As a low-expansion alloy, Invar 36 alloy is commonly used in scenarios involving significant environmental temperature changes and high-precision requirements. However, its high density and machining difficulty limit its application in the aerospace field. The use of the laser powder bed fusion (LPBF) technology to fabricate lattice-filled Invar 36 alloy structures can effectively address these issues. Existing studies pertaining to the mechanical properties of lattice-filled structures focus on energy-absorption characteristics under compressive loading. Meanwhile, studies regarding the optimization of the structural stiffness of lattice-filled structures under compressive loading are scarce. Therefore, lightweight and high-stiffness lattice-filled structures must be urgently developed. Additionally, studies regarding lattice-filled structures based on Invar 36 alloy have not yet been reported. This study uses LPBF technology to fabricate Invar 36 alloy lattice-filled structures with various structural parameters, which results in good forming performance. Furthermore, this study provides valuable insights into the lightweight design of Invar 36 alloy components.

Invar 36 alloy lattice-filled structures with different structural parameters were fabricated via LPBF. The strut diameters are 0.8, 1.0, and 1.2 mm; the cell dimensions are 4, 6, and 8 mm; and the thickness of the skin is 1 mm. Optimized fabricating parameters were used: laser power, 280 W; scanning spacing, 0.12 mm; scanning speed, 1200 mm/s; powder thickness, 0.04 mm; and interlayer turn angle, 67°. A strip-scanning strategy featuring a strip width of 7 mm and a strip overlap of 0.08 mm was adopted. After fabrication, the samples were heat treated by increasing the temperature to 750 ℃ and maintaining it for 1.5 h, followed by cooling down to room temperature under argon atmosphere. The samples were sandblasted and cleaned ultrasonically to remove adhesive powder from the sample surfaces. Quasi-static compression tests and finite-element analyses were performed on the lattice-filled structures to investigate and analyze the compression curves, deformation modes, and stress distributions.

The compression stress?strain curves of the lattice-filled structures with different structural parameters are shown in Fig. 5. For the lattice-filled structures with cell dimensions of 8 mm and 6 mm, the curves can be classified into four characteristic regions: the elastic region; the force-drop region featuring a significant decrease in compressive stress; the plateau region; and the densification region. By contrast, the lattice-filled structure with cell dimensions of 4 mm neither exhibits a peak nor a compressive-stress-decrease region, which can be classified into three regions. This difference is primarily related to the deformation mode of the lattice-filled structures. When the strut diameter and number of lattice layers are small, the compressive deformations of the lattice-filled structures are primarily determined by the behavior of the lateral skin, which results in deformation characterized by skin wrinkling and shear-type global buckling (Fig. 7). Under these deformation modes, the compression curve is reflected by the abrupt stress after the peak strength. The compression performance of the lattice-filled structure is significantly enhanced when the strut diameter and number of lattice layers are further increased. Under this condition, the stronger constraining effect between the skin and lattice core restricts the local buckling of the skin (Fig. 8). However, a larger strut diameter can facilitate load transfer (Fig. 10). The stress is uniformly distributed inside the lattice core, and the local buckling deformation does not cause the compressive stress to decrease abruptly.

The Gibson?Ashby model was used to predict the performance of the Invar 36 alloy lattice-filled structures. The higher the relative density, the better is the structural compression (Fig. 11). However, for a specified mass, if the cell dimensions increases, then the strut diameter decreases. Therefore, the coupling effects of the structural parameters must be considered when designing lightweight materials.

The effects of structural parameters on the compression performance and deformation behavior of Invar 36 alloy lattice-filled structures were investigated via quasi-static compression tests and finite-element analyses. First, the compressive elastic modulus, plateau stress, and energy-absorbing properties of the lattice-filled structures fabricated via LPBF improve significantly as the cell dimensions decreases and the strut diameter increases. Second, the lattice-filled structures with different structural parameters exhibit three deformation modes: skin wrinkling, shear-type global buckling, and local skin buckling. Their deformation behaviors are coupled with skin tensile-dominated and lattice-filled core bending-dominated deformations. Third, the deformation behaviors obtained from finite-element analysis are consistent with the compression test results. Moreover, different structural parameters significantly affect the load transfer and stress distribution, which ultimately results in lattice-filled structures exhibiting different deformation modes. Finally, the coupling effect of the structural parameters should be fully considered when using lattice-filled structures as lightweight Invar 36 alloy members. For a specified mass, a cell dimension of 6 mm and a strut diameter of 1 mm are the better parameters for optimizing the structural stiffness, which can yield 1.75 GPa·g^-1·cm^-3 for the latter.