Investigation and characterisation of aluminium alloy foams with TiH2 as a foaming agent

This article presents an account of experiments used to produce aluminium alloy foams by the melt route process using titanium hydride as a foaming agent. Powdered titanium hydride with content of 0.4–1.4% (mass fraction) was added to the molten pure aluminium and the foaming condition was controlled at 690°C (1274°F). In the process, homogeneous aluminium foams were produced with a calcium amount of 1.0–3.0% (mass fraction). The mechanical properties of the aluminium foams with diverse relative density were tested. The result indicates that the foaming agent is suitable for making small aperture aluminium foams with an average pore diameter of 1.2?mm.     

Effects of aluminium powder content and cold rolling on foaming behaviour of xAlp/Al5Si4Cu4Mg/0.8TiH2 composites

Abstract

Aluminium foams were produced by applying powder metallurgy technology. The process began by making aluminium powder and mixing it with alloy powder (Al5Si4Cu4Mg) and foaming agent (TiH₂). The mix was compacted to the form of a billet by cold pressing and then it was hot extruded to a dense foamable strip, which was cold rolled to give 40% thickness reduction. The resulting precursor composites of both the extruded strip and the extruded plus rolled strip were then freely foamed without a mould at a constant temperature of 700°C for different foaming times. The effects of aluminium powder content and cold rolling on the foaming characteristics of the foamable composite strip were studied. It is noted that aluminium powder fibre in the extruded composite strip acts as a barrier to pore initiation and evolution due to the higher melting point of pure aluminium fibre than that of the alloy matrix. Cold rolling promotes foaming of the composite strip due to the TiH₂ cracking and debonding between TiH₂ particles and metal matrix. The morphological and microstructural evolution of composite foams was also investigated. The foaming mechanism can be described by the following sequence: cracklike pore nucleation between elongated powder fibres; ellipsoidal, spherical, and polygonal pore growth; and the collapse of pores as a result of coalescence.     

Study on the forming of sandwich shells with closed-cell foam cores

The efficiency and safety of vehicles represent today one of the most important lines of developing in the automotive industry, for example by the introduction of new materials. In fact, the investment in advanced materials represents one of the most important strategies to reduce injury among vehicle occupants in traffic accidents. Associated with the development of safety systems, there is also the possibility of improving efficiency by the introduction of materials that lead to weight reduction, having a direct impact on fuel consumption and carbon dioxide emissions. Metallic foams are one of these materials, due to the excellent ratio between mechanical properties and density. The main goal of this investigation is to study the mechanical behaviour of aluminium sandwich structures, composed by a metallic foam core with two outer layers of metallic sheets. With this work, the authors intend to contribute to a better understanding and consequently to provide design guidelines for the plastic forming of these composites. In order to correctly characterize the mechanical behaviour of the sandwich structure, the foam core and sheets were tested separately. For the aluminium sheet a series of tensile tests were performed, using samples obtained along three different angles to the rolling direction. For the metal foam core, uniaxial compression tests were used. Finally, with the numerical model defined considering isotropic and anisotropic constitutive models, a set of numerical and experimental bulge tests were performed to evaluate the capacity of forming of these panels, using hydroforming processes.     

The mechanical behaviour of aluminium foam structures in different loading conditions

The use of foam has the potential for energy absorption enhancement. Many types of materials can be produced in the form of foams, including metals and polymers. Of the metallic based foams, aluminium based are among the most advanced. Aluminium foams couple good specific mechanical properties with high thermal stability. Among the various aspects still to be investigated regarding their mechanical behaviour is the influence of a hydrostatic state of stress on yield strength. Unlike metals, the hydrostatic component affects yields. Therefore, different loading conditions have to be considered to fully identify the material behaviour. Another important issue in foam structure design is the analysis of composite structures. The mechanical behaviour of an aluminium foam has been examined. The foam was subjected to uniaxial, hydrostatic stress, pure deviatoric stress, and combinations thereof. Results obtained will be presented as quasi-static and dynamic uniaxial compression and quasi-static bending and shear loading. Moreover, composite structures were made by assembling the foam into aluminium cold extruded closed section 6060 aluminium tubes. The results show that the energy absorption capability of the composite structures is much greater than the sum of the energy absorbed by the two components, the foam and the tube.     

Strain-rate effects in Ni/Al composite metal foams from quasi-static to low-velocity impact behaviour

Metal foams are used as absorbers for kinetic energy but predominantly, they have only been investigated under quasi-static load-conditions. Coating of open-cell metal foams improves the mechanical properties by forming of Ni/Al hybrid foam composites. The properties are governed by the microstructure, the strut material and geometry. In this study, the strain-rate effects in open-cell aluminium foams and new Ni/Al composite foams are investigated by quasi-static compression tests and low-velocity impact. For the first time, drop weight tests are reported on open-cell metal foams, especially Ni/Al composite foams. Furthermore, size-effects were evaluated. The microstructural deformation mechanism was analysed using a high-speed camera and digital image correlation. Whereas pure aluminium foams are only strain-rate sensitive in the plastic collapse stress, Ni/Al foams show a general strain-rate sensitivity based on microinertia effects and the rate-sensitive nano-nickel coating. Ni/Al foams are superior to aluminium foams and to artificial aluminium foams with equal density.     

Thermal conductivity anisotropy in polypropylene foams prepared by supercritical CO2 dissolution

Different relative density polypropylene foams were prepared by means of two foaming processes: chemical foaming by compression moulding and physical foaming by high pressure CO₂ dissolution. By controlling the foaming parameters, such as blowing agent concentration, foaming temperature, pressure drop and pressure drop rate, it was possible to regulate the cellular structure, foams showing from markedly isotropic-like cellular structures to ones with highly-elongated cells in the vertical foam growth direction (honeycomb-like cell orientation). The thermal conductivity was measured using the transient plane source method. Using this technique, it was possible to measure the global conductivity and the thermal conductivity in both the axial and radial directions of a given sample. Results show that the global thermal conductivity of foams was mainly regulated by their relative density. In addition, the honeycomb-like cell orientation of the CO₂ dissolution foams resulted in considerably higher values in axial direction when compared to radial, demonstrating that there was a direct influence of cellular structure on the thermal conduction behaviour of these foams, enabling the development of new polypropylene foams with direction-dependent thermal properties.     

Aluminium foams as a filler for leading edges: Improvements in the mechanical behaviour under bird strike impact tests

The use of aluminium foams as filler materials in aeronautical leading edges is investigated. Particularly, the improvement of the mechanical behaviour of the filled structure respect to the hollow one is analysed by means of standard bird strike impact tests. For this purpose, a collection of AlSi₁₀ foams were fabricated using the powder metallurgical route (PM), and introduced into leading edges profiles, maintaining or reducing the total weight of the composite structure (leading edge + aluminium foam) in comparison with the original one (hollow structure). Bird strike impact tests were carried out in both types of structures, comparing the global deformation and total load transferred in the tests. The results showed that the composite structure, a 13% lighter than the original one, showed four times better behaviour in terms of global deformation and an improvement of two times in the transmitted load.     

Thermographic Monitoring of Aluminium Foaming Process

A novel method for measuring the temperature distribution and evolution of metal foams in the molten state is proposed. Foamable AlSi9 precursor material containing 0.6 wt% TiH₂ was foamed, kept at high temperatures and solidified while its temperature distribution was monitored by a thermographic camera. Free foaming and foaming inside a closed mould were carried out and direct and screened IR monitoring have been tested. Different heating conditions were applied giving rise to homogeneous and inhomogeneous temperature distributions. The effect of oxidation was studied on a piece of pure aluminium for reference purposes. The error sources of the measured temperature were analysed. Direct monitoring of foams was shown to be associated to serious problems with quantitative temperature measurement, while screened monitoring yielded promising and accurate quantitative results.     

Manufacturing of Al/SiCp composite foams using calcium carbonate as foaming agent

Abstract

The aluminium composite foams reinforced by different volume fractions of SiC particles were manufactured with the direct foaming route of melt using different contents of CaCO₃ foaming agent. The density of produced foams changed from 0·43 to 0·76 g cm^?3. The microstructural features and compressive properties of the Al/SiC_p composite foams were investigated. Compressive stress–strain curve of Al/SiC_p composite foams is not smooth and exhibits some serrations. At the same relative density of composite foams, the plateau stress of the composite foams increases with increasing volume fraction of SiCp and decreasing weight percentage of CaCO₃. The relation between plateau stress, relative density, weight percentage of CaCO₃ and SiCp volume fraction of Al/SiCp composite foams with a given particle size was investigated.     

SiC-particulate aluminum composite foams produced by powder compacts: Foaming and compression behavior

The foaming behavior of SiC-particulate (8.6% by volume) aluminum composite powder compacts contained Titanium Hydride blowing agent was investigated by heating above the melting temperature (750°C) in a pre-heated furnace. Aluminum powder compacts were also prepared and foamed using similar compaction and foaming parameters in order to determine the effect of SiC-particulate addition on foaming and compression behavior. The linear expansions of the compacts at various furnace holding times were ex situ determined. Optical and scanning electron microscopy techniques were used to characterize prepared and deformed foams microstructures. The SiC-particulate addition was found to increase the linear expansion and reduce the extent of the liquid metal drainage and cell coarsening of the aluminum compacts. The composite foam samples also showed higher compressive stresses, but a more brittle behavior as compared with aluminum foams.     

Effect of cell shape anisotropy on the compressive behavior of closed-cell aluminum foams

The closed-cell Al–Si foams have been prepared by molten body transitional foaming process using TiH₂ foaming agent. The cell shape anisotropy ratio of specimens with various relative densities was measured. The quasi-static compressive behavior of Al–Si foams in both longitudinal and transverse directions were investigated. The results show that Al–Si foam loaded in the transverse direction exhibits a lower stress drop ratio. The relationship between plastic collapse stress ratio and cell shape anisotropy is in accordance with Gibson and Ashby model. The plastic collapse stress and the energy absorption property of Al–Si foams increase following power law relationship with relative density. Moreover, Al–Si foams exhibit higher plastic collapse stress and the energy absorption property in the longitudinal direction than that in the transverse direction.     

Ultrasonic Torsion Welding of Sheet Metals to Cellular Metallic Materials

One of the most important requirements for finding new applications for cellular metals is to integrate them in complex technical structures. The metal foams have to be joined to each other, or to sheet materials, by suitable joining techniques. The main topics of this paper are the ultrasonic torsion welding of cellular metallic materials to sheet metals and the investigation of the mechanical properties of the joints. The basic materials of foams and sheet metals were different aluminum and iron alloys. Depending on the materials used, weldings with tensile shear strengths of up to 25 MPa were realized. Using aluminum foam sandwich (AFS) and sheet metals, successful weldings were performed before and after the foaming process. Furthermore, it was possible to perform a successful foaming process with the unfoamed AFS/sheet metal joints. Microscopic investigations showed that the ultrasonic welding technique allows the joining of the metal foams with sheet metals without significant deformation of the joining partners. The temperatures during the welding process in the interface were below the melting point of the foams and the sheet metals.     

Numerical modelling of dynamically loaded metal foam-filled square columns

The crushing behaviour of dynamically loaded metal foam-filled square columns has been investigated using an extended version of the existing self-similar pressure dependent constitutive model for metal foams. The model has been implemented in ABAQUS/Explicit and analyses have been conducted using different approaches to model the uniaxial and hydrostatic hardening behaviour of metal foams. A practical and reliable procedure to approximate the observed anisotropic behaviour within the computational framework of isotropic plasticity is introduced. The comparison between the available experimental and newly generated numerical results is presented in order to illustrate the accuracy and efficiency of the implemented model in predicting the crashworthiness of filled columns.     

Investigation on the influence of cell shape anisotropy on the mechanical performance of closed cell aluminium foams using micro-computed tomography

The mechanical behaviour of closed-cell aluminium foams made by both powder metallurgy (LKR) and liquid state (Hydro) processes is investigated. Hydro foams exhibit a significant anisotropy in their mechanical behaviour. The transverse direction stands out as the most favourable one in terms of strength. In contrast, LKR foams show an almost isotropic compressive behaviour. Both foams perform at a level far below the theoretical predictions. The reduced values are a result of imperfections and defects in the cellular microstructure. X-ray microfocus computed tomography (μCT) is therefore used for internal investigation of the foam cell structure. 2D and 3D quantitative image analyses have been performed on μCT images to characterise the morphometric parameters of the foams. The main parameters of interest are cell size, cell size distribution and cell features information. A preferred cell orientation in Hydro foams is observed along the normal and the transverse directions of the specimen. This cell shape anisotropy is quantified using the dimensions of the three axes of the equivalent ellipsoids. The orientation of the cells is well characterised by pole figures of the three axes of equivalent ellipsoids. The influence of this geometrical anisotropy on the mechanical behaviour of the foam is discussed.     

Fatigue and cyclic creep of replicated microcellular aluminium

The cyclic behaviour of 400 μm pore size replicated aluminium foam is assessed in tension-tension fatigue with a stress ratio equal to 0.1, keeping the load amplitude constant, for relative density values comprised between 0.175 and 0.220. The number of cycles to failure ranges from 6 × 10² (lowest relative density) to 5 × 10⁶ (highest relative density). The foams display cyclic creep coupled with a strong influence of relative density on their general fatigue performance. Data analysis shows that the foam fatigue behaviour is dominated by cyclic creep, which governs both the deformation and the fatigue life of the cycled specimens, yielding characteristics globally in line with what is expected knowing the metal making the foam.     

Effect of the interface bonding on the mechanical response of aluminium foam reinforced steel tubes

Aluminium foams have been recently proposed as filling reinforcements to improve impact behavior of hollow components used as protection systems in vehicles. In this study, aluminium foam filled stainless steel tubes have been prepared by directly foaming metal powder compacts inside the tubes. Attention was concentrated on the interface phenomena that characterize the core–shell interaction and the process parameters determining the metallurgical reactions between the two alloys. The formation of binary and ternary intermetallic compounds was observed at the aluminium/steel interface whenever the growth of the oxide layer on the foam surface in foaming was constrained. Compression tests of the reinforced tubes confirmed a maximized energy absorption in coincidence with the formation of the interface bonding. In those cases, extended foam intrusions into compressed tube folds were observed. The microstructural investigation revealed that in the transition zone the intermetallic layer strength was comparable to that of the foamed matrix.     

Mold-Filling Ability of Aluminum Alloy Melt during the Two-Step Foaming Process

For the two-step foaming method,one of the most cost-effective ways to fabricate three-dimensional shaped aluminum alloy foams with dense outer surface skin,it is crucial to describe and predict the moldfilling behavior of the shaped aluminum alloy foams with a favorable pore-distribution accurately.In this paper,a mold-filling model for semi-solid aluminum alloy foams was initially established and subsequently employed to predict the filling height,which represents the mold-filling ability of semi-solid aluminum alloy foams in a specially designed tube-like mold.Our results indicate that the proposed model can be applied to characterize the mold-filling property of aluminum alloy melts in a quantitative manner.Theoretically,our findings actually provide a guideline for mass-production of the shaped aluminum alloy foams by using the two-step foaming process.     

Improvement of the foaming process for 4045 and 6061 aluminium foams by using the Taguchi methodology

Taguchi methodology has been applied to the production process of aluminium foams to investigate the variability detected in several properties (including bulk density, outward appearance and density homogeneity along foaming direction), for foaming tests carried out under identical conditions. The analysis of the process has been performed separately for two different alloys, the 4045 and 6061. The results have allowed finding the main factors that influence those properties. In addition, it has been possible to establish those foaming conditions able to minimize the variability in density, to improve the outward appearance and to obtain a higher homogeneity in density, all at the same time. Different final factors have been found for the two alloys; such differences have been explained in terms of the different viscosity of the aluminium melts as well as the different content of foaming agent.     

聚氨酯/环氧树脂互穿网络硬泡静态压缩塑性变形模式

采用同步法合成聚氨酯/环氧树脂互穿网络硬泡,通过FTIR和SEM研究其化学结构及微观结构形态,为研究其压缩响应及变形模式,对其进行静态压缩试验。结果表明:IPN硬泡的压缩行为表现出各向异性。平行发泡方向的压缩应力-应变曲线表现出三个变形阶段,其中平台阶段的显著特征是应变软化和局部变形。垂直发泡方向的压缩曲线则单调增加,平台阶段没有产生应变软化,整个压缩过程中试样变形均匀分布。IPN硬泡的静态压缩存在明显的应变率效应,环氧树脂含量对压缩性能影响显著。描述IPN硬泡压缩局部变形理论模型,变形带厚度和变形带前沿传播速度的理论值与试验值吻合较好,采用该模型分析研究IPN硬泡的压缩变形机理。