Effect of Scandium Addition on Recycled A356 Alloy: Experimental Investigation and CALPHAD-Based Thermodynamic Analysis

This study systematically investigates the influence of scandium (Sc) additions at 0.1, 0.2, and 0.3 wt.% on the melt quality, microstructural characteristics, fluidity, and mechanical performance of A356 aluminum alloy under recycling-oriented casting conditions. The alloy charge consisted of 20% primary aluminum and 80% recycled scrap, while the Fe content was maintained at 0.2 wt.%. Melt treatment was performed by rotary degassing using a commercial flux. Melt quality was assessed using the Reduced Pressure Test (RPT) and bifilm index analysis, whereas casting performance was evaluated through an eight-channel fluidity test. Mechanical behavior was characterized by tensile and Brinell hardness testing, and microstructural and phase analyses were conducted using scanning electron microscopy (SEM) and X-ray diffraction (XRD). To complement the experimental investigation and provide a thermodynamic basis for interpretation, phase stability, solidification behavior, and phase-fraction evolution were examined using Thermo-Calc within the CALPHAD framework. By combining experimental characterization with computational thermodynamic analysis, this study elucidates the effect of Sc content on the processing–microstructure–property relationships of recycled A356 alloys and provides a systematic basis for evaluating Sc modification in high-recycled-content aluminum castings.