Effects of Zn Content on the Microstructure and Properties of Al-12.5Si-xZn-2Cu-Mg Alloy

Zn has been added to the Al-12.5wt%Si-2wt%Cu-1wt%Mg (abbrev Al-12.5Si-2Cu-Mg) alloy. By utilizing optical microscope (OM), X-ray diffraction (XRD), and scanning electron microscope (SEM), the microstructure and phase structure of the alloy were investigated. The alloy's mechanical properties and wear resistance were evaluated. The results demonstrate that the as-cast microstructure of Al-12.5Si-xZn-2Cu-1Mg alloy consists of (alpha-Al + Si) eutectic, a minor amount of primary Si, theta-Al2Cu phase, Q-Al5Cu2Mg8Si6 phase, and pi-Al8Mg3FeSi6 phase. In the final solidification zone of an alloy with a Zn content greater than 2%, a Zn-rich granular phase is observed. With an increase in Zn content, the size of the Si phase decreases and the number of theta-Al2Cu phase decreases. Zn slightly raises the formation temperature of primary Si, decreases the (alpha-Al + Si) eutectic transition temperature, and significantly decreases the formation temperatures of theta-Al2Cu and Q-Al5Cu2Mg8Si6 phases. The yield strength and hardness of the as-cast alloy raise with increasing Zn content, whereas the elongation decreases and the tensile strength first increases and then decreases slightly. The tensile strength is the greatest when the weight percentage of Zn is 2wt%. After T6 treatment, the tensile strength, yield strength, and hardness of the alloy rise in proportion to the alloy's Zn content, and its resistance to wear is greatly enhanced.