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

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. This study compares the microstructure and hardness of solidified Al–Si–Bi alloy with a binary hypoeutectic Al–Si alloy to demonstrate the effect of bismuth. Over a range of cooling rates from 0.8 to 800 K s−1, the microstructure map shows the effect of Bi on the silicon morphology in the Al–Si–Bi alloy. Bi also results in an increased hardness of the alloy.