Metal Casting Technologies : MCT-SEPT-2014
36 www.metals.rala.com.au METAL casting Technologies september 2014 37 Some recent research into MMNCs Since 2005 an increasing number of research papers have reported work on Al or Mg matrix nano-composites . Most work has examined nanocomposites based on Al alloys [12-22] but there is also growing interest in Mg alloy matrices [12, 23-24]. In attempting to achieve consistently uniform dispersions, these studies have examined the various forms of liquid and semi- solid melt stirring via mechanical, gas bubbling, electro-magnetic or ultrasonic techniques. The use of high intensity ultrasonic agitation is reported to be most promising for commercial development to produce both Al and Mg matrix nanocomposites [12-13, 23]. For example, when sufficient ultrasonic power was used, the addition of only 1wt% of SiC nanoparticles to A356 (Al-7Si-Mg) alloy increased the UTS (in the heat treated T6 condition) from 250MPa for unreinforced A356 to 480MPa with no loss in ductility . This is a much greater improvement in mechanical properties than in the case of the larger additions of 15-20 μm sized SiC mentioned earlier. Likewise for Mg base alloy, the addition of 1wt% of SiC nanoparticles in AZ91D (Mg-9%Al) alloy increased yield and tensile strength from 65 and 133MPa to 141 and 191MPa respectively, a 117% increase in yield strength . To avoid the use of ultrasonic stirring, another approach is to “carry” the nanoparticles into the melt using a suitable metal powder. This avoids the problem of nanoparticle clustering. Nano- sized Al2O3 can be premixed with commercial Al powder by ball milling to provide a composite addition which can be then be mechanically stirred into an Al alloy melt under an Ar atmosphere [16, 17]. Conventional stir casting of 0.6wt% Al2O3 nanoparticles into Al-2024 (Al-4 .5%Cu-1 .5%Mg) alloy is reported  to give only a small increase in yield strength, from 85 to 95 MPa accompanied by a decrease in UTS, from 155 to 135MPa. Introduction of the Al2O3 nanoparticles via the composite significantly improved nanoparticle dispersion and thus led to increases in both the yield and UTS levels up to 154 and 213MPa respectively. The use of master alloy MMNC is also of interest. Master alloy MMNC containing 1.5vol% nanoparticles can be added to the melt to give final nanoparticle levels of 0.2-0.5vol% with effective dispersion providing both grain refinement and strengthening in cast Al-9%Mg alloy . Electron microscopy has shown that nanoparticles are distributed throughout the grains whereas micron sized particles tend to be pushed by solidification fronts to interdendritic and grain boundary regions [11, 13-14]. It is believed  that nanoparticles below a certain size cannot be moved by the solidification front and hence become engulfed within the grains while larger particles or particle aggregates can be pushed to be finally located at grain boundaries. When particles are engulfed they may or may not have any grain refining effects but the level of strengthening is high due to interaction of slip dislocations with the particles [11, 14]. The interaction of a distribution of dispersed incoherent particles with dislocations is known as Orowan strengthening, a schematic model of which is shown in Figure 2. Each dislocation-particle interaction contributes to the tangle of dislocations generated around particles giving a high rate of work hardening. Larger particles may cause some refinement of dendrite arm or grain refining but only moderate strengthening is achieved since the Orowan effect is consequently reduced . Ductility is retained during strengthening of a matrix by nanoparticles since the fine particles do not fracture or suffer from interface decohesion from the matrix as readily as larger micron scale sized particles. The level of dispersion hardening via a distribution of nanoparticles decreases gradually with temperature not suddenly as in the over-aging of precipitates in age hardened alloys. Hence the development of cast Al and Mg base nanocomposites is important in improvement creep resistance in automotive engine and drive train parts. Each following dislocation will leave another ring around each particle. Each ring opposes the flow of subsequent dislocations contributing to strengthening and work hardening To avoid the need for intensive stirring methods strengthening nanoparticles can be introduced into Al melts by in situ gas metal reaction with nitrogen to give Al-Al nitride nanocomposites [25- 26]. Mg or Li must be present in the melt to prevent formation of Al2O3 and to act as catalyst for AlN formation. Carbon nanotubes (CNTs) are the strongest fibres developed with stiffness up to 1000GPa but to date, due to the ease of processing, they have only been used to reinforce polymer matrix materials It is recognized that there is considerable scope to use multiwall carbon nanotubes (MWCNTs) as reinforcements in metal matrix composites  particularly to reduce friction and improve wear performance of Al alloys . As for oxide or carbide nanoparticles the critical step in processing is achieving effective dispersion of the MWCNTs within the melt and hence throughout the final microstructure [22, 24, 27]. n References: 1. A. Mortesen & I. Jin. “Solidification processing of metal matrix composites”. Int. Mat. Reviews (1992) Vol.37 pp.101-128. 2. G. Curran. “MMCs: the future”. Materials World (1998) January pp.20-21. 3. T. Zeuner. “On track with MMC brake discs” Materials World (1998) January pp. 17-19. 4. M. Javidani & D. Larouche. “Application of cast Al-Si alloys in internal combustion engine components”. Int. Mat. Rev. 92014) Vol.59 pp.132-158. 5. A. Macke et Al. “Metal matrix composites offer the automobile industry an opportunity to reduce vehicle weight, improve performance”. Advanced Materials & Processes (2012) March pp. 19-23. 6. W.H. Sillekens et Al. “The ExoMet Project: EU/ESA research on high performance light-metal alloys and nanocomposites.” Met. & Mat. Trans. A (2014) Vol.45A pp.3349-3361. 7. L. Ivanchev et Al. “Semi-solid high pressure diecasting of metal matrix composites produced by liquid state processing”. Solid State Phenomena (2013) Vol. 192-193 pp.61-65. 8. J. Campbell. “Letter to the editor”. Mat. Sci. & Technol. (2014) Vol.30 p1257. 9. N. Poolthong et Al. “Enhancing wettability of SiC in semisolid A356 aluminium matrix composites”. Int. J. Cast Metals Res. (2008) Vol. 21 pp. 203-208. 10. M.C. Gui et Al. “Microstructure and mechanical properties of cast Al-Si/SiCp composites produced by liquid and semi-solid double stirring process”. Mat. Sci. & Technol. (2000) Vol.16 pp.556-664. 11. P.K. Rohaldi et Al. “Lightweight metal matrix nanocomposites – stretching the boundaries of metals”. Material Matters (2007) Vol.2 pp16-22. 12. M. De Cicco et Al. “Semi-solid casting of metal matrix nanocomposites”. Solid State Phenomena (2006) Vol.116-117 pp.478-483. 13. X Li et Al. “Theoretical and experimental study on ultrasonic dispersion of nanoparticles for strengthening cast aluminium alloy A356”. Metallurgical Science & Technology (2008) Vol. 26-2 pp.12-20. 14. B.F. Schulz et Al. “Microstructure and hardness of Al2O3 nanoparticle reinforced Al-Mg composites fabricated by reactive wetting and stir mixing”. Mat. Sci. & Eng. A (2011) Vol.530 pp.1187-1197. 15. A. Ansary Yar et Al. “Microstructure and mechanical properties of aluminium alloy matrix composite reinforced with nano-particle MgO”. J. Alloys & Compounds (2009) Vol.484 pp.400-404. 16. H. Su et Al. “Study on preparation of large sized nanoparticle reinforced aluminium matrix composite by solid-liquid mixed casting process”. Materials Science & Technology (2012) Vol. 28 pp.178-183. 17. H. Su et Al. “Processing, microstructure and tensile properties of nano-sized Al2O3 particle reinforced aluminium matrix composites”. Materials & Design (2012) Vol. 36 pp.590-596. 18. D. Wang et Al. “Using diluted master composites to achieve grain refinement and mechanical property enhancement in as-cast Al-9Mg”. Mat. Science & Eng. A (2012) Vol.532 pp.396-400. 19. D. Weiss et Al. “The use of high pressure direct squeeze casting for semi-solid processing of aluminium base nano- composites”. Solid State Phenomena (2013) Vol. 192-193 pp.72-75. 20. S. Chatterjee et Al. “Challenges in manufacturing aluminium based metal matrix nanocomposites via stir casting route”. Materials Science Forum (20130 Vol.736 pp.72-80. 21. El Mahallawi et Al. “Influence of nanodispersions on properties and microstructure features of cast and T6 heat treated As Si hypoeutectic alloys”. Solid State Phenomena (2013) Vol. 192-193 pp. 76-82. 22. A. B. Elshalakany et Al. “Microstructure and mechanical properties of MWCNTs reinforced A356 aluminium alloys cast nanocomposites fabricated by using a combination of rheocasting and squeeze casting techniques”. J. Nanomaterials (2014) Article ID 386370 14pp. 23. X.Y. Jia et Al. “Magnesium matrix nanocomposites fabricated by ultrasonic assisted casting”. Int. J. Cast Metals Res. (2009) Vol.22 pp.196-199. 24. H. Dieringa. “Properties of magnesium alloys reinforced with nanoparticles and carbon nanotubes: a review”. J. Mater.Sci, (2011) Vol.46 pp. 289-306. 25. C. Borgonovo & D. Apelian. “Manufacture of aluminium nanocomposites: a critical review”. Mat. Sci. forum (2011) Vol.678 pp.1-22. 26. C. Borgonovo & M. Makhlouf. “Synthesis of die-castable nano-particle reinforced aluminium matrix composite materials by in-situ gas-liquid reactions”. Metallurgical Sci. & Technology (2012) Vol.30 pp. 15-21. 27. S.R. Bakshi et Al. “Carbon nanotube reinforced metal matrix composites – a review”. Int. Mat Rev. (2010) Vol.55 pp.41-64. THE UsE Of HiGH iNTENsiTY ULTRAsONic AGiTATiON is REPORTED TO BE MOsT PROMisiNG fOR cOMMERciAL DEVELOPMENT TO PRODUcE BOTH Al AND Mg MATRiX NANOcOMPOsiTEs Figure 2. The Orowan Model of dispersion strengthening of a metallic matrix. A – under applied shear stress a dislocation approaches nanoparticles. B – The dislocation is halted at each particle but can continue to move by bowing out between the particles. C – The bowing continues developing a dislocation loop around each particle. d – On passing the particles the dislocation line reforms leaving dislocation rings around each particle.