Friedrich Dausinger

Fundamental aspects in machining of metals with short and ultrashort laser pulses

On the fast growing market of precision micro-machining of metals lasers do not only compete with other methods of structuring. There is also strong competition among different laser-processing strategies and, especially, among laser sources with different pulse duration. A comprehensive study of laser micro-machining with nanosecond, picosecond, and femtosecond laser pulses will be presented with a focus on fundamental aspects of the processes and on their practical consequences. An analysis will be given of the potential or the limitations of these laser processes with respect to their industrial application.

Micromachining with ultrashort laser pulses: from basic understanding to technical applications

Ultrashort pulses appear very promising for material removal with ultra high precision. Initial investigations showed, however, several unexpected quality problems such as formation of recast, ripples and irregular hole shapes even in the femtosecond pulse regime. After describing the problems, this contribution will present some progress in fundamental understanding of the ultrafast ablation process. On the basis of this knowledge technical means have been developed allowing to achieve an unprecedented level of accuracy at acceptable expenses. The latter being strongly influenced by the shortness of the laser pulse, a comparison between pico- and femto-second regime will be presented.

Femtosecond technology for precision manufacturing: fundamental and technical aspects

During the past few years research groups demonstrated the potential of femtosecond pulses for ultra-precise machining. Soon industry became interested in the new technique which promises to get rid of precision deficits occurring when using longer pulses. As possible applications drilling of nozzles, structuring of tribological surfaces and sharpening of diamond tools are considered among others. Initial investigations of such industrial applications soon showed, however, several unexpected quality problems such as formation of recast, ripples and irregular hole shapes even in the femtosecond pulse regime. After describing the problems this contribution will present some progress in fundamental understanding of the ultrafast ablation process. On the basis of this knowledge technical means have been developed allowing to achieve an unprecedented level of accuracy at acceptable expenses. The latter being strongly influenced by the shortness of the laser pulse, a comparison between pico- and femto-second regime will be presented.

Laser ablation of metals and ceramics in picosecond–nanosecond pulsewidth in the presence of different ambient atmospheres

Abstract Ablation tests of AlN, Si3N4, SiC, Al2O3 ceramics, steel and aluminum have been carried out in vacuum, air and argon atmospheres using UV (270 nm), visible (539 nm) and IR (1078 nm) picosecond (100÷150 ps) and nanosecond (6÷9 ns) laser pulses. Ablation rate dependencies have been measured in the range of laser energy densities varied from (2÷5)×101 J/cm2 to (5÷10)×103 J/cm2. Peculiarities of laser ablation processes at different wavelengths, pulsewidths and ambient gases are discussed. In particular, the efficiencies of laser ablation in picosecond and nanosecond regions are compared. The scanning electron microscope (SEM) pictures of high quality microstructures, deep and narrow cuts and holes produced in ceramics with typical size of tens microns and aspect ratio as high as 20, are demonstrated.

Analytical investigations on geometrical influences on laser drilling

An analytical model for laser drilling is proposed which includes three-dimensional heat conduction in a simplified manner. For that purpose, the heating of the curved surface is locally described by that of a spherical cavity with comparable curvature within an infinite medium. Additionally, the absorption of laser radiation on the inclined side wall is taken into account. Using these components, it is possible to calculate the evolution of the hole shape from pulse-to-pulse in an iterative way. Therefore, this model is suitable to study the main aspects of deep drilling such as ablation rates and hole shapes without the disadvantage of long computational times. As a drilled hole deepens and the walls become steeper, its surface area grows and, thereby, in principle the absorbed intensity drops. This can lead to a considerable reduction of ablation rate. At the same time, extremely curved surface areas will heat much faster or slower than plane ones which, again, results in local changes of drilling velocity. It is shown that the former is particularly of interest for the description of the resulting hole shape while the latter has a considerable influence on the ablation velocity at the tip of the hole. To verify the analytical model, its results are compared with those of three-dimensional numerical simulations. It is shown that the simplified assumptions introduced here are, up to some extent, suitable to explain the final surface shapes for blind holes as well as the experimentally observed dependence of ablation rate on hole depth.

Aspects of keyhole/melt interaction in high-speed laser welding

The increasing availability of highest power lasers broadens the economic use of deep penetration laser welding by increasing maximum welding depth and by increasing welding speed. The problems encountered in those two areas of interest differ significantly. Whereas the maximum welding depth is strongly affected by the absorption mechanism1,2, the maximum welding speed is limited by fluid dynamic phenomena . Theoretical modelling allows to seperately investigate the different processes.

Precise drilling of steel with ultrashort pulsed solid state lasers

The accuracy of laser drilled holes in metals is limited by a relatively large amount of molten material which is produced when lasers with pulse durations in the range of nanoseconds or longer are used. In general, shortening the pulse duration down to the picosecond or femtosecond regime promises to overcome these problems. In this contribution different influences on hole quality such as energy density, beam quality, and polarization as well as processing strategies for high precision drilling of steel with ultra-short pulses are presented and discussed. A new method of polarization control is demonstrated by which the hole geometry can significantly be improved and ripples in the surface of the hole walls can be avoided during helical drilling. Furthermore, results of investigations on the influence of the ambient pressure will be presented.

High precision deep drilling with ultrashort pulses

Ultrashort laser pulses appear very promising for material removal with highest precision. However, our investigations show several unexpected quality problems such as formation of recast, ripples, and irregular hole shapes even in the femtosecond-pulse regime. In this contribution influences on drilling efficiency and hole quality by several process parameters such as repetition rate and pulse duration will be presented and discussed. Furthermore different processing techniques to increase the drilling velocity will be shown. A suitable device for that purpose is a trepanning optic which was specially designed for the drilling of injection nozzles. By a variable angle of beam incidence it allows to produce holes with a well-defined conicity in combination with the high precision achieved by helical drilling.

Hole formation process in laser deep drilling with short and ultrashort pulses

The drilling process in different materials (diamond, steel, ceramics and PMMA) was studied for a large range of pulse lengths from about 100 fs to 10 ns using different approaches. In transparent materials the penetration process was visualized with high-speed video analysis and microscopy. The drilling rate as well as the relation between processing energy density and ablation threshold were determined in situ. The penetration of the laser beam inside the channel and the influence of laser-ignited plasma were investigated by transmission measurements. Mechanisms of energy coupling and heat losses were examined by applying simple analytical calculations. Proposals for the basic understanding of the drilling process are presented.

Precision drilling of metals and ceramics with short- and ultrashort-pulsed solid state lasers

At the end of 1999 a German National Project called PRIMUS was established, the most important aim of which is to analyze the potential advantages of ultrashort pulses in combination with different drilling strategies and to obtain a better understanding of the ablation and drilling processes. This contribution will present the first results of this project. The advantages of short and ultrashort pulses in view of quality and efficiency will be discussed. It will be shown, that the use of suitable drilling technologies, such as e.g. helical drilling, and a specifically designed trepanning optic can significantly increase the quality of holes as well as expand the possible range of applications.