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Refoaming of deformed aluminum foam fabricated by precursor foaming process using remaining foaming agent and densification using friction stir welding
... According to Zu et al. [30,44], in Al-Si alloy matrix foam, the cracks are generated across the a-Al/Si eutectic zone and the brittle eutectic phases. In addition, the bending behavior of the AFS is mainly dependent on the foam core structure and cover sheet; the microstructure of the foam matrix has little effect on the mechanical behavior of the AFS [4,45]. Figure 12. ...
... According to Zu et al. [30,44], in Al-Si alloy matrix foam, the cracks are generated across the a-Al/Si eutectic zone and the brittle eutectic phases. In addition, the bending behavior of the AFS is mainly dependent on the foam core structure and cover sheet; the microstructure of the foam matrix has little effect on the mechanical behavior of the AFS [4,45]. ...
An Al-Si matrix foam sandwich (AFS) with 6063 Al alloy cover sheets was fabricated by hot rolling combined with melt foaming. A foamable AlSiMg1/SiCp matrix precursor was prepared by the melting route. Hot rolling at 480 °C was carried out to obtain a mechanical bonding interface between the cover sheet and the foamable precursor. Meanwhile, the pore structure of the AFS was deeply affected by the foaming temperature and foaming time during the foaming process. Different pore growth mechanics of the crack-like pore disappearance mechanism (CDM) and pore active expansion mechanism (AEM) were concluded based on the pressure difference in pores inside and outside. Three bending tests were applied to three types of AFSs with different pore structures to evaluate the relation between pore structures and AFS mechanical properties. The bending property of the AFS with fewer layers of pores is like that of a dense material. The bending property of the AFS with a pore size in the range of 0~1 mm presents a typical sandwich shear failure mode. The AFS with a uniform pore structure, in which the shapes of the pores are predominately polygons and the pore diameter is concentrated in the range of 0.5~3 mm, processes a good energy absorption capacity, and the bending stress–strain curve fluctuates greatly after the first stress drop.
... Since FSW is a lowtemperature welding process, the foaming agent is mixed into the aluminum without decomposition. This method is an application of the fabrication of aluminum foam by the precursor foaming method [28,29], especially, the method of preparing precursor using FSW [30][31][32][33][34][35][36][37]. It has been shown that the steel and aluminum joints can be separated at joining line with low strength by heat foaming the aluminum part of the joint. ...
The use of multi-materials, in which a wide variety of materials are used in the appropriate places, is being promoted to reduce carbon dioxide emissions. In the multi-materialization of automobiles, steel-aluminum joints are widely used. Friction stir welding (FSW) is used for joining steel and aluminum. Because of the solid state and short joining time of the FSW, the thickness of the brittle intermetallic compound layer can be minimized and high-strength joining can be achieved. Meanwhile, since recycling aluminum greatly reduces carbon dioxide emissions compared to the production of new ingots, easy disassembly technology is required. In this study, fabrication of thin foaming agent sheet was attempted. Then, we attempted to realize easy disassembly of steel/aluminum joints by FSW, which can realize strong joints, by using foaming agent sheets that can easily introduce foaming agent on joining interface during FSW. It was shown that foaming agent sheet can be prepared by solidifying foaming agent and pore morphology stabilizer powders using spark plasma sintering. It was also shown that even if a foaming agent sheet was introduced at the joining interface and FSW was performed, good jointing can be achieved. In addition, it was found that the introduction of a foaming agent sheet and foaming at the joining interface can significantly reduce the maximum disassembly load with fracturing at the joining interface. The above results indicate that the use of a foaming agent sheet can easily add an easy-disassembly characteristic to the area near the joining interface in the FSW process.
... In the case of ADC12, the solidus temperature is 515 • C and the liquidus temperature is 580 • C [43]. Because the heating temperature of the aluminum foam sheet in this study was sufficiently lower than the foaming temperature, no shape changes in aluminum foam, such as pore deformation or re-foaming behavior [44,45], were observed. ...
Aluminum foam has relatively low tensile and flexural strengths because it is composed of many pores with thin cell walls. One method of strengthening aluminum foam is to fabricate a composite material with a dense lightweight resin. In this study, the fabrication of composite materials by directly printing resin on an aluminum foam surface using a 3D printer was attempted. The resin was directly printed on both heated and unheated aluminum foam. It was shown that composite materials consisting of aluminum foam and resin can be fabricated by directly printing resin with a 3D printer on both heated and unheated aluminum foam. The resin was softened during the printing process in the case of directly printed resin on heated aluminum foam, allowing more resin to penetrate into the pores than in the case of directly printed resin on unheated aluminum foam. In addition, it was shown that resin can be directly printed on the aluminum foam with a high bonding strength, as a large amount of resin penetrated into the pores, resulting in an anchor effect. That is, composite materials consisting of aluminum foam and arbitrary-shaped resin with relatively high bonding strength can be fabricated when a large amount of resin is allowed to penetrate into the pore.
... Therefore, the pressure, which is raised during foaming, can be calculated as 0.5 kPa using Boyle-Charles's law. However, there is also TiH 2 , which remains undecomposed in the foam [30]. Therefore, the amount of the generated H 2 gas would have been much smaller. ...
A semi-solid route is expected to be a fabrication method that can fabricate aluminum alloy foams with a variety of mechanical properties, but the allowance fluctuation of the fabrication conditions of aluminum alloy foams with high reproducibility is not clear. The objective of this study was to reveal the allowance fluctuation between the setting temperature and the actual temperature of the melt to fabricate stable foams, having pores with small pores and high circularity, and the influence of the increasing volume fraction of the solid on the pore morphology. Al-Si alloy foams were fabricated five times by adding a blowing agent into a semi-solid slurry under the same setting fabrication conditions, such as the temperature and concentration of oxygen in the atmosphere. The results of small relative standard deviations of pore diameter and circularity indicated that the conducted fabrication process had high reproducibility, even if the volume fraction of the solid changed in a range of 5%. When the volume fraction of the solid exceeds the minimal fraction of primary crystals for prevention of drainage, the clogging effect works more efficiently because the ratio of clogged cell walls increases. Additionally, the preferred range of the volume fraction of the solid for the fabrication of stable foam was revealed to be around 15% to 35%.
Porous aluminum, which has many pores inside, is lightweight and has high shock absorption properties. However, once porous aluminum is impacted and deformed, the cell walls are collapsed and difficult to reuse. In this study, we attempted re-foaming of deformed porous aluminum using the undecomposed foaming agent remained during the porous aluminum fabrication process. In our previous study, it was found that porous aluminum can be re-foamed by compressing and densifying with FSP before optical heating. In this study, we attempted further foaming of refoamed porous aluminum by optical heating after densification by compression and FSP. The amount of foaming gradually decreased with each repeated foaming, and the pore structures were not the same as that of the initial foaming, but further foaming was achieved after re-foaming. From the XRD analysis, it was observed that the undecomposed foaming agent remained after re-foaming.
Auxetics are a class of structural metamaterials with a negative Poisson’s ratio. In comparison to conventional structures, auxetic structures are proven to exhibit several superior properties including higher energy absorption, enhanced indentation resistance, and improved mechanical properties. As a result, auxetic structures are gaining popularity as lightweight, high-performance protective structures to resist blast and impact loads. Although several past reviews on auxetics partially covered different aspects of auxetics, such as classification of architectures, mechanisms, mechanical properties, energy absorption behaviours, and applications, this field still requires a comprehensive overview of the anti-blast and -impact performances of the different auxetic structures. Therefore, this paper aims to review recent advances in auxetic-based lightweight, high-energy absorbing protective structures. Different design factors affecting the performance of the protective structures are critically discussed. Moreover, a classification of modern auxetic topologies, the status of large-scale auxetic fabrication, and anti-ballistic performance evaluation methods are presented in this paper. Overall, auxetic core sandwich panels offer superior protective performance than equivalent conventional protective armours. However, several limitations and challenges exist with the design, fabrication, and implementation of auxetic-based protective structures.
Aluminum (Al) foam does not recover to its initial shape once it absorbs shock energy and deforms. In this study, the refoaming of deformed A6061 Al foam was attempted. Closed-cell Al foam fabricated by a precursor foaming process was reproduced to obtain a similar closed-cell Al foam by subjecting it to the precursor foaming process again. It was found that only slight refoaming of the precursor was observed for a cold-compressed Al foam. It is considered that the low density of the precursor causes the release of the generated gases from the cracks and pores of the precursor. In contrast, sufficient refoaming of a cold-compressed precursor can be achieved by conducting spark plasma sintering (SPS). Initial Al foams with porosity of approximately 80% and closed-cell pore structures can be reproduced with similar porosity and pore structures. From these results, it was found that not all the blowing agent in the precursor was used during the initial foaming, and some of the blowing agent remained in the foamed Al foam without decomposition. Therefore, the successful reproduction of the Al foam was due to the remaining blowing agent in the initial Al foam.
This work gives an overview of the production, properties and industrial applications of metal foams. First, it classifies the most relevant manufacturing routes and methods. Then, it reviews the most important properties, with special interest in the mechanical and functional aspects, but also taking into account costs and feasibility considerations. These properties are the motivation and basis of related applications. Finally, a summary of the most relevant applications showing a large number of actual examples is presented. Concluding, we can forecast a slow, but continuous growth of this industrial sector.
The possibilities for manufacturing metal foams or other porous metallic structures are reviewed. The various manufacturing processes are classified according to the state of matter in which the metal is processed-solid, liquid, gaseous or ionised. Liquid metal can be foamed directly by injecting gas or gas-releasing blowing agents, or by producing supersaturated metal-gas solutions. Indirect methods include investment casting, the use of space-holding filler materials or melting of powder compacts which contain a blowing agent. If inert gas is entrapped in powder compacts, a subsequent heat treatment can produce cellular metals even in the solid state. The same holds for various sintering methods, metal powder slurry foaming, or extrusion and sintering of polymer/powder mixtures. Finally, electro-deposition or metal vapour deposition also allow for the production of highly porous metallic structures. The various ways for characterising the properties of cellular metals are reviewed in second section of this paper. Non-destructive as well as destructive methods are described. Finally, the various application fields for cellular metals are discussed. They are divided into structural and functional applications and are treated according to their relevance for the different industrial sectors. (C) 2001 Elsevier Science Ltd. All rights reserved.
A preliminary study of the reproducibility of aluminum foam was performed. Aluminum foam was fabricated by a sintering and dissolution process. It was found that aluminum foam containing a blowing agent can be fabricated without the decomposition of the blowing agent, namely, the densified aluminum foam can be used as a foamable precursor for refoaming. By heat treatment of the densified aluminum foam containing the blowing agent, pores were reproduced in the aluminum. © 2017 The Minerals, Metals & Materials Society and ASM International
A novel method, named two-step foaming, was investigated to prepare three-dimensional shaped aluminum alloy foam used in car industry, spaceflight, packaging and related areas. Calculations of thermal decomposition kinetics of titanium hydride showed that there is a considerable amount of hydrogen releasing when the titanium hydride is heated at a relatively high temperature after heated at a lower temperature. The hydrogen mass to sustain aluminum alloy foam, having a high porosity, was also estimated by calculations. Calculations indicated that as-received titanium hydride without any pre-treatment can be used as foaming agents in two-step foaming. The processes of two-step foaming, including preparing precursors and baking, were also studied by experiments. Results showed that, low titanium hydride dispersion temperature, long titanium hydride dispersion time and low precursors porosity are beneficial to prepare three-dimensional shaped aluminum alloy foams with uniform pores.
A1050 porous aluminum is fabricated by the FSP route and the effect of the tool rotating rate on the porosity and morphology of the pores is investigated. To fabricate high-porosity porous aluminum with a uniform pore size distribution, a certain amount of stirring action is necessary; however, excessive stirring action is ineffective. A sufficiently uniform mixture is realized by traversing the FSP tool two times at a tool rotating rate exceeding 2200rpm. The results indicate the minimum necessary amount of stirring action and will provide a guideline for improving productivity. Also, to improve the morphology of pores, optimizing the amount of Al2O3 is effective. Closed-cell porous aluminum with a porosity of about 80% was successfully fabricated by 2-pass FSP at 2200rpm with the addition of 7mass% Al2O3, a holding temperature of 998K and a holding time of 10min.
Al-TiH2 powder mixtures have been cold compacted into precursors for foaming using single action, uniaxial pressing. After heat treatment, at 800°C for 5 min, foaming was only observed for precursors with a theoretical density above 94%. This density limit corresponds to the transition from interconnected to closed porosity, confirming that isolating porosity and enveloping the foaming agent in the matrix are vital to successful foaming. Although the variation in foam density for similar samples was small, the cell structures were different indicating that a low variation in foam density is not necessarily a good indication of achieving reproducible foam structures. The orientation of the sample had little effect on the foaming response for compacts with a length to diameter ratio below 1:2. For longer samples the foaming response was not uniform along the length of the sample indicating that in complex pressings the variation in local pressure must be understood in order to avoid regions being compacted below the critical density for foaming.
The Al alloy foam was prepared by melt foaming method with addition of titanium hydride as blowing agent. The effects of thermal decomposition properties of titanium hydride on Al alloy melt foaming behavior and pore structures were studied. The decomposition properties of titanium hydride powder were investigated by using temperature programmed decomposition (TPD) set-up. By separating and simulating the TPD spectrum of titanium hydride, a set of thermal decomposition kinetics equations of titanium hydride were acquired. Combining these equations with researches by using instant freezing method, scanning method and real-time method for measuring melt pore structure, the relationship between Al alloy melt foaming process and decomposition properties of titanium hydride was revealed, according to which the Al alloy foam with different pore structures were successfully prepared.
Friction stir welding (FSW) is a relatively new solid-state joining process. This joining technique is energy efficient, environment friendly, and versatile. In particular, it can be used to join high-strength aerospace aluminum alloys and other metallic alloys that are hard to weld by conventional fusion welding. FSW is considered to be the most significant development in metal joining in a decade. Recently, friction stir processing (FSP) was developed for microstructural modification of metallic materials. In this review article, the current state of understanding and development of the FSW and FSP are addressed. Particular emphasis has been given to: (a) mechanisms responsible for the formation of welds and microstructural refinement, and (b) effects of FSW/FSP parameters on resultant microstructure and final mechanical properties. While the bulk of the information is related to aluminum alloys, important results are now available for other metals and alloys. At this stage, the technology diffusion has significantly outpaced the fundamental understanding of microstructural evolution and microstructure–property relationships. # 2005 Elsevier B.V. All rights reserved.