Method for processing powder materials at high acceleration voltage and electron beam device

Abstract

A method for processing powder materials with an electron beam device for additive manufacturing of components is provided, which solves the problem of electrostatic powder ejection and significantly reduces the process time, by using an acceleration voltage of 90 kV or more in the preheating and / or melting steps.

Description

[Technical field]

[0001] The present invention relates to a method for processing powder materials with an electron beam device at high acceleration voltages, in particular to a method for preheating powder materials at high acceleration voltages and a method for melting powder materials at high acceleration voltages.

[0002] The invention further relates to an electron beam device for carrying out such a method for processing powder material at high acceleration voltages. [Background technology]

[0003] Additive manufacturing is characterized by joining volume elements together into three-dimensional structures, in particular building them up layer by layer. In this case, in a particularly applicable method, a powdered material in a powder bed is bonded into a 3D structure by selectively melting individual powder particles from point to point and from layer to layer with an energy beam. The hardening of the material can be achieved by sintering the powder particles or by completely melting the powder particles using a laser beam or an electron beam.

[0004] The processing of metal powders by selective electron beam melting (SEBM) allows for the creation of complex geometries and structures with rapid, precise control and a high degree of automation.

[0005] However, processing powder materials with an electron beam can result in localized and temporary electrostatic charging of the exposed powder bed, since, for example, metal powder particles are often covered with a poorly conductive oxide layer, and therefore even metal powder particles, although conductive internally, can become electrically charged when struck by the electron beam.

[0006] When charging reaches a critical level, powder particles that are stationary in the area hit by the electron beam can be accelerated en masse out of the processing zone, i.e. dispersed from the powder bed into other areas of the electron beam device, before the melting process can occur, resulting in loss of material and interruption of the process, as material is expelled from the powder bed before sintering.

[0007] In order to avoid accidents due to powder release and loss of material, according to the current state of the art it is known to preheat a layer of powder material after it has been applied, so that the individual powder particles bond to each other in a manner that makes them less adhesive than in the final product.

[0008] Only then, in a second step, the powder particles are melted by an electron beam along the contour layers of the 3D structure to be generated, respectively, forming a 3D structure between the individual powder particles to the extent that it is sufficiently stable for use in the desired workpiece.

[0009] Known methods for these processes are summarized as preheating and involve heating of the applied powder layer by means of a heated plate or by irradiation with an electron beam at a typical accelerating voltage of about 60 kV.

[0010] Another efficient method for pre-heating a powder bed is described in WO 2018 / 162261 A1, where the problem of electrostatic powder release is solved by the application of an alternating electromagnetic field.

[0011] Disadvantages of this known method are the process times required for heating the powder bed and the insufficient avoidance of electrostatic powder release, since heating must be carried out again for each applied powder layer. Summary of the Invention [Problem to be solved by the invention]

[0012] It is therefore an object of the present invention to provide a method for processing powder materials, which better solves the problem of electrostatic powder release of the powder materials when processed by electron beams. It is also an object of the present invention to provide an electron beam device for processing powder materials. [Means for solving the problem]

[0013] According to the invention, this problem is solved by a method for processing a powder material, which comprises the following steps: a1) providing an electron beam apparatus having an apparatus for receiving a powder bed of powder material to be processed and an electron beam generator configured to direct an electron beam at different positions laterally of the powder bed; b1) applying a powder bed onto a substrate; c1) preheating the powder material by an electron beam; In step c1), the electron beam is driven at an accelerating voltage of 90 kV or more.

[0014] Conventional electron beam machining processes and preheating processes carried out by electron beams are usually carried out at an acceleration voltage of about 60 kV. This is because above this high voltage value, hard x-ray beams are generated that are not shielded by normal electron beam equipment. The inventors have realized that by increasing the acceleration voltage to 90 kV or more for preheating by electron beams while keeping the input power the lower beam current can be selected, which leads to a lower number of charge carriers being introduced into the material per unit time, and thus a lower static charge being generated. Thus, the temperature for premelting the powder particles can be reached with less static charge. Furthermore, a higher acceleration voltage leads to a larger penetration depth and therefore to a larger volume of electrons being distributed in the material.

[0015] Thus, the method according to the present invention solves the above-mentioned problem of electrostatic powder shedding by making the preheating step, for example by sintering, cause a change in the energy balance in the powder material. On the other hand, if the beam current remains the same as in the conventional preheating method, a significantly shorter exposure time can be achieved by increasing the acceleration voltage, which results in a shorter process time without increasing the electrostatic powder shedding. Furthermore, the inventors have realized that an increase in the acceleration voltage at a constant beam current leads to a significant improvement in the process stability due to a reduction in the electrostatic powder shedding.

[0016] Since the variables that determine the electrostatic powder bed charging are derived from the local current density and powder properties, equations can be used to derive values ​​for the acceleration voltage and beam current, which can include parameters such as the powder bed temperature, the pressure in the electron beam tool, or material property parameters such as the melting temperature, heat capacity, electrical conductivity, etc. of the powder material.

[0017] The method preferably comprises step d1) melting at least a part of the powder layer by means of an electron beam.

[0018] In the method according to the invention, the acceleration voltage is preferably from 90 kV to 150 kV, in particular 100 kV or more, preferably 120 kV or more.

[0019] The beam power is preferably at least 100 W and up to 100 kW.

[0020] The powder material preferably comprises titanium, copper, nickel, aluminum and / or alloys thereof, in particular Ti-6Al-4V, an alloy comprising titanium, 6% aluminum and 4% vanadium by weight.

[0021] The powder material preferably has an average particle size D50 of 10 μm to 150 μm.

[0022] According to the invention the problem is solved by another method for processing powder materials, comprising the following steps: a2) providing an apparatus (6) for receiving a powder bed (7) of powder material to be processed, and an electron beam apparatus (1) having an electron beam generator (3) configured to direct an electron beam (4) laterally to different positions of the powder bed (7), b2) applying a powder layer (8) onto a substrate (10); c2) melting at least a portion of the powder material (12) by the electron beam (4); In step c2), the electron beam (4) is driven by an acceleration voltage of 90 kV or more, and No pre-heating of the powder layer (8) takes place between steps b2) and c2).

[0023] In addition to the improvements mentioned above, this method allows for a further reduction in process time, due to the complete elimination of the preheating step and the operation of the electron beam at 90 kV or higher during the melting step. As with the above-mentioned method including the preheating step, this method takes advantage of the increased penetration depth and the resulting improved charge distribution of the electrons, as well as the reduced electron dosage at the same power.

[0024] The acceleration voltage in the method according to the invention is preferably from 90 kV to 150 kV, in particular 100 kV or more, preferably 120 kV or more.

[0025] The beam power is preferably at least 100 W and up to 100 kW.

[0026] The powder material preferably comprises titanium, copper, nickel, aluminum and / or alloys thereof, in particular Ti-6Al-4V, an alloy comprising titanium, 6% aluminum and 4% vanadium by weight.

[0027] The powder material preferably has an average particle size D50 of 10 μm to 150 μm.

[0028] With respect to an electron beam device for processing powder materials by means of an electron beam device, the device according to the invention comprises: an apparatus for containing a powder bed of powder material to be processed; an electron beam generator configured to direct the electron beam laterally to different locations on the powder bed; The electron beam device is designed to carry out the method according to the invention.

[0029] The electron beam device preferably further comprises an X-ray shield, which is configured in such a way that, despite high voltages of more than 90 kV, preferably more than 100 kV, in particular more than 120 kV, the X-ray radiation outside the electron beam device remains below the prescribed limits, which must in particular meet the requirements of the Radiation Protection Ordinance.

[0030] Therefore, the inventors have realized that in order to increase the acceleration voltage into the voltage range of interest for the present invention, additional measures for X-ray shielding must be taken: for example, the observation window inside the electron beam device is provided with a relatively thick shielding layer, or in particular with a separate cover, which covers the observation window during operation and whose opening terminates the process.

[0031] The electron beam apparatus preferably includes a control unit which, upon input of the powder material to be melted, determines values ​​for the acceleration voltage U and beam current I using stored formulas.

[0032] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. [Brief description of the drawings]

[0033] [Figure 1] FIG. 1 is a perspective view of an electron beam device according to the present invention having a powder container. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] FIG. 1 shows an electron beam device 1 having a vacuum housing 2 in which an electron gun 3 is arranged for generating an electron beam 4 .

[0035] In this embodiment, an electron gun 3 with an optional magneto-optical unit 5 is positioned above a lift table 6 having a lift plate and a storage frame, which serves as a spatially limited powder container to contain a powder bed 7 of the powder material to be processed.

[0036] For this purpose, a powder application device 9 with a squeegee (not shown) is arranged above the receiving frame, which can be moved above the lift table. The powder application device 9 has a container (not shown) for powder material from which it can level the material on the powder bed 7 as the respective uppermost loose layer 8 by a displacement movement.

[0037] The relative movement of the electron beam with respect to the powder bed 7 can be achieved by deflection of the electron beam in the deflection device 5 or by providing a lift table.

[0038] Within the powder bed there is a base plate 10 on which a component 11 is built layer by layer.

[0039] The components produced by the method according to the invention and by the device according to the invention are used in particular in the aerospace industry as turbine blades, pump wheels and gear holders in helicopters, in the automotive industry as turbocharger wheels and wheel spokes, in medical technology as orthopedic implants and prostheses, as heat exchangers or in the manufacture of tools and moulds.

[0040] Powder materials according to the invention include all electrically conductive materials suitable for electron beam processing. Preferred examples are metal or ceramic materials, in particular titanium, copper, nickel, aluminum and alloys such as Ti-6Al-4V, AlSi10Mg and titanium aluminide (TiAl), which contain titanium, 6% aluminum and 4% vanadium by weight.

[0041] Other examples of materials according to the invention are nickel-based alloys, such as, for example, NiCr19NbMo, iron and iron alloys, steels, in particular tool steels and stainless steels, copper and its alloys, refractory metals, in particular niobium, molybdenum, tungsten and their alloys, precious metals, in particular gold, magnesium and their alloys, cobalt-based alloys, such as, for example, CoCrMo, high entropy alloys and shape memory alloys, for example, AlCoCrFeNi and CoCrFeNiTi.

[0042] Preferably, the powder material has an average particle size D50 of 10 μm to 150 μm.

[0043] According to the invention, the acceleration voltage in the preheating and / or melting step is 90 kV or more. The effect observed is based on the formula for calculating the power P=U acc I, where U acc represents the accelerating voltage, and I represents the beam current.

[0044] Thus, for a given beam current, a higher acceleration voltage inputs more energy and reaches a given temperature more quickly, thereby shortening the process time, or allowing a higher process temperature to be achieved for the same exposure time.

[0045] If the input energy does not change, i.e., the beam current decreases inversely proportional to the accelerating voltage, a significant improvement in the process stability is observed. The number of charge carriers is P = U acc ·I decreases approximately inversely proportional to the increase in accelerating voltage. The smaller the number of charged charge carriers, the less electrostatic powder is released for the same exposure time.

[0046] Increasing the accelerating voltage causes the electrons to penetrate deeper into the powder material. Factors that affect the maximum penetration depth of electrons into a material include accelerating voltage as well as material properties such as density, atomic weight, and atomic number. The penetration depth in titanium is known to be up to about 15 μm at 60 kV, up to about 30 μm at 90 kV, and up to about 70 μm at 150 kV.

[0047] By increasing the acceleration voltage, the input energy is distributed over a larger volume in the powder bed, which results in a reduced tendency to form local temperature peaks and therefore melting already during the preheating step, which improves the quality of the residual powder and effectively increases the recycling rate of the material.

[0048] The maximum beam power of 100 kW takes into account the fact that in the interaction volume between the electrons and the powder material to be processed, the material can be brought to a molten state with typical beam parameters without undesirable effects such as evaporation of the material, which can result from overheating. The calculation is based on the material-specific energy required for heating and melting, and the interaction volume, which is a function of the accelerating voltage and the area exposed to the electron beam.

[0049] [Process of increasing accelerating voltage by preheating] In the method according to the invention, first a top loose layer 8 of powder material is applied onto a substrate by a powder application device 9. Depending on the stage of the process, the base plate 10 or the powder bed 7 and, in later process stages, the component 11 can be considered as the substrate.

[0050] In one embodiment of the method according to the invention, a preheating step is carried out. For this purpose, the uppermost loose layer 8 is irradiated by the electron beam 4. The acceleration voltage of the electron beam 4 is at least 90 kV. In a preferred embodiment of the method according to the invention, the acceleration voltage is between 90 and 150 kV, in particular an acceleration voltage of 100 kV to 120 kV is preferred.

[0051] The beam parameters are selected according to the quality of the powder material. Typically, the beam current is adjusted between a minimum of 100 W and a maximum of 100 kW. The scanning speed is between 1 m / s and 1000 m / s.

[0052] During the pre-heating process, the loose layers 8 are bonded together at the particle surface by a diffusion process, which leads to a reduction in the contact resistance between the individual powder particles 12 within the layers 8 and, consequently, a higher electrical conductivity at the surface of the powder bed, thereby allowing the charge injected by the electron beam to escape better and avoiding electrostatic powder ejection.

[0053] Then comes the melting step, in which powder particles 12 are melted by the electron gun 3 at the points of the prepared powder bed 7 or its uppermost loose layer 8, which are determined according to the 3D structure to be produced, thereby forming a solid bond.

[0054] The above steps are repeated for each layer until the 3D structure is completed.

[0055] [Process of increasing accelerating voltage without preheating] The present invention further relates to a method for processing powder materials without an additional pre-heating step.

[0056] In the method according to the invention, a top loose layer 8 of powder material is applied onto a substrate by a powder application device 9. Depending on the process stage, the base plate 10 or the powder bed 7, and in later process stages the component 11 can be considered as the substrate.

[0057] A solid bond is then formed by melting with the electron gun 3 powder particles 12 of a prepared powder bed 7 or its uppermost loose layer 8, which is determined according to the 3D structure to be formed.

[0058] The acceleration voltage of the electron beam 4 is at least 90 kV. In a preferred embodiment of the method according to the invention, the acceleration voltage is between 90 and 150 kV. Preferred examples are acceleration voltages of 100 kV and 120 kV.

[0059] The steps described above are repeated layer by layer until the 3D structure is completed.

[0060] In summary, the idea of ​​the present invention is therefore presented in a method for additive manufacturing using an electron beam, in which an acceleration voltage of 90 kV to 160 kV, in particular 100 kV or more, preferably 120 kV or more, even more preferably higher than 120 kV, even more preferably between 135 kV and 160 kV is used during preheating and / or melting. [Configuration 1] A method for processing a powder material (12), comprising the steps of: a1) a device for receiving a powder bed (7) of the powder material (12) to be processed, and providing an electron beam apparatus (1) having an electron beam generator (3) configured to direct an electron beam (4) to different positions laterally across the powder bed (7); b1) applying a powder layer (8); c1) preheating the powder material (12) of the powder layer (8) by the electron beam (4); The step includes: In step c1), the electron beam (4) is driven by an acceleration voltage of 90 kV or more. A method for processing a powder material. [Configuration 2] d1) at least a portion of the powder layer (8) is melted by the electron beam (4). [Configuration 3] 1. A method for processing a powder material, comprising the steps of: a2) a device (6) for receiving a powder bed (7) of powder material (12) to be processed, and providing an electron beam apparatus (1) having an electron beam generator (3) configured to direct an electron beam (4) to different positions laterally across the powder bed (7); b2) applying a powder layer (8); c2) melting at least a portion of the powder layer (8) by the electron beam (4); The step includes: In step c2), the electron beam (4) is driven by an acceleration voltage of 90 kV or more, and Between steps b2) and c2), no preheating of the powder layer (8) is performed; A method for processing a powder material. [Configuration 4] 4. The method of any one of configurations 1 to 3, wherein in step c) the electron beam is driven at an accelerating voltage of 90 kV to 150 kV. [Configuration 5] 5. The method of any one of claims 1 to 4, wherein the powder material comprises titanium, copper, nickel, aluminum, and / or alloys thereof. [Configuration 6] 6. The method of any one of claims 1 to 5, wherein the electron beam is driven by an accelerating voltage of 100 kV or more. [Configuration 7] 7. The method of any one of claims 1 to 6, wherein the electron beam is driven by an accelerating voltage of 120 kV or more. [Configuration 8] 8. The method of any one of claims 1 to 7, wherein the electron beam is driven by an accelerating voltage of greater than 120 kV. [Configuration 9] 9. The method of any one of the preceding claims, wherein the powder material has an average particle size D50 of 10 μm to 150 μm. [Configuration 10] 10. The method of any one of configurations 1 to 9, wherein the beam power of the electron beam (4) is at least 100 W and at most 100 kW. [Configuration 11] An electron beam apparatus (1) for processing a powder material (12), comprising: a) a device for accommodating a powder bed (7) of the powder material (12) to be processed; b) an electron beam generator (3) configured to direct an electron beam (4) laterally to different positions on the powder bed (7), c) the electron beam device (1) is designed to carry out the method according to any one of configurations 1 to 10; Electron beam device characterized by: [Explanation of symbols]

[0061] 1 Electron beam device 2 Vacuum housing 3. Electron gun 4. Electron Beam 5 Magneto-optical unit 6 Lift Table 7 Powder bed 8. Top loose layer 9 Powder Coating Equipment 10 Base plate 11 Components 12 Powder particles, powder materials

Claims

1. A method for processing a powder material (12), comprising the steps of: a1) a device for receiving a powder bed (7) of the powder material (12) to be processed, and providing an electron beam device (1) having an electron beam generator (3) configured to direct an electron beam (4) laterally to different positions across the powder bed (7); b1) applying a powder layer (8); c1) preheating the powder material (12) of the powder layer (8) by the electron beam (4); d1) at least a portion of the powder layer (8) is melted by the electron beam (4); The step includes: In step c1), the electron beam (4) is driven by an acceleration voltage of more than 120 kV. A method for processing a powder material.

2. 1. A method for processing a powder material, comprising the steps of: a2) a device (6) for receiving a powder bed (7) of powder material (12) to be processed, and providing an electron beam device (1) having an electron beam generator (3) configured to direct an electron beam (4) laterally to different positions across the powder bed (7); b2) applying a powder layer (8); c2) melting at least a portion of the powder layer (8) by the electron beam (4); The step includes: In step c2), the electron beam (4) is driven by an acceleration voltage of more than 120 kV, and Between steps b2) and c2), no pre-heating of the powder layer (8) is performed; A method for processing a powder material.

3. The method of claim 1 or 2, wherein the electron beam is driven at an accelerating voltage of 135 kV to 150 kV.

4. 4. The method of claim 1, wherein the powder material comprises titanium, copper, nickel, aluminum, and / or alloys thereof.

5. 5. The method according to any one of claims 1 to 4, wherein the beam power of the electron beam (4) is at least 100 W and at most 100 kW.

6. An electron beam device (1) for processing a powder material (12), comprising: a) a device for receiving a powder bed (7) of said powder material (12) to be processed; b) an electron beam generator (3) configured to direct an electron beam (4) laterally to different positions on the powder bed (7), The electron beam device (1) is designed to carry out the method according to any one of claims 1 to 5. Electron beam device characterized by:

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