Densification Behavior, Microstructure Development, and Mechanical Performance of Laser-Based Powder Bed Fusion Processed Al2024 Alloy With Zr Additions

High strength 2xxx series Al alloys encounter intolerable hot cracks when solidified rapidly. The addition of group IV transition metals, such as Zr, aid in preventing their formation. The effect of 1-4 wt% Zr additions on the solidification behavior, microstructural refinement, and concomitant mechanical properties of Zr-modified Al2024 alloys processed by Laser Powder Bed Fusion (L-PBF) is investigated. The rapid cooling characteristic of L-PBF facilitated the formation of metastable and fully coherent L12Al3 Zr phases, acting as heterogeneous nucleation sites during rapid solidification hence refining the L-PBF microstructure. When 1 wt% of Zr was added, crack-free samples could be obtained at high laser energy densities, yet accompanied by the presence of keyhole pores. Upon further Zr addition, higher laser scan speeds could be applied, avoiding the formation of keyhole porosity and gradually inducing a shift from a mixed columnar + equiaxed towards a fully equiaxed microstructure, accompanied by a grain size reduction down to 0.7 ± 0.3 µm. In the as-built condition, the Al2024 alloy modified with 4 wt% Zr exhibited an excellent combination of strength and ductility (yield strength of 568 ± 23 MPa, ultimate tensile strength of 584 ± 16 MPa, and elongation at break of 6.1 ± 1.6%). Besides the refined microstructures, solid solution and Orowan strengthening effects contribute to the exceptionally high strength of this new alloy, hereby highlighting the potential of the use of ultra-fine grained hyper-peritectic aluminum alloys containing transition element additions in high-strength and lightweight parts processed by L-PBF.