, wherein the beam of second charged particles is scanned over the powder bed to melt the powder according to a predetermined pattern; and (c) stopping the beam of second charged particles and forming a new layer of powder in the powder bed; is formed by A powder bed fusion apparatus, wherein the first charged particles mitigate charging of the powder bed by the beam of second charged particles. [Item 6] Item 6. The powder bed fusion apparatus of item 5, wherein the beam of second charged particles is a negatively charged electron beam and the first charged particles are positively charged ions. [Item 7] 7. The powder bed fusion apparatus of claim 6, wherein the first source of first charged particles is an ion source configured to generate positive ions of a noble gas. [Item 8] 8. The powder bed fusion apparatus of claim 7, comprising a plasma source as the first source of first charged particles for generating the positive ions. [Item 9] 9. The powder bed fusion apparatus of claim 8, wherein the plasma source is a thermionic emission plasma flood source, a radio frequency plasma source, a hollow cathode plasma source, or a duoplasmatron. [Item 10] 9. The powder bed fusion apparatus of claim 5, wherein the plasma source is a hollow cathode plasma source or a duoplasmatron, and the apparatus further comprises a neutralizing particle focusing system arranged to weakly focus the beam of first charged particles onto the powder bed in the vicinity of a position where the beam of second charged particles impinges on the powder, thereby causing the first charged particles to mitigate charging of the powder bed by the beam of second charged particles. [Item 11] Item 11. The powder bed fusion apparatus of item 10, further comprising a negative extractor electrode or grid extractor positioned at the output of the plasma source to extract positively charged ions from the plasma to form the beam of first charged particles, wherein the current of the beam of first charged particles is not affected by a bias potential applied to the plasma source. [Item 12] Item 12. The powder bed fusion apparatus of item 11, wherein the neutralizing particle focusing system comprises a series of electrostatic cylindrical or aperture lens elements aligned along the beam of the first charged particles between the plasma source and the powder bed and spaced apart to support an electric field therebetween, each lens element in the series of electrostatic cylindrical or aperture lens elements being referenced to a different respective electric potential. [Item 13] Item 13. The powder bed fusion apparatus of item 12, wherein an electrostatic lens element closest to the powder bed is configured to be at substantially ground potential, whereby the energy of the first charged particles is substantially defined by the bias potential applied to the plasma source. [Item 14] 14. The powder bed fusion apparatus of any one of items 10 to 13, wherein the neutralizing particle focusing system has a series of electromagnetic lens elements aligned along the beam of the first charged particles between the plasma source and the powder bed, and in the series of electromagnetic lens elements, each lens element passes a different respective current. [Item 15] Item 15. The powder bed fusion apparatus of item 14, further comprising one or more ferrites surrounding one or more of the electromagnetic lens elements, whereby substantially no magnetic field exists outside of the neutralizing particle focusing system. [Item 16] 16. The powder bed fusion apparatus of any one of items 10 to 15, further comprising a scanning system arranged to control the beam of first charged particles such that the beam of first charged particles is scanned over the powder bed in quasi-synchronous with the beam of second charged particles, the scanning system having an electrostatic or electromagnetic deflector positioned within the last lens element of the neutralizing particle focusing system. [Item 17] Item 17. The powder bed fusion apparatus of item 16, wherein the scanning system has an electrostatic deflector and further comprises a wire mesh configured to be at ground potential and positioned at the output of the electrostatic lens element closest to the powder bed, whereby there is substantially no electric field transmission from the neutralizing particle focusing system or electrostatic deflector to the powder bed. [Item 18] 18. The powder bed fusion apparatus of any one of items 10 to 17, wherein the focusing system utilizes a bend so that there is no direct line of sight through the focusing system between the plasma source and the powder bed. [Item 19] 19. The powder bed fusion apparatus of any one of items 10 to 18, further comprising a heat shield positioned around the powder bed, the heat shield 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. [Item 20] 20. The powder bed fusion apparatus of any one of items 8 to 19, further comprising a snubber circuit configured for use in series with a bias potential applied to the neutralizing particle source. [Item 21] 21. The powder bed fusion apparatus of any one of items 5 to 20, wherein the first source of first charged particles is housed in an auxiliary chamber connected to a main build chamber of the powder bed fusion apparatus.

Claims

1. 1. A method of additive manufacturing using a powder bed fusion apparatus, the method comprising forming a part as a series of layers, each layer comprising: (a) irradiating a powder bed with first charged particles to form a region of the first charged particles in and adjacent to the powder bed; (b) irradiating the powder bed with a beam of second charged particles of opposite charge to the first charged particles after the region of the first charged particles is formed, the method comprising scanning the beam of second charged particles over the powder bed to melt powder according to a predetermined pattern; and (c) stopping the beam of the second charged particles and forming a new layer of powder in the powder bed; is formed by the source of first charged particles is active when the powder bed is irradiated using the beam of second charged particles, whereby the first charged particles relieve charging of the powder bed by the beam of second charged particles.

2. The method of claim 1, wherein the beam of the second charged particles is a negatively charged electron beam and the first charged particles are positively charged ions.

3. 3. The method of claim 1, comprising controlling the kinetic energy and current of the first charged particles such that, as the beam of the second charged particles is scanned over the powder bed, an equilibrium potential is maintained on the powder bed due to interaction of the first charged particles and the beam of the second charged particles that is below a threshold required to induce powder motion.

4. A method described in any one of claims 1 to 3, further comprising a step of weakly focusing the first charged particles onto the powder bed near the irradiation position of the beam of the second charged particles, thereby causing the first charged particles to relieve charging of the powder bed caused by the beam of the second charged particles.

5. 1. A powder bed fusion apparatus for use in additive manufacturing, comprising: a first source of first charged particles operable to provide said first charged particles; a second source of said second charged particles operable to provide a beam of second charged particles of opposite charge to said first charged particles; a powder bed for receiving the beam of the first charged particles and the beam of the second charged particles; and a controller configured to control operation of the first source and the second source to form the portion as a series of layers. Each layer comprises: (a) irradiating the powder bed with the first charged particles to form a region of the first charged particles in and adjacent to the powder bed; (b) irradiating the powder bed with the beam of second charged particles of opposite charge to the first charged particles after the region of the first charged particles is formed, wherein the beam of second charged particles is scanned over the powder bed to melt powder according to a predetermined pattern; and (c) stopping the beam of the second charged particles and forming a new layer of powder in the powder bed; is formed by 10. A powder bed fusion apparatus, comprising: a first source of first charged particles configured to be active when the powder bed is irradiated using the beam of second charged particles, whereby the first charged particles mitigate charging of the powder bed by the beam of second charged particles.

6. A powder bed fusion bonding apparatus as described in claim 5, wherein the beam of the second charged particles is a negatively charged electron beam and the first charged particles are positively charged ions.

7. A powder bed fusion bonding apparatus as described in claim 6, wherein the first source of the first charged particles is an ion source configured to generate positive ions of a rare gas.

8. 8. The powder bed fusion apparatus of claim 7, comprising a plasma source as the first source of the first charged particles for generating the positive ions.

9. 9. The powder bed fusion bonding apparatus of claim 8, wherein the plasma source is a thermionic emission plasma flood source, a radio frequency plasma source, a hollow cathode plasma source, or a duoplasmatron.

10. 9. The powder bed fusion apparatus of claim 8, wherein the plasma source is a hollow cathode plasma source or a duoplasmatron, and the powder bed fusion apparatus further comprises a neutralizing particle focusing system positioned to weakly focus the beam of first charged particles onto the powder bed near a location where the beam of second charged particles impinges on the powder, thereby causing the first charged particles to reduce charging of the powder bed caused by the beam of second charged particles.

11. A powder bed fusion bonding apparatus as described in claim 10, further comprising a negative extractor electrode or grid extractor positioned at the output of the plasma source to extract positively charged ions from the plasma to form the beam of the first charged particles, wherein the current of the beam of the first charged particles is not affected by a bias potential applied to the plasma source.

12. 12. The powder bed fusion apparatus of claim 11, wherein the neutralizing particle focusing system comprises a series of electrostatic cylindrical or aperture lens elements aligned along the beam of the first charged particles between the plasma source and the powder bed and spaced apart to support an electric field therebetween, each lens element in the series of electrostatic cylindrical or aperture lens elements being referenced to a different respective electric potential.

13. 13. The powder bed fusion apparatus of claim 12, wherein an electrostatic lens element closest to the powder bed is configured to be at substantially ground potential, whereby the energy of the first charged particles is substantially defined by the bias potential applied to the plasma source.

14. 14. The powder bed fusion apparatus of claim 10, wherein the neutralizing particle focusing system comprises a series of electromagnetic lens elements aligned along the beam of the first charged particles between the plasma source and the powder bed, each lens element in the series of electromagnetic lens elements passing a different respective current.

15. 15. The powder bed fusion apparatus of claim 14, further comprising one or more ferrites surrounding one or more of the electromagnetic lens elements, whereby substantially no magnetic field exists outside of the neutralizing particle focusing system.

16. A powder bed fusion bonding apparatus as described in any one of claims 10 to 15, further comprising a scanning system arranged to control the beam of the first charged particles so that the beam of the first charged particles is scanned over the powder bed in pseudo-synchronism with the beam of the second charged particles, the scanning system having an electrostatic or electromagnetic deflector positioned within the last lens element of the neutralizing particle focusing system.

17. 17. The powder bed fusion apparatus of claim 16, wherein the scanning system has an electrostatic deflector and further comprises a wire mesh configured to be at ground potential and positioned at an output of an electrostatic lens element closest to the powder bed, whereby there is substantially no electric field transmission from the neutralizing particle focusing system or electrostatic deflector to the powder bed.

18. 18. The powder bed fusion apparatus of claim 10, wherein the neutralizing particle focusing system utilizes a bend such that there is no direct line of sight through the neutralizing particle focusing system between the plasma source and the powder bed.

19. 19. The powder bed fusion apparatus of any one of claims 10 to 18, further comprising a heat shield positioned around the powder bed, the heat shield configured to be at ground potential such that substantially no electric field exists between the heat shield and a portion of the neutralizing particle focusing system closest to the powder bed.

20. A powder bed fusion bonding apparatus as described in any one of claims 8 to 19, further comprising a snubber circuit configured for use in series with a bias potential applied to the first source of the first charged particles.

21. A powder bed fusion bonding apparatus as described in any one of claims 5 to 20, wherein the first source of the first charged particles is stored in an auxiliary chamber connected to a main build chamber of the powder bed fusion bonding apparatus.

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