Objective Gradient lattice structures are widely used in aerospace because they enable lightweight designs based on stress distribution; however, they cannot be manufactured easily using conventional methods. Currently, selective laser melting (SLM) is the most widely used metal additive-manufacturing method. Ti-6Al-4V alloy prepared via SLM offers the advantage of high forming precision and has been widely investigated in the aerospace field. Because graded lattice structures with different gradient directions have different mechanical properties, fracture modes, and energy-absorption capacities, quasistatic compression experiments and microscopic characterization have been performed to investigate the mechanical properties, fracture mechanisms, and energy- absorption capacities of BCC (body-centered cubic) and FCC (face-centered cubic) lattice structures with different gradient directions and cell types. The results serve as a foundation for achieving more efficient lightweight designs and better energy-absorption performance of lattice structures in the aerospace field. Methods In this study, gradient lattice structures with smooth transition of nodes were designed, and the gradient lattice structures were formed via SLM using Ti-6Al-4V powder as the raw material. The parameters set were as follows: laser power, 210 W; scanning speed, 1200 mm/s; spot diameter, 0.1 mm; scanning spacing, 0.12 mm; and powder-layer thickness, 0.03 mm. Additionally, rubber-scraper unidirectional powder laying was performed, a chamber was formed in argon as a protective gas, and 0.01 mm high-precision Z-axis control was implemented. The laser spot and positioning accuracy were corrected prior to the formation of the chamber. The forming accuracy was further controlled via a spot-compensation strategy, and the dimensional accuracy of the lattice structures was ensured via equipment and process control. The shaped samples were sandblasted and their roughness and overall size were measured. Small samples were cut at the rods and joints for microstructural characterization. Finally, the mechanical properties of the different samples were tested via quasistatic compression, and the fracture mode was analyzed via force analysis and numerical simulation. Results and Discussions Under the abovementioned process parameters and owing to the rapid heating of a specific region of the powder bed by the laser beam during SLM forming followed by the rapid cooling of the region, the formed sample deformed slightly (Fig. 4). The smaller the rod diameter, the greater the effect on both the surface roughness and overall dimensions (Table 2). The microstructure of the lattice is primarily martensitic, which can improve the strength and hardness of the lattice structure and increase its carrying capacity (Fig. 6). Based on the EBSD characterization of the lattice nodes (Fig. 7) and rods (Fig. 8), one can conclude that because the heat-transfer speed affects the rods more significantly than the nodes in terms of mechanical properties, fracture will occur preferentially at the nodes. Powder coating is observed at the rod and surface profile, powder adhesion to the surface results in a rough forming surface, and some non-fusion and hole defects are observed in the interior (Fig. 9). Most of the defects measure only 5-20 mu m and thus do not affect the mechanical properties of the gradient lattice structures. In terms of the compressive mechanical properties (Fig. 11), the weak layer contributes significantly to the bearing capacity of the gradient structures. The failure mode of the Z-axis gradient is primarily layer-by-layer fracture, whereas the axial and uniform gradients are 45 degrees shear fractures. The shear fracture mode of a homogeneous lattice is primarily determined by the combination of material properties and lattice structures (Figs. 13 and 14). The yield strength (Fig. 11) and energy-absorption capacity (Fig. 16) of the FCC structure with the same bar diameter and gradient direction are better than those of the BCC structure. The deformation of the bidirectional gradient structures has a symmetric effect; thus, its energy-absorption capacity is better than that of the unidirectional gradient structure. Conclusions By performing parametric modeling, a structure with continuous gradient changes can be designed to ensure an effective connection between cell units. By adjusting the rod diameter of each layer of the cell units in different ranges, anisotropy can be achieved, and the relative density and local mechanical properties of different regions can be controlled. The yield strength and energy-absorption capacity of an FCC structure with the same bar diameter and gradient direction changes are better than those of a BCC structure. In the strain range of 0-10%, the energy-absorption capacity of the bidirectional gradient structures is better than that of the unidirectional gradient structure, which indicates that the bidirectional gradient structures are more suitable for buffer-energy absorption scenarios than the unidirectional gradient structure. The layered failure behavior observed in the gradient structures is different from the 45 degrees shear failure observed in the uniform structure, which primarily depends on the distribution of weak layers in the structural design. Meanwhile, the mode of shear fracture in the uniform lattice depends primarily on the combination of material properties and lattice structure. Thus, the weak section of the structures can be placed in a position where energy absorption is more important than the support function, thereby enabling the optimization of the gradient lattice structure.
