Microstructural refinement in ultrasonically modified A356 aluminum castings

Two A356 aluminum alloys (Al–Si–Mg), one with 0.09 wt.% Fe and one with 0.91 wt.% Fe, were cast in a graphite mold with simultaneous local ultrasonic processing to refine the as-cast microstructure. Ultrasonication during casting transformed the morphology of primary Al grains from dendritic (~ 140–290 microns in size) to globular (~ 33–36 microns in size). The alloy with high Fe exhibited globular grains at distances up to 45 mm away from the ultrasound probe, while the alloy with low Fe exhibited globular grains at distances only up to 6 mm away from the ultrasound probe. Near the location of the ultrasound probe (< 2 mm away), a second non-dendritic microstructural morphology was observed with fine aluminum grains (~ 9–25 microns in size). This unique fine-grained morphology has not been previously reported, contains a greater area fraction of Si relative to the globular microstructure, and may be a large, fully eutectic region. Ultrasonication during casting also transformed the morphology of the β-Al₅FeSi phase particles (which are deleterious to the strength and ductility of the alloy) in the high Fe alloy from needle-like to rectangular, which could enable the greater use of secondary Al alloys. Thermodynamic simulations conducted to calculate the solidification paths of the two alloys studied predict that the β-Al₅FeSi phase begins to form earlier in the alloy with high Fe. Data suggest that the β-Al₅FeSi phase (which is more abundant in alloys with high Fe content) may enhance ultrasonically-induced grain refinement.