Effects of Cr and Cooling Rate on Segregation and Refinement of Primary Si in Al-20 WT %Si Alloy

The effect of Cr on segregation and refinement of primary Si in Al-20 wt %Si alloy under a different cooling rate was investigated with a wedge-shaped mould. In addition, the effects of P and a combination of P and Cr on Al-20 wt %Si were also studied. Thermal analysis techniques were used to calculate the cooling rate in the solidification range. The microstructures were examined by optical microscope and scanning electron microscope. The results showed that the segregation of primary Si exists in the castings. The optimal pouring temperature was 850°C (1562°F) with a range of 800–900°C (1472–1652°F). Segregation of primary Si could be inhibited with the addition of refiners and increasing cooling rate. The CrSi2 particles adhered to primary Si particles further reduced the segregation. The primary Si particles were well refined by Cr, P or a combination of P and Cr for CrSi2 and AlP served as heterogeneous nucleus for primary Si, but P had a better refinement effect than Cr. A combination of P and Cr into the melt improved the P refinement effect and substantially inhibited the segregation of primary Si.