Enhanced strength and ductility in sand-cast Al-Li-Cu-Mg-Zr alloy via synergistic microalloying with Sc and Ti

Abstract

Cast Al-Li alloys show attractive potential for application in the field of manufacturing large-size and complex parts as a result of profound weight reduction and good fluidity. However, the ductility and strength of sand-cast Al-Li alloys are relatively poor due to the low cooling rate of sand casting. In this study, Sc and Ti were added to sand-cast Al-Li-Cu-Mg-Zr casting alloy to investigate the microstructure evolution and tailor the final performance for the first time. The results showed that grain decreased from 189 µm to 56 µm and the needle Cu-rich phase increased in the vicinity of grain boundaries as the total element content of Sc and Ti increased. After aging at 175 °C, the enrichment of the T1 and S′ phases, the finer δ′, the higher number density of core-shell composites, and the narrowed δ′-precipitation free zones (δ′-PFZs) were stimulated by Sc and Ti, leading to a significant increase in strength of the peak-aged alloy. The SZT alloy exhibits the best yield strength of 359 MPa and ultimate tensile strength of 452 MPa with an adequate elongation of 4.6 %. In addition, a Li-rich zone was observed and intersected with T1 phase, which provided a new precipitation path for Li atom diffusion and nucleation of T1 precipitates. The present study suggests that microalloying with Sc and Ti is expected to be an effective strategy to simultaneously improve the strength and ductility of sand-cast Al-Li alloys.