RU205100U1 - DEVICE FOR CONTROL OF ADDITIVE MANUFACTURING OF PARTS - Google Patents

Abstract

The utility model relates to the field of additive formation of products, namely to devices for monitoring 3D printing based on holographic, interferometric measurements, structured light, as well as photogrammetry, and can be used to obtain information about the shape of an object, its deformation, the presence of internal subsurface defects in the workpiece. and stress states of the material. The device for controlling the additive manufacturing of a part contains an interconnected lighting unit in the form of a coherent radiation source with a lens, an interference unit and a computing unit. The interference unit is made on the basis of a shear interferometer, which contains an inclined mirror that moves along the optical axis. The computing unit is configured to decipher the interferograms by the phase step method and calculate the current values of the internal stresses in the part. The utility model improves the quality of manufactured parts. 3 ill.

Description

The utility model relates to the field of additive formation of products, namely to devices for controlling 3D printing based on holographic, interferometric measurements, structured light, as well as photogrammetry, and can be used to obtain information about the shape of an object, its deformation, the presence of internal subsurface defects in the workpiece. and stress states of the material.

Parts produced by additive manufacturing methods (3D printing methods) have characteristic features:

- internal layered microstructure;

- high surface roughness;

- different mechanical properties of the material of parts in different directions (anisotropy).

These features affect both the production process and the performance of finished parts. Layer-by-layer formation leads to the appearance of internal stresses in the material and subsequent deformation (warping), leading to a change in the linear dimensions, as well as the volume. Surface roughness in the vertical and horizontal directions will have different meanings and pronounced directions. To improve performance and reduce weight, a cellular structure of a complex shape (filler) is often formed inside the product. Also, often the shell of the part (skin) can be printed with a print mode that is different from the main one, or made of a different material. It is important to note that some of the defects can be located in the subsurface layer of the workpiece, in the place where the skin is connected to the filler. Thus, parts can have quite specific defects characteristic of layer-by-layer printing, such as delamination and discontinuities, which, as a rule, are detected after the part is manufactured at the stage of its control. Despite the fact that additive manufacturing processes are constantly improving, problems remain related to the quality and stability of manufactured parts.

It is possible to prevent the appearance of some defects and the out of tolerance limits of the part by making corrections directly during the production of the part through optical control and a feedback system.

From the prior art, technical solutions are known, aimed at finding the optimal orientation of the part and the configuration of supports for 3D printing. For example, this task is especially relevant in the production of cranio-maxillofacial implants (see patent EP 2398411, class A61B 17/00, publ. 28.12.2011), since the manufactured part, after removal from the platform and removal of the supports, demonstrates the effect of "Springing back". This effect is due to the presence of internal stresses in the printed workpiece, which irreversibly reveal themselves in the process of separating the part from the substrate.

The problem of optimizing the print configuration in order to reduce internal stresses in finished parts can be solved by the method of computer simulation. For example, by modeling the layer-by-layer construction with the "natural stress method", so that the finished part will have a minimum value of internal stresses (see https: //habr.eom/m/company/iqb_technologies/blog/445158/). However, it is not always possible to estimate the print parameters with high accuracy. The external conditions and mechanical characteristics of the material will play an important role, since their deviations can introduce a significant error in the simulation process and distort the results.

Holographic non-destructive testing methods (or speckle interferometry methods) allow registering the slightest deformation of objects. These methods work well with the rough surfaces of parts that are produced by additive manufacturing. A number of additive machines are known that use laser interferometric units in their composition and have feedback for making adjustments to the part manufacturing process.

For example, a device containing a chamber is known from the prior art for controlling the flatness and deformation of a liquid bath (melt) in the heating area during three-dimensional printing (see patent CN 107843205, class B22F 3/105, publ. 03/27/2018). Moreover, the deformation of the melt is directly related to the geometric shape of the product and can be used for timely recognition of the shape of the product in order to avoid subsequent ineffective processing. The bath geometry information is used to predict the geometric dimensions of the workpiece after final cooling, thereby optimizing laser processing parameters. The measuring device is attached to the side of the laser printhead of the additive machine parallel or at an angle to the printhead nozzle. Thus, the beam of light from the measuring device moves in concert with the print head. This device uses a method of processing information from the measuring unit by using the finite element method, which takes into account the characteristics of the material to further correct the printing process of layers.

The disadvantage of this device is the fact that this measurement method is applicable exclusively for production technology with the formation of a molten metal bath using a laser, where the bath temperature can reach three thousand degrees. There is a drawback associated with a change in the reflection coefficient of the bath surface during cooling, which is especially important for interferometric measurements. Material surface control is carried out only in the area around the melt in the visibility zone of one layer. The indicated arrangement of the structural elements of the device makes it possible to estimate the non-flatness only for a relatively small area of the melt surface. Thus, most of the workpiece is left without continuous monitoring. In the process of forming a corrective action, there is no comparison of the product geometry with the CAD model.

From the prior art, a system for controlling the accuracy of manufacturing is known for the additive manufacturing of three-dimensional components (see publication WO 2019138038, class В29С 64/393, publ. 18.07.2019). The system includes a combined lighting-detecting element containing a two-dimensional array of detectors, and at least one source of laser radiation directed to the area of a powdery or pasty material. The array of detectors is located in such a way that speckles arising as a result of reflection from the surface irradiated by the laser radiation source can be spatially resolved using the array of photodetectors. Information about the speckle field is transmitted to the electronic evaluation and management system. Coherent illumination to obtain a speckle pattern can be carried out using a special source or using a source used to heat the material in additive manufacturing. The system is characterized in that an optical filter and / or an optical lens is located between the material and the laser radiation source.

The advantage of the system is the use of the speckle structure created by the radiation reflected from the object to reconstruct the three-dimensional shape of the measured area. In addition, the measuring unit in such a scheme has small dimensions. The detectors are located on a single movable base - a squeegee.

However, the described device uses the control of the pool of melt and the method implemented in it is not suitable for controlling high-speed processes in confined spaces. The use of an array of detectors partially solves this known problem. The device can also include temperature sensors, from which a temperature map is created, which can be used to judge the residual stresses in the workpiece. The disadvantages of this device can also be attributed to the work directly with a powder material or paste used for sintering, while a large area of the workpiece also remains uncontrolled. In addition, the use of temperature sensors does not always allow us to assess the stress states of the material due to the low resolution of the thermometry method under additive printing conditions, and, as a rule, is applicable only to metals. The use of a laser heating light source in an additive machine to create speckle patterns simplifies the design of the control circuit, but such a laser has a much lower optical stability for precise interference measurements. As a rule, high-power laser radiation can create illumination of the sensors, and the products of ablation of the material and their movement will make it difficult to fix the interference fringes.

This problem can be solved by reducing the laser power, which is proposed in another technical solution (see patent GB 2565853, class В29С 64/268, publ. 27.02.2019). However, this solution significantly reduces the speed of additive manufacturing, since the control device is located directly on the movable doctor blade, which can be moved along one axis to smooth the powder surface. At the same time, it is not possible to illuminate and control the entire surface of the bath at a time, as well as analyze the material from different angles. To process the speckle pattern, a correlation processing method is used, which has a low spatial resolution. This squeegee-mounted control device can only be used in additive selective laser sintering technology.

A device is known from the prior art that uses interferometric methods to control the surface of a liquid material (resin) intended for photocuring in the process of additive manufacturing (see patent CN 106938544, class В29С 64/386, publ. 11.07.2017). The area of the liquid is illuminated by a laser, LED or mercury lamp. The camera registers interference fringes on the surface of the liquid bath. The solutions used make it possible to improve the quality of the parts produced.

The disadvantage of the device is the increased requirements for the surface of the reflective material (resin) to obtain a stable interference pattern. The method is applicable to SLA technology using certain printing modes at low speed. In addition, this method does not imply control of the workpiece itself during the printing process.

The prior art discloses a laser cladding method, in which the height of the cladding is controlled by interferometry, which, through feedback, makes it possible to accurately position the printhead in height, while the scanning measuring beam moves separately from the laser head for cladding material (see publication WO 2018178387, class B22F 3/105, publ. 04.10.2018). The described scheme makes it possible to control the geometry of the material and the area of phase separation in the axial direction. The disadvantage of this arrangement of the additive machine tool, measuring system and its implementations is the impossibility of monitoring several printed layers and most of the workpiece.

From the prior art, a device is known that contains an additive machine in conjunction with a 3D scanning system for rapid prototyping of products and is implemented using a polar coordinate system, which allows layer-by-layer application of material with subsequent curing in accordance with a given CAD model of the part, to carry out optical control of the part formation process and correction, if necessary, of the current print parameters (see publication US 20140271964, class В29С 67/00, publ. 09/18/2014). This additive machine contains a rotating base that is simultaneously used to rotate the product for both scanning and printing. The extruder of the printer has the ability to make only linear movements. For scanning purposes, the system has one or more cameras and lighting devices. Cameras receive a set of two-dimensional (2D) images of a three-dimensional object, from which the 3D surface is reconstructed. The processor processes the 3D surface reconstruction and generates the printer control code. This approach allows you to print a copy of the scanned item. In this case, during printing and scanning, the part or product rotates around the central axis of the base. This arrangement of the printing organs of the additive machine has advantages over the previous ones in that it allows the use of simplified and less expensive control systems; can be implemented with DC motors instead of traditional stepper motors; can be smaller in volume, occupying a smaller machine area than traditional systems based on X, Y - displacements; is quieter because there are no "jerks" in two dimensions, observed in traditional systems based on X, Y-displacement; simplifies the creation of smooth curved surfaces; the use of the same mechanisms when constructing a surface during printing and scanning.

The device may also include a temperature sensor for feedback control of the temperature of the extruder.

The described technical solution allows you to create a three-dimensional copy of the part located on the substrate of the print area of the additive machine and write it to the internal memory of the device. However, despite the convenience of using the turntable for scanning and printing curved surfaces, this solution degrades the print quality of straight surfaces.

The closest in technical essence to the proposed utility model is a control device for additive manufacturing of a part containing interconnected interference and computing units (see publication US 2018297117, class B22F 3/105, publ. 18.10.2018). In the known device, optical interferometry is used in material modification (for example, sintering and welding) using a system containing an optical source, a feedback controller based on interferometric control with processing based on homodyne filtration. The device contains an additional source of illumination for image formation and an image receiver. The principle of operation of the known device is based on the analysis of the depth of the laser cut and offers a system that allows for efficient axial control of the laser to adjust the depth of laser ablation. The method includes many implementations of optical circuits and control circuits for correcting an additive manufacturing process or a laser welding process. In some implementations, the described device further comprises a circuit for creating interferograms according to their mathematical models and comparing with the interferogram obtained by reflection from the surface under study. Further, a correlation signal is obtained with the subsequent determination of the correspondence of the obtained result to a given limit value. One implementation of the method involves analyzing the speckle image to register changes in the subsurface layer of the material. An algorithm for processing such speckle interferograms using the Fourier method, autocorrelation, and Doppler shift is described. These algorithms provide an advantage when working in real time, but have a low phase reconstruction accuracy.

In the known device, the phase transition region is monitored during laser heat treatment. The closed-loop system implies the control of the following parameters of the laser beam used for material processing: its on / off; average power; pulse duration; peak intensity; energy; wavelength; pulse repetition rate; pulse energy; pulse shape and sweep speed; focal diameter; material feed rate; flow rate of cooling media. The device is mainly applicable for the analysis of the following indicators of the additive process: the depth of penetration of the laser beam into the material; change in depth and rate of change in depth; the height of the laser head in relation to the material surface. The main application technology is SLM, as well as some implementations of the optical scheme SLS and LENS.

The above devices for interferometric control of additive manufacturing have several main disadvantages: they are not universal, their application is mainly limited to the use in printing technologies such as SLM, SLS and some implementations of SLA and LENS. Most implementations of optical inspection schemes require a reflective, smooth material surface, which is a known problem for additive manufacturing products. In addition, interferometric methods require protection from vibrations that are inevitably present in the moving parts of an additive machine. The main disadvantage of using such an arrangement of the measuring part, in relation to the workpiece, is the ability to control only visible parts of the workpiece from one angle, as a rule, this is a small area of the part surface. Some solutions presented for the FDM printing technology have the advantage of the arrangement of the control devices - the lighting and receiving unit. This arrangement allows you to capture the full length of the printed part from one or more angles. The proposed approach is used for rapid scanning and prototyping of the product. In addition, the described system has a polar coordinate system for controlling the organs of an additive machine, which is well suited for efficient scanning of a product, but poorly applicable for printing and processing parts that have a shape other than axisymmetric.

A technical problem is the elimination of these shortcomings and the creation of a technical solution that makes it possible to increase the accuracy of manufacturing parts by means of additive manufacturing, save material (by reducing scrap, timely stopping the process with critical discrepancies with tolerances) and predicting changes in the linear dimensions of a part after it is separated from the substrate. The technical result consists in improving the quality of manufactured parts.

The problem posed is solved, and the technical result is achieved by the fact that in a device for controlling the additive manufacturing of a part containing an interconnected lighting unit in the form of a coherent radiation source with a lens, an interference unit and a computing unit, said interference unit is made on the basis of a shear interferometer containing an oblique and movable along the optical axis of the mirror, and the specified computing unit is configured to decode interferograms by the method of phase steps and calculate the current values of internal stresses in the part. The device can be equipped with a three-dimensional scanning unit containing at least two cameras installed on different sides of the part and connected to the computing unit, while the computing unit is configured to convert images from these cameras into the resulting three-dimensional model of the manufactured part.

FIG. 1 shows the algorithm of the proposed device;

in fig. 2 is a general view of the proposed device;

in fig. 3 is an optical diagram of a sherographic camera as part of the proposed device.

The additive manufacturing of a part using the proposed device involves layer-by-layer application of the material followed by curing using a 3D printer in accordance with a given CAD model of the part. In the process of forming the part, optical control is carried out to correct, if necessary, the current print parameters. During optical inspection, the current values of internal stresses in the printed part are determined every n layers. To do this, speckle interferograms of the part are obtained using a shear interferometer (for example, based on a shearographic camera), decoded by the method of phase steps, and the current values of the derivative of the deformation, which characterizes the internal stresses, are calculated. Finally, the current print parameters are corrected if the specified limit value is exceeded. Additionally, in the course of optical inspection, three-dimensional scanning of the shape of the printed part is carried out every m layers, and the current printing parameters are corrected in case of exceeding the limiting difference of the scan results from the specified CAD model.

A special feature is the step-by-step control of the printed part, which is performed through a certain number of printed layers.

Typically, during additive manufacturing, the workpiece is heated by a laser or other method. In some variants of additive manufacturing, photopolymerization occurs - the hardening of the material under the influence of radiation. In both cases, during the printing process, transient and phase processes occur in the material, which are accompanied by a change in the linear dimensions of the workpiece (printing layers in relation to each other). At this stage, optical control is very important for the timely introduction of corrective actions in the additive manufacturing mode through a feedback system. At least one printed layer can be inspected. The manufacturing process may include combining a deformation derivative image with a three-dimensional reconstruction of the part using three-dimensional scanning techniques to visualize the printing process.

The utility model can be implemented on the basis of an additive machine 1 with the printing technology Fused deposition modeling (FDM) or Fused Filament Fabrication (FFF) or Selective laser sintering (SLM), or another method of additive manufacturing, where the material is heated, followed by cooling, or the phenomenon of photopolymerization is used. The additive machine 1 contains a base 2 (substrate) for growing a part, which moves along the X axis and a print head (extruder) moves along the Y and Z axes, or has a bath containing powder for further sintering. Additive machine 1 is equipped with the proposed control device for additive manufacturing of part 3.

The proposed control device consists of an interconnected lighting unit, an interference unit, a computing unit and a three-dimensional scanning unit - a 3D scanner 5 directed to the print area, which can receive information about the profile of part 3 based on the principles of using structured light and / or laser scanning and / or photogrammetry (photogrammetric profilometers, structured light profilometers, scanning laser profilometers, etc.).

The interference unit is made on the basis of a sherrographic camera 4, the illumination unit is equipped with a source of coherent radiation in the form of a laser 6 with a scattering lens 7 to illuminate the workpiece with a wide beam. The light reflected from the part 3 enters the optical circuit of the interference unit through the lens 8. The sherrographic camera 4 implements the shear interferometer circuit and contains a beam splitter 9, which divides the radiation reflected from the object into two beams. To set the transverse shift of one beam relative to the other and adjust the sensitivity, the installation uses an inclined mirror 10 installed in the reference channel and having the ability to rotate through an angle α. The lateral shift can be adjusted in two directions. To implement the method of phase steps, the interference unit is equipped with a phase-shifting element in the form of a mirror 11 moving along the optical axis, fixed in the reference channel of the interferometer and discretely moving by means of a piezo drive. In the plane of the matrix of the video camera 13 with the help of the optical system 12, consisting of two lenses, a shear speckle interferogram of the object is formed.

The use of a sherrographic camera 4 makes it possible to sequentially record a number of speckle interferograms of an object with a phase shift and, after computer processing, obtain a phase image that is a derivative of the deformation of the part. Specifying the transverse shear between the beams allows you to tune the required range of displacement (deformation) of the material [Steinchen W., Yang L. Digital Shearography: Theory and Application of Digital Speckle Pattern Shearing Interferometry. USA. 2003. - 330 p], typical for a particular production process. The creation of a rigid structure of the sharographic chamber 4, having small lengths of the interferometer arms, provides good vibration resistance, in comparison with the scheme of a conventional speckle interferometer, and therefore can be built into the additive manufacturing process.

The computing unit is configured to decode speckle interferograms and calculate the current values of internal stresses in the part. For this, the method of phase steps is used [Vishnyakov G., Levin G., Minaev V., Nekrasov N. Advanced method of phase shift measurement from variances of interferogram differences // Applied Optics. - 2015. - Vol 54. - No. 15. - P. 4797-4804.]. One of the advantages of phase reconstruction using the phase step method over spatial methods (such as the Fourier method) is less noise and higher accuracy.

The use of a 3D scanner 5 in the construction of an additive machine tool makes it possible to build a three-dimensional model of part 3 from a point cloud during printing with an accuracy of up to ten microns, which is comparable to the thickness of one printed layer. The use of a sherrographic camera 4, in turn, makes it possible to register an image that is a derivative of the micro-displacements of the surface of the part 4, thereby giving information about the field of deformations arising on the surface of the printed layers. Sherrographic camera 4 is capable of working with both metals and dielectrics. The deformation registration accuracy reaches fractions of a micrometer, which makes it possible to judge the presence of internal stresses in the material of the part and subsurface defects, such as discontinuities, porosity and delamination, typical for layer-by-layer printing, by the picture of the deformation field.

The sherrographic camera 4 and the scanner 5 may not be tied to the moving elements of the additive machine and are located so that the registration direction and the used field of view of optical devices allow obtaining an image of one projection of part 3. At the same time, using a wide illumination beam and a wide viewing angle of the sherrographic camera 4 (as a rule, this is an area of up to 2 × 2 m), it is possible to illuminate the entire part, thus obtaining at least one two-dimensional projection of the part profile, containing all the printed layers (for FDM, LENS and SLA technology) or at least one printed layer (for SLM and SLS technologies).

Corrective actions or a decision to suspend printing are taken by the control device based on information about the geometry of the workpiece, the magnitude of deviations of the linear dimensions from the CAD model and the specified limit value.

The proposed utility model can be used to obtain information on the deformation of the workpiece, the dynamics of the change in linear dimensions, as well as internal stresses in the material, the presence of voids, cavities, delamination in real time during the process of printing the workpiece. The proposed technical solution makes it possible to experimentally visualize deformations and internal stresses directly in the process of three-dimensional printing of parts. The applied technical solution makes it possible to additionally monitor the deviations of the workpiece geometry from the CAD model and make decisions about stopping the printing process or correcting the control action. Using this approach, it is possible to visualize the internal stresses of the part being grown. This allows you to make timely changes to the additive manufacturing process, reducing the waste of material and time on rejected parts and post-processing procedures to give the part the finished dimensions laid down in the design documentation. The utility model makes it possible to diagnose non-optimal orientation of a part during growth, or incorrect implementation of supports for printing, by visualizing internal stresses.

Thus, as a result of the implementation of the proposed utility model, the quality of the manufactured parts is significantly increased.

Claims (1)

A device for controlling the additive manufacturing of a part containing an interconnected lighting unit in the form of a coherent radiation source with a lens, an interference unit based on a shear interferometer containing an inclined mirror movable along the optical axis, and a computing unit characterized in that it is equipped with a three-dimensional scanning unit containing two cameras installed on different sides of the part and connected to the computing unit, while the computing unit is capable of decoding interferograms by the phase steps method and calculating the current values of internal stresses in the part, as well as converting images from these cameras into the resulting three-dimensional model of the manufactured part.

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