Analytical model for the calculation of the depth progress of V-shaped grooves obtained by laser ablation with ultrashort pulses
Daniel Holder, Rudolf Weber, Christian Hagenlocher, Thomas Graf

The depth progress and the final depth of V-shaped grooves are described by an analytical model for laser ablation of metals with ultrashort laser pulses. The model assumes that the fluence absorbed along the walls is distributed with a linear increase from the edge to the tip of the groove. The depth progress of the machined groove is recursively calculated based on the depth increments induced by successive scans of the laser beam along the groove. Experimental results
agree well with the calculated predictions in the case of titanium alloy and tungsten carbide and for different pulse energies, repetition rates, scanning speeds, and number of scans. This confirms the validity of the model and its assumptions, and highlights the model as a useful tool for estimating groove dimensions, optimizing of process windows for machining with high depth progress, and predicting the maximum achievable groove depth.

Keywords: laser ablation; grooves; ultrashort laser pulses; analytical model; depth progress

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High-temperature laser absorption of steel
Joerg Volpp

Laser light absorption is a complex process including several absorption effects to transfer photonic energy into material. Absorption values and knowledge about impacting factors are relevant to simulate laser processes and get a better understanding of laser-material interaction. However, due to the high temperatures and dynamic melt surfaces of liquid steel, absorption measurements are difficult to conduct. Theoretical predictions show differing tendencies for high-temperature absorption values. Therefore, a radiometric absorption measurement is proposed in this work to derive absorption values above melting and even above boiling temperature of steel. While a ‘heating’ laser beam was used to create the melt pool, a ‘measuring’ laser beam in combination with an intensity sensor was used to derive reflection data. In general, an increasing tendency of absorption at increasing temperature was seen. The absorption drop just above boiling temperature is assumed to derive from vapor effects.

Keywords: Boiling temperature; laser beam absorption; reflectometry; vaporization

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Automated weld seam evaluation and 2D simulation parameter calibration for absorber-free laser transmission welding
Frederik Maiwald, Johannes Tröger, Stefan Hierl

Absorber-free laser transmission welding enables clean and precise joining of plastics without additives or adhesives. It is therefore well suited to produce optical and medical devices, which place high demands on cleanliness and accuracy. However, the weld usually has an undesirably large vertical expansion, causing bulges and distortion. To improve this, the intensity distribution of the laser beam as well as the processing strategy must be adapted. Due to the complexity, this is aided by process simulation. However, simulation parameter calibration and verification are usually done considering the seam width and height, which is of limited significance. To overcome this, we propose a new method for image processing of microtome sections, determining the spatially resolved geometry of the weld. Thus, the deviation between experiment and simulation can be calculated pixel by pixel. This spatially resolved value is predestined for the calibration of the simulation parameters: For a parameter field with 18 different settings, the total deviation between experiment and simulation is less than 11 % after calibration.

Keywords: plastics welding; image processing; simulation; optimization

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Measurement of the influence of the vapor plume on laser beam characteristics during laser beam welding
Johannes Wahl, Christian Frey, Michael Sawannia, Simon Olschok, Rudolf Weber, Christian Hagenlocher, Andreas Michalowski, Thomas Graf

During deep penetration laser welding a plume of hot metal vapor and particles is ejected by the keyhole. This vapor plume interacts with the incident laser beam by the means of scattering, absorption and phase front deformation. Within this work we present a measurement setup for diagnostics of the interaction characteristics. The setup combines a high-speed video with the measurement of the emitted spectrum of the vapor plume. This allows the location and differentiation between different zones of interaction between the laser beam and the plume. This knowledge will assist in the avoidance of weld defects which are induced by the vapor plume.

Keywords: Laser welding; vapor plume; beam characteristics

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Nano-steps to new functionalities: a laser-based process chain for oxide-dispersion strengthened steel
Mareen Goßling, Silja-Katharina Rittinghaus, Markus B. Wilms, Somnath Bharech, Yangyiwei Yang, Bai-Xiang Xu and Bilal Gökce

Oxide-dispersion strengthened (ODS) alloys are a prominent representative of metal matrix composites, highly demanded in high temperature and corrosive environments, e.g., efficient combustion engines. Conventional manufacturing of these materials is typically carried out by powder metallurgical processes, offering limited freedom in the fabrication of complex geometries. Additive manufacturing technologies allow the economical utilization of the cost-intensive powder material through direct near-net-shape manufacturing and thus significantly expand the range of applications.
In this study, the generation of spherical oxide nanoparticles (ZrO2) by laser-based ablation in liquids (LAL) from bulk target material and subsequent deposition on gas-atomized steel powder (Fe-20Cr (wt.%)), a composite powder material suitable for the additive manufacturing process of laser powder bed fusion (PBF-LB/M) is produced. Thus, a complete laser-based additive manufacturing chain for metal-ceramic composites from powder particles to a final component is presented.

Keywords: Composite; ODS; Laser Ablation in Liquid; oxide; Laser Powder Bed Fusion; materials

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Scan path optimization for laser additive manufacturing with quantum computing
Thomas Bussek, Annika Völl, Jochen Stollenwerk, Andrew Mitri, Anna Stollenwerk, Carlo Holly

In laser additive manufacturing processes such as laser-based powder bed fusion (LPBF), standardized scan strategies are used in the layer-by-layer build-up of the workpieces. This non-optimized procedure results in heat accumulation and therefore in thermal stress as well as in large laser off times for multi-scanner systems. Optimizing the scan path for each layer is a computational expensive task that is difficult to solve on conventional computers within reasonable time. The recent rise of quantum computers promises to solve optimization problems in highly reduced computational time with increased solution quality in the future. To exploit these advantages for LPBF, an adaptation of the multi-vehicle routing problem is presented. This formulation allows for reducing the overall scan time while accounting for the interaction of multi-scanner systems, reducing heat accumulation, avoiding laser-plume interaction and accounting for the direction of the shield gas flow.

Keywords: LPBF; scan path optimization; multi-scanner systems; quantum computing; QUBO formulation

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Evaluation of failure modes on AM-processes
Christoph Wilsnack, Anne-Katrin Leopold, Lukas Stepien, Elena Lopez, Frank Brückner, Christoph Leyens

Additive Manufacturing (AM) has become increasingly popular in recent years, particularly in the aerospace industry, but still has challenges like ensuring high-quality parts and a reproducible process. This contribution presents an evaluation of failure modes in additive manufacturing (AM) processes. The scope of the study includes an overview of challenges in AM such as multivariate interaction and quality assurance.
A model for a generic process failure mode and effect analysis (PFMEA) is developed and applied in an industrial context, specifically in the aerospace industry. Recommendations are also derived to improve the speed, reproducibility, and stability of AM processes, with the goal of achieving first-time-right production. The study includes the demonstration of proposed recommendations on an exemplary application using laser powder bed fusion (LPBF).

Keywords: Laser Powder Bed Fusion; Additive Manufacturing; Quality Assurance; Process chains; Failure modes

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Hybrid model for the threshold of deep-penetration laser welding
Michael Jarwitz, Andreas Michalowski

The development of reliable laser welding processes within a short time and with minimum experimental effort is an important aspect for small batch-size manufacturing. A physics-informed hybrid model was applied for the prediction of the threshold of deep-penetration laser welding. A “residual model” approach was used where a machine learning model was applied to learn and compensate for the deviations of an analytical model to the experimental results. Gaussian processes were used for the machine learning part. The results show an increase in model accuracy by using such a hybrid model compared to only using the analytical model. In comparison to only using a black-box machine learning model, the amount of required training data can be reduced and the extrapolation capability can be improved.

Keywords: modelling; hybrid models; machine learning; laser welding; deep-penetration threshold

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Influence of capillary shape on spatter formation during welding with ring-core fiber systems
Felix Zaiß, Michael Haas, Jonas Wagner, Christian Diegel, Klaus Schricker, Jean Pierre Bergmann, Marc Hummel, Alexander Olowinski, Felix Beckmann, Julian Moosmann, Christian Hagenlocher, Thomas Graf

During laser welding, the formation of spatter results from instabilities of the capillary, which are associated with adverse conditions in the melt flow surrounding the capillary and in the vapor flow at the capillary opening.
A change of the characteristics of these flows in the melt pool and in the capillary requires a change of the shape of the capillary and/or the surrounding melt pool. In order to influence the stabilize the capillary the intensity distribution was varied by means of a ring-core fiber system. Analysis of the resulting welds shows a significant effect of the intensity distribution on the stability of the capillary shape and considerable changes in the melt flow direction and velocity. These findings identified an optimum capillary shape, which results from a specific power share in the ring-core fiber combination.

Keywords: Laser welding; spatter formation; synchrotron; x-ray imaging; beam shaping

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Optimization of the melt flow by adjusting the keyhole geometry with dynamic beam shaping during laser welding
Jonas Wagner, Michael Haas, Felix Zaiß, Christian Hagenlocher, Rudolf Weber, Thomas Graf

The melt flow determines major characteristics in laser welding such as temperature distribution and defects like humping and undercuts. The characteristics of the melt flow are mainly determined by its interaction with the keyhole wall. In order to optimize the melt flow, it is required to adjust the keyhole geometry. Recent fiber laser sources provide coherent beam combining in the range of several kilowatts optical power, which enables beam shaping with almost arbitrary variations of quasi-stationary intensity distributions. The influence of the beam shape on the keyhole geometry was experimentally captured by online highspeed records during welding of aluminum and steel. Subsequently the influence of the shapes on the characteristics of the fluid flow was experimentally and numerically analyzed. The obtained results prove the potential of coherent beam combining to optimize welding processes by means of arbitrary and independent adjustment of the fluid flow.

Keywords: beam shaping; laser welding; numerical analysis; melt flow; optimization

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Polarization-dependent bulk modification of SiO2 by a femtosecond laser with pulse-front tilt
Qiang Fu, Cheng-Wei Wang, Hong-Jin Li, Quan-Zhong Zhao, Huai-Hai Pan, Jing Qian, Guan-De Wang, and Yu-Xing Zhao

The polarization-dependent machining morphology in amorphous and monocrystalline SiO2 machined by femtosecond laser is explained by second harmonic and tilted plasma due to PFT. While the polarization state changed from s-polarization to p-polarization, the machined region length of amorphous and monocrystalline SiO2 increased by 25.1% and 97.9% separately, although the wave vector direction, the PFT sign, and the writing direction were the same. The plasma distribution under the irradiation of 182 fs laser pulse with PFT of 4.41 fs/mm is calculated by the plasma evolution model based on strong field and avalanche ionization. The tilted plasma leads to not only weaker reflection of p-polarized first harmonic than the s-polarized first harmonic, but also resonant absorption at the p-polarization state instead of s-polarization state. The integral of the reflected second harmonic at p-polarization was 56.5% of that at s-polarization for monocrystalline SiO2 due to larger SHG coefficient at s-polarization.

Keywords: Femtosecond laser; pulse front tilt; plasma evolution

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Laser mediated fusing of paper materials
Florian Lull, Dr. Martin Zahel, Dr. Michael Panzner, Tom Schilling

This paper presents the results of fundamental studies on the interaction of laser radiation with classical paper materials regarding the melting of the main paper components cellulose, hemicellulose and lignin. Different papers were irradiated with the laser radiation of a carbon monoxide (CO) laser. Fluence-dependent interaction regimes, the dynamics of the flash pyrolysis and chemical changes due to irradiation are discussed. Using a high-speed camera, a liquid intermediate state could be observed as a result of the irradiation. This is decomposed into gaseous reaction products by a highly dynamic boiling process. In addition to the time resolved investigations, extensive FTIR studies were performed.

Keywords: CO laser; paper; pulp; melting

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Efficient Raman pulse generation at 1117nm
Yang Yang, Liwei Li, Jianxin Lu, Weimin Shen, Yuxing Zhao

Stimulated Raman Scattering (SRS) is a third‐order nonlinear optical phenomenon, which plays an important role in wavelength expansion of light source. In this paper, a pulsed Raman optical generator based on Raman effect and a MOPA scheme is developed and its performance is tested in laboratory. This set‐up demonstrated a very high conversion from 1064nm seed wavelength into the first order 1117nm wavelength Raman output. The highest optical conversion efficiency achieved from the experiment is 91.5%. Whilst the second‐order Raman effect is well suppressed during the process with a 40 dB difference between the first and second order Raman peaks.

Keywords: SRS, non‐linear optics, electrical modulation, MOPA

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Feasibility study on laser rod end melting of stainless-steel microtubes
Yang Lu, Joerg Volpp, Tim Radel

Laser rod end melting is a process to generate a wire end finish in micro range, wherein the laser induced melt forms nearly spherically due to surface tension. The solidified melt, called preform, can e.g. further be shaped by forming. At present, this process has been successfully used in wires of stainless steel and Nitinol, with the wire diameters below 1 mm. In this paper this process is applied on stainless steel (1.4401) microtubes with 0.8 mm outer diameter and 0.25 mm inner diameter to determine the process window for forming hollow preforms. The differences compared to processing of wires is discussed and the general feasibility of laser rod end melting of microtubes is proven.

Keywords: Laser beam machining; Stainless steel; Free forming