Abstract
The present work was carried out on Al-7%Si-0.4%Mg-X alloy (where X = Mg, Fe, Sr or Be), where the effect of solidification rate on the eutectic silicon characteristics was investigated. Two solidification rates corresponding to dendrite arm spacings (DAS) of 24 and 65 μm were employed. Samples with 24 μm DAS were solution heat-treated at 540 °C for 5 and 12 h prior to quenching in warm water at 65 °C. Eutectic Si particle charateristics were measured using an image analyzer. The results show that the addition of 0.05% Be leads to partial modification of the Si particles. Full modification was only obtained when Sr was added in an amount of 150–200 ppm, depending on the applied solidification rate. Increasing the amount of Mg to 0.8% in Sr-modified alloys leads to a reduction in the effectiveness of Sr as the main modifier. Similar observations were made when the Fe content was increased in Be-treated alloys due to the Be-Fe interaction. Over-modification results in the precipitation of hard Sr-rich particles, mainly Al4SrSi2, whereas overheating causes incipient melting of the Al-Cu eutectic and hence the surrounding matrix. Both factors lead to a deterioration in the alloy mechanical properties. Furthermore, the presence of long, acicular Si particles accelerates the occurrence of fracture and, as a result, yields poor ductility. In low iron (less than 0.1 wt%) Al-Si-Mg alloys, the mechanical properties in the as cast, as well as heat treated conditions, are mainly controlled by the eutectic Si charatersitics. Increasing the iron content and, hence, the volume fraction of Fe-based intermetallics leads to a complex fracture mode.
Highlights
The eutectic silicon particle characteristics in Al-Si alloys play an important role in determining the mechanical properties of the alloys
Strontium is commonly used in Al-Si casting alloys to modify the morphology of the eutectic Si from a coarse, flake-like form to a fine fibrous one, so as to improve the mechanical properties, in particular, the ductility [1,2,3]
The possibility of the existence the planar bifilm cracks in the Si particles may accelerate fracture
Summary
The eutectic silicon particle characteristics in Al-Si alloys play an important role in determining the mechanical properties of the alloys. Strontium is commonly used in Al-Si casting alloys to modify the morphology of the eutectic Si from a coarse, flake-like form to a fine fibrous one, so as to improve the mechanical properties, in particular, the ductility [1,2,3]. With the addition of Sr, the eutectic temperature of the Al-Si eutectic reaction is depressed This depression is often used to estimate the degree of modification which has taken place in the Al-Si alloy. Other alloying elements, such as magnesium and beryllium, together with varying amounts of iron, manganese and zinc as impurity elements, form intermetallic particles during solidification [4,5]
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