Metal matrix syntactic foams produced by pressure infiltration—The effect of infiltration parameters

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Abstract

Metal matrix syntactic foams (MMSFs) were produced by pressure infiltration. Two parameters of the infiltration process (pressure and time) were varied and the infiltrated length was measured as the function of infiltration parameters in order to get data for the implementation of pressure infiltration as mass-production of MMSFs similar to injection mould casting, especially in the short infiltration time range (<10 s). The infiltrated length was found to be linear function of pressure and square-root function of time. The effect of the infiltration parameters on the microstructure and mechanical properties of MMSFs were investigated by optical microscopy and standardised compression tests. The microscopic images were used to qualify the pressure infiltration and showed that more than one combination of infiltration parameters can be found for successful production of a part with given required dimensions. Considering the compression tests, the main characterising properties were mapped as function of infiltration parameters. The registered values showed dependency on the infiltration parameters and indicated that a given infiltration length produced by higher pressure and shorter time has better mechanical properties. The infiltrated specimens were isotropic, anisotropy was not observed in the reference measurements.

Introduction

Metal matrix syntactic foams (MMSFs) are particle reinforced composites consisting of low weight metal matrix (Al or Mg for instance) and hollow spheres in closely or randomly packed structure [1], [2]. The hollow spheres' material is usually some sort of ceramic (SiO2, and/or Al2O3) or metallic (pure iron or steel) and they are available in commerce [3], [4], [5], [6], [7]. MMSFs are made for various reasons, for example to produce energy-saving lightweight components, collision and vibration dampers or core material for sandwich composites applied as hulls.

There are two common ways to produce MMSFs; both of them use the matrix in liquid state. On one hand MMSFs can be made by stir casting. In this case the matrix material is melted, overheated and the pre-heated hollow spheres are added in small amounts during continuous stirring [8], [9], [10], [11], [12], [13], [14], [15], [16]. The advantages of this process are simplicity and low cost. Shortcomings are the potential fracture of the hollow spheres due to the mechanical mixing and the lower volume fraction of the hollow spheres compared to the theoretically possible. On the other hand MMSFs can be made by infiltration. In the case of wetting matrix-reinforcement systems (e.g. Al matrix and Fe hollow spheres) the infiltration can be spontaneous and gravity casting can be successfully used to produce MMSFs [17], [18], [19], [20]. If the matrix-reinforcement system is non-wetting (e.g. Al matrix and Al2O3 hollow spheres) a threshold pressure is needed to initialise infiltration, most commonly inert gas pressure is used [21], [22], [23], [24], [25], [26], [27], [28], [29]. Gas pressure infiltration is similar to low pressure hot chamber injection mould casting and it is important as possible industrial scale production method of MMSFs. Gas pressure infiltration has three main parameters: infiltration pressure, time and temperature.

The infiltration pressure is forcing the molten matrix metal into the gaps between the hollow spheres. It is normally significantly higher than the threshold pressure because along the path of the molten metal the infiltration pressure decreases due to viscous and form drag like in usual flows. Asthana et al. [30] studied the infiltration of metal matrix composites (MMCs) and found that the infiltrated length is linearly proportional to the pressure. The same was also derived by Garcia-Cordovilla et al. [31] for packed ceramic particulates and liquid metals. According to these equations with sufficiently high pressure any size of MMSF can be produced theoretically. However an upper limit exists: namely the fracture strength of the hollow spheres. If the pressure exceeds this limit the molten metal can fill up the hollow spheres and the foam structure is lost.

A few review papers were published about the effect of infiltration time [30], [31], [32], [33], [34]. All of these papers present theoretical considerations to derive the infiltrated length as square root function of time. Besides the infiltration pressure and time the equations depend on the dynamic viscosity (η), the surface tension (γ), the contact angle (Θ) and geometrical dimension (for example the radius (r) in the case of straight capillaries). The existing models use common and serious simplifications:

  • Geometrical simplifications to get analytically solvable closed formulas (regular spatial distribution of capillaries instead of random distribution, permeability and tortuosity of the capillaries etc.).

  • The chemical reactions between the reinforcement and matrix is neglected (the reactions generally reduces the surface tension and wetting angle).

  • The time dependence of the wetting angle, the air resistance and the gravitational force are neglected.

With the above mentioned restrictions Washburn [32] analysed the dynamics of capillary flow and found that for simple cases like straight capillaries it is possible to derive closed form formulas to predict the infiltrated length as the function of pressure and time. However in more complicated cases these formulas would not apply and the infiltration length could only be determined by experiment. Semlak and Rhines [33] studied the rate of capillary rise of liquid metals in porous-metal bodies consists of parallel capillary tubes. Asthana et al. [30] studied non-reactive particle reinforced MMC systems and found that, a comprehensive theoretical framework suitable for rationalising all the observed features of pressure-infiltrated MMCs is lacking due to the extremely complex physicochemical and hydrodynamic nature of the process. In the case of complex problems direct measurements could be more practical. Garcia-Cordovilla et al. [31] investigated non-reactive ceramic particulate-liquid metal systems. Although the discussed results showed much progress in the process of infiltration many questions remained unsolved. For example the nature and reason of the observed incubation period in the infiltrated length–infiltration time diagrams. Kaptay [34] discussed some aspects of high-temperature capillarity and provided an extended set of mathematical expressions connecting different phenomena relevant to production of MMCs with interfacial energies. Muscat and Drew [35] investigated the kinetics of infiltration of molten Al in TiC preforms. Short incubation periods in the infiltrated length–infiltration time diagrams were observed. In the case of reactive matrix-reinforcement systems Kevorkijan [36] experimentally monitored the dynamics of the infiltration process in the time range of 100–3600 s. The reinforcements were SiC, Si3N4, AlN, Mg3N2, TiO2 and fused silica, while the matrix was standard A356-T6 aluminium alloy (7 wt% Si and 0.3 wt% Mg). It was found that the infiltration length increased linearly with the square root of time. Eustathopoulos et al. [37] studied the effect of oxygen-wetting transition in metal/oxide systems. From the analysis, it was shown that in the presence of oxygen a definite decrease of the contact angle (Θ) could be observed due to the adsorption of oxygen–metal clusters at the metal/oxide interface. In Al–SiO2 systems it can be explained by the decomposition of SiO2, while in the case of Al–Al2O3 system the rupture of the oxide layer on the melt causes a ∼40° decrease in the contact angle (Θ).

Finally the infiltration temperature (as third infiltration parameter) has only indirect effect on the infiltrated length through the temperature dependent properties (η, γ and Θ). Higher temperatures can also initialise or fasten possible chemical reactions.

In summary the results of existing infiltration models give satisfactory predictions on the infiltrated length as the function of pressure, time and temperature in the case of simple systems, however large deviations from the predicted results can be observed in case of reactive systems having complex geometry, changing permeability and short infiltration times, such in the case of MMSFs. Therefore the first aim of this paper is to determine infiltrated length values through experimental methods as function of infiltration pressure and time at constant temperature in order to ensure base data for the implementation of low pressure hot chamber injection mould casting as industrial scale production method of MMSFs. The measurements have been performed in the short infiltration time range (<10 s), in order to analyse the nature of the so called ‘incubation period’ and to get results from this range typically lacking or not detailed in the professional literature.

Additionally the produced MMSFs should be qualified both on the microstructure scale (to qualify the infiltration process itself) and on the aspects of mechanical properties (to ensure design parameters for engineers). The overall microstructure of MMSFs is usually investigated by optical microscopy. Extended studies apply scanning electron microscopy (SEM) with energy dispersive X-ray spectrometry (EDS) in map and line scan modes to investigate the interface layer, that responsible for load transferring from the matrix to the reinforcement [38], [39], [40]. Considering the mechanical characteristics mainly the compressive properties have been studied. Palmer et al. [21] performed compressive tests on Al based MMSFs reinforced with 45 µm or 270 µm hollow spheres. The MMSFs with smaller hollow spheres showed higher compressive strength. Rohatgi et al. [24] also investigated the size effect of the hollow spheres and confirmed the same relationship. Balch et al. [41], [42] investigated Al matrix MMSFs and they have found that, MMSFs can ensure higher mechanical properties than conventional metallic foams. Kiser et al. [43] investigated the mechanical response MMSFs under compression loading. Extremely high energy absorption capacity was observed compared to conventional metal foams. Wu et al. [44] established a new method to predict the compressive strength of MMSFs. Tao et al. [45], [46], [47] investigated the mechanical properties and failure mechanisms of MMSFs with monomodal and bimodal distribution of hollow spheres. The bimodal foams have the advantages of a flat plateau regime, high plateau stress and good ductility. Zhang et al. [25] studied the mechanical response of Al matrix MMSFs with low-cost porous ceramic spheres of diameters between 0.25 and 4 mm. They found that the amount of energy absorption was mainly determined by the volume fraction of Al and to a lesser extent by the mechanical properties of the ceramic spheres. Mondal et al. [11] measured considerably higher plateau stress on MMSFs containing 30–50 vol% hollow spheres than in the case of conventional Al foams. Rabiei and O'Neill [18] produced MMSFs reinforced by steel hollow spheres, that displayed superior compressive strength and energy absorption capacity. By compression and low-velocity impact tests Castro et al. [48] confirmed the MMSFs as potentially beneficial materials in high energy absorption applications. Peroni et al. [49], [50] investigated the mechanical behaviour of MMSFs made of hollow glass microspheres mixed in an iron matrix. Compared to other types of metal foams it showed greatly increased quasi-static compressive strength.

Besides the compressive behaviour other mechanical properties have been also studied: the wear properties of MMSFs was examined on pin-on-disc apparatus and showed much better wear behaviour, than the pure matrix [12], [15], [16], [51]. On the other hand the creep resistance has been also studied on aluminium matrix syntactic foams [52].

According to above mentioned mechanical investigations the second aim of this paper is to map the standardised compressive properties (DIN 50134 [53]) of the produced MMSFs as the function of infiltration parameters.

Section snippets

Materials

Ceramic hollow spheres of SL300 grade from Envirospheres Pty. Ltd. [4] were used as reinforcement. The average diameter, wall thickness and density of the hollow spheres were 150 µm, 6.75 µm and 0.692 g/cm3 respectively. The wall of the hollow spheres was built up from oxide ceramics (33 wt% Al2O3, 48 wt% amorphous SiO2 and 19 wt% mullite (3Al2O3·2SiO2)). AlSi12 alloy (Al4047) was used as matrix material; it contained: 12.830 wt% Si, 0.127 wt% Fe, 0.002 wt% Cu, 0.005 wt% Mn, 0.010 wt% Mg, 0.007 wt% Zn and

Infiltrated length

The measured and averaged infiltrated lengths and their scatters are plotted in Fig. 4. The scatter was relatively small, not higher than 10 mm. In order to predict the infiltrated length in the whole parameter range a nonlinear surface, defined in Eq. (2) was fitted on the points.L=AptBwhere p is the infiltration pressure in kPa, t is the infiltration time in s, A and B are fitting parameters with the values of 0.10972 and 0.49727 respectively. With the equation of the surface the infiltrated

Conclusions

From the investigations and results above the following conclusions can be drawn:

  • An equipment for the measurement of the infiltrated length as the function of infiltration pressure and time has been successfully developed. The equipment is ideal to operate in the short infiltration time range (<10 s).

  • In the short infiltration time range the infiltrated length is proportional to the infiltration pressure and square-root function of time.

  • The infiltration method has an upper and a lower pressure

Acknowledgements

This paper was supported by the János Bolyai Research Scholarship of the Hungarian Academy of Sciences. The investigations were supported by The Hungarian Research Fund, NKTH-OTKA PD 83687. This work is connected to the scientific programme of the “Development of quality-oriented and harmonised R+D+I strategy and functional model at BME” project. This project is supported by the New Széchenyi Plan (Project ID: TÁMOP-4.2.1/B-09/1/KMR-2010-0002). The work reported in the paper has been developed

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