Improved additive manufacturing monitoring method and system

Abstract

The present invention relates to a method for non-destructively detecting deviant additive manufacturing (AM) process behavior of a 3D printed solid part, the method comprising the steps of: applying an impact, preferably a mechanical impact, to the solid part; obtaining a vibration response of the solid part to the impact in the frequency domain; extracting a set of natural frequencies, and optionally damping, from the vibration response, each natural frequency corresponding to a vibration mode of the solid part; and, for at least one vibration mode, obtaining a natural frequency shift by comparing one of the set of extracted natural frequencies corresponding to the vibration mode with a reference natural frequency value of the vibration mode, thereby detecting the deviant AM process behavior.

Description

[Technical Field]

[0001] Technical Field The present invention relates to a non-destructive method for monitoring additive manufacturing processes with regard to porosity, sintering quality, elasticity, microcell clustering, defects, etc., which allows deviating process behavior to be observed in time and optionally corrected. The present invention also relates to a method for calibrating additive manufacturing equipment, for example by compensation for deviating process behavior. The present invention also relates to the field of non-destructive testing methods and systems by impact excitation measurement methods and systems. The present invention makes it possible to measure the amount of microcracks in a workpiece, thereby enabling a qualitative and quantitative assessment of long-term structural weaknesses in the workpiece. [Background technology]

[0002] background Additive manufacturing (AM), also known as three-dimensional or 3D printing, is a process that has gained significant momentum in recent years. It allows for the construction of three-dimensional objects from CAD models or digital 3D models. This allows materials to be deposited, joined, or solidified under controlled conditions to create three-dimensional objects, where materials such as liquid molecules or powder particles are added together, typically fused layer by layer. 3D printing's precision, repeatability, and material range have increased to the point where some 3D printing processes are considered viable as industrial manufacturing techniques. One of the key advantages of 3D printing is its ability to produce highly complex shapes or geometries that would otherwise be impossible to construct by hand, including hollow parts or components with internal truss structures for weight reduction.

[0003] It is often necessary to ensure that 3D printed parts meet required quality standards and be able to detect and characterize anomalies, so it is important to have metrology tools for mechanical characterization as well as proper anomaly detection, quality control, and monitoring.

[0004] Knowing when various types of deviations from expected values ​​appear increases the likelihood of early detection of critical defects in additively manufactured parts, providing the possibility for in-process intervention and thus reducing the time and cost of repair or rework. Because AM processes involve the gradual deposition of material, they offer a unique opportunity to monitor the quality of the material as it is deposited. To meet this need, there are various sensing methods and signals that can be measured. Among the available measurement modalities, acoustic-based methods have the advantage of potentially providing real-time, continuous, in-service monitoring of the manufacturing process at a relatively low cost.

[0005] Taheri, Hossein's doctoral thesis, "Nondestructive evaluation and in-situ monitoring for metal additive manufacturing" (2018), Graduate Theses and Dissertations, Iowa State University, 16675, discloses a method for in-situ monitoring of additive manufacturing processes. A proof-of-concept is demonstrated for acoustic measurements used to monitor both machine and material conditions. Analysis is performed on temporal and spectral features extracted from the acoustic signals. These features are generally related to defect formation and the acoustic noise generated, which can potentially characterize the process. A novel application of signal processing tools is used to identify temporal and spectral features of the acoustic signals. The method involves acquiring acoustic measurements during the additive manufacturing process and attempting to extract information from them. This appears to involve complex analysis and may be highly dependent on the exact manufacturing method and materials used.

[0006] U.S. Patent Application Publication No. 2019 / 234908 discloses an object analysis method that includes measuring the frequency-dependent natural vibration behavior of an object by dynamically mechanically exciting the object over a defined frequency range (f) by applying a test signal to generate object vibrations, and detecting the object vibrations generated in the object due to the excitation. The method further includes simulating the frequency-dependent natural vibration behavior of the object by generating a virtual digital representation of the object, performing finite element analysis based on the virtual representation, dynamically exciting the virtual representation in a simulated manner over a virtual frequency range to generate virtual object vibrations, calculating the virtual object vibrations generated in the object by exciting in the simulated manner, and deriving an object state based on a comparison of the measured natural vibration behavior and the simulated frequency-dependent natural vibration behavior.

[0007] EP 3309544 A1 discloses a method for in-process monitoring of a 3D manufacturing device or quality control of a 3D manufactured structure. The method includes a generating step of generating acoustic waves in the 3D manufactured structure. A receiving step receives the acoustic waves using a microphone array. An analyzing step analyzes a frequency spectrum of the acoustic waves. A determining step determines whether the frequency spectrum indicates a defect in the 3D manufactured structure.

[0008] U.S. Patent Application Publication No. 2020 / 057030 discloses a system and method for nondestructive testing of additively manufactured parts. An input mechanism excites an additively manufactured build structure, including a part on a build platform, with an excitation force (e.g., vibration) to induce a dynamic response in the part. An output mechanism (e.g., a non-contact transducer) senses the induced dynamic response in the part. A processor determines and examines a relationship between the response and the excitation to identify indications of defects in the part and communicates an alert if an indication is identified. The processor can compare the phase, magnitude, coherence, or time delay of the relationship to a reference relationship and / or compare modal frequencies or modal damping to a reference to identify deviations greater than a pre-established threshold.

[0009] EP 3658868 A1 discloses an apparatus for analysing the mechanical vibration response of a solid material sample, comprising an array of impactors arranged to deliver impacts at respective distinct points on the surface of the solid material sample, a sensor configured to capture the mechanical vibration response as a time-varying signal following impact of at least one impactor, and processing means configured to analyse the time-varying signal to determine the frequency and decay constant of the sinusoids constituting the time-varying signal. The invention also relates to a corresponding method for characterising a solid material sample.

[0010] WO2020254698 discloses a method for acoustically measuring material properties of a test part at elevated temperatures, the method comprising: a. heating the test part within a test temperature range; b. performing background measurements within the test temperature range by capturing vibration signals from the test part within a calibration period, thereby obtaining a noise signal; c. performing acoustic measurements on the test part within the test temperature range and within a test period by: c1. applying a vibration excitation to the test part; c2. capturing a vibration signal of the test part within a test period, thereby obtaining a vibration response signal to the vibration excitation; and d. obtaining material properties of the test part by analyzing the vibration response signal, thereby taking into account the noise signal. The present invention also relates to a system for acoustically measuring material properties of a test part at elevated temperatures.

[0011] Both of the latter prior art documents in the name of the applicant relate to methods and apparatus for performing impact excitation (IE) measurements. IE techniques essentially consist of impacting a workpiece and acquiring and analyzing a vibration response signal, also called an acoustic response signal. The response signal can be captured via several methods, including using a microphone, a piezoelectric displacement sensor, and / or a laser interferometer.

[0012] IE measurements can be used to non-destructively test workpieces to quantitatively assess their quality. This can be used for workpieces of any type of material, such as metals, alloys, polymers, ceramics, or any combination thereof. IE measurements, and in particular the present invention, may be particularly useful for characterizing 3D-printed workpieces, also known as additively manufactured workpieces. In such workpieces, the additive manufacturing (AM) process can result in non-uniformity in the quality of the workpiece, i.e., some areas of the workpiece may be of lower quality than other areas.

[0013] The quality of a workpiece, particularly a 3D-printed workpiece, can be characterized by measuring its porosity. Typically, higher porosity indicates more defects and thus a lower-quality workpiece. Porosity can be measured using tomography, such as X-ray micro-computed tomography (CT). X-ray micro-CT has the advantage of recreating a complete 3D model of the scanned object, allowing nondestructive testing of internal features or defects. However, X-ray micro-CT is an expensive and time-consuming measurement method. Furthermore, the inventors have found that X-ray micro-CT cannot resolve all types of quality-degrading structural characteristics. For example, micro-CT does not have good sensitivity for detecting microcracks. Therefore, the present invention aims to provide a method that can obtain a quality assessment of an AM workpiece with respect to both porosity and microcracks in a single measurement, or at least in a single measurement setup. Summary of the Invention [Problem to be solved by the invention]

[0014] The present invention aims to overcome these and other problems by providing a method for monitoring additive manufacturing processes that is easier to use and / or implement than prior art methods and that allows for accurate measurement of deviant AM process behavior based on acoustic measurements. [Means for solving the problem]

[0015] Summary of the Invention The present invention relates to a method for non-destructively detecting deviant additive manufacturing (AM) process behavior of a 3D printed solid part, the method comprising: - subjecting the solid part to an impact, preferably a mechanical impact; - obtaining the vibration response of the solid part to an impact in the frequency domain; - extracting a set of natural frequencies, and optionally damping, from the vibration response, each of said natural frequencies corresponding to a vibration mode of said solid-state part; for at least one vibration mode, obtaining a natural frequency shift by comparing one of the set of extracted natural frequencies corresponding to said vibration mode with a reference natural frequency value of said vibration mode, and preferably obtaining a porosity value of the solid part from said natural frequency shift, thereby detecting a deviant AM process behavior; Includes.

[0016] Preferably, a set of dampings is also extracted from the vibration response, each of which corresponds to a natural frequency of the set of natural frequencies. These dampings can be extracted for at least one, preferably each, natural frequency. For at least one natural frequency, at least one damping parameter is calculated from the damping corresponding to the natural frequency, thereby obtaining a microcrack volume value for the solid part that allows for detecting deviant AM process behavior that would not be detected by natural frequency shift alone. The damping thus relates to how a particular natural frequency echoes after an impact is applied to the solid part, in particular, the time after the impact during which the natural frequency can still be seen in the vibration response. The damping can be extracted from the time-domain response of the vibration response and / or from the frequency-domain response of the vibration response, preferably from the frequency-domain response. The damping parameter, also referred to as the damping coefficient, can be calculated from the damping. Typically, the damping can be quantified by looking at the natural frequency peaks in the frequency spectrum of the vibration response. Preferably, the damping parameter relates to the width of the natural frequency peaks in the vibration response, for example, the width of the natural frequency peaks at half maximum. Alternatively or additionally, the damping parameter may relate to the shape of the natural frequency peak in the vibration response, for example a skew damping parameter and / or a kurtosis parameter.

[0017] The inventors have found that the natural frequency shift thus obtained is directly related to the porosity of the solid part, which in turn is related to the amount of voids, including medium-sized and large-sized radii. Furthermore, the inventors have found that the damping parameter of the natural frequency is closely related to the amount of microcracks in the solid part, which in turn is related to the durability, i.e., long-term strength, of the solid part. It should be noted that microcracks are essentially linear in structure and therefore do not significantly contribute to the porosity of the solid part. However, the present invention makes it possible to obtain information about both porosity and microcracks with a single measurement and / or a single measurement setup, thus also making it possible to obtain a quantitative assessment of the quality of the solid part based on the natural frequency shift and damping parameter. This quantitative assessment also applies to the AM method used to fabricate the solid part. The quantitative assessment may be preferably performed by a scoring algorithm, which calculates a score depending on the values ​​of one or more natural frequency shifts and one or more damping parameters, and optionally, depending on a set of thresholds, preferably selected to ensure a minimum quality of the solid part.

[0018] The present invention also relates to a system for non-destructively detecting deviant additive manufacturing (AM) process behavior of a 3D printed solid part, the system comprising: an impactor, preferably a mechanical impactor, for impacting the solid part; a sensor for acquiring the vibration response of a solid part to an impact; a processing means, Calculate the vibration response in the frequency domain, extracting a set of natural frequencies, and optionally damping, from the vibration response, each of said natural frequencies corresponding to a vibration mode of said solid-state component; obtaining, for at least one vibration mode, a natural frequency shift by comparing one of the set of extracted natural frequencies corresponding to said vibration mode with a reference natural frequency value for said vibration mode, and preferably obtaining a porosity value of the solid part from said natural frequency shift, thereby detecting deviant AM process behavior; A processing means configured as follows: Equipped with.

[0019] Preferably, the processing means is configured to obtain a set of dampings from the vibration response, each of said dampings corresponding to a natural frequency of said set of natural frequencies, and these dampings may be extracted for at least one, preferably each, natural frequency, whereby the processing means is configured to calculate, for at least one natural frequency, at least one damping parameter from the dampings corresponding to said natural frequency, thereby obtaining a microcrack volume value for the solid-state component.

[0020] EP 3435044 A1 discloses an apparatus for analyzing the mechanical vibration response of a solid material sample, comprising at least one impactor arranged to impact each distinct point on the surface of the solid material sample, a sensor configured to capture the mechanical vibration response as a time-varying signal following the impact of the at least one impactor, and processing means configured to analyze the time-varying signal to determine the frequency and decay constant of the sinusoidal waves constituting the time-varying signal. The invention also relates to a corresponding method for characterizing a solid material sample. The method and apparatus described in this document can be used to perform measurements of the vibration response of solid parts to impacts.

[0021] In a further aspect, the present invention relates to a method for calibrating a 3D printing device, the method comprising: a) forming a set of solid parts on a printing plate by 3D printing; b) obtaining location information of the deviant process behavior by detecting the deviant process behavior in any of the solid-state components using the method described above; c) calibrating the 3D printing device taking into account the position information of the deviating process behavior on the 3D printing device; Includes.

[0022] The present invention also relates to a method of operating a 3D printing system, the method including performing a calibration method for at least one set of solid parts at a plurality of measurement events during the formation of the solid parts, whereby a reference natural frequency value at each measurement event is a predetermined target reference natural frequency for the measurement event, whereby processing parameters of a 3D printing system are manipulated based on the natural frequency shift.

[0023] In a preferred embodiment, the vibration response is acquired by an acoustic sensor. The acoustic sensor may be in contact with the solid part or may be located remotely from the solid part. Preferably, in the present invention, the acoustic sensor is located remotely from the solid part. This allows for in-situ and in-process monitoring without the difficulty of ensuring that the acoustic sensor is in contact with the solid part being built.

[0024] In a preferred embodiment, the method is applied in situ. This preferably applies the method during the additive manufacturing process. This allows for in situ monitoring of the AM process in real time, which can help identify during which part of the AM process deviant behavior occurs. If the AM process involves printing a solid part layer by layer, in situ monitoring can also identify where defects may occur in the solid part. It can also make it possible to manipulate the AM process. [Brief explanation of the drawings]

[0025] [Figure 1] 1 is a schematic diagram of an embodiment of an apparatus according to the present invention; [Figure 2]FIG. 1 illustrates a preferred embodiment of an apparatus for analyzing the mechanical vibration response of a solid material sample. [Figure 3a] FIG. 1 illustrates a method according to the invention performed on a solid part being 3D printed. [Figure 3b] FIG. 1 illustrates a method according to the invention performed on a solid part being 3D printed. [Figure 4] 1 illustrates an exemplary embodiment of a method according to the present invention; [Figure 5A] FIG. 1 shows a set of 3D printed open cube-shaped lattice structures. [Figure 5B] 1 shows a set of 3D printed open cube shaped lattice structures, and two sets of lattice structures tested according to the present invention. [Figure 6] FIG. 10 shows the natural frequencies and corresponding damping parameters of ten different AM solid parts. DETAILED DESCRIPTION OF THE INVENTION

[0026] Detailed Description of the Invention The present invention relates to methods and systems for non-destructively detecting deviant additive manufacturing (AM) process behavior of 3D printed solid parts as defined above and in the claims, and as further defined herein.

[0027] The detection method includes the steps of: applying an impact, preferably a mechanical impact, to a solid-state part; obtaining a vibration response of the solid-state part to the impact in the frequency domain; extracting a set of natural frequencies and optionally damping from the vibration response, each of the natural frequencies corresponding to a vibration mode of the solid-state part; and, for at least one vibration mode, obtaining a natural frequency shift by comparing one of the set of extracted natural frequencies corresponding to the vibration mode with a reference natural frequency value of the vibration mode, thereby detecting a deviant AM process behavior. Preferably, the detection method includes extracting a set of dampings from the vibration response, each of the dampings corresponding to a natural frequency of the set of natural frequencies; and, for at least one natural frequency, obtaining a microcrack volume value of the solid-state part by calculating at least one damping parameter from the damping corresponding to the natural frequency.

[0028] Here, the natural frequency shift refers to the change in the natural frequency peak in the observed response in the frequency domain. This change can be the difference in the peak frequency of the vibration mode between the extracted natural frequency and the reference natural frequency, but it can also refer to the change in peak width. In the latter case, the natural frequency shift can be quantified, for example, by looking at the change in frequency at half the peak height. A broadened peak tends to indicate increased damping due to increased internal friction, which may be caused by microcracks.

[0029] The system for acquiring the response can include an apparatus for analyzing the mechanical vibration response of a solid material sample, an embodiment of which is shown schematically in FIG. 1. This embodiment can be used to test 3D solid parts by placing the solid part on a sample holder. The apparatus 100 includes at least one impactor (i.e., one or more impactors) 110 configured to impact each clearly defined point on the surface of the solid material sample 99. Without loss of generality, the apparatus 100 is shown with a single impactor 110. The apparatus 100 further includes a sensor 120 configured to capture the mechanical vibration response of the sample 99 as a time-varying signal following the impact of the at least one impactor 99. A suitable sample holder 199, for example, comprising a linear support, is provided to allow the sample 99 to vibrate freely during impact. The apparatus 100 further includes processing means 130 configured to analyze the time-varying signal to determine the frequency and decay constant of the sinusoidal wave that constitutes the time-varying signal. The impactor 110 is preferably attached to the rest of the apparatus and may comprise one or more automatically actuated hammers (i.e., weights attached to an actuated, optionally spring-loaded, arm) or other projectiles. The sensor 120 may comprise a contact sensor such as a piezoelectric sensor, a non-contact sensor such as a microphone or laser vibrometer, or an accelerometer. Good results can be obtained with microphones, laser vibrometers, or accelerometers, as well as contact-based sensors such as piezoelectric sensors. The piezoelectric sensor can be located within a probe, which may further include a signal amplifier, a means for illuminating a spot where the mechanical vibration response is captured (e.g., a small laser source configured to illuminate the expected point of contact with the spot of light as the probe moves toward the sample's surface), and / or a temperature sensor. The probe is preferably equipped with a means for measuring the force applied to the sample by the sensor. The apparatus may include a means for automatically bringing the probe into contact with the sample, such as an actuated arm or a more complex robotic support. In this case, the force measurement means provides the necessary feedback to ensure good contact between the sensor and the sample.If the probe is manually manipulated by the user, the device can be configured to provide visual and / or audible feedback to help the user maintain contact force within a predetermined target range (e.g., an LED can illuminate one color if the applied force is too great and another color if the force is too small; an intermittent beep can have a different pitch or period depending on whether the applied force is too high or too low, etc.). If the device includes two or more impactors, these may be arranged, for example, as a linear array or a rectangular grid. If linear, the device can function as a scanner, moving the sample down a line of impactors for successive tests on different portions of the sample, or moving a line of impactors across the sample for successive tests on different portions of the sample. The device is preferably equipped with precisely controllable motors to provide the necessary relative motion of the sample and impactor. If the impactors are arranged as a grid, such relative motion is only necessary if the extent of the grid does not cover the entire region of interest on the sample. The apparatus preferably includes a means for determining the geometric properties of a solid material sample (not shown), such as a 3D scanner, a camera (optionally a stereo camera), or the like. As is evident from the above scheme, accurate knowledge of the sample's geometry is important for deriving the correct values ​​of Young's modulus and shear modulus from the detected frequency peaks. A scale (not shown) can be incorporated into the apparatus to determine the sample's weight, and therefore its mass. As is evident from the above scheme, accurate knowledge of the sample's mass is also important for deriving the correct values ​​of Young's modulus and shear modulus from the detected frequency peaks. The apparatus 100 can include a conventional user interface, which can include a screen 140 (preferably a touchscreen), buttons or dials 150, a keypad (not shown), or the like.

[0030] When in-situ measurements of solid parts are envisioned, a preferred embodiment of an apparatus for analyzing the mechanical vibration response of a solid material sample is shown in FIG. 2. The apparatus can be arranged in and around a 3D printing chamber (200) having a print head (201). The apparatus includes a printing plate (202) on which one or more solid parts (203) can be additively manufactured. The printing plate (202) is suspended by support structures (204, 205) that allow the test part to vibrate as freely as possible, for example by supporting it at the vibration node positions of the test part. An acoustic sensor (210, 211) or multiple acoustic sensors (210, 211), which may comprise a microphone or laser interferometer, can be arranged in the 3D printing chamber to obtain the vibration response of the solid part to an impact.

[0031] In a preferred embodiment, the acoustic sensor comprises a laser interferometer, which is particularly useful for measurements in a vacuum and allows for contactless measurements. Alternatively or additionally, the acoustic sensor may comprise an ultrasonic measurement sensor and / or a time-of-flight sensor and / or a Doppler sensor, for example as described below.

[0032] -SR.Huang, RMLerner, KJParker, "Time domain Doppler estimators of the amplitude of vibrating targets", J.Acous.Soc.Am., 91(2), 965-974(1992) -J. Tapson, "High precision, short range ultrasonic sensing by means of resonance mode-locking", Ultrasonics, 33, 6, 441-444 (1995) - R. Kazys, R. Sliteris, L. Mazeika, "Ultrasonic technique for Vibration Measurements", Proceedings of the 15th World Conference on Non-Destructive Testing, October 15-21, 2000, Rome, https: / / www.ndt.net / article / wcndt00 / papers / idn246 / idn246.htm These types of sensors may also be used for non-contact measurements.

[0033] In one embodiment, the method of the present invention is implemented such that the 3D printing chamber contains a pressure below atmospheric pressure, preferably 0.5 bar or less, more preferably 0.2 bar or less, and most preferably essentially a vacuum. This allows for non-contact vibration measurements at low to zero pressure, preferably using a laser interferometer. Measurements down to zero pressure attenuate ambient noise, thereby increasing the signal-to-noise ratio.

[0034] The apparatus preferably includes an impact system comprising a ballistic impactor (212) that can be impacted by an impactor actuator (213). The ballistic impactor (212) can be a ceramic or metal rod that can be fired upward (214) towards the test part using the impactor actuator (213). a guide tube (215) through the bottom of the heating chamber for guiding the ballistic impactor preferably along the vertical direction; an electromechanically operated hammer (216) configured to impart a preferably perpendicular impact (217) to the impactor (212); The hammer (216) may include an electric coil (218) and a movable rod or projectile (219) that can move in response to an electric current flowing through the coil (218). An example of such a system is presented in U.S. Patent No. 6,782,970, whereby in the present invention, the projectile of the impactor actuator impacts the ceramic impactor (212) rather than directly on the test part. Alternatively, a pressure-driven impactor or pressure-driven impact actuator (280) may be used to deliver the impact to the impactor.

[0035] The impactor actuator is configured to deliver an impact to the ballistic impactor. Preferably, a noise measurement step is performed whereby the ballistic impactor is raised to within 2 cm below the printing plate or solid part, but without contacting the printing plate or solid part. The impactor actuator is then preferably configured to deliver an impact to the ballistic impactor that brings it into contact with the printing plate or solid part, thereby mechanically inducing vibrational excitation in the printing plate and / or solid part.

[0036] The vibration response of a solid-state component is a time-varying signal that can be obtained via a vibration sensor. The vibration response can then be further analyzed. Therefore, the system of the present invention preferably includes processing means configured to analyze the time-varying signal to determine the frequencies and decay constants of the sinusoids that make up the time-varying signal, i.e., to solve the harmonic inversion problem. The harmonic inversion problem, more generally, consists of determining the frequencies, decay constants, amplitudes, and phases of sinusoids that make up a discrete-time, finite-length signal consisting of the sum of a finite number of such sinusoids in a given bandwidth. This problem is well known in the literature, but is rarely related to impulse excitation techniques (IET). In their paper "Harmonic inversion of time signals and its applications," The Journal of Chemical Physics 107, 6756 (1997), Vladimir A. Mandelshtam and Howard S. Taylor describe the use of Wall and Neuhauser's general filter diagonalization method to solve the harmonic inversion problem by re-evaluating it as a small matrix diagonalization problem. Computer-based implementations of this technique are known in the art, including the "Harminv" program by Steven G. Johnson of the Massachusetts Institute of Technology. The results of the analysis can be output to a screen 140 or to any other suitable interface for storage or further processing by other equipment. The processing means may be comprised of one or more dedicated hardware components (e.g., ASICs), appropriately configurable hardware components (e.g., FPGAs), microprocessors with appropriate software, or combinations of the above. The same components may perform other functions.

[0037] Thus, the vibration response of the solid part to an impact in the frequency domain can be obtained. From this response, a set of natural frequencies and, optionally, damping can be extracted, each of the natural frequencies corresponding to a vibration mode of the solid part. For at least one vibration mode, preferably two or more vibration modes, and more preferably all vibration modes, a natural frequency shift can be obtained by comparing one of the extracted natural frequencies corresponding to the vibration mode with a reference natural frequency value for the vibration mode, thereby detecting deviant AM process behavior. As mentioned above, the natural frequency shift can include a change in the width of a peak in the response, which can be quantified, for example, by looking at the change in half-peak frequency. When a set of damping is extracted for the natural frequencies, one or more damping parameters can be calculated from one or more of the damping parameters. From these damping parameters, values ​​indicative of the amount of microcracks in the solid part can be estimated. Microcrack and picrocrack propagation play a crucial role in the fatigue life and fracture strength performance of AM-manufactured parts. Without wishing to be bound by theory, it is believed that microcracks cause internal friction in the AM material, which results in vibration damping, especially attenuation of the natural frequency of the vibration response to impact. Microcracks result in spherical voids that do not create extreme stress concentrations; i.e., voids increase porosity, but are not as important for the long-term strength of the solid part as microcracks.

[0038] In a preferred embodiment, the reference natural frequency value is obtained from a database. This database can be obtained by measuring the natural frequencies of solid parts of various shapes and sizes, preferably made of the same material as the solid part being additively manufactured. The reference natural frequency value can then be obtained by direct comparison with values ​​from the database related to solid parts of the same shape, size, and material, or by interpolation based on values ​​from the database. Alternatively or additionally, the reference natural frequency can be obtained from a natural frequency locus. This natural frequency locus can preferably describe the natural frequency of a solid part having a determined cross-section as a function of the height of the solid part, where height refers to the size of the solid part as measured in a direction perpendicular to the cross-section. For example, the solid part can have a rectangular cross-section with width w and length l, and the natural frequency locus describes the dependence of the natural frequency of the solid part on its height. Alternatively or additionally, the natural frequency locus can be obtained from calibration measurements. This allows the natural frequency locus to be determined via a set of natural frequency measurements performed at different moments during the calibration AM process of a calibration part, which is essentially the same as the solid part, i.e., made from the same material and with the same size and shape. The quality of the calibration part can be sufficiently checked, and if approved, the natural frequency measurements obtained during the manufacture of the calibration part can be used to construct the natural frequency locus. The natural frequency locus can be constructed as a function of one or more characteristics of the calibration part and / or the calibration AM process. Preferably, these characteristics include one or any combination of the following: - The moment of measurement within the AM process, for example the period between the start of the AM process and the measurement, optionally taking into account dead periods during the AM process, i.e. periods when the AM process has stopped or slowed down.

[0039] the size of the calibration part when measured, for example by obtaining one or more sizes of the calibration part and / or solid part when measured, whereby these one or more sizes can preferably be obtained by size measurement.

[0040] When a solid part is being manufactured, the natural frequency values ​​obtained for the solid part during the AM process can be compared to the natural frequency values ​​on the natural frequency locus of the corresponding characteristic. For example, if the AM process for the solid part follows the same procedure as the AM process for the calibration part, the natural frequency value obtained for the solid part at time T should be compared to the reference natural frequency value obtained from the natural frequency locus at the same time T. If the AM process for the solid part does not follow the same procedure as the AM process for the calibration part, the characteristic preferably includes one or more sizes. In this case, the natural frequency values ​​obtained for the solid part when the solid part has a specific size can be compared to the natural frequency locus for that specific size.

[0041] This embodiment is shown in Figures 3a and 3b. In Figure 3a, a solid part (301) is 3D printed on a printing plate (302). Several frequency measurements using impact excitation techniques are performed at regular intervals corresponding to specific heights (303) of the 3D printed part (301). The results can be seen in Figure 3b, which shows the reference natural frequency locus (304) as a function of the height (306) of the solid part being printed, and the measured natural frequency (305) as a function of said height (306). The deviation between the natural frequency values, i.e., the observed natural frequency shift (308), indicates deviant process behavior at a specific height. Note that at the full height (309), the frequency shift may disappear, even if it exists at intermediate heights, demonstrating the importance of performing multiple measurements at different heights for complete monitoring. At the same time, the damping parameters of the natural frequency can be calculated from the damping of the vibration response at this natural frequency. One particular advantage of looking at damping is that it is not expected to change significantly during the fabrication of a solid part; that is, as the solid part is fabricated, its natural frequency changes, but the damping at the natural frequency remains fairly constant, or changes very slowly. Large deviations in damping can be seen in large changes in the damping parameters as the solid part is printed, for example, a jump in the damping parameter value when the solid part reaches a certain height, which may indicate a large amount of microcracks at that height.

[0042] It should be noted that the eigenvalue trajectory can be based on a database such as that described above, or can be based on measured eigenfrequencies of various shapes and sizes of solid parts and materials of the same solid part being additively manufactured. Embodiments in which the reference eigenfrequency values ​​are derived at least in part from the eigenfrequency trajectory are particularly well suited for observing deviant AM process behavior, particularly in situ and / or in real time.

[0043] In one embodiment, the reference natural frequency value may be obtained by numerical calculation and / or simulation. Note that such calculation and simulation may be combined with measurements of natural frequency values ​​of solid-state or calibration components to obtain the best reference natural frequency value.

[0044] In a preferred embodiment, the detected deviant process behavior is related to improper sintering of the solid part. The improper sintering results in changes in vibration parameters. Without wishing to be bound by theory, the inventors believe that a faulty sintering process has a significant effect on the stiffness of the solid part, typically a decrease in stiffness, which can be observed in different material parameters such as elastic modulus (Young's modulus, shear modulus, ...) and damping coefficient. These material parameters are closely related to the acoustic response of the piece to mechanical impacts. Therefore, the present invention is particularly suitable for detecting faulty or incomplete sintering of 3D printed solid parts.

[0045] In the present invention, the extracted natural frequencies correspond to vibration modes of the solid-state component. The vibration modes can be any modes that provide information about the distributed mechanical elastic behavior of the solid-state component. In a preferred embodiment, the vibration modes can include any or any combination of bending, torsional, longitudinal vibration modes, or any harmonics thereof, such as the first, second, or third harmonics, more preferably the first harmonic of the bending mode, the second harmonic of the bending mode, the first harmonic of the torsional mode, the second harmonic of the torsional mode, or the first harmonic of the longitudinal vibration mode.

[0046] In a preferred embodiment, the vibrational modes include torsional modes and / or any harmonics thereof.

[0047] In one embodiment, deviant process behavior is detected by obtaining one or more natural frequency shifts and / or by considering the peak width of the extracted vibration response.

[0048] The present invention also relates to a calibration method, said method comprising the following steps: a) forming a set of solid parts on a printing plate by 3D printing; b) obtaining location information of the deviant process behavior by detecting the deviant process behavior in any of the solid-state components using the method described above; c) calibrating the 3D printing device taking into account the position information of the deviating process behavior on the 3D printing device; Includes.

[0049] The solid-state components used to obtain position information are also called calibration components. In a preferred embodiment, the set of solid parts may be arranged in a predetermined pattern, preferably a matrix pattern, relative to the printing plate. This allows for obtaining location information of the deviant process behavior in the XY position of the printing plate. This is preferably done by identifying in which solid part the deviant process behavior occurs. There are several ways in which this identification can be done.

[0050] An impact can be applied to each solid part separately and the vibration response of said solid parts obtained. This process may preferably be repeated for two or more, preferably each, of the solid parts.

[0051] The impact may be applied to two or more solid state components simultaneously, preferably to all solid state components simultaneously, and the response of said two or more solid state components may be obtained, for example, using a set of vibration sensors.

[0052] The solid components of the set may preferably comprise distinct reference natural frequency values, preferably uniquely different natural frequency values. This may be achieved, for example, by solid components having different cross sections and / or different material properties. This allows two or more, preferably all, solid components to be impacted simultaneously. The distinct natural frequency values ​​of the solid components then allow the vibration responses of two or more, preferably all, solid components to be obtained essentially simultaneously.

[0053] Furthermore, from the vibration response, a set of damping parameters can be calculated that provide further information about the solid part and about deviations from an ideal AM process, especially with regard to the amount of microcracks.

[0054] The predetermined pattern may be a preferably rectangular matrix pattern in the xy plane, which is preferably a horizontal plane.

[0055] An exemplary embodiment of the method according to the present invention is shown in FIG. 4. A set of solid parts (401, 402, 403, 404, 406) is 3D printed on a printing plate (405). In the figure, four of these solid parts (401, 402, 403, 404) are printed at the corners of the printing plate, but other configurations can also be used. These four corner solid parts serve as calibration parts. In a calibration printing run, the four solid parts are printed without any additional solid parts being 3D printed. The natural frequency measurements of each of the calibration parts can be performed at regular intervals, for example, corresponding to different heights of the parts. The parts can have different cross-sections so that their natural frequencies are sufficiently separated and can be obtained by a single measurement. The measurements can be performed by impacting the printing plate (405) (407) and acquiring the vibration response with a sensor (408), such as a microphone. The obtained natural frequencies can be compared to reference natural frequencies to obtain natural frequency shifts that indicate deviating process behavior. In the event of such deviating process behavior, the 3D printing device can be calibrated and / or recalibrated. Alternatively or additionally, the natural frequency shift of the solid parts, preferably the calibration parts, can be obtained by removing the solid parts from the 3D printing device and performing the method steps on each solid part individually or together. After this frequency shift measurement, the solid parts can be reintroduced into the 3D printing device, preferably in the same position as before the parts were removed. This process can be repeated, for example, after 3D printing layer by layer or every n layers.

[0056] Therefore, the present invention also relates to a method for additively manufacturing a production part, the method comprising additively manufacturing said production and a set of calibration solid parts in the same 3D printing process, and performing the method according to the present invention on one or more calibration solid parts of said set of calibration solid parts, whereby deviant process behavior observed on one or more calibration solid parts is indicative of deviant process behavior of a production part. This is particularly useful when a production part cannot be easily tested using the method according to the present invention, for example due to lack of knowledge of the reference natural frequency or because the production part includes a unique and / or complex shape.

[0057] Furthermore, in the absence of deviating process behavior or after calibration of the apparatus, measurements of the calibration part's natural frequency at different moments and / or heights in the printing process can serve as reference natural frequency values ​​or as input for constructing a reference natural frequency trajectory. This allows any other solid part (406) to be printed with at least one, preferably all, of the calibration parts. By implementing the method of the present invention for detecting deviating process behavior for any, preferably each, of the calibration parts, it is possible to infer whether the other solid part (406) is 3D printed correctly. This can be applied to any type of other solid part (406), regardless of its shape, size, or other characteristics. This clearly provides a significant advantage when printing many different types of solid parts or solid parts with complex shapes and sizes for which it is not easy to obtain reference natural frequency values.

[0058] After the printing process, the calibration part can be further tested for defects to check whether the aligned natural frequencies correspond to a well-printed solid part. In a further run, the calibration parts, in this exemplary case four calibration parts, can be printed along with any other solid parts (406) being printed.

[0059] As described above, the deviating process behavior can be detected by preferably impacting the printing plate, thereby impacting each of the solid parts formed thereon. The vibration response of each of the solid parts can preferably be obtained simultaneously, separately, subsequently, and / or in combination thereof. When the vibration response of each of the solid parts is obtained simultaneously, the response is preferably obtained by acoustic measurement, which can then preferably be frequency analyzed.

[0060] In one embodiment, the solid parts are essentially identical and deviating process behavior is detected by obtaining one or more natural frequency shifts and / or by considering the peak width of the extracted vibration response.

[0061] The calibration method of the present invention is preferably applied at different moments during the 3D printing process, which also allows obtaining positional information regarding deviating process behavior with respect to the distance from the printing plate, thereby obtaining three-dimensional positional information.

[0062] The set of solid parts can include any one or any combination of shapes such as beams, cylinders, strands, etc.

[0063] In a preferred embodiment, the reference natural frequency value of a first solid state part of said set is obtained by taking into account the extracted natural frequency of at least a second solid state part of said set.

[0064] The deviant process behavior may preferably be due to any one or any combination of the following:

[0065] defects, for example local defects which may be point defects, line defects, surface defects, volume defects, domain defects, microcracks, macrocracks, or any combination thereof; - global deviation parameters such as density, porosity and / or sintering quality, -Material parameters such as powder quality and / or powder material variability.

[0066] In one embodiment, the solid state components of the set differ in a predetermined manner, e.g., each solid state component has a different horizontal cross section, a different thickness, and / or a different width, and defects are detected by taking into account a shift in at least one natural frequency of the test component, thereby enabling simultaneous detection of defects in the different solid state components.

[0067] In one embodiment, the calibration method is performed multiple times during the formation of a set of solid parts, preferably at different heights of the solid parts, to obtain information about defect locations in three dimensions.

[0068] In one embodiment, the calibration method is performed in situ on the 3D printing machine, or alternatively, by removing a solid part from the machine after it has been formed, measuring it, remounting it, continuing 3D printing at a particular height, and repeating this process N times, where N is at least 2, to obtain 3D dimensional positional information regarding deviant process behavior.

[0069] In a preferred embodiment, the at least one set of solid state parts includes a production part and a set of calibration parts, and the calibration method is applied to the set of calibration parts.

[0070] The method of the present invention may also include the step of measuring, or at least obtaining through measurement, a damping parameter of the vibration response to the vibration mode, and detecting defects, preferably microcrack defects, based on the damping parameter.

[0071] Figure 6 shows the natural frequencies and corresponding damping parameters of 10 different AM solid parts obtained using the same AM method; i.e., the variation between different parts arises primarily from the statistical variability of the AM method. The results are shown as small dark and light circles. Each of the 10 parts was tested for vibration mode. The measured natural frequencies range from approximately 39,800 (F1) to 43,500 Hz (F10), as can be read off the x-axis. The lower set of (dark gray) circles represents the porosity of the corresponding solid part, given as a percentage on the left y-axis. 0% porosity means no porosity, i.e., a completely solid part. Porosity was measured directly using X-ray micro-CT. The part with a natural frequency of approximately 43,500 Hz (F10) had a low porosity of 0.01%, while the part with a natural frequency of approximately 39,800 Hz (F1) had a porosity of approximately 4.3%. The F10 natural frequency of approximately 43,500 Hz can serve as a reference natural frequency. A clear correlation can be seen in the figure, illustrated by the bottom graph (601) showing a fitted cubic curve for the data points, i.e., porosity measured by X-ray micro-CT versus measured natural frequency. The greater the shift in natural frequency from the reference value (F10), the higher the porosity. Furthermore, note that the current method appears to be very sensitive to very low porosity, as shown by the data points at frequencies F6, F7, F8, F9, and F10, all of which were measured to have very low porosity but still exhibit a measurable natural frequency shift. This also indicates that for high-quality parts with low porosity, the method can still distinguish between good and excellent quality.

[0072] For each natural frequency, the damping parameter was also measured (light gray upper circle) and plotted. The damping parameter in this case is the width of the natural frequency peak at half-maximum in the frequency vibration response of the corresponding solid part. The damping parameter exhibits a frequency shift dependence, which essentially increases damping for increasing frequency shift. However, the frequency shift dependence of the damping parameter is clearly different from that of porosity, indicating that this method allows for obtaining more information than, for example, X-ray micro-CT porosity measurements, particularly revealing additional information regarding microcracks in solid parts. Note that in this case, even though the data points for parts with natural frequencies F6, F7, F8, F9, and F10 all exhibit very small porosity (less than 0.01%), the damping parameter values ​​range from approximately 13 to 25.

[0073] It should be noted that additionally or alternatively, one or more damping parameters may be calculated, such as, for example, the half-width of the natural frequency peak, damping frequency, loss factor, percentage of critical damping, quality factor, decay constant, time constant, reverberation time, decay rate, logarithmic decrement, etc. [Example]

[0074] Example 1: Sensitivity of vibration parameters to localized defects To illustrate the possibilities offered by the method of the present invention, numerical calculations have been carried out to show the sensitivity of the vibration natural frequencies to local defects.

[0075] The simulated solid part has the shape of a beam with dimensions length x width x height (L x W x H) of 100 mm x 25 mm x 10 mm. Several natural frequencies of the beam were calculated: - the bending base mode (flex), its first and second harmonics, - torsional base modes (tors), their first and second harmonics, and -Longitudinal vibration mode (long) and its first harmonic.

[0076] A small localized defect of 1 mm x 1 mm x 1 mm was introduced and the same natural frequencies were then calculated. The small defect was located at four different locations: - Corner defects (near one of the apexes of the beam) -Central defect in the center of the beam -A central zero line defect located at half the width and half the height, but 0.25 of the length -Zero boundary line defect located on the long edge of 0.25 in length The resulting natural frequencies are compared in the table below (frequencies are given in Hz and relative shifts are given in %).

[0077] [Table 1]

[0078] [Table 2]

[0079] [Table 3]

[0080] [Table 4]

[0081] Sensitivity of particular modes to particular modes can be noted, particularly torsional base mode, torsional first order mode, and torsional second order mode, for example. [Example]

[0082] Example 2: Measurement of 3D printed lattice structures The method of the present invention was also applied to a set of 3D printed open cube-shaped lattice structures. Two sets of lattice structures were tested, each containing three lattice structures. The lattice structures within a set had the same lattice type, while lattice structures from different sets had different lattice structures. The lattice structures of the first set are shown in Figure 5a, and the lattice structures of the second set are shown in Figure 5b.

[0083] The structure was measured using Archimedes' method, two types of gas pycnometry, and impedance measurements using Foucault current measurements. These methods confirm that both groups of three cubes occupy the same volume, have the same electrical resistance, and all six have the same material density. However, the method of the present invention reveals the following: the first set of three identical cubes yields very good frequency uniformity (torsion mode). However, the last cube exhibits a broader peak and therefore higher damping, indicating higher internal friction, most likely caused by microcracks. Therefore, the present invention makes it possible to identify microcrack defects by the broadening of the frequency peak. In this case, the set of extracted natural frequencies includes the peak natural frequency and the natural frequency at half the peak height. By examining the frequency shift of these three natural frequency values, it is possible to estimate the presence of deviant process behavior that may have led to microcracks.

[0084] Furthermore, the second set of three cubes shows a very large difference in resonance behavior, with the first cube having a much higher elasticity compared to the second and third cubes, as indicated by the frequency difference (1st cube: 20,700 Hz, 2nd cube: 21,000 Hz, 3rd cube: 21,150 Hz for the torsional base mode). We believe that the reason for this very significant difference is the sintering quality, which involves better / poorer adhesion of the wires on the matrix points.

[0085] Therefore, the present invention is observed to be sensitive to deviant process behavior with better sensitivity than other methods such as Archimedes measurement method, gas pycnometry and impedance measurement method.

[0086] The method of the present invention requires only about 20 seconds per measurement when performed manually, but can be performed in less than 2 seconds per measurement in fully automatic in-line mode.

Claims

1. 1. A method for non-destructively detecting deviant additive manufacturing process behavior of a 3D printed solid part, comprising: applying a mechanical impact to the solid-state component; obtaining a vibration response of the solid-state component to the impact in the frequency domain; extracting a set of natural frequencies and a set of dampings from the vibration response, each of the natural frequencies corresponding to a vibration mode of the solid-state component, and each of the dampings corresponding to a natural frequency of the set of natural frequencies; For at least one vibration mode, obtaining a natural frequency shift by comparing one of the set of extracted natural frequencies corresponding to the vibration mode with a value of a reference natural frequency of the vibration mode, and obtaining a porosity value of the solid part from the natural frequency shift; and obtaining a microcrack volume value for the solid-state component by calculating, for at least one of the natural frequencies, at least one damping parameter from the damping corresponding to the natural frequency. detecting said deviant additive manufacturing process behavior by Including, the damping parameter is calculated from the damping, the damping being quantified by a natural frequency peak of a frequency spectrum of the vibration response; method.

2. The method of claim 1 , wherein the reference natural frequency is obtained from a natural frequency locus.

3. The method of claim 2 , wherein the natural frequency locus describes the natural frequencies of a solid part having a determined cross-section as a function of the height of the solid part.

4. 4. The method of claim 2 or 3, wherein the natural frequency locus is determined via a set of natural frequency measurements performed at different moments during a calibration AM process of a calibration part.

5. A method according to any one of claims 1 to 4, wherein the extracted natural frequencies correspond to vibration modes of the solid part, and the vibration modes include any or any combination of bending modes, torsional modes, longitudinal vibration modes, or first, second or third harmonics thereof.

6. The method according to any one of claims 1 to 5, wherein the vibration response is obtained by an acoustic sensor.

7. The method of any of claims 1 to 6, applied during an additive manufacturing process for 3D printing the solid part.

8. 1. A calibration method for calibrating a three-dimensional printing device, comprising: a) forming a set of solid parts on a printing plate by 3D printing; b) detecting a deviant additive manufacturing process behavior in any of said solid state parts using a method according to any of claims 1 to 7, thereby obtaining location information of said deviant additive manufacturing process behavior; c) calibrating the 3D printing device taking into account the position information of the deviant additive manufacturing process behavior on the 3D printing device; A calibration method including:

9. 9. The calibration method of claim 8, wherein the set of solid parts are arranged in a predetermined pattern relative to the printing plate, and the position information of the deviant additive manufacturing process behavior is obtained at X-Y positions on the printing plate by identifying in which solid parts the deviant additive manufacturing process behavior occurs.

10. Identifying in which solid parts the deviating additive manufacturing process behavior occurs can be achieved by the following method: The method according to any one of claims 1 to 7 is carried out separately for each solid state component of the set of solid state components. An impact may be applied simultaneously to two or more solid state components to obtain responses of the two or more solid state components.

10. The calibration method of claim 9, comprising any one or any combination of: the solid parts of the set include distinct reference natural frequency values, whereby the impact is applied to two or more solid parts simultaneously, whereby the vibration responses of the two or more solid parts are obtained simultaneously.

11. The calibration method according to any one of claims 8 to 10, wherein the calibration method is performed multiple times during the formation of the set of solid components, the calibration method being performed at different heights of the solid components.

12. The calibration method according to any one of claims 8 to 11, carried out in the 3D printing device.

13. A calibration method described in any one of claims 8 to 12, wherein at least one of the sets of solid parts comprises a production part and a set of calibration parts, and the calibration method described in any one of claims 8 to 12 is applied to the set of calibration parts.

14. 14. A method of operating a 3D printing apparatus, comprising performing the calibration method of any of claims 8 to 13 for at least one set of solid parts at a plurality of measurement events during formation of the solid parts, whereby the value of the reference natural frequency at each measurement event is a predetermined target reference natural frequency for the measurement event, whereby processing parameters of the 3D printing apparatus are manipulated based on the natural frequency shifts.

15. 1. A system for non-destructively detecting deviant additive manufacturing process behavior of a 3D printed solid part, comprising: a mechanical impactor for impacting the solid part; a sensor for acquiring a vibration response of the solid-state component to the impact; A processing means, calculating the vibration response in the frequency domain; extracting a set of natural frequencies and a set of dampings from the vibration response, each of the natural frequencies corresponding to a vibration mode of the solid-state component, and each of the dampings corresponding to a natural frequency of the set of natural frequencies; obtaining, for at least one vibration mode, a natural frequency shift by comparing one of the set of extracted natural frequencies corresponding to the vibration mode with a value of a reference natural frequency of the vibration mode, and obtaining a porosity value of the solid part from the natural frequency shift; and obtaining a microcrack volume value for the solid component by calculating, for at least one natural frequency, at least one damping parameter from the damping corresponding to the natural frequency. processing means configured to detect said deviant additive manufacturing process behavior by Equipped with the damping parameter is calculated from the damping, the damping being quantified by a natural frequency peak of a frequency spectrum of the vibration response; system.

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