Synthesis of precipitation-strengthened Al-Sc, Al-Zr and Al-Sc-Zr alloys via selective laser melting of elemental powder blends

Abstract Selective laser melting is used to create Al-1.5Sc, Al-1.5 Zr and Al-0.75Sc-0.75 Zr (at.%) alloys from blends of elemental Al, Sc, and Zr powders. This study investigates elemental alloying elements (Sc and Zr) which are high-melting and highly reactive, unlike previous work which focused on more concentrated elemental additions of lower-melting, lower-reactivity Cu and Si to aluminum. High-speed in situ synchrotron x-ray imaging and diffraction show that the 20−30 μm Al, Sc, and Zr powders fully melt and sufficiently mix in the molten state to create, on solidification, a homogeneous distribution of primary, micron-size L12 precipitates (Al3Sc, Al3Zr, and Al3(Sc,Zr), respectively), as confirmed by SEM imaging of cross-sections. These primary precipitates show the metastable L12, rather than the stable D023, structure and they nucleate micron-size Al matrix grains. A second laser pass, simulating a realistic additive-manufacturing build condition, fully remelts the initial volume which shows, after solidification, the same Al3(Sc,Zr) L12 primary micro-precipitates and very fine Al grains. After aging at 300−400 °C, the alloys show large increases in hardness, consistent with an exceptionally high number density (1.4 × 1024 m−3) and volume fraction (2.5 %) of secondary Al3(Sc,Zr) nano-precipitates with a Sc-rich core and Zr-rich shell, as measured via atom-probe tomography.