Effect of alloying elements on the segregation and dissolution of CuAl₂ phase in Al-Si-Cu 319 alloys

Abstract

The hypoeutectic 319 aluminum alloy (Al-7%Si-3.5%Cu) was used in the present study to investigate the effect of diverse alloying elements on the dissolution of the copper phase (CuAl₂) during solution heat treatment. Elements such as Sr, Fe and P were added to the base alloy individually and in various combinations. The cooling curves of these alloys were obtained by solidifying the alloy melts in a preheated graphite mold (600°C, cooling rate ∼0.8°C/s). From these the first derivate curves were plotted and used to determine the effect of the additives on the precipitation temperature of the Al-CuAl₂ eutectic reaction. Microstructural examination was carried out using optical microscopy, image analysis, and electron probe microanalysis (EPMA), with energy dispersive X-ray (EDX) and wavelength dispersive spectroscopic (WDS) analysis facilities. Samples from different alloys were solution heat treated at 505°C for various times up to 100 hours. The results explicitly reveal that solution heat treatment plays a critical role on the dissolution of the CuAl₂ phase. It is found that Sr leads to segregation of the CuAl₂ phase away from the Al-Si eutectic regions, which slows down its dissolution during solution heat treatment. The β-Al₅FeSi phase platelets act as preferred precipitation sites for the copper phase and hence lessen the degree of segregation. Thus, addition of Fe can accelerate the copper phase dissolution. However, phosphorus addition has a negative effect on CuAl₂ dissolution due to (i) its solubility in the CuAl₂ phase particles, and (ii) the formation of (Al,P)O₂ oxide particles which act as nucleation sites for the precipitation of the block-like CuAl₂ phase. It retards the complete dissolution of this copper phase even after 100 hr solution treatment. In the case when phosphorus and iron are added together, the negative effect of phosphorus can be neutralized to some extent.