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

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Publications (555)


Fig. 4.1 a Bulk density and b powder tensile strength of layers of glass microspheres with varying particle size distribution
Fig. 4.2 Schematic illustration of a dry coating process with additives as guest particles [3]
Fig. 4.4 Maximum temperature reached during exposure depending on parameters (a), second heating after exposure (b), and the process matrix for the evaluation of suitable process parameters (c) for PA11
Fig. 4.5 Temperature profile during exposure and corresponding heating rates for PA12, b TGA analysis for PA12, crystallization curve after laser exposure for different impact times for a laser power of 3.8 W for degradation statements [34]
Fig. 4.6 Heat flow for varied isothermal crystallization (a), and progress of degree of crystallinity (b) for PA12 [35]

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Process Adapted Multiscale Material Characterization
  • Chapter
  • Full-text available

May 2025

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27 Reads

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Michael Schmidt

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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.

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Graphical Abstract
Microstructure evolution of in-situ alloyed and heat-treated stainless steels processed by DED-LB/M

May 2025

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36 Reads

Materials Today Communications

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Sebastian Frankl

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Michael Schmidt

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.



From surface roughness to crater formation in a 2D multi-scale simulation of ultrashort pulse laser ablation

April 2025

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78 Reads

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.


Stack and Structure: Ultrafast Lasers for Additive Manufacturing of Thin Polymer Films for Medical Applications

April 2025

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18 Reads

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.



Preliminary results for calibration-free laser-induced breakdown spectroscopy (CF-LIBS) for the elemental analysis of biological samples

March 2025

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13 Reads

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.



Liquid Lens Optical Design for Adjustable Laser Spot Array for the Laser-Based Three-Dimensional Reconstruction of Vocal Fold Oscillations

March 2025

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22 Reads

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.


Laser welding of polymer foils with spatially adapted intensity distributions

February 2025

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6 Reads

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.


Citations (61)


... In [17], Pham et al. tackle this issue in FD-DOS by showing that calibration factors measured at a single wavelength can be applied to other system wavelengths, reducing the need for additional source positions required by traditional self-calibrating geometries. Also aimed at quantifying both absorption and scattering, Ni et al. describe and demonstrate a novel technique that treats the medium like a random laser, but requires adding a fluorescent dye to provide optical gain [18]. On the imaging side, Kangasniemi et al. integrate Monte Carlo methods into diffuse optical tomography (DOT) for quantification of tissue optical properties in regions where the standard diffusion approximation is not valid [19]. ...

Reference:

Diffuse Optical Spectroscopy: Technology and Applications: introduction to the feature issue
Towards a sensing model using a random laser combined with diffuse reflectance spectroscopy

... From the measured transmitted and reflected power, the transmittance (Trans) and reflectance (Sca) of the laser radiation were determined. The exact formulas and their derivations can be found elsewhere [85]. The absorptance (Abs) was then calculated as the difference between 100% and the transmittance and reflectance/scattering (Abs = 100% − Sca − Trans). ...

Light matters: Quantifiable optical interaction between near-infrared laser radiation and nano-additivated polymers in all states of matter present in laser powder bed fusion

Additive Manufacturing

... According to Dioguardi et al. [2], Martens [18], and Esmaeili et al. [19], the applicability of LLLT in dentistry is consolidated, as a safe and effective option to minimize pain, discomfort, and bleeding in the patient during and after the surgical procedure, as LLLT is capable of transmitting the energy of photons to specific cells of the tissues called chromophores that will absorb the energy at different wavelengths causing effects such as heating, photoablation, photodisruption, coagulation, protein denaturation, vaporization, and carbonization resulting in the clinical signs that we see as precise cutting and hemostasis [13]. Although the benefits of LLLT are well documented in the literature, Hohmann et al. [20] report that there is still a lack of standardization in the parameters and types of semiconductors for laser surgery, in order to understand the best parameters for performing labial frenectomies avoiding complications and expanding the patient's prognosis in terms of pain and discomfort. ...

Relevant parameters for laser surgery of soft tissue

... The material response of AISI 304 is simulated with a TTM-SM model using the FEM software COMSOL (version 5.6) containing relevant EOS as HD simulations are not available. Computational details about this method can be found in a prior work [27] for calculations and verifications of Al simulations. Here, this methodology was implemented for AISI 304 with numerically interpolated thermodynamical and optical parameters taken from [24] and the EOS from the SESAM database [28], which is shown in the appendix. ...

Two-Temperature Solid Mechanics Model incorporating an Equation of State for Readily Accessible Ultrashort Pulse Laser Ablation Simulations

... For the preparation of WC-np AISI H11 powder blends, a multi-stage dry-coating approach was used. The dry-coating process was already developed and successfully evaluated in a previous study in which the processing of carbon black nanoparticles reinforcing AISI H11 powder mixtures was fundamentally proven [33,44]. Subsequently, the sub-processes involved in the dry-coating procedure are listed in bullet points. ...

Processing of Carbon Nanoparticle-Enriched AISI H11 Tool Steel Powder Mixtures in DED-LB/M for the AM of Forging Tools with Tailored Properties (Part II): Influence of Nanoscale Carbon Additives on Microstructure and Mechanical Properties

Alloys

... Beam shaping based on diffractive optical elements enables the generation of spatial light patterns that can serve as markers for the observation of the tissue's motion. Depending on what modality is to be observed, e.g., photoacoustics [1], vocal fold oscillations [2], etc., different properties of the back-scattered light should be used to evaluate the motion of the tissue. While in the case of vocal fold, 2D imaging techniques such as stroboscopy, videokymography [3], phonovibrogram [4], or high-speed videoendoscopy do not sufficiently reflect the complex 3D vocal fold dynamics (see Figure 1), the observation of the apparent motion of the spots of a laser spot array through stereo-triangulation can be used to determine the 3D motion of the vocal folds [5]. ...

Full-optical photoacoustic imaging using speckle analysis and resolution enhancement by orthogonal pump patterns projection

... Additionally, the setup was equipped with an FTIR modular unit with λ = 1.3-25 µm to analyze spectroscopically accessible structural changes in the material, such as isothermal crystallization. Further details concerning this measurement setup are provided in the publications of Schuffenhauer et al. [30] and Marschall et al. [20]. ...

Holistic Characterization of PBF-LB/P Powder Regarding Isothermal Crystallization, Rheology and Optical Properties Under Process Conditions
  • Citing Chapter
  • September 2023

... ( ) Electron thermal conductivity for three different lattice temperatures. ( ) Electron-phonon coupling factor, model described in [18]. ( ) Lattice heat capacity, both solutions obtained from numerically integrating in eqn. ...

Resolving transient temperature and density during ultrafast laser ablation of aluminum

Applied Physics A

... Gear teeth, camshafts, and some other automobile components need good surface hardness to resist wear, in combination with considerable interior toughness to withstand impact [2][3][4]. These components are generally manufactured by surface or case hardening of mild steel. ...

Development of a novel wear-resistant WC-reinforced coating based on the case-hardening steel Bainidur AM for the substitution of carburizing heat treatments
  • Citing Article
  • September 2023

Journal of Materials Research and Technology

... Regulation of the energy distribution and wavefront of the pulses provides control over the diameter and specific resistance of the created graphite structures, contributing to increased accuracy and efficiency of radiation-resistant detectors. In [127], methods for forming multiple non-overlapping spot arrays with minimized mutual influence of the points are described. ...

Spot arrays for uniform material ablation with ultrashort pulsed lasers
  • Citing Article
  • August 2023

Optics & Laser Technology