Synthesis and Characterization of Ultrasonically Treated Al/Al2O3 Nanocomposites

Aluminium-Alumina (Al/Al2O3) nanocomposites are synthesized for different amounts of reinforcement through liquid route. Ultrasonic treatment (UST) is carried out during the solidification of the melt. It is expected that the addition of nanoparticles to the melt and UST provides grain refinement for the specimens due to increased number of nucleation sites and cavitation effects respectively. The effects of UST on these nanocomposites are studied using the results obtained from optical microscopy, hardness and XRD. Reduction of porosity and grain refinement is observed in the synthesized nanocomposites.


Introduction
Aluminium and aluminium alloys owing to their low density and high strength to weight ratio finds numerous applications in automobile and aerospace industries. Mechanical properties of these alloys can be further improved by reinforcing them with nanoparticles, called nanocomposites. Various methods have been developed for synthesizing aluminium matrix nanocomposites out of which the liquid route using stir casting is considered economical [1]. Ultrasonic treatment (UST) in aluminium alloy melts is known to yield degassing effect and grain refinement. These effects are due to cavitation generated by the high frequency ultrasound wave [2]. Ultrasound treatment in the synthesis of nanocomposites can disperse the particulate reinforcements uniformly. Introduction of nanoparticles to the melt increases heterogeneous nucleation sites leading to grain refinement [3] [4]. The objective of this study is to investigate the effect of alumina nanoparticle addition in pure aluminium for ultrasonically treated melts. As received S2

Experimental Methods
With UST and no reinforcement S3 With UST and 2% reinforcement S4 With UST and 3% reinforcement To synthesize the nanocomposite specimens following method is used. Pure aluminium is melted on a table top furnace in a clay bonded graphite crucible. Melt is heated up to 740°C and alumina nanoparticle is added. The mixture is mechanically stirred for 10 minutes and is poured into a steel mould. The steel mould is then placed on the ultrasonic agitator and ultrasonic wave is propagated through the mould till the composite melt solidifies. Experiments are conducted with ultrasound frequency of 40 KHz. The schematic diagram of the experimental setup is shown in figure 1.

Optical Microscopy Analysis
The microstructures of the specimens prepared are given in figures 2 and 3. As seen from the microstructures it is evident that significant grain refinement has happened for specimens which are ultrasonically treated. The average grain sizes of the specimens are determined using the line intercept method and is given in figure 4. The average grain size of the as-received specimen is greater than 70 µm. For specimen S2 the average grain size is found to be < 50 µm. For specimen S3 also the average grain size is found to be < 50 µm. And for specimen S4 the average grain size is around 20 µm. It is also observed that in comparison with the as received specimen there is significant reduction of porosity. This can be attributed to the degassing effect that the UST has [1].  Figure 2. Microstructure of the a) as-received specimen, b) specimen with UST and no reinforcement   A significant reduction in grain size is observed in the specimen S4. This is due to the fact that adding 3% by weight alumina nanoparticle to the melt and ultrasonic treatment has increased the number of nucleation sites there by decreasing the grain size of the material.

X-ray Diffraction Studies
It has been observed from our XRD studies that the crystallographic texture strongly depends on the percentage of Al 2 O 3 in the composite. For specimen S4 where the percentage of Al 2 O 3 is 3%, a double texture of (111) and (311)

Hardness Testing
The hardness values of the ultrasonically treated specimens are given in figure 6. It is observed that the hardness values increase as the percentage of reinforcement in the composite increases. The increase in hardness values can be attributed to two main factors; one is the strengthening effect due to

Conclusions
UST aluminium and Al/Al 2 O 3 nanocomposites were successfully synthesised. It was found that UST decreases the grain size and gives a equiaxed non-dendritic structure. Significant grain refinement was observed in Al /3% Al 2 O 3 nanocomposite with ultrasonic treatment during solidification. Another observation was the reduction in porosity of the ultrasonically treated specimen. The crystallographic texture was found to be dependent on the percentage of alumina nanoparticles in the composite. It is also observed that the addition of nanoparticles and ultrasonic treatment resulted in the increase of hardness values.