Karl-Heinz Leitz

A 3D transient model of keyhole and melt pool dynamics in laser beam welding applied to the joining of zinc coated sheets

In order to get a deeper understanding of laser beam welding, a process model was developed at the Chair of Manufacturing Technology. It is based on the continuity equation, the equation of heat conduction and the Navier–Stokes equation. The model includes effects of Fresnel absorption, vapor pressure, surface tension, melting and evaporation enthalpy and energy loss due to evaporating material. This paper presents the results of a three-dimensional, transient finite volume simulation of a laser beam deep penetration welding process based on this model. The simulations show periodic keyhole oscillations and the complex fluid dynamics of the melt pool. A comparison of the evaporation rates calculated from the simulations and the experimentally observed process emissions shows good correlation. Furthermore, the simulations show pore formation at higher feed rates, the influence of a gap on the welding process and give an explanation for the welding behavior of zinc coated steel sheets.

Optical trap assisted laser nanostructuring in the near-field of microparticles

Particle based near-field nanostructuring is an excellent possibility to overcome the optical diffraction limit in laser based material processing. In the near-field of microspheres which are irradiated with pulsed laser radiation, it is possible to generate nanoholes with diameters below 100 nm using a laser wavelength of 800 nm. To improve this approach, it is possible to position the microparticles with an optical trap to generate arbitrary structure geometries. In this paper, the authors describe the basic principle of optical trap assisted nanostructuring and present simulational and experimental results demonstrating the potential of this innovative nanoscale optical material processing technology.

Simulation of process dynamics in laser beam brazing

A transient three-dimensional simulation model of laser beam brazing was developed in order to gain a deeper understanding of the laser beam brazing process. In the multi-physical model, beam-matter interaction, heat transfer as well as melting and resolidification of base and filler material are taken into account. Furthermore, the flow dynamics in the melt and the surrounding medium is calculated. By using an appropriate wetting model, the wetting of the base material by the filler material can be simulated. Additionally, for a realistic modeling of the brazing process the movement of the filler wire is also included in the model. To verify the model, the joint cross section obtained in the simulation is compared with experimental results. The agreement between experiment and simulation confirms the capabilities of the multi-physical approach. Based on the simulation model, wetting behavior as well as melt dynamics in laser beam brazing can be analyzed. Simulations were performed that show the formation of brazing errors. These results demonstrate that detailed simulations of the laser brazing process not only provide insight into process dynamics but are also a versatile tool for process optimization and process planning.A transient three-dimensional simulation model of laser beam brazing was developed in order to gain a deeper understanding of the laser beam brazing process. In the multi-physical model, beam-matter interaction, heat transfer as well as melting and resolidification of base and filler material are taken into account. Furthermore, the flow dynamics in the melt and the surrounding medium is calculated. By using an appropriate wetting model, the wetting of the base material by the filler material can be simulated. Additionally, for a realistic modeling of the brazing process the movement of the filler wire is also included in the model. To verify the model, the joint cross section obtained in the simulation is compared with experimental results. The agreement between experiment and simulation confirms the capabilities of the multi-physical approach. Based on the simulation model, wetting behavior as well as melt dynamics in laser beam brazing can be analyzed. Simulations were performed that show the formation...

Ultrafast microsphere near-field nanostructuring

Due to the steadily advancing miniaturization in all fields of technology nanostructuring becomes increasingly important. Whereas the classical lithographic nanostructuring suffers from both high costs and low flexibility, for many applications in biomedicine and technology laser based nanostructuring approaches, where near-field effects allow a sub-diffraction limited laser focusing, are on the rise. In combination with ultrashort pulsed laser sources, that allow the utilization of non-linear multi-photon absorption effects, a flexible, low-cost laser based nanostructuring with sub-wavelength resolution becomes possible. Among various near-field nanostructuring approaches the microsphere based techniques, which use small microbead particles of the size of the wavelength for a sub-diffraction limited focusing of pulsed laser radiation, are the most promising. Compared to the tip or aperture based techniques this approach is very robust and can be applied both for a large-scale production of periodic arrays of nanostructures and in combination with optical trapping also for a direct-write. Size and shape of the features produced by microsphere near-field nanostructuring strongly depend on the respective processing parameters. In this contribution a basic study of the influence of processing parameters on the microsphere near-field nanostructuring with nano-, pico- and femtosecond laser pulses will be presented. The experimental and numerical results with dielectric and metal nanoparticles on semiconductor and dielectric substrates show the influence of particle size and material, substrate material, pulse duration, laser fluence, number of contributing laser pulses and polarization on the structuring process.

Optical trap kits: issues to be aware of

An inexpensive and robust optical trap system can be built from off-the-shelf optical and opto-mechanical components or acquired as a kit to be assembled in a laboratory. The primary advantages of such a trap, besides being significantly more affordable, are its flexibility, and ease of modification and upgrade. In this paper, we consider several important issues to be addressed during development and application of a kit system. We numerically examine the performance of trapping systems equipped with oil and water immersion focusing objectives. We also investigate the effect of trapping laser beam quality on optical tweezing.

Multiphoton polymerization using optical trap assisted nanopatterning

In this letter, we show the combination of multiphoton polymerization and optical trap assisted nanopatterning (OTAN) for the additive manufacturing of structures with nanometer resolution. User-defined patterns of polymer nanostructures are deposited on a glass substrate by a 3.5 μm polystyrene sphere focusing IR femtosecond laser pulses, showing minimum feature sizes of λ/10. Feature size depends on the applied laser fluence and the bead surface spacing. A finite element model describes the intensity enhancement in the microbead focus. The results presented suggest that OTAN in combination with multiphoton processing is a viable technique for additive nanomanufacturing with sub-diffraction-limited resolution.

Laser direct-write nanopatterning by near-field multiphoton polymerization using optically trapped microspheres

We use Gaussian beam to position a microsphere in a polymer precursor environment near a substrate. Pulsed laser is focused by a microsphere to induce multiphoton polymerization in the near-field, enabling additive direct-write nanoscale processing.