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Metallurgical and Materials Transactions A-Physical Metallurgy and Materials Science


The effects of fluidized bed heat treatment on the microstructural and mechanical properties of Al-SiCu-Mg cast alloys, namely, 354 and 319, were studied. The heating rate in fluidized beds (FBs) is greater vis-a-vis conventional electrical resistance furnaces (CFs). The high heating rate in FBs increases the kinetics of metallurgical phenomena such as Si fragmentation and spherodization during solution heat treatment, as well as the precipitation rate of phases such as Al5Cu2Mg8Si6 and Al2Cu during aging. It is observed that the dissolution rate of phases such as Mg2Si and A15Cu2Mg8Si6 takes place very rapidly. The solution heat treatment of 319 alloy using FB results in complete dissolution of Mg2Si and A15Cu2Mg8Si6 particles within 45 minutes. However, for phases such as Al2Cu and Ferich intermetallics, the dissolution rate is relatively slow. Even on prolonged solution heat treatment for 6 hours, these phases do not dissolve completely. It is observed that incomplete dissolution of the Al2Cu phase does not significantly affect tensile properties of T4-treated alloys. The optimum solution heat-treatment time in FB for both 354 and 319 alloys is 45 minutes at 527 degrees C and 493 degrees C, respectively. Thermal analysis shows an exothermic peak owing to recrystallization and coarsening of eutectic grains during solution heat treatment. The high heating rate in FB causes this transformation to take place at a lower temperature than in CF. It is observed that the nucleation rate of Al5Cu2Mg8Si6 during aging in FB is greater than using CF. Thermal analysis of samples during the ramp-up stage while aging using FB did not show any phase transformation, while those using CF show two endothermic transformations, which are most likely due to the dissolution of GP zones or the co-cluster of solutes. Aging at 200 degrees C results in a greater number density of precipitates than those at 240 degrees C. The tensile strength of samples aged at 200 degrees C is greater than those aged at 240 degrees C, because the amount of precipitates formed at 200 degrees C is greater than that at 240 degrees C. The total heat-treatment time for T6 temper is less than 2 hours in FBs, which is a significant reduction in heat-treatment time, as well as energy consumption.





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Copyright 2006, ASM International. This paper was published in Metallurgical and Materials Transactions A-Physical Metallurgy and Materials Science Vol. Iss. 7 pp. 2295-2311 and is made available as an electronic reprint with the permission of, ASM International. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplications of any material in this paper for a fee or for commercial purposes, or modification of the content of this paper are prohibited.