Effect of fly ash addition on the structure and compressive properties of 4032–fly ash particle composite foams

doi:10.1016/j.jallcom.2009.08.026

Journal of Alloys and Compounds

Volume 487, Issues 1–2, 13 November 2009, Pages 618-625

https://doi.org/10.1016/j.jallcom.2009.08.026 Get rights and content

Abstract

4032–10 vol.% fly ash particle composite foams were processed using stirring technique, followed by direct foaming. Two types of fly ash particles were used; one was microballoons with two different average diameters of 80 and 140 μm. The other was solid precipitators with an average size of 20 μm. The structural analysis of the composite foams revealed that the pores were mostly closed-cell and irregular ellipses with nearly the same size in each foam sample. EDXA at fly ash–matrix interface showed excess Mg and O at the interface compared with that at the 4032 matrix alloy. The CTEs of the 4032 foam were higher than those of the composite foams. The compression tests indicated that the 4032 and composite foam stress–strain curves exhibited the behavior of typical plastic foam and the composite foam had higher yield, plateau and plastic stress in comparison to those of the 4032 foam. The composite foams showed a lower strain-hardening exponent when compared with the 4032 foam. As the result of higher strength of the composite foams, the increase in energy absorption of the composite foam sample with fly ash precipitators ranged over two times compared to that of 4032 foam.

Introduction

Metallic foams combine the properties of the metal such as strength, deformability, and electrical and heat conductivity with the structural advantages of foam such as low density and high stiffness [1], [2], [3]. Although aluminum or aluminum alloy foams have higher densities in comparison with those of polymeric foams, they have superior performance in terms of compressive properties, stiffness and energy absorption with respect to polymeric foams. Moreover, the allowable temperatures for aluminum foams are five times higher than those for polymers. Thus, they are suitable for many applications such as automotive, shipbuilding, aerospace and civil engineering [4], [5], [6].

Among aluminum alloys, AlSi alloys are suitable for the production of the foamed components because of their lower both density and melting temperature, i.e. lower foaming temperature.

Recently, many new fabrication methods for making metallic foams have been developed [7], [8]. Powder metallurgy and melt foaming are the two commonly techniques adopted for making metallic foams, mostly aluminum and aluminum alloys. In both techniques, gas bubbles are generated either through decomposition of chemical compounds or gas purging and entrapping within the metal in liquid or semisolid states. Melt foaming process is the economical method for the fabrication of metallic foams. In this method, usually, ceramic particles or fibers, such as silicon carbide, aluminum oxide, magnesium oxide and silicon oxide, are added into the molten metal to prevent the drainage of metallic melt and coalescence of bubbles [9]. The ceramic particles or fibers dispersed in the melt should remain in the melt without significant chemical and physical change and thus, can act as thickening agent. On the one hand, the addition of ceramic particles or fibers into melt increases its viscosity and stabilizes the cell wall. On the other hand, the ceramic particles or fibers contained in the cell wall significantly affect the mechanical properties of the metallic foams. Kennedy and Asavavisitchai [10] have reported that Al–10 wt.% TiB₂ particle composite foams produced by powder metallurgy have a higher yield stress and absorb larger quantities of energy than Al foam. However, it has, also, been reported that aluminum alloys reinforced with ceramic particle composite foams exhibit the characteristic of typical brittle foams. For example, Luo et al. [11] have found that the compressive stress–strain curves of AlSi9Mg–SiC particle composite foams are not smooth and exhibit some serrations in comparison with those of AlSi9Mg foams. The effect of SiC particles addition on the compression behavior of Al foams was investigated by Elbir et al. [12]. They have reported that the addition of SiC particle into Al increases the composite foam compressive strength, but induces a more brittle compression behavior.

In the last few years, considerable development has occurred in the potential use of metallic foams for automotive, shipbuilding, aerospace and civil engineering [4], [5], [6]. However, cost still remains a major barrier in the wide spread use of metallic foam components in industry. One of the options to decrease the cost of metallic foams is to use low cost particles as a thickening agent.

In view of the mentioned above, new particles, which not only increase the viscosity of the metallic melt, but also optimize the mechanical properties of the metallic foams, need to be developed. Coal fly ash particles can be extremely attractive materials because of their lower prices and interesting physical and mechanical properties. Huge quantities of coal combustion by-products are generated worldwide. The primary by-product is fly ash particles, which are solid (precipitator ash) particles and hollow, referred to as microballoons with very low densities similar to expensive microballoons of glass, carbon and ceramics. Fly ash has been combined with aluminum and magnesium alloys to produce a class of metal matrix composites called fly ash reinforced aluminum metal matrix composites or ash alloys [13], [14]. Therefore, fly ash particles could be used as thickening agent because of their chemical and physical stability in the aluminum melt.

Accordingly, the present study aims at examining the feasibility of using fly ash precipitators or microballoons as a new thickening agent for foaming process. The effects of fly ash addition on the foam structure, compressive properties and energy absorption of 4032–fly ash composite foams were evaluated and discussed.

Section snippets

Experimental details

Two types of fly ash particles were used; one was microballoons with two different average diameters of 80 and 140 μm. The other was solid precipitators with an average size of 20 μm. The major constituents of the fly ash include SiO₂, Al₂O₃ and Fe₂O₃, while the minor ones are Na₂O, CaO and ZnO. The chemical composition of the aluminum matrix alloy (4032) is given in Table 1. The 4032–10 vol.% fly ash particle composites were produced by the stirring method according to the following procedure. A

Macrostructure of the foams

Macrographs of the 4032 and 4032-based composite foams containing 10 vol.% of small or large fly ash microballoons, or fly ash precipitators are shown in Fig. 1. It can be seen that the pores are mostly closed-cell and irregular ellipses with nearly the same size in each foam sample. Although all the produced foams have relatively uniform cell size and distribution, the 4032 foam exhibits more defects such as crack, void and cell wall corrugation. The effect of microballoon size (within the

Conclusions

1.
4032–10 vol.% fly ash composite foams are successfully processed using stirring technique, followed by direct foaming.
2.
The structure of the composite foams reveals mostly closed-cell and irregular ellipses with nearly the same size in each foam sample. All the produced foams have relatively uniform cell size and distribution, but the 4032 foam exhibits more defects such as crack, void and cell wall corrugation. EDXA at fly ash–matrix interface shows excess Mg and O at the interfaces compared with

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Effect of fly ash addition on the structure and compressive properties of 4032–fly ash particle composite foams

Abstract

Introduction

Section snippets

Experimental details

Macrostructure of the foams

Conclusions

Scripta Materialia

Acta Materialia

Materials Science and Engineering A

Materials Science and Engineering A

Progress in Materials Science

Scripta Materialia

Journal of Alloys and Compounds

Composites Science and Technology

Composites Part A: Applied Science and Manufacturing

Materials Science and Engineering A

Scripta Metallurgica et Materialia

Acta Metallurgica et Materialia