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Metallic samples with unique micro- and nano-scale surface structures can easily be fabricated with Direct Laser Interference Patterning. Like in all laser processes, the material interacts with the laser radiation and as a result, thermal effects occur. These effects have a significant influence on the resulting quality of the surface patterns. In this study, the thermal effects occurring during Direct Laser Interference Patterning of stainless steel and aluminum sheets are investigated. The used experimental setup consisted of a picosecond pulsed laser source operating at 532 nm wavelength, combined with a two-beam interference optical head. An infrared camera in an off-axis position is used to detect the resulting thermal radiation of the laser process varying different process parameters such as laser power and repetition rate. The obtained results reveal a correlation between the recorded signal by the infrared camera and the reached surface quality. They show an impact of the thermal effects on the quality of the surfaces and the amount of solidified material on the resulting line-like pattern. Threshold values of the detected infrared signal detected are determined to classify the obtained surface conditions.
Recently, process monitoring emerges as a breakthrough technology in industrial laser machines applications to enhance process stability and economic efficiency while ensuring high-quality pro-cessing parts and significantly reducing scrap rate. Furthermore, the latest advances in monitoring systems open a broad range of new opportunities to increase the capabilities of laser surface structur-ing. In this study, stainless steel and aluminum substrates are structured with a line-like geometry by Direct Laser Interference Patterning. A high-speed infrared camera is used to detect the thermal effects throughout the laser process. Simultaneously, a diffraction measurement system is implemented to analyze the quality of the fabricated periodic patterns by comparing the diffraction order characteris-tics. This specific combination of the systems allows a remarkably high-performance process moni-toring and quality assurance. The obtained results reveal a correlation between the signals detected by the infrared camera and the intensity of the diffraction orders recorded with the quality of the surface reached.
Recently, monitoring systems have become crucial components in industrial-scale laser machines to increase process reliability and efficiency. Particularly, monitoring methods have the potential to optimize and ensure the quality of laser surface patterning by indirectly characterizing the surface topography. Here, a diffraction measurement system, based on scatterometry, is used to determine the mean depth of laser-induced periodic surface structures (LIPSS) on stainless steel by analyzing the characteristics of the resulting diffraction patterns. To this end, LIPSS were produced with a ps-pulsed laser system operating at a wavelength of 1064 nm. The results reveal that the mean depth of LIPSS can be extracted from the intensity of the captured diffraction orders down to approximately 14 nm. This compact monitoring tool can be easily adapted to industrial-scale laser systems to improve the quality control and stability of surface microtexturing processes.
The combination of direct laser interference patterning (DLIP) with laser-induced periodic surface structures (LIPSS) enables the fabrication of functional surfaces reported for a wide spectrum of materials. The process throughput is usually increased by applying higher average laser powers. However, this causes heat accumulation impacting the roughness and shape of produced surface patterns. Consequently, the effect of substrate temperature on the topography of fabricated features requires detailed investigations. In this study, steel surfaces were structured with line-like patterns by ps-DLIP at 532 nm. To investigate the influence of substrate temperature on the resulting topography, a heating plate was used to adjust the temperature. Heating to 250 ∘C led to a significant reduction of the produced structure depths, from 2.33 to 1.06 µm. The reduction is associated with the appearance of a different LIPSS type, depending on the grain orientation of the substrates and laser-induced superficial oxidation. This study revealed a strong effect of substrate temperature, which is also to be expected when heat accumulation effects arise from processing surfaces at high average laser power.
Lightweight materials such as aluminum alloys have an important role to play in weight reduction. However, their limited formability at room temperature poses a major challenge and restricts their use. Significant improvements in formability can be achieved by heat-assisted forming processes. However, this improvement in formability is generally associated with a change in microstructure that leads to a reduction in strength. Alternatively, improved ductility and formability can be achieved at cryogenic temperatures without the disadvantages of warm forming processes. In this project, the focus is on developing a new process for forming aluminum alloys at cryogenic temperatures without active cooling. For this purpose, macro-structured tools are used to reduce the contact area between the tool and the blank. The aim is to minimize the heat flux to the blank to maintain low temperatures during forming. This is to take advantage of the improved formability of aluminum alloys at cryogenic temperatures and thus extend the process window for deep drawing of aluminum alloys. This data collection contains material characterization data required for numerical process modelling. The focus is on the characterization of thermal and mechanical properties at blank temperatures.
High-speed processes can lead to significant technological advantages like an increased formability, reduced springback or an improved quality of cutting edges. For conventional forming processes, quasi-static conditions are a good approximation and numerical process optimisation is state of the art. However, there is still a need for research in the field of material characterisation for high speed forming and cutting processes. Production technologies with high velocities leads to high strain rates and the dependency of strain hardening and failure behaviour on the forming velocity cannot be neglected. Therefore, the data of the material behaviour at high strain rates is required for modelling high velocity processes. The challenge here is the measurement of relevant process quantities due to short process time that requires a very high sampling rate and the limited size and accessibility of the specimen. In this context, an inverse method for determining material characteristics at high strain rates was developed. The approach here is the measurement of auxiliary test parameters, which are easier to measure and then used as input data for an inverse numerical simulation. Two devices were implemented for different ranges of strain rates: a pneumatically driven device for strain rates up to 1.000 1/s and an electromagnetically driven accelerator for strain rates up to 100.000 1/s. The method developed by Psyk et al. is presented in detail in the contribution "Determination of Material and Failure Characteristics for High-Speed Forming via High-Speed Testing and Inverse Numerical Simulation". https://doi.org/10.3390/jmmp4020031. In order to test and comprehend the inverse method for material characterisation the experimental data and the FE-model (LS-Dyna) are presented in case of the electromagnetically accelerated unit. The experimental data are the displacement curve of the flyer and the recorded elastic strain curve of the solid rod for determining the force. The FE-model contains the whole test setup (flyer, specimen, measurement rod) and the determined flow curves as well as the data for the damage behaviour.
In high strain rate forming processes two superposing and opposing effects influence the flow stress of the material: strain rate hardening and thermal softening due to adiabatic heating. The presented FE-model and experimental results are based on https://doi.org/10.3390/app12052299 where uniaxial tensile tests at different high strain rates are analyzed experimentally and numerically to understand the influence of adiabatic heating of the workpiece during deformation under high-speed loading. A thermal camera and a pyrometer were used for temperature measurement in the fracture region in addition to the measurement of force and elongation. The numerical simulations are carried out in LS-Dyna using the GISSMO model for modeling damage and failure.