Effect of metallurgical parameters on the hardness and microstructural characterization of as-cast and heat-treated 356 and 319 aluminum alloys

The present study was undertaken to investigate the effect of metallurgical parameters on the hardness and microstructural characterization of as-cast and heat-treated 356 and 319 alloys, with the aim of adjusting these parameters to produce castings of suitable hardness and Fe-intermetallic volume fractions for subsequent use in studies relating to the machinability of these alloys. By measuring the amount of Fe- and Cu-intermetallics formed and the changes in the eutectic Si particle characteristics resulting from alloying additions (Fe, Mn, Mg), Sr-modification, and heat treatment of the 356 and 319 alloys, and the corresponding hardness values, it was possible to determine which conditions or metallurgical parameters yielded the required Fe-intermetallic volume fractions of 2 and 5% and hardness levels of 85 and 115 BHN. These levels conform to the most common levels observed in the commercial application of these alloys. The 356 and 319 alloys were examined in the as-cast and heat-treated conditions, using different combinations of grain refining, Sr-modification, and alloying additions. Aging treatments were carried out at 155, 180, 200, and 220 degrees C for 4 h, followed by air cooling, as well as at 180 and 220 degrees C for 2. 4, 6. and 8 h to determine conditions under which the specified hardness levels of 85 and 115 HBN could be obtained. Hardness measurements were carried out using a Brinell hardness tester. Peak hardness was observed in the 356 and 319 alloys at different aging conditions, depending upon the Fe-intermetallic type present in the alloy and whether the alloy was modified or not. Aging at 220 degrees C revealed a hardness peak at 2 h aging time in both 356 and 319 alloys. Addition of Mg to 319 alloys produced a remarkable increase in hardness at all aging temperatures. This may be explained on the basis of the combined effect of Cu- and Mg-intermetallics in the 319 alloys, where hardening during aging occurs by the cooperative precipitation of Al2Cu and Mg2Si phase particles [P. Ouellet. F.H. Samuel. J. Mater. Sci. 34 (1999) 4671-4697; P.N. Crepeau, S.D. Antolovich, J.A. Worden, AFS Trans. 98 (1990) 813-822]. Iron-intermetallic volume fraction measurements were carried out on polished specimens of the 356 and 319 alloys using electron probe microanalysis, for both as-cast and heat-treated conditions. Copper-intermetallic volume fractions were also measured for the 319 alloys to determine the amount of undissolved CuAl2, phase. It was observed that the unmodified alloys displayed higher Fe-intermetallic surface fractions than the modified alloys. The copper-intermetallic surface fractions, on the other hand, were higher in the Sr-modified alloys than the unmodified alloys. These observations may be attributed to the effect of Sr on (a) the dissolution and fragmentation of the beta-Fe-intermetallics in the matrix, the solution heat treatment also contributing to this effect; (b) severe segregation of Al2Cu and Al2MgCu phases in areas away from the eutectic Si regions, slowing down the dissolution of the Al2Cu phase during solution treatment; (c) altering the precipitation sequence of alpha-Al alpha-Al-15(Fe, Mn)(3)Si-2 from post-dendritic to pre-dendritic, the latter being expected to improve the alloy strength due to its precipitation within the alpha-Al dendrites. (c) 2006 Elsevier B.V. All rights reserved.