Emission of X-rays during ultrashort pulse laser processing
Herbert Legall, Jörn Bonse, Jörg Krüger

Ultrashort pulse laser materials processing can be accompanied by the production of X-rays. Small doses per laser pulse can accumulate to significant dose rates at high laser pulse repetition rates which may exceed the permitted X-ray limits for human exposure. Consequently, a proper radiation shielding must be considered in laser machining. A brief overview of the current state of the art in the field of undesired generation of X-ray radiation during ultrashort pulse laser material processing in air is presented.

Keywords: ultra-short pulse laser processing; laser-induced X-ray emission; radiation protection


Scaling the throughput of high-quality silicon laser micromachining using a 1-kW sub-picosecond laser
Daniel Holder, Rudolf Weber, Christoph Röcker, Gerhard Kunz, David Bruneel, Martin Delaigue, Thomas Graf, Marwan Abdou Ahmed

Recently, laser processing of silicon with ultrafast lasers has gained widespread attention for manufacturing of optics for THz radiation, an emerging topic with applications in medical imaging, security and communication. Such THz-optics require high-quality surfaces with low roughness in order to provide high transmission and low scattering. In the past, the low average power of ultrafast lasers limited the achievable throughput in silicon laser micromachining.
In this work a processing strategy for high-quality high-throughput micromachining of silicon with a 1-kW sub-picosecond laser is presented, which takes benefit of pulse bursts, low fluences and high feed rates.As a result, laser micromachining could be demonstrated as a suitable technology for manufacturing of smooth structures on silicon while maintaining a high throughput. Surfaces with an appropriate roughness of Sa ≤ 0.6 μm were produced with a high material removal rate of 230 mm³/min and a machining depth of up to 313 μm.

Keywords: Ultrafast laser ablation; silicon micromachining; upscaling; pulse burst; high quality; high throughput


Comparison of ultrafast laser ablation of CrMnFeCoNi high antropy alloy to the conventional stainless steel AISI 304
D. Redka, C. Gadelmeier, J. Winter, M. Spellauge, J. Minár, H. P. Huber

Ultrafast lasers as tools are nowadays mainly used in the field of high-precision laser micromachining. However, for technical applications, besides process optimization, the development of new functional materials is of crucial importance. In this work, we present a novel study on single-pulse laser ablation (530 fs, 1056 nm) of CrMnFeCoNi high entropy alloy (HEA), and compare results to the conventional stainless steel AISI 304. While HEAs are known to have a high damage resistance against high-energy particle radiation we find that this is not true for electromagnetic radiation, as the damage threshold of CrMnFeCoNi HEA is 0.24 J/cm², which is lower than that of AISI 304 (0.27 J/cm²). A detailed analysis of the crater morphology, ablation depths as well as ablation volumes shows that the ablation mechanisms for both alloys are comparable, but contrary to expectations, CrMnFeCoNi HEA laser ablation is energetically more efficient in comparison to AISI 304.

Keywords: High entropy alloy; Stainless steel; Ultrashort-pulse; Threshold fluence; Ablation energetic;


Influence of the build angle dependent surface quality on the ultra-short pulsed laser ablation of additive manufactured AlSi10Mg samples
Simon Ruck, David K. Harrison, Anjali De Silva, Max-Jonathan Kleefoot, Harald Riegel

Additive Manufacturing (AM) been proved as a method to offer new possibilities for the production of highly complex parts. One new interesting but yet small field of application is the 3D printing of complex optical elements, e.g. complex reflective mirror optics with integrated lightweight structures.
However, to achieve surfaces with an optical quality on additive manufactured metal parts, mostly mechanical machining processes such as diamond turning or pad polishing are used. The studying of laser material processing, e.g. ultra-short pulsed laser ablation as a post-processing method for additive manufactured optical components is of great importance. In this study, we investigate the influence of the initial surface quality on the ultra-short pulsed laser ablation process. Therefore, we varied the build angles of our samples and used different laser parameter setups to determine e.g. the material removal rate, process efficiency and achievable surface quality.

Keywords: laser ablation; post-processing; additive manufacturing; ultra-short pulsed laser


Theoretical analysis of the incubation effect on the ablation behavior using spatial shaped ultra-short pulse laser
Marco Smarra, Cemal Esen, Andreas Ostendorf, Evgeny Gurevich

Ultra-short laser pulses are well known for their low thermal effect on the ablation process and therefore are used in numerous applications like surface texturing and functionalization. However, high peak fluence can lead to a reduction in ablation efficiency. Beam shaping can be used to solve this issue. Beam errors, like defocus or astigmatism, lead to a larger beam radius and a decrease of the fluence on the surface of the workpiece. This paper focuses on the theoretical study of the incubation effect and its influences on the ablated volume per pulse by analyzing the effects of the waist position of the laser beam and the ablation threshold of the sample material. This work is fundamental for handling the ablation process using high pulse energies.

Keywords: Laser Ablation; Laser Beam Shaping; Laser Micro Processing; Ultra-Short Laser Pulses


Electric field-assisted laser ablation of silicon in air by using ultrashort laser pulses
Yiyun Kang , Pavel N. Terekhin, Garik Torosyan, Hicham Derouach, Mareike Schäfer, Bärbel Rethfeld, Johannes A. L’huillier

Precise processing on silicon, as widely used material in electronic devices, has shown a rising interest in laser micromachining. To optimize this process, we apply an external electric field parallel to the laser beam axis during irradiation of samples with ultrashort laser pulses. When the electric field is turned on, the free electrons and holes are redistributed within silicon due to electron drift. The electrons can be localized near the surface area which is supposed to influence the laser excitation process. Thus, the external electric field influences the absorption process due to the reorganized spatial distribution of electrons. We investigated the effect of an applied static strong electric field during single- and multi-pulse treatment of silicon. An enhancement of the ablation depth is observed by applying the electric field in the direction of the laser radiation.

Keywords: silicon; ablation; external electric field; femtosecond laser fabrication


Magnetic field assisted laser ablation of silicon by using short and ultrashort laser pulses
Yiyun Kanga, Pavel N. Terekhin, Garik Torosyan, Falicienne G. Keabou, Hicham Derouach, Mareike Schäfer, Bärbel Rethfeld, Johannes A. L’huillier

A controlled machining process with optimum energy-matter coupling in micro-scale by short and ultrashort laser pulses brings great benefits in industrial applications. The influence of external magnetic field on the ablation process of silicon irradiated by laser pulses in the infrared range was investigated. The external field is applied parallel to the laser beam to prevent escaping of the charge carriers in the laser-induced plasma plume from the ablation area. We performed single-pulse irradiation in three different duration ranges: femtosecond, picosecond and nanosecond. We observed that the effect of the external magnetic field depends strongly on the pulse duration. An essential improvement in the efficiency of material removal was achieved for pulses in the nanosecond range.

Keywords: Silicon; magnetic field assisted ablation; short laser pulses; ultrashort laser pulses


Fundamental characteristics of fiber laser beam sawing of 10 mm thick stainless steel
Madlen Borkmann, Achim Mahrle, Patrick Herwig, Andreas Wetzig

AISI 304 stainless steel plates of 10 mm thickness were sectioned by fibre laser beam sawing trials. The applied sawing technique as a new variant of laser cutting with an oscillating beam relies on periodical changes of the focusing length of the optical setup by integrated mirrors with adjustable curvature radius. As a result, the focal plane position can be forced to oscillate with a frequency of up to 4800 Hz and amplitudes up to 6 mm. The resulting temporally averaged beam profile is characterized by a nearly constant beam diameter over the whole spatial oscillation range. Variations of oscillation frequency, amplitude and nominal focal layer position were performed to get first insights into the effect mechanisms of fibre laser beam sawing. It is found that the cut kerf geometry can be adjusted to improve cutting gas flow characteristics and melt removal.

Keywords: laser beam sawing; laser beam oscillation; fiber laser fusion cutting; cut kerf geometry; cutting gas flow; gas flow simulation


Polymer film processing with a high-power industrial femtosecond laser
Chandra Sekher Reddy Nathala, Victor Matylitsky, Jim Bovatsek

Polymer materials are increasingly important for medical device, flat panel display and microelectronics applications. Due to the high thermal sensitivity of polymers, femtosecond laser processing can minimize heat deposition, and high powers are needed to achieve fast processing. In our work, we present ablation thresholds and cutting speeds for two common polymer materials, polyethylene terephthalate (PET) and polyimide, processed with a 100 W femtosecond laser with single and burst pulses and at infrared and green wavelengths. Cutting speeds were determined for both single-pass and multi-pass strategies. In addition to determining the ablation thresholds and the maximum cutting speed, the processed samples were analyzed for kerf width and heat affected zone (HAZ). With optimized parameters, high speed, high quality cutting of PET and polyimide films was demonstrated with a high-power femtosecond laser.

Keywords: High power femtosecond laser; Femtosecond laser processing; Polymers; High throughput processing


Laser shock micro-forming of stainless steel: thermal effects at high repetition ps-pulses
David Munoz-Martin, José Manuel López, Miguel Morales, Laura Rivera, Juan José Moreno-Labella, Arturo Chávez-Chávez, Gilberto Gomez-Rosas, Carlos Molpeceres

A solid-state ps-pulsed laser, emitting at 1064 nm and repetition rate from 0.4 to 10 kHz, was used to laser peen form thin stainless steel metal sheets (50 μm thick). The laser repetition rate and the scanning speed were adjusted to keep the pitch distance between consecutive laser pulses constant.
The effect of the treatment was measured by the bending angle induced. When using the lowest repetition rate, up to 90o bending angles are achieved. As the laser repetition rate increases, the bending angle is dramatically reduced although every sample was processed with the same total number of pulses and with the same pulse energy.
Despite the small temperature increase in the whole sample, the local accumulative thermal effect at high pulse repetition has a strong influence on the bending angle. High temperature relaxes the stress induced by laser peen treatment and thus prevents bending the sample.

Keywords: Laser peen forming; simulation; residual stress; LSP


Combining LPBF and ultrafast laser processing to produce parts with deep microstructures
Manuel Henn, Matthias Buser, Volkher Onuseit, Rudolf Weber, Thomas Graf

Laser Powder Bed Fusion (LPBF) is limited in the achievable accuracy, surface quality and structure size due to its inherent melting process. The achievable structure sizes are mainly dependent on the focal diameter and the grain size of the powder. Smaller structures, especially deep and narrow slits with a width below 100 μm, are still a major challenge. Combining continuous wave and ultrashort pulsed lasers in the same optical system enables consecutive additive and subtractive processes. This results in a quasi-simultaneous manufacturing process, where the emerging part can be precisely machined with ultrafast laser ablation after each additively added layer. In the talk the system technology used for the superposition of the lasers as well as the results of the combined additive and subtractive processes for the fabrication of deep and narrow slits in stainless steel parts will be shown.

Keywords: laser powder bed fusion; ultrafast laser ablation; additive manufacturing; microstructures


Monitoring of the melt pool for fiber laser cutting using a high-speed camera
Max Schleier, Benedikt Adelmann, Cemal Esen, Uwe Glatzel, Ralf Hellmann

We demonstrate an in-situ and coaxial monitoring system based on a high-speed camera with a spectral response in the visible range, which is designed to be integrated into a cutting head between the collimator and processing lens. The thermal radiation from the melt pool is measured in the visible spectral range, without external illumination, spatially and temporally resolved from the top view. The dependencies of the laser power and feed rate on the spectral and geometric information captured from the images of the melt pool in the cut kerf are evaluated. In addition, we developed and show an algorithm to detect incomplete cuts caused by laser power and feed rate from the captured images.

Keywords: In-situ coaxial monitoring; high-speed camera; image processing;


Laser turning using ultra-short laser pulses and intensity distribution techniques
Julian Zettl, Christian Bischoff, Stefan Rung, Cemal Esen, Andrés Fabián Lasagni and Ralf Hellmann

We report on micromachining of rotationally symmetric parts using ultra-short laser pulses by a laser turning process. In particular, we compare the application of a focused Gaussian beam shape and a Top Hat beam profile as being generated by a concave freeform beam shaping optical element. The beam-shaping element is placed in the beam path with a specified lateral displacement to create a shifted intensity profile within the focal spot in order to specifically improve the efficiency of the laser turning process. Assessment criteria are the ablation rate and the ablation efficiency. Our study reveals a distinct increase of the process efficiency by using Top Hat beam shaping, which especially for low laser powers amounts to an increase of the ablation rate by about 57 %.

Keywords: ultra-short laser pulses, laser turning, beam shaping, ablation rate, process efficiency;


Advanced metal ablation based on highly flexible ultra-short pulsed laser platforms
M. Sailer, A. Fehrenbacher, A. Budnicki, S. Rübling, U. Quentin

Considering the energy efficiency and the ablation quality, micromachining of several metals is examined for different processing regimes depending on timescales from fs to μs. Choosing an optimized temporal energy deposition can address a variety of machining aspects like ablation efficiency and surface morphologies. Using the unique features of the TRUMPF TruMicro series, the temporal energy deposition can be influenced during operation on a femto- up to a microsecond timescale by tuning parameters such as the ultrashort pulse duration or employing bursts in the MHz- and GHz-regime. This enhanced flexibility paired with patent-pending process strategies leads to cutting-edge processing speeds and surface qualities.

Keywords: micromachining; industrial lasers; metals; ultrafast; temporal energy deposition; pulse tunability; burst optimization;


Ultra-short pulse laser micro-machining by spatially shaped ps- and fs-pulses for depth-selective μ-TLM resistivity test structures in TCO contact layers
Stephan Krause, Stefan Lange, Gao Yiding, Volker Naumann, Christian Hagendorf, Paul T. Miclea

Precise measurements of the electrical sheet and contact resistivity of individual layers, isolating trenches and homogenous ablation areas are required in many applications ranging from photovoltaics, opto-electronic devices to sensor technologies. We applied spatially shaped ultra-short pulse laser micro-machining for a high precision micro-machining approach for μ-TLM test structures. μ-TLM structures are fabricated with enhanced layer selectivity by matching of pulse overlapping based on rectangular spots in μm-dimensions. Ultrashort pulses by 10 ps and 200 fs (515/532/1030 nm) as well as optical beam shaping elements for redistribution to top-hat intensity profiles enables a selective removal of the top TCO. Thus, thermal damage is minimized in the underlying material and multilayer adjacent region of the laser trenches by ultrafast ablation mechanism. Morphology and microstructure of heat-affected zones were characterized by high-resolution transmission electron microscopy to optimize laser recipes for enhancing ablation selectivity. Finally, the optimized structures were tested in resistivity measurements of various multilayer systems with highly resistive interface layers.

Keywords: ultra-sort pulses; micro-machining; μ-TLM; top-Hat profile; transparent conductive layers


Laser ablation of thermoplastic composite for aerospace application
Hagen Dittmar, Simon Hirt, Verena Wippo, Peter Jaeschke, Stefan Kaierle, Ludger Overmeyer

Carbon fibre reinforced plastic (CFRP) is a well-established material in modern aerospace products. While the majority of primary structural components contains a thermoset matrix material, secondary components are increasingly made of thermoplastic matrix systems. Due to their superb performance and advantageous production properties, thermoplastics matrices are now also pushing into primary structural applications like fuselage, cowlings, and wings.
These structures are subject to an increased risk of damage during operation. Thus, repair strategies that address thermoplastic CFRP come to the fore. The repair by conventional tooling faces challenges that result from the thermoplastics’ abilities to melt, which cause the tools to clog and decrease process efficiency. Laser ablation poses an alternative approach allowing precise material removal without material related wear and thus a constant process quality.
This study demonstrates process efficiency of a laser ablation process on a CFRP with polyphenylene sulphide (PPS) matrix and the used processing parameters on the repair quality.

Keywords: laser scarfing; CFRP; repair; rework; thermoplastic; aerospace


Ultrafast pump-probe microscopy reveals the influence of a water layer on the early stage ablation dynamics of gold
Maximilian Spellauge, Carlos Doñate-Buendía, Stephan Barcikowski, Bilal Gökce, Heinz P. Huber

Despite the tremendous amount of research carried out in the field of pulsed laser ablation in liquids, there are only a few works available regarding the early stage ablation dynamics. A description how the liquid influences ablation on time scales ranging from pico- to microseconds would give valuable information regarding the physical processes involved. This work reports on ultrafast pump-probe microscopy measurements of a high purity bulk gold sample immersed in air and water. Our setup enables the observation of transient dynamics ranging from pulse impact on a picosecond timescale to shock wave and cavitation bubble propagation on the nanosecond timescale. We find that the water layer significantly influences the ablation dynamics on the whole investigated temporal range.

Keywords: Laser ablation in liquids; Ultrafast lasers; Pump-probe microscopy; Gold, Nanoparticles


Experimental investigations of a helical laser drilling process for pilot holes on complex surfaces
Sebastian Michel, Pascal Volke, Dirk Biermann

Increasing requirements and a high degree of freedom in design increasingly demand the manufacturing of bore holes with small diameters and high length-to-diameter ratios on complex shaped surfaces. Injection nozzles, medical tools and implants, cooling holes or oil channels are just a few examples. Unlike mechanical drilling tools, the laser beam is not deflected on inclined or curved surfaces and can therefore be used to create pilot holes for a subsequent mechanical drilling process. In this paper, the generation of pilot holes on flat and inclined surfaces using a Nd:YAG laser is investigated. A helical laser drilling process is used to drill holes with a diameter of 1.5 mm in X2CrNiMo17-12-2 stainless steel. Hole depth, diameter, roundness and conicity are evaluated. Application tests with single-lip drilling tools prove the potential of the laser holes to serve as a drilling guide for the mechanical deep hole drilling process.

Keywords: laser drilling; manufacturing technology; laser assisted processes; process combination; deep hole drilling


Fluence dependence of the edge quality of microhole exits for percussion drilling with ultrashort laser pulses
Anne Feuer, Rudolf Weber, Thomas Graf

For many applications, edge quality and shape accuracy of microholes are crucial. One assumption is that the fluence at the tip of the microhole during drilling is a key parameter for the quality of the microhole’s exit. It was therefore investigated experimentally how the fluence affects the edge quality. The experiments were performed in 0.5 mm thick steel using a Ti: sapphire laser system operating at a wavelength of 800 nm, a pulse duration of 1 ps, and a repetition rate of 1 kHz. For the quantitative analysis of the edge quality, microscope images were evaluated using a machine learning approach. Two key figures, striations amplitude and perimeter ratio, were defined that proved to be significant in characterizing the edge quality of exits. In the current talk it will be shown that the quality of exits of percussion-drilled microholes could be significantly improved if the fluence dependence is considered.

Keywords: Micro processing; Percussion drilling; Hole quality; Image analysis


Striation formation at the cut edge of oxygen assisted fibre laser cutting
Handika Sandra Dewi

Laser cut edges show parallel grooves features or striations. A high-quality cut edge is identified by fine striations at the cut edge. In order to gain better understanding in striation formation at the cut edge and improve the cutting quality, striations at the cut edge and melt flow during laser cutting processes were investigated. Oxygen assisted fibre laser cutting processes were carried out on 1-mm-thick and 20-mm-thick steels at varied processing parameters and recorded using high speed imaging with borosilicate glass as replacement edge. The size of striations at the cut edge and frequency of the molten ripples were measured. The striation widths show a linear correlation with the gas pressure and cutting speed, but inverse correlation was found between striation widths and the nozzle diameter. Gas pressure is most likely the main influencing factor affecting the striation widths.

Keywords: gas density, pressure, fluid dynamics, steels;


Potential health risks due to emission of hazardous substances during outdoor laser cutting
Jürgen Walter, Christian Hennigs, Michael Hustedt, Jörg Hermsdorf, Stefan Kaierle

In contrast to well-defined industrial laser processes, there is limited knowledge regarding secondary hazards due to emission of gaseous and particulate hazardous substances during outdoor-laser applications, such as facades cleaning, pipelines repair and rescue from crashed vehicles, including hazardous-substances capturing and handling.According to the German Clean-Air Act (TA-Luft), results of emission measurements in the exhaust air of a 3.0 kW laser-cutting process of typical automotive-multilayer structures were correlated with assessment criteria for the main hazardous components found, leading to requirements for exhaust-air cleaning. Complementary, air measurements at the operation site according to TRGS 402 were performed to evaluate whether the inhalation-exposure limits for hazardous substances released from the laser-process zone and not captured by the exhaust equipment were complied with, considering assessment criteria according to TRGS 900 and TRGS 910. The investigations showed that additional measures to reduce hazardous-substance concentrations are dispensable, if the exhaust unit is dimensioned correctly.

Keywords: laser outdoor application; multilayer cutting process; secondary hazard; process emissions; hazardous substance; mobile capturing; exhaust air analysis; workplace measurement; protection measure


Laser structuring of PVD multi-layer systems for wear reduction
Andreas Stephen, Bastian Lenz, Andreas Mehner, Tim Radel

Surface texturing is an effective way of improving tribological properties. Its main effect mechanisms are to trap wear particles and store lubricants. One of these technologies is texturing the surface with micro dimples by laser ablation. In this paper, the selective texturing of multi-layer systems, i.e., removing only the top layer by ultra-short pulse laser processing is presented. The removal of the top layer of the multi-layer systems is proven by laser confocal microscopy and EDX analysis. The selective laser structuring of the multi-layer systems generated by PVD synthesis developed for tribological applications, among others for the aerospace industry, results in precise structures with depth deviations of less than 0.2 μm without burrs or melt residues. These textures will further on result in reduced wear of thereby treated components depending on the structured layer systems and the geometry of the textures regarding dimple diameter and density.

Keywords: Ultra-short pulsed laser machining; multi-layer deposition; laser convocal microscopy; EDX analysis


Roll-to-roll laser processing of flexible devices
Christian Scholz, Maurice Clair

In the manufacturing of flexible thin-film devices precision, throughput, and machining quality on ever-smaller structures are playing an important role. The presentation will give a brief overview of different case studies where roll-to-roll laser processing achieve new dimensions in terms of precision, quality and process efficiency.An example application is the ablation of thin-film layers for medical sensors. In this case, the on-the-fly laser ablation takes place by using an excimer laser and mask projection. The layout of the products is adaptable by various projection masks. The high-repetition rate of the excimer laser allows the production of up to 150 sensors per second.Innovative laser micro processes paired with sophisticated machining concepts from 3D-Micromac fulfill exactly all requirements for such sensitive and advanced devices.

Keywords: roll-to-roll processing; laser micromachining; flexible thin-fim; solar cells; laser annealing; sensors


Advanced beam shaping for high power cutting and welding
Alexander D. Griffithsa, Eoin Murphya, Gilles Diedericha, Natalia Trela-McDonalda,*

Beam shaping of light from high power fiber lasers to improve cutting or welding performance can be challenging, due to (i) high power density; (ii) short length optical train; (iii) uniformity of the shaped beam over the tolerance range of the application. We present novel approaches to the design and manufacture of high efficiency beam shaping elements for multi-mode and single-mode fiber lasers. Beam structures that are emerging as important for industrial processing – such as rings – are considered. Importantly, these designs can be manufactured with sufficient low loss and low scatter by a laser-writing freeform optical manufacturing technique.

Keywords: beamshaping; freeform


Optimized laser cutting processes and system solutions for separation of ultra-thin glass for OLED lighting and display applications
René Liebers, Mandy Gebhardt, Dr. Bernd Keiper, Jacqueline Hauptmann

For some years now, laser cutting processes based on filament technology with ultrashort pulse (USP) lasers have been increasingly adopted in industrial applications. The main reasons for this are the good edge quality that can be achieved with simultaneous easy automation and free-form capability. This ability to be automated is of critical importance, especially for applications that target the mass market with their end products. However, the real advantage of the technology comes from its almost unlimited free-form capability. In addition to established manufacturing processes for glasses of medium thickness from 0.2-2 mm, an increasing number of applications with ultra-thin glasses of 30-100 μm are entering the market. These applications also require further development of the process and fab technology.
This presentation covers the possibilities of laser technology based on applications for OLED-based lighting and glass components in the display area.

Keywords: Ultrashort pulse Laser; glass cutting; OLED lighting; Display


Modeling of selective laser ablation of lithium-ion battery electrodes
Max-Jonathan Kleefoot, Simon Ruck, Jiří Martan, Jens Sandherr, Marius Bolsinger, Volker Knoblauch, Harald Riegel

Lithium-ion batteries are an important component of the current energy and mobility transition. Various approaches are being pursued in current research regarding the production of fast chargeable electrodes. These electrode layers consist of various components that – besides the current collector – can be divided into two groups. One is the active material phase and the other is the inactive binder material phase. In addition to laser perforation, selective laser ablation to remove inactive electrode components is also being investigated in research. Within this study, a simulation model was developed that predicts the spatial temperature distribution during the laser process inside the electrode. Experimental investigations were also able to show that the active material is permanently damaged by an excessively high energy input. A comparison with the model shows good agreement here. Thus, with the help of the model, a parameter optimization can be carried out in which the active material particles are exposed but not damaged. The thereby increased active surface area promises an enhancement of the fast charging capabilities of the electrode.

Keywords: selective laser ablation; binder ablation; microstructure; thermophysical modeling; lithium-ion battery


High-speed offline and real-time monitoring and control for laser micro-drilling of large Ti sheets
Roberto Ocaña, Joseba Esmoris, Carlos Soriano

High-throughput laser micro-drilling is a highly demanded technology for several applications, including making filters, creating surfaces with better aerodynamic performance, etc. However, it is usually found that the sensitivity of the laser process to small deviations is quite high. That is why, while parameterizing, it is convenient to have techniques that allow us to monitor and control the process to ensure reproducible results. For this, we have developed several methods that combine monitoring and control in real-time and offline. For real-time control and monitoring, we have used optical coherence tomography and captured the scattered laser radiation during the process by means of photodiodes. Regarding offline monitoring, a procedure using a high-speed camera and an algorithm for measuring the dimensions of the micro-holes provides us the quality characteristics and statistical information of complete micro-perforated Ti sheets. Both methods work as fast as the laser process, i.e., 300 holes per second.

Keywords: Laser-Microdrilling; Monitoring; Controlling;


Laser microdrilling of thin aluminum sheets for metal-composite adhesion promotion
Ares, Félix; Coto, Ivette; Delgado, Tamara; Gontad, Francisco; Mera, Laura; Romero, Pablo; Vidal, Sara; Massé, Pascal; Otero, Nerea

Thin (200 μm) aluminum sheets were drilled using a 1070 nm, CW fiber laser to improve hybrid metal-composite adhesion. The laser beam was guided by a BEO D35 laser cutting head. Micro holes of several diameters (40 – 220 μm) were generated with different spacing among them. The aluminum sheets were later coated with an adhesion promotion spray and thermoformed with a thin (200 μm) Carbon Fiber Reinforced Polymer (PA66) tape. InterLaminar Shear Strength (ILSS) and Single Lap Joint (SLJ) tests were performed on the following thermoformed samples: 1) Drilled, uncoated samples, 2) Non-drilled, coated samples, 3) Drilled, coated samples. The results show that samples that were neither coated nor drilled lack the necessary adhesion for the tests. Additionally, a significant adhesion improvement for the drilled, coated aluminum samples is accomplished, reaching up to 100% higher apparent interlaminar shear strength than plain, coated samples. Finally, the pattern that provided the best ILSS values was replicated with a ns pulsed fiber laser, resulting in an equally strong bonding, while increasing productivity tenfold.

Keywords: Micro drilling; Hybrid joining; Laser functionalization;


Automated cutting by water jet-guided laser using a break-through sensor
Falk Braunmüllera, Jérémie Diboine, Amédée Zryd, Bernold Richerzhagen

The Laser Micro-Jet® is now a well-established technology among others for micro-machining and high-quality machining of hard and composite materials, with the advantages of narrow parallel cut walls without focus adaptation, minimizing the heat-affected zone and the avoidance of burrs.
This contribution describes the development of a break-through sensor measuring light from the machining point through the light-guiding water jet. By detecting a completed cut, additional safety cutting passes can be reduced and the cutting is stopped just in time, thus minimizing processing time and maximizing feature completion rate.
The sensor enables an optimized, automated cutting as well as remote monitoring which represents a significant step towards industry 4.0. The technology is now employed on an industrial scale by several customers, showing the high potential of the technology: The processing time is reduced by 5-20 %, the success rate reaches at least 99.3 %.

Keywords: sensor; water jet guided; laser; WJGL; automation;


Cut edge quality in laser ablation of micrometer-scale grating structures
Meike Zilma, Tim Radel

During the production of grid structures heat accumulation occurs due to the limited surface area for heat dissipation via the ridge structures. This impairs the edge quality, affecting the ridge width and rectangularity. Due to thermal stresses and expansions, in combination with the low stiffness of the ridges, distortion or failure of these can occur. The aim of this study is to quantify the influence of the strategy for cutting out a single cutout with regard to the cut edge quality. For this purpose, 100 μm thick aluminum foils are processed by means of picosecond laser with different scanning strategies and subsequently examined for their ridge width. It is shown that both the global and local scanning strategy must be taken into account. Thereby, the standard deviation of the ridge width can mainly be reduced by a suitable global strategy. Imperfections at the ridges i.e. necking at the account points, local thickening and ridge deformation can be reduced or even prevented by the local scanning strategy.

Keywords: micro cutting; aluminum foils; microstructure


Expanding perspectives for processing with agile high power femtosecond lasers
E. Audouard, G. Bonamis, M. Delaigue, B. Tropheme, J. Pouysegur, F. Basin, J. Sanabria, E. Mottay, C. Hönninger

Femtosecond lasers are reaching the kW level. At the same time new applications are continuously emerging in various industrial sectors: health, production, energy, transport. Femtosecond laser micro processing is today a technology enabling new fields of use and new methods of production. The "agility" of lasers and associated beam engineering make them relevant tools for flexible, reconfigurable production. Temporal and spatial shaping, amplitude and phase control, precise temporal synchronization, and digital processing will see increasing use for applications and be key sources of innovation

Keywords: femtosecond laser, Pulses shaping, femtosecond processing


Plasma dynamics induced by single-pulse femtosecond laser ablation of dielectrics and metals
Bruno Gonzalez-Izquierdo, Haruyuki Sakurai, Ryohei Yamada, Kuniaki Konishi, Makoto Kuwata-Gonokami, Junji Yumoto

The generation of plasma is inherent to subtractive laser processing phenomena and often determines energy absorption and damage characteristics. However, few experimental studies have directly focused on quantitatively analyzing such laser-induced plasma dynamics. Here, shadowgraph and interferogram techniques were combined in a pump-probe configuration to investigate experimentally the temporal (from sub-ps to ns) and spatial dynamics of ablation plasma generated under single-pulse, ultrashort (90-400fs) laser ablation of sapphire and copper. From the measurements, we derived the electron-plasma density and plasma distribution for each time delay. The results clearly reveal time evolution differences between the materials. In sapphire, the plasma expands predominantly perpendicularly to the surface and attains higher electron-plasma densities. In copper, more shocked air-plasma is produced which, instead, expands uniformly. These results provide new insights into the underlying physics of laser-induced plasma dynamics which could facilitate, for instance, the optimization of recent high repetition rate configurations in laser processing.

Keywords: laser-ablation, plasma dynamics, single-pulse, dielectric, metal;


Towards in situ monitoring and feedback control of femtosecond laser-induced nanogratings formation in dielectrics
Olivier Bernard, Andrea Kraxner, Assim Boukhayma, Ata Golparvar, Yves Bellouard, Christian Enz

Tightly focused non-ablative femtosecond laser pulses induce a variety of structural modifications in the bulk of dielectrics. Among those, sub-wavelength nanogratings are particularly interesting as a means not only to locally enhance the material etching selectively (and thus, enabling bulk 3D-micro-fabrication), but also for encoding rich information in high-density permanent data storage media. Femtosecond laser-based processes are subject to perturbations, affecting the repeatability and accuracy of the results. To increase the performance of these processes, we explore a feedback method based on direct monitoring of the laser-affected zone (LAZ) using a probe beam. Specifically, we report on the use of weak signals resulting from the interaction of a femtosecond laser probe-beam with the nanogratings index-modulation as objective functions in feedback loop algorithms.

Keywords: femtosecond lasers, laser-induced modifications, dielectrics, feedback control


3D simulation of spatial and temporal modulation in laser beam fusion cutting
Ulrich Halm, Wolfgang Schulz

Mean profile height and perpendicularity are, along with adherence of dross, the main quality features in laser fusion cutting of sheet metals. Research indicates that the dynamics of the thin melt film and the beam shape have a strong effect on these quality features. Recent measures to reduce the mean profile height include spatial and temporal modulation of the laser beam. A 3D simulation of the melt film dynamics is used to analyze the effect of these measures on the mean profile height and the perpendicularity of the cut surface. Insight into the temperature distribution inside the solid material allows a deeper understanding of how spatial and temporal modulation of the laser beam act on the cut surface. Furthermore, the effect of artificial additional beam sources can be analyzed. The most positive effect on the cut surface was created by a homogenous illumination of the side of the cutting front.

Keywords: Laser Fusion Cutting; Melt Film Dynamics; Simulation


Formation of smooth and flat area for monocrystalline diamond by ns pulsed laser
Yasuhiro Okamoto, Tubasa Okubo, Atsuya Kajitani, Akira Okada

The combination of ns pulsed laser and acid cleaning can achieve a smooth and flat surface below Ra=0.2 μm for monocrystalline diamond, when laser fluence is controlled around the threshold of removal. Although Gaussian mode is used, shiny and flat surface can be obtained in parallel direction to top surface of workpiece. Therefore, formation method of smooth and flat surface was experimentally investigated by repeating linear grooving, when ns pulsed laser of top-hat mode (1060 nm) was employed. However, subsequent linear grooving to previous one made it difficult to create flat surface with a constant depth, and two-step irradiation method was proposed. Non-removal areas were kept between processed lines in the first step, and the remained area between processed lines of the first step was removed in the second step. The two-step irradiation method was effective to achieve a wide flat-area with smooth surface, and it could improve the controllability of groove depth.

Keywords: micro machining; monocrystalline diamond; smooth surface; flat area; ns pulsed laser


Laser machining of different steel grades with 10ps laser pulses: The influence of carbides onto the surface roughness and structures for different laser parameters
S. M. Remund, S. N. Herren, J. Zuercher, A. Kipka, B. Neuenschwander

When steel is machined with ultra-short pulses the specific removal rate strongly depends on the pulse fluence, the
wavelength and, in case of bursts, on the number of pulses whereas the steel grade has a minor influence. This situation
changes for the surface roughness. Beside the laser parameters, the initial surface, and the number of machined layers
the obtainable surface roughness also depends on the carbides located in the steel as well as their size and distribution
and therefore it is strongly influenced by the steel grade. E.g. for a given set of parameters a surface roughness Sa value of
0.18 μm, 0.25 μm and 0.39 μm was achieved for CK75 (no carbides), M390 (small carbides) and K100 (large carbides). We
present the results of a systematic study for different steel grades in the application of surface structuring and
smoothening of surfaces machined by alternative technologies as e.g. electrical discharge machining (EDM).

Keywords: ultra-short pulses; steel; surface roughness; influence of carbides;


Ultrashort pulsed laser micro processing: multi-pulse and beam forming strategies for high throughput at high average powers
Beat Neuenschwander, Stefan M. Remund, Markus Gafner, Michalina W. Chaja

The development of ultrashort pulsed laser systems actually goes far beyond the kW level. But e.g. for metals and single pulses todays standard methods like galvo scanners are not suited for higher average powers and alternative approaches have to be developed. We will get an insight into actual developments using multi-pulse strategies in temporal representation as pulse bursts and in spatial representation as multi-beams or with direct beam forming. A combination of these methods with synchronized scanning or real pulse on demand option could pave the way for using high average powers.

Keywords: Ultrashort pulse; micro processing