Abstract
Magnesium and silicon concentrations in the interior of primary α-Al of Al-7Si-Mg alloys were studied at temperatures in the liquid-solid range and just after solidification was completed. Analysis of the results showed that the magnesium concentration in the interior of primary α-Al is very low in the temperatures range from the liquidus to the start of the Al-Si eutectic reaction. Formation of silicon-rich phases during eutectic reactions, such as eutectic silicon and β-Al5FeSi, phases trigger a significant increase in the magnesium concentration in the interior of primary α-Al, when sufficient time is allowed for solid-state diffusion to occur. When fast cooling rates are applied during the Al-Si eutectic reaction, most of the magnesium is retained in π-Al8FeMg3Si6 and Mg2Si phases formed during solidification. Semi-solid Al-7Si-Mg castings were produced with varying magnesium contents, and the mechanical properties were evaluated in the as-cast, T5 and T6 conditions. It was found that the 0.2% offset yield strength of the semi-solid Al-7Si-Mg castings in the T5 and T6 conditions increases linearly with the square root of the magnesium concentration in the interior of the α-Al globules formed during the slurry preparation process.
Highlights
Al-7Si-Mg alloys are commonly used in the automotive and aerospace industries due to their excellent castability, high specific strength and corrosion resistance [1,2,3,4]
The results of this study suggest that the formation of eutectic silicon and β-Al5 FeSi phases during eutectic solidification of Al-7Si-Mg samples generate a sharp increase in the magnesium concentration in the interior of primary α-Al
The magnesium and silicon solubilities in primary α-Al phase were studied in Al-7Si-Mg alloys isothermally held at different temperatures in the liquid-solid range for 120 h
Summary
Al-7Si-Mg alloys are commonly used in the automotive and aerospace industries due to their excellent castability, high specific strength and corrosion resistance [1,2,3,4]. 0.5 wt.% magnesium to these alloys improves the response to precipitation hardening during aging of these alloys [2,5,6,7]. Magnesium addition decreases ductility and fracture toughness [8]. The effectiveness of T5 and T6 heat treatments of Al-7Si-Mg alloys is strongly affected by the level of silicon and magnesium supersaturation of the primary α-Al at the artificial aging temperature [1]. Increasing the supersaturation of magnesium and silicon in α-Al will promote the formation of larger number density and a fraction of metastable and coherent β” precipitates, and enhance the hardening response of the Al-7Si-Mg alloys during aging [1]. In the T6 heat treatment, a solution treatment is carried out to homogenize the primary α-Al, dissolve intermetallic phases such as Mg2 Si and π–Al8 FeMg3 Si6 , releasing magnesium into solid solution, and spheroidize eutectic silicon [2,3]
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