Hot tearing of aluminum–copper B206 alloys with iron and silicon additions

Hot tearing of B206 aluminum alloys with additions of iron and silicon was studied with constrained mould casting (CRC) to investigate the combined effect of these additions on hot tear resistance. Susceptibility to hot tearing was found to increase gradually with iron content when the conditions were favorable to the formation of the β(FeCu) phase. Additions of silicon with a Fe/Si mass ratio ≤ 1 and high cooling rates, which together promote the α(MnFe) phase at the expense of the β(FeCu) phase, were found beneficial to the hot tearing resistance. Hot tearing sensitivity (HTS) of the alloys was evaluated with a new index defined to reflect the compliance of the torn specimens. This index showed a very good correlation with the Katgerman's hot tearing index (HCS), providing that one defines the temperature where inadequate feeding starts to be the temperature where 2% of the interdendritic volume is occupied by intermetallic phases. Examinations of the tear surfaces and profiles revealed that a premature crack opening created by insufficient healing correlates well the explanations based on the theoretical hot tearing index. The deleterious effect of iron on hot tearing was demonstrated on alloys having a coarse grain microstructure having Ti contents below or equal to 0.01 wt%. Above this limit, fine grain microstructures were obtained and the influence of iron was not strong enough to have a significant impact on the castings produced.

Research highlights▶ Hot tearing in B206 alloys is promoted by the formation of the β-Al7FeCu2 phase. ▶ Fe/Si mass ratio ≤ 1 and high cooling rates were beneficial to hot tearing resistance. ▶ Healing was inadequate when secondary phases filled 2% of the interdendritic volume. ▶ Ti wt% ≤ 0.01 in B206 alloys is insufficient to provide efficient grain refining.