Transition From High Cooling Rate Cells To Dendrites In Directionally Solidified Al-Sn-(Pb) Alloys

The preprogramming of the solidification cooling rate during casting of some Al-based bearing materials, has been used to achieve microstructural patterns conducive to better wear responses. For Al-Sn castings, improvements in the wear resistance were reported to occur when higher amounts of Sn remained segregated in the spacings of the dendritic Al-rich matrix. In this sense, a cellular microstructure could be more adequate since the segregated Sn would be contained along the cells boundaries and not spread through the interstices of multiple dendritic arms. This study investigates a range of Al-xSn alloys compositions (x = 1, 2.6, 7.5, 9, 10 wt.%) and an Al-(9 wt.%)Sn-(1 wt.%Pb), with a view to determining the ranges of Sn concentrations and solidification cooling rates ((T) over dot) for which high cooling rate cells are stable, as well as the reverse cellular/dendritic transition. For (T) over dot from 1 to 40 K/s, it is shown that for Al-Sn alloys having Sn < 7.5 wt.% the morphology of the Al-rich matrix is fully cellular and for Sn > 10 wt.% it is completely dendritic. The reverse transition from high cooling rate cells to dendrites is shown to occur for the Al-9 wt.%Sn alloy, with (T) over dot < 3.3 K/s resulting in a dendritic microstructure and (T) over dot > 6 K/s in a fully cellular Al-rich matrix. The Al-9 wt.%Sn-1 wt.%Pb ternary alloy casting also exhibits a reverse cellular/dendritic transition, with dendrites occurring also for (T) over dot < 3.3 K/s and cells for (T) over dot > 8 K/s.