Additively Manufactured Cellular Materials

It is well known that porous metals have potential application as absorbers of energy generated by high velocity impact or explosive loading. Lattice materials in which the porosity arises from a regular array of nominally identical cells consisting, for example, of planar walls or cylindrical struts are an important class of porous materials. The choice of a lattice structure to meet the requirements demanded by a particular loading scenario requires a careful balance of strength, stiffness and cost in addition to optimised response to high-rate loading. The state of the art on manufacturing, mechanical properties and applications of metallic micro-lattice materials was reviewed by Rashed et al. [1]. One of the manufacturing techniques highlighted by Rashed was additive manufacture (or AM). AM has emerged as a versatile technique to closely define the structure and properties of lattice, or cellular, materials. In particular the technique allows components to be manufactured in which the properties vary with position in order, for example, to tailor the shape and strength of a compression wave passing through the component. A brief review of additive manufacturing processes, with particular emphasis on cellular metals, is presented in Sect. 2. Clearly, to optimise cellular materials for energy absorbing applications it is important to determine how their energy-absorbing capability in the loading regime of interest depends on the architecture and material of the lattice. It will be argued here that, in studies aiming to optimise the properties of lattice materials, AM technology offers significant advantages relative to traditional manufacturing methods. One of the tools available to understand and quantify the impact response of lattice materials are computer simulations which have been validated against experiments. Two regimes, which will be termed here, “intermediate” rate loading and “high” rate loading, will be discussed in Sects. 3 and 4, respectively. Intermediate-rate loading is loosely defined as situations in which the elastic and plastic stress waves generated by the initial impact propagate through the full thickness of the specimen before it is fully densified. It is assumed that if intermediate-rate loading were to be maintained for a long period it would eventually lead to full densification as a result of multiple wave reflections within the sample. In our intermediate-rate studies, samples are loaded at velocities ranging from 105 to 157 ms−1 using a direct impact Hopkinson bar apparatus. In high-rate loading, by contrast, the material tends to be fully densified by the first plastic shock. It is assumed that at the shock front there is a density discontinuity with uncompressed (or only elastically compressed material) ahead of the front and fully densified (solid) material behind it. In the studies reported in Sect. 4 a 100 mm light gas gun is used to load cellular samples at velocities in the range 300–700 msμ1. Note that Sect. 4 is based on the work presented in [2, 3]. Conclusions and possible future applications of additively manufactured cellular materials are presented in Sect. 5.