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.     

Laser additive manufacturing foam aluminium-12 wt-% silicon with different addition TiH2 foaming agent

The aim of this work is to study the effects of laser additive manufacturing on microstructure and mechanical properties of foam Al–12?wt-%Si aluminium alloy with/without TiH₂ foaming agent. The results showed that low porosity closed foam Al–12Si was successfully obtained. The effect of processing parameters on the porosity is discussed. The porosity was changed from initial 20.9% without foaming agent to 32.3 and 45.9% with 5 and 10% addition of foaming agent, respectively. Average micro-hardness values of the obtain foam Al–Si alloy is varied from 100 to 130?HV and the shape of compressive stress–strain curve is as the same as foam aluminium made by powder metallurgy and casting methods.     

Characterisation of Strength Behaviour of Aluminium Foam Sandwiches Under Static Load

Abstract: Cellular metals, particularly aluminium foams, are increasingly used in automotive, railcar and aircraft industries due to their advantages such as low density, comparatively high stiffness, noise damping or non‐flammability. From this point of view a new powder‐compact forming and foaming technique has been developed to manufacture 3D‐aluminium foam sandwich (AFS) parts for components of railcars without glued joint between the foamed core layer and both cover sheets. Three different stages of foam expansion have been analysed to describe the material properties. We have characterised samples made of plane AFS panels by compression, bending and shear tests. The shear strain is optically measured by digital image correlation to estimate the shear modulus of foamed sandwiches. Furthermore, these experimentally determined values and curves are the basis for the verification and optimisation of finite element models by design of experiments. As a result of this work, recommendations could be derived for improving technological parameters.     

Metal foaming by a powder metallurgy method: Production, properties and applications

 A method for fabricating metal foams based on the powder metallurgy process is presented. This foaming process allows for the production of complex-shaped foam parts, metal foam sandwich panels and foam filled hollow profiles. A range of alloys can be foamed using this method including aluminum, zinc, tin, lead and steel. The as-produced part has a closed-cell microstructure and a high fraction of porosity (typical range from 40–90% porosity). Selected mechanical properties of metal foams are evaluated, including the loading of foam samples with and without face skins and the axial crushing of tubular structures with foam reinforcement. Potential applications are discussed such as light-weight construction and energy absorption for both military and civilian uses. Received: 22 July 1998 / Accepted: 4 September 1998     

金属泡沫材料研究进展

综述了金属泡沫材料的各种制备方法。液相法制备金属泡沫材料包括气体吹入法、固体发泡剂法和固体—气体共晶凝固法、熔模铸造法、渗流铸造法、喷射沉积法以及粉末加压熔化法等制备方法。采用金属粉末烧结法、浆料发泡法等制备工艺可以从固相制备金属泡沫材料。电沉积法以及气相沉积法可用于制备高孔隙率的金属泡沫材料。最后简要总结了金属泡沫材料的应用。     

Microstructure of high-strength foam concrete

Foam concretes are divided into two groups: on the one hand the physically foamed concrete is mixed in fast rotating pug mill mixers by using foaming agents. This concrete cures under atmospheric conditions. On the other hand the autoclaved aerated concrete is chemically foamed by adding aluminium powder. Afterwards it is cured in a saturated steam atmosphere.New alternatives for the application of foam concretes arise from the combination of chemical foaming and air curing in manufacturing processes. These foam concretes are new and innovative building materials with interesting properties: low mass density and high strength. Responsible for these properties are the macro-, meso- and microporosity. Macropores are created by adding aluminium powder in different volumes and with different particle size distributions. However, the microstructure of the cement matrix is affected by meso- and micropores. In addition, the matrix of the hardened cement paste can be optimized by the specific use of chemical additives for concrete.The influence of aluminium powder and chemical additives on the properties of the microstructure of the hardened cement matrices were investigated by using petrographic microscopy as well as scanning electron microscopy.     

Comparison of a low molecular weight and a macromolecular surfactant as foaming agents for injectable self setting hydroxyapatite foams: Polysorbate 80 versus gelatine

Hydroxyapatite foams are potential synthetic bone grafting materials or scaffolds for bone tissue engineering. A novel method to obtain injectable hydroxyapatite foams consists in foaming the liquid phase of a calcium phosphate cement. In this process, the cement powder is incorporated into a liquid foam, which acts as a template for macroporosity. After setting, the cement hardens maintaining the macroporous structure of the foam. In this study a low molecular weight surfactant, Polysorbate 80, and a protein, gelatine, were compared as foaming agents of a calcium phosphate cement. The foamability of Polysorbate 80 was greater than that of gelatine, resulting in higher macroporosity in the set hydroxyapatite foam and higher macropore interconnectivity. Gelatine produced less interconnected foams, especially at high concentrations, due to a higher liquid foam stability. However it increased the injectability and cohesion of the foamed paste, and enhanced osteoblastic-like cell adhesion, all of them important properties for bone grafting materials.     

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.     

Production of metallic foam under low gravity conditions during parabolic flights

Metallic foams are a recently developed light weight material. They are porous structures consistent of metals like aluminum, tin, zinc, lead etc. or their alloys. The pore sizes are in the range of millimeters and relative densities down to 10% of the original material can be achieved. Since metallic foams combine relative low weight with high stiffness, their applications are mainly for means of light weight structures as used for example in cars. Also other applications like sandwich structures and metallic filters are of interest. To make metallic foams applicable for an industrial use, still some technical and principle problems have to be solved. These concern mostly the structure of the resulting foam, which is very inhomogeneous and not well understood. Our aim is to contribute to the understanding of the physical processes that take place during the foaming process. In this paper we will introduce the powder metallurgical production method we used for producing metallic foams. Then we will describe the two main physical processes during the foam genesis and present our experimental idea and setup used to obtain information on these processes. Finally we will discuss the first results we got from parabolic flights and terrestrial experiments.     

泡沫铝芯三明治板的粉末冶金制备及其板/芯界面研究

采用粉末冶金发泡法制备了Fe/Al/Fe、Ti/Al/Ti泡沫铝芯三明治结构,研究了泡沫铝芯的膨胀规律,分析了面板与泡沫铝芯的冶金结合过程,提出了微观结合机制.试验发现,结合界面由扩散反应形成的金属间化合物以及冷却得到的凝固组织两部分组成,从而形成良好的冶金结合.     

Enhancing the strength of pre-made foams for foam concrete applications

Chemical and mechanical foaming techniques are commonly used in foam concrete technology for developing lightweight construction materials. The characteristics of the foam before the lightweight structure sets and maintains its shape has a great impact on the properties of foamed concretes. The tendency of the foams to coalesce and collapse during the preparation process brings some challenges in controlling the properties of cellular structures. Consequently, it is critical to improve the stability of fresh foams in order to produce high quality cellular structures using a predictable and reliable approach. Aggregating the liquid film around bubbles is known to be effective in improving the stability of foams, but the impact of this stabilizing method has not been investigated for foam concrete applications. In this paper, Xanthan gum (with a thickening capacity) has been utilized as the foam stabilizer to aggregate the liquid film. This stabilizing method is shown to significantly enhance the pore size distribution of foam concretes. The resulting pre-made foams are remarkably more stable than the control foam, and the mechanical properties of the final cellular structure are considerably improved (about 34% in mechanical foaming and 20% in the chemical foaming technique).     

SiC-particulate aluminum composite foams produced from powder compacts: foaming and compression behavior

The foaming behavior of SiC-particulate (SiC_p) aluminum composite powder compacts containing titanium hydride blowing agent was investigated by heating to 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_p-addition on the foaming and compression behavior. The SiC_p-addition (10 wt%) was found to increase the linear expansion of the Al powder compacts presumably by increasing the surface as well as the bulk viscosities. The compression tests conducted on Al and 10 and 20% SiC_p foams further showed a more brittle compression behavior of SiC_p/Al foams as compared with Al foams. The collapse stresses of Al and 10% SiC_p/Al foams were also predicted using the equations developed for the open and closed cell foams. Predictions have shown that Al foam samples behaved similar to open cell foams, while 10% SiC_p/Al foam collapse stress values were found between those of open and closed cell foams, biasing towards those of the open cell foams.     

Grey-fuzzy algorithm to optimise machining parameters in drilling of hybrid metal matrix composites

Metal matrix composites (MMCs) are difficult to machine due to their abrasive properties. With the projected widespread application of MMCs, it is necessary to develop an appropriate technology for their effective machining. The present investigation focuses on finding the optimal machining parameters setting in drilling of hybrid aluminium metal matrix composites using the grey-fuzzy algorithm. This proposed algorithm, coupling the grey relational analysis with the fuzzy logic, obtains a grey-fuzzy reasoning grade to evaluate the multiple performance characteristics according to the grey relational coefficient of each performance characteristics. The Taguchi method of experimental design is a widely accepted technique used for producing high quality products at low cost, therefore a L₂₇ 3-level orthogonal array is used for the experiments. The optimisation of multiple responses in complex processes is common; therefore, to reduce the degree of uncertainty during the decision making, fuzzy rule-based reasoning is integrated with the Taguchi’s method. The response table, response graph and analysis of variance (ANOVA) are used to find the optimal setting and the influence of machining parameters on the multiple performance characteristics. Experimental results have shown that the required performance characteristics in the drilling process are improved by using this approach.     

Impact of melt rheology on zein foam properties

Zein, the main protein fraction in maize, is left as a by-product from bio-ethanol production. The protein has been investigated as a material for a long time, but mainly in the form of films. In contrast, foamed zein is presented in this article. Zein foams may perhaps be used, e.g. as trays for biodegradable food packages or as scaffolds for tissue engineering. A batch method for manufacturing solid foams was successfully developed, the foams being manufactured by evaporation of solvent from zein resins. In order to be suitable for foam formation, a resin must possess gas-retaining properties, which can be predicted by extensional rheology. The presence of plasticizer in some of the resins decreased their extensional viscosity, and this in turn affected the foaming process. Although all the resins displayed strain-hardening behaviour, there was coalescence of pores in all the foams. Insufficient extensional viscosity resulted in the collapse of pore walls during foam expansion. Structure analysis showed, e.g. that most pores were elongated along the main axis of the mould in which the foams were manufactured. The plasticizer content in the resins had no significant effect on the mechanical properties of the foams.     

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.     

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.     

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.     

Deformation characteristics of metal foams

The deformation behaviour of a series of aluminium and zinc foams was investigated by uniaxial testing. Because the deformation behaviour of metal foams is expected to be anisotropic owing to the existence of a closed outer skin and with respect to the foaming direction, a series of measurements was carried out where the orientation of the outer skin and the foaming direction were varied. Stress–strain diagrams and corresponding compression strengths were determined for aluminium- and zinc-based foams. The influence of an age-hardening heat treatment was investigated. Finally, the axial deformation behaviour of aluminium tubes filled with aluminium foam was tested under uniaxial loading conditions. The results of the measurements are discussed in the context of possible applications of metal foams as energy absorbers. © 1998 Chapman & Hall     

泡沫金属的制造方法

概要叙述了泡沫金属的特性，制造方法及用途，并以泡沫铝为例，着重介绍了发泡法的原理及过程，指出了泡沫金属制造技术上应该解决的问题。     

The FOAMCARP process: foaming of aluminium MMCs by the chalk-aluminium reaction in precursors

A brief outline is presented of the factors involved in the search for gas-generating agents offering superior performance for foaming of liquid aluminium alloys. These include kinetic and thermodynamic characteristics of decomposition reactions, the ease of dispersion of the powdered foaming agent in the melt, the nature and likely effect of decomposition products on melt flow, potential reactions between the foaming gas and the melt and the availability, cost and ease of handling of the powder concerned. There is one very promising candidate material, calcium carbonate, which offers advantages compared to currently-employed hydride powders in virtually all aspects of their performance. It is shown that foams can be produced having appreciably finer cells (<1 mm diameter) and more uniform cell structures than currently-available melt route foams, a potentially lower ceramic content in the cell walls and dramatically reduced raw material costs. The presence of an oxidising foaming gas in the cells leads to reaction with the liquid cell surface, forming a continuous oxide film. The presence of this film has a significant effect on foam stabilisation, slowing down cell coalescence and melt drainage.