United States

The surface tension of metals governs essential mechanisms in thermal processing techniques, including laser materials processing. For computational fluid dynamics accurate surface tension data are essential. Because of certain reasons like limitations to lower temperatures for existing surface tension measurement methods, and too few measurements, far too few data are available. A new method variant is proposed, to extend surface tension measurement to higher temperatures, up to the boiling temperature and even beyond. A laser beam generates a falling metal drop. From high-speed imaging, the oscillation frequencies of the free-falling drops can be measured and the surface tension can be derived. Two different methods are presented to generate the drop, single pulse laser melting of a suspended metal piece, then falling, and drop detachment from a laser molten metal wire tip. The approach, which will be developed stepwise, has also the aim to generate and measure a desired drop temperature to provide surface tension data as a function of temperature for different metal alloys. First results with iron sheets or with stainless steel wire show that drops with suitable oscillations can be generated but that certain challenges have to be solved. Sufficient recording duration is needed to take into consideration a higher frequency resolution and in turn accurate calculation by considering drop rotation and the third oscillation dimension. Laboratories equipped with a high-power laser, a high-speed camera and a temperature measurement instrument, e.g. a pyrometer, will be capable to measure desired surface tension values in an efficient manner.

ICALEO 2023

Laser Generated Drops – Towards a Method Variant for Extended Surface Tension Measurement of Metals

metals

laser droplet generation

drop oscillation

surface tension

high-speed imaging

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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Whether this is the first time you are hearing about ICALEO or you are a longtime attendee, you will find something of interest, new ideas, and an excellent opportunity to network with other laser specialists worldwide. 

**Klaus Löffler **  
ICALEO General Chair   
Managing Director, Precitec GmbH

**What is ICALEO?**

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.

Participants will be immersed in a dynamic environment that fosters collaborative learning, networking, and idea exchange. Through engaging plenary presentations, technical sessions, an industry business panel, and networking events, attendees gain insights into the forefront of laser technology. From industrial applications to medical breakthroughs, additive manufacturing to materials processing, ICALEO covers the entire spectrum of laser-related disciplines. Moreover, the event serves as a launching pad for synergistic partnerships, paving the way for professionals to forge connections that shape the future of our industry.

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ICALEO is tailor-made for professionals, researchers, academics, and enthusiasts who are deeply passionate about the world of lasers and electro-optics. If you're an industry trailblazer, an academic luminary, an aspiring expert, or a postgraduate student looking to dive into the latest developments and trends, this is an event you won't want to miss.

Industrial leaders seeking to stay ahead of the curve in their fields, engineers working on laser applications, and manufacturing experts interested in additive manufacturing and materials processing will find ICALEO to be an invaluable platform. Researchers and scientists exploring the forefront of laser technology, from fundamental principles to groundbreaking applications, will also discover a wealth of knowledge and networking opportunities.

Medical professionals investigating laser-based medical advancements, entrepreneurs seeking to innovate with lasers, educators aiming to stay updated with the latest in laser education, and policy makers shaping the industry's landscape are all essential attendees. Whether you're an established expert or a rising star, the 42nd annual ICALEO promises to offer a diverse ecosystem of expertise, connections, and knowledge sharing that caters to the laser and electro-optics communities.

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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.

Skin-pass rolls are used for setting the final sheet thickness and surface texture. For sheet metal that is produced for forming, textured skin-pass rolls featuring a high-low-structure are used in order to improve the formability and paint adhesion of the sheet. In this paper, new textures for skin-pass rolls generated by high-speed laser melt injection (HSLMI) are presented and characterized. Furthermore, it is studied how the texture of the roll is transferred to steel and aluminum sheets. With HSLMI, metal-matrix composite (MMC) layers featuring spherical fused tungsten carbide (SFTC) particles with a high hardness could be produced on skin-pass rolls. For generating an increased high-low structure, laser ablation and electrolytic etching were carried out after HSLMI and grinding of the rolls. An analysis of the topography showed that different height differences between SFTC particles and matrix can be set. The textures generated by laser ablation showed a topography featuring two height levels, whereas a texture with spherically-shaped particles could be generated by electrolytic etching. Furthermore, it was found that all textures were transferred from the roll to both steel and aluminum sheets. The transfer of the textures mainly depended on the roughness and the SFTC particle content of the roll.

Texturing Skin-Pass Rolls by High-Speed Laser Melt Injection, Laser Ablation and Electrolytic Etching

The use of air bubbles injected into viscous fluid flows to reduce frictional drag has been extensively studied for industrial applications, especially in the marine industry. However, the lack of control over air bubble stability, size, and shape has limited its widespread adoption. This research explores the use of laser-based surface wettability modification techniques to enhance control over air bubble behavior in water flows. 
Using nanosecond laser and chemical immersion, metal plates were processed to create wettability patterns consisting of superhydrophobic and superhydrophilic regions. Water tunnel experiments were conducted to observe the behavior of air bubbles over different wettability patterns. The results show that surface wettability can be used to control the size and spatial distribution of air bubbles, potentially enhancing the energy cost-benefit of drag reduction methods in the marine industry. Small circular bubbles were retained over untreated and hydrophilic surfaces, while immediate bubble coalescence was observed over superhydrophobic surfaces. 
Computational fluid dynamics (CFD) modeling was also performed to verify and further understand the role of surface wettability. The results demonstrate the potential of laser-based surface wettability modification as a solution for improving the control of air bubble behavior in large-scale applications. This research provides insights into the feasibility of laser-based surface wettability modification as a novel approach for controlling air bubble behavior in water flows and improving the efficiency of drag reduction methods.

Enhancing Control of Air Bubbles in Water Flows through Laser-based Surface Wettability Patterning

With the ever-growing global demand for renewable energy sources, Cu2ZnSnS4 (CZTS) thin films can complement the existing Silicon-based photovoltaic (PV) cell market. However, low carrier mobility and lifetime, non-uniformity, and defects in the film structure have reduced the widespread application of CZTS. During the manufacture of CZTS solar cells, the initial as-deposited films have a nanocrystalline structure. Therefore post-deposition annealing is critical to make the efficiency of the cell commercially viable. Conventional furnace annealing methods have failed to enable this technology's commercialisation, and the heating of the complete PV cell limits the selection of available substrates. This research explored alternative approaches, specifically diode and pulsed fibre laser annealing for CZTS films deposited on flexible Mo foils in different gas atmospheres. SEM, XRD, and Raman spectroscopy techniques were used to analyse the laser-annealed samples. Processing parameters were established by which grain enlargement was observable with the pure CZTS phase retained. The key findings build confidence in laser annealing as an alternative method to enhance the crystallinity, surface morphology, chemical composition, and electrical properties of CZTS thin films for next-generation PV cells. Laser annealing can potentially reduce the reliance on vacuum furnace heating of PV cells and enable annealing on more thermally sensitive substrates.

Exploring Laser Annealing as an Alternative Approach to Improve CZTS Thin Film Quality for Photovoltaic Cells

Selective copper generation on polymers by reductive laser sintering represents an advantageous alternative to conventional mask-based approaches such as lithography or processes based on noble metals. A variety of applications in sensing, thermoelectronics and optoelectronics have already been demonstrated. For laser sintering, ultrashort-pulsed lasers enable precise control of the introduced energy and a reduced heat-affected zone. In turn, this type of laser can also be used to implement other processes such as ablation, modification, or the integration of embedded microfluidics and micro-optical systems. In previous studies, copper patterns with high spatial resolution have been achieved by using microscope objectives. However, design of microelectronic circuits also consists of broad conductive paths and contact areas, which result in time-consuming processing when hatching fine lines. The use of galvanometric scanners with long focal length lenses results in larger focal diameters that enable faster processing. However, this is also accompanied by higher laser powers, loading the heat-sensitive substrates and promoting reoxidation of the resulting copper structures, which can dramatically increase the achieved resistivities. 
Against this background, we report on the possibility to accelerate femtosecond reductive laser sintering of copper(II) oxide on the surface of transparent cyclic olefin copolymers by varying the focus size. By using different focusing conditions, different process speeds are achieved and the structural accuracies are controlled. The generated copper layers are investigated using scanning electron microscopy, energy-dispersive X-ray spectroscopy and 4-tip resistivity measurements to achieve high copper content at low resistivities in the resulting copper films at increased metallization rate.

Acceleration of Femtosecond Reductive Laser Sintering of Copper(II) Oxide for Conductive Copper Patterns on Cyclic Olefin Copolymers

Localized metallization of flexible and rigid dielectric materials is a powerful tool for surface engineering and modification. Metallic patterns are of great interest in various fields of science and technology, where they can find applications in the production of electronics, sensor devices, etc.&nbsp; Along with this, the vector of modern industrial microelectronics technologies is shifting annually towards the development and application of more environmentally friendly approaches. 
Deep eutectic solvents have been gaining popularity for the last 10 years as “green” solvents. Thus, the use of such systems seems extremely relevant, taking into account the replacement of traditional approaches with more environmentally friendly ones. 
We present a new approach based on deep eutectic solvents (DES)-assisted synthesis of conductive copper structures using regular technique of laser-induced liquid deposition of metals (LCLD). It was shown that it is possible to increase the deposition rate of copper by an order of magnitude by using deep eutectic solvents (DESs) as a medium for laser synthesis.&nbsp; As a results, the developed approach overcomes the previous limitations of the one-stage laser-assisted deposition technology, and opens the opportunities for new applications of this method, e.g. in micro-LEDs and displays technologies. We anticipate further development of this approach for fabrication of a wide range of conductive materials on various substrates with higher deposition rate.

Direct Laser Writing of Highly Conductive Copper Micropatterns from Deep Eutectic Solvents

Artificial intelligence is a trend that has been on innovation leader's radar for a long time. It is a technological enabler for a more sustainable future and will significantly increase business efficiency if used meaningfully. It affects our daily lives, and the way people and companies will work now and in the future. Insights into practical applications with various maturity degree will be presented from the perspective of a leading solution provider in the sheet metal industry. In times when there is a shortage of experienced operators, it is especially important to have a solution that enables machine operators to find the optimal machine settings and parameters to achieve the best performance and quality in shortest time. Artificial intelligence also supports the increase of the automatization degree of systems achieving a low-man production and ensures 24/7 production with assisting systems for monitoring the status of the laser cutting machines in operation and especially, during cutting. In addition to the monitoring, control procedures are presented to enhance the performance and quality demands of a laser cutting machine. A method for in-process quality control is presented prior to bending the sheet metals avoiding the processing of defect parts at an early stage.

Novel Multi-Material Lattice Structures Fabricated via Laser-Powder Bed Fusion Process

The capability to produce complexly and individually shaped metallic parts is one of the main advantages of the laser powder bed fusion (PBF‑LB/M) process. Development of material and machine specific process parameters is commonly based on results acquired from small cubic test coupons of about 10 mm edge length. Such cubes are usually used to conduct an optimization of process parameters to produce dense material. The parameters are then taken as the basis for the manufacturing of real part geometries. However, complex geometries go along with complex thermal histories during the manufacturing process which can significantly differ from thermal conditions prevalent during the production of simply shaped test coupons. This may lead to unexpected and unpredicted local inhomogeneities of the microstructure and defect distribution in the final part and it is a root cause of reservations against the use of additive manufacturing for the production of safety relevant parts. In this study, the influence of changing thermal conditions on the resulting melt pool depth of 316L stainless steel specimens is demonstrated. A variation of thermographically measured intrinsic preheating temperatures was triggered by an alteration of inter layer times and a variation of cross section areas of specimens for three distinct sets of process parameters. Correlations between the preheating temperature, the melt pool depth, and occurring defects were analyzed. The limited expressiveness of the results of small density cubes is revealed throughout the systematic investigation. Finally, a clear recommendation to consider thermal conditions in future process parameter optimizations is given.

On the Limitations of Small Cubes as Test Coupons for Process Parameter Optimization in Laser Powder Bed Fusion of Metals

Additive manufacturing of copper using laser powder bed fusion, enables the production of highly complex components, with excellent heat and electrical conductivity. However, the processing of copper by means of near-infrared laser radiation, which is commonly used, is challenging due to its high reflectivity. Nevertheless, it has been demonstrated that high densities and electrical conductivities can be achieved using high power laser systems. In order to process pure copper with reliable quality with different machines, it is essential to understand the conditions at which a continuous weld track is formed. For this purpose, weld tracks with varying laser power and scan speeds were welded on a copper substrate plate with an applied powder layer. The preheating temperature of the substrate plate and the beam size were varied to test different process conditions. The melt pool depths and widths were measured and a relationship was elaborated. Based on these results, cube samples with discrete weld tracks on top were manufactured. The melt pool depth was measured and compared with the predicted melt pool depth to investigate the transferability of the elaborated relationship from substrate to process conditions. It was found that with rising preheating temperature and for larger beam diameters at the same peak intensity, the weld width and weld depths increase. Furthermore, continuous weld tracks formed reliably in the keyhole welding regime. A good agreement between the weld depth of weld tracks on substrate and the elaborated relationship was revealed. However, the weld tracks were shallower than predicted.

Influence of Temperature and Beam Size on Weld Track Shape in Laser Powder Bed Fusion of Pure Copper Using Near-Infrared Laser System

The demands of the EV industry on the manufacturing processes are increasing rapidly in terms of productivity, strength, and electrical performance. Common laser tools are already reaching their performance limits in demanding fields of application, such as joining of copper and aluminum, or high-speed welding of thin metal foils. Innovative laser beam sources with new degrees of freedom in laser material processing are therefore needed to meet the increasing requirements. In this article, we examine selected laser applications in the EV industry, particularly in the areas of battery, e-motor and fuel cell production. In the case of electrical contacts, in addition to the requirements of high strength and low electrical resistance, there is also the difficulty that the welding depth may only be within a very narrow tolerance band, otherwise the parts would be destroyed if the component was shot through. In addition to the precise welding depth, the heat input is also a major factor in determining whether the component remains intact during the joining process. In this contribution results of fundamental process investigations when using two superimposed laser beams of high brilliance of the latest generation are presented. It is shown that both productivity and process stability can benefit from this new type of laser tool.

Latest Application Developments Across EV-Industries with Overlaying Highly Brilliant Laser Beams of the Next Generation

In order to achieve sustainable development goals, SDGs, decarbonization and low-carbonization are required. Electric and hybrid vehicles are indispensable for the conservation of natural environment, and the lightweight construction and the effective transfer of electricity become important. Thus, copper and aluminum have been becoming important materials because of their excellent electrical properties. However, in the welding of these materials, it is difficult to obtain strong joints, since there are problems in the brittle intermetallic compounds and the welding defects due to different melting points between copper and aluminum. Especially in joining of copper and aluminum by copper side irradiation, aluminum rich intermetallic compounds (IMC) shows brittle state, and mechanical strength decreases drastically. Therefore, mild heat input from copper to aluminum would be necessary to reduce the brittle IMC. Angled irradiation might result in the mild energy input to aluminum, because it can be expected that aluminum would be heated by the reflected light inside the keyhole generated in copper according to high light reflection of copper. In addition, stable welding can be expected by the superposed irradiation of blue and near-infrared lasers because of high light absorption rate of blue laser to copper. The angled and the superposed irradiation could achieve stable welding state, and the generation of aluminum rich IMC became smaller. Angled irradiation of near-infrared laser showed equivalent joining strength to the superposed irradiation of two wavelengths, and the combination of angled and superposed irradiation achieved the remarkable increase of joining strength in cross tensile test by 80%.

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Laser Generated Drops – Towards a Method Variant for Extended Surface Tension Measurement of Metals

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Texturing Skin-Pass Rolls by High-Speed Laser Melt Injection, Laser Ablation and Electrolytic Etching

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