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

Abstract

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 non-equilibrium 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.