2026 Rocha AlSi AlSiBi Solidification Microstructure

This work investigates the effects of growth rate (GR), cooling rate (CR), and eutectic microstructure on the machinability of Al-12.6Si and Al-12.6Si-3.2Bi (wt.%) alloys. Directional solidification experiments were performed using a water-cooled upward solidification apparatus, enabling a range of GR and CR values along the length of the ingots. The as-solidified microstructures consisted of primary α-Al dendrites and interdendritic eutectic Si, while the Bi-containing alloy exhibited additional Bi globules dispersed within the eutectic regions. Machinability was evaluated based on maximum cutting temperature (Tmax), heating rate (HR), and tool flank wear (Vbmax). In the binary alloy, CR and eutectic spacing (kSi) had negligible influence on Tmax, which remained approximately constant at approximately 46°C. On the other hand, the ternary alloy exhibited Tmax values ranging from 50 to 63°C. Higher HR values were associated with higher CR and finer kSi for both alloys. Although Bi addition did not significantly affect HR, it promoted the formation of built-up edge (BUE), which contributed to a higher Tmax. The addition of Bi improved machinability by reducing Vbmax by nearly 40% compared with the binary alloy.