United States

We present a framework for developing laser weld schedules featuring circular oscillation in the beam trajectory designed to reduce keyhole induced porosity in aluminum welds.&amp;nbsp; Our approach focuses on minimizing the frequency of keyhole collapses during the welding process by stabilizing the keyhole with circular beam oscillation.&amp;nbsp; The approach is guided by the hypotheses that if the oscillation period is less than the characteristic time of keyhole collapse, the keyhole can remain stable.&amp;nbsp; The characteristic keyhole collapse time is governed by the melt density and surface tension, as well as the keyhole radius, which can be increased by utilizing a larger oscillation radius.&amp;nbsp; By eliminating keyhole collapse, the creation of vapor bubbles in the melt flow can be avoided, thus eliminating a major source for weld porosity.&amp;nbsp; &amp;nbsp;To verify these hypotheses, experimental results from laser welding of 3 mm thick aluminum alloy (AA 5754) sheets are presented. The test results confirm that porosity is substantially reduced when the oscillation amplitudes are increased beyond predicted threshold values for different oscillation frequencies.&amp;nbsp; These findings help to elucidate a new mechanism for improving keyhole stability in laser welding processes via laser beam oscillation and provide a framework for developing improved laser weld schedules.

ICALEO 2023

Mechanism-Guided Approach for Porosity Reduction in Laser Welding of Thick Aluminum Sheets

porosity

keyhole stabilization

laser welding

We present a framework for developing laser weld schedules featuring circular oscillation in the beam trajectory designed to reduce keyhole induced porosity in aluminum welds.&nbsp; Our approach focuses on minimizing the frequency of keyhole collapses during the welding process by stabilizing the keyhole with circular beam oscillation.&nbsp; The approach is guided by the hypotheses that if the oscillation period is less than the characteristic time of keyhole collapse, the keyhole can remain stable.&nbsp; The characteristic keyhole collapse time is governed by the melt density and surface tension, as well as the keyhole radius, which can be increased by utilizing a larger oscillation radius.&nbsp; By eliminating keyhole collapse, the creation of vapor bubbles in the melt flow can be avoided, thus eliminating a major source for weld porosity.&nbsp; &nbsp;To verify these hypotheses, experimental results from laser welding of 3 mm thick aluminum alloy (AA 5754) sheets are presented. The test results confirm that porosity is substantially reduced when the oscillation amplitudes are increased beyond predicted threshold values for different oscillation frequencies.&nbsp; These findings help to elucidate a new mechanism for improving keyhole stability in laser welding processes via laser beam oscillation and provide a framework for developing improved laser weld schedules.

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.

Silicon heterojunction (SHJ) solar cells take advantage of the passivation properties of amorphous silicon: it can be easily doped, deposition temperatures are very low (below 200◦C), and it has very good passivation properties, but it has the drawback of very high sheet resistance for heterojunction emitters. An alternative to the use of a-Si:H in SHJ cells are transition metal oxide (TMO) films. These films have interesting properties as passivators and selective contacts for novel semiconductor devices, i.e. hole transport layers (HTL) or electron transport layers (ETL), and can be deposited on both electrodes of a solar cell. In this work, we present a complete study of the laser ablation of three transition metals that show good properties as p-type front contacts for n-type crystalline silicon heterojunction solar cells: WOx, VOx and MoOx. We characterize the ablation process of films deposited on crystalline silicon using laser sources of three different wavelengths (355 nm, 532 nm, and 1064 nm) and in two temporal regimes(picoseconds and nanoseconds). We measure the fluence threshold to remove the transition metal oxide (TMO) film and to induce the damage in the crystalline silicon substrate, determining the parametric window of the process. We also studied the morphology to analyze the relationships between crater dimensions and ablation parameters using confocal microscopy. Complete removal of the three different TMO films was achieved without damaging the silicon substrate. Results of diode isolation and their electrical characteristics are presented, showing the quality of the laser isolation processes.

Selective Laser Ablation of Transition Metal Oxide Thin Films for SHJ Solar Cells

Thermal barrier coating (TBC) has been widely applied to jet engine parts as the protection layer for underneath metal working at high temperature. The ceramic based coating, however will suffer from harsh environment such as hot air and sands can lead to undesired coating spallation and crack failure. This paper introduced a new laser process to improve the TBC resistance to spallation and thermal strain tolerance. Ultrafast laser system&nbsp;&nbsp;has been used to slot micro-scale grooves on coating surface with optimized geometry and function property. Coating life extension is demonstrated with laser slotted coupon and parameter study. Pros and cons of laser slotting compared with other machining methods is analyzed as well.

Pulsed UV Laser-Assisted Doping of Semi-insulating 4H-SiC Substrates for MWIR Detector.

Aerospike engines are a promising type of rocket engine with the potential to significantly improve the efficiency of rocket propulsion systems. Unlike traditional bell-shaped engines, aerospikes have a cone-shaped external contour that allows for more efficient combustion of the propellant in a variety of environmental conditions. The design of aerospike engines presents a challenging task due to their complex internal and external geometries. However, the recent advancements in additive manufacturing (AM) have made it possible to create these complex shapes with greater accuracy and precision. This contribution addresses the complete process&nbsp;chain of an additively manufactured aerospike&nbsp;breadboard engine using kerosene and hydrogen&nbsp;peroxide as propellants. The aerospike was&nbsp;fabricated by laser powder bed fusion (LPBF) using the&nbsp;nickel-based superalloy INCONEL® 718. In order to&nbsp;qualify the material and process for this application,&nbsp;an extensive material characterization campaign&nbsp;including density and roughness measurements,&nbsp;tensile tests at room temperature, 700 °C and&nbsp;900 °C, was conducted. In addition, various&nbsp;geometric features such as triangles, ellipses and&nbsp;circular shapes were generated to determine the&nbsp;maximum unsupported overhang and geometric&nbsp;accuracy. The results were taken into account in the&nbsp;design maturation of the manifold and the cooling&nbsp;channels of the aerospike breadboard engine. Postprocessing&nbsp;included heat treatment, milling, turning&nbsp;and eroding of interfaces, thermal barrier coating (TBC) of&nbsp;thermally stressed surfaces and laser welding of&nbsp;spike and shroud as well as quality assurance.

Process Qualification, Additive Manufacturing, and Post Processing of a Hydrogen Peroxide / Kerosene Aerospike Breadboard Engine

The feedstock capture efficiency on powder laser directed energy deposition is becoming a big challenge on the industrial use of L-DED process for the manufacturing of large-scale AM parts. The powder capture efficiency is dependent of process optimization and the tool-path. The current literature presents a huge range of usual powder efficiency, between 3 – 32% and some specific cases exceeding 90%. The powder-gas jet stream interact with the laser beam adding material locally onto the substrate. Part of this material is captured by the melt pool. The not captured material is affected by the laser beam, causing a material degradation. There is a lack on the literature for studies on the reusability of powder on the L-DED process. This paper presents a comprehensive study on the consequence of laser interaction with SS 316L metal powder particles during the L-DED process using a range of different powder characterization techniques to access the powder morphology, particle size distribution, chemical composition, followability and particle porosity. The study was conducted within eight powder reuse cycles, without adding virgin material to the powder batch. Reduction of PSD range, increase of circularity, improvement on the powder flowability were identified as consequences of powder reuse. The consequence of the laser interaction with particles was further explored using SEM, presenting the continuous modification of the particles across the eight reuse cycles. The oxygen content of the particles was also measured to access the O2 pick-up as a consequence of particle heating.

Powder Degradation as a Consequence of Laser Interaction: A Study of SS 316L Powder Reuse on the Laser Directed Energy Deposition (L-DED) Process

Improving mechanical topology optimization (TO) results by substitution of biomimetic beams is one possibility to achieve designs of mechanical components that are highly sustainable and show good mechanical performance. Due to their geometric complexity such designs were found to be well suited for production by laser additive manufacturing. 
One obstacle of incorporating biomimetics beams in TO designs is the lack of detailed design methodologies. [1] proposed a corresponding design concept. Building on their concept this work presents a detailed methodology for abstraction of TO results to a design consisting of ball nodes and cylindrical beams. Using such an auxiliary design, the internal forces and moments of the beams can be evaluated to allow for the substitution of suitable biomimetic beams to generate biomimetic component designs in a next step. 
The work presents a skeletonization algorithm based on the potential field approach. Using the skeletonization and an additional analysis of the dimensions of the beams in the TO result, the algorithm develops an auxiliary design of the original TO result. 
The final algorithm was applied to three common TO results to obtain one auxiliary component design, each. The developed algorithm was found to generate abstractions that were well-suited for use in the methodology proposed in [1] as internal forces and moments in the abstracted beams could be evaluated with low effort. 
Therefore, the work contributes to a detailed design methodology for biomimetic mechanical components in the field of design for additive manufacturing (DfAM)

Development and Assessment of a Methodology for Abstraction of Topology Optimization Results to Enable the Substitution of Optimized Beams

The biggest challenges for laser-based powder bed fusion of metals (PBF-LB/M) are the slow production and the high part prices combined with process and component defects. Gaussian beams with fixed beam diameters and their characteristic high peak intensities are state-of-the-art in PBF-LB/M. However, Gaussian beams can cause spatter formation, keyholing, and resulting keyhole porosity due to evaporation of the melt. Laser beam shaping has been used in various publications for laser welding and PBF-LB/M to address the challenges mentioned above. Our research at the Professorship of Laser-based Additive Manufacturing focuses on developing beam-shaping technologies to improve the production speed and process quality in PBF-LB/M. The different beam shaping strategies we are evaluating are based on spatial light modulators (= change in wavefront due to local change in refractive index by tilting liquid crystals), acousto-optic deflectors (= ultra-fast deflection of the laser radiation by frequency change of acoustic sound waves), and a multi-core fiber concept (= change in relative intensity distribution via redistribution across fiber cross-section). The study will compare the performance of three different beam shaping technologies for PBF-LB/M and evaluate their design freedom and complexity. The aim is to use beam shaping to achieve a stable melt pool, increase production speed, and reduce resource and energy consumption.

Flexible and Highly Dynamic Beam Shaping Technologies for Additive Manufacturing

Despite the maturity of laser-based powder bed fusion of metals (PBF-LB/M), some barriers prevent the manufacturing process from fully being established in the industry. One drawback is spatter formation, which is disadvantageous to PBF-LB/M for three main reasons. First, adhering spatter can initiate coater collision, resulting in process failure. Second, large ad­hering spatter may cause lack-of-fusion defects as they require more energy to remelt sufficiently compared to unprocessed powder. Furthermore, big non-adhering spatter cannot be recycled as powder. The recycling of small spatter particles potentially results in degraded material properties. Ring-shaped beam profiles have been established for deep penetration welding to reduce spatter formation. Investigations on ring-shaped beam profiles in PBF-LB/M focus on improving productivity and process robustness. Quali­tative spatter reduction in PBF-LB/M using ring-shaped beam profiles has also been reported. This publication quantitatively examines the influence of ring-shaped beam profiles on spatter formation in PBF-LB/M. Image processing algorithms of on-axis high-speed images are utilized for spatter detection and tracking. A self-developed spatter segmentation is used to determine the spatter size. A Laplacian of Gaussian (LoG) filter is combined with a Kalman tracker to count and track the spatter. The results show that spatter formation is highly influenced by the beam profile and the chosen process parameters. Considering the melt track width, ring-shaped beam profiles could reduce the number of spatter per fused area. High numbers of spatter are generated when parameter sets result in balling. Moreover, spatter velocity is primarily dependent on the introduced dimensionless enthalpy.

Influence of Ring-Shaped Beam Profiles on Spatter Characteristics in Laser-Based Powder Bed Fusion of Metals

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.

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

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.

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Mechanism-Guided Approach for Porosity Reduction in Laser Welding of Thick Aluminum Sheets

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Selective Laser Ablation of Transition Metal Oxide Thin Films for SHJ Solar Cells

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