Overview of In Situ X-Ray Studies of Light Alloy Solidification in Microgravity

Abstract

Gravity has significant effects on alloy solidification, primarily due to thermosolutal convection and solid phase buoyancy. Since 2004, the European Space Agency has been supporting investigation of these effects by promoting in situ X-ray monitoring of the solidification of aluminium alloys on microgravity platforms, on earth, and in periodically varying g conditions. The first microgravity experimentinvestigating foaming of liquid metalswas performed on board a sounding rocket, in 2008. In 2012 the first ever X-raymonitored solidification of a fully dense metallic alloy in space was achieved: the focus was columnar solidification of an Al-Cu alloy. This was followed in 2015 by a similar experiment, investigating equiaxed solidification. Ground reference experiments were completed in all cases. In addition, experiments have been performed on board parabolic flightswhere the effects of varying gravity have been studied. We review here the technical and scientific progress to date, and outline future perspectives.

Impact of gravity on directional solidification of refined Al-20wt.%Cu alloy investigated by in situ X-radiography

“…The Indeed, in horizontal configuration buoyancy effects were strongly minimized by the confinement of the grain flotation in the small thickness of the sample. Thus, the grain motion was significantly restricted as reported in [24,37] and numerically studied by [53]. In a same way, thermally and solutally induced convection was also expected to be significantly limited, as previously discussed by Nguyen-Thi et al using an order of magnitude analysis [50].…”

“…Directional solidification experiments were performed in the SFINX (Solidification Furnace with IN Situ X-radiography) laboratory device, which is a duplicate of the facility used during several parabolic flight campaigns [36] as well as during the sounding rocket experiment MASER-12 [17]. These facilities were developed within the ESA (European Space Agency) project entitled XRMON (X-ray MONitoring of advanced metallurgical processes under terrestrial and microgravity conditions) to study the microstructure formation dynamics of metal alloys onboard of microgravity platform using X-radiography [37]. A detailed description of those facilities has been given in our previous papers [17,19].…”

Impact of growth velocity on grain structure formation during directional solidification of a refined Al-20 wt.%Cu alloy

“…However, as solidification velocity increases, mechanical blocking is increasingly observed due to grain motion toward the cold side of the sample. In this case, the grains are transported by fluid flow induced by solidification shrinkage as recently reported for solidification experiment carried out during the MASER-12 and MASER-13 sounding rocket experiments in microgravity [39,40,48].…”

In situ observation of the impact of hydrogen bubbles in Al–Cu melt on directional dendritic solidification

“…Due to the thin (150 lm) sample geometry, the microstructure can be distinctly observed, as the dendrites usually do not superimpose each other, while at the same time, convective flow and buoyancy are reduced. It has been shown that the geometrical constraints in the nearly two-dimensional specimen, if oriented horizontally, effectively restrict gravitational effects such as buoyancy on bubbles and dendrites, and, to a certain extent, convection [22,34].…”