Time-resolved pump-probe analysis of metal ablation using single and double ultrashort laser pulses
Maximilian Spellauge, Jan Winter, Cormac McDonnell, Stephan Rapp, Michael Schmidt, Heinz P. Huber

In recent years several works have been investigating the temporal distribution of pulse energy with the aim of maximizing ablation efficiency in terms of energy specific ablation volume by applying double pulses or pulse bursts. Here we study the energy specific ablation volume of double pulses in dependency of increasing pulse separation on aluminum samples. To avoid multi-pulse incubation effects the double pulse irradiation is repeated only three times at one position. Ultrafast pump-probe ellipsometry and microscopy are presented in order to study the surface material motion after single laser pulse impact. The comparison of time-resolved measurements and double pulse ablation volumes suggests that between 5 ps and 20 ps an interaction of the second pulse with the rarefaction wave decreases double pulse energy specific ablation volume. For longer double pulse spacing up to 1 ns the decrease can be attributed to re-deposition of ablated material.

Keywords: ultrashort pulses, double pulses, ellipsometry, pump-probe, ablation, metals

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Time-resolved pump-probe microscopy of ultrashort laser pulse irradiated bulk aluminum and stainless steel
Jan Winter, Stephan Rapp, Cormac McDonnell, Maximilian Spellauge, Michael Schmidt, Heinz P. Huber

Metals irradiated with ultrashort laser pulses pass through a sequence of physical processes over a wide range of timescales, from femtoseconds to microseconds. Especially, the relaxation of the photomechanical material removal, known as spallation, has not been well investigated experimentally on this timescale. In this article, the complete timescales for the processes involved in ablation of industrially relevant metals, Al and the stainless steel alloy (AISI304), are analyzed and visualized, from the initial pulse absorption to the material removal occurring on a microsecond time scale. These results advance our understanding of a key aspect of the laser–material interaction pathway and can lead to optimization of associated applications ranging from material processing to laser surgery.

Keywords: ultrashort pulses, pump-probe microscopy, ablation dynamic, metals, aluminium, stainless steel, femtosecond ablation

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Simulation of ultrafast laser ablation topography of metals
L. Cangueiro, P. E. Martin, J. A. Ramos-de-Campos, A. Kupisiewicz, D. Bruneel

It has been demonstrated in the past years that ultrafast lasers are excellent tools for materials micromachining. The ablation topology resulting by irradiation with these lasers depends on both the processing parameters and the material properties. The resulting thermal effects are negligible only if a good combination of processing parameters is chosen. Consequently, optimizing the processing parameters that lead to the required ablation dimensions and surface quality on a given material can be rather complex and time consuming. To enhance this parameters research, we developed the web-based tool LS-PLUME®, based on a numerical model that allows estimating the ablation profiles while preserving the surface quality. The simulated results were validated by comparison experimental ones and show that LS-PLUME®
allows optimizing the micromachining process, both energy and time wise.

Keywords: femtosecond laser ablation; micromachining simulation; metals

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CFD simulations for laser welding of Al alloys
Pareekshith Allu

To meet stricter fuel economy standards in the automotive industry, automakers have increasingly turned to using aluminum diecast parts and panels due to their light weight and superior castability. Laser welding is a process by which die cast parts are connected to other semi-finished parts, such as profiles or tubes, through pressure-tight joints. However, gasses trapped during casting present challenges to the laser welding process and can cause additional defects such as porosity and melt pool blow-outs. Computational fluid dynamics (CFD) simulations of laser welding help improve the process and achieve superior weldability. In this presentation, case studies on CFD simulations successfully employed to optimize process parameters such as welding speed, laser heat flux profiles, and angles of inclination to mitigate porosity formation are studied. These CFD models serve as an initial, crucial step towards optimization of the process of laser welding of die-cast parts.

Keywords: CFD simulations, laser keyhole welding, FLOW-3D, melt pool dynamics

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Application of an analytical model to predict the grain structure of laser beam welds in aluminum alloys
Christian Hagenlocher, Artur Leis, Florian Fetzer, Daniel Weller, Rudolf Weber, Thomas Graf

The grain structure of a weld influences its hot crack susceptibility during welding and its strength after completion of the welding process. Simple analytical equations to predict the grain structure of laser beam welds in AlMgSi aluminum alloys are proposed. The model describes the influence of the welding parameters on the morphology as well as on the size of the grain structure. Metallographic analysis of laser beam welds show, that the analytical model predicts the grain structure in sufficient accuracy. These findings explain how to optimize laser beam welding processes in order to obtain a reliable formation of an equiaxed dendritic grain structure or to reduce the grain size. Further, these findings outline the parameter field for the formation of an equiaxed dendritic grain structure in additive manufacturing processes.

Keywords: Laser beam welding; grain structure; solidification; aluminum alloys; selective laser melting; additive manufacturing

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Experimental results and modelling of element loss in continuous laser beam welding of aluminum alloys
Florian Hugger, Eric Punzel, Michael Schmidt

In aluminum alloys the elements magnesium and zinc have a much lower evaporation temperature than the base element. This leads to selective evaporation and loss of these elements in the fusion zone. Moreover, the evaporation rate of volatile elements is determined by a combined process of evaporation and diffusion of these elements from the melt pool towards the capillary surface. In this paper the influence of welding parameters like feed rate, laser power, focal diameter as well as alloying element concentration on volatile element loss and evaporation rate is investigated. Measuring the keyhole and weld pool geometry, the interaction time of the melt with the laser beam can be calculated. Combining results from element loss and interaction time the diffusion rate of volatile elements towards the keyhole surface is estimated. The evaporation rate of volatile elements is invers proportional to interaction time.

Keywords: Aluminum; laser beam welding; element loss; diffusion; vaporazation

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A Genetic Algorithm for the Correlation of the Keyhole and the Melt Pool Depth in Laser Beam Welding of AA6082
Maximilian Schmoeller, Maximilian Neureiter, Christian Stadter, Michael F. Zaeh

The aluminum alloy AA6082 is frequently used in high-voltage energy storage systems due to its favorable electrical and mechanical properties. Laser beam welding provides a flexible process for producing the welds. In order to protect the sensitive components of the battery cells, the required welding depth must be maintained. Optical Coherence Tomography (OCT) is a promising method for inline monitoring of the deep welding process. The measured depth of the keyhole can be used to set up a process control. A thermal simulation model was developed to take the difference between the melt pool depth and the keyhole depth into account. By adapting the keyhole geometry in the simulation based on genetic algorithms, a correlation was found through the optimization. The model was calibrated by comparing metallographic cross-sections with the computed melt pool geometries. Based on these results, a database can be created to improve the performance of the signal processing algorithms.

Keywords: FEM; Numerical Simulation; Deep Penetration Laser Beam Welding; Genetic Algorithm; Optical Coherence Tomography

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A theoretical model for reactive gas laser cutting of metals
M. H. Brügmann, M. Muralt, B. Neuenschwander, S. Wittwer, T. Feurer

We present a theoretical model for the process of reactive gas laser cutting of metals with emphasis on the roughness profile of the cutting edges, i.e. striation. Theoretical results are verified against experimental observations for 10 mm DD11 steel plates. Striation spectra were obtained by performing a numerical Fourier transform of the measured roughness profiles. The spectra were then fitted to those of a driven damped harmonic oscillator yielding information on amplitudes, resonance frequencies and damping constants. These parameters were determined for different focal positions and cutting speeds. Generally, we find a good agreement between the theoretical model and experiment. From this we conclude that our model is able to predict important quality parameters of reactive gas laser cutting and how to optimize them.

Keywords: Macro-Processing ; Cutting; Fundamentals and Process Simulation

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Laser Transmission Welding of Thermoplastics – Analytical Model and its Experimental Evaluation
D. Hölzemann, M. Scholl, U. Russek

An analytically model for laser transmission welding of thermoplastics will be set up. The local evolution of the temperature field and the formation of the weld joint are modeled in terms of explicit analytical expressions. Necessary approximations will be pointed out keeping computations manageable. The model yields explicit expressions for a process window, weld seam width and depth as well as the tensile strength in terms of the process and materials parameters. The predictions of the model fit well to results obtained from systematic welding experiments. Furthermore, the results show that the analytical model enables a much better understanding of the welding process and allows a much quicker process parameter adaptation within industrial applications.

Keywords: laser welding thermoplastics; analytical model; process parameters; experimental verification;

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Optimization of Reactive Gas Laser Cutting Parameters based on a combination of Semi-Analytical modelling and Adaptive Neuro-Fuzzy Inference System (ANFIS)
M. H. Brügmann, M. Muralt, B. Neuenschwander, S. Wittwer, T. Feurer

We demonstrate an optimization procedure to determine optimal process parameters in reactive gas laser cutting of metals. The optimization procedure is based on a combination of a semi-analytical model for reactive gas laser cutting and an adaptive neuro-fuzzy inference system (ANFIS). The semi-analytical model was employed to generate training and testing data for ANFIS. Exemplarily, we show results for 10 mm thick DD11 steel plates. The system parameters consisted of two inputs: the cutting speed and the focal position of the laser with respect to the workpiece. The optimization was done with respect to the striation amplitude. For each output a corresponding ANFIS-network was built. The optimal process parameters were extracted via a 3D surface or control plot of the generated fuzzy output for the striation amplitudes as a function of the two input parameters.

Keywords: Macro-Processing; Cutting; Fundamentals and Process

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Temperature-dependent reflectivity of unpolished rolled copper for near infrared lasers
Manuel Mattern, Andreas Ostendorf

Data about the temperature-dependent reflectivity of materials are essential for the modeling of laser-material interactions. Most of the calculated and experimentally acquired data currently available for copper consider only polished and oxygen free copper surfaces. However, the surface of industrially processed copper is usually unpolished, therefore having a slightly oxidized surface. In this paper, the reflectivity of unpolished rolled copper foils for 1064 nm wavelength is measured for temperatures up to 800 °C, using an integrating sphere. Measurements with recently polished copper are conducted for reference. The measurements for the unpolished copper show a behavior of the reflectivity over temperature which is completely different from the one for the polished copper samples. After cooling, the reflectivity for the unpolished samples is increased by about 2.5 percentage points compared to the initial values. This shows the necessity to measure the reflectivity also of oxidized samples when modeling a real process.

Keywords: reflectivity; copper; temperature; near infrared; oxidation

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Numerical simulation of residual stresses in laser welding: application to Ti6Al4V/316L steel assembly with vanadium insert
Antoine Mannucci, Rodolphe Bolot, Alexandre Mathieu, Iryna Tomashchuk, Eugèn Cicala, Cyril Roudeix, Sébastien Lafaye

Prediction of residual stresses in laser welding is an important challenge allowing better understanding of forces involved in the bonding strength mechanisms. This type of simulation is particularly complementary with measurements. Direct welding of titanium to stainless steel is challenging due to the formation of brittle intermetallic
compounds. A good strategy to solve this problem consists in using a compatible insert material. The case of laser welding with vanadium insert was thus considered in the present work. A FEM model was developed to simulate the two-step welding process (titanium/vanadium weld followed by vanadium/steel weld). The case of 1 mm thick plates with a 2 mm large vanadium insert was considered to avoid any contact between the two welds. A semi-coupled model was developed: the thermal problem considering equivalent heat sources is solved in a first step, and the mechanical problem in a second one. A complete cooling of the titanium/vanadium assembly is considered before manufacturing of vanadium/steel weld.

Keywords: dissimilar welding ; numerical simulation ; FEM ; stainless steel ; titanium ; vanadium;