Characterization of the solidification path and microstructure of secondary Al-7Si-3Cu-0.3Mg alloy with Zr, V and Ni additions

The effect of individual and combined Zr, V and Ni additions on solidification path and microstructural evolution of secondary Al-7Si-3Cu-0.3Mg alloy were investigated using microscopy and thermal analysis techniques. The results show how the Zr addition caused remarkable grain refinement due to the precipitation of primary pro-peritectic Al3Zr particles that lead to a peritectic reaction to yield α-Al during solidification; however, the peritectic transformation was incomplete due to a limited solid-state diffusion. Moreover, with Zr addition, few numbers of flaky (AlSi)3(ZrTi) compounds appeared in the microstructure. Vanadium addition showed also grain refinement level similar to that of Zr-added alloy. However, the role of V cannot be explained by the precipitation of primary pro-peritectic particles that would provide potent nucleation sites; instead, vanadium addition substantially increases the degree of constitutional undercooling, thus activating certain particles to more easily nucleate α-Al. Excess vanadium were mainly bound to pro-eutectic and polyhedral-shaped (AlSi)2(VMnTi) compounds. Combined Zr and V addition exerted better grain refinement level than the individual Zr or V addition; this occurred due to both increased the population of Al3Zr phase particles as a result of its enrichment with V, and enhanced the degree of constitutional undercooling. Nickel addition exerted no apparent influence on the formation of both α-Al and eutectic Si; however, significant changes occurred in the sequence of post-eutectic reactions: the script-like Al6Cu3Ni and flaky-like Al9(FeCu)Ni phases formed in addition to the Al2Cu and Al5Si6Cu2Mg8 particles; this led to a substantial reduction in the alloy's freezing range. The findings of this study can benefit and contribute to developing a new Al-Si based foundry alloys intended for high-temperature applications.