In-orbit additive manufacturing (AM) is a promising approach for fabrication of large structures. It allows to expand and accelerate human space exploration possibilities. Extrusion-based AM was demonstrated in zero gravity, while the realization of such a process in orbit-like vacuum conditions is currently under exploration. Still, a solution for protection of the UV and IR radiation sensitive polymers is needed in order to prevent their early mechanical failure under space conditions. Vacuum arc plasma based process is widely applied on earth for thin protective coating deposition. Its major advantage is its scalability—from tiny size used in electric propulsion to large scale coating devices. The usability of the vacuum arc process in space conditions was shown in electric propulsion applications in nano-satellites. In this work we discuss and demonstrate the integration of vacuum arc process as a post processing step after Fused Filament Fabrication (FFF) for additive manufacturing and functionalization of long polymer structures. Here we address the concept for technical realization, which integrates the vacuum arc into additive manufacturing process chain. More over we present a laboratory prototype, which implements this concept together with a use case, where a previously printed PEEK structure is coated with aluminum based coating suitable for UV radiation protection.

The diagnostics of atmospheric welding plasma is a well-established technology. In most cases the measurements are limited to processes using pure shielding gas. However in many applications shielding gas is a mixture of various components including metal vapor in gas metal arc welding (GMAW). Shielding gas mixtures are intentionally used for tungsten inert gas (TIG) welding in order to improve the welding performance. For example adding Helium to Argon shielding gas allows the weld geometry and porosity to be influenced. Yet thermal plasmas produced with gas mixtures or metal vapor still require further experimental investigation. In this work coherent Thomson scattering is used to measure electron temperature and density in these plasmas, since this technique allows independent measurements of electron and ion temperature. Here thermal plasmas generated by a TIG process with 50% Argon and 50% Helium shielding gas mixture have been investigated. Electron temperature and density measured by coherent Thomson scattering have been compared to the results of spectroscopic measurements of the plasma density using Stark broadening of the 696.5 nm Argon spectral line. Further investigations of MIG processes using Thomson scattering technique are planned.

In-space additive manufacturing (AM) offers significant potential to expand human space exploration beyond low Earth orbit and the moon. Although extrusion-based AM has proven feasible in zero gravity, the functionality of such a process in orbit-like vacuum conditions with practically no convective heat loss remains barely explored. To this end, a Fused Filament Fabrication (FFF) system was designed that successfully operated in high vacuum at 10^− 4 mbar where convective heat transfer is negligible. Polylactic acid (PLA) tensile specimens were fabricated in three orthogonal print orientations under high vacuum conditions. Tensile testing, scanning electron microscopy, and micro-computed tomography were employed to assess tensile strength, elongation at break, void content, and the effects of vacuum on thermal dissipation. The objective was to evaluate how the vacuum environment influences the anisotropy of mechanical properties in printed parts. The absence of convective losses improved the layer bonding strength of specimens printed in the vertical (z) direction with load applied perpendicular to the filament strand orientation (V90), compared to specimens printed in horizontal (x or y) direction with load applied perpendicular to filament strand orientation (H90). This was attributed to the considerably slower cooling process in the case of V90 specimens. Moreover, thermal insulation provided by the vacuum environment had a beneficial influence on heat break and hot end. In vacuum, the set extrusion temperature was closer to the temperature measured at the tip of the nozzle. This study also identifies a temperature undershoot during decrease in hot end temperature in the printing process, which adversely affected interlayer adhesion. The findings indicate that the current PID-based hot-end control requires optimization to enhance the temperature control rate and minimize thermal deviations. These insights contribute to a deeper understanding of thermoplastic processing under convection-free conditions using fused filament fabrication (FFF).

The application of fused filament fabrication (FFF) in vacuum changes the heat transfer of the process. This work investigates the influence of the working ambient pressure conditions in FFF-based 3D printing of polyetheretherketone (PEEK) specimens, and its impact on the resulting part strength. Layer adhesion drastically improves with decreasing pressure, maximum layer adhesion is reached for ambient pressure below $$10^{-3}$$ 10 - 3 mbar. We show that simple and low-cost vacuum equipment is sufficient to achieve such pressure conditions, making this process interesting for the general processing of high-temperature polymers using FFF.

A simple and time efficient model for atmospheric arc plasmas is presented in this work. It uses experimentally recorded emission spectra in order to estimate radial temperature and electron density profiles in welding arc plasmas. The plasma parameters are calculated using the Elenbaas–Heller and Saha equations, together with welding current, gas composition and simplified cylindrical arc geometry with a particular radius as input parameters. From these parameters an emission spectrum is calculated under the assumption, that the individual lines are mainly broadened due to the Stark effect. Here only second order perturbation theory for the Stark effect without any further influences of ion collision or other additional effects is considered. The comparison of measured and calculated spectra leads to a cost function. Its minimisation is used to find the radius of the arc model. This model was successfully applied for the estimation of plasma parameters in gas tungsten arc welding processes with argon and argon–helium as shielding gases. The calculated gas temperature and electron density profiles were compared with experimental data obtained by Thomson scattering. Further extension of the model for dynamic processes containing metal vapours and its application in feedback control are planned in the future.