System Technology and Process Control (LiM 2019)

Monitoring of laser welding/AM processes combining high speed X-ray imaging, acoustic sensors and artificial intelligence
K. Wasmer, T. Le-Quang, B. Meylan, M.P. Olbinado, A. Rack, S.A. Shevchik

Laser processing (welding / additive manufacturing) is known for its complexity and high dynamics, thus challenging for
in situ and real-time quality monitoring and control.
To tackle this challenge, we collected the signals from acoustic sensors. We then analyzed these signals by state-of-the-art artificial intelligence technics to classify the laser process in terms of quality. Despite promising results, many questions remain open relating the hidden mechanisms that makes such classification approach feasible. Hence, to get a fundamental understanding of the laser process and background physical origins of the acoustic emission, in situ and real-time high-speed X-ray imaging of real laser processing was carried out at European Synchrotron Radiation Facility (ESRF-Grenoble). The presentation makes an overview of this approach and results from ESRF.

Keywords: Additive manufacturing; laser welding; quality control; acoustic emission; high-speed X-ray imaging; artificial intelligence;


Adaptive technique for signals in the monitoring of laser welding
Giuseppe D’Angelo, Gianmarco Genchi, Giorgio Pasquettaz

Laser weld monitoring is usually based on the feedback from sensors (photodiodes, cameras) able to provide
information about radiation from plume, the reflected laser light and the thermal condition of the melt. By using the
optical emissions, it is possible to evaluate laser process quality, in particular, to find out the relationship between
emission characteristics and weld quality characteristics. However, the optical signals detected during the laser welding
are typically contaminated by different kind of noises that affect the sensor. To avoid this phenomenon, it is necessary to
de-noise the signal for getting a “clean” signal. A plethora of inspection systems have being developed to improve weld
quality and reduce overall costs. However, the signal analysis technique to be applied, for inferring information about
the condition of the weld, is still an open field. This work presents a technique based on an adaptive method for cleaning
up the signal, named Modified Singular Spectrum Analysis (MoSSA), combined with the Teager-Huang Transform (THT)
for better inferring information about the condition of the weld. The proposed technique significantly improves
components localization. For giving practical applicability to the proposed method, we compare the methods by
analyzing signals detected during the laser welding, demonstrating the expected advantages.

Keywords: singular spectrum analysis; transient signal analysis; Teager Kaiser energy operator;


Laser integrated process monitoring
Steffen Belke, Marcus Bode, Peter Kallage, Wolfram Rath, Simon Aleryd, Fredrik Johannson

In many material processing applications the detection of process emission is the basis for process monitoring, reliable
process control and quality control systems. Coherents laser integrated process monitoring system (HighLight SQD)
detects back reflected laser power and visible process light (plasma). The sensors are placed inside the process fiber
connector of the high power fiber laser and ensure a high quality signal detection due to a high signal to noise ratio and
a measurement coaxial to laser beam. A wide range of applications benefit from the availability of these signals. For
cutting processes, events like piercing end, cut interruption and self-burning can be detected. Decreased cut quality like
burr formation and plasma-cut is observable as well. For hairpin welding applications, failure detection is possible as
misalignments like height offset, gap, v-gap or lateral offset are visible in the process monitoring signals.

Keywords: process monitoring; fiber laser; cutting; welding; HighLight SQD; HighLight FL


Holistic sensor concept for process control and quality assurance in laser beam welding based on Optical Coherence Tomography
Christian Stadter, Maximilian Schmoeller, Markus Kogel-Hollacher, Ulrich Munzert, Michael F. Zaeh

Optical Coherence Tomography (OCT) enables the acquisition of direct information about the geometry of the process
zone during laser beam welding. The measuring principle allows for spatial and temporal highly accurate measurements
that are hardly affected by process emissions or metal vapor/plasma. A novel sensor concept based on OCT was
evaluated for both multi-mode and single-mode laser radiation as well as for fixed and remote optics. The sensor was
attached to a 3D scanner unit allowing for measurements before, in, and after the process zone. Fundamental studies on
aluminum, copper, and galvanized steel were carried out to analyze the effect of the material, the process, and the
measurement parameters. Scan lines before and after the process zone were applied for tracking edges and assessing
the weld seam topography during the process in real-time. Measurements coaxial to the laser beam revealed the
capillary depth for spot sizes down to 55 μm.

Keywords: Optical Coherence Tomography; In Situ Weld Depth Measurement; Process Control; Quality Assurance; Laser Beam Welding


High dynamic beam shaping by piezo driven modules for efficient and high quality laser beam cutting and welding
A. Jahn, C. Goppold, P. Herwig, C. Reinlein, P. Boettner, D. Stoffel, M. Bach

Today’s laser remote processing e. g. cutting and welding is often limited by comparatively low oscillation frequency of
the beam focus. Traditionally, high frequency in plane oscillations of the beam is realized with galvo-scanners. New
requirements in industry are demanding for not only fast in plane motion but for high-speed active vertical distribution
of the beam energy within the workpiece. For laser beam cutting of thick plates, an increase of cutting edge quality is
expected. In terms of laser beam welding of aluminium material, a significant process stability improvement is aimed in
connection with a weld seam quality enhancement. This paper describes a novel concept of beam shaping optics using
piezo actuators for fast focus modulation in z-direction with working frequencies above 2.5 kHz. The development of the
actuator concept will be presented as well as the technical realization of the piezo driven modules and their integration
into commercial cutting and welding optics. Optical characterization of the beam propagation and the possibilities for
dynamic z-modulation will be demonstrated. First application-oriented experimental investigations are presented too,
proving the functional capability and showing basic interrelations between z-modulation and process behaviour for
typical cutting and welding processes.

Keywords: material processing, highly dynamic beam shaping, piezo driven moduls, laser beam cutting, laser beam welding;


Increase of deposition rates in laser hot wire cladding (LHWC) by use of beam-oscillation for appropriate energy deposition and thermal closed loop control
Dieter Tyralla, Thomas Seefeld

Laser hot wire cladding (LHWC) obtains high deposition rates by the combination of electrical preheating of a wire and
the power of a laser. However, the quality of the weld usually was not as good as in processes like laser powder
The present study demonstrates LHWC with high quality and high deposition rate. Therefore appropriate deposition of energy by beam-oscillation strategy is used. In addition, a novel temperature field measurement system observes the process zone. The melt pool size is measured by an emissivity compensated camera and is controlled by the help of the laser power. The electrical heating is matched to the laser power to maintain the total energy in the process and to keep the thermal conditions constant. High deposition rates of more than 2 kg per hour were obtained with less than 1 kW of laser power.

Keywords: laser cladding; hot wire; beam oscillation; temperature field measurement; melt pool geometry


Design, manufacturing and test of a highly dynamic piezo-driven metal mirror for laser material processing
P. Böttner, C. Reinlein, A. Jahn, P. Herwig, C. Goppold, D. Stoffel, M. Bach

This paper reports on the opto-mechanical design and manufacturing of a highly dynamic piezo-driven metal mirror. This
metal mirror replaces the last 90° deflecting mirror in conventional laser material processing heads. The central element
of the mirror is a diamond turned free-form membrane. The membrane design is optimized to correct astigmatic
aberrations which arise from the 90° beam deflection while simultaneously shifting the laser focus over a large range. In
combination with a 200 mm focus lens, this allows diffraction-limited focus movements of up to 20 mm. A coating is
applied to the mirror surface which has been successfully tested for cw laser loads up to 4 kW. The mirror is activated by
a controlled piezoelectric stack actuator allowing beam oscillations up to 2.5 kilohertz. Optical, static and dynamic
properties of a protoype mirror are characterizedin order to qualify the mirror for laser cutting and welding processes.

Keywords: focus shifting; material processing; highly dynamic beam oscillation; aberration correction


OCT – A versatile technology for laser material processing
F. Dorsch, W. Dubitzky, J.-P. Hermani, A. Hromadka, T. Hesse, T. Notheis, K. Schmitt, J. Seebach, M. Stambke

Optical coherence topography (OCT) is a 3D imaging technique based on low-coherence interferometry. In recent days,
it became a key technology in laser material processing. The OCT beam is coupled co-axially to the laser beam into the
processing optics and provides surface information of the probe. Additional degree of freedom is obtained when the
OCT beam is deflected using a small field scanner attached to the processing optics. This report elucidates the manifold
use cases for an OCT-based sensor system in laser processing applications.

Keywords: Optical coherence topography; OCT; low-coherence interferometer; weld depth; seam tracking; image processing; 3D imaging


Correlation between camera image and photodiode signal during laser welding
Petr Horník, Libor Mrna, Hana Šebestová

Photodiodes are often used to monitor weld quality. This solution has the advantage of simplicity and low cost, but it
may be more difficult to interpret the measured data. Moreover, measurement with multiple photodiodes indicates
dependency of the measured intensity on the photodiode position. The purpose of this study is to better understand the
data measured during the laser welding process. Therefore we explore which part of the high-speed camera image best
corresponds to photodiode intensity during welding. Regions of interest including weld pool and vapour plume are
selected from camera images. The regions are compared with the photodiode using correlation coefficient.
Subsequently, the task is generalized, and the photodiode is compared directly with each pixel of the camera. This
creates a map describing which parts of the image most closely correspond to the photodiode signal. In addition, the
results are also viewed in relation to back-reflected laser radiation.

Keywords: laser welding; photodiode; high-speed camera; correlation; monitoring


Waveguide polarization and mode selection technique for CO2 laser
Peter Dyer, Jason Lee, Gavin Markillie

To address the simultaneous demands for high mode quality and polarization stability in the laser applications market, a CO2 diffusion cooled gas laser operating at the 600W level is presented using a novel planar waveguide surface made from a combination of dielectric and metallic areas. The dielectric generates sufficient higher order mode waveguide loss to ensure only lowest order, fundamental mode oscillation, whilst the metallic area provides enough polarization selectivity to ensure a linearly polarized output. Experiments to optimize the metallic area while maintaining high levels of waveguide mode selection have resulted in metallic lengths between 5 and 30% of the waveguide that extend across its entire width. A ZnSe Brewster window operating with a small offset away from Brewster’s angle to sample the beam demonstrates a laser that always operates in the expected TE mode while giving highly stable linearly polarized light at a purity > 5000:1.

Keywords: waveguide; mode; polarization stability; selectivity


In-Line, Real-Time Laser Process Monitoring Using Low-Coherence Interferometry
Christoph A. Riedel, Philippe Ackermann, Rouwen E. Kunze, Robert H. Schmitt

With the ubiquity of laser material processing as a high-accuracy tool in modern production, in-line and real-time
monitoring of such process becomes increasingly important. A key technology for such monitoring is low-coherence
interferometry, which can be implemented into the same optical beam path as the process laser. This metrology system
integration has to be custom tailored for each individual system. In recent years, we demonstrated successful
integrations of LCI into several different laser material processing methods, such as fixed focus optics, scan heads and
polygon scanners. We give an overview over the boundary conditions of such an integration and the challenges that
were encountered. The results pave the way for future real-time process feedback for self-focusing and self-correcting
laser process systems.

Keywords: Laser material processing; High-precision measuring; In-line metrology; Process monitoring; Process feedback control


Monitoring micro-drilling of large Ti plates using single laser pulses for HLFC applications
Roberto Ocañaa, Carlos Sorianoa, Jose I. Esmorisa and Rafael Sánchezb

The laser single pulse drilling (SPD) is a very promising technique for the construction of hybrid laminar flow control
(HLFC) structures for the aerospace industry. However there are numerous issues that have to be addressed before this
technique can be applied in industrial production. The requested characteristics of the micro-holes in a Ti panel for a
HFLC structure are quite demanding. A typical panel with size 2 x 5 m containing more than 24 million of micro-holes
must fulfill a standard deviation for the diameters less than 10 μm at the beam entrance and 5 μm at the exit. Here we
have studied the main aspects that govern the laser process at a stable production rate of 300 holes per second. For this,
not only in situ monitoring techniques but also off-line measurement systems have been developed. The results provide
a feedback for the laser process in order to fabricate large Ti panels.

Keywords: laser drilling; single pulse drilling; HLFC; micro-holes; monitoring micro-drilling;


Laser Lapping of Piezoelectric Ceramics using Ultrashort Pulse Laser and Closed Loop Control Algorithm
Marco Smarra, Matthias Lautenschläger, Sven Verpoort, Klaus Dickmann

A key part of a deformable mirror is a piezoelectric ceramic, which provides the deformation of the surface by a supply
voltage. The mechanical surface flattening of these ceramics leads to unpredictable new deformation of the ceramic due
to the mechanical stress during the lapping process. Ultrashort pulse lasers can be used to ablate a small amount of
material volume with low thermal influence and low mechanical stress to the processed material. In this study, we
demonstrate the combination of inline topographic measurement and ultrashort pulse laser ablation for local material
removal of a piezoelectric ceramic. A differential model of the surface was built, and the surface deformation after the
laser treatment was measured. The material properties regarding the ablation efficiency and roughness after laser
treatment are investigated. The flatness and the roughness of the surface after the laser process are compared to the
mechanical process.

Keywords: Laser Micro Processing; Laser System Technology and Process Control; Deformable Mirror; Laser Ablation


Laser doping for 4H-SiC power-device fabrication with laser pulse-duration controller
Toshifumi Kikuchi, Kaname Imokawa, Akihiro Ikeda, Daisuke Nakamura, Tanemasa Asano, Hiroshi Ikenoue

We have investigated a KrF excimer laser doping method for high-concentration, low-temperature doping of 4H-SiC. We reported that heavy N or Al doping of the 4H-SiC can be achieved by laser ablation of thin source films (SiNx or Al) formed on the SiC substrate. Recently, we developed a laser doping system capable of processing the entire SiC wafer with high productivity for manufacturing systems. In this system, a gas supply nozzle for ambient environment control was installed to prevent oxidation of the SiC surface, achieving the same oxidation-suppression effect as a vacuum chamber. Furthermore, the system contains an optical pulse stretcher that
lowers the laser peak pulse and stretches the pulse width. By using the KrF excimer laser doping system, we achieved high concentration and high throughput doping of 4H-SiC at low temperature and low contact resistance.

Keywords: laser doping, power device, semiconductor, excimer laser, 4H-SiC;


Reducing process variation of laser powder bed fusion by real-time closed-loop control
Volker Renken, Daniel Gleichauf, Felix Pastors, Lutz Lübbert, Axel von Freyberg, Andreas Fischer

The powder bed fusion process continuously increases market share, as it offers the production of metal parts with complex geometries by selectively melting a metal powder. However, the process is disturbed by inconsistent geometrical heat flow and atmospheric variations. Since commonly used sensors are not able to detect crucial states in-process and no possibility is given to adapt the process parameters in a closed-loop control strategy, a real-time control is developed with a high-speed pyrometer as sensor and Field Programmable Gate Arrays (FPGA) as control unit. The approach is validated at scanning speeds up to 1400 mm/s for the powder bed
fusion process. The standard deviation of the pyrometer signal as a measure of the melt pool temperature was reduced by 25 % for simple cubic geometries and by 40 % for more complex geometries. Hence, the control concept allows to stabilize the process and to reduce manufacturing errors, especially in ambitious production tasks.

Keywords: Control; closed-loop; pyrometer; powder bed fusion; real-time


Multi beams fs processing with high power laser
E. Audouard, M. Delaigue, A. Chambinaud, K. Mishchik, C. Hönninger, E. Mottay, Y. Di Maio, S. Landon, B. Dusser

Translating the high available power for industrial lasers in the 100 W range into high throughput micro-processing is of high importance for future industrial applications. Beam divisions into multi beams using diffractive optics is a promising investigation direction. Programmable Spatial Light Modulators (SLM) can bring flexibility while maintaining a high spatial resolution compatible with complex multi spots shaping or user defined beam profiles. The recent technological progress in LCOS based systems enables an optical transmission greater than 95 %, high diffraction efficiency around 80 % but also high average power handling up to at least 100 W. However, the dependence of the shaping performances on laser bandwidth places specific requirements when using ultrafast lasers. The usable field size with respect to the chromatic effects is for instance a specific parameter to manage. Optimization of the machining results by software driving of the SLM phase map can provide original and efficient solutions for parallel processing.

Keywords: femtosecond laser processing, spatial light modulator, parallel processing


OCT-controlled generation of complex geometries on stainless steel using ultra-short laser pulses
Daniel Holder, Steffen Boley, Matthias Buser, Christoph Irion, Rudolf Weber, Thomas Graf

The generation of complex surface geometries with a defined depth, high contour accuracy, and low roughness in steel with ultra-short pulse laser ablation is still a challenge. In the present work, optical coherence tomography (OCT) providing high-resolution optical distance measurement of the ablation depth was combined with a Galvanometer-Scanner for fast beam deflection. The actual ablation depth was measured during processing. The online comparison with the target ablation depth was used to select areas that have to be processed in the subsequent pass. This selective material ablation allowed to smooth surface defects caused by inhomogeneous material removal. Furthermore, the selective material ablation was used to generate locally different ablation depths in order to create complex geometries. As a result, the surface roughness could be significantly reduced compared to the uncontrolled process, and precise ablation of complex geometries on steel was demonstrated.

Keywords: Process Control; Optical Coherence Tomography; Ultra-short Laser Pulses; Controlled and Selective Ablation of Steel; Complex Geometries; Stainless


Process controller for scanning laser surface machining on cylinder surfaces
Jan Düsing, Jürgen Koch, Oliver Suttmann, Stefan Kaierle, Ludger Overmeyer

Laser beam scanners are often combined with a mechanical axis stage in order to take advantage of both the high
dynamics of the scanner device as well as the motion flexibility of the stage system. However, synchronization is yet a
challenging control task because of diverging operation principles and incompatible operation frequencies. In this study
we investigate a laser process control strategy for laser surface machining which is implemented on a field-
programmable gate array (FPGA) data processor. The control system reads geometrical data from memory, performs
real-time coordinate transformations based on current stage position sensor signals and generates control signals for
the laser scanner and beam modulator. The system is able to solve all computation tasks with an update rate well
beyond 100kHz and thus allows for processing strategies which involve continuous motion of scanner and rotation axis
for seamless circumferential laser processing on cylindrical surfaces.

Keywords: laser micro machining; surface processing; real-time process controller; FPGA


Productivity optimization of scanner based laser ablation processes with adaptive scan paths
Matthias Buser, Daniel Holder, Steffen Boley, Volkher Onuseit, Thomas Graf

In this study, productivity optimization was investigated for laser based ablation processes with online depth control
using a fast scan path generation algorithm. In ablation processes, a depth control system utilizing optical coherence
tomography can be implemented to improve geometrical accuracy and reduce surface roughness. The actuating variable
for the control loop is pulse energy, which in the simplest case involves switching laser pulses on and off along the initial
scan path as required. However, every suppressed pulse contributes to a decrease in productivity, which is a critical
criterion for industrial applications. For this reason, an approach was taken to deploy the scan path as an additional
actuating variable. The developed algorithm is designed for improving the scan path during the process by omitting
already finished areas. Theoretical verification with sample geometries shows a significant improvement in productivity
compared to bare pulse toggling.

Keywords: Laser ablation; tool path planning; system technology; process control


Selective Multibeam micro processing of metal with a 1x8 beam array
Alexander Meyer, Johannes Finger, Oliver Nottrodt, Michael Jüngst

Processing metals by means of ultrashort pulsed laser radiation with pulse durations below approx. 12 ps yields higher
surface quality, higher precision and smaller thermal stress compared to processing with pulsed laser radiation of longer
pulse duration. To increase the productivity and use the available laser power efficiently, an approach for parallelization
of the processing using a flexible multibeam setup is presented. The laser beam is split into 8 beamlets by a Diffractive
Optical Element (DOE). All beams are moved across the workpiece using a high speed galvanometer scanner in
combination with an f-theta lens. Each beam is individually switchable by means of an acousto-optic modulator (AOM)
enabling multibeam bitmap structuring. Real-time control of the multi-channel modulator is realized with an FPGA and a
time-based control scheme. Grey-scale information of the bitmaps is used as layer information, enabling the generation
of arbitrary 2.5D surface structures. A threefold decrease in processing time is achieved.

Keywords: Multibeam; ultrafast; parallel processing; micro processing


Detection of heat accumulation in laser surface texturing by fast infrared detectors
Jirí Martan, Denys Moskal, Lucie Prokešová, Milan Honner

Laser surface texturing (LST) with high average power ultrashort pulsed lasers is starting to be used in industry. Using
high repetition frequency of laser pulses leads to significant overlapping of laser spots and increasing of heat
accumulation in the textured material. Heat accumulation has great influence on quality of the resulting surface texture.
In majority of the research works the heat accumulation is analysed by mathematical modelling. In this work, a
measurement system for time-resolved temperature measurement during the texturing process was developed. The
system is based on high speed infrared (IR) detectors with sensitivity in near and mid IR wavelength (NIR and MIR).
Measurement results during machining of straight lines and surface texturing of dimples on stainless steel are
presented. The system can be used as a tool for technology developers for comparing different scanning strategies and
material removal approaches and choosing the right one for the desired application.

Keywords: Laser surface texturing; heat accumulation; infrared detectors; process monitoring; temperature measurement


Laser beam shaping and stabilization for single-mode laser material processing based on Multi-Plane Light Conversion
Clément Jacquard, Olivier Pinel, Pu Jian, Jean-François Morizur, Guillaume Labroille

Laser beam parameters vary over time: tilt, shift, defocusing or modal imperfections can negatively impact the
applications. It can drastically degrade the performance of a beam shaping setup (DOE, SLM, etc.), especially with pulsed
laser processing.
We present a novel approach for beam shaping coupled with passive modal filtering that compensates for the
imperfections and temporal variations of the laser beam parameters. The non-ideal, misaligned beam is projected on an
adapted transverse mode basis. The unwanted energy is routed onto higher order modes and dumped or used as a
feedback signal, while the energy in the fundamental Gaussian mode is shaped into the spatial mode of interest (e.g.
flat-top, Bessel beam, etc.). This approach is implemented with Multi-Plane Light Conversion technique, whose high
flexibility and shaping performance enable efficient, versatile and robust shaping for applications where beam pointing
accuracy and beam quality are crucial, such as micro-processing and ultrafast laser processing.

Keywords: Multi Plane Light Conversion ; modal filtering ; laser beam shaping ; USP laser ; laser microprocessing


Aging of laser protective filters concerning laser resistance
Rico Bühring, Winfried Janßen, Uwe Urmoneit, Hans-Joachim Krauß

Various influencing factors concerning the laser resistance of laser protective filters have already been investigated, but
the influence of the age of the filters has not been studied yet. Therefore, the focus of the experimental work was to
determine the laser resistance of artificially aged laser protective filters and to compare the experimental results with
new filters of the same product. The investigations were carried out on filters made of Polymethylmethacrylate
to examine, whether aging has an effect on the protective properties of the filter material or not. The test samples were
aged artificially by UV irradiation. On all filter samples the spectral transmittance (optical density), luminous
transmittance and diffusion of light were first measured in the initial condition and afterwards in the aged state. The
laser resistance was then tested by using Nd:YAG and CO2 laser radiation. These laser resistance tests were carried out
according to the certified scale numbers of the filters by the norm EN 207.

Keywords: Aging effects; laser resistance test; laser protective filter