Effect of Simultaneous Mg and Zn Addition on the Solidification and Microstructure of Multi-Element Hypoeutectic Al-Si Alloys

Microalloying is a crucial method for enhancing alloy properties. Magnesium (Mg) is the primary strengthening element in 6xxx alloys, while zinc (Zn) plays a similar role in 7xxx alloys. However, the combined addition of Mg and Zn and its impact on the multi-element hypoeutectic Al-Si cast aluminum alloys remain uncertain. This paper investigates the effects of Mg and Zn additions on the solidification and microstructure of multi-element hypoeutectic Al-Si alloys. With Mg and Zn additions, the size and distribution of eutectic silicon transform from individual long platelets to a finer, more compact structure due to increased undercooling resulting from lower eutectic silicon formation temperatures. Additionally, the needle-like phases were AlSi(Mn, Cr)Fe and (Al, Zr, Si) in the A1 alloy, respectively. The incorporation of Zn into the AlSi(Mn, Cr)Fe phase, the AlSi(Mn, Cr)Fe phase has the preferred growth direction and finally presents a needle-like structure. The formation of the new phase (Al, Zr, Si) is attributed to increased partial Gibbs energy of Zr, reducing its ability to remain in the liquid phase and promoting reaction with Si upon addition of Mg and Zn. The hardness and tensile strength increase with the addition of Mg and Zn due to their solid solution into the aluminum matrix, while elongation decreases. The room temperature tensile strength, hardness, and elongation of the as-cast alloy under gravity casting reach 221.04 MPa, 84.1 HBW, and 2.12%, respectively, upon the addition of Mg and Zn in multi-element hypoeutectic Al-Si alloys. This paper provides a new direction and reference value for the development of solution-free high-strength aluminum alloys.