Aging and Strength Properties of Highly Rigid Al-Si-Ni-Cu-Mg Cast Alloys

<p>  Due to growing demands for better working accuracy and productivity in Al alloy products with high Young’s modulus in the precision machinery and robot industries, we developed Al-Si-Ni-Cu-Mg alloys with a Young’s modulus of 100GPa. The solidification of this alloy proceeded in the order of primary Si, primary-like Ni₂Al₃, Al FeMn₂, eutectic Al (α) ＋ Si, Al (α) ＋ Ni₂Al₃, Al (α) ＋ CuAl₂ and Al (α) ＋ Al₄Cu₂Mg₇Si₈. To investigate the effects of these Si and intermetallic compounds on strength and Young’s modulus, we prepared three hypereutectic alloys with varying amount of primary phases, a eutectic alloy with a structure similar to the eutectic in the hypereutectic alloys and an alloy with almost the same composition as the Al (α) matrix. The hardening processes in T6 and T5 heat treatments were examined on each test alloy specimen, and tensile tests were performed on the T5-treated specimens. The eutectic alloy specimen exhibited a Young’s modulus of 90GPa, about 30GPa higher than that of the Al (α) matrix alloy due to the presence of the eutectic Si and intermetallic compound phases. Furthermore, in the hypereutectic alloy, the Young’s modulus increased to 100GPa with the increase in primary Si and primary-like N₃Al to 25vol%. The effects of these primary phases were well evaluated by the rule of mixture in which primary Si and Ni₂Al₃ work as reinforcing particles and fibers respectively. On the other hand, the tensile strength of the eutectic alloy specimen was about 300MPa, and the strength decreased about 60MPa due to the distribution of coarse primary phases in the hypereutectic alloy.</p>