On the growth of the minority phase during downward transient directional solidification of hypomonotectic and monotectic Al–Pb alloys

Abstract

Al-Pb alloys were solidified under non steady heat flow conditions using a casting assembly in order to promote vertical downward directional solidification. The downward configuration enables the effects of gravitydriven convection on the final microstructure to be evaluated since the collective movement of Pb-rich particles downwards is favored, due to density differences between the two coexisting liquid phases. Investigations have been made of the obtained solidification structures. Growth rates (v) and cooling rates (T Á ) of the Al-Pb alloys solidified downwards were experimentally determined by the cooling curves recorded along the casting length. The monotectic structure was characterized by metallography and a microstructural transition has been observed in all cases. The microstructure was characterized by well-distributed Pb-rich droplets in the aluminum-rich matrix from the casting cooled surface up to a certain position in the casting, followed by a mixture of fibers and strings of pearls from this point to the bottom of the casting. The increase in alloy lead content delays the formation of fibers for alloys solidified downwards, which occurs for v \ 0.48 mm/s and v \ 0.15 mm/s for Al-0.9 wt%Pb and Al-1.2 wt%Pb alloys, respectively. Experimental power laws relating the interphase spacing, k, to v and characterized by -2.0 and -6.5 exponents, were found to represent the growth of droplets and fibers, respectively, for both alloys solidified downwards.

Proposition of a Growth Law as a Function of Solidification Parameters for Monotectic Alloy Systems

“…In the other cases, different power functions were obtained λ = a. (V) Àb , with b reaching the following values: À2.2 for the occurrence of fibers in the upward vertical solidification [20]; À2 and À6.5 for the occurrence of droplets and fibers in the downward vertical solidification, respectively [29].…”

“…In this way, following the same methodology used for the analysis of the interphase spacings and droplets diameters with positions from metal/mold interface, the ordered pairs of the curves that relate the interphase spacing and the droplet diameters with the corresponding growth rates obtained in the studies were used [6,7,20,28,29,34,36,40]. Again, the values of interphase spacing, and droplet diameters used in the present analysis were dimensionless.…”

“…Yang and Liu [17] analyzed the particle diameter of hypermonotectic alloys and found results that lead to power-type functions, however, with different exponents from those proposed ever, with different exponents from those proposed for the quantification of interphase spacings. Silva et al [6,7,18,[26][27][28][29] tried to evaluate the microstructural behavior of hypomonotectic and hypermonotectic alloys solidified in devices with upward and downward configurations. They found that the alloys that showed cellular configuration presented mathematical expressions correlating cellular spacing with growth rate in agreement with the one found and merged in the literature, λ 1 = C.v À1,1 , where v is the growth rate and C is a constant [30][31][32][33].…”

“…There are also few studies that analyze the influence of the growth direction on the interphase spacings and on the dimensions of bismuth, lead, and indium particles [37]. The use of the relationships of [23] may not be valid under certain conditions, as verified by [29], in the case of downward directional solidification. Moreover, the relationship proposed by [15] for dendritic growth cannot be suitable to be employed in the case of downward directional solidification where the heat transfer can occur not only by conduction but also by convection.…”

Steady and unsteady state peritectic solidification

“…A number of recent studies on unsteady solidification of alloys of binary eutectic [27][28][29][30] and monotectic metallic systems [31][32][33] have been carried out with a view to characterising microstructural features (cellular, dendritic and interphase spacings) as a function of the initial alloy solute concentration (C 0 ), growth rate (V L ), thermal gradient (G L ) and cooling rate ( : T) ahead of the solidification front. These studies are fundamental for the design of microstructures as a function of solidification thermal parameters, since it has been shown that the predictive growth models existing in the literature are, in most cases, not capable to furnish reliable predictions under transient heat flow conditions.…”