Spurious grain formation due to Marangoni convection during directional solidification of alloys in µ-g environment of International Space Station

Abstract

During directional solidification of alloys in the microgravity environment of the International Space Station (ISS), growth of dendritic array is expected to be under purely diffusive transport conditions. The resulting microstructure is expected to make up uniformly arranged primary dendrites on sample cross-sections without any defects, such as macrosegregation and spurious (misoriented) grains. However, spurious grains have been seen in recently directionally solidified Al-7 wt% Si samples under the joint NASA-ESA project (MICAST) and also in SCN-0.24 wt% H2O samples grown under the NASA-PFMI project. Careful examination of both these experiments shows the presence of voids at the melt-crucible interface which are the likely source of the convective flows responsible for the fragmentation of dendrite-side arms, and their rotation which creates the nucleus for the spurious grains. In this paper, we assume that side-arm of a primary dendrite has gotten detached from the primary trunk during remelting and isothermal hold, prior to the onset of directional solidification on the Space Station, in the vicinity of a pore at the melt-crucible interface. We numerically simulate the influence of the Marangoni convection on the trajectory of such a dendrite fragment in the alloy melt and compare with the experimental observations. The experimentally observed rotation behavior of the fragmented side-arm in transparent SCN-0.24 wt% H2O, observed from the PFMI video, shows a good agreement with simulation results. The predicted rotational speeds of broken-off secondary arm in the Al-7 wt% Si (MICAST) samples is significantly higher than those in the SCN-0.24 wt% H2O (PFMI). The strength of the convection is dependent on the Marangoni Number and location of the surface pore relative to the Mushy zone. Marangoni convection is strong enough to force the rotation of broken-off side arms and should be considered an important source of microstructural inhomogeneity during terrestrial solidification processing applications.