Lopez-D'Angelo O (2021)
Publication Language: English
Publication Type: Thesis
Publication year: 2021
As human reach into space expands, need arises for versatile technologies, working under extreme conditions – notably, in absence of gravity. In-Space Manufacturing (ISM) would allow payload optimisation by enabling on-demand production of tools, spare parts or building elements, increasing self-sufficiency of long-endurance missions. While powder-based three dimensional (3D) printing processes offer high printing quality and adaptability to multiple raw materials, for all techniques available today, constrains remain on feedstock flow properties and recyclability of base-material. The work presented here proposes a new method for handling and additively manufacturing granular materials, independently of the gravitational environment, and emancipating from requirements on powder-feedstock flow properties. Technical evolutions go hand in hand with an understanding of the physical phenomena that underlie them. The development of a powder-based Additive Manufacturing (AM) process was hence preceded by a study of the factors influencing granular rheological response. To be used as demonstrator, a spherical polystyrene (PS) powder was modified to increase its surface roughness, resulting in two powders similar in all respects but their surface state. The link between physical and rheological properties of powders was explored, through typical rheometry tests (flow start-up and stationary flow curves, powder-bed tensile strength), but also through phenomenological testing methods found in industrial applications (notably the flow energy test). Observing and quantifying essential differences in flow-behaviour emerging from this modification of the frictional interactions between particles, allowed to approach the ill-defined concept of powder flowability, to propose a definition which encompasses not only the inherent powder physical properties, but also the environmental factors surrounding the material. To query the influence of such environmental factors, the same powder was used to study the effect of gravity (and absence thereof) on piston-probing of granular material. Using parabolic flight as a microgravity platform, this experiment revealed that, in absence of the secondary force field provided by gravity, powder flow deteriorates and packing fraction at jamming lowers, with dramatic consequences for powder handling in reduced gravity. Results from this first experimental campaign led to the design of the mentioned AM process. To deposit the granular base-material, this process uses solely driving mechanisms in- dependent of gravity (namely shear and shaking). To allow versatility in feedstock material’s flow-behaviour, adaptability of the process printing parameters was implemented through a control loop fed by in-situ probing during powder handling. The material deposition method was first validated through Discrete Element Method (DEM) simulation, showing no dependence of the process efficiency on the gravitational environment, nor on the increased interparticle cohesive forces. Experimental demonstration was also provided, through the construction of two prototype 3D printers, used on-ground and on parabolic flights to manufacture parts from the two powders mentioned above: PS powders of high and mediocre flowability. The successful manufacturing of parts under gravity as well as in weightlessness, from both powders, regardless of their flow properties, represents the main achievement of this work. X-ray computed tomography (CT) analysis of the sintered parts showed no systematic change in porosity between the samples manufactured under different gravitational environments, with near-perfect density and isotropic porosities. Parts were also produced directly from recycled material (under 1g), obtained by closed-loop recycling of former 3D printed parts; an achievement that no commercially available additive manufacturing process to date could realise, and that will be a critical asset for increasing the self-sustainability of space exploration missions. The work presented lead to filing a patent in September 2020, endorsed by the German Aerospace Center, under the application number 10 2020 123 753.7 for an “Apparatus and Method for Additive Manufacturing of Components in Environments with Various Gravitation- levels and with Materials of Different Flowability”.
Lopez-D'Angelo, O. (2021). Powder-based Additive Manufacturing for Space (Dissertation).
Lopez-D'Angelo, Olfa. Powder-based Additive Manufacturing for Space. Dissertation, 2021.