Solidification Analyses and Heat Treatment of Modified And Refined Al-Si Alloys -A Study

Abstract

The present work involved the study of the effect of addition of cerium (Ce), phosphorus (P) and strontium (Sr) on cooling curve parameters, heat transfer characteristics, microstructure and mechanical properties of Al-8Si, Al-13Si, Al-14Si and Al-22%Si alloys. Melt treated alloys were solidified against sand base and against stainless steel, brass, and copper chills to assess the effect of cooling rate on silicon morphology. Thermal analysis parameters of melt treated alloys were determined by recording the temperature of the liquid metal solidifying against the chill. The addition of Ce significantly influenced both primary and eutectic phase nucleation temperatures and increased the undercooling temperatures. Newtonian and Fourier analysis techniques were used for the calculation of solid fraction and latent heat of solidification. The heat flux across the casting/chill interface was estimated using inverse modeling technique. The interfacial heat flux between the chill and the alloy increased with Ce and Sr melt treatment and decreased on addition of P. The effect of melt treatment and chilling on microstructure was assessed by measuring (i) the grain size (ii) roundness of eutectic silicon (iii) fineness of primary silicon. Ce treated alloys solidified against sand base at a slow cooling rate resulted in refinement of eutectic silicon along with the formation of AlSi-Ce ternary intermetallic compound. The addition of Ce to alloys solidified against chills resulted in complete modification of the eutectic silicon. The addition of Ce also resulted in fine equiaxed grains in Al-8Si alloys and refined the primary silicon in hypereutectic Al-Si alloys. The addition of Ce simultaneously modified and refined both primary and eutectic phases, whereas, Sr and P modified eutectic silicon and refined primary silicon respectively. On addition of Ce, P and Sr there was a significant improvement in the mechanical properties of the alloys. The combined addition of Ce with Sr to Al-Si alloys resulted in further improvement of mechanical properties. Al-8Si, Al-14Si and Al-22Si alloys were subjected to T6 heat treatment. The ageing temperature and time were selected based on a series of trial experiments. The tensile strength of all untreated alloys increased when subjected to heat treatment, whereas, the heat treatment had deleterious effect on the mechanical properties of melt treated alloys except on P treated alloys.