Architectural design in stretch-formed microtruss composites

Abstract

Structural coatings of nanocrystalline metal can significantly enhance the mechanical properties of microtruss cellular materials, creating new types of cellular composites. This study investigated the optimal architectural design under a compressive load for stretch-formed low carbon steel cores reinforced with ∼20 nm grain size nanocrystalline Ni–35 wt%Fe. During stretch-forming, the struts in the starting perforated low carbon steel sheet are elongated and thinned as higher internal truss angles are formed, leading to an optimal internal truss angle. While a nanocrystalline coating enhances the weight specific mechanical properties of an optimally designed conventional microtruss, the resulting composite microtruss becomes sub-optimal since the optimal internal truss angle changes with coating thickness. A strut design framework was therefore developed to determine the optimal truss angle for a given combination of starting material, reinforcing material, sheet geometry, and coating thickness.

Introduction

Cellular metals, i.e. hybrids of space (air) and metal, are an attractive option for weight-limited engineering applications. Microtruss cellular materials can be used as the core in lightweight sandwich panel structures having multifunctional characteristics such as load bearing [1], [2], [3], [4], impact energy absorption [5], pressure and fluid flow control [6], and heat dissipation [7]. These microtruss materials can be fabricated using techniques such as lithography [4], [8], rapid prototyping and investment casting [2], [3], [9], [10], weaving [11], [12], diffusion bonding and brazing [13], [14], [15], and sheet forming [16], [17], [18], [19], see reviews by Wadley [20]. Microtruss materials are specifically designed such that externally applied loads are resolved axially along the internal struts, resulting in failure mechanisms such as inelastic buckling [21], [22]. As a result, at a given relative density, the stiffness and strength of microtruss materials can be up to an order of magnitude higher than those of conventional metal foams which fail in bending [21], [22].

The open and low-relative density internal architecture of microtruss cellular metals, combined with the non-line-of-sight characteristics of electrodeposition also allows them to be reinforced by thin sleeves of nanocrystalline metal [23], [24], [25]. Nanocrystalline materials exhibit large strength increases by virtue of grain size reduction to the nm-scale [26]. By controlling the bath chemistry and the cathodic current waveform, the nucleation of new grains can be favoured over the growth of existing grains [27], allowing an ultra-fine and equiaxed nanostructure to be deposited. In the case of Ni, decreasing the grain size from the μm-scale to nm-scale can result in an order of magnitude yield strength increase (e.g. from 86 MPa [28] to 910 MPa [29] when the grain size decreases from 10 μm to 10 nm). In effect, reinforcing a conventional microtruss metal by electrodeposition creates a metal/metal cellular composite in which the ultra-high strength nanocrystalline sleeve forms an interconnected network of conformal nanocrystalline tubes that controls the overall mechanical properties.

Nanocrystalline electrodeposition also adds a new dimension of complexity to the architectural/material design. While previous studies have outlined the sequence of relevant failure mechanisms such as inelastic buckling, sleeve/core delamination, sleeve fracture, and local wrinkling [23], [24], [25], none of these have addressed the critical question of architectural/material optimization in the presence of this additional degree of freedom. The purpose of the present study is to set up a framework for the optimal design of stretch-formed metal/metal microtruss composites. At the same time, it addresses the key questions of when it is beneficial to reinforce by electrodeposition, and what is the optimal truss angle for a given combination of starting material, reinforcing material, sheet geometry, and coating thickness.