System Engineering and Process Control (LiM 2023)

Measurement and control system for compensation of thermal effects in laser material processing
J. Hofmann, V. Taube, J. Stollenwerk, C. Holly

Thermal lensing leads to quality losses or, in extreme cases, to the abortion of the whole laser machining process. Here, we present a device for the inline measurement and compensation of the thermal focus shift. For the measurement, the diffraction pattern visible in the working plane created by an amplitude mask is used. By a readjustment of the position of the focusing element the thermal focal shift is compensated.

Keywords: laser material processing; thermal effects; focus compensation, optical design


Anamorphic beam shaping for glass nanofibers production by continuous fiberizing by laser melting (Cofiblas)
Joaquín Penide, Félix Quintero, José Luis Fernández, Mónica Fernández-Arias, Raúl Barciela, Jesús del Val, Fernando Lusquiños, Juan Pou

The use of glass nanofibers as reinforcements in composite materials promises a substantial enhancement of their performance thanks to their superior mechanical properties. The continuous fiberizing by laser melting (Cofiblas), makes possible to obtain ultrafine fibers with virtually infinite length and capability to feasibly scaling up for mass production. The effectiveness of this technique relies on several factors, being the laser beam shaping system the more relevant one. In this work we compare two different optical systems: one constituted by spherical lenses, and the second one based on an anamorphic system which generates an elliptical Gaussian distribution of the laser beam irradiance. Both configurations are evaluated in several sets of experiments with the main target of obtaining continuous glass nanofibers with the smallest diameter. As a result, continuous silica nanofibers with diameters 84.5% smaller are obtained with the anamorphic system thanks to the increase of energy absorbed by the filament.

Keywords: anamorphic optics; laser beam shaping; glass nanofibers; laser glass processing; Cofiblas


Monitoring Direct Laser Interference Patterning of metallic substrates using an infrared camera system
Lukas Olawsky, Stephan Moghtaderifard, Clemens Kuhn, Andrés Fabián Lasagni

Direct Laser Interference Patterning (DLIP) is a technique that enables the fabrication of homogeneous microstructures in the micrometer and sub-micrometer range. Typically, the topography of these structures is evaluated ex-situ, using methods such as confocal microscopy or white light interferometry. However, these techniques are not suitable for real-time process observation due to their long measurement time. In this study, an Infrared camera system is used to explore the correlation between the captured average temperature during DLIP treatment and topographical parameters in real-time. The results show a linear relationship between the applied laser fluence (0.7 to 4.9 J/cm²) and the measured average temperature, as well as significant changes in surface roughness, skewness, and kurtosis within this fluence range. These findings suggest that the presented method could be used for in-situ indirect monitoring of topography during DLIP treatment, enabling quick identification of process fluctuations.

Keywords: Direct Laser Interference Patterning; in-line monitoring; surface topography; infrared camera


System development for active deviation compensation in laser-based wire micro deposition using image recognition
Tobias Schmid, Adrian Spahn, Florian Hüsing, Henning Janssen, Christian Brecher

Gold plating is used to protect electrical components from corrosion and enhance functionality. A sustainable alternative is localized laser-based wire deposition: Using a laser scanning system, micro gold wires are welded into gold spots locally on the component surface. However, process and system tolerances lead to deviations between the target and actual weld positions. This paper presents a system for active deviation compensation using an on-axis camera and image recognition algorithms. Different adaptive thresholding methods and image processing algorithms are used to determine the welding target position on the part. The actual welding point of the wire on the component is calculated and transferred to the scanner as the new welding position. An approach for designing such a system for active deviation compensation and the possibility of higher-level communication with industrial controls is presented.

Keywords: Laser-based gold coating; electrical contact; micro gold wire deposition; laser-based micro deposition; accuracy improvement; image recognition; adaptive laser process


Investigation on the influence of beam steering frequency on keyhole behavior using a dynamic beam laser
Florian Hugger, Robin Sättele, Jonas Wagner, Eric Punzel, Andreas Bürger

Dynamic beam lasers are the latest tool to influence keyhole behavior in laser welding. Civan’s OPA6 laser can change spot position within the working field with a frequency up to MHz range. During these time intervals, the laser beam interacts with the melt at different positions and with rapid position changes. The material surface heats up and cools down in dependence of shape frequency and number of shape points.
A 1D analytical calculation is carried out to determine the influence of different thermal cycles on the surface temperature. From this calculation the time dependent ablation pressure is derived, and the dynamic reaction of the capillary surface is estimated.
This theoretical investigation is compared with experimental results. The behavior of the keyhole surface is analysed by means of high-speed imaging with a frame rate of 400 kHz, applying different beam steering frequencies from 1 kHz to 1,000 kHz.

Keywords: Dynamic beam laser; laser beam welding; keyhole; interaction time


Design of freeform optic module creating a ring-shaped laser beam profile for localized heating of sheet metals
Teng Chen, Tobias Schmid, Florian Hüsing, Henning Janssen, Christian Brecher

Laser-based preheating is an energy-efficient method to locally enhance the formability of a metal sheet. A ring-shaped beam can be used for preheating in the flange forming process. According to the tailored heat principle, only the area to be formed is heated. In this paper, a method is presented to design modules with reflective freeform optics (FFO) that converts a collimated Super-Gaussian beam into a ring-shaped beam at a working distance of e.g. 170 mm and deflects the laser beam by 90° simultaneously. The FFO design is based on a 2D geometric model of the optical system and is iteratively optimized according to the process requirements. The FFO was manufactured by fast tool diamond turning and placed into an integration model within a progressive die tool. The design was verified by beam profile measurement as well as by analyzing the formed part quality.

Keywords: Flange forming; sheet metal working; Tailored Heat; freeform optics design; ring-shaped intensity profile


Online determination of the local hardness during laser beam welding of steel
David Traunecker, Michael Jarwitz, Andreas Michalowski, Thomas Graf

Due to the increasing product variety, flexible production facilities are required that can already work productively from batch-size 1 onwards. This requires flexible and online-capable quality assurance of the processes to be able to detect defective parts as soon as possible.
One important quality feature in laser welding is the resulting hardness, which usually must be determined post-process. In this paper, a method for the spatially resolved online determination of the hardness is presented at the example of laser welding of mild steel. The dependence of the hardness on the cooling rate was utilized for this purpose.
Therefore, the cooling curves were determined spatially resolved during the welding process using a scanning pyrometer and the heat loss coefficient was determined from these cooling curves as a measure of the cooling rate. The corresponding local hardness was determined post-process. The results show a correlation between the local hardness values and the local heat loss coefficients within a set of process parameters, making this a promising approach for the spatially resolved online determination of the hardness.

Keywords: process monitoring; online monitoring; temperature measurement; hardness; laser beam welding


Identification of new kinematic systems for laser materials processing
Thomas Kaster, Philipp Walderich, Leon Gorißen, Felipe Arango Callejas, Christian Hinke

Laser-based manufacturing systems have become increasingly popular in recent years due to their ability to achieve high precision and accuracy in a wide range of applications. However, the kinematic systems used are often adapted from other manufacturing technologies like milling. These do not exploit the advantages of laser technology, e. g. the contact-free and thus restoring force-free process. A systematic search for new kinematic systems geared towards laser materials processing is carried out in accordance with the method of the technology intelligence process. The available technologies are evaluated with respect to various criteria, in particular their suitability for processing large-area components. It is determined that mobile robots should be particularly suitable for laser material processing of large-area components. The potential advantages of mobile robots are derived and discussed in a systematic method and validated with a prototype.

Keywords: laser materials processing; technology intelligence; kinematic systems; robotic


Monitoring and control of laser-based additive manufacturing using convolutional neural nets and reinforcement learning
Frederic Cousin, Vishnuu Jothi Prakash, Julian Ulrich Weber, Ingomar Kelbassa

Directed Energy Deposition (DED) processes using industry robots enable the production of large structures through additive manufacturing technology. One major challenge hindering the industrialization of DED processes is online process monitoring and control. Melt pool characteristics are one of the key identifiers regarding the stability of DED processes. Typically, these characteristics are monitored visually by camera systems using standard image processing toolboxes. To improve the robustness, the image processing of melt pools has been replaced by a convolutional neural net.
Furthermore, a control strategy, focusing on laser power and processing speed, has been introduced through a Reinforcement Learning Framework. Wherein, a second neural net, predicting melt pool characteristics, has been used as the simulation environment. Based on this, the reward metric for the reinforcement system has been determined by comparing target melt pool characteristics with the predicted ones.

Keywords: Laser-based Processing; Reinforcement Learning; Convolutional Neural Net; Artificial Intelligence; Additive Manufacturing


In-situ monitoring of the laser powder bed fusion process by thermography, optical tomography and melt pool monitoring for defect detection
Nils Scheuschner Frank Heinrichsdorff, Simon Oster, Eckart Uhlmann Julian Polte, Anzhelika Gordei, Kai Hilgenberg

For the wide acceptance of the use of additive manufacturing (AM), it is required to provide reliable testing methods to ensure the safety of the additively manufactured parts. A possible solution could be the deployment of in-situ monitoring during the build process. However, for laser powder bed fusion using metal powders (PBF-LB/M) only a few in-situ monitoring techniques are commercially available (optical tomography, melt pool monitoring), which have not been researched to an extent that allows to guarantee the adherence to strict quality and safety standards.
In this contribution, we present results of a study of PBF-LB/M printed parts made of the nickel-based superalloy Haynes 282. The formation of defects was provoked by local variations of the process parameters and monitored by thermography, optical tomography and melt pool monitoring. Afterwards, the defects were characterized by computed tomography (CT) to identify the detection limits of the used in-situ techniques.

Keywords: Thermography; Optical tomography; Melt-pool-monitoring; Laser powder bed fusion; Haynes 282; Additive Manufacturing


Controlled laser hardening and laser metal deposition with flexible beam shaping
Manuel Marbach, Fabian Gärtner, Ramon Daetwyler, Matthias Julius, Matthias Hoebel

Productivity and reliability of laser transformation hardening (LTH) and laser metal deposition (LMD) processes benefit from optimizing the energy input into the part. We report on a new high-performance laser controller enabling flexible beam shaping for laser material processing. A dynamic galvo-scanner is integrated into the beam path and rapid 2-dimensional beam oscillations can be superimposed to the movement of the laser head. Our FlexiBeam-Controller analyses beam positions at 25 kHz update rates. Optimized intensity distributions for laser hardening and LMD applications are generated by adapting oscillation parameters and synchronizing laser output power with the trajectories of the laser beam. In combination with a two-colour pyrometer the laser control system can monitor and maintain desired temperature profiles in the laser interaction zone. Our study presents first applications of this advanced laser process control for laser hardening on steel 1.1191 (C45E) and LMD of Hastelloy X on stainless steel 1.4404 (AISI 316L) indicating clear benefits for industrial laser processing.

Keywords: Laser Metal Deposition; LMD; Laser Transformation Hardening; LTH; Pyrometer; Closed Loop; Temperature Controlled; FPGA; Hastelloy X; 1.4404; 1.1191


Real-time monitoring and control of the edge quality in metal laser cutting
Nino Di Pasquale, Roland Bader, Andreas Lüdi, Gabriele Maroni, Loris Cannelli, Dario Piga

A real-time monitoring of the edge quality during laser cutting enables a permanently good and robust cut. Since direct measurements of edge quality, particularly burr and roughness, are hardly possible, monitoring of the process has been realized via a cutting head camera on a flatbed high-power laser cutting machine. The acquired in-process images are used together with an independent quality measurement to train an artificial neural network. The latter is trained to classify the cutting quality based on the process camera images into high and low burr and roughness, respectively. This feedback from the cutting process allows the implementation of a closed-loop controller to adapt cutting parameters and will enable to realize a self-learning machine. Several monitoring setups, including various illumination concepts were investigated. It was found that the cut quality estimation with a selectable acceptance threshold is reliably feasible with an F1-score of about 0.85, on average. Further, with a simple calibration or fast adaptation step, the model transferability to different machines, thicknesses, and alloys are well possible.

Keywords: laser metal cutting; real-time; in-process quality monitoring; deep neural network; closed loop control; camera


Real-time roughness estimation in laser oxidation cutting via coaxial process vision
Matteo Pacher, Leonardo Caprio, Giulio Delama, Davide Gandolfi, Sergio Matteo Savaresi, Barbara Previtali, Mara Tanelli

Laser cutting is an established technology for the processing of metal sheets and tubes given its elevated productivity and high part quality. However, external influences or variations in the process conditions may affect the quality of the final product. In oxidation cutting, cuts are typically evaluated by means of profile roughness whilst critical defect formation consists in loss of cut. Real-time estimation of the cut quality via process monitoring is of great interest since it enables inline evaluation of the manufactured components and identification of defected parts. Such capabilities were investigated during the cutting of high thickness mild steel, acquiring process emission images with a coaxial monitoring system and correlating them to the profile roughness via Machine Learning algorithms. Results indicate roughness predictions with good fitting (R2>80%) and with a Mean Absolute Error below 10 μm (𝑅𝑅𝑧𝑧 parameter).

Keywords: Laser cutting; Oxidation cutting; Monitoring; Roughness; Machine Learning


Towards dynamic shaping of high power laser beam intensity profile by means of a deformable mirror
Scholte J. L. Bremer, Martin Luckabauer, Ronald G.K.M. Aarts, Gert-willem R.B.E. Römer

In high power laser material processing, such as laser welding, laser transformation hardening and laser cladding, the material properties depend on laser-induced thermal cycles during the process. Traditionally, these thermal cycles are controlled by modulating laser power and beam velocity. However, the thermal cycle is also strongly affected by the laser beam intensity profile in terms of dimensions and shape. Adapting this intensity profile during processing allows to control and tailor the thermal cycles. Therefore, this work presents a dynamic beam shaping setup for high power laser applications. An optical setup, based on a deformable mirror, has been designed and implemented. The prototype of the setup is evaluated in a lab environment by measuring the laser intensity distribution with a CCD camera based beam profiler at low laser powers. Results show that the setup makes it possible to create various distinct laser intensity profiles dynamically and with a large design freedom.

Keywords: Laser material processing; Laser intensity profile; Dynamic beam shaping; Deformable Mirror


Design of a research system for process monitoring and closed-loop control of laser powder bed fusion
R. Wenger, M. Hofele, A. Oster, D. Reitemeyer, H. Kriz, H. Riegel

Additive manufacturing by means of Laser Powder Bed Fusion (L-PBF) enables the fabrication of highly complex metal components. Currently, the L-PBF process is performed by predefined parameters that interact in complex manners and are determined by time-consuming and costly trial-and-error parameter studies. Nevertheless, geometry-dependent differences in heat balance can occur in the L-PBF build-up process, leading to local differences in microstructure, and further temperature-dependent process failures such as cracks, balling and pores. Therefore, a closed-loop control system using temperature feedback is an interesting approach to homogenize the heat balance and stabilize the melt pool during micro-welding. This contribution describes the design of an L-PBF research system using the Autodesk Machine Control Framework as the overarching software that provides closed-loop process data feedback of monitoring sensors and combines a modular L-PBF process unit with a fiber laser, 3D-scanner components (SCANLAB) and a Sensortherm two-color pyrometer for temperature measurement and control.

Keywords: Additive Manufacturing; Laser Powder Bed Fusion; Closed-Loop Control; Two-Color Pyrometer; Temperature Feedback; Process Data Feedback


Spatially resolved temperature field measurement in powder bed fusion for online detection of porosity
Dieter Tyrall, Thomas Seefeld

As many quality issues in powder bed fusion (PBF) may be related back to processing issues, there is a strong demand for online process monitoring that allows for a complete documentation of the AM build-up processes, and preferably an in-situ detection of imperfections like porosity.
Thermal monitoring with a 2-color thermal camera allows for a determination of melt pool dimensions with high lateral resolution during PBF. From this, a correlation of online melt pool size data and built density was established. A spatially resolved temperature field measurement with a frame rate of 500 Hz enables the layer wise evaluation of melt pool geometry with a spatial resolution of 9 μm within the entire build-up area of 250 mm by 250 mm. This method ensures a continuous process monitoring of the entire build-up process and enables the recognition of deviations in process behavior. For the first time, a correlation of these online monitoring data with spatially resolved porosity distribution data from micro-X-ray tomography measurements is established, indicating the potential of online porosity detection and monitoring in PBF.

Keywords: additive manufacturing, laser pwoder bed fusion, process monitoring, temperature measurement, porosity


An intelligent quality inspection system to detect laser welding defects
Patricia M. Dold, Meiko Boley, Fabian Bleier, Ralf Mikut

This paper deals with a laser welding process with a high welding speed of 500 mm/s and thin metal plates with a thickness of 75 μm. While the welding process is observed by a photodiode and a high-speed camera in-sito, various in production occurring defects such as spatter, gap or defocus were provoked. To detect welding defects, deep learning has achieved great success and therefore is used as a baseline. However, the results of deep neural networks are difficult to interpret, and their inference times are long. Therefore, our approach empirically extracts and selects relevant features and classifies using decision trees. Results show that our approach leads to competitive results to deep neural networks, but with the advantage of more interpretable results and shorter inference times.

Keywords: laser welding; quality monitoring; machine learning; decision trees; feature importance; signal and image processing


Diffractive neural networks as a tool for three-dimensional beam shaping in laser materials processing
Paul Buske, Annika Völl, Jochen Stollenwerk, Carlo Holly

Diffractive neural networks are a design method for cascading diffractive optical elements or spatial light modulators that is based on neural network training techniques. In previous work, we have shown that these systems can be used for complex beam shaping tasks like combined beam splitting and shaping, high depth-of-focus through simultaneous optimization of amplitude and phase and the shaping of multiple target planes for effective 3D profiles. Additionally, alignment errors can be addressed and compensated during training and other optical elements like lenses can be included into training without effort.
Here, we present the method, advantages compared to other beam shaping techniques and applications in laser materials processing like for example in surface treatment.

Keywords: beam shaping, artificial intelligence, spatial light modulator, diffractive optics


Development of Fast Temperature Measurement System for Ultrashort Pulse Laser Material Processing
Jiri Martan, Denys Moskal, Carlos Beltrami, Milan Honner, Inigo Ramon- Conde, Ole Peters, Joerg Schille, Vladislav Lang, Udo Loeschner

Laser material processing with high repetition frequencies of ultrashort laser pulses is able to initiate heat accumulation effects that can decrease processing quality. In order to gain deeper insights into these effects, a temperature measurement system with nanosecond time resolution was developed using infrared detector and a set of parabolic mirrors. This system was usable for scientific measurements on one small area (0.5 mm) of the sample. For measurement in more industrially relevant processes on larger areas (100 mm), alternative configurations were developed: 1) measurement through scanning head and 2) special multifocal ellipsoidal mirror placed beside a polygon scan head. This work focusses on comparison of advantages and limitations of the developed measurement configurations by sensitivity, signal to noise ratio, field of view and measurable temperature range. The measurement system was used for analysis of laser surface texturing of steel and ceramics substrates as a preparation for thermal spraying of coatings.

Keywords: heat accumulation measurement; laser micromachining; ultrashort pulsed lasers; infrared optics and detectors; process monitoring


Battery recycling – Exploitation of laser technologies for dismantling and recycling processes
Max Rettenmeier, Mauritz Möller, Oliver Bocksrocker, Vinayakrajr Salvarrajan, Christoph Neugebauer

The ramp-up of new production infrastructure to manufacture lithium-ion batteries for battery electric vehicles is moving ahead at a rapid pace. These enormous quantities of vehicle batteries must be recycled in a fast loop due to the increasing shortage of critical raw materials. Laser technologies offer the possibility to perform many of the necessary process steps of dismantling and recycling. In this paper, an application overview and analysis of laser technologies in the field of cutting and ablating processes will be presented. The cutting processes are primarily focused on the dismantling of metal and metal-plastic components of battery packs. Furthermore, in the ablative processes, the ablation of active material of the battery electrode foil using ns-pulsed lasers is investigated. Within the scope of this application, the elaboration of laser-technological parameter fields will be pursued in particular.

Keywords: Recycling; cutting processes, ablating processes, dismantling, battery pack


Measurements of weld quality with OCT during laser beam oscillation welding of aluminum alloy for battery production
Nataliya Deyneka Dupriez

Successful mass production is closely linked to a quality assurance procedure tailored to the joining process. The oscillating laser beam was used for welding aluminum alloy. Continuous weld depth monitoring was realized with OCT by synchronously controlling the position of the OCT scanner and the processing scanner optics. On the one hand, this ensured the required penetration, which benefits the stabilization of the molten pool and keyhole and thus better weld quality. On the other hand, control of the weld penetration depth is essential to avoid the risk of piercing the battery cell when welding the battery system. The variation of weld depth within the oscillation period was observed and analyzed in conjunction with video recordings. OCT enables highly accurate non-contact process monitoring and quality assurance during remote laser welding and can serve as a corrective and/or preventive measure to achieve zero scrap and optimal battery performance without compromising safety.

Keywords: industrial laser processing; welding, oscillation; optical sensor; optical coherence tomography; process monitoring; quality assessment; weld depth; aluminum alloy; battery; electric vehicle; efficiency