Solidification of hypereutectic Al-38 Wt Pct Cu alloy in microgravity and in unit gravity

Abstract

Solidification in microgravity aboard the space shuttle Endeavour resulted in a dramatic change in the morphology of the primary Al 2 Cu phase compared to ground-based solidification in unit gravity. An Al-38 wt pct Cu ingot directionally solidified at a rate of 0.015 mm/s with a temperature gradient of 1.69 K/mm exhibited large, well-formed dendrites of primary Al 2 Cu phase. Ingots solidified under similar conditions in unit gravity contained primary Al 2 Cu phase with smooth, faceted surfaces. The primary Al 2 Cu phase spacing in the microgravity ingot was much greater than that in the unit gravity ingot, 670 m compared to 171 m. It is suggested that thermosolutal mixing in the unit gravity ingot reduces the buildup of an Al-rich layer at the solid/liquid interface, which increases the stability of the interface resulting in smooth, faceted particles of Al 2 Cu phase. It is also suggested that the large difference in primary phase spacings is due mostly to the difference in morphology rather than changes in parameters that might influence dendrite ripening mechanisms. The presence or absence of gravity had no effect on the interlamellar spacing of the inter-Al 2 Cu phase eutectic. The ingot solidified in microgravity exhibited almost no longitudinal macrosegregation, in agreement with the theory of inverse segregation in the absence of thermosolutal convection. The ingot solidified in unit gravity exhibited considerable longitudinal macrosegregation, with the chilled end having about 6 wt pct more Cu than the average composition. It is not clear whether the segregation results from thermosolutal convection during solidification or from sedimentation during melting.

Mushy zone morphology during directional solidification of Pb-5.8 wt pct Sb alloy

“…This plot indicates that convection remarkably reduces primary dendrite spacing. This observation is in agreement with low gravity exper-REFERENCES iments [31,32] where primary spacings were observed to be about 4 times larger in the low gravity specimens as com-…”

“…The extent of decrease in primary dendrite spacing is indicated by the ratio, ( exp : th ), which is directly related to the increasing value of this parameter. A decrease in the primary V. CONCLUSIONS dendrite spacing due to convection has also been observed in Pb-Sn [15] and Al-Cu [31,32] alloys. The following conclusions can be drawn from this study In order to explain the influence of convection in the where entire mushy zone morphology, from the tip of the interdendritic region on primary spacing, Doupouy et al [33] cellualr/dendritic array to its base, was examined in direcintroduced a convection parameter given below tionally solidified and quenched Pb-5.8 wt pct Sb alloy.…”

A mushy-zone rayleigh number to describe interdendritic convection during directional solidification of hypoeutectic Pb-Sb and Pb-Sn alloys

“…[15] It also appears to reduce the primary dendrite spacing. [13][14][15][16][17][18] Numerical simulations of the transport phenomena during alloy solidification have shown some success in predicting some of the solidification defects in metal alloys, [19][20][21][22] but they have two serious limitations. First, the two-order-of-magnitude difference between the thermal and solutal length scales makes the three-dimensional computation very time consuming, and, second, the interdependence of the mushy-zone morphology, its permeability, and its convection does not allow an exact analysis of the problem.…”

The Effects of Thermosolutal Convection on Macrosegregation during Alloy Solidification

“…As microgravity conditions can be experienced in space, researches on the effect of microgravity on solidification are becoming important. 8,[16][17][18] Timchenco et al 8) studied the transient plane front solidification of alloys under microgravity conditions. Nastac 16) developed a comprehensive numerical approach for modeling of macrosegregation and applied it to the Pb-10wt%Sn alloy solidification under normal and reduced gravity.…”