On the sintering mechanisms and microstructure of aluminium–ceramic cenospheres syntactic foams produced by powder metallurgy route

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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.

Introduction

Metal matrix syntactic foams (MMSFs) are a class of close cell foams which also satisfies the definition of particle reinforced metal matrix composites (MMC). The desired porosity is provided by embedding ceramic cenospheres in the metallic matrix [1], [2]. Ceramic cenospheres consisted of alumina–silicates (mullite) recently emerged as reinforcement materials for aluminium-based syntactic foams (Al-MMSF). Their main advantage, compared to conventional MMCs, in general, is their high energy dissipation capacity in relation to their weight [3], [4], [5]. MMSFs can be fabricated by either pressure infiltration, stir casting or by powder metallurgy processes [4], [6]. Pressure infiltration and stir casting are two common ways to produce MMSFs in which the matrix is in liquid stage [4], [7]. In the case of stir casting process, the matrix material is melted (overheated) and pre-heated cenospheres are added in relative small quantities during continuous stirring [7], [8]. The advantage of the relative simplicity of this method is tempered with the potential fracture of the cenospheres during mixing and the low volume fraction of the cenospheres that can be achieved compared to the theoretical one [7], [9]. In pressure infiltration process, generally, a “bed” of ceramic cenospheres is infiltrated with molten matrix material under pressure [10]. High volume fraction of cenospheres and production of large amounts of MMSFs are two characteristics of this technique [7], [10]. The disadvantages of pressure infiltration are the complexity of the production equipment, the difficulty of producing MMSFs with different volume fraction of cenospheres and the cenospheres’ potential to be crushed or filled with matrix as the infiltration pressure passes an upper limit, namely the fracture strength of the cenospheres [7].

A review of the current state of the production of MMCs and specifically MMSFs reinforced with cenospheres, indicates a literature gap concerning powder metallurgy (PM) techniques despite the deeper understanding of PM mechanisms over the last decades and their cost efficient nature [5], [11], [12], [13].

Guo and Rohatgi [14] firstly attempted to produce and evaluate Al-MMCs by PM, using ceramic particles and cenospheres (fly ash particles) as reinforcements. They demonstrated that the proper selection of the compact pressure plays a vital role in PM of cenospheres MMSFs. Furthermore, Neville and Rabiei [15] used steel particles and steel cenospheres as reinforcements to produce a new type of MMSF with superior mechanical properties compared to many existed foams. Also, Vogiatzis et al. [16] produced and evaluated, in terms of mechanical properties, ceramic cenospheres/Al-MMSFs by a PM route. They [16], also, investigated the optimization of compacting pressure during cold compaction of the “green” body. The selection of a proper compaction pressure is a vital aspect of a PM route because if this pressure is high enough, a significant amount of cenospheres might be crushed. Furthermore, Tao et al. [2] and Zhao et al. [5] produced ceramic cenospheres/Al-MMSF and ceramic cenospheres/Fe or Ti-MMSFs by PM route. Both [2], [5] investigated MMSF’s microstructure and compressive behaviour. A special sintering technique (microwave energy) was used by Ananda Kumar et al. [17] to produce MMSFs through PM route. Finally, Mondal et al. and Jha et al. [18], [19] examined the influence of porosity on the mechanical response of ceramic cenospheres/Ti MMSFs produced by PM route, under uniaxial compression.

As reported by Orbulov and Majlinger [20] the quality and the chemical composition of the cenospheres apart from having an important influence on many of their mechanical properties they also affect to a great extent the production of the MMSFs. This is because any chemical interaction between the cenospheres and the matrix material has a detrimental effect on any load transfer and thus on the mechanical properties of the composite [1], [20]. What is of great interest and not, to the best of our knowledge, been studied, is how these chemical interactions influence the PM technique (sintering process) of a cenospheres/Al-MMSF.

As Tao and Zhao [21] have shown, the maximum energy absorption capability for MMSFs can be achieved as the volume fraction of cenospheres increases up to 50%. High volume fractions of cenospheres are certainly related to low density (inversely proportional) and therefore to the porosity (proportional) of the final composite. Nevertheless, Vogiatzis et al. [16] showed, that in the case of cenospheres/Al-MMSF produced by PM technique, a lower limit of the volume fraction of the reinforcement exists (around 30% v/v). Above this limit the energy absorption capability of the composites reduces regardless of the initial production parameters (like the compact pressure of the “green body”) mainly due to the presence of high number of fragments as the cenospheres’ volume fraction increases [16].

Based on these and in order to investigate the influence of the composition (primarily for relative low volume percentage of the reinforcement) and the sintering temperature on the microstructure of the cenospheres/Al-MMSFs produced by PM technique, composites with 10–40% v/v ceramic cenospheres were produced. Furthermore, a description of the sintering stages (mechanisms) is presented for different composites produced at different sintering temperatures. Scanning electron microcopy (SEM), X-ray diffraction measurements (XRD) and X-ray (EDS) analysis were applied to characterize the microstructure of the composites.

Section snippets

Raw materials

Commercial pure aluminium (Al-powder) of a −325 mesh size (particle size ⩽44 μm, average particle size 4.3 μm with physical Malvern: D10 = 4.58 μm, D50 = 9.89 μm and D90 = 20.16 μm) and 99.5% wt. purity along with commercial ceramic cenospheres (Omega minerals-Germany) were selected as matrix and reinforcement material, respectively. The physical and chemical characteristics of cenospheres are presented in Table 1. The compressive strength of cenospheres is about 25 MPa (product data) [16].

The ratio of

Characteristics of ceramic cenospheres

Data, for the cenospheres, obtained by XRD and EDS analysis together with their morphology are illustrated in Fig. 1.

Fig. 1b demonstrates almost spherical cenospheres with generally smooth exterior surfaces. It should be noted that the cenosphere cells are porous by nature. A statistical analysis of a square area with one hundred cenospheres indicated that less than 10% of these particles are broken or have double celled walls. EDS analysis (Fig. 1b) of cenospheres revealed the presence of O

Density and porosity evaluation

Mullite is the only intermediate phase in the Al2O3–SiO2 system at atmospheric pressure and one of the most important ceramic materials [22]. Typically, single-phase mullite ceramics are difficult to be produced with amounts of silica-rich glassy phases or α-alumina present in the structure of mullite ceramics [23]. As already has been mentioned, in this work mullite has a silica-rich bulk composition that can favour any SiO2 activity during the PM procedure (Fig. 1).

In the area of MMSFs the

Conclusions

  • 1.

    Metal–ceramic (mullite) syntactic foams were fabricated by powder metallurgy technique with different sintering temperatures and compositions. For the compact pressure of 250 MPA significant deviations were observed between the theoretical estimated and experimental measured density–porosity values for relative low cenospheres volume fraction (<30% v/v) composites.

  • 2.

    Reaction between aluminium particles and silica is occurred leading to the formation of silicon and precipitation of silicon

Acknowledgements

The authors gratefully acknowledge Dr. -Ing. Fani Stergioudi, Αssoc. Prof. G. Vourlias and Αssoc. Prof. E. Pavlidou for assisting with the XRD and SEM measurements.

References (42)

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