Metal Casting Technologies : MCT-2NDQRT-2017
32 www.metals.rala.com.au METAL Casting Technologies 2nd Quarter 2017 33 Nano size ilmenite (FeTiO3) particles have been incorporated into commercially pure Al by stir casting technique. Reasonably uniform distribution of the particles was observed although some of the clusters could not be broken. It was observed that 5 wt% nano ilmenite reinforcement exhibited the maximum tensile strength and hardness and the ductility of the Al matrix could be maintained probably due to the fact that pure Al was used as the matrix of the nano-composites. In another study, nano-ZrO2 particles were dispersed in Al alloy by Disintegrated Melt Deposition (DMD) technique. The ZrO2 particles were incorporated into the Al alloy by stir casting technique followed by deposition of disintegrated liquid composite stream. In secondary processing, extrusion of the deposited composite was conducted. The microstructure showed fairly uniform distribution of reinforcement particles. Reinforcement content up to 12% revealed improvement in mechanical properties including fracture toughness. Exceptionally high strength of Al matrix nano-composites has been reported that contained 10 vol% of 50 nm sized Al2O3 particles in an Al alloy. The yield strength of the nano- composite is 515 MPa which is 15 times stronger than that of the base alloy and over 1.5 times stronger than 304 stainless steel. It is interesting to note that the friction coefficient and wear rate of this nano-composite are less than the respective micro-composites. Comparing the mechanical properties of the micro- composites and nano-composites, substantial improvement in mechanical properties is possible by replacing the micro- particles by nano-particle reinforcements in the composites even at a lower content. This is possibly due to the exceptional properties of the individual nano-particle reinforcements, smaller mean free path between adjacent nano-particles in the matrix of nano-composites and greater constraint to plastic deformation under load due to higher surface area of nano-particles. In some cases, the nanoscale reinforcement leads to property changes in the matrix itself. For instance, nanoscale reinforcements can lead to nanosize grains in the matrix, which will increase the strength of the matrix. Due to their size, properties of nanomaterials are dominated by their surface characteristics, rather than their bulk properties, which is the case in micronscale reinforcements. The potentially unique interfaces between nanosized reinforcements and the matrix can lead to even greater improvements in the mechanical properties due to the strong interface between the reinforcement phase and the matrix, and through secondary strengthening effects such as dislocation strengthening. Though nanocomposite materials exhibit ultra high-strength, it suffers from the drawback of decreased ductility. This may be attributed to currently used processing methods that result in the formation of voids and defects. One of the methods that has been used to overcome the lack of ductility in nanostructured materials is to incorporate nanosize dispersoids in a bimodal or trimodal microstructure. Using pure Al as the matrix with high ductility is another approach to maintain ductility and fracture toughness of the nano-composites. Conclusion Al alloy matrix nano-composites are still in the developing stage. Uniform distribution of nano-particles has been achieved by high energy ball milling in case of powder metallurgy and by ultrasonic assisted processing by liquid metallurgy route. Exceptionally high strength has been attained in some nano- composite systems. However further work is to be carried out to produce nano-composites free from defects such as voids to have acceptable ductility and fracture toughness and hence to find its applications in automotive, aerospace and electronic devices. By exploring lower cost and more versatile methods to manufacture metal matrix nanocomposites with improved ductility, these materials are expected to become commercially viable for a variety of applications, particularly where weight savings is essential. n TECHNICAL FEATURE TECHNICAL FEATURE REFERENCES 1) X. C . Tong and H. S. Fing, Met. And Mater. Trans. 29A (1998) 875. 2) X. C . Tong and H. S. Fing, Met. And Mater. Trans. 29A (1998) 893. 3) Q. C . Jiang, X . L . Li and H. Y. Wang, Scripta Materialia 48 (2003) 713. 4) T. Yanosaki, Y. J. Zheng, Y. Ogino, M. Terasawa, T. Mitamura and T. Fukami, Mater. Sci. and Engg. 350A (2003) 168. 5) S. Hirsosawa, Y. Shingemoto, T. Miyoshi and H. Kanckiyo, Scripta Materialia, 48 (2003) 839. 6) B. Prabhu, C . Suryanarayana, L . An and R. Vaidyanathan, Mater. Sci. Engg. 425A (2006) 192. 7) J. Hemanth, Mater. Sci. Engg. 507A (2009) 110. 8) S. Zhou, X . Zhang, Z . Ding, C . Min, G. Xu and Composites Part A 38 (2007) 301. 9) Ma E., Journal of Metals 58 (2006) 49. 10) M. S . El-Eskandarani, J. Alloys Compds. 279 (1998) 263. 11) G. Nandipati and N. R . Damera J. Mech. Civ. Eng., 7 (2013) 1. Stir casting Stir casting process was initiated to produce Al alloy matrix micro – composites in the 1970’s with the incorporation of graphite, Al2O3, SiC, B4C, zircon etc. into Al alloy matrix. In stir casting process, the reinforcing particles of required amount are added slowly into the vortex formed in the molten Al alloy in a crucible, generally placed inside a furnace, by stirring the molten metal by an impeller driven by a motor. About 500 rpm of the impeller has been found to be suitable for effective incorporation of the ceramic particles. Other process parameters are ratio of crucible -to- impeller dia., height of liquid metal bath and the position of the impeller along the height of the liquid metal bath. All these process parameters need to be controlled for successful incorporation of the ceramic particles in the Al alloy. On the other hand, stirring time is the factor to distribute the particles in the liquid alloy uniformly. Generally 4 to 10 minutes stirring is required for uniform distribution of the particles in the alloy. Since liquid Al has high oxidation tendency in open atmosphere, stirring is often conducted under inert atmosphere such as argon gas so as to prevent oxidation of Al and hence to produce high quality composite. After mixing of the particles, the liquid composite can be cast by sand casting, die casting and squeeze casting processes. Wettability of the solid ceramic particles by liquid Al during incorporation is a hindrance to this process. If the wettability is poor, the particles are rejected during addition into the vortex of liquid Al. About 2 wt% Mg added freshly into liquid Al alloy before incorporation of the ceramic particles improves the wettability and the particles are incorporated successfully. Stir casting can be used for incorporating nano-particles into Al alloy matrix. However, as mentioned earlier, nano – particles forms clusters and it is difficult to break these clusters of nano-particles by conventional stir casting process. Clusters of particles in the matrix of composite act as void and hence deteriorate the mechanical properties of the composites. Hence other modified stir casting processes named compo-casting and ultrasonic-assisted processing have been developed and these are discussed below. Compo-casting Compo-casting is a combination of rheocasting and stir casting. Rheocasting was developed in early 1980’s to produce globular microstructure. In this process the alloy is held at a temperature in between its solidus and liquidus so as to have around 0.4 solid fraction and agitated mechanically to break dendritic structure and to transform to globular structure. Globular structure improves the mechanical properties of the rheocast alloy. Other benefits of rheocasting are lower shrinkage and less die wear in subsequent squeeze casting due to lower processing temperature than conventional casting. Compo-casting is incorporation of ceramic micro- particles into agitated semi-solid alloy. Although it has been used for micro-particle composites, the same can be extended to nano-particles for its uniform distribution due to breaking the clusters of nano-particles by the solid fraction of the alloy. Ultrasonic assisted processing To ensure uniform dispersion of nano-particles in a liquid Al alloy, ultrasonic cavitation assisted fabrication has been applied in the recent past. After incorporation of nano- particles in an Al alloy by stir casting, an ultrasonic probe is immersed in the liquid Al. Cavitation from the ultrasonic waves can produce transient (in the order of nanoseconds) micro- ‘hot spots’ that can have temperatures of about 5000°C, pressures above 1000 atms. and heating and cooling rates above 1010 K/s. The strong impact of this cavitaion combined with local high temperatures can break the clusters of nano- particles. The air entrapped within the loosly bonded particles in a cluster serve as nuclei for cavitations. Several studies have been conducted in the recent past using ultrasonic assisted processing of nano-composites containing SiC, B4C, CNT etc. and improved dispersion of the nano-particles in the Al alloy matrix have been reported. Properties of Al matrix nano-composites Mechanical properties of powder processed Al matrix composites containing γ-Al2O3 (50 nm) and rutile (30 nm) nano-particles has been reported. Dry mixing method was used to mix the powder of Al-12% Si alloy with γ-Al2O3 and rutile nano-particles. The mixing time was for 4 hrs. At milling speed of 650 rpm to achieve uniform particle distribution. Cold isostatic pressing was carried out of the milled powder at 10 MPa followed by sintering at 520°C for 90 minutes with argon flow to avoid oxidation Al matrix. Mechanical milling produced uniformly distributed nano- particles in the matrix. The nano-composites reinforced with 4 wt% Al2O3 nano-particles showed the highest hardness and higher wear resistance compared to base alloy and other nano-composites. OTHER BENEFITS OF RHEOCASTING ARE LOWER SHRINKAGE AND LESS DIE WEAR IN SUBSEQUENT SQUEEZE CASTING DUE TO LOWER PROCESSING TEMPERATURE THAN CONVENTIONAL CASTING. ONE OF THE METHODS THAT HAS BEEN USED TO OVERCOME THE LACK OF DUCTILITY IN NANOSTRUCTURED MATERIALS IS TO INCORPORATE NANOSIZE DISPERSOIDS IN A BIMODAL OR TRIMODAL MICROSTRUCTURE.