DE102016209065A1 - Method and device for process monitoring in the additive manufacturing of components - Google Patents

Abstract

The present invention relates to a method and a device for process monitoring in the generative production of components by stratified solidification of a building material by means of energetic radiation. In the method, data of at least one area of the processing area used for solidifying the building material is recorded with a radiation-sensitive sensor arrangement before and / or after application of a new layer of the building material. The recording of the data takes place with at least one line sensor as the radiation-sensitive sensor arrangement which is moved over the processing surface. The method enables the recording of image data with high resolution and high measurement rates without interference from the process emission and without additional non-productive time.

Description

Technical application

The present invention relates to a method for process monitoring in the generative production of components by layerwise solidification of a building material by means of energetic radiation, in which with a radiation-sensitive sensor array before and / or after applying a new layer of building material on a processing surface data at least one of the Solidification of the building material used area of the processing surface are recorded. The invention also relates to a device for the additive production of components with which the method can be used.

Generative production with its almost unlimited geometrical degrees of freedom, the complete absence of forming tools and material utilization of almost 100% offers the possibility of manufacturing individual products in a resource- and energy-efficient manner. One of the most widely used methods of additive manufacturing of metal components (also ceramics) is the selective laser melting (SLM: Selective Laser Melting), in which the components are built up by layer-wise solidification of a powdered building material on a building platform.

The layered construction directly from CAD data enables SLM to produce highly complex components from metallic series materials without forming tools that can not be produced using conventional production techniques such as casting or machining. Since existing production-related restrictions are eliminated, completely new components with innovative functionalities can be realized.

The starting material for the SLM process is in powder form. It is applied in a closed process chamber as a thin layer (about 15 to 200 microns) on a substrate plate, referred to in the present patent application as a construction platform. According to the calculated surfaces of the CAD model, the powder is selectively melted by local heat input with the laser beam. Thereafter, the substrate plate is lowered and applied a new powder layer. The next layer is selectively reflowed with laser radiation and fusion metallurgically bonded to the lower layer. In this way, the customized component is produced in layers from the powdered building material. Components produced by selective laser melting are characterized by large component densities (> 99%). This ensures that the mechanical properties of the generatively produced component largely correspond to those of the base material. In the field of metal processing, this process principle is partly known by other names, such as DMLS (Direct Metal Laser Sintering), LaserCUSING ^® or laser metal fusion.

However, it has not yet been possible to exploit the potential of generative manufacturing to a greater extent than manufacturing processes for functional components. This is mainly due to the lack of quality assurance and control in generative production, which causes errors such as insufficient powder supply, pore and binding defects in the component, distortion of the component or defects on overhangs during the sometimes several hours of production process can not be detected. As a rule, the errors that occur are geometry-dependent, which is why a general avoidance of errors through improvements in the process control is not possible. In addition, these errors can occur during the entire process at various points in the component and are therefore detectable only after completion with complex test methods such as computed tomography. This also applies to cases in which the defect has already arisen in the first layers, since the already solidified or built-up part is concealed by the surrounding powder bed during the manufacturing process. So it is usually necessary that in most cases several attempts are made until a new, previously unknown geometry can be manufactured without errors. This is not acceptable for single and small batch production. Likewise, it is still very time-consuming to secure and document the quality of the generatively manufactured components. Especially with load-bearing structures in the field of aerospace or medical technology, this is an exclusion criterion in the selection of suitable manufacturing processes.

State of the art

In industry and research different approaches to quality assurance are pursued. Here, a distinction can be made between two approaches with regard to the sensor integration, on the one hand the off-axis construction and on the other hand the on-axis construction. In the off-axis solution, the entire installation space is detected and monitored by the sensor system. The sensor system can be arranged inside or outside the process chamber. In the on-axis solution, the sensor is integrated in the machining head and detects the machining surface coaxially with the beam path of the machining laser. As a result, the processing zone can be imaged with the molten bath on the sensor. In both approaches, the monitoring can be done in layers, so that the end of Machining 3D information on component formation is available.

The on-axis structure is based on a coaxial arrangement of the sensors, such as cameras and photodiodes, to the processing axis of the processing laser beam. The sensors use part of the same optical beam path as the processing laser. An example of such a structure can be found in the publication of U. Thombansen et al., "Process Observation in Selective Laser Melting (SLM)", Proc. SPIE 9356, High-Power Laser Materials Processing: Lasers, Beam Delivery, Diagnostics, and Applications IV, 93560R (March 9, 2015) , Such an arrangement has the advantage that the Schmelzbademissionen can always be focused on the sensor surface and the image size can be chosen so that high sampling rates are achieved. This allows continuous process documentation. The disadvantages of this arrangement are due to the integration in the optical beam path. The complex optical design leads to limitations in the quality of the image. The observation over the scanner requires high measuring frequencies, which leads to a limitation in the resolution of the sensor surface used. This arrangement is used exclusively to document the process. Monitoring methods, such as pulse thermography or monitoring of powder application, which are disturbed by process emissions, are not used.

An example of an off-axis setup for monitoring powder application can be found in the publication of T. Craeghs et al., "Online Quality Control of Selective Laser Melting", from http://sffsymposium.engr.utexas.edu/Manuscripts/2011/20 11-17-Craeghs.pdf, pages 221-226 , The advantage of an off-axis solution is the simple system integration of the system and the sensor system. An off-axis structure allows statements to be made about the entire installation space, for example about the overall melting and cooling behavior. However, a detailed statement about the molten bath or individual component structures is not derivable. This is due to the lower resolution, since the entire space is imaged on the sensor surface. For this reason, the sensors are usually used over the entire area, resulting in a lower detection rate. The DE 102014212246 B3 shows a method and a device for quality assurance with such an off-axis structure in which recorded with the camera used a thermal image of the processing surface and thus an evaluation by means of thermography is possible. Again, however, the above disadvantages of the off-axis solution apply.

The object of the present invention is to provide a method and a device for process monitoring in the generative production of components by layerwise solidification of a building material by means of energetic radiation in which recorded image data of each layer applied with high resolution and high measuring rate without interference from process emissions for process monitoring can be.

Presentation of the invention

The object is achieved with the method and the device according to claims 1 and 10. Advantageous embodiments of the method and the device are the subject of the dependent claims or can be found in the following description and the embodiment.

In the proposed method, with a radiation-sensitive sensor arrangement and optionally by means of a suitable imaging optics before and / or after the application of a new layer of building material on the processing surface data recorded at least one used for the consolidation of the building material area of the processing surface, from which a if necessary Image of this area can be created. The method is characterized in that the recording of this data takes place with at least one line sensor as a radiation-sensitive sensor arrangement which is moved over the processing surface. The working surface represents the surface of the last applied and partially solidified build material before applying a new layer of the build material and after the application of a new layer the surface of this new layer. Below the area used for the consolidation of the build material is the area the processing surface that the component requires for the construction, also referred to below as the construction area.

With this method, it is possible to document the entire structure of the component in layers or layers as in the known methods of the prior art. For this purpose, an image is preferably created of each individual layer before and / or after solidification by movement of the line sensor over the processing surface. The movement takes place parallel to the processing surface. By means of a suitable imaging optics, the surface of the respectively newly applied and / or just solidified layer is imaged onto the line sensor. As a result of the continuous movement or passage of the line sensor over the construction area of the processing area, an image of the entire building area or of the entire processing area can be generated via the take-up rate of the line sensor. Thus, two major advantages can be achieved. For one thing, the Line sensor arranged at a very short distance of preferably ≤ 400 mm above the working surface and moved synchronously with the layer over the working surface, without causing a collision or interference with or by the application of a new layer of building material. As a result, a very high resolution of the data or the respective image can be achieved. On the other hand, this arrangement allows the acquisition of data of the entire construction area or the entire processing area without the influence of process emissions. By capturing the respective data during the order of a new layer, there are no additional waiting times for the data acquisition. As a result, the process enables the entire structure of the component to be documented in 3D by means of a layer-by-layer analysis in very high resolution, without generating additional non-productive time.

In a preferred embodiment of the proposed method, the at least one line sensor is moved synchronously with the application device for the layered application of the building material over the processing surface. Preferably, this movement is carried out by the at least one line sensor is attached to the applicator itself, so that with the movement of this applicator for applying the new layer and the line sensor is moved to the data recording on the processing surface.

The proposed method and the associated device can be used in all generative manufacturing processes in which the construction of the component takes place by stratified solidification of a building material by means of energetic radiation, wherein the building material or the material required for the structure layer or is supplied in layers. This may be a powdery material which is distributed over the processing surface, for example via a suitable slider. In addition to the SLM process, this also includes similar processes for plastics and methods for laser sintering. The method is also suitable for production techniques in which the building material in layers or layer by layer is supplied in liquid form. In addition to these methods, which use laser radiation as energetic radiation for stratified solidification, the proposed method and the associated apparatus are also suitable for additive manufacturing techniques which use other types of energetic radiation, for example electron beams, for the layered solidification of the building material.

The device designed for carrying out the method comprises at least one construction platform, an application device for coating the build-up material on a processing surface above the build platform, a guide device for the energetic radiation and a radiation-sensitive sensor arrangement, with the at least one used for the solidification of the building material area of Machining surface can be detected. The device is characterized in that the sensor arrangement comprises at least one line sensor which is connected to a mechanical movement device which moves the line sensor over the processing surface to detect the construction area.

The mechanical movement device can be designed specifically for the movement of the line sensor on the processing surface, for example, using a linear axis or a swing arm. In a preferred embodiment, however, the mechanical movement device is used for the movement of the line sensor, with which the application device for the order of the building material is guided over the processing surface. The line sensor is then attached directly to this application device in this embodiment. This offers the particular advantage that the movement of the line sensor and thus also the image recording takes place automatically synchronously with the movement of the application device, so that no additional measuring runs or waiting times occur. The at least one line sensor is preferably arranged in the direction of movement of the application device during the layer application in front of the application device. With this arrangement, it is then possible to create an image of the already processed or partially solidified layer, for example, during the outward journey of the application device for coating, and / or to create an image of the newly applied layer on the return journey.

The guide device for the energetic radiation may be a fixed guide device which generates, for example, a projection of the energetic radiation onto the processing surface, or also a dynamic guide device, for example a scanning device, with which one or more processing beams are guided over the processing surface , The construction platform is usually formed lowerable or liftable. An application of a layer "over" the build platform is not necessarily to be understood in the present patent application relative to the direction of gravity, but rather refers to the construction direction relative to the build platform.

In a synchronous to the applicator performed or coupled to the applicator movement of the line sensor on the processing surface high recording rates of 10 kHz, for example, are required to achieve a high resolution. To implement these high line frequencies with appropriate exposure times, additional lighting of the be observed area. Preferably, this illumination is effected by at least one line of suitable radiation-emitting components, which is arranged parallel to the line sensor and also connected to the mechanical movement device. For example, components such as LEDs, laser diodes or Vertical Cavity Surface Emitting Lasers (VCSELs), which can emit in different wavelength ranges and powers, can be used for this illumination. The at least one line sensor can be designed, for example, for the detection of the visible spectral range or else for the detection of thermal images. In the case of capturing images or image data in the visible spectral range, the radiation sources used for illumination are then also light-emitting components which emit in the visible spectral range. When recording thermal images or thermal image data, corresponding infrared-emitting components are used.

The structure of the line with radiation sources and the line sensor can be done in different ways. In the simplest case, the entire or almost entire processing area is covered in one dimension by continuous lines. For cost or monitoring reasons, it may also be advantageous to offset shorter line sensors and / or to arrange them in the overlap. Several continuous lines are also possible, for example to obtain image sequences at defined time intervals. Both the line of radiation sources and the at least one line sensor can be located in the entire wavelength spectrum depending on the desired application. Different wavelength ranges can also be combined with each other locally or temporally. The evaluation of the signals can be correlated with other process variables and sensor signals or evaluated in their dependency.

With the proposed method and the associated device different measuring principles can be implemented. Thus, the method offers the possibility of recording image data of the upper or working surface in the visible spectral range and to analyze the detection of visible effects or faulty locations when applying the building material with suitable image processing algorithms. Another possibility is the use of pulse thermography. Pulse thermography is a test method that makes it possible to detect process errors below the surface, such as pores, cracks or connection errors. The short test duration and the high detection sensitivity enable the use of pulse thermography in non-destructive material testing. This is of particular importance in generative manufacturing processes such as the SLM, since layered buildup leads to remelting of deeper layers and process defects can only occur in the range of 100 μm to 150 μm depth. By applying the pulse thermography in the proposed method and the associated device, a reliable statement about the product quality, in particular about the strength of the components, can be made, as errors in deeper layers can be detected. The proposed method allows the use of pulse thermography, since there are no disturbances due to process emissions during image recording and a sufficiently high resolution is possible in order to be able to detect pores etc. in the size from 200 μm down to 50 μm. The measurement principle is based on a disturbance of the thermal equilibrium of the test specimen, by being briefly briefly locally heated with the aid of a thermal excitation source, for example by means of diode lasers or VCSELs. The line sensor used is then an IR sensor, for example a bolometer, an InGaAs detector or a PbSe detector, which records image sequences after the heating pulse, which are subsequently analyzed. The heat spreads in all spatial directions. Defects such as pores behave like a thermal barrier and hinder heat flow. Recognition depends on the size of the defect (cross section) and the distance to the surface. The propagation speed depends on material properties such as thermal conductivity, specific heat capacity and density. The time until an error in the depth of the surface makes itself felt depends on the speed of propagation. However, the effect of an error on the surface temperature must be detectable. Due to the small layer thickness in the generative production of typically 30 μm to 100 μm, this test method is ideally suited for process monitoring.

Brief description of the drawings

The proposed method and the proposed device will be explained in more detail below with reference to an embodiment in conjunction with the drawings. Hereby shows:

1 a schematic representation of an exemplary structure of the proposed device.

Ways to carry out the invention

The proposed method and the associated apparatus will be explained in more detail below using an example in which a component with the technique of selective laser sintering (SLM) is established. The construction device has a process chamber 1 on, in which a building platform 2 in a building container intended for the construction 3 is lowerable. The process chamber 1 has an inlet 4 and an outlet 5 for a process gas. The powdery building material 7 each with a powder slide 6 applied as a new layer over the processing surface. The order of each new layer takes place after solidification of the respective preceding layer by lowering the build platform 2 by a corresponding layer thickness and movement of the powder slide 6 over the component container 3 ,

The storage container for the building material and the guide means for the laser beam are not shown in this figure. These components of the device are known from the prior art and can be formed here in the same way. The in 1 represented powder slide 6 in every position over the already solidified component areas 8th and the remaining powder 7 is used for attaching the used for image recording process monitoring line sensor 9 , For process monitoring, each individual layer is detected by the line sensor 9 Captured image data. The image data is recorded during the forward and / or return journey of the powder slide 6 , Due to the continuous passage of the line sensor 9 over the building container 3 by means of the powder slide 6 is about the frame rate of the line sensor 9 creates an image of the entire surface of the installation space.

Along the line sensor 9 the resolution is typically in the range of the pixel size of the line sensor, for example at 10 microns. In the direction of the feed of the powder slide 6 the resolution depends on the feed rate and the pickup frequency of the line sensor. In order to achieve a spatial resolution of 10 μm at the typical feed rate of the powder slide of 100 mm / s, a take-up rate of 10 kHz is required. Commercially available camera lines (CMOS, CCD, InGAs, etc.) are capable of converting line frequencies of 40 kHz. In conjunction with a pixel size of 10 μm, it is possible to achieve excellent spatial resolution for monitoring the applied powder layer and the melted or solidified areas.

1 also shows an optical or thermal line source 10 , which are parallel to the line sensor 9 also on the powder gate 6 is appropriate. This line of illumination or line source can be used to illuminate the surface due to the short exposure times of the line sensor 9 may be required.

In one possible embodiment, with the line sensor 9 Image data of the processing surface recorded for inspection. The recorded image data are evaluated via at least one image processing algorithm and relevant features extracted. This can be in different wavelength ranges, for example in the visible or in the near infrared wavelength range, as well as in the return and / or return of the powder slide 6 for powder application. On the way out, the already exposed or processed surface and on the way back the newly applied powder layer is added.

Another possibility is the use of pulse thermography in this device and the associated method. For this purpose, the line sensor 9 formed for recording thermal image data and the line source 10 as a thermal line source. Using pulse thermography, defects can be removed 11 Detect below the surface in already solidified building material. Such a defect 11 generates a detectable on the surface response of the line source 10 irradiated heat pulses. The on slide 6 attached sensor unit with line source 10 and line sensor 9 is with a control and evaluation 12 connected. In the control and evaluation device 12 the control of the sensors and the signal processing of the response take place. Upon detection of an error, an analysis result 13 output, indicating the position and depth of the respective defect.

With this method and the device described, a process monitoring can be carried out during the powder application without taking any process time. This allows spatially resolved measurement data of the entire processing area to be generated in the off-time of powder application. It can thereby perform a complete layer by layer analysis during the manufacture of a component. By combining the individual shots, the spatial internal structure of the component becomes visible and documented.

LIST OF REFERENCE NUMBERS

1

 process chamber

2

 building platform

3

 Baubehältnis

4

 gas inlet

5

 gas outlet

6

 powder slide

7

 powder

8th

 Solidified component areas

9

 line sensor

10

 line source

11

 malfunction

12

 Control and evaluation device

13

 analysis result

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102014212246 B3 [0008]

Cited non-patent literature

• U. Thombansen et al., "Process Observation in Selective Laser Melting (SLM)", Proc. SPIE 9356, High-Power Laser Materials Processing: Lasers, Beam Delivery, Diagnostics, and Applications IV, 93560R (March 9, 2015) [0007]
• T. Craeghs et al., "Online Quality Control of Selective Laser Melting", from http://sffsymposium.engr.utexas.edu/Manuscripts/2011/20 11-17-Craeghs.pdf, pages 221-226 [0008]

Claims (19)

Method for process monitoring in the additive production of components by stratified solidification of a building material ( 7 ) by means of energetic radiation, in which with a radiation-sensitive sensor arrangement in each case before and / or after application of a new layer of the building material ( 7 ) on a processing surface data of at least one for the consolidation of the building material ( 7 ) used area of the processing surface, characterized in that the recording of the data with at least one line sensor ( 9 ) takes place as a radiation-sensitive sensor arrangement which is moved over the processing surface. Method according to claim 1, characterized in that the at least one line sensor ( 9 ) in synchronism with an order device ( 6 ) for the layered application of the building material ( 7 ) is moved over the working surface. Method according to claim 2, characterized in that the at least one line sensor ( 9 ) mechanically with the application device ( 6 ) is coupled. Method according to one of claims 1 to 3, characterized in that with the at least one line sensor ( 9 ) Image data in the visible spectral range and / or thermal image data are recorded. Method according to one of claims 1 to 4, characterized in that for the solidification of the building material ( 7 ) used area with at least one parallel to the line sensor ( 9 ) and with the line sensor ( 9 ) over the working surface moving line ( 10 ) is irradiated with radiation sources, in particular optical radiation sources. Method according to claim 5, characterized in that the at least one line sensor ( 9 ) is used together with the radiation sources for pulse thermography on the processing surface. Method according to one of claims 1 to 6, characterized in that the at least one line sensor ( 9 ) is moved over the working surface for recording the data at a distance of ≤ 400 mm. Method according to one of claims 1 to 7, characterized in that the recorded data on defects in an already partially solidified and / or disturbances in a newly applied layer of the building material ( 7 ) are analyzed. Method according to one of claims 1 to 8 for process monitoring during selective laser melting. Device for the additive production of components by stratified solidification of a building material by means of energetic radiation, comprising at least one construction platform ( 2 ), - an order device ( 6 ) for the layered application of the building material ( 7 ) in a processing area above the build platform ( 2 ), - a guiding device for the energetic radiation and - a radiation-sensitive sensor arrangement, with which at least one for the solidification of the building material ( 7 ) used area of the processing surface can be detected, characterized in that the sensor arrangement at least one line sensor ( 9 ), which is connected to a mechanical movement device, which the line sensor ( 9 ) for detecting the solidification of the building material ( 7 ) area moved across the working area. Apparatus according to claim 10, characterized in that the at least one line sensor ( 9 ) at the order facility ( 6 ) for the layered application of the building material ( 7 ) is attached as a mechanical movement device. Apparatus according to claim 11, characterized in that the at least one line sensor ( 9 ) in a direction of movement of the application device ( 6 ) before the order device ( 6 ) is arranged. Apparatus according to claim 11 or 12, characterized in that the application device ( 6 ) a powder slide for the layered application of powdered building material ( 7 ), on which the at least one line sensor ( 9 ) is attached. Device according to one of claims 10 to 13, characterized in that the at least one line sensor ( 9 ) is a line sensor for the visible spectral range. Device according to one of claims 10 to 13, characterized in that the at least one line sensor ( 9 ) is a line sensor for the infrared spectral range, with which a thermal image of the for the solidification of the building material ( 7 ) area can be recorded. Device according to one of claims 10 to 15, characterized in that the at least one line sensor is attached to the mechanical movement means such that it is used for detecting the for the solidification of the building material ( 7 ) is moved over the working area at a distance of ≤ 400 mm. Device according to one of claims 10 to 16, characterized in that parallel to the line sensor ( 9 ) at least one line ( 10 ) is arranged with radiation sources, in particular optical radiation sources, for irradiation of the processing surface and connected to the mechanical movement device. Device according to one of claims 10 to 17, characterized in that a control and evaluation device with the line sensor ( 9 ) connecting the line sensor ( 9 ) drives during the movement over the processing surface for data recording and with the line sensor ( 9 ) recorded for defects in an already partially solidified and / or disturbances in a newly applied layer of the building material ( 7 ) analyzed. Device according to one of claims 10 to 18, which is designed for generative production by selective laser melting.

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