Solidification of Sn-Pb alloys: Experiments on the influence of the initial concentration

A comparative study among solidification experiments for three selected alloys (Sn-3 wt.%Pb, Sn-6.5 wt.%Pb and Sn-10 wt.%Pb) was conducted on a benchmark experiment model under the same experimental conditions. The goal of this paper is to analyze the effect of variation in concentration on the solidification process with respect to different aspects: thermal, dynamic, structure, and morphology of segregation. Experimental results consist of instantaneous temperature maps provided by a lattice of 50 thermocouples welded on the large crucible side and post-mortem characterizations of the samples, such as X-ray imaging, solute local composition and metallography. Measurement of the instantaneous temperature field and numerical computation of liquid-solid interface evolution allows us to evaluate the effect of variation in concentration on thermosolutal convection behavior. Experimental results show that an increase in concentration greatly enhances the mechanism of the columnar-to-equiaxed transition (CET) and leads to refinement of the equiaxed structure. However, a significant effect of solutal element (lead) stratification is observed, which can slow down thermosolutal convection, in particular for large concentrations. Especially, without any stirring lead segregation which likely occurs during the melting phase may suppress natural convection. Furthermore, lead stratification is significantly reduced when electromagnetic stirring is opposed to natural convection before the solidification phase begins.