Solidification-Controlled Microstructure and Its Effect on the Machinability of Directionally Solidified Al-Si and Al-Si-Bi Alloys

Abstract 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 ( T max ), heating rate (HR) and tool flank wear (Vb max ). In the binary alloy, CR and eutectic spacing ( λ Si ) had negligible influence on T max , which remained approximately constant at 46 °C. On the other hand, the ternary alloy exhibited T max values ranging from 50 to 63 °C. Higher HR values were associated with higher CR and finer λ Si 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 T max . The addition of Bi improved machinability by reducing Vb max by nearly 40% compared with the binary alloy.