UC Santa Barbara

Additive manufacturing (AM) empowers the creation of novel open-celled architectures that can achieve dramatic performance gains through topology. Lattice structures with millimeter-scale features, specifically, yield the most notable performance gains in applications that require high specific strength and stiffness, enhanced energy absorption ability, and improved heat transfer. For metal AM features at this length scale, process-structure-property relationships become intrinsically linked to geometry because the widths of the struts in the lattices consist of only several melt pools. This process-properties coupling greatly complicates the design and development cycle. One strategy to speed up the design process is to characterize lattice sub-components, or “primitives”, and use their effective properties in process-informed models of larger scale structures. To explore the feasibility of this approach, extensive characterization of the effective properties of both lattice primitives and multi-celled lattices is needed.

In this work, experimental and computational techniques are used to advance the understanding of the mechanical response of metal AM lattices. The effective properties needed to predict the response of lattice primitives made of AM Ti-6Al-4V were determined using CT measurements and validated against experimental and FEA. The effective properties are shown to be highly dependent on printed geometry. The methods used to define effective properties are then applied to AM SS316L to quantify the effect of processing conditions upon effective properties of struts and nodes. Samples built with processing conditions that resulted in a higher energy density were found to have increased distributed porosity but not necessarily decreased strength. In addition, thin strut-based samples were found to display increased hardening relative to bulk properties. After understanding how to determine effective properties in struts and nodes, AM Ti-6Al-4V lattices are examined with CT and tested using in situ DIC and evaluated to determine if lattice primitives behave as expected in larger scale lattice structures. The results of these tests demonstrate some unexpected localizations in the lattices, warranting further exploration of local material properties.

This work makes several critical advances towards improving the design of printed lattice structures. Its foremost contribution is a method of determining the effective geometry of printed strut-based structures based on intuitive, direct measurements of the structure from CT. By using this method, the effective mechanical properties can be inferred, and advances in the understanding of the influence of print orientation, processing condition, and sample size upon the mechanical properties of lattice primitives are made. This work also provides insight into the behavior of strut intersections (nodes) in isolation and within lattice structures. Nodal behavior within lattice structures was found to heavily depend upon degree of constraint, orientation within the lattice, and nodal geometry.