It is a challenging task to effectively incorporate graphene nanoplatelets (GNPs) which have recently emerged as potential reinforcement for strengthening metals into magnesium-based matrices by conventional solidification processes due to their large surface areas and poor wettability. A solidification processing which combines mechanical stirring and ultrasonic dispersion of reinforcements in liquid matrix was employed to develop AZ91 magnesium alloy matrix composites reinforced with 0.25 and 0.5 wt.% GNPs. The microstructural studies conducted with scanning and transmission electron microscopes revealed that fairly uniform distribution and dispersion of GNPs through the matrix were achieved due to effective combination of mechanical and ultrasonic stirring. The GNPs embedded into the magnesium matrix led to significant enhancement in the hardness, tensile strength and ductility of the composites compared to those of unreinforced AZ91 alloy. The strength enhancement was predominantly attributed to the grain refinement by the GNP addition and dislocation generation strengthening due to the coefficient of thermal expansion mismatch between the matrix and reinforcement. The improved ductility was attributed to the refinement of b eutectics by transforming from lamellar to the divorced eutectics due to the GNP additions. In addition, the strengthening efficiency of the composite with 0.25 wt.% GNP was found to be higher than those of the composite with 0.5 wt.% GNP as the agglomeration tendency of GNPs is increased with increasing GNP content. These results were compared with those of the GNP-reinforced magnesium composites reported in the literature, indicating the potential of the process introduced in this study in terms of fabricating light and high-performance metal matrix composites.

Thixoforming is a type of semi-solid processing which is based on forming metals in the semisolid state rather than fully liquid or solid state. There have been no reports of the thixoforming of nanocomposites in the literature. The incorporation of ceramic nanoparticles into liquid metals is a challenging task for the fabrication of metal matrix nanocomposites due to their large surface-to-volume ratio and poor wettability. Previous research work by a number of workers has highlighted the challenges with the incorporation of nanoparticles into liquid aluminum alloy. In the present study, SiC and TiB 2 nanoparticles with an average diameter between 20 and 30 nm were firstly incorporated into green compacts by a powder forming route, and then the compacts were melted and treated ultrasonically. The microstructural studies reveal that the engulfment and relatively effective distribution of the nanoparticles into the melt were achieved. The hardness was considerably improved with only 0.8 wt pct addition of the nanoparticles. The nanocomposites were successfully thixoformed at a solid fraction between 0.65 and 0.70. The microstructures, hardness, and tensile mechanical properties of the thixoformed nanocomposites were investigated and compared with those of the as-received A356 and thixoformed A356 alloys. The tensile properties of the thixoformed nanocomposites were significantly enhanced compared to thixoformed A356 alloy without reinforcement, indicating the strengthening effects of the nanoparticles.

Metal matrix nanocomposites (MMNCs) could be strong candidates for use in the automotive and aerospace industry, where the mechanical performance is a crucial factor. This study aims to determine the feasibility of Al/SiC nanocomposite fabrication by the ultrasonic technique for the production of thixoforming feedstock material. Aluminium (A356) matrix composites were fabricated with the addition of 0.2 wt.% SiC nanoparticles using an ultrasonic method. Two different particle feeding mechanisms; the Al foil double capsulate method and the crucible placement approach, were employed. Better results were obtained for the double capsulate method. Also, Ti and Nb probe tips were tried to provide ultrasonic vibration in the melt. It has been shown that Nb is a promising ultrasonic probe tip material to produce MMNCs above 650 °C as it is thermodynamically stable in comparison with Ti.

In this work, A357/0.5 wt.% SiC nanocomposites were fabricated with a combination of ultrasonic processing and a nanoparticle feeding mechanism that involves the introduction of a closed end aluminium tube filled with the ball-milled SiC nanoparticles (20–30 nm) and aluminium powders (<75 µm) into the melt for complete deagglomeration and uniform dispersion of nanoparticles through the matrix. The microstructural and mechanical properties of the fabricated nanocomposites were investigated. The microstructural studies conducted with optical and advanced electron microscopes indicate that relatively effective deagglomeration and uniform dispersion of SiC nanoparticles into the molten alloy were achieved. The hardness and tensile properties of the nanocomposites were notably improved compared to those of the ultrasonically processed A357 alloy without reinforcement, showing the strengthening potency of nanoparticles and the good bonding obtained at the particle-reinforcement interface.

Bu çalışmada, birkaç grafen tabakasından oluşan, 100 nm'nin altında kalınlığa ve olağanüstü mekanik özelliklere sahip grafen nanolevhaların (GNL) endüstride sıkça kullanılan AlSi10Mg alaşımına katkısının mikroyapı ve mekanik özellikler üzerine etkisi incelenmiştir. Büyük yüzey alanı ve sahip oldukları yüksek yüzey enerjileri nedeniyle GNL'ların sıvı metaller içinde homojen olarak dağıtılması güçtür. GNL'ların sıvı alüminyum alaşımına geçişi yarı-katı mekanik karıştırma ile matris içinde dağılımı ise ultrasonik proses ile gerçekleştirilmiştir. Dökülen kompozitlerin yapılan mikroyapı analizleri sonucunda, yüksek yoğunluktaki ultrasonik dalgalar ile GNL'ların aglomerasyonlarının önlenerek matris içinde göreceli olarak homojen dağıldığı ve matris-GNL'lar arasında iyi bir tutunma yüzeyinin elde edildiği gösterilmiştir. Gerçekleştirilen çekme deneylerinde, ağırlıkça %0.25 GNL takviyesinin alaşımın mukavemetini önemli oranda arttırdığı tespit edilmiştir. Mukavemetteki iyileşme ağırlıklı olarak GNL'ların dislokasyonların ilerlemesinde bariyer vazifesi görmesine dayandırılmaktadır. Bu sonuçlar GNL takviyeli yüksek performanslı metal matrisli nanokompozitlerin seri imalata uygun olarak sıvı fazda üretilebilirliklerini göstermektedir.

Metal matrix nanocomposites (MMNCs) are promising materials to produce engineering components for the automotive and aerospace industry. This study aims to determine the feasibility of Al/TiB2 nanocomposite fabrication by the combination of the ultrasonic method and flux-assisted particle incorporation for the production of thixoforming feedstock material. Flux assistance has been invoked to attempt to overcome challenges with the presence of oxide on the surface of the foil in the aluminium foil capsulate method. A356 alloy has been reinforced with 0.25 wt.% TiB2 nanoparticles using different methods; 1. Flux-assisted casting, 2. Flux-assisted casting with ultrasonic cavitation, and 3. The Al foil capsulate method for particle feeding with ultrasonic cavitation. The composite fabricated by the ultrasonic method with the use of flux agent provided a non-dendritic microstructure which is the requirement for thixoforming. It was found that the flux-assisted casting method is not appropriate for achieving nanoparticle entry into the melt in MMNC fabrication due to the buoyancy forces of nanoparticles, unlike micron-sized particles.

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