Microstructure and Hardness of an Al–8 wt%Si–2.5 wt%Bi Alloy Subjected to Solidification Cooling Rates from 0.1 to 800 K s−1

Abstract

This work explores the effect of the addition of bismuth (Bi) to Al–8 wt%Si alloys. Bi in Aluminum based alloys works as a self‐lubricating agent, improving machining and wear properties. As Bi is a soft material, it is essential to evaluate how it affects microstructural features and the resulting properties of Al–8 wt%Si alloys. Herein, this hypoeutectic alloy is modified by the addition of 2.5 wt%Bi and subjected to three solidification techniques: differential scanning calorimetry, transient directional solidification, and impulse atomization. Thus, this work investigates the effect of Bi in samples solidified under a wide range of cooling rates. Of specific interest is how Bi modifies the eutectic silicon morphology and alloy hardness compared with a hypoeutectic Al–10 wt%Si alloy from the literature. The silicon (Si) morphology of Al–8 wt%Si–2.5 wt%Bi transitions from flaky (coarse) to fibrous (fine) at a critical cooling rate of 1100 K s−1. Through the combination of Bi addition and processing through impulse atomization, the ternary Al–Si–Bi alloy achieves improvements in hardness of up to 20% compared to Al–10 wt%Si. This is despite having a coarser eutectic microstructure than the binary hypoeutectic Al–Si alloy. This is due to Bi modifying the morphology of the eutectic Si.