Constituent phase diagrams of the Al-Cu-Fe-Mg-Ni-Si system and their application to the analysis of aluminium piston alloys

The evaluation of phase equilibria in quinary systems that constitute the commercially important Al-Cu-Fe-Mg-Ni-Si alloying system is performed in the compositional range of casting alloys by means of metallography, electron probe microanalysis, X-ray diffractometry, differential scanning calorimetry, and by the analysis of phase equilibria in the constituent systems of lesser dimensionality. Suggested phase equilibria are illustrated by bi-, mono- and invariant solidification reactions, polythermal diagrams of solidification, distributions of phase fields in the solid state, and isothermal and polythermal sections. Phase composition of as-cast alloys is analyzed in terms of nonequilibrium solidification. It is shown that the increase in copper concentration in piston Al-Si alloys results in the decrease in the equilibrium solidus from 540 to 505 °C. Under non-equilibrium solidification conditions, piston alloys finish solidification at ~505 °C. Iron is bound in the quaternary Al8FeMg3Si6 phase in low-iron alloys and in the ternary Al9FeNi and Al5FeSi phases in high-iron alloys.