Structural Modification of Sand Cast Eutectic Al-Si Alloys with Sulfur/Sodium and Its Effect on Mechanical Properties

Chikezie W. Onyia, Boniface A. Okorie, Simeon I. Neife, Camillus S. Obayi

Department of Metallurgical & Materials Engineering, Enugu State University of Science and Technology, Enugu, Nigeria.
Department of Metallurgical & Materials Engineering, University of Nigeria, Nsukka, Nigeri.
Department of Metallurgical & Materials Engineering, University of Nigeria, Nsukka, Nigeria.
Department of Metallurgical & Materials Engineering, University of Nigeria, Nsukka, Nigeria..

DOI: 10.4236/wjet.2013.12002                

Abstract

In the present study, the structural modification of sand cast Al-12wt%Si alloy with sulfur/sodium and its effect on mechanical properties were investigated. Different addition levels of sulfur and sodium were used to modify and produce castings of the same shape and size from the alloy. The results indicated that the addition of sodium or sulfur to eutectic Al-Si alloy can modify the Al-Si eutectic morphology from needle-like eutectic silicon structure to fine-scale eutectic silicon structure with significant improvement in mechanical properties of the alloy. The optimum levels of modification by sodium flux (60% NaF and 40% NaCl) and sulfur were found to be 0.6% - 1.0% and 0.02% - 0.05% of the weight of the alloy respectively. The alloy modified with 0.6% Na flux had the best mechanical properties closely followed by the one modified with 0.02% sulfur. Over modification of the alloy with sodium produced over modification band which consisted of aluminum dendrites and coarse silicon particles in the microstructure of the alloy. Increase in concentration of sulfur decreased the degree of fineness of the eutectic silicon structure with significant decrease in mechanical properties of the alloy and this is suggested to be as a result of the presence of a brittle sulfur compound at the grain interfaces of the alloy.

Keywords

Aluminum-Silicon Alloy; Sand Casting; Modification; Morphology; Mechanical Properties

Conflicts of Interest

The authors declare no conflicts of interest.

References

[1]	A. Pacz, US Patent 1387900, 1921.
[2]	M. D. Hanna, S. Z. Lu and A. Hellawell, “Modification in the Aluminum Silicon System,” Metallurgical and Materials Transactions A, Vol. 15, No. 3, 1984, pp. 459-469. doi:10.1007/BF02644969
[3]	C. B. Kim and P. W. Heine, “Fundamentals of Modification in the Aluminum-Silicon System,” Journal of the Institute of Metals, Vol. 92, 1963, pp. 367-376.
[4]	F. Yilmaz and R. J. Elliott, “The Microstructure and Mechanical Properties of Unidirectionally Solidified Al-Si Alloys,” Journal of Materials Science, Vol. 24, No. 6, 1989, pp. 2065-2070. doi:10.1007/BF02385422
[5]	B. S. Pena and J. A. Lozano, “Microstructure and Mechanical Property Developments in Al-12Si Gravity Die Castings after Ti and/or Sr Additions,” Materials Characterization, Vol. 57, No. 4-5, 2006, pp. 218-226. doi:10.1016/j.matchar.2006.01.015
[6]	Q. Y. Liu, Q. C. Li and Q. F. Liu, “Modification of Aluminum-Silicon Alloys with Sodium,” Acta Metallurgica et Materialia, Vol. 39, No. 11, 1991, pp. 2497-2502. doi:10.1016/0956-7151(91)90064-8
[7]	A. K. Dahle and M. Hillert, “Reply to Discussion on Nucleation Mechanism of Eutectic Phases in Aluminum-Silicon Hypoeutectic Alloys,” Metallurgical and Materials Transactions A, Vol. 37, No. 4, 2006, p. 1353.
[8]	S. Shankar, Y. W. Riddle and M. M. Makhlouf, “Eutectic Solidification of Aluminum-Silicon Alloys,” Metallurgical and Materials Transactions A, Vol. 35, No. 9, 2004, pp. 3038-3043. doi:10.1007/s11661-004-0048-1
[9]	P. E. Crosley and L. F. Mondolfo, “The Modification of Aluminum-Silicon Alloys,” AFS Transactions, Vol. 74, 1966, pp. 53-64.
[10]	S. Z. Lu and A. Hellawell, “Modification of Aluminum-Silicon Alloys: Microstructure, Thermal Analysis, and Mechanisms,” JOM, Vol. 47, No. 2, 1995, pp. 38-40. doi:10.1007/BF03221405
[11]	S. Z. Lu and A. Hellawell, “The Mechanism of Silicon Modification in Aluminum-Silicon Alloys: Impurity Induced Twinning,” Metallurgical and Materials Transactions A, Vol. 18, No. 10, 1987, pp. 1721-1733. doi:10.1007/BF02646204
[12]	A. K. Dahle, K. Nogita, S. D. McDonald, C. Dinnis and L. Lu, “Eutectic Modification and Microstructure Development in Al-Si Alloys,” Materials Science & Engineering A, Vol. 413-414, 2005, pp. 243-248. doi:10.1016/j.msea.2005.09.055
[13]	M. G. Day, “The Modification of Aluminum-Silicon Eutectic Alloys by Metallic Sodium,” Journal of the Institute of Metals, Vol. 98, 1970, pp. 57-59.
[14]	H. Fredriksson, M. Hillert and N. Lange, “The Modification of Aluminum-Silicon Alloys by Sodium,” Journal of the Institute of Metals, Vol. 101, 1973, pp. 285-299.
[15]	H. Iwahori and K. Yonekura, “Occurring Behavior of Porosity and Feeding Capabilities of Sodium- and Strontium-Modified Al-Si Alloys,” AFS Transactions, Vol. 98, 1990, pp. 167-173.
[16]	Raymond A. Higgins, “Engineering Metallurgy, Part 1, Applied Physical Metallurgy,” The English Language Book Society and Hodder and Stoughton, London, 1973.
[17]	M. M. Makhlouf and H. V. Guthy, “The Aluminum-Silicon Eutectic Reaction: Mechanisms and Crystallography,” Journal of Light Metals, Vol. 1, No. 4, 2001, pp. 199-218. doi:10.1016/S1471-5317(02)00003-2
[18]	G. Chai and L. Backerud, “Factors Affecting Modification of Al-Si Alloys By Adding Sr-Containing Master Alloys,” AFS Transactions, Vol. 100, 1992, pp. 847-857.
[19]	E. Martínez, D. M. Cisneros, G. S. Valtierra and J. Lacaze, “Effect of Strontium and Cooling Rate upon Eutectic Temperatures of A319 Aluminum Alloy,” Scripta Materialia, Vol. 52, No. 6, 2005, pp. 439-443. doi:10.1016/j.scriptamat.2004.11.012
[20]	K. Kobayashi, P. H. Shingu and R. Ozaki, “Over-Modification Band in Aluminum-Silicon Eutectic Solidified Structure Modified with Sodium,” Journal of Japan Institute of Light Metals, Vol. 22, No. 3, 1972, pp. 165-174. doi:10.2464/jilm.22.165
[21]	B. Linchevsky, A. Sobolevsky and A. Kalmenev, “Iron & Steel Making,” MIR Publishers, Moscow, 1983, p. 92.