Metal Casting Technologies : MCT-JUNE-2014
24 www.metals.rala.com.au / metalsonline.rala.com.au METAL casting Technologies June 2014 25 TEcHNicAL fEATURE of Al7SiMg A356 alloy intended for use as diesel engine cylinder heads but a Mg free AlSi7Cu3-4Mn alloy containing small amounts of Ti, V and Zr is said to offer a better combination of strength, ductility and creep resistance . Other work has also shown that there is no benefit from adding over 0.5%Mg to AlSi7Cu3 alloy . The role of Ni additions, first used to improve eutectic piston alloys, is of interest with respect to the short fibre reinforcement of the matrix and load transfer effects mentioned earlier. Ni as eutectic Al3Ni in combination with eutectic Si forms an interconnected network which is not broken down during solution treatment since the Al3Ni resists changes in shape . If it does not encourage cracking then a hard eutectic network can assist in retaining strength at high temperatures since as the age hardening precipitates in the matrix coarsen and reduce in number with time at temperature more of the loading becomes transferred to the network. Without the presence of Ni the interconnectivity of the eutectic Si is partially removed. The reinforcing effect of Al3Ni is more significant in near eutectic alloys (e.g. piston alloys) where more eutectic Si is present. When Fe is present with Ni interdendritic Al9FeNi is also formed. During creep strain this phase can crack even at 300oC during the early stages of creep but it has been shown that up to 0.3%Mn additions can prevent such cracking and increase creep resistance . Using creep testing at 250oC (130MPa) and 400oC (20MPa) it has been shown that increasing the Cu and Ni contents of Al-Si12 alloy from 1 and 1.2% respectively to 4.9 and 2.8% produced a significant decrease in creep strain rate at both temperature/load conditions, increasing time to failure by about 8x . Alloy developments Among the more recently developed non Si-containing alloys there is continuing interest in Al-Ni-Mn compositions with 1-6%Ni + 1-3%Mn , Al-Sc and Al-Li-Sc alloys . In the UK a new casting called A20X alloy has been commercially developed. This is based on A201 Al-Cu alloy with Mg and Ag additions to control the nature of precipitation hardening giving improved ambient and high temperature strength [17-18]. Solidification control overcomes the poor castability associated with conventional Al-Cu alloys. The alloy is claimed to be the strongest commercially available cast Al alloy in the world and can compete with forged and fabricated 2000 and 7000 series alloys. High temperature strength and creep resistance can be increased by short fibre reinforcement of the matrix using silicon carbide, C nanotubes, boron, alumina, etc., for example in pistons . Reinforcement can also be achieved using cast-in rigid but porous preforms made of alumina, silicon carbide and titanium dioxide. There is now increasing interest in reinforcing cast Al alloys using ceramic nanoparticles (less than 100nm in size) with R&D concentrating on investigating suitable casting techniques such as stir casting, use of ultrasonic vibration, or semi-solid processing in order to obtain predictable and consistent dispersion of the nano-particles throughout all sections [e.g. 20-22]. n ReFeRences 1. J.T.H . Pearce. “An outline of creep in cast metals: superalloys and steels”. Metal casting Technologies (2014) Vol. 60, March 2. J.s. Robinson et al. “creep resistant aluminium alloys and their applications”. Mat. sc. & Tech. (2003) Vol.19 pp.143-155. 3. R . Molina et al. “Mechanical characterization of aluminium alloys for high temperature applications. Part 1: Al-si-cu alloys”. Met. sc. & Tech. (2011) Vol. 29 pp. 5-15. 4. F. stadler et al. “effect of main alloying elements on strength of Al-si foundry alloys at elevated temperatures”. Int. J. cast Met. Res. (2012) Vol. 25 pp. 215-224. 5. A .M .A . Mohamed et al. “Microstructure, tensile properties and fracture behaviour of high temperature Al-si-Mg-cu cast alloys”. Mat. sc. & eng. A (2013) Vol.577 pp. 64-72. 6. M . Javidani & D. Larouche. “Application of cast Al-si alloys in internal combustion engine components”. Int. Mat. Reviews (2014) Vol.59 pp.132-158. 7. K .e . Knipling et al. “criteria for developing castable, creep- resistant aluminium-based alloys – A review”. Z . Metallkd (2006) Vol. 97 pp, 246-265. 8. T. Jaglinski & R. Lakes. “creep behaviour of Al-si die-cast alloys”. Trans. AsMe (2004) Vol.126 pp.376-383 . 9. R .F. Gutierrez & G.c . Requena. “The effect of spheroidisation heat treatment on the creep resistance of a cast Alsi12cuMgni alloy”. Mat. sc. & eng. A (2014) Vol.598 pp.147- 153. 10. A .R . Farkoosh & M. Pekguleryuz. “The effects of Mn on the T-phase and creep resistance in Al-si-cu-Mg-ni alloys”. Mat. sc. & eng. A (2013) Vol.582 pp.248-256. 11. M .Garat & G.Laslaz. “Improved aluminium alloys for common rail diesel cylinder heads’. AFs Trans, (2007) Paper 07-002, 8pp. 12. L . Heusler et al. “Alloy and casting process optimization for engine block application”. AFs Trans. (2001) Paper 01-050, 9pp. 13. W. Prukkanon et al. “effect of sc on precipitation hardening of Alsi6Mg alloy”. J. Mater. sci. Technol. (2008) Vol.24 pp.17- 20. 14. c .Y. Jeong. “effect of alloying elements on high temperature mechanical properties for piston alloy”. Mat. Trans. (2012) Vol.53 pp.234-239. 15. s.M . Miresmaeli & B. nami. “Impression creep behaviour of Al-1 .9%ni-1 .6%Mn-1%Mg alloy”. Materials & Design (2014) Vol.56 pp. 286-290. 16. M .e . Krug et al. “creep properties and precipitate evolution in Al-Li alloys microalloyed with sc and Yb”. Mat. sc. & eng. A (2012) Vol.550 pp.300-311. 17. B . stott & J. Forde. “A20X-alloy of the future?” Materials World (209) november pp.27-28. 18. www.aeromet.co.uk/a20X 19. G . Requena & H.P. Degischer. “creep behaviour of unreinforced and short fibre reinforced Alsi12cuMgni piston alloy”. Mat. sc. & eng. A (2006) Vol.420 pp.265-275. 20. 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. 21. D. Wang et al. “Using diluted master nanocomposites to achieve grain refinement and mechanical property enhancement in as-cast Al-9Mg”. Mat. sc. & eng. A (2012) Vol.532 pp.396 -400. 22. H . su et al. “study on preparation on preparation of large sized nanoparticle reinforced aluminium matrix composite by solid-liquid mixed casting process”. Mat.sc. & Tech. (2012) Vol.28 pp.178-183. BacktoBasics Introduction luminium-silicon alloys are widely used in the casting industry due to their excellent castability and good mechanical properties. The alloys consist predominantly of aluminium with silicon addition up to 11% (hypoeutectic), 11 to 13% (eutectic) and over 13% (hypereutectic) with various other elements such as magnesium, zinc, iron, copper and nickel added to achieve desired casting and mechanical properties. Silicon is the most significant alloying ingredient and is primarily responsible for the excellent castability of these alloys. Silicon increases fluidity, wear resistance and strength, whilst it reduces melting temperature, solidification contraction and hot-tears. Aluminium-silicon casting alloys are essential to a wide range of industries including engineering, aeronautical and automotive and are suitable for sand or die casting processes. Solidification mechanism Al-Si is a simple eutectic system of two solid solution phases where each element has little or no solubility in the other. Aluminium melts at approximately 660oC while silicon melts at 1414oC. The diagram in figure 4 shows the eutectic at 12.6 wt % Si and 577oC. The maximum solubility of Si in Al is approximately 1.65% at 577oC. The typically mushy solidification mode of most sand moulded aluminium alloy castings presents difficulties in feeding considerations for sand moulded aluminium- silicon alloy castings A J. f. Meredith, casting solutions Pty Ltd Figure 1. pouring of aluminium-silicon alloy casting in sand mould. Figure 2. Automobile engine block casting. ALUMiNiUM-siLicON cAsTiNG ALLOYs ARE EssENTiAL TO A WiDE RANGE Of iNDUsTRiEs iNcLUDiNG ENGiNEERiNG, AERONAUTicAL AND AUTOMOTiVE AND ARE sUiTABLE fOR sAND OR DiE cAsTiNG PROcEssEs.