Microstructural modification of recycled aluminium alloys by high-intensity ultrasonication: Observations from custom Al–2Si–2Mg–1.2Fe–(0.5,1.0)Mn alloys

The effect of ultrasonication on the solidification microstructure of recycled Al-alloys is investigated using custom Al–2Si–2Mg–1.2Fe–xMn alloys (x = 0.5 and 1%, in wt.%) through cooling curve measurement, optical and electron microscopy, X-ray diffraction, differential scanning calorimetry and computational thermodynamic calculations. Applying ultrasonication throughout the primary-Al nucleation stage resulted in refined non-dendritic grain structure. Cooling curves indicate a noticeable reduction in primary-Al nucleation undercooling and reduction of the recalescence peak under ultrasonication. However, terminating ultrasonication prior to the nucleation of primary-Al led to dendritic grains with marginal refinement. Without ultrasonication, coarse Chinese-script α−Al15(Fe,Mn)3Si2 intermetallics developed from initially polygonal particles due to interface growth instability under thermo-solutal undercooling. In contrast, ultrasonication produced refined and polygonal α−Al15(Fe,Mn)3Si2 particles by promoting nucleation and growth stabilisation under strong fluid flow. The enhanced nucleation from ultrasonication is presumably due to the pressure-induced shift of freezing point along with improved wetting of insoluble inclusions under cavitation. The present results show that ultrasonication can effectively modify the Fe-intermetallics and refine the grain structure in recycled Al-alloys.