Microstructure, Microhardness, and Wear of a Rapidly Solidified Al–20Sn–1Cu Alloy with Bi Addition

This study investigates the effects of Bi addition on the microstructure, hardness, and wear behavior of a rapidly solidified Al–20Sn–1Cu alloy, aiming to clarify Bi’s role under cooling rates comparable to those experienced during twin-roll casting (TRC). Alloys with and without 2 wt % Bi were solidified in a stepped graphite mold, under cooling rates between 260 and 409 °C/s. Microstructural analyses revealed that Bi did not alter the primary cellular/dendritic growth of the α-Al matrix but significantly modified the interdendritic Sn-rich phase, promoting its fragmentation. CALPHAD predictions confirmed the occurrence of liquid phase separation in the Bi-containing alloy, consistent with the formation of finely dispersed Bi-rich regions observed experimentally. Microhardness increased with Bi addition and with increasing cooling rate. Wear ball-cratering tests showed that Bi addition markedly reduced the worn volume, particularly for rapidly solidified samples, and led to a distinctive evolution of the dimensional wear coefficient (k). This improvement is attributed to the combined effects of microstructural refinement and increased hardness. The worn surfaces exhibited predominantly adhesive wear, characterized by material transfer from the steel counterface and minor oxidation. Overall, the results demonstrate that Bi addition is an effective strategy to restore or enhance the tribological performance of Al–Sn alloys produced under rapid solidification conditions relevant to the TRC process.