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PublicationPrecision Glass Molding of infrared optics with anti-reflective microstructures( 2020)
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PublicationOptimized Temperature Distribution for Laser Hardening with Freeform Mirrors( 2019)Laser beam shaping by the usage of freeform mirrors for laser hardening is a known technology which has been used for CO2-Lasers. Nevertheless, due to developments in manufacturing technologies and computer simulation the industrial relevance has increased recently. By now, it is possible to manufacture freeform mirrors precise enough for the diode laser wavelength. With these mirrors, complex intensity distributions can be generated. Combined with a solution of the Inverse Heat Conduction Problem, the temperature distribution in the process zone can be designed according to the process necessities. This study will compare the results of three different profiles which were designed to optimize the process. The first optic generates a rectangular isothermal profile. The second is a rectangle with a reduction of temperature in the direction of the movement to avoid grain growth. The third is optimized to increase the desired compressive residual stresses.
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PublicationPulsed Laser Influence on Two-Beam Laser Metal Deposition( 2019)While pulsed lasers are generally used to process solid materials, there are other technologies based on interaction between pulsed lasers and liquids. One example is the double-beam laser brazing where material is molten by a cw-laser and spread using a pw-laser. This technology can be transferred to enhance process stability during laser metal deposition by adjusting the force applied on the liquid surface. It is known that the liquid's dynamic is controlled by the pressure caused by vaporization and opposed influences such as surface tension and gravity, but the magnitude of the force applied on the surface by the pw-laser remains unknown. This study tries to quantify the mean value of this force over time by placing a metal ball into the laser beam. The whole system is mounted on a stage, which is gradually tilted during the experiment. When the gravity forces exceed the pulsed laser pressure, the ball rolls down. Thus, laser parameters can be correlated to acting forces.
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PublicationReplicative manufacturing of glass optics with functional microstructures( 2019)Microstructuring of glass optics enables a large variety of benefits for miscellaneous fields of application. From an enhancement of the performance of optical systems to the haptic improvement of coverglasses the advantages of structured glass are obvious. Especially in the field of high-precision optics, microstructured optical surfaces can carry out important functions, such as beam shaping in laser systems or the correction of dispersive color alterations. Besides enhancements regarding optics of the visible light spectrum, microstructures can compensate disadvantages of infrared(IR)-transmissive lenses such as chalcogenide glasses. As these optics suffer high transmission losses due to their high refractive index the integration of an anti-reflective (AR) function is necessary. Moth-eye-structures are a promising way to avoid the currently used AR-coatings. So far, microstructures are brought into the lens surface by lithography mainly. The therefore additional processing step follows the previous shaping. An efficient production of the structured components is the key to success for applications aside science and research. The technology precision glass molding (PGM) is able to combine the contradicting aspects of high precision and high volume production. PGM is a replicative manufacturing method that allows the macroscopic molding and the manufacturing of microscopic structures to be carried out simultaneously. Based on a representative PGM process chain, the paper at hand describes differences, challenges and current research results regarding molding microstructures.
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PublicationNumerical-experimental investigation of load paths in DP800 dual phase steel during Nakajima test( 2018)Fuel efficiency requirements demand lightweight construction of vehicle body parts. The usage of advanced high strength steels permits a reduction of sheet thickness while still maintaining the overall strength required for crash safety. However, damage, internal defects (voids, inclusions, micro fractures), microstructural defects (varying grain size distribution, precipitates on grain boundaries, anisotropy) and surface defects (micro fractures, grooves) act as a concentration point for stress and consequently as an initiation point for failure both during deep drawing and in service. Considering damage evolution in the design of car body deep drawing processes allows for a further reduction in material usage and therefore body weight. Preliminary research has shown that a modification of load paths in forming processes can help mitigate the effects of damage on the material. This paper investigates the load paths in Nakajima tests of a DP800 dual phase steel to research damage in deep drawing processes. Investigation is done via a finite element model using experimentally validated material data for a DP800 dual phase steel. Numerical simulation allows for the investigation of load paths with respect to stress states, strain rates and temperature evolution, which cannot be easily observed in physical experiments. Stress triaxiality and the Lode parameter are used to describe the stress states. Their evolution during the Nakajima tests serves as an indicator for damage evolution. The large variety of sheet metal forming specific load paths in Nakajima tests allows a comprehensive evaluation of damage for deep drawing. The results of the numerical simulation conducted in this project and further physical experiments will later be used to calibrate a damage model for simulation of deep drawing processes.
