Metal Casting Technologies : MCT-2NDQRT-2017
30 www.metals.rala.com.au METAL Casting Technologies 2nd Quarter 2017 31 TECHNICAL FEATURE Figure 5 compares the micro-hardness of the matrix in A356 (Al – 7.32%Si – 0.3%Mg) that had been solution treated at 540oC for 4 hr. and then given conventional T6 ageing treatment for various times at 160oC or an interrupted ageing treatment, denoted as T6I6 . The T6I6 cycle involved ageing at 160oC for 20 minutes, then cooling to and holding at 65oC for 2 weeks before resuming ageing at 160oC . In this case, it was found that peak hardness was about 8% higher for the T6I6 treatment compared to T6 ageing, but the in the T6I6 treatment softening from over-ageing occurred at shorter times. TEM study has shown that at the peak ageing condition T6I6 results in a higher number density of the strengthening needle shaped GP phase . Further work is needed to understand the effects of interrupted ageing on fatigue resistance since it has been found that interrupted ageing treatments give lower fatigue life in alloy 354 (Al – 9%Si – 1 .8%Cu – 0.5%Mg) compared to conventional T6 treatment . n REFERENCES: 1. J .T .H. Pearce. “Heat Treatment of Aluminium Alloys”. Metal Casting Technologies (2002) Vol. 48 No 2 June pp.40-43. 2. P . Apichai et Al. “Effect of Solution Treatment and Aging on Microstructure and Mechanical Properties of Cast Aluminium Alloy A319”. Proceedings of the European Conference on Heat Treatment 2011 “Quality in Heat Treatment”, 23-25 March 2011, Wels, Austria, pp. 218-225. 3. A . Wiengmoon et Al. “HRTEM and HAADF-STEM of precipitates at peak ageing of cast A319 aluminium alloy”. Micron (2013) Vol.45 pp.32-36. 4. V.R . Knobloch. “The most recent advances in innovative heat treatment systems”. Proceedings 7th Asian Foundry Congress, Taipei, Taiwan (2001) pp.603-613. 5. L.L.M. 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TECHNICAL FEATURE Introduction luminium matrix particle composites exhibit improved hardness, strength at room temperature and elevated temperature and wear resistance compared to unreinforced Al alloys and have been widely studied since 1980’s. Various particles such as SiC, Al2O3, graphite etc. have been incorporated in Al alloy in the range of 5 to 30 wt% to improve their mechanical properties. At the initial phase of the development of Al alloy matrix composites, micro-particles (particles having size range up to few hundred microns) were used. Although substantial improvements in the mechanical properties could be achieved by incorporation of micro-particles in Al alloys, ductility and fracture toughness of the composites were inferior to those of Al alloys. Hence in recent years, attempts are being made to fabricate Al alloy matrix nano-composites by incorporation of nano-particles (particles in the size range up to 200 nano-meter) in it. It has been envisaged that due to the finer size of nano-particles, much less wt% of the nano-particles would be sufficient to enhance the mechanical properties to same extent and hence the ductility and fracture toughness of the Al alloy matrix nano-composites could be restored. Nano-particles of Al2O3, SiC, MgO, ZrO2, B4C, FeTiO3 have been incorporated into Al alloy matrices and the properties of the nano-composites have been studied in the recent past. One of the challenges to produce the nano-composites is to distribute the nano-particles uniformly in the matrix of Al alloys. Nano-particles forms clusters due to the surface charge of the particles that hold the low weight fine particles together. Hence advanced techniques have been developed to distribute the nano-particles uniformly and hence to improve the mechanical properties of the nano-composites to the maximum possible extent. Fabrication techniques of Al matrix nano-composites Powder metallurgy based processes Powder metallurgy has been used to produce Al alloy based micro-composites. Metal powders and reinforcing particles are blended, compacted and sintered to produce the composites. Whereas double cone blenders are used for micro-composites, it is not effective to produce the nano-composites, since the clusters of nano-particles are not uniformly dispersed in the metal powders. Hence high energy ball milling is practised to produce nano-composites. Earlier, high energy ball milling has been used for mechanical alloying. Now it is being used for nano-composites. In this technique, repeated cold welding, fracturing and re-welding of particles takes place. This method is of primary importance since it allows achieving a uniform distribution of nano- particles in the composite by breaking up the cluster of nano-particles. Both cold isostatic pressing and hot isostatic pressing of blended particles have been used to compact the composite powders. Pressures in the range of 500 to 600 MPa are common and sintering is done at around 600°C under inert atmosphere such as argon gas. A Aluminium alloy matrix nano-composites By P. C. Maity Metal casting and materials Engineer; E: email@example.com FIGURE 5. Micro-hardness variation with time of ageing at 160oC of solution treated A356 for T6 and interrupted T6I6 treatment . 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.