In the endeavor to maximize the refinement effect of primary Si and alleviate the inherent brittleness of hypereutectic Al-Si alloy, the approach of coating P as a modifier on powder was adopted. The ulti-mate aim was to create more heterogeneous fine AlP nucleus and enhance the nucleation efficiency of primary Si on AlP to refine the coarse primary Si to nano-scale during 3D printing. In the combination of large undercooling and high density of nucleation sites, the size of primary Si was successfully refined to 20 0-30 0 nm and the divorced eutectic was also induced to modify the microstructure of matrix. In the presence of nano-scale primary Si, the melting pool boundary (MPB) feature disappeared and the fracture mechanism also changed from load transfer to interfacial fracture. Compared with the pristine alloy, the ductility was increased four times without significantly changing the ultimate tensile strength (UTS) and wear resistance. The improvement of ductility is attributed to the refinement of primary Si, the disappearance of MPB features and the formation of divorced eutectic. The optimal tensile properties were: UTS-482 MPa, yield strength-320 MPa and ductility of 8.1% at 0.05 wt.% P. These are comparable to those for high-strength Al alloys. (c) 2021 Published by Elsevier Ltd on behalf of Chinese Society for Metals.