Niobium nanoparticle-enabled grain refinement of a crack-free high strength Al-Zn-Mg-Cu alloy manufactured by selective laser melting

Abstract

Grain refinement is an effective way to avoid hot cracking of non-weldable high strength aluminum alloys during additive manufacturing. In the present work, primary Al3Nb with the tetragonal D022 structure was discovered to be an extremely effective inoculant for Al-Zn-Mg-Cu alloys manufactured by selective laser melting (SLM). The columnar crystals were transformed into fine equiaxed crystals with an average grain size of ~1.9 µm through adding 1.5 wt% Nb nanoparticles, eliminating the defects such as cracks and porosity. The high number density of primary Al3Nb and the coherent interface with high lattice matching between the primary Al3Nb and the Al matrix are the main reasons for grain refinement. The Time-Temperature-Transformation (TTT) curves of the precipitation kinetics curves of primary Al3X (XNb, Zr, Ti, Sc) show that primary Al3Nb has the advantage in grain refinement compared with Al3Sc and Al3Ti. Due to grain refinement and defect elimination, the 1.5 wt% Nb-modified Al-Zn-Mg-Cu alloy after T6 heat treatment exhibits excellent mechanical properties, with a tensile strength of ~ 505 ± 12 MPa, and an elongation after fracture of ~ 12.3 ± 1.3%.