Effects of Cooling Rate and Silicon Content on Microstructure and Mechanical Properties of Directional Solidification of Eutectic Al-Si Alloys

Abstract

This study investigates the directional solidification of hypoeutectic aluminum-silicon (Al-Si) alloys, with a focus on the influence of solidification parameters and silicon content (1 to 11 wt.%) on structural and mechanical properties. Among significant conclusions regarding the influence of thermal gradients and cooling rates on the position of the columnar-to-equiaxed transition (CET) and the conditions governing its formation, it can be resumed: CET is observed only under low or negative thermal gradients, with critical values ranging between 2.4 and 2 2.6 C/cm above which, and assuming constant cooling rates, the structure remains entirely columnar. Variations in cooling rate, in turn, have a strong impact on the CET position, while silicon content influence is only significant at lower cooling rates. Relationship between microstructure and mechanical properties of Al-Si alloys was analyzed. As cooling rate and Si content increased, an improvement in microhardness and tensile strength was observed, with maximum tensile strength values ranging from 58.54 MPa measured in the equiaxed zone of an Al-1wt.% Si sample to 188 MPa measured in the columnar zone of an Al-11wt.% Si sample. Mechanical properties were found to be superior in the columnar zones for all measured samples. Eutectic phase showed similar values independently of its placement in columnar or equiaxed zone, but the values were incremented with silicon content while keeping cooling rate constant at 9.5 ºC/min. In effect, measurements of 53.41 and 44.48 kg/ cm2 were obtained for an Al-1wt.%Si alloy sample while 91.4 and 84.5 kg/cm2 values were measured for an Al-11wt.% Si alloy sample, for columnar and equiaxed zone, respectively.