Effect of GBF Process Conditions on the Microstructural Characteristic, Melt Quality and Mechanical Properties of Al-Si Alloys with Scrap Addition

In this study, the applicability of an Al-Si alloy with 30 wt% added scrap for automobile pistons was evaluated by investigating the melt quality, microstructural characteristics, and tensile properties under modified GBF (gas bubbling filtration) process conditions, including increasing rotor rotation speed and adjusting the air-line supply and the inclination angle of the impeller blade. The melt quality was dramatically improved under modified GBF process conditions, resulting in a very clean melt, with the D.I. value decreasing by 28%, the length of the oxide layer per kilogram decreasing by 65%, and inclusion content decreasing by 97% compared to that of the conventional GBF process conditions. Additionally, the size of primary Si decreased from 40 µm to 27 µm, and the eutectic Si and intermetallic compounds were refined, showing a very fine microstructure. The identified phases included Al₄Cu₂Mg₈Si₇, Al_xCu_yNi_z, and MgO. The ultimate tensile strength was 275 MPa, and the elongation was 6.0%, indicating improved tensile properties compared to those of the conventional GBF process conditions. The fracture behavior changed from a brittle microcleavage fracture mode to a ductile dimple fracture mode as the primary Si, eutectic Si particles, and intermetallic compounds were refined under modified GBF process conditions. These results confirmed that Al-Si alloy with added scrap can be used as a material for automobile pistons.