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. 6 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 °C for 4 h, followed by air cooling, as well as at 180 and 220 °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. ardness 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 °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]. ntermetallic 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 β-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 α-Al15(Fe, Mn)3Si2 from post-dendritic to pre-dendritic, the latter being expected to improve the alloy strength due to its precipitation within the α-Al dendrites.