On the macro- and micro-deformation mechanisms of selectively laser melted damage tolerant metallic lattice structures

Abstract

316L stainless steel lattice structures with several designs were prepared by selective laser melting at different laser powers. The surface structure, internal porosity distribution and microstructure of the lattice structures were investigated using a range of characterisation techniques and their properties evaluated by compressive testing. It was found that the as-fabricated lattice structures all showed much larger strut diameters than the design value. Increased laser power let to slight increase in strut size and increased relative density. The lattice struts contained a number of keyhole pores and consist of large columnar grains which led to development of strong <001> texture. During compression, all the samples demonstrated a stretch dominated deformation mode and experienced stable plastic deformation prior to densification. The Z-struts-bearing lattice structures showed much higher elastic modulus, 0.2% yield strength, steeper stress-strain slope and higher compressive strengths in plastic deformation regime than the Z struts-free lattice structures, leading to higher specific energy absorption. Deformation of the Z-struts-bearing lattice structures progressed by buckling of vertical struts along the vertical direction together with development of plastic hinges in regions near nodes whereas the deformation of Z-struts-free lattice structures proceeded only through the development of plastic hinges near nodes and the rotation of slanted struts around the nodes. The macroscopic deformation of struts was found to be coordinated by extensive internal grain movement, particularly grain rotation, which was sustained by the activation of massive twinning and slipping systems.