Microstructure and geometry effects on the compressive behavior of LPBF-manufactured inconel 718 honeycomb structures

Abstract

This work discusses the combined effect of the microstructure and geometry on the deformation modes and energy-absorbing characteristics of laser powder bed fusion (LPBF)-manufactured Inconel 718 (IN718) hexagonal honeycomb structures tested under quasi-static compression. Three different geometries of the hexagonal cells, varying only in the cell wall thickness (0.4, 0.6 and 0.8 mm) were manufactured using LPBF. Electron backscatter diffraction (EBSD) imaging of the three studied geometries revealed three distinct zones of grain morphologies and textures across the 0.6 and 0.8 mm cell walls and only two zones and higher overall <001> texture across the 0.4 mm cell walls. Miniature tensile tests were performed on 0.4 and 0.8 mm thick tensile samples to evaluate the thickness and orientation effects on the parent material behavior. Each hexagonal geometry was loaded in three different directions resulting in nine study sets. Exhibiting monotonically increasing plateau stress and specific energy absorbed (SEA) in addition to the high SEA/plateau stress ratios, LPBF-manufactured IN718 hexagonal honeycomb structures were demonstrated to be a viable candidate for additively-manufactured (AMed) metallic lattice structures in energy absorption applications. The reduction in the cell wall thickness influenced the instability failure mechanism for the in-plane load direction X1 but no pronounced effect was observed for the in-plane direction X2. As a result of the coupled change of the material properties with the variation in the cell wall thickness, a non-normalized anisotropic form of the Gibson-Ashby model for stochastic foams was proposed to characterize the honeycomb-structure mechanical properties. The findings of this paper more generally provide useful insights into optimizing the design of metallic AMed lattice structures.