Lars Hartwig

Selective Laser Micro Sintering with a Novel Process

Microparts with a structural resolution of <30μm and aspect ratios of >12 have been generated by selective laser sintering. The technique includes sintering under conditions of vacuum or reduced shield gas pressures. In a novel set-up the material is processed by a Q-switched 1064nm Nd-YAG laser after a special raking procedure. The procedure allows the work pieces to be generated from powders of high melting metals like tungsten as well as lower melting metals like aluminium and copper. Contingent on the parameters, the generated bodies are either firmly attached to the substrate or can be dissevered by a non-destructive method.

Process assembly for μm-scale SLS, reaction sintering, and CVD

A novel device suited for the generation of sintered microparts of metal and ceramics, for reaction sintering and for CVD has been developed and successfully tested. With the production of a functional component it has evidenced professional performance. The set-up is vacuum tight; unstable substances can be processed under various shield gases and pressures; it is equipped with a device suited to rake thin layers of fine powders as well as slurries. Sub micrometer powder can be processed in steps of 1 μm thick sintered layers. In combination with a proprietary sintering regime, micro parts with a structural resolution of <30μm, and aspect ratios of >10 have been achieved.

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...

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...

Laser microsintering of tungsten in vacuum

Laser microsintering of tungsten powder is investigated as a function of laser output power, pulse interval and vacuum level. The intensities are calculated for the evaporation thresholds of tungsten powder particles of various sizes. In addition, the powder layer generation and the resulting layer thicknesses are calculated. The powder abrasion occurring during the process was taken into consideration. Polished sections and REM images were prepared in order to analyse the experimental outcomes. The dependence of sinter density on the parameters is discussed.

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.

Highspeed laser welding of steel using a high-power single-mode continuous-wave fiber laser

Since a few years, high brilliance laser sources find their way into laser material processing. Laser micro processing by applying high brilliance laser radiation up to 3 kW of continuous wave laser power in combination with ultrafast beam deflection systems has been successfully demonstrated in 2008 for the first time. In the fields of laser welding, high brilliant laser radiation was mainly used for micro welding, but up to now the macro range is still insufficiently investigated. Hence, this study reports on detailed investigations of high speed laser welding of different steel grades, performed with a high power single mode fiber laser source. The laser beam was deflected relative to the sample by using both a fast galvanometer scanner system with f-theta focusing objective and a linear axis in combination with a welding optic, respectively. In the study, the mainly process influencing parameters such as laser power, welding speed, thickness of the metal sheets, angle of incidence and laser beam spot size were varied in a wide range. The weld seam quality was evaluated by structural analyses, static tensile tests and EDX measurements. Finally, the laser welding process has been optimized for different weld seam geometries, for example bead-on-plate welds and butt welds.

Quality of remote cutting

Remote cutting is stated to be a very fast laser cutting process for the cutting of thin materials. As the main characteristics of remote cutting have been studied only in past couple years, the quality of the cut is still not well compared and specified against other processes. Process parameters of remote cutting differ from the traditional laser cutting because of the multi-pass and vaporizing characteristics of the process. Process parameters and performance also vary depending on the used equipment and processed material. Parameters have to be optimized for each set up to achieve the best cut quality.Quality of laser cut kerf can be measured in various ways. In this study, various metals were processed using remote cutting. The quality of the cut kerfs were measured by burr height, kerf width, perpendicularity tolerance, surface roughness, 90 degree corner sharpness, and heat affected zone. The quality of the cut kerf is also compared to those of conventional CO2 laser cutting and remote fusion cutting with multimode fiber laser. When possible the comparison was carried out according to valid standard of thermal cutting.Remote cutting is stated to be a very fast laser cutting process for the cutting of thin materials. As the main characteristics of remote cutting have been studied only in past couple years, the quality of the cut is still not well compared and specified against other processes. Process parameters of remote cutting differ from the traditional laser cutting because of the multi-pass and vaporizing characteristics of the process. Process parameters and performance also vary depending on the used equipment and processed material. Parameters have to be optimized for each set up to achieve the best cut quality.Quality of laser cut kerf can be measured in various ways. In this study, various metals were processed using remote cutting. The quality of the cut kerfs were measured by burr height, kerf width, perpendicularity tolerance, surface roughness, 90 degree corner sharpness, and heat affected zone. The quality of the cut kerf is also compared to those of conventional CO2 laser cutting and remote fusion cutti...