Method and system for processing powder materials for additive manufacturing of workpieces

Abstract

A method for processing powder material for additive manufacturing of workpiece (22) includes the following steps: [Solution] a) A device (15) for receiving a powder bed (20) made of powder material to be processed, and a beam generator (12) adjusted to direct an energy beam (13) to different positions laterally on the powder bed (20). b) A step of coating the powder material in layers onto the powder bed (20). c) A step of irradiating surfaces (30, 30a, 30b, 30c) within the powder bed (20) with an energy beam (13), wherein the surfaces (30, 30a, 30b, 30c) consist of a predetermined number n of points P1···Pn arranged in two dimensions that are sequentially irradiated. To improve the scanning strategy in step c), at least once during the irradiation of the surfaces, two sequentially irradiated points Pi, Pi+1 are spaced apart from each other, so that at least one other irradiated point P1···Pi-1, Pi+2···Pn in each dimension is located between the two sequentially irradiated points Pi, Pi+1.

Description

Technical Field

[0001] The present invention relates to a method for processing powder materials for additive manufacturing of workpieces. In particular, the present invention relates to a method for preheating powder materials and a method for melting powder materials.

[0002] Furthermore, the present invention relates to a system for implementing such a method for processing powder materials.

Background Art

[0003] Additive manufacturing methods such as 3D printing are characterized by joining volume elements to each other to form a three-dimensional workpiece, particularly by layer-by-layer shaping. In this case, in particular, a method is used in which the powder material in the powder bed is selectively fused by an energy beam, point by point and layer by layer, of the individual powder particles of the material and bonded into a strong three-dimensional structure. Solidification of the material can be carried out by sintering, i.e., partial melting or complete melting of the powder particles by a laser beam or an electron beam, followed by solidification. Hereinafter, the term "melting" should be understood to mean both of these.

[0004] When powder materials, particularly metal powders, are processed by selective melting using an electron beam (selective electron beam melting; SEBM) or selective melting using a laser (selective laser melting; SLM), complex shapes and structures can be manufactured with higher speed, precision operation, and high automation.

[0005] However, complex shapes are also a major challenge in process management, which may only be partially addressed by known scanning strategies for guiding the energy beam laterally on the powder bed.

[0006] Conventional scanning strategies are based on guiding the energy beam along a parallel path. In this case, asymmetry in the workpiece shape, such as corners and tapers, is not energetically considered. This can lead to the accumulation of heat and / or charged particles in specific areas of the workpiece, resulting in unexpected changes in process conditions and material properties, potentially compromising the quality of the manufactured workpiece. This is because, in both laser beam and electron beam methods, the large temperature gradient during manufacturing causes distortion and residual stress in the finished product. On the other hand, monitoring process conditions and material properties would result in frequent process interruptions.

[0007] In the case of selective electron beam melting (SEBM), there are yet another problem. When powder materials are processed with an electron beam, for example, metal powder particles are often surrounded by an oxide layer with low conductivity, so the irradiated powder bed becomes electrostatically charged in a spatially and temporally limited manner by the colliding electrons. Therefore, even if the metal powder particles are conductive internally, they can become charged when struck by an electron beam.

[0008] The charge can reach supercritical levels, accelerating stationary powder particles in the electron beam collision region out of the processing region, meaning they may be distributed from the powder bed to other areas of the electron beam system before the fusion process begins. As a result, the material is ejected from the powder bed before it reaches a sufficient degree of sintering, leading to material loss and process interruption.

[0009] To avoid abnormalities and material loss due to powder discharge, conventional techniques have shown that after a layer of powder material is applied, it is preheated by irradiating it along a parallel path with less energy, particularly less electron flow, to bond individual powder particles with a smaller adhesive force than that of the final product.

[0010] In the second step, the melting step, the powder particles are selectively melted laterally by an electron beam in the contour layer of the resulting three-dimensional structure, thereby forming sufficient three-dimensional structural stability between individual powder particles for the subsequent use of the workpiece.

[0011] In the melting step, regardless of whether a laser beam or an electron beam is used, the energy beams are directed along parallel paths to the top powder layer to form a melting bath, which then moves linearly together according to the irradiation pattern.

[0012] Many of the drawbacks of known methods stem from non-uniform temperature fields, which are also due to the scanning strategy. [Overview of the Initiative] [Problems that the invention aims to solve]

[0013] Therefore, an object of the present invention is to provide a method for processing powder materials for additive manufacturing of workpieces, which is improved with respect to the scanning strategy. Preferably, this can also reduce the non-uniformity of the workpiece. An object of the present invention is to provide a corresponding system for processing powder materials. [Means for solving the problem]

[0014] The above problem relates to a method for processing powder materials for additive manufacturing of workpieces, a) A device for receiving a powder bed consisting of powder material to be processed, A beam generator adjusted to direct energy beams to different positions laterally across the powder bed, The steps to provide, b) A step of applying the powder material in layers to the powder bed, c) A step in which an energy beam is irradiated onto a surface in the powder bed, and the surface consists of a predetermined number n points P1···Pn arranged in two dimensions that are sequentially irradiated, In a method including, d) This is solved by a method characterized in that, during the illumination of the plane, two sequentially illuminated points Pi and Pi+1 are spaced apart from each other at least once, and at least one other illuminated point P1···Pn in each dimension is located between the two sequentially illuminated points Pi and Pi+1.

[0015] The inventors have observed that in known powder processing processes, energy beams are typically directed along parallel paths toward the powder surface (see Figure 2a). Furthermore, they have observed that the energy thus input is strongly concentrated around the area being processed. By irradiating a single point on the powder surface, energy is transferred to neighboring areas by heat conduction. However, since so many processes are carried out in a vacuum and under negative pressure, heat dissipation is mainly limited to the powder. In contrast, with linear irradiation, additional energy is introduced into already heated areas, where heat conduction in the molten bath becomes more effective, thus forming localized hot spots. Therefore, known methods often operate with little to no energy input to avoid effects such as powder ejection and uncontrolled melting, resulting in longer processing times.

[0016] The method according to the present invention solves the problem of the non-uniform temperature distribution described above in such a way that energy is delivered in a point-like distribution, thereby avoiding local heat and charge accumulation. To this end, the points to be irradiated in the plane are irradiated sequentially, so that points on the grid of irradiated points that are not directly adjacent are irradiated sequentially at least once, preferably 20 or more times. The points on the surface are not directly and sequentially irradiated column by column and row by row within their grid. Instead, the points on the grid are initially omitted in both dimensions and then irradiated during the subsequent irradiation scanning process.

[0017] The interval between the two sequentially illuminated points described above ensures that at least one other illuminated point P1···Pn lies between the two sequentially illuminated points Pi and Pi+1 at least once in both dimensions, although this interval is considered an absolute lower limit compared to conventional scanning strategies. The advantage of this method is, of course, that it becomes more effective the larger the interval between two sequentially illuminated points within a plane is selected.

[0018] Therefore, preferably, step d) includes at least 10 times, preferably 20 times, and more preferably 50 times, that at least 1 point, preferably at least 5 points, more preferably at least 10 points, and even more preferably at least 20 points are located between two points Pi and Pi+1 that are sequentially irradiated in both dimensions of a grid formed by all points in the irradiated surface. In this case, the number of points to be initially omitted from irradiation may differ in both dimensions of the grid, for example, at least 5 points in the x direction and at least 10 points in the y direction.

[0019] This method allows for more uniform heating of the irradiated surface. This is because the introduced thermal energy, distributed in a point-like manner, spends more time distributed around each irradiated point, preventing localized excess energy from being injected through direct, adjacent irradiation.

[0020] In this invention, a point or point-like area is considered to be a location irradiated by an energy beam without being actively moved by a deflection coil, coordinate table movement, or similar device.

[0021] Within the irradiated surface of the powder bed, a finite number of possible points are determined. A suitable boundary condition for the distribution of irradiated points is, for example, that each point on the surface is irradiated at least once or exactly once.

[0022] Preferably, at least 10%, preferably at least 30%, more preferably at least 60% of the distance between two sequentially irradiated points Pi and Pi+1 is different from the distance between the subsequently sequentially irradiated points Pi+1 and Pi+2.

[0023] Among the three sequentially irradiated points Pi, Pi+1, and Pi+2, if the distances between each two points are different from each other, irregularities are introduced into the irradiation step, which additionally prevents the occurrence of a locally concentrated energy excess inside the irradiated surface. In this case, the distance may be different from the previous distance in only one dimension or in both dimensions. In particular, the amount of the next distance may be different from the previous distance by 10% or more, preferably 30% or more, more preferably 60% or more.

[0024] It goes without saying that for such dot-by-dot separated irradiation, a very diverse range of algorithms for selecting points can be considered. These algorithms can take into account the shape of the irradiated surface, but it is not necessarily required to do so.

[0025] However, preferably, the selection of the next point Pi+1 irradiated in step c) is made randomly, pseudo-randomly, or quasi-randomly.

[0026] By randomly, pseudo-randomly, or quasi-randomly selecting the points to be continuously irradiated from the entire set of irradiation target points in the plane (hereinafter also abbreviated as probabilistic irradiation), the above-mentioned intervals of at least some of the consecutive points can be automatically obtained. In this way, by irradiating the powder surface point by point and making the selection of the next point with a random component, it becomes possible to distribute the input energy more uniformly without following a specific distribution plan in advance.

[0027] The starting point of irradiation may be any point within the irradiated surface of the powder bed. In this case, a random element is involved in determining the next point to be irradiated. This random selection may be random in the classical sense, but it can also be pseudo-random or quasi-random using a random number generator or similar function.

[0028] Therefore, the criteria for selecting and ordering the points to be illuminated may be one of the known quasi-random or pseudo-random orders, such as the Mersenne Twister, permutation congruence generator, carry multiplication, Fibonacci generator, arithmetic random number generator, Well random number generator, XOR shift, block cipher or stream cipher, cryptographic hash function, van der Korpt sequence, additive recurrence relation, Hulton sequence, Hammersley set, Sobol sequence, Faule sequence, Nieder-Leiter sequence, Poisson disk sampling, and / or similar deterministic low-coincidence sequences.

[0029] Quasi-random point sequences also have the advantage that the density distribution of the continuously irradiated points is more uniform, and this unfolds more uniformly compared to random or pseudo-random sequences.

[0030] Preferably, step c) is part of a heating step in which the amount of energy input introduced into the powder bed by the energy beam is not sufficient to completely melt the powder material.

[0031] As already explained, heating steps that result in a temperature rise insufficient for a complete melting process are known from the prior art. Since such heating steps are usually used over a wider area and not limited to a localized area, the spaced irradiation strategy according to the present invention is particularly suitable for this purpose.

[0032] In one embodiment of the present invention, spaced irradiation with an energy beam is applied during the preheating step. This homogenizes the heat and electric field (in the case of SEBM) during the preheating step. Preheating with an electron beam, in particular, significantly improves process stability. Preheating with spaced irradiation prevents charge accumulation, and consequently suppresses the tendency for powder ejection due to static electricity.

[0033] The heating step may be a preheating step, an intermediate heating step, and / or a post-heating step.

[0034] In this specification, the preheating step is understood to be any process step in which a still powdery material is prepared for the original melting process, which uses a higher energy input, with less energy input (shorter irradiation time at one point or less radiant energy), in particular by the energy input of the preheating step, so that the powdery material does not yet solidify into the final workpiece.

[0035] If necessary, after the preheating step, once a portion of the surface has already melted, it may be necessary to reheat the powder bed temperature to further melt a portion of the surface for the duration required for the melting step. This should be understood as an intermediate heating step.

[0036] The post-heating step should be understood as all process steps that apply controlled tempering to the actual workpiece by applying energy to specific surface areas or the entire surface of the layer as needed after solidification.

[0037] Preferably, the heating step includes a two-stage heating process.

[0038] The heating step may be a multi-stage process. In the first stage, energy is introduced to a large surface by an energy beam to achieve or maintain the desired molding temperature. In the second stage, sintering is performed according to the shape to locally sinter more strongly in the subsequent molten region or to mechanically support protrusions and structural parts on the unsolidified powder. Especially for large surfaces, a multi-stage heating process can prevent temperature fluctuations due to localized cooling. The second stage can be performed immediately after the first stage, in parallel with melting, or after melting and before applying a new powder bed. In particular, the planar preheating step can be performed by stochastic irradiation or by classical raster scanning along the path.

[0039] Localized preheating, particularly locally enhanced sintering, enables more advanced powder recycling and facilitates blasting and unpacking of the finished workpiece.

[0040] Preferably, step c) forms a molten bath, and preferably, the formed molten bath is prevented from being induced.

[0041] In another embodiment of the present invention, intermittent irradiation is used to melt the powder material. By controlling the heat field during melting, temperature peaks are avoided and localized alloy changes are prevented. By altering the microstructure, material properties can be improved or controlled as needed. The microstructure substantially influences the material properties of the workpiece and affects characteristic values ​​such as hardness, strength, and modulus. A smaller complete molten bath results in a faster solidification rate, which in turn leads to a wide variety of phase properties and / or finer microstructures.

[0042] Therefore, by controlling the energy input, pores, surface irregularities, and variations in workpiece characteristics can be avoided. Another advantage is that the randomized irradiation pattern is shape-independent. Probabilistic irradiation can evenly cover large and small cross-sections, avoiding non-uniformity due to changes in cross-sectional area.

[0043] Preferably, the formed molten bath is prevented from being induced.

[0044] Statistically, point-like melting creates an uninduced molten bath. That is, lateral movement of the center of melting does not occur. Because there is no lateral movement, material transport along the melting path is avoided, making it easier to control the fluid dynamics of the molten bath.

[0045] Single-stage or multi-stage heating processes using energy beams can precede or superimpose on the melting process. The latter can be achieved, for example, by rapidly alternating between point melting and planar preheating.

[0046] In another embodiment, there may be additional conditions for selecting the following points, for example, a condition that a certain radius centered on a previously irradiated point should not be irradiated. The scanning control algorithm may also take into account the amount of energy input at the points to be irradiated and the frequency at which a predetermined energy input should be performed. For example, it may be possible to generate strong irradiation at the periphery of the irradiated surface or irradiate non-uniformly based on an energy model.

[0047] If two sequentially irradiated points are placed too close together, the two resulting molten baths will combine, causing mass transport between them. This results in defects such as alloy changes, unevenness, and coarsening of the microstructure. Therefore, a minimum local distance can be defined between two sequentially irradiated points. Preferably, the two sequentially irradiated points Pi and Pi+1 are spaced apart from each other, and at least two other irradiated points P1···Pi-1, Pi+2···Pn in each dimension are located between the two sequentially irradiated points Pi and Pi+1.

[0048] Therefore, a minimum temporal interval centered on the irradiated point can also be defined. Depending on environmental and beam parameters, it takes several milliseconds for the molten bath to solidify completely or at least partially. During this time, points within the local minimum interval should not be irradiated. In other words, a condition is imposed that irradiated points that are subsequent in the irradiation sequence, or even further subsequent, must not be within the minimum interval of preceding points. In this case, the minimum temporal interval defines how many subsequent points this condition applies to.

[0049] Furthermore, the maximum spatial distance to the following points can be defined. A smaller beam jump width results in a more uniform molten bath and a more homogeneous surface structure. The maximum spatial distance can be achieved by superimposing functions or by dividing the irradiated surface into sub-surfaces or cells. These sub-surfaces or cells can be irradiated completely or in specific proportions, so that the points to be irradiated in any one sub-surface or cell are irradiated first according to the method of the present invention, and then another sub-surface or cell is irradiated.

[0050] In a preferred embodiment of the method according to the present invention, an energy-dependent function is superimposed on a random function.

[0051] Energy-dependent models allow for consideration of local conditions, the locations of points already irradiated in the preceding powder bed, and their residual energy, particularly energy and / or temperature. This enables control of energy input in response to demand and better tuning of the microstructure throughout the workpiece. In addition, energy-dependent statistical irradiation allows for variable tuning of the microstructure inside the workpiece.

[0052] To achieve demand-driven control of energy input, a model of the workpiece to be manufactured is created, allowing for querying of energy states in spatial and temporal contexts. This model is used to identify the regions where energy should be supplied. Then, precisely tuned energy amounts are randomly introduced into these specific regions by an energy beam.

[0053] This energy model includes data specifically on the electric field, thermal field, workpiece shape, installation space shape, and / or the material composition of the powder bed, gas space, and pressure within the process chamber. The position of preceding points in the layer being manufactured, the spacing between them considering expected heat transfer, and the thickness of the powder material layer can be taken into account. Energy effects from the previous position can also be considered to calculate heat transfer from the molten region at the previous position to the new powder layer.

[0054] In addition to the location of the melting point, beam parameters can also be adjusted, particularly the holding time, lens current, and / or beam intensity. These parameters can also be variably adjusted on a point-by-point basis, for example, by a lamp.

[0055] By adjusting energy input and demand-oriented control, the combined effects of heat and matter can be effectively utilized. Significant material loss due to temperature peaks and evaporation can be avoided, and the original chemical composition can be maintained.

[0056] Preferably, the selection of the next irradiation point is made randomly, pseudo-randomly, or semi-randomly, and depends on the energy input, particularly the energy balance or thermal balance.

[0057] The order of the points to be irradiated and / or the corresponding beam parameters can be calculated and determined before the start of fabrication, or they can be determined point by point during fabrication, in particular, before each new powder bed to be irradiated, or while the current point is being irradiated. This can include data measured in real time.

[0058] Preferably, the path between the two irradiated points Pi and Pi+1 is exposed.

[0059] To approach the calculated irradiation points, the beam requires a certain amount of time to traverse the path between the points. This time varies depending on the beam technology used. Electron beams can be deflected on the order of 1° / μs, while lasers take much longer due to the inertia of the deflection mirrors. The method according to the present invention can also expose the path between the irradiation points. Generally, a continuously fed energy beam is preferred over a pulsed energy beam because it creates a static state.

[0060] The time between points can be pre-set, or made as short as possible depending on the length of the path that must be traversed, or a combination of these can be selected. However, the time each point on the path is illuminated is much shorter than the time at which the beam is held, which is determined randomly. As a result, the energy input at the held points becomes significantly larger. For this reason, we also say that the path is exposed, only to emphasize the difference between points on a surface that are intentionally illuminated and intermediate spaces that are only traversed for a short time.

[0061] The length of the paths between points can be made as short as possible, or can be freely selected within a certain time, and / or can have a specific geometric shape, in particular, an arc. The length of the paths and / or time can be independently selected for each point, for example, by alternately selecting the shortest path and an arc-shaped path. To avoid energy accumulation in the center of the irradiated surface of the uppermost powder layer by multiple intersecting paths, the paths between points can be adapted so that the same amount of energy is introduced to the periphery of the cross-section.

[0062] Preferably, the energy beam is an electron beam.

[0063] In a preferred embodiment of the present invention, the powder material is processed under vacuum or negative pressure, and the process does not use auxiliary gases. Preferably, no additional gases, such as helium, are introduced into the process chamber. The homogeneous thermal and electric fields formed in the method according to the present invention eliminate the need to introduce gases to further stabilize the process. This avoids the disadvantages associated with auxiliary gases in the process chamber, such as beam expansion, additional costs during the system and operation, and additional contamination.

[0064] Preferably, the acceleration voltage in the method according to the present invention is 90kV to 150kV, and more preferably 100kV or higher, and more preferably 120kV or higher.

[0065] Preferably, the beam output is at least 100W and at most 100kW.

[0066] Preferably, the powder material includes titanium, copper, nickel, aluminum, and / or alloys thereof, in particular, an alloy Ti-6Al-4V consisting of titanium, 6% by weight of aluminum, and 4% by weight of vanadium.

[0067] Preferably, the powder material has an average particle size D50 of 10 μm to 150 μm.

[0068] With respect to a system for processing powder materials with an electron beam apparatus, the system according to the present invention comprises a device for receiving a powder bed consisting of the powder material to be processed, and a beam generator adjusted to direct an energy beam to different positions laterally across the powder bed, and the system is designed to carry out the method according to the present invention.

[0069] Workpieces manufactured using the method and system according to the present invention are used particularly in the aerospace industry as turbine blades, pump wheels and helicopter transmission mounts; in the automotive industry as turbocharger wheels and wheel spokes; in medical technology as orthopedic implants and artificial joints, as heat exchangers, and in tool and mold manufacturing.

[0070] The embodiments of the present invention will be described in more detail below with reference to the drawings. [Brief explanation of the drawing]

[0071] [Figure 1] Figure 1 is a schematic diagram of the system according to the present invention, which includes a powder container. [Figure 2] Figure 2 shows schematic diagrams of different irradiation strategies. [Figure 3] Figure 3 is a schematic diagram for creating a randomized sequence of points. [Figure 4] Figure 4 is a schematic diagram of the preheating step by stochastic irradiation. [Figure 5] Figure 5 is a schematic diagram of the melting step by stochastic irradiation. [Figure 6] Figure 6 is a schematic diagram of multi-stage preheating by stochastic irradiation. [Figure 7] Figure 7 is a schematic diagram of stochastic irradiation, in which the energy input is intentionally increased or decreased in a specific region. [Figure 8] Figure 8 is a schematic diagram of irradiation in which the irradiated surface is divided into smaller cells. [Figure 9] Figure 9 is a schematic diagram of irradiation in which the cell shown in Figure 8 is further divided into subcells. [Modes for carrying out the invention]

[0072] Figure 1 shows an electron beam system 10 having a process chamber 11 in which an electron beam generator 12 for generating an electron beam 13 is located.

[0073] In this embodiment, an optional deflection device 14, for example, an electron beam generator 12 equipped with a magneto-optical unit, is positioned above a lifting table 15 equipped with a lifting plate and a receiving frame. The receiving frame functions as a spatially limited powder container that receives a powder bed 20 consisting of the powder material to be processed.

[0074] Above the receiving frame, a powder coating device 16 having a squeegee (not shown) is positioned and can move on a lifting table. The powder coating device 16 has a container (not shown) for powder material, and from here, by moving, the material can be applied evenly to the powder bed 21 as a non-solidified top layer 21.

[0075] Moving the electron beam relative to the powder bed 20 can be achieved by deflecting the electron beam within the deflection device 14 or by moving the lifting table.

[0076] Furthermore, a base plate 17 is located within the powder bed 20, on which the workpieces 22 are formed layer by layer.

[0077] The control unit 23 is connected to the main components of the electron beam system 10, particularly the electron beam generator 12 and the magneto-optical unit 14, via one or more signal transmission lines in order to control the entire manufacturing process.

[0078] Another system according to the present invention includes a laser beam system in a vacuum, in air, or under overpressure, as well as a system using an auxiliary gas.

[0079] Figure 2 shows different strategies for irradiating the powder material in the powder bed 20.

[0080] Figure 2a shows a conventional irradiation strategy, where the beam is scanned linearly within the irradiation surface 30. That is, in the melting step, the beam, and consequently the melting bath, is guided along the parallel path 31 schematically shown in Figure 2a. In this case, the material is transported along the path, and energy is input at a very high density. The accumulation of heat due to locally concentrated energy input leads to drawbacks such as powder discharge and the resulting process interruption, as well as defects in the workpiece due to uneven energy input, both in the preheating and melting steps.

[0081] Figures 2b, 2c, 2d, and 2e illustrate the irradiation strategy according to the present invention.

[0082] Figure 2b shows a probabilistically distributed point irradiation. The position of the next irradiated point is randomly selected and can be any point on the predefined irradiation surface. The energy beam irradiates the defined point for a certain period of time, and then jumps to irradiate the next point.

[0083] In another embodiment, there may be additional conditions for selecting the following points, such as a certain radius (minimum interval) centered on previously irradiated points not being irradiated in the next step of the point sequence, or stronger irradiation occurring at the periphery of the surface based on the energy model.

[0084] Figures 2c, 2d, and 2e show embodiments of the present invention in which probabilistically distributed points are irradiated for a certain period of time, as shown in Figure 2b, and the paths between those points are also irradiated.

[0085] In Figure 2c, the shortest path between the two irradiation points is selected for this purpose. To achieve this, one point is irradiated with specific beam parameters for a certain period of time, and the beam is then directed to the next point to be irradiated. This can be done in the shortest possible time, i.e., within a time window of a few microseconds, within a predetermined time, or at a specific speed. The energy input varies depending on the variable speed and irradiation time, but the energy input at the irradiated point is significantly large.

[0086] In the embodiment shown in Figure 2d, an arc-shaped path is selected rather than the shortest path. This affects the energy distribution within the irradiated surface, in addition to the position of the holding point, opening up the possibility of placing the path in less irradiated areas, particularly at the periphery.

[0087] Figure 2e schematically illustrates an embodiment in which the path between points can be freely selected and determined only by time and / or velocity. The path selection can similarly be done randomly or based on an energy model.

[0088] Figure 3 is a schematic diagram for creating a random sequence of points according to one embodiment of the present invention.

[0089] The irradiated surface 30 is transformed into a set of points P1 to P9 by a discretization algorithm. The set of points P1 to P9 contains all the points that need to be melted in order to melt the entire surface 30. In Figure 3, the set of points P1 to P9 is represented by a circular region. This set of points P1 to P9 is moved to point sequence A. Point sequence B is formed from point sequence A by substitution.

[0090] A={P1,P2,P3,P4,P5,P6,P7,P8,P9} B={P5,P3,P9,P6,P8,P1,P2,P7,P4}

[0091] The order of the points in point sequence B indicates the order in which the points are melted consecutively. As a result, the following time points occur for each point, as shown in the table below.

[0092] Point P1 P2 P3 P4 P5 P6 P7 P8 P9 Time t5 t3 t9 t6 t8 t1 t2 t7 t4

[0093] In particular, a random permutation from the first sequence of points A to the second sequence of points B can be achieved as follows. 1. A random number generator creates a random number X ∈ [0,1] between 0 and 1. 2. Multiply this random number X by the number of elements in the first column and round up. 3. The dot in the first column at the location of the multiplied number is added to the second column and removed from the first column. 4. Repeat steps 1-3 above until the first column is empty.

[0094] Particularly advantageous is that the scanning strategy according to the present invention can be used to guide the energy beam in the preheating step.

[0095] Figure 4 schematically shows the heating surface 30a, within which the powder bed is heated by energy input from an energy beam. The heating surface 30a can have any shape and size, but in this embodiment it is square and is entirely within the deflection field 40 of one energy beam. Inside the heating surface 30a, the energy beam is guided over holding points P1 to Pn in a probabilistically selected order. The energy beam moves to the first holding point P1 thus determined at time t, and remains there for a holding time Δt1 defined at this point. Subsequently, the energy beam is deflected to position P2 at a faster speed, preferably at maximum speed, and held there for a predetermined holding time Δt2. In a single heating step, this procedure is applied at least once for all holding points P1 to Pn.

[0096] Typically, holding points P1 to Pn are arranged on the heating surface 30a in a regular grid 41 with a grid spacing of 42, so as to completely fill it. The grid spacing between holding point Pi from the set {P1, P2, ..., Pn} and the points directly adjacent to it can be adjusted arbitrarily. Preferably, the grid spacing is within the diameter of the focused beam, and for a normally distributed beam intensity, the standard deviation is 0.5 to 2.

[0097] The beam parameters during the preheating step are selected to locally heat the powder bed, preferably sintering, without the energy beam transferring the material to the molten phase. Preferably, the energy beam is used in a defocused state during the preheating step. If the energy beam is an electron beam, the beam current is preferably 20 to 100 mA, depending on the powder material, beam diameter, and acceleration voltage, and the holding time at each holding point is 1 to 100 μs. In another embodiment, the beam current is 300 mA, the holding time Δt1 to Δtn varies between 0.1 and 10 μs, and a defocused beam is typically used for heating.

[0098] The heating surface, point holding time, and beam parameters can be changed for each layer of the additively manufactured workpiece.

[0099] Figure 5 shows an embodiment of probabilistic point irradiation of a surface 30c to be melted by an energy beam, preferably entirely within the deflection field 40 of a single energy beam. This melting surface 30c can be obtained, for example, from cross-sectional data of a 3D workpiece to be additively manufactured. Within the melting surface 30c, the energy beam is guided over a predetermined number of holding points P1 to Pn, so that the powder bed is locally melted at these locations, at least for a short time. Between the holding points, the energy beam is deflected at high speed, preferably at its maximum speed. Preferably, the energy beam is focused during the melting step. The holding points can be assigned to any position within the melting surface 30c, preferably located on a grid 41. To ensure that the melting surface 30c is completely melted, the grid 41 is exemplary regularly designed, having a constant grid width 42 and a normally distributed beam intensity with a standard deviation of 0.5 to 2.

[0100] In one embodiment of the melting step, the positions of the holding points P1 to Pn are precisely controlled exactly once in a probabilistically set order, and the energy beam resides at each holding point for a predetermined residence time Δt1 to Δtn. Preferably, the beam current of the electron beam is 5 mA to 50 mA, and the residence time is variable in the range of 1 to 100 μs.

[0101] In another embodiment of the present invention, the positions of the holding points P1 to Pn are controlled with low beam current and holding time, such as in the case of a probabilistically set sequence, at least once, preferably a one-time movement, and the beam current and / or holding time decrease as the number of repetitions increases.

[0102] The melting surface, point holding time, and beam parameters can be varied for each layer of the additively manufactured workpiece.

[0103] Figure 6 shows an embodiment of multi-stage preheating using probabilistic spot irradiation.

[0104] The preheating process consists of, for example, two preheating steps performed in direct succession. In the first preheating step, the first heating surface 30a is heated, as shown in Figure 6a, and in the second preheating step, the second heating surface 30b is heated, as shown in Figure 6b. In this case, it is important that the second heating surface 30b is completely contained within the first heating surface 30a.

[0105] In one embodiment, the first heating surface 30a extends across the entire deflection field 40 of a single energy beam. The second heating surface 30b is preferably larger by a defined distance than the workpiece cross-section 30c where the layer being manufactured is melted, taking into account the 3D shape of the workpiece being additively manufactured.

[0106] Energy input is performed by energy beams that are induced over holding points P11~P1n inside the first heating surface and over holding points P21~P2m inside the second heating surface, with the order of the holding points being set probabilistically.

[0107] In one embodiment, preheating of the first heating surface 30a is preferably performed with a strongly defocused beam at a high power input, and heating of the second heating surface 30b is preferably performed with a weakly defocused beam at a low power input.

[0108] In another embodiment of the present invention, at least two preheating steps are performed alternately with at least one process step that is not primarily used for preheating.

[0109] Figure 7 shows an example of preheating while controlling stochastic spot irradiation to achieve a homogeneous energy field. When the entire surface 30 is irradiated with a uniform grid using constant beam parameters, energy dissipates to the lower temperature outer region, resulting in a non-uniform temperature field within the heated surface. The temperature difference between the center and the vicinity of the periphery of the heated surface is typically around several tens to several hundreds of K.

[0110] Figure 7a shows an embodiment of the present invention in which the grid spacing 42 of the grid 41 that determines the position of the holding points is designed to be indirectly proportional to the temperature gradient. The holding points are controlled in a probabilistic order. The density of the grid 41 defines the local power input and, consequently, the temperature field, by the number of holding points per unit area.

[0111] Figure 7b schematically shows another embodiment in which local power input is set by beam parameters. In this case, the grid 41 has a constant grid width 42. The holding point in the low-temperature region shows at least one of the following changes compared to the holding point in the high-temperature region: (a) higher beam power, (b) longer holding time, or (c) more repetitions.

[0112] In another embodiment, the multiple embodiments described above can be superimposed by the energy model such that the total residence time of the region results in additional control of the temperature field.

[0113] In another embodiment, the irradiation sequence schematicly shown in Figures 7a and 7b can be used in the melting step with adapted beam parameters.

[0114] The above steps are repeated for each layer to complete the 3D structure.

[0115] Figure 8 shows another embodiment of the present invention in which the molten surface 30 is divided into cells 3. This division can consist of hexagons of the same type, as shown in Figure 8. However, the molten surface can also be divided into other geometric shapes that are different from each other (e.g., squares, circles, etc.), and their sizes may also be different from each other.

[0116] In the first step, the molten surface 30 is divided into 3 cells. Each cell is completely filled with points that will be irradiated during the process. The order in which the cells are irradiated may be random, pseudo-random, semi-random, or in a specific order, as may be the order in which the points are irradiated. As each point P1.n in the first cell C1 is irradiated, the beam jumps to the next cell Cn, irradiating all the points that are to be irradiated there.

[0117] In another embodiment, the process could be designed so that only a certain percentage of points within one cell are illuminated, and the beam jumps to one or more other cells, then returns to the first cell to illuminate the remaining points.

[0118] Repeat this procedure until all points in all cells are exposed.

[0119] Figure 9 shows the method according to the present invention, which is particularly suitable for preheating a powder bed.

[0120] The entire heating surface 30a is divided into cells F1 to Fn. These cells F1-Fn are then divided into subcells. Each cell is completely filled with points Fn.n.1-Fn.nn that are irradiated during the process. During preheating and / or postheating, the beam irradiates points Fn.n.1-Fn.nn randomly or in a specific order. After all points in subcell Fn.nn have been irradiated, the beam jumps to the next subcell Fn.n.n+1. In this case, it is possible to irradiate all subcells F1-n sequentially, or to irradiate all n subcells first. The order of cells and / or subcells may be random or sequential. [Explanation of Symbols]

[0121] P, P1, P2...Pn are the points that are irradiated. 10 Electron Beam Systems 11 Process Chamber 12 Electron beam generator 13 Electron beam 14. Magneto-optical unit 15 Height-adjustable table 16 Powder coating apparatus 17 Base Plate 20 powder bed 21 Top powder layer 22 Work 23 Control Unit 24 Signal transmission path 30 Irradiation surface 30a heating surface 30b Contour-fitting heating surface 30c molten surface 31 Straight path 32 Path between two points to be irradiated 40 Deflection field 41 grid 42 grid intervals 50. Points irradiated with a constant power input. 51. Points irradiated with variable power input.

Claims

1. A method for processing powder material for additive manufacturing of a workpiece (22), a) A device (15) for receiving a powder bed (20) consisting of the powder material to be processed, A beam generator (12) is adjusted to direct energy beams (13) to different positions laterally across the powder bed (20), The steps to provide, b) A step of applying the powder material in layers to the powder bed (20), c) A method comprising the step of irradiating a surface (30; 30a, 30b, 30c) in a powder bed (20) with an energy beam (13), wherein the surface (30, 30a, 30b, 30c) is composed of a predetermined number n of points P1...Pn arranged in two dimensions that are sequentially irradiated, A method characterized in that, during irradiation of the aforementioned surface (30, 30a, 30b, 30c), two points Pi and Pi+1 that are sequentially irradiated at least once are spaced apart from each other, and at least one other irradiated point P1...Pi-1, Pi+2...Pn in each dimension is located between the two sequentially irradiated points Pi and Pi+1.

2. The method according to claim 1, characterized in that at least 10%, preferably at least 30%, of the distance between two sequentially irradiated points Pi and Pi+1 is different from the distance between those points Pi+1 and Pi+2 that are subsequently irradiated.

3. The method according to claim 1 or 2, characterized in that the selection of the next point Pi+1 to be irradiated in step c) is performed randomly, pseudo-randomly, or quasi-randomly.

4. The method according to any one of claims 1 to 3, characterized in that step c) is part of a heating step in which the amount of energy introduced into the powder bed by the energy beam is not sufficient to completely melt the powder material.

5. The method according to claim 4, characterized in that the heating step includes a two-stage heating process.

6. The method according to any one of claims 1 to 5, characterized in that step c) forms a molten bath, preferably without induction of the formed molten bath.

7. The method according to any one of claims 1 to 6, wherein the selection of the next irradiated point is carried out randomly, pseudo-randomly, or quasi-randomly as a function of the energy input, particularly as an energy balance or thermal balance.

8. a) The order of the irradiated points P1...Pn is determined before the workpiece (22) is manufactured. or b) The order of the irradiated points P1...Pn is determined layer by layer during the fabrication of the workpiece (22). or c) While the current point Pi is being irradiated or after it has been irradiated, the calculation of the next points P1...Pn to be irradiated is performed. The method according to any one of claims 1 to 7, characterized in that

9. The method according to any one of claims 1 to 8, characterized in that the path between two irradiated points Pi and Pi+1 is exposed.

10. A system for processing powder material for additive manufacturing of a workpiece (22), a) A device (15) for receiving a powder bed (20) consisting of the powder material to be processed, b) A beam generator (12) adjusted to direct energy beams (13) to different positions laterally across the powder bed (20), It has, c) The system is designed to carry out the method described in any one of claims 1 to 9. A system characterized by the following features.

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