The influence of slight energy density variation within the optimized parameters scope on the grain growth, precipitation behavior and mechanical property of laser direct energy deposited AA7075 alloy

Abstract

The laser direct energy deposition (LDED) of high-strength AA7075 alloy is highly challenging owing to its inherent cracks and porosity, exhibiting narrow low-defect processing window. Meanwhile, its precipitate behavior and dislocation density, the primary source of strength, are significantly influenced by the slight variation of energy density. Therefore, this study firstly explored the operatable parameter scope for the crack-free and low-porosity fabrication of AA7075 alloy through the stepwise regulation of single-track, single-layer, and multi-layer deposition. Results indicated that the porosity was strongly related to the mass flow rate, while the pore type was more affected by energy density. Subsequently, the effects of energy densities on microstructural evolution and mechanical properties were explored among this scope. The average grain size, aspect ratio, texture intensity, and internal strain were all enhanced with the increase of energy densities, while the density of geometrically necessary dislocations peaked at 155.6 W/mm². The point-shaped coherent precipitates pinning dislocations identified as η/η' phase of MgZn₂ were observed under the energy density of 155.6 W/mm², while the stripe-shaped non-coherent precipitates identified as S phase of Al₂CuMg dominated under the energy density of 183.9 W/mm². The precipitation behavior models under different energy densities were then obtained through the LDED thermal history analysis. The largest ultimate tensile strength of ∼347.7 MPa occurred at 155.6 W/mm²-2 mg/mm², which can be attributed to the joint effect of its relatively low porosity, fine grain size, high dislocation density, denser precipitation distribution and η/η' phase pinning dislocation structure. This study reveals the evolution pattern of grain growth, precipitate, and mechanical property of AA7075 alloy among the optimized parameters scope, providing a feasible solution for its in-situ microstructural regulation.