DE102016209065B4 - Method and device for process monitoring in the additive manufacturing of components - Google Patents

Abstract

Method for process monitoring in the generative manufacturing of components by layer-by-layer solidification of a building material (7) by means of energetic radiation, in which a radiation-sensitive sensor arrangement is used before and/or after a new layer of the building material (7) is applied to a processing surface, at least one data for solidification of the construction material (7) are recorded on the processing surface, and in which the recording of the data takes place with at least one line sensor (9) as a radiation-sensitive sensor arrangement, which is moved over the processing surface, characterized in that the solidification of the construction material ( 7) the area used is irradiated with radiation sources with at least one line (10) arranged parallel to the line sensor (9) and moved with the line sensor (9) over the processing surface, and that the at least one line sensor (9) together with the radiation sources for pulse thermography at the processing surface is used.

Description

Technical field of application

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

With its almost unlimited geometric degrees of freedom, the complete absence of shaping tools and a material utilization of almost 100%, generative manufacturing offers the possibility of manufacturing individual products in a resource- and energy-efficient manner. One of the most widespread methods of generative manufacturing of metal components (including ceramics) is selective laser melting (SLM: Selective Laser Melting), in which the components are built up on a construction platform by layer-by-layer solidification of a powdered construction material.

Due to the layered construction directly from CAD data, highly complex components can be produced from metallic series materials without shaping tools using SLM, which cannot be produced with conventional manufacturing techniques such as casting or machining. Since previous production-related restrictions are no longer applicable, 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 (approx. 15 to 200 μm) on a substrate plate, also referred to as a construction platform in the present patent application. According to the calculated areas of the CAD model, the powder is selectively melted by applying local heat with the laser beam. Then the substrate plate is lowered and a new layer of powder is applied. The next layer is selectively melted again with laser radiation and connected to the lower layer by melt metallurgy. In this way, the tailor-made component is created in layers from the powdered construction material. Components manufactured by selective laser melting are characterized by high component densities (> 99%). This ensures that the mechanical properties of the generatively manufactured component largely correspond to those of the base material. In the field of metal processing, this process principle is sometimes also known under other names, such as DMLS (Direct Metal Laser Sintering), LaserCUSINGO or Laser Metal Fusion.

However, it has not yet been possible to use the potential of additive manufacturing on a larger scale as a manufacturing process for functional components. This is essentially due to a lack of quality assurance and control in additive manufacturing, which means that errors such as insufficient powder supply, pore and bonding defects in the component, distortion of the component or defects in overhangs occur during the production process, which sometimes lasts several hours cannot be detected. The errors that occur are usually geometry-dependent, which is why it is not possible to generally avoid errors by improving process control. In addition, these defects can occur at various points in the component during the entire process and can therefore only be detected after completion using complex test methods such as computed tomography. This also applies to cases in which the defect has already appeared in the first layers, since the part that has already solidified or built up is hidden by the surrounding powder bed during the manufacturing process. As a rule, it is usually necessary to make several attempts before a new, previously unknown geometry can be manufactured without errors. This is unacceptable for single and small series production. It is also still very time-consuming to ensure and document the quality of the additively manufactured components. In the case of load-bearing structures in the aerospace and medical technology sectors in particular, this is an exclusion criterion when selecting suitable manufacturing processes.

State of the art

Different approaches to quality assurance are pursued in industry and research. A distinction can be made between two approaches with regard to sensor integration, on the one hand the off-axis structure and on the other hand the on-axis structure. With the off-axis solution, the entire installation space is recorded and monitored by the sensor system. The sensor system can be arranged inside or outside the process chamber. With the on-axis solution, the sensor is integrated into the processing head and records the processing surface coaxially to the beam path of the processing laser. This allows the processing zone with the melt pool on the sensor be mapped. With both approaches, the monitoring can take place layer by layer, so that at the end of the processing 3D information on the component creation is available.

The on-axis design 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 partly use the same optical beam path as the processing laser. An example of such a structure can be found in the publication by 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 melt pool emissions can always be focused on the sensor surface and the image size can be selected in such a way that high scanning rates can be achieved. This enables continuous process documentation. The disadvantages of this arrangement are based on the integration into the optical beam path. The complex optical structure leads to limits in the quality of the image. Observation via the scanner requires high measuring frequencies, which leads to a limitation in the resolution of the sensor area used. This arrangement is only used to document the process. Monitoring methods such as pulse thermography or monitoring of the powder application, which are disturbed by process emissions, are not used.

An example of an off-axis setup for powder deposition monitoring can be found in the publication by 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 plant and sensor system. An off-axis setup enables statements to be made about the entire installation space, for example about the overall melting and cooling behavior. However, a detailed statement about the weld pool or individual component structures cannot be derived. This is due to the lower resolution, since the entire installation space is mapped onto the sensor surface. Since for this reason the sensors are usually used over the entire surface, this also results 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 a thermal image of the processing surface is recorded with the camera used, thus enabling evaluation by means of thermography. However, the above disadvantages of the off-axis solution also apply here.

The US 2015/0273583 A1 describes a method and a device for process monitoring in an additive manufacturing method, in which sensors are arranged in a line on an application device with which the powder is brushed over the processing surface. With these, 2D image data of the solidified component structures are recorded on the return journey of the application device, from which a 3D image of the workpiece can be obtained.

The US 2015/0158249 A1 discloses radiation sources and detectors which are each arranged in a direction parallel to the coating direction and are used to detect disturbing elevations on the already solidified layer.

The object of the present invention is to specify a method and a device for process monitoring in the generative manufacturing of components by layer-by-layer solidification of a construction material by means of energetic radiation, in which image data of each applied layer is recorded with high resolution and high measuring rate without interference from process emissions for process monitoring can become.

Presentation of the invention

The object is achieved with the method and the device according to patent claims 1 and 8. Advantageous configurations of the method and the device are the subject matter of the dependent patent claims or can be found in the following description and the exemplary embodiment.

In the proposed method, a radiation-sensitive sensor arrangement and, if necessary, suitable imaging optics are used to record data before and/or after the application of a new layer of the building material to the working surface of at least one region of the working surface used for solidifying the building material, from which a Image of this area can be created. This data is recorded with at least one line sensor as a radiation-sensitive sensor arrangement, which is moved over the processing surface. Before a new layer of build-up material is applied, the processing area represents the surface of the last applied and partially solidified build-up material and after the application of a new layer, it represents the surface of this new layer. Below the area used for solidifying the build-up material is the area of the processing area that the component requires for assembly, hereinafter also referred to as assembly designated area. The method is characterized in that the area used for the solidification of the construction material is irradiated with at least one line of radiation sources arranged parallel to the line sensor and moved over the processing surface with the line sensor, and that the at least one line sensor is used together with the radiation sources for pulsed thermography the processing surface is used.

With this method, as with the known methods of the prior art, the entire structure of the component can be documented layer by layer. For this purpose, an image is preferably created of each individual layer before and/or after solidification by moving the line sensor over the processing surface. The movement takes place parallel to the processing surface. The surface of the respectively newly applied and/or just solidified layer is imaged onto the line sensor by means of suitable imaging optics. Due to the continuous movement or crossing of the line sensor over the build-up area of the processing area, an image of the entire build-up area or the entire processing area can be generated via the recording rate of the line sensor. Two essential advantages can be achieved in this way. On the one hand, the line sensor can be arranged at a very small distance of preferably ≦400 mm above the processing surface and can be moved over the processing surface synchronously with the layer application without a collision or disruption occurring with or due to the application of a new layer of the construction material. As a result, a very high resolution of the data or the respective image can be achieved. On the other hand, this arrangement makes it possible to record data from the entire build area or the entire processing area without the influence of process emissions. By capturing the respective data during the order of a new shift, there are no additional waiting times for data acquisition. The process makes it possible to document the entire structure of the component in 3D using 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 over the processing surface synchronously with the application device for the layered application of the building material. This movement preferably takes place by attaching the at least one line sensor to the application device itself, so that when this application device is moved to apply the new layer, the line sensor is also moved over the processing surface for data acquisition.

The proposed method and the associated device can be used in all generative manufacturing methods in which the component is built up by layer-by-layer solidification of a building material by means of energetic radiation, with the building material or the material required for the structure being added in layers or layers. This can be a powdered material that is distributed, for example, using a suitable slide on the processing surface. In addition to the SLM process, this also includes similar processes for plastics and processes for laser sintering. The method is also suitable for manufacturing techniques in which the building material is added in liquid form in layers or layers. In addition to these methods, which use laser radiation as energetic radiation for layer-by-layer solidification, the proposed method and the associated device are also suitable for generative manufacturing techniques that use other types of energetic radiation, for example electron beams, for layer-by-layer solidification of the building material.

The device designed to carry out the method comprises at least one construction platform, an application device for applying the construction material in layers to a processing surface above the construction platform, a guide device for the energetic radiation and a radiation-sensitive sensor arrangement with which at least one area of the construction material used for solidifying the construction material Processing area can be detected. 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 in order to detect the build-up area.

The mechanical movement device can be designed specifically for moving the line sensor over the processing surface, for example using a linear axis or a swivel 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 application of the construction material is guided over the processing surface. In this configuration, the line sensor is then attached directly to the application device. This offers the particular advantage that the movement of the line sensor and thus also the image recording is automatically synchronized with the movement of the application device, so that there are no additional measuring runs or waiting times. The at least one line sensor is preferably in the direction of movement of the up carrying device arranged in layer application in front of the application device. With this arrangement, an image of the layer that has already been processed or partially solidified can then be created, for example, on the outward journey of the application device for layer application and/or on the return journey, an image of the newly applied layer.

The guiding device for the energetic radiation can be a fixed guiding device, which, for example, generates a projection of the energetic radiation onto the processing surface, or a dynamic guiding device, for example a scanning device, with which one or more processing beams are guided over the processing surface . As a rule, the construction platform can be lowered or raised. In the present patent application, an application of a layer “above” the construction platform is not necessarily to be understood relative to the direction of gravity, but rather relates to the direction of construction relative to the construction platform.

When the line sensor moves synchronously with the application device or is coupled to the application device over the processing surface, high recording rates of, for example, 10 kHz are also required to achieve high resolution. In order to convert these high line frequencies with corresponding exposure times, additional lighting of the observed area is required. In the proposed device, this illumination is provided by at least one line of suitable radiation-emitting components, which is arranged parallel to the line sensor and is also connected to the mechanical movement device. For example, components such as LEDs, laser diodes or VCSELs (Vertical Cavity Surface Emitting Lasers) can be used for this illumination, which can emit in different wavelength ranges and powers. The at least one line sensor is designed in the proposed device for capturing thermal images. For this recording of thermal images or thermal image data, appropriate infrared-emitting components are used for lighting.

The line with radiation sources and the line sensor can be constructed in different ways. In the simplest case, the entire or almost the entire processing area is covered in one dimension by continuous lines. For cost or monitoring reasons, it can also be advantageous to arrange shorter line sensors offset and/or in an overlap. Several continuous lines are also possible, for example to obtain image sequences at defined time intervals. The evaluation of the signals can be correlated with other process variables and sensor signals or evaluated as a function of them.

Pulse thermography technology is used in the proposed method and the associated device. Pulse thermography is a test method that makes it possible to detect process errors that lie 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 SLM, since layer-by-layer construction leads to remelting of deeper layers and process errors can only occur there in the range of 100 µm to 150 µm depth. By using pulse thermography in the proposed method and the associated device, a reliable statement can be made about the product quality, in particular about the strength of the components, since defects in deeper layers can be detected. The proposed method enables the use of pulse thermography, since there are no disturbances caused by process emissions during image recording and a sufficiently high resolution is possible to detect pores etc. with a size of 200 µm down to 50 µm. The measuring principle is based on disrupting the thermal equilibrium of the test object by briefly heating it locally with the aid of a thermal excitation source, for example using diode lasers or VCSELs. An IR sensor, for example a bolometer, an InGaAs detector or a PbSe detector, is used as a line sensor, which, after the heating pulse has taken place, records image sequences which are subsequently analyzed. The heat spreads out in all spatial directions. Defects such as pores behave like a thermal barrier and impede the flow of heat. The detectability depends on the defect size (cross-section) and the distance to the surface. The speed of propagation depends on material properties such as thermal conductivity, specific heat capacity and density. The length of time before a fault becomes noticeable at depth on the surface thus depends on the propagation speed. However, the effect of a defect on the surface temperature must be detectable. Due to the low layer thickness in additive manufacturing of typically 30 µm to 100 µm, this test method is ideal for process monitoring.

character list

• 1 eine schematische Darstellung eines beispielshaften Aufbaus der vorgeschlagenen Vorrichtung.

The proposed method and the proposed device are explained in more detail below using an exemplary embodiment in conjunction with the drawing. This 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 device are explained in more detail below using an example in which a component is constructed using selective laser sintering (SLM) technology. The construction device has a process chamber 1 in which a construction platform 2 can be lowered into a construction container 3 provided for construction. The process chamber 1 has an inlet 4 and an outlet 5 for a process gas. The powdery building material 7 is applied in each case with a powder pusher 6 as a new layer over the working surface. Each new layer is applied after the solidification of the previous layer by lowering the construction platform 2 by a corresponding layer thickness and moving the powder pusher 6 over the component container 3.

The storage container for the construction material and the guiding device for the laser beam are not shown in this figure. These components of the device are known from the prior art and can be designed in the same way here. the inside 1 The powder pusher 6 shown, which is guided over the already solidified component areas 8 and the remaining powder 7 in each layer, is suitable for attaching the line sensor 9 used for image recording for process monitoring. For process monitoring, image data is recorded from each individual layer by the line sensor 9 . The image data is recorded during the outward and/or return journey of the powder pusher 6. Due to the continuous crossing of the line sensor 9 over the construction container 3 by means of the powder pusher 6, an image of the entire surface of the construction space is generated via the frame rate of the line sensor 9.

The resolution along the line sensor 9 is typically in the range of the pixel size of the line sensor, for example 10 μm. In the direction of advancement of the powder pusher 6, the resolution depends on the feed rate and the recording frequency of the line sensor. A sampling rate of 10 kHz is required to obtain a spatial resolution of 10 µm at the typical feed rate of the powder pusher of 100 mm/s. 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, excellent spatial resolution can be achieved for monitoring the applied powder layer and the melted or solidified areas.

1 also shows a thermal line source 10, which is also mounted parallel to the line sensor 9 on the powder pusher 6. This line of illumination or line source serves to illuminate the surface, which may be necessary due to the short exposure times of the line sensor 9 .

To use pulse thermography, line sensor 9 is designed to record thermal image data and line source 10 is designed as a thermal line source. Defects 11 below the surface in the already solidified building material can be detected by means of pulse thermography. Such a defect 11 produces a response to the heat pulses radiated in via the line source 10 that can be detected on the surface. The sensor unit with line source 10 and line sensor 9 attached to slide 6 is connected to a control and evaluation device 12 . The control of the sensors and the signal processing of the response take place in the control and evaluation device 12 . If a fault is detected, an analysis result 13 can be output which indicates the position and depth of the respective fault.

With this method and the device described, process monitoring can be carried out during the powder application without taking up any process time. This means that locally resolved measurement data of the entire processing area can be generated during the non-productive time of powder application. This allows a complete layer-by-layer analysis to be carried out during the manufacture of a component. The spatial internal structure of the component becomes visible and documented through the combination of the individual layer recordings.

Reference List

1

process chamber

2

build platform

3

building container

4

gas inlet

5

gas outlet

6

powder pusher

7

powder

8th

Hardened component areas

9

line sensor

10

line source

11

malfunction

12

Control and evaluation device

13

analysis result

Claims (14)

Method for process monitoring in the generative manufacturing of components by layer-by-layer solidification of a building material (7) by means of energetic radiation, in which a radiation-sensitive sensor arrangement is used before and/or after a new layer of the building material (7) is applied to a processing surface for at least one data for the solidification of the construction material (7) of the working surface is recorded, and in which the recording of the data takes place with at least one line sensor (9) as a radiation-sensitive sensor arrangement, which is moved over the working surface, characterized in that the solidification of the construction material (7) the area used is irradiated with radiation sources with at least one line (10) arranged parallel to the line sensor (9) and moved with the line sensor (9) over the processing surface, and that the at least one line sensor (9) together with the radiation sources for pulse thermography used on the processing surface. procedure after claim 1 , characterized in that the at least one line sensor (9) is moved synchronously with an application device (6) for the layered application of the construction material (7) over the processing surface. procedure after claim 2 , characterized in that the at least one line sensor (9) is mechanically coupled to the application device (6). Procedure according to one of Claims 1 until 3 , characterized in that the at least one line sensor (9) is used to record image data in the visible spectral range and/or thermal image data. Procedure according to one of Claims 1 until 4 , characterized in that the at least one line sensor (9) for recording the data is moved at a distance of ≤ 400 mm over the processing surface. Procedure according to one of Claims 1 until 5 , characterized in that the recorded data are analyzed for defects in an already partially solidified layer and/or faults in a newly applied layer of the building material (7). Procedure according to one of Claims 1 until 6 for process monitoring in selective laser melting. Device for the additive manufacturing of components by layer-by-layer solidification of a construction material by means of energetic radiation, which has at least - a construction platform (2), - an application device (6) for applying the construction material (7) in layers in a processing area above the construction platform (2), - a guide device for the energetic radiation and - has a radiation-sensitive sensor arrangement with which at least one area of the processing surface used for solidifying the building material (7) can be detected, and - in which the sensor arrangement comprises at least one line sensor (9) which is equipped with a mechanical movement device is connected, which moves the line sensor (9) for detecting the area used for solidifying the building material (7) over the processing surface, 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 area used for solidifying the construction material (7) can be recorded, and that at least one line (10) with radiation sources for irradiating the processing surface is arranged parallel to the line sensor (9) and is connected to the mechanical movement device. device after claim 8 , characterized in that the at least one line sensor (9) is attached to the application device (6) for layered application of the construction material (7) as a mechanical movement device. device after claim 9 , characterized in that the at least one line sensor (9) is arranged in front of the application device (6) in a given direction of movement of the application device (6) during layered application. device after claim 9 or 10 , characterized in that the application device (6) has a powder pusher for the layered application of powdered construction material (7), to which the at least one line sensor (9) is attached. Device according to one of Claims 8 until 11 , characterized in that the at least one line sensor is attached to the mechanical movement device in such a way that it is moved over the processing surface at a distance of ≤ 400 mm to detect the area used for solidifying the construction material (7). Device according to one of Claims 8 until 12 , characterized in that a control and evaluation device is connected to the line sensor (9), which controls the line sensor (9) during the movement over the processing surface for data recording and with the line sensor (9) recorded data for defects in an already partially solidified and/or faults in a newly applied layer of the construction material (7) are analyzed. Device according to one of Claims 8 until 13 , which is designed for additive manufacturing by selective laser melting.

Images