Udo Loeschner

Micro processing of metals using a high repetition rate femto second laser: from laser process parameter study to machining examples

The paper presents a study of laser micro processing of metals by using a high repetition rate femto second laser. On stainless steel (AISI 304), copper and aluminium the impact of the significant laser processing parameters onto the machining process was investigated, such as laser fluence, repetition rate, lateral pulse distance and polarisation. The machining results were evaluated by the ablation rate, surface roughness, process efficiency, material removal rate and the wall-angle. For complementary discussions the experimental data were compared with results achieved in theoretical analysis. Outgoing from the results appropriate laser processing parameters were derived in order to optimise the machining process. With the application of ultra short laser pulses high-quality machining results with a minimal thermal load and a roughness Ra of the laser processed surface of only some hundreds nano meter were obtained. On other hand high machining throughputs were achieved due to application of high repetition rates. Finally, the possibilities and the limits of the high repetition rate femto second laser technology in laser micro processing are demonstrated by means of three-dimensional micro structured machining examples.The paper presents a study of laser micro processing of metals by using a high repetition rate femto second laser. On stainless steel (AISI 304), copper and aluminium the impact of the significant laser processing parameters onto the machining process was investigated, such as laser fluence, repetition rate, lateral pulse distance and polarisation. The machining results were evaluated by the ablation rate, surface roughness, process efficiency, material removal rate and the wall-angle. For complementary discussions the experimental data were compared with results achieved in theoretical analysis. Outgoing from the results appropriate laser processing parameters were derived in order to optimise the machining process. With the application of ultra short laser pulses high-quality machining results with a minimal thermal load and a roughness Ra of the laser processed surface of only some hundreds nano meter were obtained. On other hand high machining throughputs were achieved due to application of high repet...

High-rate laser microprocessing using a polygon scanner system

This paper discusses results obtained in high-rate laser microprocessing by using a high average power high-pulse repetition frequency ultrashort pulse laser source in combination with an in-house developed polygon scanner system. With the recent development of ultrashort pulse laser systems supplying high average power of hundreds watts and megahertz pulse repetition rates, a significant increase of the productivity can potentially be achieved in micromachining. This permits upscaling of the ablation rates and large-area processing, gaining increased interest of the ultrashort pulse laser technology for a large variety of industrial processes. However, effective implementation of high average power lasers in microprocessing requires fast deflection of the laser beam. For this, high-rate laser processing by using polygon scanner systems provide a sustainable technological solution. In this study, a picosecond laser system with a maximum average power of 100 W and a repetition rate up to 20 MHz was used. I...

High repetition rate femtosecond laser processing of metals

Previously, in high repetition rate femto second laser processing novel laser matter interacting effects were reported, such as heat accumulation and particle shielding. In this study, high repetition rate laser processing was investigated to discuss and understand the impact of laser repetition rate and accompanied accumulative laser material interacting effects. Therefore, a high repetition rate femto second fibre laser setup joint together with galvo scanner technology was applied in laser micro machining of metals (copper, stainless steel, aluminium). High repetition rate laser processing of aluminium and stainless steel lead to considerably lowered ablation thresholds accompanied with higher ablation rates. Laser ablation behaviour of copper was almost independent of the repetition rate with neither considerable lower ablation thresholds nor higher ablation rates. For explanation, heat accumulation caused by higher repetition rates were assumed as mainly ablation behaviour influencing effect, but thermal material properties have to be considered. Furthermore laser machining examples demonstrate the possibilities and limits of high repetition rate laser processing in 3d micro structuring. Thus, by using innovative scanning systems and machining strategies very short processing times were achieved, which lead to high machining throughputs and attract interest of the innovative laser technology in Rapid Micro Tooling. For discussion, high repetition rate processing results are evaluated by means of comparative machining examples obtained with 1 kHz femto second laser system.

Investigation of cw and ultrashort pulse laser irradiation of powder surfaces: a comparative study

The paper presents results obtained in a comparative study of laser irradiation of tungsten powder surfaces using a continuous wave fiber laser and a high repetition rate femtosecond laser. Depending on the energy input per unit length different melt structures have been produced. In general, if the same average laser power level was applied the structures show the same appearance independent from the laser source. But there was both a little higher degree of initial fusing and cross-linking along the processed path when the powder surface was irradiated with ultrashort pulses. Further, with increasing laser intensity a change in structure formation as well as a broadening of the laser processed path has been occurred, although the energy input per unit length remains constant. However, accumulation of slab-like structures, which was previously observed in high-intense ultrashort pulse laser irradiation, has been become more pronounced in cw laser irradiation above a certain number of consecutive scans. Moreover, characteristic effects, such as formation of ripples and nanomelt structures appearing in ultrashort pulse laser processing have been not detected in cw laser irradiation.

Characterisation of interaction phenomena in high repetition rate femtosecond laser ablation of metals

The paper discusses results obtained in ultrashort pulse laser irradiation of metals in order to characterise interaction phenomena occurring in highly repetitive laser processing, such as heat accumulation and particle shielding. The impact of the temporal pulse-to-pulse distance on the ablation process was investigated using repetition rates ranging between 25.8 kHz and 2.05 MHz. Interacting effects were studied by means of industrial grade metal sheets with various thermo-physical characteristics. The experimental results obtained were evaluated by theoretical calculations of both the ablation rate and surface temperature. Furthermore ultra high speed camera images were taken into discussion.Ablation rates obtained empirically for stainless steel and aluminium indicate increasing material removal at higher repetition rates and, hence, heat accumulation is proven as influencing effect. Thus in case of stainless steel and shorter pulse-to-pulse distances, temperature calculation yields the rise of the surface temperature. Additionally, ultra high speed camera images give evidence of more voluminous ablation plumes at shorter pulse-to-pulse distances, induced by intense laser matter interaction.In contrast, for copper only a marginal impact of the repetition rate on the material removal was found. Thus for highly heat-conductive materials the ablation rate is assumed almost independent from the temporal pulse-to-pulse distance. Even high speed camera images show minor impact of the repetition rate on the ablation process.Finally the application of the laser micro machining technology in micro-mould manufacturing is presented. As a result micro-featured plastic demonstrators were produced by micro injection moulding, offering a wide range of sensor applications, for example in microfluidic systems.The paper discusses results obtained in ultrashort pulse laser irradiation of metals in order to characterise interaction phenomena occurring in highly repetitive laser processing, such as heat accumulation and particle shielding. The impact of the temporal pulse-to-pulse distance on the ablation process was investigated using repetition rates ranging between 25.8 kHz and 2.05 MHz. Interacting effects were studied by means of industrial grade metal sheets with various thermo-physical characteristics. The experimental results obtained were evaluated by theoretical calculations of both the ablation rate and surface temperature. Furthermore ultra high speed camera images were taken into discussion.Ablation rates obtained empirically for stainless steel and aluminium indicate increasing material removal at higher repetition rates and, hence, heat accumulation is proven as influencing effect. Thus in case of stainless steel and shorter pulse-to-pulse distances, temperature calculation yields the rise of the su...

FEM calculations on laser bending of silicon with a moving laser source

In this paper we are going to present FEM calculations applied for laser bending of silicon microstructures and compare them with our experimental results. According to the mechanisms in plastic deformation of metals with laser radiation we performed calculations to find out, if there are similar mechanisms in forming of silicon. To model the laser heating up mechanism we have to take into account several physical effects like heat radiation or reflection of silicon for the used laser radiation. To transform the laser bending process into a FE model the boundary conditions include compromises and simplifications of the geometry and the energy input. In our calculations we modelled the laser beam as a moving heat source in order to get information about the temperature distribution, the temperature gradient and the heat flow in dependence on the position on the sample and the time. The calculated essentially higher temperatures at the edges of the structure compared to the middle of the structure, exceeding the melting point there, are in very good agreement to the melting areas observed at the edges in the experi-ments. After a number of consecutive scans we reach a balanced temperature field moving with the laser beam across the surface. The calculations revealed that there is a steep temperature gradient in the depth of the structure indicating a similar temperature gradient mechanism observed in forming of metals with laser radiation. Additionally we carried out temperature field calculations to determine the influence of the process parameters like the laser power, the velocity of the heat source, the material thickness or the position of laser treatment on the temperature field generated in the material. The results of the calculations are in very good agreement with our experiments. Next time stress field calculations are intended. At the moment there are not enough data on the plastic behaviour of silicon available to the authors in order to get reliable results.

Laser bending of silicon

We are going to present a new technology for laser machining of silicon developed at the Laser Institute of Mittweida by a suggestion and in cooperation with the Technical University of Chemnitz, Department of Electrical Engineering and Information Technology. It allows the laser induced bending of microstructural silicon elements prepared by anisotropic wet etching. Bending of the element toward the incident laser beam occurs as a result of the laser induced thermal stresses in the material. We investigated the influence of various process parameters on the bending angle. There is only a small range of laser power to generate bendings in silicon. We will show a variety of examples including multiple and also continuous bendings. There are several essential advantages compared to conventional bending technologies: Laser bending is a contactless process without using additional tools or external forces. Because of the local laser treatment the heat flux to neighbouring material can be minimized so that the technology is suitable for machining of already finished microsystems. This new technology opens up a new field of applications in microsystem technologies. It is possible to generate a clip-chip-mechanism to clip a chip in a holder. Other examples are the exact positioning of optical mirrors or other components, the production of electrostatic drives and sliding chips for micro optical benches.

High-throughput machining using a high-average power ultrashort pulse laser and high-speed polygon scanner

Abstract. High-throughput ultrashort pulse laser machining is investigated on various industrial grade metals (aluminum, copper, and stainless steel) and Al2O3 ceramic at unprecedented processing speeds. This is achieved by using a high-average power picosecond laser in conjunction with a unique, in-house developed polygon mirror-based biaxial scanning system. Therefore, different concepts of polygon scanners are engineered and tested to find the best architecture for high-speed and precision laser beam scanning. In order to identify the optimum conditions for efficient processing when using high-average laser powers, the depths of cavities made in the samples by varying the processing parameter settings are analyzed and, from the results obtained, the characteristic removal values are specified. For overlapping pulses of optimum fluence, the removal rate is as high as 27.8  mm3/min for aluminum, 21.4  mm3/min for copper, 15.3  mm3/min for stainless steel, and 129.1  mm3/min for Al2O3, when a laser beam of 187 W average laser powers irradiates. On stainless steel, it is demonstrated that the removal rate increases to 23.3  mm3/min when the laser beam is very fast moving. This is thanks to the low pulse overlap as achieved with 800  m/s beam deflection speed; thus, laser beam shielding can be avoided even when irradiating high-repetitive 20-MHz pulses.

High-pulse repetition frequency ultrashort pulse laser processing of copper

This paper presents results obtained in high-pulse repetition frequency ultrashort pulse laser microprocessing of copper. In the study, a variety of ultrashort pulse laser systems supplying high average laser power were applied in order to investigate the influence of the laser parameters on copper ablation. For this, laser pulses of different wavelengths (515 nm, 1030 nm) and pulse durations, ranging between 200 fs and 10 ps, were irradiated to the sample surface by raster scanning of the laser beam. The dependencies of average laser power, pulse energy, and the pulse repetition rate on the ablation rate, the ablation efficiency, and the productivity were studied. A maximum average laser power of 31.7 W was applied in this work. The pulse repetition rate was varied in the rage between 0.2 and 19.3 MHz. Finally, the machining qualities obtained were evaluated by means of surface roughness measurements and scanning electron microscope micrograph analysis.

Highspeed laser ablation cutting of metal

In laser ablation cutting, irradiation of high-intense laser beams causes ejection of molten and evaporated material out of the cutting zone as a result of high pressure gradients, induced by expanding plasma plumes. This paper investigates highspeed laser ablation cutting of industrial grade metal sheets using high-brilliant continuous wave fiber lasers with output powers up to 5 kW. The laser beam was deflected with scan speeds up to 2700 m/min utilizing both a fast galvanometer scan system and a polygon scan system. By sharp laser beam focusing using different objectives with focal lengths ranging between 160 mm and 500 mm, small laser spot diameters between 16.5 μm and 60 μm were obtained, respectively. As a result high peak intensities between 3*108 W/cm² and 2.5*109 W/cm² were irradiated on the sample surface, and cutting kerfs with a maximum depth of 1.4 mm have been produced. In this study the impact of the processing parameters laser power, laser spot diameter, cutting speed, and number of scans on both the achievable cutting depth and the cutting edge quality was investigated. The ablation depths, the heights of the cutting burr, as well as the removed material volumes were evaluated by means of optical microscope images and cross section photographs. Finally highspeed laser ablation cutting was studied using an intensified ultra highspeed camera in order to get useful insights into the cutting process.