United States

Laser beam welding is a very efficient process for joining copper pins in electric drives, which are referred to as hairpins. The occurrence of pores reduces the conducting cross-sectional area, which increases the electric resistance and further impairs the tensile strength of the joint.&lt;br&gt;
The present work compares different welding strategies regarding the formation of pores and spatters during welding cu-hairpins. The welding strategy was varied by different geometries of the scan contour, different beam shapes and an adaptive line energy. Different beam shapes were realized using a two-in-one beam delivery fiber and using a civan laser. In order to capture the formation of pores, the processing zone was observed by means of X-ray imaging at the IFSW in Stuttgart and at Petra III at Desy in Hamburg. The formation of spatters was observed using a high-speed camera. With the observation it was possible to separately determine the influence of scan contour and beam shape on the formation of pores and spatters. Finally, these findings were applied to identify beneficial welding strategies to reduce the formation of pores and spatters. Furthermore, tensile testing proves the increase of the tensile strength of the hairpins, which were welded with the optimized welding strategies.

ICALEO 2023

High-Speed X-Ray Imaging of Pore and Spatter Formation During Welding of Copper Pins

hairpin winding

in-process x-ray

welding

copper

synchrotron

Laser beam welding is a very efficient process for joining copper pins in electric drives, which are referred to as hairpins. The occurrence of pores reduces the conducting cross-sectional area, which increases the electric resistance and further impairs the tensile strength of the joint. 
The present work compares different welding strategies regarding the formation of pores and spatters during welding cu-hairpins. The welding strategy was varied by different geometries of the scan contour, different beam shapes and an adaptive line energy. Different beam shapes were realized using a two-in-one beam delivery fiber and using a civan laser. In order to capture the formation of pores, the processing zone was observed by means of X-ray imaging at the IFSW in Stuttgart and at Petra III at Desy in Hamburg. The formation of spatters was observed using a high-speed camera. With the observation it was possible to separately determine the influence of scan contour and beam shape on the formation of pores and spatters. Finally, these findings were applied to identify beneficial welding strategies to reduce the formation of pores and spatters. Furthermore, tensile testing proves the increase of the tensile strength of the hairpins, which were welded with the optimized welding strategies.

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 **  
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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.

Pulsed laser material processing with high repetition frequencies is able to initiate heat accumulation effects that can decrease processing quality even for ultrashort laser pulses. In order to gain deeper insights into these effects, a fast temperature measurement system was developed using infrared detectors and dedicated optics. The system evolved through years when responding to different challenges, like homogeneous nanosecond laser melting of metals and semiconductors or laser marking of stainless steel for corrosion resistance conservation. Appropriate calibration methods were developed for temperature evaluation from measured signal to be used in different application fields. Evaluation algorithms were invented for analyses of thermal characteristics from the signal. Hardware implementation of the analyses using field programmable gate array (FPGA) was prepared for online evaluation of heat accumulation in nanosecond time resolution during the laser micro-processing. The system was used for example for real-time analysis of thermal processes in laser surface texturing by nanosecond laser and polygon scanner for increasing adhesion of thermal spray coatings, LIPSS formation by high power multibeam picosecond laser system or processing of lithium-ion battery electrodes for enhancing charging speed. The presented diagnostics system can be applied for analysis, monitoring and control of processes in a wide range of pulsed laser applications, e.g. laser surface texturing, micromachining, polishing, drilling or 3D printing.

Infrared online diagnostics for pulsed laser micro-processing

This presentation showcases various methods for producing laser-induced micro and nano structures with controllable spatial features, scale, and shape. These structures can be directly fabricated on dielectric surfaces and thin metallic films to enable efficient light manipulation, polarization control, and perfect absorption. By selecting the appropriate structure and processing conditions, we achieve optimal combinations of super-transmissivity, anti-glare, and extreme wetting properties for variable dielectric materials and optical fiber tip use cases. The nanostructured glass surfaces exhibit a high lased damage threshold, even more, that double in particular wavelengths. In addition, we demonstrate a method for producing polarizing plates exclusively via direct laser nano-structuring. Our grating-type planar metasurfaces, consisting of periodically aligned infinite ridges on a uniform transparent substrate, can be used as wire grid polarizers for far or mid-infrared electromagnetic frequencies. We also show that perfect narrow-band absorption can be achieved with gap plasmons polarizing metasurface plates operating in reflection at IR and mid-IR via laser structuring, specifically, laser-induced periodic surface structures (LIPSS) formed on sub-micrometer metallic films. The polarizing efficiency along with the optical responce of these structures was validated through numerical calculations and experimental results. Our study demonstrates that exploiting nanostructure formation on sub-micron metal films and dielectric materials is a promising approach for sophisticated light manipulation and may pave the way toward laser-induced meta-optics.

Advanced Laser-Induced Micro & Nano Surface Structuring for Photonic Applications

Antibacterial surfaces are a huge concern for many applications and uses. To solve this, chemical coatings with limited lifetime are often used to inhibit bacterial attachment, like E.Coli and S.Aurus, bacteria commonly found in hospitals.Hospital Acquired Infections (HAI) affect 1 patient out of 217 in Canada [1], cost up to 15 billion $US every year in the US and Europe, and caused 250,000 deaths in 2020[2]. Many works [3][4][5] highlighted the potential of laser textured surfaces to reduce bacterial adhesion. This passive approach consists in femtosecond laser-induced submicron structures such as Laser Induced Periodic Surface Structures (LIPSS), which can be smaller than the bacteria size. Through this novel approach, it has been demonstrated that bacterial attachment and thus biofilm formation can be significantly reduced. On the other hand, superhydrophobic surfaces could be mimicked by laser texturing of macrostructures, offering antibiofouling behavior to the treated surface. 
Many laser parameters could be tweaked to tailor structures sizes and morphologies, offering mixtures of structures for multifunctional surfaces. In this work, different strategies to obtain biofouling and antibacterial laser textured surfaces for medical applications shall be presented. 
 
[1]https://www.canada.ca/en/public-health/services/publications/science-research-data/healthcare-associated-infection-rates-canadian-hospitals-infographic.html 
[2] World Health Organization. Report on the Burden of Endemic Health Care-Associated Infection Worldwide. 2011 
[3] Peter, Alexander, et al. "Direct laser interference patterning of stainless steel by ultrashort pulses for antibacterial surfaces." (2020) 
[4] Romoli, Luca, et al. "Influence of ns laser texturing of AISI 316L surfaces for reducing bacterial adhesion." (2020) 
[5] Lutey, A.H.A., Gemini, L., Romoli, L. et al. Towards Laser-Textured Antibacterial Surfaces.(2018).

Structures Tailoring of Laser Textured Stainless Steel for Antibiofouling and Antibacterial Applications

The ability to deliver high energy and high average power laser radiation through a flexible fiber is a key enabler for numerous processing applications. This provides better space management, direct delivery to the processing area and better serviceability. Silica fibers were successfully used to deliver signals in the near-infrared and visible spectra, but they suffer notably from high absorption in the UV region. Hollow-core fibers alleviates this issue as the mode of propagation minimally overlaps with the surrounding guiding microstructure. However, their power scaling was currently limited by two factors: beam quality/stability of available UV sources and power handling of the thin capillary microstructure, especially due to the limited control of the internal surface roughness. 
In this work we will present recent results achieved by the combination of a high beam quality UV nanosecond lasers and low-internal roughness hollow-core fibers to achieve efficient UV fiber delivery. After giving experimental characterizations of the improved hollow-core fiber, we will detail the characterization of the 343nm UV source, including beam stability measurements at full power and evolution of beam quality on a large range of parameters. Fiber delivery with power above 20W (150 µJ energy) and transmission above 90% will be presented. We will also present preliminary results achieved at 266 nm with output power of 20.5mW (20.5 µJ) and transmission efficiency above 90%.

Fiber Delivery of UV Nanosecond Lasers using Hollow-Core Fibers

The railway industry has a high demand for lightweight structures in new trains. We present an overview of 20 years of progress in using laser-based manufacturing techniques to realize such structures, with the recent implementation of Additive Manufacturing (AM) enabling rapid advances. We show how AM-designs reduce car body weight by ~50% over conventional solutions. Moreover, the lifetime of rolling stock is scaling towards 50 years, creating a long-term demand for spare parts. AM offers a strategic solution, creating a digital warehouse, which produces parts only when needed, increasing resilience for the mobility industries.

Further advances in AM-techniques in lightweight manufacture are also being enabled by direct-diode laser technology. Kilowatts of optical power can now be efficiently generated in a compact module mounted on a lightweight process robot and delivered in a small spot onto the workpiece. New functions and operation modes are enabled, e.g. alternative wavelengths, with 780nm operation wavelength selected here, targeting the absorption maximum of aluminum. The 5-fold higher absorption over conventional 1060nm sources will shorten processing times, reduce energy consumption and minimize deformation.

Direct diode techniques are used to produce ultra-compact process heads that integrate laser source, simplified optics for imaging the optical output onto the work surface, and a wire-feed system. The resulting compact AM unit promises high flexibility and low cost and will enable complex shapes such as topology-optimized stiffening structures to be fabricated in limited accessibility environments typical for car body manufacturing. This will support the wide adoption of AM and raise sustainability.

Topology Optimized Lightweight Design through Additive Manufacturing: 780nm Laser Wire Deposition for Train Side Wall Structures

Hybrid Laser Arc Welding (HLAW) integrates the advantages of laser and arc welding, providing high process stability, high welding speed and penetration, narrow weld beads with a low heat input, low thermal distortion and good metallurgical properties. It is an outstanding joining process for medium and high thickness structural steels, being specially interesting for naval and offshore applications in which large panels must be 1G butt welded to manufacture larger flat panels. Although this technology has already provided appropriate results in terms of microstructure, weld morphology and static mechanical behaviour of HLAW butt joints, few studies have been yet reported regarding the fatigue behaviour. 
 
In the present research, HLAW process was applied to join 8 mm thick structural S355J2N steel under 1G configuration. Welding tests were performed at the Advance Laser Welding Center (CASOL) available at the University of Cadiz, Spain. Different experimental welding parameters were fitted to obtain sound butt welds. The welds were subjected to different quality control tests, including visual inspection, metallographic characterization, microhardness measurements, and tensile and fatigue tests. The HLAW tests were performed at higher welding rates in 1G configuration than previously reported for 8 mm thick steels. The present contribution reports novel fatigue results for these butt hybrid welds.

Laser Glass Deposition as an efficient Tool For the Production of Structural Components and Optical Systems

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.

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

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.

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Infrared online diagnostics for pulsed laser micro-processing