On the sintering mechanisms and microstructure of aluminium–ceramic cenospheres syntactic foams produced by powder metallurgy route
TL;DR: In this paper, the in situ reaction between silica (phase of mullite) and aluminium particles, apart from any degradation of the mullite cell wall and the precipitation of silicon in the matrix, changes the sintering mechanism of syntactic foams due to the formation of eutectic aluminium-silica liquid quantities.
Abstract: Metal matrix syntactic foams of ceramic (mullite) cenospheres (10–40% v/v) embedded in pure aluminium matrix, were fabricated by powder metallurgy technique at different sintering temperatures (610–710 °C). Density, vis-a-vis porosity and microstructural characteristics were examined by using scanning electron microcopy and energy dispersive X-ray spectroscopy. A first systematic description of the sintering mechanism of the composites is presented. The in situ reaction between silica (phase of mullite) and aluminium particles, apart from any degradation of the mullite cell wall and the precipitation of silicon in the matrix, changes the sintering mechanism of syntactic foams due to the formation of eutectic aluminium–silica liquid quantities. Transient liquid phase sintering, with surface material diffusion, and liquid phase sintering, with bulk diffusion, selected to be the main sintering mechanisms, are described in detail for sintering temperatures below and above the melting point of aluminium matrix, respectively.
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TL;DR: Friction stir processing (FSP) is a rapidly emerging newer solid-state technique for composite fabrication as discussed by the authors, which involves surface modification which in turn enables successful adaptation of surface p...
Abstract: Friction stir processing (FSP) is a rapidly emerging newer solid-state technique for composite fabrication. It involves surface modification which in turn enables successful adaptation of surface p...
106 citations
TL;DR: In this article, a comprehensive review on the mechanism of pore formation, mechanical properties, and applications of metallic porous materials is presented, which highlights some important factors for advanced wear-resistant tool and biomedical implant applications of porous metallic materials.
Abstract: This paper presents a comprehensive review on the mechanism of pore formation, mechanical properties, and applications of metallic porous materials. The different manufacturing techniques of metallic porous materials using various pore-forming agents (e.g., sodium chloride, polymethyl methacrylate, magnesium, and cenosphere) are highlighted in the first part of this review. Subsequently, the pore formation mechanism and pore morphology in final products as well as corresponding pore-forming agent removal techniques (e.g., sintering-dissolution process, thermally stimulated decomposition, thermally melted elimination, and embedding cenosphere technique) are specifically discussed. Then, some major influential factors on the mechanism of pore formation, including pore size, shape, distribution, and porosity, are analyzed in detail. Meanwhile, the primary mechanical properties such as compressive strength, elastic modulus, fatigue properties, and flexural strength of metallic porous materials depending on pore morphology and porosity are explored in detail. Furthermore, their applications in structural and functional aspects according to their pore morphology and mechanical properties are emphatically summarized. Finally, this review article highlights some important factors for advanced wear-resistant tool and biomedical implant applications of porous metallic materials.
87 citations
TL;DR: In this paper, a low-cost metal matrix syntactic foams (MMSFs) have been developed and characterised in the structure and mechanical properties relationship aspect, and the mechanical properties were characterised by compressive tests.
Abstract: Metal matrix syntactic foams (MMSFs) are high-performance foams consisting of a light-weight matrix and a set of porous fillers. The obstacle against the wide-spreading of MMSFs is their high cost (raised by the fillers’ price). In this study, low-cost MMSFs have been developed and characterised in the structure – mechanical properties relationship aspect. The MMSFs were produced by low-pressure infiltration from the combination of two matrices (Al99.5, AlSi9MgMn) and three nominal diameters (4, 8 or 11 mm) light expanded clay agglomerate particles (LECAPs). The density of the MMSFs was 1.38–1.53 g.cm−3. The mechanical properties were characterised by compressive tests. The effects of the alloying and T6 heat treatment were evaluated. Exponential relationships were found between the LECAPS’ fracture force values and the strength and absorbed energy values. The produced MMSFs proved equal or higher properties than the top materials investigated in the literature. The produced MMSFs showed plastic collapse failure.
60 citations
TL;DR: In this paper, the CaAl12O19 reinforced Al 2O3-Cr2O3 composites were prepared by pressureless sintering process, and the influences of CaO content on the sinterings properties, mechanical properties, and microstructure evolution of the composites are studied.
Abstract: Al2O3-Cr2O3 refractories have excellent slag corrosion resistance and can adapt to the oxidation/reduction atmosphere in the smelting reduction ironmaking furnace. However, Al2O3-Cr2O3 refractories have poor mechanical properties and sintering properties. In order to improve the mechanical properties of Al2O3-Cr2O3 materials, the CaAl12O19 reinforced Al2O3-Cr2O3 composites were prepared by pressureless sintering process, and the influences of CaO content on the sintering properties, mechanical properties, and microstructure evolution of the composites were studied. The results show that a small amount of CaO can significantly improve the compactness of the composites, which is mainly due to the formed sheet-like CA6 fill the gap between the solid solutions, and reduces the porosity of the composites. In addition, the sheet-like CA6 makes the connection between solid solutions closer, and the intergranular fracture gradually transforms into a mixed mode of intergranular and transgranular fracture. The best mechanical propertie is observed at S4 with the CaO content of 2 wt.%. Compared with sample S0 without CaO, the hardness, compressive strength and flexural strength of the S4 were increased by 35.19 %, 49.69 %, and 68.34 %, respectively. The addition of excessive CaO will deteriorate the mechanical properties of the composites, because the formation of a large number of layered CA6 increases the porosity of the composites. Furthermore, a small amount of CaO addition can significantly improve the thermal shock resistance of the composites. After 10 and 20 thermal shock cycles, the strength loss rates of S4 are only 5.83 % and 8.74 %, respectively.
42 citations
TL;DR: In this article, high performance metal matrix syntactic foams (MMSFs) have been produced by liquid state infiltration of AlSi12 alloy into two loosely packed sets of different nominal diameter (150μm and 1500μm) oxide hollow spheres (CHSs) and their mixed packs.
Abstract: High performance metal matrix syntactic foams (MMSFs) have been produced by liquid state infiltration of AlSi12 alloy into two loosely packed sets of different nominal diameter (150 μm and 1500 μm) oxide hollow spheres (CHSs) and their mixed packs. The structure of the MMSFs has been studied by X-ray computer tomography and microscopy. The mechanical properties were mapped by compressive tests. The structural investigations showed proper infiltration. In the case of pure small and large CHS filler, the compressive tests revealed high strength levels all over the deformation of the materials up to 50% compressive strain, with the dominance (higher strength values) of the smaller CHSs filled MMSFs. In the case of bimodal CHS filling, the compressive features of the MMSFs have been mixed with two local peaks corresponding to the failure of the smaller and larger CHS filled MMSFs. Rule of mixtures is applicable to estimate the compressive strength.
41 citations
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990 citations
TL;DR: A review of the literature on liquid phase sintering can be found in this paper, with a focus on the densification and microstructure evolution events of a mixture of powders.
Abstract: Liquid phase sintering (LPS) is a process for forming high performance, multiple-phase components from powders. It involves sintering under conditions where solid grains coexist with a wetting liquid. Many variants of LPS are applied to a wide range of engineering materials. Example applications for this technology are found in automobile engine connecting rods and high-speed metal cutting inserts. Scientific advances in understanding LPS began in the 1950s. The resulting quantitative process models are now embedded in computer simulations to enable predictions of the sintered component dimensions, microstructure, and properties. However, there are remaining areas in need of research attention. This LPS review, based on over 2,500 publications, outlines what happens when mixed powders are heated to the LPS temperature, with a focus on the densification and microstructure evolution events.
940 citations
TL;DR: Mullite has achieved outstanding importance as a material for both traditional and advanced ceramics because of its favourable thermal and mechanical properties as discussed by the authors. But it is not a suitable material for many applications.
Abstract: Mullite has achieved outstanding importance as a material for both traditional and advanced ceramics because of its favourable thermal and mechanical properties. Mullite displays various Al to Si ratios referring to the solid solution Al 4+2 x Si 2−2 x O 10− x , with x ranging between about 0.2 and 0.9 (about 55 to 90 mol% Al 2 O 3 ). Depending on the synthesis temperature and atmosphere mullite is able to incorporate a number of transition metal cations and other foreign atoms. The crystal structure of mullite is closely related to that of sillimanite, which is characterized by chains of edge-connected AlO 6 octahedra running parallel to the crystallographic c -axis. These very stiff chains are cross-linked by tetrahedral chains consisting of (Al,Si)O 4 tetrahedra. In more detail: Parallel to a the tetrahedra are linked to the relatively short more stiff Al–O(A, B) bonds, whereas parallel b they are linked parallel to the relatively long more compliant Al–O(D) bonds. In mullite some of the oxygen atoms bridging the tetrahedra are removed for charge compensation. This gives rise to the formation of oxygen vacancies and of T 3 O groups (so-called tetrahedral triclusters). The anisotropy of the bonding system of mullite has a major influence on the anisotropy of its physical properties. For example: • the highest longitudinal elastic stiffness is observed parallel c , but lower ones parallel a and especially parallel b , • the maximum of the thermal conductivity occurs parallel c , but maller ones parallel a and especially parallel b , • large thermal expansion especially parallel b , • fastest crystal growth and highest corrosion parallel c . Heat capacity and thermal expansion measurements of mullite display reversible anomalies in the temperature range between about 1000 and 1200 °C. It is believed that tetrahedral cations, bridging O atoms, and O vacancies undergo dynamical site exchange processes at high temperatures. At lower temperatures the dynamic disorder may transform to a static one. Diffraction experiments revealed that also partially ordered states may exist.
901 citations
TL;DR: In this paper, the effect of alumina particle size, sintering temperature and time on the properties of Al-Al 2 O 3 composite were investigated, including density, hardness, microstructure, yield strength, compressive strength and elongation to fracture.
345 citations
TL;DR: In this paper, a closed cell composite metal foam has been produced using a powder metallurgy technique, which is processed by filling the vacancies between densely packed steel hollow spheres with steel powder and sintering them into a solid cellular structure.
174 citations