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

Abstract

Gravity effects such as natural convection in the liquid phase and buoyancy forces acting on the solid phase have a strong influence on the grain structure and microstructure formation dynamics during the solidification of metal alloys. It is thus very useful to undertake experimental studies that will provide benchmark data for a deeper understanding of the role of such gravity effects. In this paper, we study the formation of the equiaxed grain structure during refined Al-20wt.%Cu solidification in a temperature gradient for three different configurations: horizontal, vertical upward and vertical downward solidification. The key grain characteristics, namely grain size, grain elongation and grain growth orientation, were determined for all experiments and a comparative study was performed to identify the dominant effects of gravity 2 for each case. The present study provides quantitative information on the impact of grain flotation and solute flows on the equiaxed microstructure characteristics by means of in situ laboratory X-radiography.

In situ investigation of the Columnar-to-Equiaxed Transition during directional solidification of Al–20 wt.%Cu alloys on Earth and in microgravity

“…The solute plumes impacted the growth of some columnar dendrites by locally increasing the Cu concentration ahead of the dendrite tip and thus retarding their growth. Similar observations have been reported in detail in previous studies [22,26,33,34].…”

“…• Vertical downward solidification (in-gravity direction). In this configuration, both thermal and solutal gradients are destabilising and strong convective flow is expected [22].…”

“…A quantitative analysis was made to determine the grain size and grain elongation distribution of the equiaxed microstructure. The procedure was described in detail in [22,29] and only the main steps are summarised hereafter.…”

“…Over the last couple of decades, X-radiography has become a relevant technique to investigate microstructure formation dynamics in metal alloy solidification [19], and especially gravity-induced effects [20][21][22]. For such studies, synchrotron sources are mainly used because they provide a continuous range of photon energies and an intense X-ray beam [23].…”

“…Recent developments made compact microfocus X-ray sources, X-ray sensitive cameras and optics more efficient, allowing home laboratory alternatives with substantial brilliance to be used [13,24,25]. Solidification experiments could then be performed on ground, in various directions (vertical, horizontal) [22,[25][26][27], and also in microgravity either in sounding rockets or on board of parabolic flights [12][13][14]28].…”