Phase precipitation in transition metal-containing 354-type alloys

The present study was carried out to investigate the effects of Ni, Mn, Zr, and Sc additions, individually or in combination, on the microstructure of 354 casting alloy ( Al-9 wt.% Si-1.8 wt.% Cu-0.5 wt.% Mg). Microstructural examination and thermal analysis data showed that the main reactions detected during the solidification of the six 354 alloys ( G1, G6-G10) investigated are: formation of the alpha-Al dendritic network; precipitation of Al-Si eutectic and post-eutectic beta-Al5FeSi; Mg2Si phase; transformation of the beta-phase into pi-Al8Mg3FeSi6 phase; and precipitation of Al2Cu and Q-Al5Mg8Cu2Si6 phases. With 2 wt.% Ni addition, the formation of Al9FeNi and Al3CuNi phases is observed. In the base 354 alloy the main phases are restricted to Cu-, Mg-, and Fe-rich intermetallic phases. The Si particle characteristics and volume fraction of intermetallics are influenced by the solidification rate and Mg level, whereas addition of Fe and/or Mn has no significant influence. In alloy G9, Fe, Mn and Ni interact to form new intermetallic phases. An increased Fe content leads to formation of polyhedral/star-like sludge particles in addition to alpha-Fe and beta-Al5FeSi phases; the presence of the hard sludge particles within the soft alpha-Al dendrites improves the alloy properties.