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Effect of semi-solid compression test on refinement of Si grains in hypereutectic Al-Si alloy

This paper presents results of studies carried out to evaluate the probability of silicon particle refinement in hypereutectic Al–25 mass% Si alloy through a uniaxial compression test using cylindrical specimen at over the eutectic melting point T m = 850 K (577 ºC). The test conditions were a combination of temperatures of 580 and 590 ºC, compression rates of 5, 25, and 125 mm/min, and compression stroke of 5, 10, and 12 mm. The effects on silicon particle refinement assessed by measuring equivalent strain, mean strain rate, power rate, and shear rate. Specimens show the barreled shape due to low compression rate and also the load versus displacement curves showed the work softening nature. The results indicated that the finer silicon grains were obtained in cases of lower test temperature, higher compression rate, and larger compression stroke. Greater applied strain, strain rate, power rate, and shear rate were affected on the refinement of silicon grain. The only satisfying result showing strong correlation was obtained from the relationship between shear rate and silicon grain size on a double logarithmic plot ； the effect of the shear rate, g [s –1 ], and tested temperature, T [K], on silicon grain size, G [mm], was expressed using power-law model G = 0.08( T - T m ) 0.4 g - 0.24/( T - T m ) . The relation showed that high shear rate operation at just above the melting point must be the optimum condition.