Aluminium Matrix Composites: Challenges and Opportunities

Aluminium matrix composites (AMCs) refer to the class of light weight high performance aluminium centric material systems. The reinforcement in AMCs could be in the form of continuous/discontinuous fibres, whisker or particulates, in volume fractions ranging from a few percent to 70%. Properties of AMCs can be tailored to the demands of different industrial applications by suitable combinations of matrix, reinforcement and processing route. Presently several grades of AMCs are manufactured by different routes. Three decades of intensive research have provided a wealth of new scientific knowledge on the intrinsic and extrinsic effects of ceramic reinforcement vis-a-vis physical, mechanical, thermo-mechanical and tribological properties of AMCs. In the last few years, AMCs have been utilised in high-tech structural and functional applications including aerospace, defence, automotive, and thermal management areas, as well as in sports and recreation. It is interesting to note that research on particle-reinforced cast AMCs took root in India during the 70’s, attained industrial maturity in the developed world nd is currently in the process of joining the mainstream of materials. This paper presents an overview of AMC material ystems on aspects relating to processing, icrostructure, roperties and applications

Microstructure evolution during solidification of DRMMCs (discontinuously reinforced metal matrix composites): State of art

Distribution of particle reinforcements in cast composites is determined by the morphology of the solidification front. Interestingly, during solidification, the morphology of the interface is intrinsically affected by the presence of dispersed reinforcements. Thus the dispersoid distribution and length scale of matrix microstructure is a result of the interplay between these two. A proper combination of material and process parameters can be used to obtain composites with tailored microstructures. This requires the generation of a broad data base and optimization of the complete solidification process. The length scale of soldification microtructure has a large influence on the mechanical properties of the composites. This presentation addresses the concept of a particle distribution map which can help in predicting particle distribution under different solidification conditions Future research directions have also been indicated

World university rankings and subject ranking in engineering and technology (2015-2016): a case for greater transparency

There has been a great deal of interest in the ranking of academic and research universities across the world, especially in developing economies including India. Ranking lists are periodically released by many for-profit agencies, including Times Higher Education (THE) and Quacquarelli Symonds (QS). All universities vie for a ranking in them, and wish to be listed in the top 100. These ranking exercises are based partly on hard data and partly on the perception of the university. Ranking helps students choose universities and funding agencies supporting research, and add to the overall reputation of universities. Lack of transparency in providing access to the critical data used in the ranking exercise for public scrutiny/verification has drawn intense criticisms on the accuracy and credibility of the process. In this paper, fallacy of recent ranking exercise carried out by THE is illustrated with a study of two institutions in Asia

Studies on age-hardening characteristics of ceramic particle/matrix interfaces in Al-Cu-SiCp composites using ultra low-load-dynamic microhardness measurements

Ultra low-load-dynamic microhardness testing facilitates the hardness measurements in a very low volume of the material and thus is suited for characterization of the interfaces in MMC's. This paper details the studies on age-hardening behavior of the interfaces in Al-Cu-5SiC(p) composites characterized using this technique. Results of hardness studies have been further substantiated by TEM observations. In the solution-treated condition, hardness is maximum at the particle/matrix interface and decreases with increasing distance from the interface. This could be attributed to the presence of maximum dislocation density at the interface which decreases with increasing distance from the interface. In the case of composites subjected to high temperature aging, hardening at the interface is found to be faster than the bulk matrix and the aging kinetics becomes progressively slower with increasing distance from the interface. This is attributed to the dislocation density gradient at the interface, leading to enhanced nucleation and growth of precipitates at the interface compared to the bulk matrix. TEM observations reveal that the sizes of the precipitates decrease with increasing distance from the interface and thus confirms the retardation in aging kinetics with increasing distance from the interface

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

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 \mu m) and wide size range (0.5–400 \mu m) 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

Dry sliding wear of fly ash particle reinforced A356 Al composites

In the present study aluminium alloy (A356) composites containing 6 and 12 vol. % of fly ash particles have been fabricated. The dry sliding wear behaviour of unreinforced alloy and composites are studied using Pin-On-Disc machine at a load of 10, 20, 50, 65 and 80N at a constant sliding velocity of 1 m/s. Results show that the dry sliding wear resistance of Al-fly ash composite is almost similar to that of $Al_2O_3$ and SiC reinforced Al-alloy. Composites exhibit better wear resistance compared to unreinforced alloy up to a load of 80 N. Fly ash particle size and its volume fraction significantly affect the wear and friction properties of composites. Microscopic examination of the worn surfaces, subsurfaces and debris has been done. At high loads (>50 N), where fly ash particles act as load bearing constituents, the wear resistance of A356 Al alloy reinforced with narrow size range $(53-106 \mu m)$ fly ash particles were superior to that of the composite having the same volume fraction of particles in the wide size range $(0.5-400 \mu m)$

Microstructure evolution during multidirectional solidification of Al-Cu-SiC composites

The present investigation is aimed at understanding the solidification behaviour of Al-Cu-Sic, composites under multidirectional solidification conditions. Macro- and microstructures of composites solidified using small and large size moulds have been studied. It was observed that an increase in particle volume fraction and/or an increase in cooling rate reduced the extent of macrosegregation of reinforcements in the composites. In the case of composites solidified in large size permanent moulds, the differential cooling rate along the transverse direction led to the formation of a conical-shaped particle-depleted zone in the bottom part of the casting. Columnar grains were observed in the particle-depleted zone, while the particle- ontaining region exhibited equiaxed grain morphology. Sic particles did not facilitate heterogeneous nucleation of matrix phases. In the absence of convection, Sic particles promoted grain refinement in the matrix. However, when there was a significant amount of convection, the presence of Sic particles led to a coarser grain structure. Matrix dendrite arm spacing (DAS) was not significantly altered by the presence of Sic particles

Damping Behavior of Al−Li−SiCpAl-Li-SiC_p Composites Processed by Stir Casting Technique

Damping characteristics of 8090 Al alloy and its composites reinforced with 8, 12, and 18 vol pct SiC particles were investigated using a dynamic mechanical analyzer (DMA). Tests were done at different frequencies over a temperature range of $27^oC$ to $300^oC$. Composites show higher damping capacity than the unreinforced alloy. Damping capacity is found to increase with decreasing frequency. An equation relating damping capacity with frequency has been proposed. The damping data are analyzed in the light of matrix microstructure and different operative mechanisms

Age-Hardening Behavior Of Al-Cu-Sicp Composites Synthesized By Casting Route

Age hardenin,g behaviour of discontinuously reinforced aluminium matrix composites has been a subject of great interest both from scientific and technological view points. The nature of change in kinetics and magnitude of hardening during aging of these composites depend on (a) matrix material(l,2], (b) type of reinforcement including its size, shape and volume fraction[2-41, ( c ) method of processing the composite[2],(d) post fabricati’on treatment [5,6] and (e) temperature of aging[l,2]. In the case of Al-Mg-Si alloy ( 6xxx scriea) based composites, it hna IWCII conclllsivdy sl~owtl that thr prcscnrc of cc*rnlnic rc.illforcc?lrlc.111,~ .SIICII as SiC/Al,& (whiskers, pdiclcs or short fibers ) Icd to accclcrntim in the aging kiuctics whw compared with the unreinforced alloy [4-61. However, in contrast, different results have been reported in the case of hardening behaviour of ceramic reinforced Al-Cu alloy composites. Suresh et al.[3] reported rapid hardening of Al-3.5 Cu-Sic, composites during aging at 190°C compared to the unreinforced alloy. On the contrary, Kim et a1.[7] reported significant retardation in the kinetics of hardening (i.e. increased time to reach peak hardness) during aging of Al-4Cu-SiC whisker composites. However, Harris et a1.[2], h ave shown that Sic, additions have no influence on room temperature aging kinetics of Al-4Cu-Sic, composites compared to the unreinforced alloy. Furthermore, they also report that in case of high temperature aging (aged at 135”C, 17O”C, and 19O”C), composites attain peak hardness much earlier compared to the unreinforced alloy. In the light of the above inconsistencies in these previous investigations, the present work is aimed at understanding the aging bebaviollr of AI-3.2Cu-Sic, composites synthesized by casting route