Hollow Struts Research Articles

Improving the properties of ceramic foams by a vacuum infiltration process

Reticulated ceramic foams are widely used for industrial applications such as metal filtration, exhaust gas and air purification, catalyst support and others. In this work, the compression strength and specific surface area of reticulated foams have been improved, while at the same time maintaining a high level of permeability in the final foam structure. In particular, a vacuum infiltration step by using a suitable slurry, followed by a pre-sintering cycle was adopted for filling up the hollow struts, generated due to the burnout of the PU foam. Furthermore, various mixtures of fine and coarse-grained alumina as well as in combination with zirconia, were utilised with the aim of controlling the foam properties such as compression strength, specific surface area and permeability. The compression strength was improved by a factor of two for alumina foams by infiltrating the hollow struts, and by a factor of four when infiltrating the struts of ZTA foams, with the composition 70 mol% Al 2O 3 and 30 mol% ZrO 2. The weight gain resulting from the vacuum infiltration process was in the order of 10 wt%.

Cost‐Efficient Metal–Ceramic Composites—Novel Foam‐Preforms, Casting Processes and Characterisation

AbstractBecause of their dissatisfactory cost‐performance ratio metal matrix composites (MMCs) are still not established in industry, although they show improved properties compared to pure metals in some application fields. The present paper describes the development of enhanced MMCs based on silicon carbide (SiC) foams made by the Schwartzwalder process. Therefore, foams with cell sizes of 30, 45 and 60 ppi based on pressure less sintered SiC (SSiC) were developed. They were coated with layers of coarse SiC particles, which form a rough strut surface. The ceramic content of the foams could be increased to values of 20–30 mass%. Additionally, foam preforms based on clay‐bonded SiC (as they are known from molten metal infiltration) were tested.The preforms were infiltrated with aluminium alloys AlSi9Cu3 and AlSi7Mg0.6 and cast iron EN‐GJSA‐XNiCr35‐5‐2 and EN‐GJL‐250. For aluminium alloys high pressure die casting (HPC) as well as gravity casting was applied, whereas iron was only infiltrated by gravity casting.For HPC an excellent interlocking of metal and preform was observed because of the microporosity of the rough surface of the SSiC foam struts. By the use of gravity casting preform cells up to 45 ppi could be well infiltrated. Microporosity in the ceramic coating and the typical hollow struts of the foams did not show metal infiltration.Even by use of moderate ceramic volume fractions pressure‐infiltrated aluminium matrix composites showed a high specific stiffness of up to E/ρ = 42 GPa cm3 g−1 compared to conventional Al or Mg alloys (E/ρ = 25–27 GPa cm3 g−1). Ceramic foam based MMCs produced by pressure less casting showed no advantages in mechanical properties compared to pure metals. Nevertheless it can be expected that they can provide improved wear resistance and lower thermal expansion coefficients.

Enhancement of mechanical properties of hollow-strut foams: Analysis

Hollow-strut foams are predicted to have superior mechanical properties compared with solid-strut foams of the same relative density. Due to the enlarged bending stiffness of the hollow strut, the enhancements of stiffness, buckling strength, plastic collapse strength, and brittle failure strength and fracture toughness were substantial (even an order of magnitude) according to the analysis and comparisons. The hollow-strut foam is much more damage tolerant than the solid-strut foam. The enhancement of the robustness to waviness influence is dramatic. The stiffness enhancement of the wavy hollow-strut foam will exceed an order of magnitude. With hollow struts, foams have the comparable (even stiffer and stronger) mechanical properties compared with stretching dominated lattice truss materials.

Failure modes in open-cell foams with hollow struts

Failure modes in open-cell foams with hollow struts are analyzed. As the walls of the struts become thin, the failure mode of the foam can change from plastic collapse of struts loaded in bending to plastic collapse of struts loaded in direct compression. For very thin strut walls an elastic strut wrinkling failure mode is possible. Expressions for the failure modes are derived, and a failure mode map is presented to clarify when the various modes are expected. The analysis suggests that a hollow strut foam can be substantially stronger than a solid strut foam with the same relative density.

Micromechanical Models of Metallic Sponges with Hollow Struts

Coating of a precursor structure, which is subsequently removed by chemical or thermal treatment, is a technology for producing cellular materials with interesting properties, for example in the form of metallic sponges with hollow struts. In this paper idealized models for determining the effective elastic properties of such materials are presented. The chosen models for the structures are space-filling, periodically repeating unit cell models based on idealized models of wet foams, which were generated with the program ‘Surface Evolver’. The underlying topology is that of a Weaire-Phelan structure. The geometry of the micro-structures can be described by two principal parameters, viz. the volume fraction of solid material in the precursor structures, which determines the shape of the final structures, and the thickness of the metallic coating, which defines their apparent density. The influence of these two parameters on the macro-mechanical behavior is investigated. The elastic properties of the micro-structures are described by three independent elastic constants owing to overall cubic material symmetry. The dependence of the effective Young’s modulus on the direction of uniaxial loading is investigated, and the elastic anisotropy of the structures is evaluated.

Open-cell foams with hollow struts: Mechanical property enhancements

Open-cell foams with hollow struts are predicted to have superior mechanical properties compared to a solid strut foam with the same relative density. The enhancement in Young's modulus and plastic collapse strength is modest while for the case of power law creep deformation the enhancement can be very substantial.

Strong ceramic foams from polyurethane templates

The most conventional manufacturing route for ceramic foams with open cells consists of coating reticulated polyurethane foams. The resulting material has well defined cell size, but suffers from low mechanical strength owing to the presence of hollow struts. As a result, the performance and utilisation of ceramic foams manufactured by this method are limited in certain applications. One solution to overcoming this inherent disadvantage is to use a stronger ceramic coating material and to ameliorate the wetting of the polyurethane template. Such a material is reaction bonded Al2O3 (RBAO), which has high mechanical strength as a result of its fine grain structure, with less or even without glassy phases at the grain boundaries. The present work demonstrates the feasibility of producing strong ceramic foams by using a reaction bonded modified replica technique starting from polyurethane templates varying in both composition and cell size distribution.

Synthesis, structure, and mechanical properties of Ni–Al and Ni–Cr–Al superalloy foams

Reticulated nickel foams were alloyed with 8–9 wt% Al or 14–18 wt% Cr and 5–9 wt% Al in a three-step method consisting of (i) gas-phase deposition of Cr and/or Al onto the struts of a pure Ni foam by pack-cementation at 1000 °C; (ii) homogenization at 1200 °C to remove concentration gradients and to solutionize the alloyed struts; (iii) aging at 900 °C to form γ ′ precipitates within the struts. The resulting alloyed foams retain the low relative densities (less than 3%) and open-cell structure with hollow struts of the original pure Ni foams, and they exhibit struts with the γ/γ ′ structure typical of nickel-base superalloys. The compressive behavior of the superalloy foams, measured at ambient temperature, is compared to model predictions. The creep behavior of the superalloy foams, measured between 680 and 825 °C in the stress range of 0.1–0.3 MPa, is discussed using two models assuming strut compression or strut bending as deformation modes.

Pack aluminisation kinetics of nickel rods and foams

The kinetics of pack aluminisation has been described through a multiphase, finite difference model considering diffusion of aluminium into nickel under conditions of concentration dependent diffusivity and finite specimen dimensions, allowing complete conversion of the specimen to the equilibrium intermetallic phase. For a cylindrical geometry, the model accurately predicts the growth of an outer Ni2Al3 shell, the increase in diameter, and the average composition as measured for 0.5 and 1 mm diameter nickel rods aluminised at 1000°C for up to 4 h. For nickel foams with hollow struts, which are modelled as thin sheets, good agreement is found with the average aluminium concentration measured on two foams with strut wall thicknesses of 41 and 84 νm. The measured aluminisation kinetics for a foam with strut thickness of 6 νm, however, is much slower than predicted owing to the high surface area of this foam, which prevents the centre of the specimen from reaching the equilibrium aluminium surface concentration.

Fixed geometry controlled entrainment ventilated convergent nozzle and method

A combination convergent jet engine aircraft nozzle (86, 142, 146) and a ventilation tube (88, 136, 144) are fixed in place and have no moving parts. Ambient air is entrained through duct inlets (96, 140, 154, 160), through the nozzle walls, through hollow struts (90, 138), the struts supporting and being connected to a hollow central tube (102, 144) or a plurality of tubes (136), to adjacent the downstream end of the nozzle to mix at takeoff with the jet forming gas exhaust. The ducts (102, 136, 144) are positioned with respect to the nozzle exhaust so that entrained ambient air flows into the gas exhaust during takeoff and so that the ambient air flow is substantially restricted or shut off during cruise flight. Ambient air inlet openings (160) are formed as a porous surface, the holes through the surface being generally in the shape of louvers (166) of which downstream end portions (174) are depressed inwardly of the outer wall surface of the nozzle to provide a minimum drag along the outer nozzle periphery during cruise flight and at takeoff. A method of positioning and fixing the outlet end of a noise suppressor within a convergent nozzle so that entrained ambient air flows into the gas exhaust during takeoff to substantially reduce the engine noise and so that ambient air is substantially restricted or is shut off entirely during cruise flight. The method in which ambient air is entrained in a noise suppressor within a convergent nozzle and is exited downstream adjacent the nozzle downstream end to mix with the gas exhaust during takeoff to suppress the engine noise, and at cruise flight substantially restricting or shutting off the ambient air through the noise suppressor by turning engine exhaust gas toward the noise suppressor axis downstream of the ambient air exit.

Cylindrical and boat-tailed afterbodies in transonic flow with gas ejection

The measurements were made in an intermittent ejector driven 9 X 8 in. perforated wall transonic wind tunnel with atmospheric stagnation pressure and Reynolds number of 4 X 10/ft. The model consisted of a 1 in. diam 5 caliber centerbody and interchangeable 2 caliber afterbodies. The centerbody was supported on a pair of 45° swept forward hollow struts (see Fig. la), one strut contained pressure tubes