United States

In this contribution, we investigate the influence of ambient atmosphere on the formation of laser-induced periodic surface structures (LIPSS) on the metal surface using 8 picosecond and 45 nanosecond pulses. We patterned the surface of AISI 216 stainless steel in air, nitrogen, and argon and observed that the properties of the fabricated LIPSS were affected by changing the beam scanning velocity and pulse fluence while keeping the pulse duration constant. The LIPSS phenomenon was found to be strongly dependent on the atmosphere surrounding the irradiation spot during fabrication. When patterned with picosecond pulses, the dependence was less pronounced and manifested itself mainly in the curvature of the fabricated ribs. The spatial period of the fins patterned with picosecond pulses was smaller in air than in nitrogen and argon atmospheres. On the other hand, periodic structures were observed with nanosecond pulses only when laser processing was performed in air, suggesting that the structures were related to surface oxidation. Further analysis using EDS, XPS, AFM, and FIB cross sections revealed that LIPSS obtained with nanosecond pulses on samples consisted mainly of periodic parallel ridges of accumulated oxides, while in the case of picosecond pulses LIPSS consisted of topographically modulated bulk material with an oxide layer deposited uniformly over the entire surface of the sample. The results suggest that the formation of stainless steel LIPSS is significantly influenced by the surrounding atmosphere and the pulse duration used for laser processing.

ICALEO 2023

The Influence of Ambient Atmosphere on LIPSS Formation in Stainless Steel: A Comparison between Ps- And Ns-Pulse Processing

laser-induced periodic surface structures

laser surface engineering

lipss

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.

A fundamental understanding of ablation in different incidence angles is indispensable to expand the result to volume ablation where non-perpendicular irradiation exists. So far, no study with this orientation has been conducted in the category of volume laser machining. In this study, a nanosecond laser with different fluencies was utilized for single-point ablation experiments. The effect of incidence angles of 0°, 30° and, 60° on the ablation depth and the crater geometry was evaluated. Different laser pulse numbers are also considered. The results show that the ablation depth for the 0° and 30° angles is almost in the same range for the initial pulses, but afterward, the ablation depth for the incidence 30° drops considerably. As the number of incident pulses increases, the ablation depth first develops approximately linearly and then grows exponentially. By changing the incident from 0° to 60°, the affecting area changes as well. The affecting area could be categorized into two distinct areas: (1) ablation area (A.A) where the crater ablation depth rapidly increases for the first 20 pulses and then, as more incident pulses arrive, it does not grow anymore and reaches a plateau due to the increase in the ablation depth. The second area (2) is the heat-affected area (H.A.A) of the crater where no further ablation occurs, but due to heat accumulation, it becomes constantly bigger when more incident pulses strike the crater.

Tackling Micro-Processing Challenges with Beam Shaping

Femtosecond laser technology has opened a large variety of industrial applications. The short duration of the laser pulses enables material minimal collateral damages due to thermal effects, which makes possible to reach a high precision in micro-processiing. This is sometimes at the expense of industrial productivity, since a single femtosecond pulse will only remove a limited amount of matter. Increasing process throughput productivity can be achieved by increasing the&nbsp; repetition rate of the laser, but he onset of thermal accumulation in the material limits this approach. 
Femtosecond pulses with bursts of GHz repetition rate can significantly improve the ablation efficiency of femtosecond lasers. Freely adjustable bursts involve thermal and non-thermal ablation mechanisms. The long GHz bursts can remove very efficiently heated matter, with non-thermal ablation leading to a high-quality material processing. We report an experimental study on crystalline silicon line scribing with GHz burst regime of femtosecond pulses at 515 nm. The number of pulses per bursts varied from 50 ppb to 200 ppb. A second harmonic generation module was placed at the output of a 100 W femtosecond GHz laser to generate up to 50W of second harmonic (SHG,515nm) at 200 kHz. The results show the highest ablation efficiency obtained on silicon with fs pulses (more than 10 mm3/min/W), and the crucial role of the pulse overlap.

GHz Long Burst for Femtosecond Laser Processing Optimization

Micro-processing with ultra-short pulse lasers has become a vital tool in various industrial and scientific applications, such as surface texturing, surface treatment, metal drilling, glass drilling, and micro-welding. While these applications offer significant advantages over traditional processing methods, they also present unique challenges that must be overcome to take micro-processing to the next level. These challenges include improving process speed, enhancing process precision, and enabling easy integration of the laser into industrial applications. 
In this paper, we discuss how beam shaping can be used to tackle these challenges in various micro-processing applications. We describe how beam splitting, non-diffractive beams, top-hat shaping, and other original beam shapes such as U-shaped beams and triangular beams have been used to improve the process speed, precision, and integration of ultra-short pulse lasers into various micro-processing applications. We also discuss the challenges of fibering the laser for easy integration into industrial applications and describe how beam shaping can be used to overcome these challenges. 
In conclusion, this paper demonstrates how beam shaping can be used to tackle the unique challenges of micro-processing with ultra-short pulse lasers. With the appropriate beam shaping technology, we can significantly enhance the process speed, precision, and integration of ultra-short pulse lasers into various micro-processing applications.

Investigation of ns Single-Point Laser Ablation of Bronze under Different Incidence Angles and Pulses

Additive manufacturing processes have undergone significant development in regard to process management and reproducibility. Porosity, crack formation and similar defects can be reliably avoided through process control for a multitude of materials. This also applies to wire laser direct energy deposition (wire L-DED), which offers standardized feedstock material and direct controllable flow of mass. However, the process inherent microstructure remains fundamentally unaffected by parameter variations within typical process configurations. This microstructure is characterized by coarse columnar grains with a distinct fiber texture along the build direction. The underlying cause is the presence of a strong directional heat flux from the melt pool through the deposited material to the substrate. Consequently, as-built components demonstrate anisotropic mechanical properties with significantly lower tensile strength in build direction.&nbsp;One possible approach to inducing grain refinement and promoting the formation of an isotropic microstructure is the application of ultrasonic waves to the melt pool during solidification This lecture presents an approach to introducing ultrasonic waves to the wire L-DED process. An ultrasound system was developed, integrated in a laser wire deposition machine and a suitable process window has been established. A disruption of the highly directional solidification, resulting in a randomized texture as well as finer grains, has been confirmed by means of scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) (dm = 284,5 μm without ultrasound, dm = 130,4 μm with ultrasound).

Grain Size Manipulation by Wire Laser Direct Energy Deposition of 316L with Ultrasonic Assistance

Stainless steels are established in various fields with challenging environments, e.g. offshore, petrochemical, and automotive industries. The combination of high-performance properties and high-value added applications makes stainless steels attractive for Additive Manufacturing (AM). In powder-based AM processes such as Laser Directed Energy Deposition (DED-LB/M) typically pre-alloyed powders are used for part fabrication. By an innovative approach called in-situ alloying, the chemical composition of pre-alloyed powder can be adjusted by mixing it with an additional powder material. This allows the material properties to be flexibly and efficiently tailored for specific applications. In this work, a standard duplex stainless steel (DSS) is modified for the first time with elemental powders in order to systematically adjust the resulting phase formation, mechanical properties and corrosion resistance. For this, powder mixtures were generated consisting of pre-alloyed DSS 1.4462 and additions of pure chromium (+1.0 to +7.0 wt.-%) or nickel (+1.0 to +5.0 wt.-%) powder. Processing by means of DED-LB/M resulted in specimens (relative density &gt; 99.7 %) with ferrite-austenite phase ratios ranging from almost 10:90 % to 90:10 %. Increasing the Cr-content successively increased the ferrite percentage, resulting in higher material hardness, higher strength and resistance against pitting corrosion, but poor ductility and toughness compared to the unmodified DSS. In contrast, an increased Ni-content resulted in an increased austenite formation with lower hardness and strength, but increased ductility and toughness. This strategy was shown to add flexibility to powder-based AM processes by enabling an on-demand material design for stainless steels.

In-Situ Process Monitoring of Directed Energy Deposition Powder Flow to Detect Anomalies

Parts with complex geometry, elevated mechanical and corrosion resistance can be obtained by martensitic stainless steels (MSS) using additive manufacturing by laser directed energy deposition (AM L-DED) process. In this work, a proposed method was employed for choosing L-DED parameters based on design of experiments (DoE): Single beads, to define windows of powder feed rate, laser power, and travel speed; Single layers, to find the ideal hatch spacing for lateral bead overlapping; and Multilayer, to determine the ideal layer height for layer overlapping. Using the optimized parameters, AM L-DED samples were produced of AISI 410L MSS feedstock on AISI 410 MSS substrate. Microstructure, hardness, tensile strength, and impact toughness were evaluated in the as built and heat-treated conditions and compared to the AISI 410 substrate. Four heat-treatment routes were investigated: annealed (1050 °C for 90 min and water quenched), and annealed, water quenched and tempered at 300 °C, 450 °C, and 600 °C for 90 min, then air cooling. From results, as built samples showed the highest hardness, yield and ultimate tensile strength, but the lowest toughness. Ductility was progressively recovered with tempering temperature increase. Best compromise was seen in samples tempered at 600 °C that showed toughness improvement, keeping a tensile performance higher than AISI 410 substrate.

Influence of Post-processing Heat-Treatment on the Mechanical Performance of Aisi 410L Stainless Steel Manufactured by L-Ded Process

Batteries and other energy storage and energy transformation devices require a dedicated functionalisation of the subcomponents. From surface structuring to cutting and assembly of the different components, every single processing step should not influence the basic properties of the material and should even more increase the the characterics of the entire device. Ultrashort pulsed lasers provide technologies to optimize the electrical properties of the electrodes in batteries but also allow the modification of components in redox flow batteries and also in fuel cells. With micro and nano scaled structured electrodes based on ultra short ablation and modification the efficiency of batteries and electrolysers can be increased significantly. Especially in new types of batteries, like solid state batteries, short wavelengths in the UV provide the necessary power and physical properties to allow thin film modification an large scales. With ultra short pulsed lasers in the UV nano scaled modification of electrodes can be realized without influencing the electical properties of the electrodes without loosing active surface areas. With high power ultra short pulsed lasers and adapted beam guiding technologies large area surfaces can be processed. The presentation will show different approaches for the use of high power ultra short and UV lasers for the manufacturing of battery components.

Ultrashort Pulsed Lasers and New Laser Wavelengths for Improved Battery Manufacturing

Laser welding is a key enabling technology the transition towards electric mobility, enabling joints with elevated electrical and mechanical properties. In the production of battery packs, cell to busbar connections are challenging due to the strict tolerances and zero-fault policy. Hence, it is of great interest to investigate how beam shaping techniques may be exploited to enhance the electro-mechanical properties as well as improve material processability. Industrial laser systems often provide the possibility to oscillate dynamically the beam or redistribute the power in multi-core fibers. Although contemporary equipment enables elevated flexibility in terms of power redistribution, further studies are required to indicate the most adequate solution for the production of high performance batteries. 
Within the present investigation, both in-source beam shaping and beam oscillation have been exploited to perform 0.2-0.2 mm Ni-plated steel welds in lap joint configuration, representative of typical cell to busbar connections. An experimental campaign allowed to define process feasibility conditions where partial penetration welds could be achieved by means of in-source beam shaping. Hence, dynamic beam oscillation was explored to perform the connections. In the subset of feasible conditions, the mechanical strength was determined via tensile tests alongside electrical resistance measurements. In-situ high speed videography allowed to disclose the process dynamics. Linear welds with a Gaussian beam profile enabled joints with the highest productivity at constant electro-mechanical properties. Spatter formation due to keyhole instabilities could be avoided by redistributing the emission power via multi-core fibers whilst dynamic oscillation did not provide significant benefits.

Effect of In-Source Beam Shaping and Laser Beam Oscillation on the Electro-Mechanical Properties of Ni-Plated Steel Joints for E-Vehicle Battery Manufacturing

We discuss laser micromachining as a means of fabricating dielectric-based electrospray thrusters that use ionic liquid propellants for nanosatellite propulsion applications. Because of their small size, nanosats typically lack means of propulsion, which limits their lifetimes and mission scopes and is problematic for deorbit at end of life. Electrospray thrusters, however, can be very compact, using electric potentials to accelerate and eject ionic matter, thereby achieving high specific impulse. Traditional fabrication methods involve numerous steps, including lithography and/or chemical etching, resulting in microscale emitter structures that protrude from a substrate and can easily be damaged during assembly and/or launch. This format also requires a separate, detached grid electrode that serves as an extraction grid needed to induce ionic emission of propellant from the emitters. Our fabrication approach uses Bessel-beam laser micromachining to form embedded, micron-diameter capillaries up to several hundred microns long through a transparent glass substrate, without chemical etching. One surface of the glass is coated with a transparent conductive layer that serves as an integrated extraction electrode and which needs no alignment. Further laser machining with Gaussian-beam, water-assisted cavitation is used to excavate cylindrical electrospray cavities in this surface around each capillary, to catch errant propellant and prevent shorting while optimizing the capillary emitter-extractor separation distance. The resulting array of capillary emitters and cavities with integrated extractor comprises an electrospray emitter chip that forms the core of an electrospray thruster, and provides the potential to achieve significantly lower onset voltages than traditional electrospray thruster designs.

Laser Microfabrication for Ionic Propulsion Systems

Liquid immersion is used in laser machining to cool and remove debris from samples.&nbsp; The use of fluid media such as water, oils, or air for machining is well researched; however, the application of low-vapor-pressure fluids, such as ionic liquids, for laser micromachining has not been researched in depth.&nbsp; Ionic liquids are electrically neutral salts that are liquid at room temperature.&nbsp; These liquids tend to have low vapor pressure, which impacts a fluid’s ability to vaporize and cavitate during laser machining.&nbsp; Fluids with high vapor pressures, on the other hand, can be considered volatile and easily vaporize with minimal added energy.&nbsp; This paper discusses the effects of various immersion fluids on laser micromachining.&nbsp; &nbsp;Different immersion media were tested on a borosilicate glass cover slip using ultrafast (300 fs) laser pulses focused by a high numerical aperture (NA 1.2) immersion lens.&nbsp; For each immersion fluid, rectangular arrays of holes were machined with single, well-separated pulses, where the cover slip was translated laterally and stepped axially to change the focal position and bracketing the glass surface with positions both above (in the fluid) and below it (in the glass).&nbsp; The machined samples were then replicated with polymer imprint lithography, and both the substrate and replica were imaged using optical and scanning electron microscopies.&nbsp; The results suggest that fluids with lower vapor pressures, like ionic liquids, can expand the usable machining focal range, while also reducing the diameter of the holes and perhaps even mitigating collateral surface damage.

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The Influence of Ambient Atmosphere on LIPSS Formation in Stainless Steel: A Comparison between Ps- And Ns-Pulse Processing

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Tackling Micro-Processing Challenges with Beam Shaping