United States

An accepted approach to computing laser-induced peak surface temperature is to employ the enthalpy formulation of the transient heat conduction equation [Grigoropulos et al, 1995; Iyer et al, 2017]. This approach is generally implemented using an explicit numerical scheme to solve the thermal transport equation. While it offers the advantage of modeling the solid-melt phase transition automatically, the approach results in instability-like behavior in the computed surface temperature. When laser-induced ablation becomes significant, the heating rate in the surface cell becomes unrealistically large. This results in spikes in the computed peak surface temperature due to large errors in calculating the heating rate. In this paper, we present a new approach, which we refer to as the Moving Frame Solver, that employs a moving coordinate frame of reference, located at the receding evaporating surface. We also use an analytical representation for the phase transition region of the enthalpy-temperature relationship. The Moving Frame Solver combined with an implicit scheme leads to a stable solution without surface temperature, pressure or velocity spikes. In other words, any instability in these computed parameters due to use of an explicit scheme has been eliminated. Details of the new thermal solver and example calculations are presented. Numerical experiments suggest that the surface cell size needs to be small, ~ 0.1 mm, to obtain a highly accurate solution with a typical metal such as aluminum. Using the Moving Frame Solver with a refined grid near the surface, but coarse elsewhere, enables accurate and stable surface temperature computation.

ICALEO 2023

A New Thermal Solver for Mitigating Surface Temperature Instability Induced by Laser-Induced Heating

moving frame of reference

surface temperature instability

pulsed laser

ablation

poster

**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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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.

The global mobility and regenerative power revolution is in full swing. Demand for components for electric cars, alternative drives and energy storage is rising continuously. 
At the heart of many solutions is the laser. Especially high power and mid power nanosecond Laser for surface treatment in many different applications. 
Another field to enhance power efficiency is to avoid electrical losses. One main driver of losses in EV’s or high currant cabinets are screwed busbar connections. State of the art solutions to prevent contact resistances losses are expensive environmental critical coating processes with deformable high conductive metals like nickel. Several papers have shown that mechanical pretreatment of the busbar surface in the interaction zone of screwed joining can lead to a significant drop down of the contact resistance. In our investigation we show that Laser surface treatment is an ideal tool to transfer these results in a highly flexible and productive way. 
We will demonstrate with an industrial relevant case study the potential to significantly decrease the contact resistance on screwed connections of copper busbars by means of laser surface treatment. 
Therefore, 3 different Laser sources, different treatment strategies were performed. All busbars were screwed and electrically qualified by a calibrated µ-Ohmmeter. The investigation leads to the conclusion, that a tailored laser surface treatment combines highest productivity, lowest contact resistance without any additional coating steps, a long shelf lifetime and highest reproducibility.

Optimizied Contact Resistance at Bolted Copper Busbars by Means of Ns-Laser Surface Treatment

Tungsten has been used in many industrial fields such as automobile industry, medical industry and so on, because of having its high creep strength and high corrosion resistance. Recently, the W is expected to apply for divertor surface of nuclear fusion reactor to prevent of wear and tear from charged and neutral particles form plasma. In case of thermal shielding properties such as divertors, it is important to smooth the surface, but the tungsten is difficult to process because of its high hardness and susceptibility to cracking and chipping. Therefore, we focused on laser processing. Pulsed laser can produce high power density over 1015 W/cm2 as ablation process, while the average power relatively low. On the other hand, continuous wave (CW) laser produces continuous power, an output power became high over 10 kW.&nbsp; Although the difference between CW and pulsed lasers is obvious in processing tungsten, it is necessary to gain a better understanding of which processing is more suitable for surface processing. 
　In this study, we tried to smooth the tungsten plate surface by irradiating it with a high-power CW laser and a femtosecond laser which is an ultrashort pulsed laser and investigated the differences in the processing characteristics of tungsten.

Comparison of Surface Integrity for Tungsten Plate with CW and Ultra Short Pulse Laser

In this paper, the necessity and theoretical analysis of the flat-top nanosecond green laser annealing process are overviewed for semiconductor Si wafers. The previous technology for rapid thermal annealing on silicon wafers was based on flash ramp annealing (FLA) with uniform illumination, which involves volume heating for the entire Si wafer and uses 3–10 msec heating time. Due to the continuous technological development toward advanced semiconductor chips with narrower linewidth and pitch, top-surface-only heating demand with a very short period has been increasing to minimize the diffusion and get clear and good pattern lines. It made the development of nanosecond 532nm laser annealing technology possible, but there is a lack of information on the required energy, energy density, and relevant technological analysis. After considering the Si absorption coefficient of 7850/cm at 532nm, the specific heat capacity, the heating area of 10mm x 10mm, the heating depth of 5um, the heating volume, the heating mass, and the 4% initial reflectivity, the required pulse energy using a 1J/10nsec/532nm pulse green laser was analyzed to get various heating temperatures. Also, considering 600 ea 10mm by 10mm chips on a 300mm Si wafer, a possible ideal maximum annealing speed was calculated, which resulted in an annealing speed of one minute per wafer. Regarding green laser annealing, the technology necessity, the market trend, the power budget simulation for volume production, and the comparison to FLA are discussed.

Analysis of Flat-Top Nanosecond Green Laser Annealing Process for Semiconductor Si Wafers

Experiments were carried out to investigate the surface morphology and ablation threshold of titanium in air and in water irradiated with Nd:YAG laser. In the experiment, a Nd:YAG laser (wavelength of 532 nm, a pulse width of 10 ns, and a repetition rate of 10 Hz) was focused to a diameter of 33 μm (FWe-1M) by a convex lens with a focal length of 200 mm, and was irradiated to titanium surface at a incident angle of 0 degree in the atmosphere or water. Titanium surface was mechanically polished and to be obtain an optical grade.Laser energy was controlled by a&nbsp;polarizing prism and a half wave plate in the range of&nbsp; 1μJ to 40μJ (corresponding laser fluence&nbsp;of 0.11J/cm2 to 4.61J/cm2). The melting threshold was estimated from the crater diameter depended on laser fluence.The melting threshold was 0.30 J/cm2&nbsp;in air. On the other hand, the melting threshold was 0.43J/cm2&nbsp; in water. The higher melting threshold in water might be suggested due to the cooling effect of water. The cooling effect in water was also confirmed in the melting threshold calculation using the one-dimensional thermal diffusion model. In addition, as comparing the surface morphology of the crater produced by laser in air and in water, it was confirmed by observation with FE-SEM that the formed shapes were different.

Surface Morphology and Ablation Threshold of Titanium in Air and Water Irradiated by Nd:YAG Laser

YAG laser (wavelength of 532 nm and a pulse width of 10 ns) was irradiated to SUS430 surface to produce a periodic structure with an interspace of approximately laser wavelength. 
The material is used mirror-polished SUS430 (25 mm x 25 mm, thickness 1 mm). 
For the periodic structure formation experiment, the melting threshold fluence (FMelt) of SUS430 was evaluated from the laser fluence dependence of the crater diameter, and it was found to be FMelt = 1.18 J/cm2. 
A periodic structure was produced at a laser fluence F=0.8 J/cm2 below the melting threshold and near the ablation threshold (Fth=0.96 J/cm2). 
Escherichia coli DH5α with a size of about 500 nm to 1000 nm was used for film adhesion method as an antibacterial method. The periodic structure given to the material surface was about 500 nm, which was smaller than that of Escherichia coli DH5α. 
In the film adhesion method, first, the cultured bacterial solution was dropped on the surface, covered with a film, and cultured for 24 hours. 
After that, the bacterial solution was diluted and cultured again in a solid medium for 24 hours, and the antibacterial function was examined from the number of colonies formed. 
As a result, it was found that the number of colonies of SUS430 with microstructure was 198 compared to the number of colonies of 1244 of the bacterial solution, which is about an order of magnitude decrease.

Antibacterial Effect of Periodic Structure Formed on SUS430 by Using Nanosecond Pulsed Laser

The ISO 11146-1 is a well know standard for measuring the M-Square value of a laser or laser system. The multi-spot method of measuring the M-Square against the ISO 11146-1 standard has not, to date, been measured one to one against this standard using both the traditional scan method and the multi-spot method. This work validates the multi-spot method against the standard by using the identical optical setup and simply changing the software algorithm to accommodate both methods while staying within the ISO 11146-1 parameters.&nbsp; We then compare the results for the same laser within a very short span of time. This work covers the results of three (3) different ISO compliant measurement methods: full caustic multi-spot, scan method of one spot at a time and a smaller number of multi-spots within 3 different Rayleigh ranges of the beam caustic. The multi-spot method has the significant advantage of providing an ISO compliant measurement within a single laser pulse whether it be a femtosecond, picosecond, nanosecond, or millisecond pulse. The traditional scan method can only provide a time averaged measurement and is not possible in making a single pulse M2 measurement.&nbsp; The multi-spot, therefore, provides instant feedback of the laser performance and reduces the optimization time.

Validation of a Multi-spot M2 Measurement to the ISO 11146-1

Solidification cracking is one of the most critical weld defects in laser welding of Al 6000 alloys. It occurs at the end stage of solidification due to the shrinkage of weld metal and deteriorates the joint strength and integrity. Filler metal can control the chemical composition of weld metal, which mitigates the solidification cracking; however, the chemical composition is difficult to control in autogenous laser welding. Temporal/spatial laser beam modulations have been introduced to control the solidification cracking in autogenous laser welding because the welds morphology is one of the factors that influence the solidification cracking initiation and propagation. Solidification cracks generate thermal discontinuity and visual flaws on the bead surface. In this study, a high-speed infrared (IR) camera and a coaxial charge-coupled device (CCD) camera with an auxiliary illumination laser (808 nm) were employed to identify solidification cracking during laser welding. Deep learning models were developed using two sensor images of the solidified bead. The output of deep learning models were location-wise cracking formation. Convolution neural network (CNN)-based deep learning models showed excellent cracking identification accuracies. In this study, the multiple sensor system of high-speed IR camera and low-cost CCD camera demonstrated its capability and potential in laser welding inspection.

Identification of Solidification Cracking Using Multiple Sensors and Deep Learning in Laser Overlap Welded Al 6000 Alloy

Global initiatives and legal frameworks to achieve net-zero carbon emissions will require the development of manufacturing processes significantly different from current commercial methods. The efficiency and fine control of laser-material interactions have proven themselves as disruptive manufacturing technologies that are ideally suited to replace conventional thermal processes. Conventional textile dyeing is known to be resource-intensive, relying on slow-acting thermal dye-fixation processes. These are inherently inefficient as they indirectly heat the material's surface through steam chambers or boiling vats to chemically fixate the dye onto the fabric, resulting in large lag system responses. The typical system response time for these manufacturing processes is minutes, which equates to a work-in-progress of hundreds of linear metres of product. This paper presents a method of directly heating the textile surface with a Continuous Wave CO2 laser in combination with existing commercially available Polyamide textiles and Nylosan Acid Dye. Results from this new process show that the essential industrial quality control standards for colour fastness and mechanical properties are maintained. The key finding discussed in this paper is that using a laser source to provide a direct and rapid heating mechanism at the site of dye fixation enables the implementation of a fast response system. The heating control of the laser is effectively instantaneous, at &lt;100 µs, resulting in control responses that affect less than one linear metre of material. The results presented here represent a product family with annual manufacturing volumes of 1.5 billion meters of textile, creating potential for significant improvements to manufacturing.

Rapid Response Textile Dyeing: A Laser-enabled Manufacturing Process

Additive manufacturing is an enticing way for producing complex geometries and optimized parts for special applications. Even though the achievable static properties for the printed material are usually good when compared to wrought materials, in many cases dynamic properties are known to be much worse. Often the quality is sacrificed in respect of printing speed. Further, printed materials have usually anisotropic behavior, caused by the remelting and fast cooling of each deposited layer. This means that the mechanical properties are needed to be measured in several directions in respect of the printing direction for attaining a more holistic approach on the achieved static and dynamic behavior. As a demonstration, this study focuses on determining the properties of 316L stainless steel manufactured with laser powder bed fusion. Comprehensive set of samples for various testing methods were manufactured to investigate the effect of layer thickness and printing orientation on the microstructure, mechanical properties, impact strength and fatigue life. Fatigue performance of the material was evaluated both in axial and flexural bending comparing as built and polished surface conditions. Bending fatigue testing revealed that a fatigue limit of 100 MPa at best can be achieved with the as built surface quality, but with a polished surface and lower layer thickness it could be doubled. Impact toughness and mechanical strength of the material are heavily dependent on the layer thickness and while the best results were obtained with the lower layer thickness the printing orientation can have a detrimental effect on it.

Fatigue Strength and Impact Toughness Dependence of PBF-LB Manufactured 316L Stainless Steel on Orientation and Layer Thickness

Laser Powder Bed Fusion (LPBF) is a critical additive manufacturing process for producing complex parts with high accuracy and precision. However, LPBF faces significant challenges in increasing the process yield and printing new powders while reducing hot cracking. These challenges limit the production speed, reduce the manufacturing efficiency, and restrict the range of materials that can be used in LPBF. 
Beam shaping offers a potential solution to overcome these challenges by modifying the laser beam's intensity distribution. In this paper, we explore the application of Multi-Plane Light Conversion (MPLC) to LPBF to improve the process speed and quality. MPLC is a novel beam-shaping technique that enables the creation of complex intensity profiles by redirecting the laser beam through multiple planes of phase modulators. 
Our study demonstrates the successful application of MPLC to LPBF, enabling a significant increase in the process speed while maintaining high-quality parts. We use an appropriate ring shape for printing the inner part and a small Gaussian beam for the contour of the part. We compare the performance with no beam shaping as well as with beam shaping based on shaping within the fiber of the laser technology. Our results show that MPLC-based beam shaping improves the process yield, leading to faster printing and improved manufacturing efficiency.

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A New Thermal Solver for Mitigating Surface Temperature Instability Induced by Laser-Induced Heating

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Optimizied Contact Resistance at Bolted Copper Busbars by Means of Ns-Laser Surface Treatment