Effect of Sb Addition on the Al–Si Eutectic of Hypoeutectic Al–Si Casting Alloys under Different Cooling Rates

Abstract

The effect of antimony (Sb) additions (0, 0.2 mass%) and cooling rates (0.56°C·s⁻¹, 2.03°C·s⁻¹, 7.42°C·s⁻¹) on the Al–Si eutectic of hypoeutectic Al–Si alloys were investigated. The results show that Sb combines with Mg to form a compound Mg₃Sb₂ which poisoned the nucleation particle AlP, resulting in the nucleation undercooling of the eutectic silicon increased. In unmodified alloy, the sensitivity of eutectic temperatures to cooling rate decreases with the increase of cooling rate. The reason is that the growth rate of eutectic silicon increases and the latent heat of crystallization is released in large quantities. After adding 0.2%Sb, there was a good linear relationship between eutectic temperatures and cooling rates. Due to the inhibition of silicon growth by Sb and Mg₃Sb₂ particles, the growth of the eutectic silicon was slower and the latent heat of crystallization was released slowly. Hence, Sb increased the sensitivity between 2.03°C·s⁻¹ and 7.42°C·s⁻¹. In unmodified alloy, the eutectic nucleated on the primary dendrites and interdendritic liquid, after adding 0.2%Sb, the eutectic only nucleated independently in the interdendritic regions.

Fig. 7 EBSD images of (a)(c)(e) Al–7Si–0.6Mg alloy, (b)(d)(f) Al–7Si–0.6Mg–0.2Sb alloy at different cooling rates ((a)(b) correspond to 0.56°C·s⁻¹, (c)(d) correspond to 2.03°C·s⁻¹ and (e)(f) correspond to7.42°C·s⁻¹).