Pradeep K. Rohatgi

Cast aluminum-matrix composites for automotive applications

The potential automotive applications of metal-matrix composites, particularly aluminum-matrix composites, are numerous. Although some composite components have reached the demonstrator stage, there is still much work to do and many barriers to conquer before widespread application can be expected. These challenges include such issues as processing for specific properties, compiling property databases and addressing recyclability.

Crystallinity and selected properties of fly ash particles

Abstract The morphology, composition and crystallinity of both precipitator (solid) and cenosphere (hollow) fly ash particles of different sizes were studied with scanning electron microscopy (SEM), EDX, and X-ray diffraction (XRD). Bulk density, tap density and real density of both precipitator and cenosphere particles of different sizes as well as the wall thickness to diameter ratio of cenosphere particles were measured. The microhardness of individual fly ash particles embedded in the matrix of aluminum alloy was also measured. The crystalline to amorphous ratio weight percentage in fly ash particles, and the weight or volume fraction of each crystalline component varies with the particle size. The crystallinity of precipitator particles increased as the particle size increases, whereas the crystallinity of cenosphere decreased as the particle size increases. The elastic modulus of fly ash was estimated from the crystallinity of fly ash and the volume fraction of each component, using the rule of mixtures. The calculated upper limits for Youngs modulus of precipitator particles were 126 GPa for particles in the size range 150–250 μm and 98 GPa for particles in the size range 5–10 μm. Youngs modulus of cenosphere particles was estimated to be approximately in the range of 13–17 GPa in all particle size ranges. The hardness of the larger precipitator fly ash particles (120 μm) exhibited a wide scatter in the range of 160–400 kg mm−2, while the hardness of the smaller size precipitator particles (20 μm) were in a narrow range from 250 to 270 kg mm−2.

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

Abstract 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.

A map for wear mechanisms in aluminium alloys

A quantitative wear map for aluminium and its alloys has been constructed using normalized test variables and the physical modelling approach proposed by Lim and Ashby for steels. New model equations based on a different state-of-stress criterion suitable for aluminium alloys have been developed and found to match well with reported experimental wear data on aluminium alloys. The field boundaries between various interfacing wear mechanisms were constructed by using critical values of experimental wear data which manifest themselves in discontinuities in the slope of wear curves. However, within a given wear regime, the model equations developed here agreed fairly well with the reported wear data. The wear mechanisms successfully modelled here include oxidation dominated wear, delamination wear, severe plastic deformation wear, and melt wear.

Wear performance of copper–graphite composite and a leaded copper alloy

Abstract The wear behavior of new lead free metal matrix composite (MMC), centrifugally cast copper alloy graphite (C90300–10%graphite) composite (CG) is studied in comparison to a commonly used leaded copper (LC) alloy (18–22% Pb). Tribological tests were conducted with pins made from these materials and tested against a SAE 1045 steel counterface. The CG material showed higher wear resistance than the LC in the load range investigated (27–118 N). The CG and the LC showed similar friction coefficient (0.38) values at a low load of 27 N, where as at 118 N the CG had a slightly higher friction coefficient than the LC against the 1045 steel counterface. Transfer of material from the CG and the LC pins resulted in lowering the wear rate of the counterface as measured by the weight loss of the steel counterface. At the load of 27 N, the CG composite seems to be a viable substitute for the LC. For other loads modified versions of the CG are likely to provide optimum substitutes for the LC. Observations on structure, composition and morphology of surface, subsurface and wear debris was utilized in understanding the wear properties in each material.

Thermal Expansion of Aluminum-Fly Ash Cenosphere Composites Synthesized by Pressure Infiltration Technique

The coefficients of thermal expansion (CTEs) of commercially available pure aluminum and aluminum alloy composites containing hollow fly ash particles (cenospheres) of average size 125 mm are measured using a dilatometer. Three types of composites are made using the pressure infiltration technique at applied pressures and infiltration times of 35 kPa for 3 min, 35 kPa for 7 min, and 62 kPa for 7 min. The volume fractions of the fly ash cenospheres in the composites are around 65%. The CTE of the composites is measured to be in the range of 13.1×10-6-11×10-6/°C, which is lower than that of pure aluminum (25.3×10-6/°C). The infiltration processing conditions are found to influence the CTE of the composites. A higher applied pressure and a longer infiltration time lead to a lower CTE. The theoretical value of the CTE of fly ash cenospheres is estimated to be 6.1×10-6/°C.

Preparation of aluminium-fly ash particulate composite by powder metallurgy technique

Aluminium-fly ash mixtures containing different weight percentages of fly ash were prepared and compacted at pressures from 138–414 MPa. The compacts prepared at 414 MPa were sintered in nitrogen atmosphere at 600, 625 and 645°C, respectively. The time of sintering ranged from 0.5–6 h. The densification parameter and the green densities of the compacts were determined as a function of compacting pressure and fly ash weight per cent. Density, hardness and strength of the sintered compacts were determined as a function of weight per cent of fly ash particles. Volume changes during sintering of green compacts were also evaluated as a function of increasing fly ash weight per cent. Microscopic studies of green and sintered compacts were done to study the effectiveness of sintering. Green and sintered density of the compacts were found to decrease with increasing weight per cents of fly ash. Sintering results in slight decrease in density and increase in volume of green compacts within the range investigated. Strength of the sintered compacts decreased with increasing weight per cent of fly ash under the present experimental conditions; however, the hardness was found to increase slightly up to 10 wt% fly ash, beyond which it decreased.

Prediction models for the yield strength of particle-reinforced unimodal pure magnesium (Mg) metal matrix nanocomposites (MMNCs)

Particle-reinforced metal matrix nanocomposites (MMNCs) have been lauded for their potentially superior mechanical properties such as modulus, yield strength, and ultimate tensile strength. Though these materials have been synthesized using several modern solid- or liquid-phase processes, the relationships between material types, contents, processing conditions, and the resultant mechanical properties are not well understood. In this paper, we examine the yield strength of particle-reinforced MMNCs by considering individual strengthening mechanism candidates and yield strength prediction models. We first introduce several strengthening mechanisms that can account for increase in the yield strength in MMNC materials, and address the features of currently available yield strength superposition methods. We then apply these prediction models to the existing dataset of magnesium MMNCs. Through a series of quantitative analyses, it is demonstrated that grain refinement plays a significant role in determining the overall yield strength of most of the MMNCs developed to date. Also, it is found that the incorporation of the coefficient of thermal expansion mismatch and modulus mismatch strengthening mechanisms will considerably overestimate the experimental yield strength. Finally, it is shown that work-hardening during post-processing of MMNCs employed by many researchers is in part responsible for improvement to the yield strength of these materials.

Interfaces in cast metal-matrix composites

Abstract A detailed analysis of possible interfaces in cast aluminum silicon base reinforced particle composites containing SiC, Al 2 O 3 and C indicates that several different kinds of interfaces can form. The reinforcements may be totally surrounded by primary-phase, or primary silicon, or by the eutectic between Al and Si. In addition, some of the original coatings or their reaction products in the case of coated particles (such as nickel or nickel-aluminum intermetallics in nickel coated reinforcements Cu or Cu-aluminum intermetallics in Cu coated reinforcements) may also form the interface. The reaction between dispersoids and the alloy itself can form a complex interface. These different interfaces have also been experimentally observed in the microstructures of cast particulate composites, with the exception of primary α-aluminum surrounding the reinforcement. The absence of α-aluminum on the reinforcements is attributed to possible lack of nucleation, persistent lateral growth and a thermal lag between the reinforcement and the matrix. Estimates of works of adhesion for the different interfaces observed in cast composites have been made using London-van der Waal equation, correlated to the properties of the composites, and used to identify the possibilities of further improving these properties.

Factors affecting the damping capacity of cast aluminium-matrix composites

The damping capacity of stir-cast aluminium-matrix composites containing graphite and silicon carbide particles, were studied using a cantilever beam specimen and an HP 5423A Structural Dynamics Analyser. Damping data were determined in the first mode of vibration. Aluminium-matrix composites containing 5–10 vol % graphite particles and 10 vol % silicon carbide particles were prepared by the stir-casting technique and die cast to obtain standard samples (6 mm×25 mm100 mm). Graphite particles were found to be more effective in enhancing the damping capacity of composites compared to silicon carbide particles. The damping capacity of composites increased with the volume percentage of graphite within the range studied. However, no notable improvements in damping capacity were observed by dispersion of silicon carbide in aluminium alloy. The results have been analysed in terms of the effect of size, shape, nature and volume fraction of particles on the damping capacity of the aluminium matrix particulate composites and compared with the damping capacity data available in the literature. The effects of frequency, strain amplitude, temperature and processing on damping capacity of the aluminium matrix composites are reviewed.