Fig. 2. The conventionally cast composite (left) featured silicon carbide in large clusters among the interdendritic regions. The ablation composite sample (right) showed more evenly distributed silicon carbide and graphite particles with higher volume fraction.
settling of the reinforcement (Fig. 2). T e ablated sample’s microstructure was more cellular, meaning reinforce- ment particles were trapped in a tight network of cell boundaries, increasing the local concentrations of reinforce- ment and reducing the average inter- particle spacing. Although the combination of
silicon carbide and graphite particles in the liquid aluminum resulted in a near neutrally buoyant suspension, some settling and fl otation occurred over time when melting a large batch. If the melt is insuffi ciently mixed, the reinforcements can migrate, producing areas that are rich in silicon carbide or graphite. T is can result in large diff erences in the volume percent- age of reinforcement in the solidi- fi ed microstructure. T e increase in volume percentage in the thin section of the ablated component compared to the conventional sand casting may have partly contributed to the higher strength of the ablated samples and could be due to the casting process, if metal is ladled from the top, middle or bottom of the original melt charge. T e ablated samples featured an
improved distribution of particles in the casting compared to the conven- tionally cast samples. Additionally, ablation appeared to refi ne the den- dritic cell size, which is in contrast to early reports in a monolithic A356 alloy. T e particulate reinforce- ment within the liquid melt during solidifi cation may have restricted the dendrite cell size while it was pushed by the solidifying aluminum. In
Jan/Feb 2012 | METAL CASTING DESIGN & PURCHASING | 39
ablation, the impingement of solvent on the solidifying surface increases cooling rates, entrapping the particles more rapidly without allowing time to substantially migrate, leading to increased branching of the dendrites and fi ner overall cell size. Strength of the ablated sample
was higher than that of the con- ventionally cast sample (Table 2). Further improvements to ablation castings can be achieved by design- ing and optimizing the pouring technique and mold geometry spe- cifically for the ablation process to reduce turbulence in the melt.
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