Microstructural Development and Hardness Response in Al–3Cu–1Bi Alloy under Controlled Solidification and T6 Heat Treatment

Abstract In this study, solidification and heat treatment experiments were conducted on an Al–3Cu–1Bi (wt.%) alloy to investigate the effects of thermal processing parameters on microstructural evolution and mechanical response. Directional solidification (DS) was conducted under transient heat-flow conditions and monitored by means of thermocouples positioned at predefined locations along the casting length from the heat-transfer surface. The resulting in situ thermal data were used to determine the local growth rate (V L ) and cooling rate (T R ) throughout the solidification process. Subsequent T6 heat treatment was applied using different solution treatment and aging conditions. Microstructural characterization was carried out using optical microscopy (OM), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The results demonstrated that transient directional solidification significantly affected the dendritic microstructural scale of the Al–3Cu–1Bi alloy over a wide range of growth rates (V L = 0.50–2.26 mm/s) and cooling rates (T R = 0.37–15.36 °C/s). The microstructure consisted of α-Al, Al 2 Cu, Al 7 Cu 2 Fe, and Bi phases, comprising an Al-rich dendritic matrix surrounded by eutectic constituents and interdendritic Bi globules. Hardness followed a Hall–Petch-type relationship with λ 2 , indicating that dendritic refinement contributed significantly to strengthening, with higher hardness associated with lower λ 2 values. Solution treatment at 495 °C for 3 h increased hardness by approximately 92% relative to the as-cast condition, while the T6 treatment (495 °C/3 h + 155 °C/5 h) provided an additional increase of about 39% due to precipitation strengthening. Overall, Bi addition influenced both microstructural evolution and heat-treatment response, highlighting its potential for monotectic Al-based alloy systems.