METHOD AND ELECTRON BLASTING PLANT FOR THE ADDITIVE MANUFACTURING OF A WORKPIECE

Description

1. Field of the invention

[0001] The invention relates to a method for the additive manufacturing of a workpiece.

[0002] The invention further relates to an electron beam system for the additive production of a workpiece from a powdered material. 2. Description of the state of the art

[0003] Additive manufacturing processes for workpieces are currently receiving considerable attention under the term 3D printing. It is already known to use a laser beam to selectively melt a powdered material, such as plastic granules, layer by layer in a material bed, so that a coherent workpiece is gradually created from the molten and then recurved plastic. At the end of the additive manufacturing process, this workpiece is freed from the remaining powdery material.

[0004] To monitor such a manufacturing process, it is also known from EP 2 032 345 B1 to use a camera which optically monitors the processing zone.

[0005] By recording individual slice images, image analysis can then determine whether the workpiece is progressing as desired, layer by layer. By combining the individual slice images, image processing can also create a tomographic image of the workpiece.

[0006] Furthermore, it is also known to use so-called electron beam systems, in which an electron beam is directed onto a material bed in a vacuum vessel, for the additive manufacturing of workpieces.

[0007] However, it has been found that the use of an optical monitoring method does not lead to satisfactory results.

[0008] EP 2 832 473 A1, EP 2 832 475 A2, and EP 2 832 474 A1 therefore disclose methods in which two electron beams are used simultaneously. A first melting electron beam is used for selective melting, while a second scanning electron beam is directed at the surface and scans it. The backscattered electrons generated during this scanning are captured to create an image of the surface.

[0009] DE 10 2010 011 059 A1 and CN 206 794 757 U disclose similar scanning processes using a second electron beam. SUMMARY OF THE INVENTION

[0010] The object of the invention is therefore to provide an improved method.

[0011] The object of the invention is also to provide a corresponding electron beam system.

[0012] According to the invention, this object is achieved by a method according to claim 1.

[0013] When additively manufacturing workpieces using electron beam systems, different materials are typically used than those used with laser beam systems. For example, metals or metal alloys are often used in electron beam systems.

[0014] The inventor recognized that the well-known approach of using a camera is not applicable for melting using electron beams. This is because the material to be melted in the powder bed often has to be preheated during electron beam melting to bring the powder close to or even close to its melting temperature. Otherwise, a powdered material would be "blown" out of the powder bed due to electrostatic repulsion and the vacuum-controlled process.

[0015] The powdered material is therefore already red-hot and would visually be barely distinguishable from a white-hot melting point, or would overwhelm the dynamics of optical detection. Furthermore, the workpiece cools only slowly in a vacuum, so the boundary between the workpiece and the powdered material would be barely detectable during optical detection without longer waiting times.

[0016] The inventor has further recognized that waiting for the workpiece to cool down for optical detection is not applicable, since undesirable structural changes would then occur in the workpiece.

[0017] The inventor has therefore started using backscattered electrons to monitor the additive manufacturing process.

[0018] In contrast to the optical acquisition of an entire slice with a single image, the backscattered electrons are scattered back only at one point, the point of impact of the electron beam. Therefore, the method according to the invention only produces an image comparable to that obtained with optical acquisition through a corresponding scanning movement.

[0019] In particular, the intensity of the backscattered electrons, ie the backscattered electron current, can be evaluated.

[0020] Overall, the method according to the invention allows better control of the entire additive manufacturing process when using an electron beam for locally selective melting.

[0021] Preferably, the density of the workpiece can be determined by evaluation.

[0022] The backscattering behavior of electrons is strongly determined by the density of the material at the respective point of impact of the electron beam. Therefore, the analysis of the detected backscattered electrons is particularly suitable for determining the density of the material. This is a particular advantage over optical detection using lasers in manufacturing processes, as it allows the internal structure of the workpiece material to be determined.

[0023] Preferably, a layer image of the workpiece can be generated by evaluating the backscattered electrons.

[0024] By capturing backscattered electrons layer by layer during the additive manufacturing process, a type of tomographic image of the resulting workpiece can be generated. Since the backscattering behavior of electrons depends primarily on the density of the material, this image is particularly useful for revealing the density distribution within the workpiece.

[0025] Preferably, the evaluation can determine a change in the composition of the material.

[0026] If the workpiece material is composed of different components, such as an alloy, the change in the alloy can be determined in this way, provided this change affects the backscattering behavior of the backscattered electrons. For example, an alloy in the original, powdered material can have a first density, while the molten and resolidified alloy in the workpiece can have a second density. Changes in the composition of the powder bed can also be detected and used, for example, to control the temperature during preheating. In particular, the nitrogen depletion of a material due to the melting process can also be measured.

[0027] Preferably, the microporosity of the workpiece can be determined by evaluating the backscattered electrons.

[0028] By appropriately parameterizing the electron beam and / or controlling the temperature of the powder bed, a microporous workpiece can be produced in which the powder particles are only partially fused together. The method according to the invention can be used to monitor the microporosity. Independently of this, however, microporosity can also be used to monitor a complete melting process in order to achieve the lowest possible microporosity of the workpiece.

[0029] Preferably, the surface quality of the component can be determined by evaluation.

[0030] Since the backscattered electrons react strongly to the density of the material, the analysis is particularly good at detecting the transition from the less dense powder material to the molten, and thus denser, material of the workpiece. In particular, this allows for the detection of sinter necks that extend from one surface of the workpiece into the adjacent hot powder. This is crucial, for example, for controlling the temperature of the powder bed to achieve a smooth surface.

[0031] The additive manufacturing process using electron beams has the disadvantage of inferior surface quality due to preheating, especially compared to the laser process, which uses cold powder. However, the process according to the invention can produce a workpiece with a higher-quality surface, while at the same time exhibiting less distortion than a workpiece produced using a laser process.

[0032] According to the invention, the backscattered electrons are detected during melting. Since electrons are also backscattered by the electron beam during the melting process, detection and analysis can, in principle, also take place simultaneously at the respective melting point. In particular, the density can be measured continuously at the respective point during the melting process.

[0033] However, acquisition and analysis can also be performed, for example, with a reduced electron beam intensity after the actual melting process. In this case, a scanning movement can also be performed, for example, to generate a complete layer image.

[0034] Preferably, the result of the evaluation can be used for process control. In general, the entire additive manufacturing process can be controlled based on the evaluation of the backscattered electrons, in particular the evaluations mentioned above. Controllable process parameters include, for example, temperature control for preheating the powder, the electron beam intensity, and / or the speed at which the electron beam is moved.

[0035] According to the invention, the backscattered electrons are evaluated in a direction-dependent manner using several backscattered electron detectors arranged at different locations.

[0036] For this purpose, at least one of the detectors can be positioned laterally, for example, between 30° and 60° relative to the vertical backscatter direction, in order to detect the backscatter cone direction-dependently. Furthermore, a typically single-piece backscattered electron detector can be divided into several sectors. This allows shadow topographies to be generated, which can be evaluated for further process control.

[0037] This direction-dependent measurement of backscattered electrons can also be used in electron beam processing in general, independent of the additive manufacturing process according to the invention.

[0038] With regard to the electron beam system, the object is achieved by an electron beam system according to claim 8.

[0039] Such an electron beam system can perform the processes described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In the following, exemplary embodiments of the invention are explained in more detail with reference to the drawings. In these drawings: Figure 1 shows a perspective view of an electron beam system according to the invention for the additive production of a workpiece with a backscattered electron beam detector and an evaluation unit; Figure 2 shows a sectional view of the electron beam system from Figure 2 Figure 3 shows a schematic representation of the sequence of a method according to the invention for the additive production of a workpiece with a step for detecting and evaluating the backscattered electrons. DESCRIPTION OF PREFERRED EMBODIMENTS

[0041] Figure 1 shows an electron beam system 10 with a vacuum housing 12 in which an electron beam gun 14 is arranged for generating an electron beam 16.

[0042] In the present embodiment, the electron beam gun 14 is arranged above a lifting table 18 with an electrically conductive lifting plate 20 and with a receiving frame 22, which serves as a spatially limited powder container which receives a powder bed 24 made of a powdery material to be processed.

[0043] If the lifting plate 20 is moved downwards, the powder bed 24 can gradually occupy an increasing area, so that the powder bed 24 is enlarged layer by layer.

[0044] For this purpose, a powder application device 26 with a doctor blade 28 is arranged above the receiving frame 22, which can be moved over the lifting table 18 by a displacement device 30. The powder application device 26 has a container (not shown) for the powdery material, from which the material can be doctored evenly onto the powder bed 24 as the uppermost loose layer 32 by means of a displacement movement.

[0045] About a Figure 2 The powder bed 24 can be heated by means of the heating element 34 at the upper edge of the receiving frame 22.

[0046] The reference number 80 in Figure 1 furthermore, a workpiece, represented by way of example as a star, is designated, which was gradually formed or "printed" by fusing individual particles in the powder bed 22 with the aid of the electron beam 16.

[0047] Out of Figure 2 It can be seen that the workpiece 80 is also more complexly structured in the perpendicular direction to the horizontal surface of the powder bed 22 with recesses and bulges.

[0048] In addition, smaller sinter necks 82 are shown on the workpiece 80 as examples, which protrude from the otherwise smooth surface of the workpiece 80.

[0049] Furthermore, a microporous region 84 of the workpiece 80 is shown in which only a partial fusion of the powder particles occurred.

[0050] The electron beam system 10 also has an evaluation and control unit 36, which is electrically connected to two backscattered electron detectors 38 and 40.

[0051] In the simplest case shown here, the backscattered electron detectors 38 and 40 are metal rings of different diameters arranged concentrically around the main axis of the electron beam 16. The backscattered electron detector 40 is divided into four sectors, each of which (not explicitly shown here) is individually electrically connected to the evaluation and control unit 36.

[0052] As from Figure 2 As can be seen, so-called backscattered electrons 42 can be detected with the aid of the backscattered electron detectors 36 and 38, which are scattered back from the powder bed 24 or the workpiece 80 due to the electron input by the electron beam 16.

[0053] The evaluation and control unit 36 ​​is then configured to determine various properties of the powdered material in the powder bed 24 or of the workpiece 80.

[0054] In addition, the evaluation and control unit 36 ​​can use this information to control the additive manufacturing process. For this purpose, the evaluation and control unit 36 ​​is, for example, electrically connected to the heating element 34 for heating the powder bed 24.

[0055] The electron beam system 10 according to the invention operates as Figure 3 can be seen as follows: First, in step A, the uppermost loose layer 32 of powdered material is applied to the powder bed 24 using the powder application device 26. Following the movement of the powder application device 26, excess powder is scraped off during the movement.

[0056] Subsequently or at the same time, according to step B, the powder bed 24 is heated with the aid of the heating element 34 to just below or at the melting temperature of the material.

[0057] Then, in step C, the electron beam 16 is generated with the electron beam gun 14, which then defines the 3D structure of the workpiece 80 to be created by locally selective melting of the powder particles and the subsequent solidification of the material. Locally selective melting means a selective melting of the powdered material at the point of impact of the electron beam 16. Depending on the scanning strategy, the workpiece is then created point by point, line by line, and layer by layer.

[0058] In step D, the electron beam 16 is then guided over the powder bed 24, in particular over the entire powder bed 24, and the backscattered electrons 42 are detected. For this purpose, the evaluation and control unit 36 ​​records and evaluates the current detected by the backscattered electron detectors 38 and 40.

[0059] This allows various properties of both the powder bed 24 and / or the workpiece 80 to be determined. This includes, in particular, the determination of the density of the material and / or the workpiece 80, the surface quality of the workpiece 80, the presence of sinter necks 82, the microporosity of the workpiece 80, the composition of the material, and / or other properties.

[0060] The steps A to D described above are repeated layer by layer until the 3D structure 80 is completed.

[0061] Since the backscattered electrons 42 are backscattered to different degrees in different directions, for example depending on the orientation of a surface of the workpiece 80, the evaluation and control unit 36 ​​can obtain additional information by appropriately evaluating the different currents, for example in the four sectors of the backscattered electron detector 40 or between the two detectors 38 and 40.

[0062] This allows various properties of both the powder bed 24 and / or the workpiece 80 to be determined. This includes, in particular, the determination of the density of the material and / or the workpiece 80, the surface quality of the workpiece 80, the presence of sinter necks 82, the microporosity of the workpiece 80, the composition of the material, and / or other properties.

[0063] The steps A to D described above are repeated layer by layer until the 3D structure 80 is completed.

[0064] Since the backscattered electrons 42 are backscattered to different degrees in different directions, for example depending on the orientation of a surface of the workpiece 80, the evaluation and control unit 36 ​​can obtain additional information by appropriately evaluating the different currents, for example in the four sectors of the backscattered electron detector 40 or between the two detectors 38 and 40.

Claims

1. Method for additive manufacturing of a workpiece (80) comprising the following steps: a) providing a powdery material in a powder bed (24) in an electron beam system (10) comprising a single electron beam generator (14) which is configured to direct an electron beam (16) to laterally different locations of the powder bed (24) in order to selectively melt the powdery material locally there; b) locally selective melting of the material in the powder bed (24) with the moved electron beam (16); c) detecting backscattered electrons (42), which are scattered back from the material or the workpiece (80), with a backscattered electron detector (38, 40); d) evaluating the backscattered electrons (42) with an evaluation unit (36); characterized in that e) the detection of the backscattered electrons (42), which are backscattered due to the electron input by the electron beam (16), takes place during melting and the evaluation of the backscattered electrons (42) takes place in a direction-dependent manner by means of a plurality of backscattered electron detectors (38; 40) arranged at different locations.

2. Method according to claim 1, characterized in that the density of the workpiece (80) and / or of the powdery material is determined by the evaluation of the backscattered electrons (42).

3. Method according to one of the preceding claims, characterized in that a layer image of the workpiece (80) and / or of the powdery material is generated by the evaluation of the backscattered electrons (42).

4. Method according to one of the preceding claims, characterized in that a change in the composition of the material is determined by the evaluation of the backscattered electrons (42).

5. Method according to one of the preceding claims, characterized in that the microporosity of the workpiece (80) is determined by the evaluation of the backscattered electrons (42).

6. Method according to one of the preceding claims, characterized in that the surface quality of the workpiece (80) is determined by the evaluation of the backscattered electrons (42).

7. Method according to one of the preceding claims, characterized in that the result of the evaluation is used for process control.

8. Electron beam system (10) for the additive manufacturing of a workpiece (80) from a powdery material with a) a container (20, 22) for receiving a powdered material in a powder bed (24), b) a single electron beam generator (14) which is set up to direct an electron beam (16) onto laterally different locations of the powder bed (24) in order to selectively melt the powder material locally there, characterized in that c) the electron beam system (10) has an evaluation unit (36) and a plurality of backscattered electron detectors (38, 40) arranged at different locations, which are configured to detect backscattered electrons (42), which are scattered back from the material or the workpiece (80) due to the electron input by the electron beam (16), during melting and to evaluate them direction-dependent by means of the backscattered electron detectors (38, 40).