Microstructural evolution and mechanical properties of nano-SiC reinforced A356 aluminum composites fabricated using ultrasonication-coupled gas-induced semi-solid processing

A dual processing technique combining ultrasonication (UT) and gas-induced semi-solid (GISS) processing was developed to fabricate A356 aluminum matrix composites reinforced with 2.5 vol.% nano-SiC. Ultrasonic treatment (20 kHz, 2.5 kW) was applied at a low-superheat near-liquidus temperature (620°C) under two conditions: (i) ultrasonication alone (UT-620) and (ii) ultrasonication with simultaneous argon gas injection through a specially designed ultrasonic gas diffuser (UGD-620). Optical metallography revealed a transformation from a coarse dendritic structure in the base A356 alloy (~120 µm) to refined near-globular grains of 60–65 µm (UT-620) and 40–45 µm (UGD-620), with the shape factor increasing from 0.65 to 0.74 and 0.82, respectively. SEM and TEM analyses showed dendrite fragmentation, nSiC agglomeration in UT-620, and uniform nanoscale dispersion in UGD-620. XRD confirmed the presence of α-Al, Si, and Mg2Si phases without Al4C3 formation. Yield strength and hardness increased from ~78 MPa and 81 HV to 158 MPa and 154 HV, while porosity decreased from 2.1% to 0.7%. The results demonstrate that combining UT with GISS is essential for achieving a stable, densified microstructure and superior mechanical performance in nSiC-reinforced A356 composites.