Grain refinement of Al–Si alloy (A356) by melt thermal treatment

In order to increase the casting quality and material plasticity of hypoeutectic Al–Si alloys, the effects of melt thermal treatment on the solidification structure of the A356 alloy were analyzed. Unlike the conventional melt addition treatment, no refinements are required during this novel processing. The structural characteristics and mechanical properties (plasticity and strength) were demonstrated. Results of quantitative metallorgraphic analysis showed that the primary dendrite size obviously reduces and the dendrite grains change into equiaxed ones. However, the secondary dendrite arm spacing changes little. Experimental results show that there is an optimal cooling rate range for thermal treatment, in which a maximum microstructure refining effect can be obtained. By the optimized parameters obtained with orthogonal experiments, the elongation ratio of the castings increases by 46.2% and the tensile strength by 8.6%. These results are explained by means of microheterogeneous melt theory, which is a result of the multiplication of the nuclei in the melt thermal treating procedure.