Investigation on the evolution of deposition layer grain structure and its effect on mechanical properties in aluminum alloy fabricated by laser directed energy deposition

In recent years, aluminum alloy additive manufacturing has played a significant role in promoting lightweight applications in aerospace and automotive industries due to its efficiency and cost advantages. In order to study the evolution of grain structure in the deposition layer, single-variable single-pass deposition experiments was conducted to optimize the process parameters of laser directed energy deposition (LDED). Subsequently, multi-layer multi-pass depositions were performed to fabricate the components. The deposition layer was divided into three distinct regions: Top coarse grain zone (TCZ), Central equiaxed grain zone (CEZ), and Bottom columnar grain zone (BCZ). It was observed that subsequent depositions led to recrystallization, resulting in the refinement of the original equiaxed grain region and the coarsening of the grain structure in the deposition layer, leading to the formation of the latter two characteristic grain regions. Interestingly, within the TCZ and BCZ, we observed the presence of aluminum alloy twins that are difficult to form. This observation may be attributed to a significant reduction in the required twin shear stress within the coarse grains. The stress concentration at the grain boundaries of the coarse grains and the rapid strain rate during the LDED process could also be contributing factors to the formation of aluminum alloy twins. In terms of mechanical properties, the tensile strength of the samples parallel to the deposition direction is 268.25 MPa, while the tensile strength of the samples perpendicular to the deposition direction is 276.68 MPa. The fracture surfaces exhibit numerous equiaxed dimples, displaying characteristics of ductile fracture. There is almost no anisotropy observed in the mechanical properties, this is attributed to the alternating arrangement of different crystal regions within the deposition layer and the role of twins in suppressing dislocation motion.