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

TL;DR: In this article, metal matrix syntactic foams (MMSFs) were produced by pressure infiltration and 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.

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 (

Summary

1. Introduction

• Metal matrix syntactic foams 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].
• 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.
• For example the nature and reason of the observed incubation period in the infiltrated length-infiltration time diagrams.
• Eustathopoulos et al. [37] studied the effect of oxygen-wetting transition in metal/oxide systems.
• Rabiei and O’Neill [18] produced MMSFs reinforced by steel hollow spheres, that displayed superior compressive strength and energy absorption capacity.

2.1 Materials

• Ceramic hollow spheres of SL300 grade from Envirospheres Pty. Ltd. [4] were used as reinforcement.
• This measured composition is in the range of the standardised nominal values [54].
• The driving force of the above diffusion reaction is the Si concentration mismatch between the matrix and the wall material of the hollow spheres [38-40, 55].

2.2 Equipment and experimental method

• An infiltration system has been designed and assembled to determine infiltrated length values as the function of infiltration parameters.
• In the beginning of the process the equipment was continuously flushed by Ar gas and the heating of the matrix and upper tube started.
• The 11   pressure has linear effect on the infiltration length (Fig. 5a).
• Another important question is the interface layer between the hollow spheres and matrix.

Figures

Table 2. Fitting parameters for infiltrated length as the function of infiltration time

Full text

Accepted for publication in Materials Science & Engineering A
Published in July 5, 2013
DOI: 10.1016/j.msea.2013.06.066
Metal matrix syntactic foams produced by pressure infiltration – the effect of infiltration
parameters
Imre Norbert ORBULOV
a,b,c
*
a
Department of Materials Science and Engineering, Faculty of Mechanical Engineering,
Budapest University of Technology and Economics, Bertalan Lajos utca 7., Budapest,
Hungary, 1111
b
MTA–BME Research Group for Composite Science and Technology, Műegyetem rakpart 3.,
Budapest, Hungary, 1111
c
orbulov@eik.bme.hu
*Corresponding author
Address: Department of Materials Science and Engineering, Faculty of Mechanical
Engineering, Budapest University of Technology and Economics, Bertalan Lajos utca
7., Budapest, Hungary, 1111
Tel: +36 1 463 2386
Fax: +36 1 463 1366
E-mail: orbulov@eik.bme.hu, orbulov@gmail.com
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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.
Keywords: metal foam; infiltration; compression test; microstructure
1. 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 (SiO
2
,
and/or Al
2
O
3
) or metallic (pure iron or steel) and they are available in commerce [3-7].
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.
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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 [8-16]. 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 [17-20]. If the matrix-reinforcement
system is non-wetting (e.g. Al matrix and Al
2
O
3
hollow spheres) a threshold pressure is
needed to initialise infiltration, most commonly inert gas pressure is used [21-29]. 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.
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A few review papers were published about the effect of infiltration time [30-34]. 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 rationalizing 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
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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, Si
3
N
4
, AlN, Mg
3
N
2
, TiO
2
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-
SiO
2
systems it can be explained by the decomposition of SiO
2
, while in the case of Al-Al
2
O
3
system the rupture of the oxide layer on the melt cause a ~40° decrese 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
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Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Metal matrix syntactic foams produced by pressure infiltration – the effect of infiltration parameters" ?

In this paper, 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 mold casting, especially in the short infiltration time range. 

Q2. What is the main characterising parameter of the MMSFs?

The absorbed mechanical energy (W (J/m3)) is the fourth main characterising parameter of the MMSFs, as it indicates the damping and protecting capability of the MMSFs against a blast, collision or simple vibration. 

Q3. How many steps were used to validate the models?

In order to validate the models preliminary measurements were done with constant infiltration time (5 s) and relatively small pressure steps (25 kPa increments from 50 to 250 kPa). 

Q4. What is the common term used in the literature for the infiltration period?

This period is generally called ‘incubation time’, and usually related to the pressure built up in the infiltration chamber or to the existence of some kind of chemical exchange reaction between the reinforcement and the matrix in the case of reactive material pairs. 

Q5. What is the effect of oxygen on the contact angle of metals?

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. 

Q6. What was the process of heating the matrix and upper tube?

In the beginning of the process the equipment was continuously flushed by Ar gas and the heating of the matrix and upper tube started. 

Q7. What is the definition of the hydraulic radius?

The hydraulic radius is defined as the ratio of pore volume and pore surface area and although it is dimensionally correct, it has no fundamental basis on correctly describing even the average pore size [19]. 

Q8. What is the effect of the pressure on the mechanical properties of the infiltrated pieces?

In order to analyse the potential anisotropy of the infiltrated pieces, reference measurements were done perpendicular to the infiltration direction of the infiltrated specimens. 

Q9. What are the main simplifications used to get the infiltration pressure?

The existing models use common and serious simplifications:• Geometrical simplifications to get analytically solvable closed formulas (regular spatialdistribution of capillaries instead of random distribution, permeability and tortuosity of the capillaries etc.).• 

Q10. What was the procedure for measuring the length of the infiltrated rods?

After measuring the length, plate specimens for microstructural9  investigations and cylindrical specimens for compression tests were manufactured from the infiltrated rods. 

Q11. What is the effect of the chemical reactions between the reinforcement and matrix?

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

Q12. How can The authorestimate the density of a MMSF?

The ideal density of the produced MMSFs can be estimated by the rule of mixtures (Eq. 9).( ) 121 AlSiHSMMSF VV ρρρ −+= (Eq. 9)where ρMMSF is the estimated optimal density of the MMSF, ρAlSi12 is the density of the AlSi12 matrix (2.680 g/cm3 [54]), ρHS is the density of the reinforcing hollow spheres (0.692 g/cm3) and V is the volume fraction of the reinforcement (64 vol%). 

Q13. What is the method for measuring the mechanical properties of the MMSF?

These outstanding mechanical properties (among metallic foams) and the possibility of the application of injection mould casting as production method makes the MMSFs good choice for low weight structural parts with high mechanical absorbtion capability. 

Q14. What is the effect of temperature on the infiltration length?

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).