Enhancing the mechanical performance of additively manufactured lattice structure via locally reinforcing struts: Coupled influence of structure and microstructure

Lattice structures with excellent mechanical properties are always desirable for high-end manufacturing fields. The existing methods such as innovative unit cell design and unit cell hybridization show potential for enhancing the performance of strut-based lattice structures. However, these methods are generally associated with complicated and time-consuming design processes and challenges in fabricating high-quality lattices. In this study, selectively optimizing the primary load-bearing struts within a strut-based lattice structure is proposed. Based on a face-centered cubic lattice with z-struts (FCCZ), a series of variants with locally enhanced z-struts, including the strut-reinforced (FCCZ-SR), the edge-reinforced (FCCZ-ER), and the node-reinforced (FCCZ-NR) lattice structures were designed and fabricated using laser powder bed fusion (L-PBF). The effects of reinforcement strategies on mechanical properties, deformation behavior, and failure modes of lattice structures during the compression process were investigated. Furthermore, the effects of strut geometrical characteristics and aging treatment on microstructures, and further on the performance of lattice structures were revealed. Results show that the FCCZ-ER structure exhibits the highest yield strength in the as-built state, which is 60.8 % higher than that of the FCCZ structure. The deformation behavior of the FCCZ-ER structure is primarily governed by the nodes, leading to a layer-by-layer failure mode, while other structures ultimately display shear failure due to strut-dominated behaviors. An increased diameter of struts can induce a transition of grain morphology from equiaxed to columnar. Aging treatment at 325 degrees C for 4 h can significantly enhance the strength of lattice structures, attributed to the precipitation of secondary coherent Al3(Sc, Zr) nanoparticles in the equiaxed grain regions. Consequently, the FCCZ-SR structure with high proportion of equiaxed grains exhibits the highest strength in the heat-treated state, providing a 58.6 % increase compared to the FCCZ structure. The aging treatment also decreases the ductility of lattice structures, which causes shear failure to be more likely to form in heat-treated lattice structures.