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

## TL;DR

Adding bismuth to an aluminum alloy improves its hardness and wear resistance when rapidly solidified, making it suitable for twin-roll casting processes.

## Contribution

The study reveals that bismuth enhances tribological performance by refining microstructure and increasing hardness in rapidly solidified Al–Sn alloys.

## Key findings

- Bismuth addition promotes fragmentation of the Sn-rich phase without altering the α-Al matrix structure.
- Microhardness and wear resistance increase with bismuth and higher cooling rates.
- Wear surfaces show adhesive wear with material transfer from the steel counterface.

## Abstract

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.