Stimulating multi-scale gradient heterostructure in laser direct energy deposited and hybrid additive manufactured AA7075 alloy via double-aging heat treatment to realize strength-ductility synergy

The inverse relationship between strength and ductility has long been a major challenge in the field of metal research. Recently, the design and fabrication of non-uniform heterostructures, which involve the spatial distribution of different constituents via external thermal or mechanical means, have proven to be effective strategies for simultaneously enhancing strength and ductility. In this work, double-aging heat treatment (HT) was employed to activate the alternating columnar-equiaxial and cross-scale coarse-fine gradient heterostructures in laser direct energy deposited (LDED) and hybrid additive manufactured (HAM) AA7075 aluminum alloy. This approach led to a synchronous improvement in strength and ductility. Specifically, the ultimate tensile strength (UTS) and elongation (EL) of the LDED-HT samples were enhanced by approximately 20.1 % and 58.8 %, respectively. Meanwhile, the HAM-HT samples exhibited an UTS of ∼ 591.5 MPa and an EL of ∼ 15.1 %, which are superior to those of commercial wrought plates. Compared with the standalone LDED samples, The HAM ones with interlayer friction stir processing (IFSP) demonstrated completely pore elimination, significantly refined grains, and homogeneously fragmented inherent coarse precipitates. Following the double-aging HT, the width of columnar grains in LDED samples increased slightly, whereas the HAM samples exhibited abnormal grain growth (AGG) in the inter-track (IT) zones and normal grain growth (NGG) in the interlayer (IL) zones. Additionally, double-aging HT induced a much higher density of precipitations, such as η/η' phases, in the HAM samples compared to the LDED ones. The width of the precipitate-free zone (PFZ) in the HAM-HT samples was also significantly narrower than that in the LDED-HT ones. This work provides a novel approach for preparing multi-scale spatially heterostructures in thermally sensitive aluminum alloys, thereby breaking the inverse relationship between strength and ductility.