Additive manufacturing using powder bed fusion bonding and high-efficiency charge neutralization

Abstract

A powder bed fusion apparatus for use in additive manufacturing is provided, 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 containing 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 parts as a series of layers, each layer being formed by scanning the electron beam over the powder bed to melt a layer of powder in the powder bed into a desired shape; wherein the plasma source is connected to the anode of the power supply, enabling a circuit in which the ion current formed by the positively charged ions in the plasma is balanced by an electron flow supplied from the plasma and returned to the power supply through the plasma, thereby automatically adjusting the reduction of the charge of the powder bed by the electron beam by the ion current.

Description

[Technical Field]

[0001] The present invention relates to the use of a powder bed fusion apparatus in additive manufacturing, and more particularly to charge control when irradiating metal powder in electron beam additive layer manufacture. [Background technology]

[0002] One of the most well-known techniques used in additive manufacturing is powder bed fusion, in which thin layers of powder, typically metal or plastic, are selectively melted by an energy source such as a laser or electron beam. The melted areas of the powder layer form the cross-sectional portions of the article to be built. After a layer is selectively melted, a new layer of powder is deposited and then selectively melted again, thereby constructing a complete article layer by layer.

[0003] Metal powders are typically metal alloys that have the disadvantage of tending to oxidize and become insulating or semi-insulating. In this insulating or semi-insulating state, irradiation with a charged particle beam, such as a high-energy electron beam, during a powder bed fusion process can cause the metal powder particles themselves to become charged and retain some or all of their charge. As the charge accumulation increases, the metal powder particles experience increasing Coulomb repulsion, which can cause them to overcome gravity and frictional forces acting from the underlying powder layer or molten material. The charged powder layer may then become mobile and even eject from the powder bed, potentially disrupting the layer-by-layer additive process and potentially damaging the equipment. For example, the powder may contaminate and fuse to equipment components. High-voltage electric arcs may also form, and the mobile powder may scatter the electron beam. [Overview of the project]

[0004] According to one aspect of the present invention, a powder bed fusion apparatus for use in additive manufacturing is provided, 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 containing 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 parts as a series of layers, each layer being formed by scanning the electron beam over the powder bed to melt a layer of powder in the powder bed into a desired shape; wherein the plasma source is connected to the anode of the power supply, and a circuit is provided which enables the ion current formed by the positively charged ions in the plasma to be balanced by an electron flow supplied from the plasma and returned to the power supply through the plasma, thereby automatically adjusting the reduction of the charge of the powder bed by the electron beam by the ion current.

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

[0006] During the build process, particles with the opposite charge to those used to irradiate the powder bed during additive manufacturing act to neutralize the charge on the metal powder particles resulting from the molten charged particle beam. Thus, excessive charging of the powder bed is avoided. Consequently, instances of charge-induced migration of metal powder particles are significantly reduced, thus avoiding associated adverse effects.

[0007] The technology disclosed herein enables the optimization of this mechanism by providing an electrical configuration in which the ion current supplied by the plasma source is the same as the electron current generated by the electron source.

[0008] Since the ionic current required to neutralize the negative charge on the powder is substantially equal in magnitude to the electron beam current, and the plasma between the plasma source and the powder bed acts as a conduit for charge transfer, this method enables an efficient charge neutralization process. [Brief explanation of the drawing]

[0009] Embodiments of the present invention will be described simply by reference to the accompanying drawings.

[0010] [Figure 1] This shows an additive layer manufacturing apparatus according to an embodiment of the present invention. [Figure 2] This shows the electrical configuration used with an additive layer manufacturing apparatus according to an embodiment of the present invention. [Figure 3] This illustrates the effect of changes in the neutrizer coupling potential of a plasma discharge chamber used in embodiments of the present invention. [Figure 4] This document describes a method for operating an additive manufacturing apparatus according to an embodiment of the present invention.

[0011] For the sake of clarity, please understand that some elements of the drawings are not shown to scale. [Modes for carrying out the invention]

[0012] Figure 1 shows a powder bed fusion bonding apparatus 1 according to an embodiment of the present invention. The apparatus 1 shown in Figure 1 is configured to perform additive manufacturing by melting metal powder using an electron beam 17 and forming parts 3 layer by layer.

[0013] The powder bed fusion apparatus 1 comprises an electron-optical assembly 21 for forming, conditioning, and guiding an electron beam 17. The electron-optical assembly 21 comprises an electron gun cathode 7, referred herein to as an "electron source," configured to emit electrons. The electron-optical assembly 21 further comprises an electron extraction and focusing element 8 for forming an electron beam 17 focused from the emitted electrons onto the powder bed, which moves along what is shown in Figure 1 as the z-axis of the apparatus 1. The electron-optical assembly 21 further comprises an electron deflection system 9 for scanning the electron beam 17 over a bed of metal powder 2 to melt the powder into desired additive manufacture parts 3. The electron deflection system 9 comprises an electromagnetic deflector positioned around the electron beam 17.

[0014] The operation of the electro-optical assembly 21 is controlled, as is known in the art, by signals originating from one or more preferably programmed build controllers (not shown), such as a computer or processor, according to a scan file for a desired part 3.

[0015] The apparatus 1 further comprises at least one hopper 4 operable to distribute powder via a distribution mechanism (not shown), and a stage 20 for supporting a build tank 19 positioned to receive the distributed powder into a volume defining a powder bed 2. The stage 20 is movable in the z direction via a piston, and the hopper 4 and the piston are controlled in conjunction with signals originating from a build controller (not shown).

[0016] Apparatus 1 further comprises a plasma source 11 that generates and emits a plasma or mixture of ions, electrons and neutral atoms (shown as 16a in Figure 1) which will be used in a charge neutralization mechanism used in an additive manufacturing method according to embodiments of the present invention, which will be described in more detail below. The operation of the plasma source 11 is controlled via signals originating from a build controller (not shown).

[0017] In embodiments of the present invention, additive manufacturing is performed under vacuum conditions. Therefore, the apparatus 1 further comprises a build vacuum chamber 5 through which a focused electron beam 17 and ions 16 travel to a powder bed 2. The ions 16 travel through a region 16a of plasma and neutral atoms present between a plasma source 11 and the powder bed 2. A first auxiliary vacuum chamber 6, containing an electron-optical assembly 21, is coupled to the build vacuum chamber 5. While the plasma source 11 is shown as being contained within the build vacuum chamber 5, it may alternatively be contained within a second auxiliary vacuum chamber coupled to the build vacuum chamber 5. Vacuum conditions are 1 × 10⁻⁶, as known in the art of powder fusion systems. -3 mbar(1×10 -3 hPa) ~ 1 × 10 -7 mbar(1×10 -7 It is maintained at a vacuum pressure on the order of hPa.

[0018] The hopper 4 distributes the powder so that the measured amount of powder is deposited on the surface of the powder bed 2. Mechanisms such as scrapers or blades (not shown) are used to evenly disperse the powder on the movable stage 20. The electro-optical assembly 21 forms and guides the electron beam 17 so that the electron beam 17 scans the powder bed 2 to heat and melt the powder and form solid layers of part 3. After each layer of part 3 is formed, the stage 20 is lowered in the z direction to accommodate the increasing height of part 3 and allow the next layer to diffuse.

[0019] Charge neutralization As described above, the interaction between the negatively charged electron beam 17 and 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.

[0020] In the absence of a charge neutralization mechanism used in embodiments of the present invention, this can lead to the accumulation of negative charges on the powder, which can have an adverse effect on the build process, including a build end event where powder particles are displaced from the powder bed 2 due to the Coulomb repulsion exerted by other charged powder particles of the same charge polarity in the powder bed 2 and move throughout the build chamber 5.

[0021] According to embodiments of the present invention, by simultaneously irradiating the powder bed 2 with both the electron beam 17 and the ions 16 via the region 16a from the plasma source 11, the powder is prevented from accumulating excessive negative charges, thus avoiding a plurality of potential build end events caused by the excessive charges on the powder bed 2 as described above.

[0022] During the build process, electrons from the incident electron beam 17 can elastically backscatter from the surface of the powder bed 2, initiating a process herein referred to as cascade ionization, which can have an adverse effect on the build. Electrons from the incident electron beam 17 may also generate secondary electrons by ionizing the material at the build surface, and these secondary electrons may be ejected from the surface of the powder bed 2. The backscattered electrons and secondary electrons can both cause further ionization of the ions and atoms (neutral atoms and / or ions generated from the plasma source 11 or neutral atoms evaporated from the melt pool 18) present in the build area, generating additional electrons, which in turn can cause further ionization events.

[0023] 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, an electric field exists near the build surface. This combined electric field is large enough to add additional energy to the secondary electrons within the build area, resulting in an increased electron-atom interaction and thus playing an important role in the occurrence of cascade ionization.

[0024] As a result of the process described above, in the absence of the charge neutralization mechanism used in the embodiments of the present invention, a large electron and ion current is generated within the build volume. The large electron flow generated in the build area can interfere with the operation of the plasma source 11 and the power supply attached to the plasma source 11. If the operation of the plasma source is interfered with, and the electric field generated by ions 16 surrounding the electron beam 17 is altered, this may result in a shift in the position of the electron beam.

[0025] The optimization of the charge neutralization mechanism is achieved through the implementation of a circuit that ensures the ion current supplied by the plasma source is substantially the same as the electron flow generated by the electron source. The electrical circuit automatically provides the correct amount of positive ions necessary to neutralize the negative charge of the electron beam and is automatically adjustable in response to changes in conditions within the build area.

[0026] Electrical configuration Figure 2 shows an electrical configuration 100 used with a powder bed fusion fusion apparatus 1 according to an embodiment of the present invention. This system enables efficient neutralization of electron charges deposited on powder particles by a fusion electron beam 17 (generated by electron source 7) using positively charged ions 16 (generated by plasma source 11).

[0027] A single high-voltage power supply 101 drives the system, which is connected to both the electron source 7 and the plasma source discharge chamber 105 as shown in Figure 2, and sets the overall operating voltage range across the system. The power supply 101 may, for example, provide a potential difference of 60 kV between its terminals.

[0028] As shown in Figure 2, the electron gun cathode 102 is held at a negative potential relative to the electron gun anode 103, accelerating electrons away from the electron source 7 towards the powder bed to generate the electron beam 17. For ease of explanation, the electron source 7 is positioned below the build surface so that its negative potential is relative to the powder bed fusion equipment ground voltage (referred to herein as the equipment ground), in contrast to the more positive potential on the plasma discharge chamber. However, physically, the electron source 7 is located on the same side of the build surface as the plasma source discharge chamber 105, as shown in Figure 1. The electron gun anode 103 is relative to the equipment ground. The powder bed is represented in Figure 2 as a series of layers of powder particles insulated from the build tank 19 and stage 20 so that areas of the powder bed are negatively charged when the powder bed is irradiated with the electron beam 17. After melting by the electron beam 17, the resulting conductive additive manufacture part 3 is relative to the equipment ground via the support platform on which the part 3 is built.

[0029] The plasma source discharge chamber 105 is connected to the anode of the power supply 101 and is positioned to provide a high-density plasma containing ions and electrons in a space bridging the plasma source 11 and the powder bed surface. This region of plasma present in the build chamber is referred to herein as the “plasma bridge” 104, as shown in Figure 2. The plasma bridge 104 corresponds to region 16a in Figure 1. The plasma bridge 104 functions as a conduit for charge conduction, thus effectively completing the circuit between the plasma discharge chamber 105 and the electron gun cathode 102 via the powder bed, and ions in the plasma are transmitted through the plasma bridge 104 to one or more points on the powder bed where negative charges are accumulating due to the incident electron beam 17.

[0030] The effective resistance of the above conduit is a function of the density of the plasma bridge 104. In this regard, the plasma bridge 104 is shown in Figure 2 as including a variable resistor. With respect to a given current flowing through the above conduit, it can be understood that there exists a potential difference across the conduit, which is expressed as the product of the current and the effective resistance. This potential difference across the plasma bridge conduit 104 is offset from the potential difference between the plasma source discharge chamber 105 and the equipment ground, and is referred to herein as the neutralizer coupling potential (V NCP ) is referred to as.

[0031] During operation, the neutralizer coupling potential of the plasma source discharge chamber 105 automatically adjusts relative to the equipment ground to provide sufficient ionic current to neutralize the electron charge accumulating on the powder bed during build (as described in more detail below). The greater the required ionic current, the greater (more positive) the neutralizer coupling potential.

[0032] The fact that the plasma discharge chamber 105 and the electron gun cathode 102 are connected to the same terminal of power supply 101, and that they do not have separate current return paths as a result of the circuit described above, ensures that the same current flows through both. As a result, the plasma source provides the correct ion current via the plasma bridge 104 so as to react with the current from the electron source. Thus, the system can be considered to be "self-regulating". More specifically, due to the fact that both the electron gun cathode 102 and the plasma discharge chamber 105 are connected to power supply 101, electrons are supplied by the plasma bridge 104 to the electron gun cathode 102 and, consequently, into the electron beam 17, via the plasma source discharge chamber 105. Ions travel around the circuit in the opposite direction (specifically, for every single electron supplied by the plasma bridge 104, one ion is supplied to the powder bed, where the flow of electrons in one direction and the flow of ions in the opposite direction may be considered the same current flow in the circuit). As a result, the ionic current is attracted only to the areas of the powder bed where negative charge neutralization is needed, and only in the required amount, leading to a highly efficient charge neutralization process.

[0033] When the plasma source 11 is not operating or is malfunctioning (for example, when a zero or low-density plasma bridge 104 exists, representing a high effective resistance pipeline), the neutralizer coupling potential cannot exceed, for example, approximately 100V relative to the equipment ground, due to the back-to-back Zener diodes used and depending on their type. In such a situation, when the electron source 7 is switched on, the neutralizer coupling potential becomes +100V, and electrons are supplied into the electron beam 17 from ground rather than from the plasma bridge 104.

[0034] This "automatic adjustment" is shown in Figure 3, and it involves adjusting the neutrizer coupling potential (V) of the plasma source discharge chamber 105. NCP This shows the change (Δ) of ). The elements in Figure 3 are not shown to scale, and VNCP It should be noted that it represents a value significantly smaller than the values indicated by the two voltage rails (-V, +V).

[0035] The first configuration shows two voltage rails (-V, +V) provided with a 60 kV potential difference by the power supply 101. The back-to-back Zener diode 106 ensures that the maximum value for the top rail is +100 V with respect to ground. The neutralizer coupling potential of the plasma source discharge chamber 105 with respect to ground has a value V NCP and has.

[0036] Larger or smaller neutralization currents may be required in response to changes in the electron beam current incident on the powder bed, and the system adjusts accordingly. The second configuration shows an example where the two voltage rails have adjusted values (-V+Δ, +V+Δ). The 60 kV potential difference between the rails is maintained. As a result of this "shift", the neutralizer coupling potential V of the plasma source discharge chamber 105 with respect to the facility ground NCP +Δ increases, resulting in an increase in the ion current provided to the powder bed 2.

[0037] When a smaller neutralization current is required, the neutralizer coupling potential of the plasma source discharge chamber 105 with respect to the facility ground can decrease to V NCP -Δ.

[0038] It is clear from FIG. 3 that the voltage between the electron source 7 and the plasma source discharge chamber 105 is constant. In a likely scenario, during the build process, due to Δ being small with respect to the 60 kV voltage utilized by the electron source, the change in the voltage of the electron gun cathode 102 with respect to the facility ground is only minor.

[0039] The ion current provided by the plasma source, which matches the electron flow generated by the electron source, can have a value of approximately 50 mA. In the absence of the electrical configuration 100 described above, a higher ion current is generated between the plasma source discharge chamber 105 and ground during operation, resulting in a current requirement (e.g., as high as 1 A) that is significantly higher than what is needed to neutralize the electron beam.

[0040] Plasma source In the embodiment shown in Figure 1, a plasma source 11 is shown, which is implemented 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 argon, helium, or xenon, which is selected so as not to cause interlattice contamination of the metal lattice of the metal part 3 that is consequently formed on the build surface. Using helium, which has the smallest mass and highest fluidity among the noble gases, may improve the efficiency of the neutralization process. The atomic ionization process can be based on thermionic emission from an energized tungsten filament that ionizes the gas in a discharge chamber located at a positive bias potential with respect to ground. The plasma thus generated exits the discharge chamber through an aperture in the plasma source 11.

[0041] As shown in Figure 1, the plasma source 11 is contained within the build vacuum chamber 5. Alternatively, the plasma source 11 may be contained within a separate vacuum chamber attached to the build vacuum chamber 5.

[0042] In alternative embodiments, the plasma source is a radio frequency plasma source, a hollow cathode plasma source, or a duoplasmatron, but any other suitable plasma source can be used.

[0043] Additive Manufacturing Method A method of additive manufacturing using a powder bed fusion apparatus 1 according to an embodiment of the present invention is also provided, which is illustrated with reference to Figure 4 and described in relation to the powder fusion apparatus 1 described with reference to Figure 1.

[0044] 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.

[0045] In step S20, the build controller obtains an instruction file for part 3 to be created. The instruction file contains computer executable instructions that the controller should follow to form part 3, such as electron beam build parameters (e.g., beam energy, current, scanning speed, spot size) and a sequence of addresses on the powder bed 2 where the electron beam 17 should be positioned to form each layer of part 3.

[0046] In step S30, the electron source 7 is activated. The build controller starts the electron source 7 according to the specified build parameters and positions the electron beam 17 to the first address obtained from the instruction file. Embodiments of the present invention are adaptable to any particular scanning strategy. When the electron beam 17 is incident on the powder bed 2, it begins to melt the powder. Before melting the powder, step S30 may further include, in some embodiments, a preheating stage that heats the area to be melted before melting to assist the melting process. When the electron source 7 is activated, the neutralizer coupling potential of the plasma source 11 automatically adjusts to react with the electron beam 17.

[0047] The positive ions 16 react to the negative charges on the powder caused by the electron beam 17, establishing an equilibrium potential over the area of ​​the melting powder.

[0048] In step S40, the build controller obtains the following 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 across the powder bed 2, it melts the powder to form the desired additive manufacture part 3.

[0049] In step S50, the build controller determines whether there are further addresses in the instruction file where the electron beam 17 should be positioned within the layer of the generated part 3. If further positions exist, the method returns to step S40 (yes in S50) and moves the electron beam 17 to the next position in the sequence of addresses in the instruction file. If no further positions exist within the layer (no in S50), the method proceeds to step S60.

[0050] In step S60, the build controller determines whether there are any more layers to be processed in the instruction file. If there are no more layers to be processed (S60-N), the method proceeds to step S70, where the electron source 7 and then the plasma source 11 are switched off, and the method then terminates. However, if not all layers have been processed, the method returns to step S40 via step S80 (S60-Y). In step S80, build parameters for the next layer for the electron beam 17 are obtained from the instruction file, the stage 20 descends, and new powder is diffused to form the powder bed 2 for the next layer of part 3. Returning to step S40, the build controller obtains the next address in the next layer from the instruction file, moves the electron beam 17 to the specified address on the powder bed 2, and the build continues.

[0051] In this way, the electron beam 17 can scan all addresses specified in the instruction file for each layer of part 3, so that part 3 is formed by the additive layer manufacturer. As mentioned 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; thus, the plasma source 11 generates the precise ion current necessary to neutralize the negative charge caused by the incident electron beam 17 while scanning over the powder bed 2.

[0052] Powder bed fusion fusion equipment can be configured in several different ways depending on the user's requirements for a specific build process, and it is understood that compatible features of different embodiments can be easily combined.

Claims

[Claim 1] A powder bed fusion apparatus used in additive manufacturing, wherein the powder bed fusion apparatus is A power supply having an anode and a cathode; An electron source capable of operating to provide an electron beam, wherein the electron source is biased by the power supply; A plasma source capable of providing a plasma containing 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 series of layers, each layer being formed by scanning the electron beam over the powder bed to melt the powder layers of the powder bed into a desired shape; Equipped with, The plasma source is connected to the anode of the power supply, enabling a circuit in which the ion current formed by the positively charged ions in the plasma is balanced by an electron flow supplied from the plasma and returned to the power supply through the plasma, thereby automatically adjusting the reduction of the charge of the powder bed by the electron beam by the ion current. Powder bed fusion bonding apparatus.

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