Effects of cooling rate and TiC nanoparticles on the microstructure and tensile properties of an Al–Cu cast alloy

In this work, the effects of cooling rate and the addition of TiC nanoparticles on the as-cast microstructure, precipitation behavior during subsequent heat treatments and mechanical properties were investigated. The grain size (GS), secondary dendritic arm spacing (SDAS) and primary particle size are the function of cooling rate v, following a similar power law, D = Av-n (D is the size of grains/SDAS/primary particles, A and n are constants). The TiC addition reduced the A value for grain size and primary particles but increased A for SDAS. The addition of TiC nanoparticles promoted the coarsening of secondary dendritic arms, leading to the reduced microsegregation and smaller interdendritic/intergranular secondary phase (Al2Cu). Consequently, the precipitation of the θ′ phase during ageing treatment was promoted, and the precipitates were refined and more dispersive, contributing to the increased tensile strength. The improved ductility was ascribed to the combination of reduced secondary phases and micropores. With the combination of high cooling rate and TiC addition (cooling rate: 93 °C/s; 0.5% TiC addition), the yield strength, ultimate tensile strength and uniform elongation of the Al–5Cu cast alloy were simultaneously improved, and reached those of Al–5Cu rolled sheet.