Solidification Sequence of High-Stiffness Al-14%Si-5.5%Ni-15%Cu-0.5%Mg Cast Alloy

<p>  Aluminum casting alloys are widely used for various machine parts for the purpose of weight reduction and energy saving. Because of the strong demands for high-stiffness Al alloy to improve their operating accuracy in the fields of precision machinery and robots, we have developed Al-12.5～14%Si-5～6%Ni-14～15%Cu-0.5%Mg alloys with a stiffness of 100GPa. The solidification sequence of this alloy was investigated to clarify the structural constituents that contribute to its higher stiffness. A series of quenched Al-14%Si-5.5%Ni-15%Cu-0.5%Mg samples were analyzed by optical microscope, EPMA and X-ray diffraction. The results revealed that the alloy solidifies in the order of primary Si, primary-like Ni₂Al₃, Al (α) ＋ Si eutectic, Al (α) ＋ Ni₂Al₃ eutectic, Al (α) ＋ CuAl₂ eutectic and Al (α) ＋ Al₄Cu₂Mg₇Si₈ eutectic. Just after the crystallization of primary-like Ni₂Al₃, Al (α) hollow and α-dendrites developed around primary Si and Ni₂Al₃, since these primaries hardly nucleated the above eutectics. The thermodynamic calculation software used provides information on the solidification temperatures and volume fraction for primary Si, Ni₂Al₃, and other eutectic structures. However, it is unable to predict the appearance of Al (α) hollow and α-dendrites because these are non-equilibrium phenomena. In addition, some discrepancies were observed especially in the final stage of solidification. The evaluation performed by the thermodynamic calculation software using the Scheil's equation provides details on the final solidification reactions similar to experiment findings.</p>