Ultrafast laser surface structuring presents a compelling avenue for finely tuning material surface properties, such as coloration, wettability, tribological properties, etc. While nanoripples, the conventional laser-induced periodic surface structures, offer control over surface coloration, their effectiveness is limited to a narrow viewing angle. Addressing this limitation requires the development of techniques for generating alternative surface structures like nanobumps. Current methods for generating nanobumps, such as the double-pulse technique, are characterized by their complexity and high costs, posing significant challenges to widespread implementation. In this investigation, we introduce a novel approach for creating periodic nanobump structures through a straightforward double-scan method. This method involves subjecting the same surface to two consecutive laser scans with differing polarization directions.

Our study delves into the formation dynamics of these intriguing nanostructures, exploring the influence of laser parameters such as peak fluence, overlapping ratio, and polarization angle between scans. We propose and validate a formation mechanism to elucidate the observed phenomena. The resultant nanobump structures exhibit superior optical characteristics compared to nanoripples, particularly in terms of enhanced visibility across multiple viewing angles. By broadening the scope of achievable surface structures, our findings significantly expand the potential applications of ultrafast laser surface structuring. This research not only advances our understanding of laser-material interactions but also paves the way for the development of innovative surface engineering strategies with implications across diverse fields.

A Double-Scan Technique for Femtosecond Laser-Induced Nanobumps

The laser peening is a surface treatment technology that improves the mechanical performance of metals. For instance, it has been utilized to increase the resistance of a metal to crack initiation, extend the fatigue life, and enhance the fatigue strength. These effects are imparted by shockwave generated by the expansion of laser-induced ablation plasma. Utilizing double-pulse laser ablation is one of the methods to improve a coupling of the laser beam with the plasma plume and target material. With double-pulse laser ablation, a target material is irradiated by two pulsed laser beams separated by a nanosecond inter-pulse delay time. It is reported that this irradiation method allows for a more effective energy delivery to the plasma plume and target material. In this study, double-pulse laser ablation has been adopted for laser peening. The experiments were performed by two Nd: YAG Q-switched pulsed lasers (wavelength: 532 nm) in water ambient. Austenitic stainless steel SUS316L was used as the target sample. The pulse durations of the first pulse and second pulse were 6 and 4 ns, respectively. Varying the inter-pulse delay time, the surface hardness on the sample after laser peening was evaluated. The results indicated that the surface hardness depends on the inter-pulse delay time. It is found that optimizing the inter-pulse delay time for a double-pulse laser enables enhancement of the laser peening effect.

Double-Pulse Laser Peening as a Surface Enhancement Technology

The electrification of the mobility sector is vital to meet the targets for reducing greenhouse gas emissions. Besides battery-based mobility solutions, polymer electrolyte membrane fuel cells (PEMFCs) are a promising technology for electrifying drive trains, especially in heavy-duty applications, such as maritime or logistics. Bipolar plates, a key component of PEMFCs, can consist of two stainless-steel foils that must be welded to be gas-tight. In order to join the two metal foils, laser beam welding is the state-of-the-art technology. Current challenges include process instabilities at higher welding speeds, such as the humping effect, which can cause weld seam imperfections. Therefore, applying sensors for laser beam welding is a promising approach to monitor the welding process. AISI 316L foils were welded within the scope of this work with various process parameters using an adjustable ring mode laser beam source. Additionally, an optical microphone was used as a process monitoring system. By applying different parameter settings and due to the introduction of artificial faults, weld seam defects, such as a burn-through or a gap, were induced. After utilizing a noise reduction algorithm for the acoustic signals, numerous features in the time and frequency domain were extracted, with which multiple machine learning algorithms were trained and compared concerning their performance. A light gradient boosting machine was identified as a suitable machine learning model for the weld seam classification. Finally, a hyperparameter tuning was conducted, which resulted in a cross-validation accuracy of 94.78 %, depending on the quality categories considered.

Acoustic Process Monitoring During the Laser Beam Welding of Stainless-Steel Foils Using an Adjustable Ring Mode Laser Beam Source

Chromatic dispersion is a wavelength dependence of a group velocity of an optical fiber, and is an important fiber characteristic in fabricating pulse fiber lasers because it affects spectrum and pulse width. One of the chromatic dispersion measurement methods is a phase shift technique, which measures a relative phase shift between intensity-modulated lasers with different wavelengths. A disadvantage of this measurement method is that fiber lengths of several hundred meters or more are required to obtain high time resolution. Therefore, specialty fiber with short production lengths and amplification fiber that absorbs light of a specific wavelength, used for pulse fiber lasers, is difficult to measure with the method.

This study aims to develop a chromatic dispersion measurement method that can evaluate specialty fiber and amplification fiber using a short fiber under test of a few meters. The measurement light source used is a broadband light of high brightness and fiber optics that we developed. The feature of this study is that the fiber under test constitutes a Fabry-Perot resonator using the Fresnel reflections at both endfaces of the fiber. Enables measurement with the short fiber under test because it is measured from a standing wave of the resonator. The wavelength dependence of reflection characteristics from the standing wave is measured to obtain a relative group delay. Chromatic dispersion can be obtained by measuring the group delay difference at different wavelengths. Also, the fiber under test is the resonator, therefore measurement can be done without connectors and end face polishing.

Development of Wavelength Dispersion Measurement Method Using Fresnel Reflection of Fiber Under Test at Both End Faces

Laser welding is widely used in the automotive industry to join various materials due to its advantages of minimal thermal deformation and high productivity, applied in different joint

configurations, such as T-fillet and single-lap. The tensile properties of these joints are crucial for the stability of vehicle bodies, prompting research into both experimental and analytical evaluations. In particular, to experimentally evaluate the tensile properties of single-lap joints, a load cell-based force and crosshead displacement are generally used. However, the reliability of crosshead displacement data is affected by machine compliance and the specimen's dimensions, resulting in low reliability of displacement-related tensile properties such as structural stiffness. This can lead to discrepancies between experimental results and numerical analysis, which are critical for evaluating vehicle stability. Therefore, this study introduces a novel numerical modeling method for predicting structural stiffness using crosshead displacement during tensile testing of single-lap joints. Additionally, it presents modeling techniques to enhance the computational efficiency of the numerical model. Laser welding was performed on two types of materials, SPFC 590 and A6061, to make single-lap joint specimens. To evaluate the structural stiffness of these specimens, lap-shear tensile test was conducted. A three-dimensional numerical model based on ABAQUS software was developed, and a modeling method involving the application of an elastic modulus in the numerical model was proposed to enhance the correlation with the structural stiffness derived from lap-shear tensile test. Furthermore, modeling techniques such as optimizing mesh size and element type were suggested to enhance the computational efficiency.

A study on numerical modeling method for predicting structural stiffness using crosshead displacement in laser welded joints

In this paper, the necessity and theoretical analysis for Nanosecond-Scale Temperature Sensing Technology during Flat-Top Nanosecond Green Laser Annealing Process of Semiconductor Si-Wafer is overviewed. When a 532nm laser pulse with 10 ns or less pulse duration and several 100s mJ pulse energy is irradiated on the Si-wafer chip surface, the surface and internal temperature inside the chip is increased to several 100s or thousands quickly and cooled down during the next several 100s nanoseconds. The fastest commercial temperature sensor based on IR energy sensing has 40 us response time at which time the cooling process was already finished. So, the annealing process using 10 ns pulse laser annealing needs a proper method of ultrafast temperature measurement technology to understand the relationship between heated temperature and annealing quality which can make an impact on semiconductor chip performance. To resolve this issue, two-wavelength temperature measurement technology using fiber-guided sensing is selected among several technologies which is analyzed to know re-emission energy distribution from the Si-Chip and the captured energy into sensing fiber. Then, a proper focusing lens or lens set with optimum entrance pupil size to enhance the captured energy is determined. Then, a prototype setup for this two-tone temperature measurement is prepared and its performance is tried to compare with the results using a fast commercial temperature sensor.

Analysis of Nanosecond-Scale Temperature Sensing Technology for Flat-Top Nanosecond Green Laser Annealing Process of Semiconductor Si Wafers

Pure copper has antimicrobial effect, and the development of pure copper coating technology for product surfaces is needed to prevent the spread of infections. To achieve high antimicrobial performance in coatings, it is essential to form a coating with high copper purity and minimal defects. In this study, we attempted to form a pure copper coating layer on a SS304 substrate using multi-beam directed energy deposition with blue diode lasers. In the coating layer formation process, multiple bead layers are overlapped to form a smooth surface and two different types of joining is conducted simultaneously: the similar material joining area with the remelted previous layer, and the dissimilar material joining area with the substrate. However, it was challenging to conduct this process for SS304 and pure copper, due to their significantly different thermal properties. Therefore, we varied the hatching distance to control the heat input to both joining areas and investigated the effect on the quality of the coating layer. The results showed that there is an optimal ratio of similar and dissimilar joining areas, at which a high-quality coating layer with few voids and low dilution is formed. Furthermore, antimicrobial tests showed that the pure copper coating layer formed in this study exhibited the antimicrobial performance equivalent to those of a pure copper plate.

Pure Copper Coating by Multi Beam Directed Energy Deposition With Blue Lasers for Antimicrobial Effect

Busbar welds play a pivotal role in battery pack assembly, as their quality significantly impacts the structural integrity, electrical conductivity, and thermal efficiency of the battery pack. Current in-line quality evaluation methods encompass diverse techniques, including resistance measurement, thermographic imaging for fusion area assessment, machine vision for weld surface scanning, and monitoring of spectral line intensities during the welding process. This research delves into the efficacy of several approaches across various production scenarios, considering the inherent variations in the production processes. Typical variations in laser power, gap condition, laser focus condition, surface contamination, weld location etc are considered and result in welds with different qualities. Process spectrum, weld surface morphology and resistance measurement data are meticulously analyzed by correlating them with joint strength derived from tensile testing. Porosity distributions characterized by X-ray are used for quality evaluation. The study reveals that weld surface morphology from machine vision and spectral line intensity data exhibit strong correlations with process variations and weld strength. Conversely, resistance data proves to be less sensitive to process changes and does not correlate well with weld strength and porosity levels. In summarizing the findings, this study offers recommendations on effective in-line inspection for ensuring the quality of busbar welds in production environments.

Study of In-line Busbar Weld Quality Inspection Methods

Recent Research in Laser Welding & Ablation at GM R&D

To design future laser manufacturing processes for welding of copper materials, more and more high-end analysis methods are required. A fundamental process understanding by analyzing cause-and-effect relations of process dynamics using inline in situ diagnostics allows for an improved description of laser material interaction. Strategies for a reliable and robust welding process are derived from the findings. In this study, a four-step advanced methodical approach is presented and discussed. In the first step, a fundamental process description of the geometry of the vapor capillary and the formation of weld defects is developed. Therefore, welds on electrolytic tough pitch copper (Cu-ETP) and CuSn6 are carried out to analyze the temporal and spatial vapor capillary dynamics depending on laser power, welding speed, and focal diameter. This fundamental process understanding is transferred to the welding of copper pins in the form of I-pins. For this purpose, impurities and imperfections were applied to the pin surface to investigate the effects on the process result. As a third step, strategies by means of laser intensity distributions were adapted to compensate for imperfections in the welding process. Finally, a sensor vision system is adapted for ideal welding results. Investigations are based on in situ synchrotron analysis at Petra III, DESY in Hamburg. For the experiments, a TRUMPF TruDisk laser (100/400 μm fiber diameter), a TRUMPF TruFiber 6000P (50/100 μm fiber diameter), and a single-mode fiber laser (14 μm fiber diameter) were used. The focal diameter was adjusted with the optical system depending on the investigation.

A Development Workflow Based on In Situ Synchrotron Investigations to Optimize Laser-Processing of Copper Pins

Femtosecond lasers have over the past ten years seen a tremendous increase in average power, with industrial laser reaching today the kW level. At the same time new applications are continuously emerging in various industrial sectors: health, production, energy, transport. Femtosecond laser processing is today a technology enabling large volume, high precision industrial manufacturing. The "agility" of lasers and associated beam engineering now available, enables flexible, reconfigurable production. High average power femtosecond lasers allow higher productivity and thus open access to new and large markets unreachable so far (Aeronautics, Energy and Mobility, etc.). Making full use of the available average power without losing the high quality of ultrafast laser processing is not straightforward and require advances in laser design and associated beam engineering. For instance, high laser repetition rate up to 10 MHz or temporal shaping of the pulse train with busts of pulses, allows specific process optimization with the use of beam deviation at very high speed if suited pulses synchronisation is possible. Spatial shaping, for instance by dividing a single high-power beam into multiple beamlets, will be also a “must have” in the following years. Fixed diffractive optics (DOE), user configurable optical tools (spatial light modulators, SLM, or AOM), and new shaping systems using multi-plane light conversion (MPLC) technology allow flexibility to transform both phase and amplitude of a laser beam without major losses.

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A Double-Scan Technique for Femtosecond Laser-Induced Nanobumps

Double-Pulse Laser Peening as a Surface Enhancement Technology

Acoustic Process Monitoring During the Laser Beam Welding of Stainless-Steel Foils Using an Adjustable Ring Mode Laser Beam Source

Development of Wavelength Dispersion Measurement Method Using Fresnel Reflection of Fiber Under Test at Both End Faces

A study on numerical modeling method for predicting structural stiffness using crosshead displacement in laser welded joints

Analysis of Nanosecond-Scale Temperature Sensing Technology for Flat-Top Nanosecond Green Laser Annealing Process of Semiconductor Si Wafers

Pure Copper Coating by Multi Beam Directed Energy Deposition With Blue Lasers for Antimicrobial Effect

Study of In-line Busbar Weld Quality Inspection Methods

Recent Research in Laser Welding & Ablation at GM R&D

A Development Workflow Based on In Situ Synchrotron Investigations to Optimize Laser-Processing of Copper Pins

Coupling High-power Femtosecond Lasers and Beam Engineering for Industrial Application Challenges

Application of Hollowcore Fiber coupled Femtosecond laser for 6 Degree of Freedom Robotic Applications

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