Fabrication and characterisation of Al–7Si–0.35Mg/fly ash metal matrix composites processed by different stir casting routes

Loose beds of hollow fly ash particles (cenospheres) were pressure infiltrated with A356 alloy melt to fabricate metal-matrix syntactic foam, using applied pressure up to 275 kPa. The volume fractions of cenospheres in the composites were in the range of 20–65%. The processing variables included melt temperature, gas pressure and particles size of fly ash. The effect of these processing variables on the microstructure and compressive properties of the synthesized composites is characterized. Compressive tests performed on these metal-matrix composites containing different volume fractions of hollow fly ash particles showed that their yield stress, Young's modulus, and plateau stress increase with an increase in the density. Variations in the compressive properties of the composites in the present study were compared with other foam materials.

Compressive characteristics of A356/fly ash cenosphere composites synthesized by pressure infiltration technique

Syntactic foams were fabricated by liquid metal infiltration of commercially pure and 7075 aluminum into preforms of hollow ceramic microspheres. The foams exhibited peak strengths during quasi-static compression ranging from −100 to −230 MPa, while dynamic compression loading showed a 10–30% increase in peak strength magnitude, with strain rate sensitivities similar to those of aluminum–matrix composite materials. X-ray tomographic investigation of the post-compression loaded foam microstructures revealed sharp differences in deformation modes, with the unalloyed-Al foam failing initially by matrix deformation, while the alloy–matrix foams failed more abruptly through the formation of sharp crush bands oriented at about 45° to the compression axis. These foams displayed pronounced energy-absorbing capabilities, suggesting their potential use in packaging applications or for impact protection; proper tailoring of matrix and microsphere strengths would result in optimized syntactic foam properties.

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Plasticity and Damage in Aluminum Syntactic Foams Deformed under Dynamic and Quasi-Static Conditions

Dry sliding wear of fly ash particle reinforced A356 Al composites

A356 Al–fly ash particle composites were fabricated using stir-cast technique and hot extrusion. Composites containing 6 and 12 vol.% fly ash particles were processed. Narrow size range (53–106 μm) and wide size range (0.5–400 μm) fly ash particles were used. Hardness, tensile strength, compressive strength and damping characteristics of the unreinforced alloy and composites have been measured. Bulk hardness, matrix microhardness, 0.2% proof stress of A356 Al–fly ash composites are higher compared to that of the unreinforced alloy. Additions of fly ash lead to increase in hardness, elastic modulus and 0.2% proof stress. Composites reinforced with narrow size range fly ash particle exhibit superior mechanical properties compared to composites with wide size range particles. A356 Al–fly ash MMCs were found to exhibit improved damping capacity when compared to unreinforced alloy at ambient temperature.

Synthesis of fly ash particle reinforced A356 Al composites and their characterization

Beds of nickel coated and uncoated cenosphere fly ash can be successfully infiltrated by molten aluminum under very low pressures. The density of the resultant composite is ≈1.4 and 1.2 g cm−3 respectively, compared to 2.68 g cm−3, the density for aluminum. The threshold pressure was found to be between 20.68 and 27.58 kPa for infiltration of molten pure aluminum into uncoated fly ash, and around 6.7 kPa for Ni-coated fly ash. These data were used to back calculate an effective wetting angle between molten pure aluminum and fly ash using Young’s and Washburn equations. The average value of the wetting angle calculated for uncoated fly ash is 111°. The microstructures and chemical composition of the composites made with coated and uncoated fly ash were studied using optical and scanning microscopy; they show good infiltration except for regions between contacting cenospheres. They also show transport of nickel from the bottom to the top of the sample when nickel coated particles were used.

Pressure infiltration technique for synthesis of aluminum–fly ash particulate composite

The major constituents of fly ash are various oxides of silicon, aluminum, and iron present in complex glass and ceramic forms. Thermodynamic analysis indicates the possibility of exothermic reduction of silica and iron oxides by molten aluminum; however, no data is available on the thermodynamics and kinetics of these reactions in a complex glass ceramic particles like fly ash, and this makes predictions about their stability difficult. In view of this, differential thermal analysis was used to establish the influence of processing and reheating on the stability of the aluminum-fly ash composite system using aluminum-fly ash composite-aluminum couples. During cooling of the sample after reheating in DTA equipment, several exothermic peaks were recorded, suggesting that rapid chemical reactions occur between aluminum and fly ash after pressure infiltration at 700°C. After 1 h of holding at 850°C, the DTA showed less than 1/2 of the exothermic peaks indicating rapid reaction during that period. After 10 h of holding at 850°C, a similar DTA plot showed no exothermic peaks except that for aluminum phase transformation. This suggests that the reactions between aluminum and fly ash were close to completion. The liquidus temperature of the once-pure aluminum matrix was seen to decrease from 655 to 644°C because of solute enrichment from reduction and dissolution of the silicon and iron oxides in the fly ash. The results of the present DTA study indicate that pressure infiltrated aluminum–40 vol.% fly ash composite made in this study was chemically stable after holding for 10 h at 850°C, demonstrating the usefulness of DTA to establish the chemical stability of aluminum-fly ash composites during synthesis and reheating.

Differential thermal analysis to establish the stability of aluminum-fly ash composites during synthesis and reheating

The use of fly ash particles as a filler in aluminum was found to result in a composite of lower density and lower thermal expansion. The thermal expansion coefficients (CTEs) of aluminum matrix composite, reinforced with cenosphere fly ash or precipitator fly ash, were measured between 25 to 400 C. The CTEs of fly ash preforms were also measured under same temperature range and compared with the calculated values of CTE of fly ash from CTE measurements of aluminum-fly ash composite using rule of mixture model. The results show that additions of fly ash significantly reduce the coefficient of thermal expansion of matrix aluminum alloys which could be a major advantage in several applications.

Thermal expansion of aluminum-fly ash composites

There is an increasing interest in metal matrix composites where the reinforcements are lower in cost and density than the matrix; an example is the possible use of solid or hollow spherical fly ash particles as fillers or reinforcements in aluminum matrix composites. We have recently developed aluminum-fly ash composites with low density, low cost, and improved hardness and wear resistance [1,2]. Here we summarize the results of structure investigations employing a combination of optical metallography and transmission electron microscopy (TEM) methods [3].

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R.Q Guo

Papers

Pressure infiltration technique for synthesis of aluminum–fly ash particulate composite

Differential thermal analysis to establish the stability of aluminum-fly ash composites during synthesis and reheating

Thermal expansion of aluminum-fly ash composites

Electron Microscopy Characterization of Aluminum Alloy - Fly Ash Composites