More Than Beam Profiling

Transcription

1 More Than Beam Profiling A new approach for beam diagnostics in 3D additive manufacturing systems Andreas Koglbauer Meet us at Lasys Hall 4 Booth 4E52 With its unique design and fabrication possibilities, additive manufacturing (AM) is rapidly gaining relevance in a wide field of scientific and industrial applications. Especially when considering industrial exploitation, a sound knowledge and control of the influential factors dominating the process is crucial, to ensure constant quality. In addition to the properties of the laser spot as the actual tool processing the powder, also the behavior of the deflection unit plays a key role. However, current state-of-the-art diagnostic devices do not fully address these issues. Company PRIMES Pfungstadt, Germany While in other laser-based manufacturing processes, like laser welding or cutting, laser beam diagnostics is well established and a substantial element of quality management, this statement does not hold true for additive manufacturing, e.g., selective laser melting (SLM). This is not due to a lack of necessity, but rather of a suitable PRIMES has more than 25 years of experience in the development of diagnostics solutions for the complete range of laser technology and especially in custom tailored OEM beam analysis devices and absorbers. We offer laser beam diagnostics for quality assurance in industrial manufacturing, maintenance, trouble shooting as well as for R&D applications. Our focus lies on measuring devices for the characterization of lasers used for material processing and scientific applications. Products range from power meters and scanning beam diagnostics systems over to camera based beam profilers for high power lasers. Fig. 1 A novel measuring concept, based on stray-light detection of a scattering pattern allows unrivaled analysis possibilities for laser scanner systems. possibility to measure all the relevant parameters, which exceed the abilities of conventional beam diagnostics. Although a variety of beam diagnostic devices that are especially designed for 3D scanner applications exist, they all suffer from certain limitations. Some of the major obstacles are the space constraints inside an additive manufacturing machine and the variety of possible beam positions and angles of incidence, which typically restricts the analysis to the scanner zero point position. Also, the necessity of absorber cooling and umbilicals hinders the ease of use in the enclosed environment of an AM machine. In addition to that, the characterization of the optical setup of a scanner system is more comprehensive, since it also involves the deflection unit. This significantly extends the list of relevant parameters, linked to the properties of the beam delivery system. There are, of course, the conventional beam parameters like spot size, Rayleigh length, or beam quality, preferably not only in the central position but over the complete field of view. As in every high power laser application, thermally-induced focal shift is an issue worth investigating which, due to the elaborated optical setup of the imaging objective, may also be position-dependent. Beyond that, there are the scannerspecific questions: Field distortion or flatness, accuracy, and reproducibility with which positions or marking speeds are met compared to the programmed target values, or the accurate synchronization of scanner motion and laser illumination. And with regard to large additively manufactured work pieces requiring multiple laser machines, there is the nontrivial calibration task of precisely overlapping two (or more) individual scanning fields. Especially in industrial sectors with stringent safety regulations, like medicine or aerospace applications which aspire to a high degree of process stability, standards exist (e.g [1]), demanding 4 Laser Technik Journal 3/ WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2 Additive Manufacturing the regular monitoring of a majority of the above-mentioned measurands. Since quite a lot of these cannot be determined by a mere measurement of the power density distribution, various alternative methods, like the writing of test patterns and subsequent visual inspection of the results, are still common practice. Their working range on the other hand often is in a parameter regime which differs from the actual parameters optimized for the process. In conclusion, there is a need for a more complete system characterization with the ability to address the mentioned challenges. To meet these requirements, we developed a compact measuring instrument based on a novel measuring concept, capable of addressing scanner specific measurement tasks. We present proof-of-principle measurements performed with a laboratory prototype the ScanFieldMonitor and the determination of quantities so far inaccessible to conventional beam diagnostics. Working principle of the ScanFieldMonitor As stated earlier, the behavior of the beam steering apparatus is an essential component when considering the process quality in an AM machine. It should thus be under the scope of investigation when analyzing the optical beam delivery system down to the processing zone. This is why, in contrast to conventional beam diagnostics, our patented measuring concept uses a moving beam (Fig. 2a) instead of a stationary one [2]. The beam is scanned in a straight line (here-in after referred to as a vector) across an in-glass pattern, defining the measuring plane. A signal photodiode (PD) collects the scattered light when the beam crosses the pattern. The transmitted beam is expanded via a diffuser plate to reduce the power density to a noncritical level on the building plate below, which serves as an absorber. A second trigger photodiode provides us with information about the laser switching times. The relative temporal spacing and the width of the signals in the peak sequence (Fig. 2b) allow us to determine not only the beam width, but an absolute position measurement of the vector with respect to the pattern (Fig. 2c) and the marking speed. Since the position of the signal a signal plate pattern diffusor plate plate is referenced and the position of the device on the building plate in turn can be referenced, an absolute position measurement in the framework of the building plate is possible. So, while this novel measuring concept only provides an integrated power-density distribution, rather than a two-dimensional representation, the amount of hereby accessible, additional information is highly beneficial as will be elaborated on the basis of several typical application examples in the following. Beyond beam profiling laser beam signal PD trigger PD We set up a prototypical measuring device, based on the described technique. It is capable of handling power levels and spot sizes typical for current 3D metal printing machines working with laser systems in the NIR (up to 1 kw of laser power and beam waists down to 25 µm radius). The measuring concept allows a very compact mechanical setup and equipment footprint and can thus be positioned at different transverse locations in the scanning field. We present several different measurement examples, illustrating the basic functionality, performance and opportunities the device can provide for scanner characterization and calibration. The new possibilities and advantages include: access to measurands beyond the reach of conventional beam diagnostic devices, e.g., the marking speed or beam analysis in different transverse locations on the building plate, merging isolated applications for different calibration tasks into one b t on time t t 1 t 2 t 3 t off c (x i, y i ) y device, reducing investment, complexity and working hours, severe simplification and saving of time in certain measurement tasks, possibility to measure at the actual process parameters, where alternative methods often rely on a narrow process window, a compact and powerful service tool suitable for comprehensive scanner analysis in the field. As a very basic example to illustrate the functionality of this new concept, we performed several measurements with the device in one fixed position. A statistical number of measured vectors with varying orientation and increasing scanning speed is featured in Fig. 3a. The device was located in the scanner zero-point position, with the measuring plane approximately coincident with the focal plane of the objective. The color gradient indicates the chronological order (from red to purple) in which the lines were written. There is an apparent shortening of the vectors with increasing scanning speed. This is not an artefact of the measurement but an actual effect and can also be observed when illuminating a work piece with the same exposure scheme. The reason for this are inadequately set laser switching delay times in the scanner driver software, causing a constant timing error, which manifests as a velocity dependent length error. The ability to detect these synchronization issues and the accompanying opportunities for scanner calibration will be discussed in the next section. The calculated marking speed and inclination angle with respect to the x (,) ϑ Fig. 2 Working principle of the ScanFieldMonitor: schematic setup (a), signal sequence (b), and vector coordinates in relation to signal pattern (c). (x f, y f ) 218 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Laser Technik Journal 3/218 41

3 y position [mm] a b c waist [µm] marking speed [m/s ] angle ϑ[ ] x position [mm] index Fig. 3 Measurement of 41 vectors with different orientations and marking speeds in one axial plane in the scanner zero-point position: graphical representation (a), measured beam waist (b), and marking speed and angle (c). Δx start [mm] horizontal over the complete series is displayed in Fig. 3c. The high accuracy of the apparatus is apparent, regarding an only slight scattering of the data points compared to the linear parameter sweep of the programmed values, indicated by the dashed lines. On the other hand, the determination of the beam width does not show any significant dependency on orientation or marking speed, as can be seen from Fig. 3b where the color code resembles the one in Fig. 3a. The average value for the beam radius accounts for w = 35.8 µm with a standard deviation (light blue band) of only ±.7 µm. This impressively demonstrates the robustness of the concept to varying measuring conditions. Requiring solely the time-resolved photodiode signal, data acquisition is inherently fast and only limited by the time the scanner needs to write the vectors. In the case of the illumination pattern in Fig. 3, this amounts to less than Δt = 59 µs Δt < 1 µs marking speed [m/s] Fig. 4 Delay-time calibration measurement. Position shift of the vector s starting point with the marking speed for uncorrected (red circles) and corrected (green triangles) settings. The slope of the linear regression (dashed lines) directly translates into the timing error. 35 µs for the complete sequence. Thus, the method can also be a versatile tool for time resolved analysis like the investigation of thermal lensing. Timing and synchronization In order to accurately position a melt trace in the powder bed, not only the beam positioning system has to work properly. Also the laser illumination sequence has to be synchronized precisely to the motion of the steering mirrors. Since the ScanFieldMonitor provides absolute positioning information, it can be used to investigate issues of that kind: Let us, for example, assume that there is a switch-on delay of the laser, which is not sufficiently accounted for in the scanner programming. Such a constant timing error will translate into an increasing length error with the marking speed, resulting in a linear shift of the vector s starting point. The results of such a measurement are depicted in Fig. 4, where the deviation Δx start of the vector s starting point at the lowest marking speed (.1 m/s) is plotted as a function of the measured marking speed. The red data points show the behaviour with the initial scanner settings. There is a significant shift, amounting to more than half a millimeter displacement for the maximum speed. Now, the slope of this curve has the unit of time and directly equals the timing-error, in this example, Δt = 59 µs. Repeating the measurement, taking into account this mismatch, results in the green data points, showing an almost perfect compensation, except for a slight systematic upturn of the values for speeds exceeding 8 m/s. The same data set can, of course, be applied to perform an end-point calibration. Measurement time for such a calibration curve (here with 21 different velocities and a ten-fold repetition) is less than three seconds. This, in combination with the fact that the correction values are directly accessible via this kind of measurement and do not need a successive approximation to the optimal parameter set, can constitute a significant time saving when calibrating or inspecting scanner systems. Another advantage is, that the calibration can be performed at the process conditions, be it 2 W or 1 kw,.1 m/s or 1 m/s, and is not restricted to the operating point of, for example, a special test specimen. It is also not unheard of that lasers can show a power dependant switching behavior. Thus, the possibility to probe at different power levels can be essential to ensure a satisfying compensation. Beam propagation parameters Having talked about the possibilities to ensure a precise positioning of the beam in the working chamber, we want to turn our attention now towards the spot size on the powder bed. In order to obtain a viable fusion of the metal powder, you have to ensure a sufficient power density, this means, apart from the correct power level, the desired focal size and axial position of the laser focus on the powder surface. The determi- 42 Laser Technik Journal 3/ WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

4 Additive Manufacturing nation of the laser propagation parameters like focal position, divergence angle, Rayleigh length, or beam quality, requires measurements at several different axial positions along the direction of propagation [3]. In an AM machine, the movement of the building platform naturally provides this translation. Such a measurement was performed with the ScanFieldMonitor, yielding propagation parameters in excellent agreement with one of our industry proven camera-based measuring instruments (Primes MircoSpotMonitor). Of particular interest is now what goes beyond the scope of conventional beam diagnostics: For example, an extended interpretation is possible, when we include the behavior of the marking speed as a function of the axial position in our considerations. Due to the partial telecentricity, the use of an f θ objective leads to a linear increase of the observed speed with the distance to the lens. This is confirmed by the data depicted in Fig. 5, where the determined marking speed (green triangles) follows the predicted behavior. The plane in which the set marking speed of v set =.1 m/s is achieved (vertical green dashed line) can be interpreted as the working plane, because here the scanner satisfies the proper geometric dimensions. Comparing this axial position to the determined focal position of the laser, we see a deviation of Δz = 2.6 mm in our system. This essential alignment property of a scanner system bringing those two planes in coincidence can be reviewed and adjusted in a simple manner, using the presented device, since spot size and marking speed are always determined simultaneously in one measurement. Time resolved thermal focal shift Absorption driven thermal focal shift is an ubiquitous issue in high-power laser applications [4]. The transmitting beam locally heats the glass substrate due to residual absorption in coatings and bulk material. The change in refractive index waist [µm] z position[mm] and shape typically leads to a shortening of the effective focal length and thus to a shift of the beam waist closer to the objective, and away from the working plane. This may reduce the power density below the process threshold and cause insufficient fusion. Moreover, in scanner applications, the swift changes in position and exposure entails that the thermal equilibrium state is typically not reached. Hence, an analysis marking speed [m/s] Fig. 5 Caustic measurement with the SFM provides more than just the beam propagation parameters. Here a discrepancy between focal and working plane. 218 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Laser Technik Journal 3/218 43

5 waist [µm ] of the steady state may not be a conclusive description of the relevant system behaviour. As pointed out before, the measurement frequency of our device is primarily limited by the time to scan the vector. Hence, with typical vector length of less than 1 cm for a measurement and mark / jump speeds up to 1 m/s, measuring frequencies of up to 5 Hz are feasible. This temporal resolution gives us the opportunity to investigate not only the steady state, but the transient behaviour of thermally-induced shifts. A measurement of the beam waist in a plane approximately three Rayleigh lengths above the focus is presented in Fig. 6. In this asymptotic section of the beam caustic, the change in radius is directly proportional to an axial shift of the focal position. Marking- and jump speeds of 1 m/s and 1 m/s, respectively result in a measurement frequency of about 1 Hz. The blue data, measured at 2 W laser power, do not show any significant change in radius over the observation time (w = 12.3 ± 2.2 µm). In contrast, the 4-W data (red) show a rapid decrease in radius. This is associated with a shift of the focus closer to the measuring plane. The data can be modelled via a two-phase exponential decay, with an equilibrium value of 81.2 µm and time constants in the range of a few seconds. Conclusion and outlook Our novel beam analysis concept for 3D additive manufacturing systems provides analysis tools for essential scanner calibration tasks, so far inac- 4 W 2 W time [s] Fig. 6 Time-resolved focal shift analysis. Temporal behaviour of the beam radius in a fixed measuring plane, about three Rayleigh lengths above the focus, for two power levels. cessible to conventional beam diagnostics. This includes, among others, the precise determination of a plurality of measurands demanded by the DIN for aerospace applications [1], like marking speed, axial focal position, thermal lensing, or absolute position. In addition to the measurements presented in this article, the possibilities of the device extend to the characterization of scanner properties associated with different transverse positions: field distortion, when analysing one axial plane, or a complete analysis of the flat field and a comparison to the working plane, when recording full beam caustics. And, of course, the method is predestined for multihead laser systems where the device can simply be placed in the overlapping region of two scanning fields. It should also be mentioned that all illustrated results are based on one very universal but only one type of scattering pattern. The prospects of other patterns, designed for specific problems, is yet to be investigated. In addition, the technology is directly transferrable to other scanner-based processing methods like remote welding, for example. The universal applicability entails a range of applications across a wide area of potential uses: the manufacturers of AM machines and scanner heads for calibration tasks during commissioning, R&D engineers, field service technicians or end users, and quality assurance to document the required standards of accuracy. Hence, we believe this new kind of beam analysis can unite in one device what so far is accomplished by a combination of diverse, specialized measurement procedures. It can answer many questions in a convenient way and has the potential to become a versatile tool for quality control in 3D additive manufacturing. Acknowledgments We gratefully acknowledge financial support from the German Federal Ministry of Education and Research (BMBF) within the GenChain research project under the funding code 13N1359. DOI: 1.12/latj [1] DIN 35224:216-9: Welding for aerospace applications Acceptance inspection of powder bed based laser beam machines for additive manufacturing [2] A. Koglbauer et. al.: A compact beam diagnostic device for 3D additive manufacturing systems, Proc. SPIE: Laser 3D Manufacturing V, 1523; (218) [3] DIN EN ISO :25-4: Lasers and laser-related equipment Test methods for laser beam widths, divergence angles and beam propagation ratios Part 1: Stigmatic and simple astigmatic beams [4] N. Harrop et. al.: Absorption driven focus shift, Proc. SPIE: High-Power Laser Materials Processing: Lasers, Beam Delivery, Diagnostics, and Applications V; Volume 9741, 9741 (216) Author Andreas Koglbauer is head of the physical concepts research department at Primes. He studied laser physics in Mainz (Germany) and graduated in the field of ultracold quantum gases. Afterwards, he joined the ATRAP collaboration, where, in 214, he received his PhD in physics on the subject of vacuum-ultraviolet laser development for fundamental research on anti-hydrogen. After a short postdoc position, developing an UV laser system for the GBAR antigravitation experiment, he started with Primes in 215 as a development engineer for high-power laser beam diagnostic devices. Since 217, he is heading the research department for physical concepts, addressing novel beam diagnostic concepts for industrial laser systems. Dr. rer. nat. Dipl.-Phys. Andreas Koglbauer, PRIMES GmbH, Max-Planck-Str. 2, Pfungstadt, Germany; phone: +49 () ; a.koglbauer@primes.de, 44 Laser Technik Journal 3/ WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim