Influence of Tin Content on the Correlation Between Thermal Solidification Parameters, Macrostructure, and Machinability of Al-Sn Alloys Using the Surface Roughness Criterion

The machinability of aluminum-tin (Al-Sn) alloys is strongly influenced by solidification history, although solidification behavior and machining performance are often addressed independently. This study investigates the influence of tin content on the relationship between thermal solidification parameters, macrostructural evolution, and machining response of directionally solidified Al-Sn alloys. Al-10wt.%Sn and Al-20wt.%Sn alloys were solidified under identical transient heat extraction conditions, and liquidus isotherm velocity (VL) and cooling rate (T) were correlated with macrostructure and surface roughness after machining. Distinct macrostructural morphologies were obtained, with the Al-10wt.%Sn alloy exhibiting columnar, plumose, and equiaxial grains along the ingot length, whereas the higher Sn content promoted a predominantly equiaxial structure due to enhanced constitutional undercooling. Machinability was assessed by average surface roughness (Ra), measured using a non-contact optical method, with Ra values ranging from approximately 10.9 to 12.4 µm, depending on the macrostructural region. The results show that surface roughness is governed primarily by macrostructural morphology rather than alloy composition alone. Plumose and transition regions, typically located around P = 20 mm and P = 60 mm from the chilled surface, presented inferior surface finish (Ra ≈ 12.3-12.4 µm), whereas equiaxial regions exhibited lower and more homogeneous roughness levels (Ra ≈ 10.9 µm). Higher VL and T conditions, which favor equiaxial grain formation, were consistently associated with improved machining response. These comparative findings provide a physically grounded framework linking solidification parameters, macrostructure, and machining response, supporting the development of machinability databases based on controlled solidification.