Furnace development for TEXUS and MAXUS: Directional Solidification of Al-cast alloys in ARTEX

Abstract

A new experimental design for directional solidification experiments under microgravity conditions was developed at DLR: The aerogel-based furnace ARTEX. This setup has demonstrated during the sounding rocket campaign TEXUS 39 that a furnace equipped with fragile aerogels as a crucible material can be used in microgravity. The scientific goal of the experiment consisted in establishing diffusive solidification conditions in an Al-6wt.% Si alloy in order to compare convection free growth in space with results on earth. Since heat and mass transport determines fully the solidification microstructure, it is essential to perfectly control both in a controlled manner. One extreme is experimentation under microgravity conditions, in which all sources of fluid flow are eliminated; the other extreme is microgravity with an artificial fluid flow being fully controlled by the experimenter. The first move was done with the experiment on TEXUS 39. Now ARTEX is developed further to an ARTEXplus facility flown on TEXUS 41 (December 2004). Three pairs of coils around the sample can induce a homogeneous rotating magnetic field being able to generate a controlled fluid flow in the melt. The scientific objective was defined by the ESA MAP project MICAST studying the influence of controlled convection on the microstructure evolution. The experiment on TEXUS 41 leads to a direct comparison of the effect of controlled convective conditions and pure diffusive growth conditions (TEXUS 39) on microstructure development using the same alloy. The samples solidified in microgravity are evaluated with regard to the processing parameters and to the primary dendrite spacing and secondary dendrite arm spacing. These results are compared with the microstructure from lab experiments – samples solidified with and without the influence of a rotating magnetic field over a range of velocities – and accepted steady-state growth models. The primary spacing decreases with increasing fluid flow whereas the secondary dendrite arm spacing increases. Both furnace facilities ARTEX and ARTEXplus are foreseen for reflight in May 2006 on the sounding rocket MAXUS 7. The forthcoming flight experiments will be carried out in 12-15 minutes microgravity time leading to half the solidification velocity of the TEXUS flights. From the laboratory experiments and the (re-)flights of the ARTEX and ARTEXplus facilities with a rotating magnetic field device an experimental data base will be available which can be used for numerical calculations of dendrite growth under convective conditions. This will be an important step towards a better theoretical understanding of the influence of convective transport processes on the microstructure formation.