Michael Schmidt’s research while affiliated with Friedrich-Alexander-University Erlangen-Nürnberg and other places

Since the laser based powder bed fusion of polymers process involves previously unknown boundary conditions, process-adapted powder characterization at the particle and powder layer levels is essential. In this chapter, new measurement methods are presented, and the results are discussed within the context of process-adapted boundary conditions for powder flow behavior and optical and thermal properties.

To date, the number of stainless steels available for additive manufacturing (AM) is limited and the range of material properties achievable by AM is inadequate. This limits the industrial applicability of AM technologies such as laser-based directed energy deposition of metals (DED-LB/M). However, the approach of in-situ alloying enables efficient alloy design for AM. This study provides novel insights into the microstructure formation of in-situ alloyed stainless steels manufactured by DED-LB/M. Therefore, 1.4462 duplex stainless steel (DSS) powder was blended with chromium and nickel powder to adjust the chemical composition and stabilize either ferrite or austenite formation. Defect-free specimens were generated and then heat-treated by solution annealing and quenching. Chemical analysis demonstrated that the target composition could be precisely adjusted via in-situ alloying. In-situ alloying of DSS with ferrite- or austenite-stabilizing elements shifted the phase ratio from a ferritic-austenitic (50%:50%) duplex microstructure to a predominantly ferritic (88% ferrite) or a completely austenitic microstructure (100% austenite). The measured phase fractions (electron backscattered diffraction, EBSD) of all compositions investigated were in good agreement with the initially calculated phase equilibria. Optical light microscopy and EBSD analysis also showed that the dominating austenite morphology changes along with the phase ratio. In addition, the phase-specific average grain sizes and textures were significantly influenced by Cr- or Ni-additions. Increasing the ferrite fraction from 50% to 88% enhanced the material hardness from 245 ± 4 HV10 to 270 ± 6 HV10. In contrast, increasing the austenite fraction from 50% to 100% reduced the material hardness to 173 ± 4 HV10.

Surface roughness plays a critical role in ultrashort pulse laser ablation, particularly for industrial applications using burst mode operations, multi-pulse laser processing, and the generation of laser-induced periodic surface structures. Hence, we address the impact of surface roughness on the resulting laser ablation topography predicted by a simulation model and compared to experimental results. We present a comprehensive multi-scale simulation framework that first employs finite-difference-time-domain simulations for calculating the surface fluence distribution on a rough surface measured by an atomic-force-microscope followed by the two-temperature model coupled with hydrodynamic/solid mechanics simulation for the initial material heating. Lastly, a computational fluid dynamics model for material relaxation and fluid flow is developed and employed. Final state results of aluminum and AISI 304 stainless steel simulations demonstrated alignment with established ablation models and crater dimension prediction. Notably, Al exhibited significant optical scattering effects due to initial surface roughness of 15 nm - being 70 times below the laser wavelength, leading to localized, selective ablation processes and substantially altered crater topography compared to idealized conditions. Contrary, AISI 304 with RMS roughness of 2 nm showed no difference. Hence, we highlight the necessity of incorporating realistic, material-specific surface roughness values into large-scale ablation simulations. Furthermore, the induced local fluence variations demonstrated the inadequacy of neglecting lateral heat transport effects in this context.

Overcoming the limitations of powder-based additive manufacturing processes is a crucial aspect for the manufacturing of patient-specific sophisticated implants with tailored properties. Within this work, a novel manufacturing process for the fabrication of polymer-based implants is proposed. This manufacturing process is inspired by the laminated object manufacturing technology and is based on using thin films as raw material, which are processed using an ultrafast laser source. Utilizing thin films as a starting material helps to avoid powder contamination during additive manufacturing, thus supporting the generation of internal cavities that can be filled with secondary phases. Additionally, the use of medical materials mitigates the burden of a later certification of potential implants. Furthermore, the ultrafast laser supports the generation of highly resolved structures smaller than the average layer thickness (from 50 to 100 µm) through material ablation. These structures can be helpful to obtain progressive part properties or a targeted stress flow, as well as a specified release of secondary phases (e.g., hydrogels) upon load. Within this work, first investigations on the joining, cutting, and structuring of thin polymer films with layer thickness of between 50 and 100 µm using a ps-pulsed laser are reported. It is shown that thin film sizes of around 50 µm could be structured, joined, and cut successfully using ultrafast lasers emitting in the NIR spectral range.

Laser-induced Breakdown Spectroscopy (LIBS) has been demonstrated to be an effective tool in industrial contexts for the analysis of a diverse range of materials, including metals, polymers, and the identification of material composition on Mars. Although LIBS has already demonstrated its value in industrial contexts, it remains relatively unexplored in medical applications, where its potential advantages have not yet been fully realized. Building upon the successful implementation of a LIBS system for metal analysis, this research extended the methodology to biological samples. The objective of this study is to evaluate the requisite conditions for calibration-free LIBS (CF-LIBS) analysis of porcine skin. This tissue was chosen because it is similar to human tissue. The three requisite conditions for CF-LIBS are stoichiometric ablation, optical thin plasma, and local thermodynamic equilibrium (LTE). To fulfill these conditions the temperature was evaluated using the Boltzmann plot, and the electron density was determined using the McWhriter and the Saha-Boltzmann equation. All three methods were evaluated for the elements sodium (Na) and calcium (Ca). The results demonstrated that all three conditions for CF-LIBS could be fulfilled for all investigated elements. Therefore, it is promising that CF-LIBS for biological tissue will be possible in the future.

Standard endoscopy of vocal folds is in general limited to two-dimensional imaging. Laser-based 3D imaging offers not only absolute measurements but also the possibility of assessing all three spatial directions. However, due to human inter-individuality, a fixed grid configuration (with fixed edge length and spot size) does not necessarily provide the best coverage and resolution. We present a liquid lens optical design for a diffractive spot array generator with dynamic adjustment capabilities for both array size and spot size. The tunable nature of the liquid lenses enables precise control over the spot array generated by a diffractive optical element (DOE). The first liquid lens controls the spot divergence in the observation plane, while the second liquid lens adjusts the zoom factor. The optical configuration provides a dynamic range of 1.8 with respect to array size, significantly enhancing adaptability in imaging across various applications.

Absorber-free laser transmission welding enables the precise and clean joining of polymer films without absorbing additives or adhesives. For a large process window and thus a robust process, a homogeneous weld seam temperature is required. For this reason, the typically Gaussian intensity distribution of the laser beam was converted into a ring-shaped Donut distribution and a rectangular Flat-top distribution. The donut-shaped intensity distribution leads to a significant homogenization of the weld seam temperature and enlarges the process window. This was confirmed in welding experiments: The process window for welding PP (polypropylene) was expanded by a factor of three. Peel test show that a high strength is achieved if a laser power in the middle or at the upper end of the process window is used.