Honeycombs are ultra-light materials with outstanding mechanical properties, which mainly originate from their unit cell configurations rather than the properties of matrix materials. Honeycombs are triggering in numerous promising applications in the fields of architecture, automotive, railway vehicle, marine, aerospace, satellite, packaging and medical implants, etc. Here we provide an in-depth overview of some important advances in basic structural design to obtain novel honeycombs with various improved mechanical properties. Firstly, the review introduces the classical honeycomb configurations. Then, a clear classification of advanced designs is established and discussed according to the design scale. The designs on the macro scale are divided into hierarchical, graded and disordered, while the designs on the meso scale are grouped as hybrid, curved ligament and reinforced strut. Moreover, the effects of different designs on the mechanical properties of honeycomb are discussed quantitatively. Finally, we summarize the important potential designs to improve the mechanical properties of honeycombs and the challenges in further research.

Advanced honeycomb designs for improving mechanical properties: A review

In this research, the mechanical behavior of closed-cell aluminum (Al)-alloy foams was investigated at different temperatures in the range of 25-450 °C. The main mechanical properties of porous Al-alloy foams are affected by the testing temperature, and they decrease with the increase in the temperature during uniaxial compression. From both the constant/serrated character of stress–strain curves and macro/microstructural morphology of deformed cellular structure, it was found that Al foams present a transition temperature from brittle to ductile behavior around 192 °C. Due to the softening of the cellular structure at higher temperatures, linear correlations of the stress amplitude and that of the absorbed energy with the temperature were proposed. Also, it was observed that the presence of inherent defects like micropores in the foam cell walls induced further local stress concentration which weakens the cellular structure’s strength and crack propagation and cell-wall plastic deformation are the dominant collapse mechanisms. Finally, an energy absorption study was performed and an optimum temperature was proposed.

The Temperature Effect on the Compressive Behavior of Closed-Cell Aluminum-Alloy Foams

Laser additive manufactured WC reinforced Fe-based composites with gradient reinforcement/matrix interface and enhanced performance

This study proposed a new bio-inspired hierarchical circular honeycomb (BHCH), mimicking the hierarchical structures from wood. The mechanical properties and energy absorption characteristics of the proposed structures were investigated by means of quasi-static compression tests and finite element (FE) analysis. Subsequently, the influences of geometric parameters and material selections on the energy absorption capacity were investigated. The experimental results and parametric study indicated that the BHCH outperformed the conventional circular honeycomb (CH) in terms of energy absorption capacity. Specifically, the BHCH had a higher specific energy absorption (SEA) than the corresponding CH with the same wall thickness (i.e. 45.3%) and the same volume of honeycombs (i.e. 71.2%). Furthermore, the relative stiffness, relative strength, and relative energy absorption of the BHCH were significantly higher than other popular cellular structures. Finally, a theoretical model was developed to estimate the mean crushing stress (MCS) and interaction effect of the proposed structure, which further demonstrated its enhanced energy absorption mechanisms. A good correlation between the predicted values and the numerical results proved the reliability of the proposed analytical models. With its excellent performance, the proposed BHCH provided a promising solution for the enhanced energy absorption of the honeycomb structure used in a wide range of applications from structural components of defence structures to protection systems in vehicles. 

Mechanical properties and energy absorption of bio-inspired hierarchical circular honeycomb

Crashworthiness performance and microstructural characteristics of foam-filled thin-walled tubes under diverse strain rate

Investigating the internal structure and mechanical properties of graphene nanoflakes enhanced aluminum foam

In this study, the microstructural control methods and mechanical properties of a β-solidified TiAl alloy Ti-46Al-2Nb-2V-1Mo-Y (at%) were investigated systematically. It was found that a pretreatment of annealing in α + β + γ phase field contributed to decomposition of α/γ lamellar structures via α/γ → α + γ + β. After further nearly isothermal forging at 1240 °C, a fine-grained and fully recrystallized β + γ microstructure was produced. Grain size of the wrought microstructure was totally below 25 µm. We considered that the decomposition transformation of α/γ→γ + β induced by both pretreatment and hot forging, and dynamic recrystallization of β and γ grains directly resulted in the present equiaxed microstructure. By a final annealing treatment of 1360 °C/1 h/FC, fully-lamellar (FL) microstructure with colony size smaller than 100 µm was obtained, while an even finer duplex one was produced by two-step treatment of 1320 °C/2 h/FC + 1250 °C/4 h/FC. This duplex microstructure (DP) displays a superior tensile elongation of 2.2% at room temperature, while the FL one exhibits a yield strength of 700 MPa and a medium elongation 1.5%.

Microstructural control and mechanical properties of a β-solidified γ-TiAl alloy Ti-46Al-2Nb-1.5V-1Mo-Y

Silicon (Si), which exists in the form of solid solution and silicide, is an important component in high temperature titanium (Ti) alloys. The content of Si is often limited to less than 0.5wt.%, but powder metallurgy shows the potential for the preparation of titanium alloys with high Si content and high temperature. The addition of Si is beneficial to the improvement of strength, creep resistance and oxidation resistance of high temperature titanium alloy, but it will reduce the plasticity, especially the plasticity at room temperature. In this review, with particular stress on the influence of heat treatment and hot working process on the precipitation behavior of silicide, as well as on the effect of Si on the properties and strengthening mechanism of high temperature titanium alloys. The development direction of high temperature titanium alloy containing Si is to increase the content of Si and to control the precipitation behavior of silicide through reasonable process.

Recent advances in silicon containing high temperature titanium alloys

Characterization of metal grid-structure reinforced aluminum foam under quasi-static bending loads

The unique thermal history of different metal additive manufacturing processes would have profound impacts on the resulting microstructure and material properties. However, few have conducted benchmark research on the impacts. This work provides a comprehensive benchmark comparison on microstructure, mechanical properties, and their underlying mechanisms in selective laser melting (SLM), electron beam melting (EBM), and mill-annealing of Ti–6Al–4V alloys. The results have shown that the SLMed and EBMed samples possess very fine acicular α′ martensite while the conventional mill-annealed ones have granular α phase. The SLMed samples exhibit the highest tensile and yield strength resulted from the combined effects of refined α’ martensite and high microscale residual stress. The lowest tensile and yield strength and intermediate elongation of the EBMed samples are attributed to the relatively high number of type-II pores and in-situ annealing for residual stress relief during the printing process. The mill-annealed samples have the highest elongation due to the fully dense structure, the negligible microscale residual stress, and favorable grain orientation. It is expected to improve the ductility of SLMed samples via appropriate post-annealing and enhance the strength of EBMed samples by reducing the number of type-II pores through process optimization. The fundamental differences in microstructure and properties are attributed to the unique thermal histories of the concerned processes.

Ertuan Zhao

Papers

Investigating the internal structure and mechanical properties of graphene nanoflakes enhanced aluminum foam

Microstructural control and mechanical properties of a β-solidified γ-TiAl alloy Ti-46Al-2Nb-1.5V-1Mo-Y

Recent advances in silicon containing high temperature titanium alloys

Characterization of metal grid-structure reinforced aluminum foam under quasi-static bending loads

A comparative study on mechanical properties of Ti–6Al–4V alloy processed by additive manufacturing vs. traditional processing