Additive manufacturing using powder bed melting and efficient charge neutralization

Abstract

A powder bed melting apparatus for additive manufacturing is provided, the apparatus comprising: a power source having an anode and a cathode; an electron source operable to provide an electron beam wherein the electron source is biased by a power source; a plasma source operable to provide a plasma comprising electrons and positively charged ions; a powder bed arranged to receive the electron beam and the plasma; and a controller configured to control operation of the electron source and the plasma source to form a part as a series of layers, each layer being formed by scanning an electron beam over the powder bed to melt powder layers of the powder bed into a desired shape; wherein a plasma source is connected to an anode of a power source, thereby enabling a circuit in which an ion current formed by positively charged ions in the plasma is balanced by an electron current originating from the plasma and returned to the power source via the plasma, therefore, the charging of the powder bed by the electron beam is relieved through the ion current by self-regulation.

Description

Technical Field

[0001] The present invention relates to the use of a powder bed fusion apparatus for additive manufacturing, and more particularly to charge control during the irradiation of metal powder during electron beam additive layer manufacturing. Background Art

[0002] One of the most prominent technologies for additive manufacturing is powder bed fusion, in which thin layers of powder (usually metal or plastic) are selectively melted by an energy source such as a laser or an electron beam. The melted areas of the powder layer form a cross-sectional part of the article to be built. After selectively melting the layer, a new powder layer is deposited and then also selectively melted, thus building the complete article layer by layer.

[0003] Metal powders are usually metal alloys, which suffer from the disadvantage that they tend to oxidize and become 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 the powder bed fusion process causes the metal powder particles themselves to become charged and retain that charge or a portion thereof. As the charge accumulation increases, the metal powder particles experience an increasingly large Coulomb repulsion force, which may cause the metal powder to overcome the gravitational and frictional forces from the lower powder layer or the melted material. Then, the charged powder layer may become mobile and may even be ejected from the powder bed, immediately disrupting the layer-by-layer additive process and potentially damaging the apparatus. For example, the powder may contaminate and fuse to the components of the apparatus. 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, there is provided a powder bed fusion apparatus for additive manufacturing, the apparatus comprising: a power supply having an anode and a cathode; an electron source operable to provide an electron beam, wherein the electron source is biased by the power supply; a plasma source operable to provide a plasma comprising electrons and positively charged ions; a powder bed arranged to receive the electron beam and the plasma; and a controller configured to control the operation of the electron source and the plasma source to form a part as a series of layers, each layer being formed by scanning the electron beam over the powder bed to fuse the powder layer of the powder bed into a desired shape; wherein the plasma source is connected to the anode of the power supply, thereby enabling a circuit in which an ion current formed by the positively charged ions in the plasma is balanced by an electron current originating from the plasma and returning to the power supply via the plasma, thereby self-regulating the ion current to mitigate charging of the powder bed by the electron beam.

[0005] Based on the above, embodiments of the present invention provide an apparatus and method for additive manufacturing that employ charge neutralization techniques to prevent excessive charge accumulation on a powder bed caused by a beam used to melt powder.

[0006] During the build process, particles having a charge opposite to the particulate charge used to irradiate the powder bed during additive manufacturing act to neutralize the charge on the metal powder particles due to a melting charged particle beam. As a result, overcharging of the powder bed is avoided. Consequently, instances of charge-induced movement of the metal powder particles can be significantly reduced, thereby avoiding associated adverse effects.

[0007] The technology disclosed in the present application optimizes this mechanism by providing an electrical configuration in which the ion current provided by a plasma source is the same as the electron current generated by an electron source.

[0008] This method enables an effective charge neutralization process because the ion current required to neutralize the negative charge on the powder is substantially equal in magnitude to the current of the electron beam, where the plasma between the plasma source and the powder bed acts as a conduit for charge transfer. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Embodiments of the present invention will be described by way of example only with reference to the accompanying drawings, in which:

[0010] Figure 1 shows an additive layer manufacturing apparatus according to an embodiment of the present invention;

[0011] Figure 2 shows an electrical configuration for an additive layer manufacturing apparatus according to an embodiment of the present invention;

[0012] Figure 3 illustrates the effect of a change in the neutraliser coupling potential of a plasma discharge chamber used in an embodiment of the present invention; and

[0013] Figure 4 shows a method of operating an additive manufacturing apparatus according to an embodiment of the present invention.

[0014] It should be understood that, for the sake of explanation, some elements in the drawings are not shown to scale. DETAILED DESCRIPTION

[0015] Figure 1 shows a powder bed melting apparatus 1 according to an embodiment of the present invention. Figure 1 The apparatus 1 shown therein is configured for additive manufacturing of melting metal powder using an electron beam 17 to form a part 3 layer by layer.

[0016] The powder bed melting apparatus 1 includes an electro - optical assembly 21 for forming, regulating, and manipulating an electron beam 17. The electro - optical assembly 21 includes a cathode 7 of an electron gun (referred to herein as an “electron source”) that is arranged to emit electrons. The electro - optical assembly 21 further includes an electron extraction and focusing element 8 for forming an electron beam 17 focused on the powder bed from the emitted electrons, and the electron beam 17 travels along Figure 1 the z - axis of the apparatus 1 as shown. The electro - optical assembly 21 further includes an electron deflection system 9 for scanning the electron beam 17 over the metal powder bed 2 to melt the powder into a desired additive - manufactured part 3. The electron deflection system 9 includes an electromagnetic deflector arranged around the electron beam 17.

[0017] As is known in the art, the operation of the electro - optical assembly 21 is controlled via signals derived from a build controller (not shown) (e.g., one or more suitably programmed computers or processors) according to the scan file of the desired part 3.

[0018] The apparatus 1 further includes at least one hopper 4 and a platform 20. The hopper 4 is operable to dispense powder via a dispensing mechanism (not shown), and the platform 20 is for supporting a build tank 19 that is positioned to receive the dispensed powder so as to define the volume of the powder bed 2. The platform 20 is movable in the z - direction via a piston, and the hopper 4 and the piston are controlled in conjunction with signals derived from a build controller (not shown).

[0019] The apparatus 1 further includes a plasma source 11 for generating and emitting a plasma or a mixture of the following items ( Figure 1 represented by 16a in

[0020] ): ions, electrons, and neutral atoms for a charge - neutralization mechanism used in an additive - manufacturing method according to an embodiment of the present invention, which will be described in more detail below. The operation of the plasma source 11 is controlled via signals derived from a build controller (not shown). -3 mbar to 1×10 -7 mbar.

[0021] The hopper 4 distributes the powder, thereby depositing a measured amount of powder 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 platform 20. The electro-optical assembly 21 forms and manipulates the electron beam 17 such that the electron beam 17 scans over the powder bed 2 to heat and melt the powder and form a solid layer of the part 3. After each layer of the part 3 is formed, the platform 20 is lowered in the z-direction to accommodate the increasing height of the part 3 and to allow the next layer to be deployed.

[0022] Charge neutralization

[0023] As described above, due to the insulating or semi-insulating oxide layer on the metal powder particles, the interaction of the negatively charged electron beam 17 with the powder particles causes the unmelted powder particles to become negatively charged.

[0024] In the absence of the charge neutralization mechanism used in the embodiments of the present invention, this may lead to the accumulation of negative charge on the powder, which may have an adverse effect on the build process, including end-of-build events in which powder particles are displaced from the powder bed 2 due to the Coulomb repulsive force exerted by other charged powder particles with the same charge polarity in the powder bed 2 and travel throughout the build chamber 5.

[0025] According to an embodiment of the present invention, the powder bed 2 is irradiated simultaneously with both the electron beam 17 and the ions 16 from the plasma source 11 via the region 16a, preventing the powder from accumulating too much negative charge and thus avoiding many potential end-of-build events caused by excessive charge on the powder bed 2 as described above.

[0026] During the build process, electrons from the incident electron beam 17 may elastically backscatter from the surface of the powder bed 2, thereby initiating a process referred to herein as cascade ionisation, which may have a negative impact on the build. Electrons from the incident electron beam 17 can also generate secondary electrons by ionising the material at the build surface, and these secondary electrons can be ejected from the surface of the powder bed 2. Both the backscattered electrons and the secondary electrons can cause further ionisation of the ions and atoms present in the build region (neutral atoms and / or ions emitted from the plasma source 11, or neutral atoms evaporated from the melt pool 18), thereby generating additional electrons, which in turn cause additional ionisation events.

[0027] An electric field exists near the build surface due to the proximity of the positively biased discharge chamber of the plasma source 11 and the powder bed 2, and due to the positively charged ions 16 surrounding the negatively charged conduit of the electron beam 17. The strength of this combined electric field is sufficient to provide additional energy to the secondary electrons in the build area, resulting in increased electron-atom interactions, thereby playing an important role in the occurrence of cascade ionization.

[0028] As a result of the above process, in the absence of the charge neutralization mechanism used in embodiments of the present invention, large electron and ion currents are generated within the build volume. The large electron currents generated in the build area may interfere with the operation of the plasma source 11 and the power supply attached to the plasma source 11. If the interference with the operation of the plasma source causes the electric field generated by the ions 16 around the electron beam 17 to change, this may cause the position of the electron beam to shift.

[0029] Optimization of the charge neutralization mechanism is achieved through the implementation of a circuit that ensures that the ion current provided by the plasma source is substantially the same as the electron current produced by the electron source. The circuit automatically provides the correct number of positive ions required to neutralize the negative charge of the electron beam and is able to self-adjust in response to changing conditions in the build area.

[0030] Electrical configuration

[0031] Figure 2 An electrical configuration 100 for a powder bed fusion apparatus 1 according to an embodiment of the invention is shown. The system is capable of using positively charged ions 16 (generated by plasma source 11) to effectively neutralize the electron charge deposited on powder particles by a melting electron beam 17 (generated by electron source 7).

[0032] A single high voltage power supply 101 drives a system that Figure 2 The means shown are connected to both the electron source 7 and the plasma source discharge chamber 105 and set the operating voltage range of the whole system. The power supply 101 may provide a potential difference of, for example, 60 kV between its terminals.

[0033] like Figure 2 As shown, the electron gun cathode 102 is maintained at a negative potential relative to the electron gun anode 103 to accelerate electrons away from the electron source 7 and toward the powder bed, thereby generating the electron beam 17. For ease of explanation, the electron source 7 is shown below the build surface to represent the negative potential relative to the powder bed melting device ground voltage (referred to herein as the facility ground) compared to the more positive potential on the plasma discharge chamber. However, in physical terms, the electron source 7 is at a negative potential. Figure 1 The arrangement shown is located on the same side of the build surface as the plasma source discharge chamber 105. The electron gun anode 103 is referenced to facility ground.Figure 2 is represented as a series of layers of powder particles insulated from build tank 19 and platform 20 such that when the powder bed is irradiated by electron beam 17, the regions of the powder bed become negatively charged. After being melted by electron beam 17, the resulting electrically conductive additive manufacturing part 3 is referenced to the facility ground via the support platform on which part 3 is built.

[0034] The plasma source discharge chamber 105 is connected to the anode of power supply 101 and is arranged to provide a high-density plasma including ions and electrons in the space bridging the plasma source 11 and the powder bed surface. As Figure 2 shown, this plasma region present in the build chamber is referred to herein as the "plasma bridge" 104. The plasma bridge 104 corresponds to Figure 1 region 16a. The plasma bridge 104 acts as a conduit for charge conduction, thus effectively completing the circuit between the plasma source discharge chamber 105 and the electron gun cathode 102 via the powder bed: the ions in the plasma are transported via the plasma bridge 104 to one or more points on the powder bed that accumulate negative charge due to the incident electron beam 17.

[0035] The effective resistance value of the conduit is a function of the density of the plasma bridge 104. In this regard, the plasma bridge 104 is represented in Figure 2 as including a variable resistor. For a given current passing through the conduit, it can be understood that there is a potential difference across the ends of the conduit represented by the product of the current and the effective resistance value. The potential difference across the ends of the plasma bridge 104 conduit cancels out the potential difference between the plasma source discharge chamber 105 and the facility ground, which is referred to herein as the neutralizer coupling potential (V NCP ).

[0036] In operation, the neutralizer coupling potential of the plasma source discharge chamber 105 is automatically adjusted relative to the facility ground to provide sufficient ion current to neutralize the electron charge that accumulates on the powder bed during build (as will be described in more detail below). The greater the required ion current, the greater (more positive) the neutralizer coupling potential.

[0037] The plasma source discharge chamber 105 and the electron gun cathode 102 are connected to terminals of the same power supply 101, and the fact that there is no independent current return path due to the above circuit ensures that the same current flows through both. As a result, the correct ion current is provided by the plasma source via the plasma bridge 104 to counteract the current from the electron source. Thus, the system can be considered "self-regulating". More specifically, since both the electron gun cathode 102 and the plasma source discharge chamber 105 are connected to the power supply 101, electrons are supplied to the electron gun cathode 102 by the plasma bridge 104 via the plasma source discharge chamber 105 and thus enter the electron beam 17. Ions travel in the opposite direction around the circuit (specifically, for each electron provided by the plasma bridge 104, one ion is provided to the powder bed, where the electron flow in one direction and the ion flow in the opposite direction can be considered the same current within the circuit). This results in an efficient charge neutralization process because the ion current is only attracted to the negatively charged regions of the powder bed and needs to be neutralized, and is only attracted in the required amount.

[0038] In the case where the plasma source 11 is not operating or is operating poorly (e.g., in the case of a zero-density or low-density plasma bridge 104 representing a high-efficiency resistive duct), depending on and according to the type of back-to-back Zener diode used, the neutralizer coupling potential cannot exceed a voltage of, for example, about 100 V with respect to the facility ground terminal. In such a case, with the electron source 7 turned on, the neutralizer coupling potential will be +100 V, and electrons will enter the electron beam 17 from the ground rather than from the plasma bridge 104.

[0039] This "self-regulation" is as Figure 3 shown Figure 3 and shows the variation (Δ) of the neutralizer coupling potential (V NCP ) of the plasma source discharge chamber 105. It should be noted that Figure 3 the elements in NCP are not shown to scale, and the value represented by V

[0040] is significantly less than the values indicated by the two voltage rails (-V, +V). The first configuration shows the two voltage rails (-V, +V), with the power supply 101 providing a potential difference of 60 kV across its ends. The back-to-back Zener diode 106 ensures that the maximum value of the top voltage rail is +100 V with respect to the ground. The neutralizer coupling potential of the plasma source discharge chamber 105 with respect to the ground has the value V NCP .

[0041] In response to changes in the electron beam current incident on the powder bed, a larger or smaller neutralization current may be required, and the system is accordingly readjusted. The second configuration shows the two voltage rails having adjusted values Example. Maintain a 60 kV potential difference between the voltage rails. This "offset" increases the neutralizer coupling potential of the plasma source discharge chamber 105 relative to the facility ground by (V NCP +Δ), resulting in an increase in the ion current supplied to the powder bed 2.

[0042] In cases where a smaller neutralization current is required, the neutralizer coupling potential of the plasma source discharge chamber 105 relative to the facility ground may be reduced to V NCP -Δ.

[0043] As can be clearly seen from Figure 3 , there is no change in the voltage between the electron source 7 and the plasma source discharge chamber 105. In a possible scenario, during construction, Δ is very small relative to the 60 kV voltage applied to the electron source, so the voltage of the electron gun cathode 102 relative to the facility ground has only a very small change.

[0044] The ion current provided by the plasma source matches the electron current generated by the electron source, with a value of approximately 50 mA. In the absence of the above electrical configuration 100, during operation, a higher ion current will occur between the plasma source discharge chamber 105 and the ground, resulting in a current demand (e.g., up to 1 A) significantly higher than the current required to neutralize the electron beam.

[0045] Plasma source

[0046] In Figure 1 the illustrated embodiment, 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 (e.g., one of the inert gases such as argon, helium, or xenon), and the selected gas will not cause interstitial contamination to the metal lattice of the resulting metal part 3 formed at the build surface. The use of helium (the lowest mass and highest mobility among the inert gases) can contribute to the efficiency of the neutralization process. The atomic ionization process can be based on thermionic emission of a current-carrying tungsten wire to ionize the gas in the discharge chamber, and the gas is at a positive bias potential relative to the ground. The plasma generated in this way exits the discharge chamber through the holes in the plasma source 11.

[0047] As Figure 1 shown, the plasma source 11 is contained within the build vacuum chamber 5. Alternatively, the plasma source 11 can be contained in a separate vacuum chamber attached to the build vacuum chamber 5.

[0048] In an alternative embodiment, the plasma source is a radio frequency plasma source, a hollow cathode plasma source, or a duoplasmatron, but any other suitable plasma source can also be used.

[0049] Additive manufacturing method

[0050] There is also provided an additive manufacturing method according to an embodiment of the present invention using a powder bed melting apparatus 1, as shown in reference Figure 4 shown and described in conjunction with the powder melting apparatus 1 shown in reference Figure 1 shown.

[0051] In step S10, the plasma source 11 is activated, and a region 16a of plasma and neutral atoms is formed between the plasma source 11 and the powder bed 2.

[0052] In step S20, the build controller obtains an instruction file for the part 3 to be manufactured. The instruction file contains computer-executable instructions for the controller to follow to form the part 3, such as electron beam build parameters (e.g., beam energy, current, scan speed, spot size) and a series of addresses on the powder bed 2 to position the electron beam 17 to form each layer of the part 3.

[0053] In step S30, the electron source 7 is activated. The build controller starts the electron source 7 according to the specification of the build parameters and positions the electron beam 17 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 17 impinges on the powder bed 2, the electron beam 17 starts 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. When the electron source 7 is activated, the neutralizer coupling potential of the plasma source 11 is self-regulated to cancel the electron beam 17.

[0054] Positive ions 16 cancel the negative charge generated by the electron beam 17 on the powder, thereby establishing an equilibrium potential in the powder melting region.

[0055] In step S40, the build controller retrieves the next address from the instruction file and moves the electron beam 17 to the specified address on the powder bed 2. As the electron beam 17 moves on the powder bed 2, the electron beam 17 melts the powder to form the desired additive manufacturing part 3.

[0056] In step S50, the build controller determines whether there are more addresses in the instruction file at which the electron beam 17 will be positioned within the layer of the part 3 being produced. If there are more positions, the method returns (S50 - Y) to step S40 and moves the electron beam 17 to the next position in the address sequence in the instruction file. If there are no more positions within the layer (S50 - N), the method proceeds to step S60.

[0057] In step S60, the build controller determines whether there are any more layers to process in the instruction file. If there are no more layers to process (S60-N), the method proceeds to step S70, in which the electron source 7 and the plasma source 11 are turned off, and then the method ends. However, if not all layers have been processed, the method returns (S60-Y) via step S80 to step S40. In step S80, the build parameters for the next layer of the electron beam 17 are retrieved from the instruction file, and the platform 20 is lowered and a new powder is spread to form a powder bed 2 for the next layer of the part 3. When returning to step S40, the build controller retrieves the next address in the next layer from the instruction file and moves the electron beam 17 to the specified address on the powder bed 2, and then the build continues.

[0058] In this way, the electron beam 17 can be scanned through all the addresses specified for each layer of the part 3 in the instruction file, thereby forming the part 3 by additive layer manufacturing. As described above, since the electron source 7 and the plasma source 11 are connected via the same power supply, it is ensured that the same current flows through both of them; thus, when the incident electron beam 17 scans on the powder bed 2, the plasma source 11 generates the exact ion current required to neutralize the negative charges generated by the incident electron beam 17.

[0059] It should be understood that the powder bed melting apparatus can be configured in a variety of different ways according to the user's requirements for a specific build process, and the compatible features of different embodiments can be easily combined together.

Claims

1. A powder bed melting apparatus for additive manufacturing, the apparatus comprising: a power supply having an anode and a cathode; an electron source operable to provide an electron beam, wherein the electron source is biased by the power supply; a plasma source operable to provide a plasma comprising electrons and positively charged ions; a powder bed arranged to receive the electron beam and the plasma; and a controller configured to control the operation of the electron source and the plasma source to form a part as a series of layers, each layer being formed by scanning the electron beam over the powder bed to fuse a powder layer of the powder bed into a desired shape; wherein the plasma source is connected to the anode of the power supply to effect a circuit in which an ion current formed by the positively charged ions in the plasma is balanced by an electron current originating from the plasma and returning to the power supply via the plasma, thereby self-regulating the ion current to mitigate charging of the powder bed by the electron beam.

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