United States

On-the-fly evaluation and process control of laser surface microprocessing can be realized with the detection of infrared (IR) thermal radiation. In this study, a new multi-focus mirror was developed and tested for large-area detection of IR-thermal radiation during ultrashort pulsed laser surface micromachining. The IR-thermal detection in the laser micromachining was for the first time combined with a fast hardware signal analysis and laser power control using a field programmable gate array (FPGA) module. The new mirror has a long-line distribution of focused points on one side to collect the emitted thermal radiation along a laser scanning line and focuses the captured IR-thermal radiation to a point on the other side for signal analysis. The length of the detection line of the developed multi-focus mirror was 120 mm and it can be expanded up to tens of centimeters. This IR-thermal radiation signal is processed by the FPGA module with a fast evaluation of heat accumulation at a cycle time loop of 50 ns and on-the-fly control of the laser power during surface microprocessing. When the detected heat accumulation signal grows higher than the required value, then the average laser power is automatically decreased, and vice versa. This lR-thermal tracking system was applied for laser surface micromachining of metals and ceramics with polygon- and galvo-scanners. The presented lR-thermal tracking system with the multi-focus mirror can be used also in other applications, such as beam shaping, line scanning spectroscopy, linear surface irradiation, or non-destructive testing (IR-NDT).

ICALEO 2023

Fast IR-Thermal Tracking of Laser Microprocessing with Multi-focus Mirror

straight-line focusing

multi-focus mirror

on-the-fly laser power control

lr-thermal tracking

laser micromachining

technical paper

**General Chair Welcome**

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Welcome to ICALEO® 2023!
	
We were excited to host the world’s leading international conference on Lasers and Electro-Optics in Chicago. The Laser has been shown to have the potential to revolutionize solutions for major global challenges like energy usage and supply, global warming, and CO2 footprint. Even the fusion energy breakthrough is based on laser technology. The conference, with peer-reviewed scientific and technical presentations, focused on the newest results in Micro Applications, Macro Applications, Additive Manufacturing, Beam Shaping and Frontiers in Laser Applications. Additionally, the Battery-Manufacturing Track gave an overview of the process combined with the newest advances in laser application results. 

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**Klaus Löffler **  
ICALEO General Chair   
Managing Director, Precitec GmbH

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ICALEO® brings together the brightest minds, seasoned professionals, and pioneering researchers from around the world. With a laser-focused agenda, ICALEO offers an exceptional platform to delve into the cutting-edge advancements, diverse applications, and transformative research that define lasers and electro-optics.

Modern beam sources and system technology allows to adjust a large number of parameters relevant to the machining process, and often these parameters can even be varied as a function of time. In principle, a large parameter space makes it possible to find and use particularly well-suited parameter regimes for the specific application, but this generally increases the experimental effort considerably. The efficient optimization of process parameters, incorporating all the possibilities of modern flexible laser beam sources and system technology, in perspective also opens the door to high quality and high throughput.<br>
Semiconductors as Silicon and Germanium show on the one hand significantly higher ablation efficiencies when they are machined with bursts but then suffers from lower machining quality. On the other hand, especially (100) oriented wafers, can be machined with very low surface roughness due to a melting effect ocuring if a certain peak fluence is exceeded. Combination of these two processes would further exceed the parameter space for a process optimization when high throughput and surface quality is demanded. We will demonstrate the potential of machine learning to significantly shorten the demanded time and effort for the process optimization when these materials are machined with ultra-short laser pulses.

Optimizing the Ultra-Short-Pulsed Laser Machining of Silicon and Germanium With Machine Learning for Highest Throughput and Quality

Industrially widespread laser processes such as laser cutting and laser welding are nowadays usually only monitored and at best controlled in partial areas such as process aborts. On the other hand, process data can be obtained with the aid of acoustic and optical sensors as well as thermal cameras and then processed further. Process data obtained in this way are linked to quality characteristics such as burr formation and edge roughness in cutting and structure depth and spacing in micro structuring on the one hand, and to process parameters such as laser power, feed rate, focus size and position on the other. A prerequisite for controlled laser processes is whether AI-supported algorithms can be used to make reliable predictions about the expected process result based on the process data. Within an internal Fraunhofer project "AI Beam", large amounts of data originating from cutting and welding are used to train an AI (and the correct selection of the appropriate mathematical method) and linked to quality criteria. First results of this project related to cutting will be presented. In another research project, similar approaches for laser micro structuring are being pursued together with industrial partners. Results from this ongoing project will also be presented.

Use of AI for Laser Cutting and Structuring

Laser heat treatment is a process that changes the strength and hardness of the material. Unlike general heat treatment processes, laser heat treatment has the advantage of being able to apply heat locally and having a fast-cooling time. However, due to the high-intensity heat source, excessive heat can be applied to the material, resulting in surface melting, and softening. In this study, a deep learning model based on generative adversarial network was applied to predict the hardening and softening phenomena that occur during laser heat treatment of AH36 carbon steel. As input data, the temperature field of the cross-section, which is obtained by solving the 3-D heat conduction equation using COMSOL, was applied. The input data consist of two temperature distributions: one obtained when the surface temperature is at its maximum, and the other obtained after 0.5-second. The hardness and phase distributions of the cross-section used as ground truth were obtained from a 2 kW multi-mode fiber laser experiment. The model trained with only one-temperature field predicted the hardness distributions well, but the prediction of the softened region was inaccurate. On the other hand, in the case of the model trained with two-temperature fields, it showed high prediction accuracy for hardness and phase distributions in both the softened and hardened regions.

Prediction of Hardness and Phase Distributions in Laser Heat Treatment of AH36 Steel Using Deep Learning

In laser cutting, the fundamental role of the gas flow for melt removal and kerf formation is generally accepted. Beyond this vague understanding, however, the underlying physical mechanisms are not yet fully understood. In particular, detailed data concerning the momentum and heat transfer between the gas and melt have seldom been reported. This study addresses the local interactions between the cutting gas and kerf surface (melt film surface) in a fundamental way based on a combined experimental, theoretical, and numerical approach. Typical solid-state laser cut edges are analyzed considering the characteristic surface structures and the basic influences of the gas flow on the global and local melt movement. Here, apparent structures in the micrometer range indicate the effect of vortical gas structures close to the wall. Theoretical investigation of the gas boundary layer is conducted by semi-empirical equations and the transfer of basic results from boundary layer theory. It is shown that the boundary layer is in transition between laminar and turbulent flow, and local flow separations and shock-boundary layer interactions primarily induce spatially periodic and quasi-stationary instability modes. An improved numerical model of the cutting gas flow confirms the theoretical results and exhibits good agreement with experimental cut edges, reproducing the relevant instability modes and quantifying the local momentum and heat transfer distributions between gas and melt. With the knowledge gained about the underlying physical mechanisms, promising approaches for improvements of the fusion cutting performance are proposed.

Laser Fusion Cutting: The Missing Link Between Gas Dynamics and Cut Edge Topography

Laser cutting process is widely used across the engineering sector for processing metals and alloys. Over the last few years there has been an increasing interest in exploiting ceramic matrix composite (CMC) based materials for both high and low-temperature applications in aerospace, power generation and nuclear applications, due to their superior thermal and structural properties. However, laser processing of anisotropic composites such as CMC is difficult due to its high melting temperature, and the differences in the physical and thermal properties of the resin/matrix and fibre. This research discusses about laser cutting of silica based CMC material using a continuous wave fibre laser operating at an average power of 2kW.<br>
The influence of various laser, kinematic and assist gas parameters on cut quality and speed was investigated, and the effect of various cutting strategies was studied in detail. The laser cut samples were analysed for surface and sub-surface cut characteristics using optical microscope, surface profiler, scanning electron microscope and X-Ray CT scan. Irrespective of the conventional wisdom (that high-power long pulse lasers are not suitable for composites) CW laser can be used to cut a 5 mm thick CMC material in line with the speed and quality requirements of the manufacturing industries. Multi-pass laser cutting shows better cutting performance, in terms of both speed and quality than the conventional single pass cutting technique. Assist gas composition plays a major role in controlling the material removal mechanism, and significantly influences the cut quality.

Laser Cutting of Ceramic Matrix Composites

High gas pressures (1.0-1.6 MPa) are employed in conventional inert laser cutting to achieve efficient material removal and high cut quality. However, this approach results in the emission of large quantities of by-products, which can pose a risk to human health and the environment. For applications such as nuclear decommissioning, where global extraction and containment can be challenging, hazardous by-product formation, rather than process efficiency, is the main priority. This paper demonstrates low-pressure (0.3 − 0.6 MPa) laser cutting techniques developed to reduce by-products. This study investigates the causal links between melt ejection and gas dynamic interactions in low-pressure laser cutting. Experiments were conducted using a 300 W Nd:Yb fibre laser to cut 304 stainless steel samples. Melt ejection and breakdown profiles were captured using a FASTCAM mini AX 200 camera. The lens combination fitted to the camera provided a spatial resolution of approximately 1 µm. The gas dynamic interactions were assessed through comparisons to existing studies of schlieren imaging in idealised environments. The results show that gas dynamics are crucial in melt ejection and breakdown mechanisms during laser cutting. The key findings of this study are images of breakdown mechanisms linked to low-pressure gas dynamics. The impact of this work is that breakdown mechanisms more favourable to reducing environmental risk have been demonstrated. A greater understanding of the risk is indispensable to developing new laser-cutting control methods for hazardous materials.

Reducing Environmental Risks in Laser Cutting: A Study of Low-Pressure Gas Dynamics

During laser fusion cutting, burr forms when the molten metal does not sufficiently exit the interaction zone. When it forms on the lower edge of the cut flank, burr becomes a factor limiting quality. Previous research has shown that a temporally regular and spatially localized melt flow can prevent the formation of burr. However, the high dynamics of the sub-processes involved can cause intrinsic instabilities that disrupt the flow and reduce the efficiency of the melt ejection. This paper presents a study on the correlation between process parameters, melt flow properties, and burr formation. It includes an experimental observation of the melt-flow dynamics using high-speed videography. In addition, a numerical CFD model was set up to examine fundamental flow properties, some of which are not observable experimentally. The dependency of the burr formation on the liquid Weber and Reynolds numbers is analyzed and it is demonstrated how the magnitude and allocation of vapor pressure gradients in the kerf decisively affect melt ejection and burr formation. Additionally, a previously unknown melt ejection regime is identified in the thick section range, which occurs at feed rates close to the maximum cutting speed under specific high-power process conditions. This regime is characterized by a significantly increased process efficiency that could open up a new high-speed process window.

Numerical and Experimental Investigation of the Melt Removal Mechanism and Burr Formation During Laser Cutting of Metals

Laser-based production systems have become more and more popular in recent years due to their potential to achieve high precision and accuracy in a wide range of different applications. However, the kinematic systems used for laser material processing are often inherited from other production technologies such as milling. The use of mobile robots equipped with laser processing optics could disprove the current paradigm of adapted kinematic systems: scaling the size of the material processing system with the size of the components being processed, and thus the resources used. The trend of autonomous mobile robotic systems replacing classical kinematic systems in the field of material handling in industrial applications has been evident for years due to their higher flexibility, efficiency and lower operating costs. In this paper, the prototype of a corresponding mobile robot system is presented. In addition, the general design of the mobile robot is presented. One challenge is the accuracy of a mobile robot system, for a common LMP process such as laser cutting, the mobile robot must be able to follow a predefined trajectory as accurately as possible. For this purpose, two different measurement systems are presented and compared. To demonstrate the potential of the mobile robot, an exemplary laser material processing is also performed and evaluated. Finally, possibilities for improvement or further development, such as the integration of scanner optics or the use of several autonomous mobile robots to increase productivity, are shown.

An Approach Towards the Application of Mobile Robots in Laser Materials Processing

Exploiting the Potentials of Laser Beam Shaping for Welding Applications via Numerical Simulations

In the attempt to produce lighter battery packs at lower cost, replacing common copper parts with aluminium components has been a popular approach in recent years. With regards to joining technologies, there is a growing interest in applying laser beam welding in battery manufacturing due to several advantages such as single-sided and non-contact access, whilst maintaining a narrow heat affected zone. Motivated by the need to control and reduce weld porosity in AA1060 battery busbar welding with the ultimate goal to enhance durability and reduce electrical resistance, this paper has been developed with the aim to study the mechanisms of porosity formation and implement a novel beam shaping approach to reduce the volumetric pores distribution. Porosity formation is attributed to two mechanisms: metallurgical porosity caused by chemical reactions and process porosity related to the instability of the keyhole and fast solidification rates. In this study, a multi-physics Computational Fluid Dynamics (CFD) model has been developed and calibrated to study the effect of a circular and a tailing laser beam shape on melt pool dynamics and evolution of porosity due to instability of the keyhole, and hence generate knowledge about the underlying physics of the welding process itself. The study will discuss roots of actions for optimisation of weld quality via integration of laser beam shaping lens in “off-the-shelf” welding heads.

Elucidating the Effect of Circular and Tailing Laser Beam Shapes on Keyhole Necking and Porosity Formation During Laser Beam Welding of Aluminium 1060 Using a Multi-Physics CFD Approach

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Optimizing the Ultra-Short-Pulsed Laser Machining of Silicon and Germanium With Machine Learning for Highest Throughput and Quality

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Optimizing the Ultra-Short-Pulsed Laser Machining of Silicon and Germanium With Machine Learning for Highest Throughput and Quality

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