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Until now, the processes involved in secondary ice production which generate high concentrations of ice crystals in clouds have been poorly understood. However, collisions that involve rimed ice particles or crystal aggregates have the potential to effectively produce secondary ice from their fragmentation. Unfortunately, there have only been a few laboratory studies on ice–ice collision so far, resulting in an inaccurate representation of this process in microphysical schemes. To address this issue, experiments were conducted at the wind tunnel laboratory of the Johannes Gutenberg University, Mainz, on graupel–graupel and graupel–snowflake collisions under still-air conditions at −15 ∘C and water supersaturation. The particles were synthetically generated within a cold room through two distinct methods: by riming and vapor deposition for graupel with diameters of 2 and 4 mm and by manually sticking vapor-grown ice which was generated above a warm bath to form snowflakes with a diameter of 10 mm. All fragments resulting from graupel–graupel collisions were collected and investigated by means of a digital optical microscope, while fragments from graupel–snowflake collisions were observed and recorded instantly after collision using a holographic instrument. From these experiments, distributions were obtained for fragment sizes, cross-sectional areas, and aspect ratios. The results showed a higher number of fragments at lower kinetic energy compared to those presented in the literature. A total of 150 to 600 fragments were observed for graupel–graupel with dendrites collisions, as well as 70 to 500 fragments for graupel–snowflake collisions for collision kinetic energies between 10−7 and 10−5 J. Parameterizations for fragment size distributions are provided with a mode at 75 µm for graupel–graupel with dendrites collisions and at 400 µm for graupel–snowflake collisions. We also propose new coefficients fitted on our experiments to parameterize the number of fragments generated by collisions based on the theoretical formulation of Phillips et al. (2017). These results can be used to improve the representation of ice collision breakup in microphysical schemes.

In powder bed fusion with laser beam (PBF-LB), two process-induced defects by pore formation are known: local spherical pores by the keyhole effect and geometrically undefined pores caused by lack of fusion. Both pore types are heterogeneously distributed and can be used for lightweight or damping design applications. The achievable porosity is limited to around 13%. This article presents a novel process-controlled method enabling the targeted and reproducible manufacturing of solid parts with regularly distributed cavities, currently up to 60% porosity in AlSi10Mg, using the balling effect. This eliminates the need for time-consuming digital pre-processing work.

As part of an international research project (HiPTSLAM), the development and holistic processing of high-performance tool steels for AM is a promising topic regarding the acceptance of the laser powder bed fusion (PBF-LB) technology for functionally optimized die, forming and cutting tools. In a previous work, the newly developed maraging tool steel FeNiCoMoVTiAl was qualified to be processed by laser powder bed fusion (PBF-LB) with a material density of more than 99.9% using a suitable parameter set. To exploit further optimization potential, the influence of dual-laser processing strategies on the material structure and the resulting mechanical properties was investigated. After an initial calibration procedure, the build data were modified so that both lasers could be aligned to the same scanning track with a defined offset. A variation of the laser-based post-heating parameters enabled specific in-situ modifications of the thermal gradients compared to standard single-laser scanning strategies, leading to corresponding property changes in the produced material structure. An increase in microhardness of up to 15% was thus obtained from 411 HV up to 471 HV. The results of the investigation can be used to derive cross-material optimization potential to produce functionally graded high-performance components on PBF-LB systems with synchronized multi-laser technology.