Microstructure morphologies during the transient solidification of hypomonotectic and monotectic Al–Pb alloys

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

“…After a complete analysis of the sizes and morphologies found in all evaluated conditions, it was possible to correlate the regions along the castings length with distinct microstructure morphologies with the solidification growth rate. Based on the present results and those found recently by Silva et al [22] for upward solidification conditions, the regions can be synthesized as follows: droplets prevailing for v [ 1.0 mm/s and fiber-like structures found for v \ 0.65 mm/s in the case of the Al-0.9 wt%Pb alloy solidified upwards [22]; v [ 1.1 mm/s and v \ 0.85 mm/s in the case of the Al-1.2 wt%Pb alloy solidified upwards [22]; v [ 0.57 mm/s (droplets)/v \ 0.48 mm/s (fibers) and v [ 0.18 mm/s (droplets)/v \ 0.15 mm/s (fibers) found for directionally solidified downwards Al-0.9 wt%Pb and Al-1.2 wt%Pb alloys, respectively.…”

“…Some studies in literature [20,21] also reported that such decrease in thermal contact during downward solidification, is conducive to lower and constant interfacial heat transfer coefficients. Indeed, during solidification of immiscible Al-Pb alloys in this study, the growth rates profiles along the castings lengths for both alloys solidified downwards are lower than those experimentally determined for the upward cases [22]. For instance, the experimental power functions relating the growth rate with position for the Al-0.9 wt%Pb alloy are v = 2.4(P) -0.…”

“…A region of microstructure transition (mixture of droplets and fibers) was found along the castings lengths, with the following locations: from 30 to 65 mm for the Al-0.9 wt%Pb alloy (downwards) and from 35 to 85 mm for the Al-1.2 wt%Pb alloy (downwards). Silva et al [22] have reported the following ranges: from 20 to 85 mm from the bottom of the casting for the Al-0.9 wt%Pb alloy solidified upwards; from 15 to 30 mm for the Al-1.2 wt%Pb alloy (upwards); After this transition region, which means higher positions for upward cases and lower positions for downward cases, Pb-rich fibers and strings of pearls are the predominant morphologies.…”

“…Points are experimental results and lines represent empirical fits to the experimental points, with the spacing being expressed as a power function of the growth Fig. 9 Interphase spacing of droplets and fibers as a function of the growth rate for Al-0.9 wt%Pb and Al-1.2 wt%Pb alloys solidified a upwards [22] and b downwards. R 2 is the correlation coefficient rate.…”

“…The final as-cast microstructure can be affected due to the thermal and constitutional gradients in the melt typically found in the downward case. The classical relation proposed by Jackson and Hunt [26], i.e., k 2 v = constant was found to be ineffective to encompass both the fibrous and droplets growth during the downward directional solidification of Al-Pb alloys despite the application reported of such relation for the growth of droplets during upward solidification [22]. Thus, other experimental power functions relating the interphase spacing with the growth rate have been derived from the present experimental results: k = 8.2(v) -2.0 and k = 0.9(v) -2.0 for the growth of droplets during downward directional solidification of Al-0.9 wt%Pb and Al-1.2 wt%Pb alloys, respectively; k = 0.5(v) -6.5 and k = 6 9 10 -4 (v) -6.5 for the growth of fibers during downward directional solidification of Al-0.9 wt%Pb and Al-1.2 wt%Pb alloys, respectively.…”