A major reason to conduct solidification experiments in space is that the unique conditions accessible in reduced-gravity allow investigation of fundamental questions while limiting the influence of sedimentation or buoyancy-induced convection. When processing metallic alloys using containerless electromagnetic levitation, convection may be controlled over a wide range, spanning the laminar-turbulent transition, by proper selection of facility operating conditions. By measuring key thermophysical properties such as density, viscosity, and electrical resistivity on-orbit, the specific sample being processed may be characterized and the results used to update pre-mission magnetohydrodynamic model predictions of induced stirring within the droplet. Thus, convection becomes a controlled experimental parameter that can be applied to an investigation of how stirring influences the metastable-to-stable transformation during rapid solidification of FeCrNi alloys. For these alloys, the incubation or delay time is observed to be a weak function of undercooling and a strong function of applied convection.

One of the most important factors during Laser Powder Bed Fusion is a robust and stable process to generate reproducibly dense and crack-free components with the same homogenous and fine-grained microstructure. Among other factors, surface tension influences the robustness of this process. Three aluminum alloys were investigated by means of 3D laser printing with respect to their ability to form lightweight and high-performance structures. The alloys include two Al-Cr based alloys (Zicromal®: Al-Cr-Zr-Mn, Scancromal®: Al-Cr-Sc-Zr) and an Al-Mg based alloy (Scalmalloy®: Al-Mg-Sc-Mn-Zr). The surface tensions of their liquid melts were measured by means of the oscillating droplet method in electromagnetic levitation. Scalmalloy® exhibited a surface tension that was by 3-8% smaller than the other two alloys, over a temperature range of 500 K above their liquidus temperature; the evaporation analysis showed that Scancromal® exhibits significantly less evaporation compared to the other two alloys by an order of magnitude. During the Laser Powder Bed Fusion process, Scalmalloy® presented the most unstable melt dynamics, and Scancromal® appeared to be more robust and stable with less weld spatters and negligible dust deposition, as evidence from the high-speed video observations. The results support the idea that surface tension and evaporation are potentially crucial factors influencing the melt dynamics and also demonstrate the capability of additive manufacturing that produces customized aluminum alloys for aerospace applications.

Surface tension is an essential thermophysical property of liquids, and the oscillating droplet method is particularly effective for investigations involving reactive molten alloys. The Rayleigh equation is commonly used to evaluate surface tension from measurements of the damping frequency response of an oscillating droplet with small deformation, but non-linear effects are expected to arise for larger deformation. This work describes an improved method for interpreting frequency analysis and validates previous numerical simulation and theoretical analyses which predict a decrease in observed frequency at moderate deformation amplitude. Experimental results from microgravity tests are used to determine a correction of the Rayleigh equation to eliminate the influence of finite deformation.

Oscillating droplet experiments are conducted using the Electromagnetic Levitation (EML) facility under microgravity conditions. The droplet of molten metal is internally stirred concurrently with the pulse excitation initiating shape oscillations, allowing viscosity measurement of the liquid melts based on the damping rate of the oscillating droplet. We experimentally investigate the impact of convection on the droplet’s damping behavior. The effective viscosity arises and increases as the internal convective flow becomes transitional or turbulent, up to 2–8 times higher than the intrinsic molecular viscosity. The enhanced effective viscosity decays when the stirring has stopped, and an overshoot decay pattern is identified at higher Reynolds numbers, which presents a faster decay rate as the constraint of flow domain size becomes influential. By discriminating the impact of convection on the viscosity results, the intrinsic viscosity can be evaluated with improved measurement accuracy.

Transitions from the liquid to the solid state of matter are omnipresent. They form a crucial step in the industrial solidification of metallic alloy melts and are greatly influenced by the thermophysical properties of the melt. Knowledge of the thermophysical properties of liquid metallic alloys is necessary in order to gain a tight control over the solidification pathway, and over the obtained material structure of the solid. Measurements of thermophysical properties on ground are often difficult, or even impossible, since liquids are strongly influenced by earth’s gravity. Another problem is the reactivity of melts with container materials, especially at high temperature. Finally, deep undercooling, necessary to understand nucleus formation and equilibrium as well as non-equilibrium solidification, can only be achieved in a containerless environment. Containerless experiments in microgravity allow precise benchmark measurements of thermophysical properties. The electromagnetic levitator ISS-EML on the International Space Station (ISS) offers perfect conditions for such experiments. This way, data for process simulations is obtained, and a deeper understanding of nucleation, crystal growth, microstructural evolution, and other details of the transformation from liquid to solid can be gained. Here, we address the scientific questions in detail, show highlights of recent achievements, and give an outlook on future work.