Ultrasonic-Assisted Stir Casting of SiO2-Reinforced AA7075: Microstructure, Strengthening, and Wear

Abstract The present work examines the effect of SiO2 nanoparticle addition on the microstructure, mechanical properties, and dry-sliding wear behavior of AA7075-based metal matrix nanocomposites. Four reinforcement levels, 0.5, 1.0, 1.5, and 2.0 wt %, were investigated. AA7075 was reinforced with SiO2 using an ultrasonic-assisted stir-casting process. The process produced relatively uniform SiO2 distributions up to 1.5 wt % and a refined aluminum matrix, whereas localized particle-rich regions became evident at 2.0 wt %. The tensile strength increased from 190 ± 15 MPa for the unreinforced alloy to 255 ± 22, 310 ± 10, 360 ± 22, and 385 ± 12 MPa for composites containing 0.5, 1.0, 1.5, and 2.0 wt % SiO2, respectively, corresponding to strength improvements of approximately 34.2%, 63.2%, 89.5%, and 102.6%. Microhardness also increased from 86 ± 3 VHV for AA7075 to 164 ± 12 VHV for the composite containing 2.0 wt % SiO2. The improvements in strength and hardness are consistent with matrix refinement, interfacial load transfer, restriction of dislocation motion, and thermal-expansion-mismatch-induced dislocation strengthening. Dry-sliding wear experiments were conducted in accordance with ASTM G99 at a sliding velocity of 3.14 m s–1, a total sliding distance of 6 km, and applied loads of 10–50 N. The reinforced materials generally showed lower specific wear rates than the unreinforced alloy. At 50 N, the specific wear rate decreased from approximately 6900 to 4500 μm3 N–1 mm–1 with the addition of 2.0 wt % SiO2, corresponding to a decrease of approximately 34.8%. The coefficient of friction generally decreased with increasing SiO2 content, and the 2.0 wt % composite exhibited the lowest values within the investigated range. SEM and TEM examinations provided evidence of changes in particle distribution and microstructural features with increasing reinforcement content, while SEM examination of the worn surfaces revealed corresponding changes in wear morphology. Overall, the present findings show that the ultrasonic-assisted stir-casting route used in this study produced AA7075–SiO2 metal matrix nanocomposites with increased strength and hardness and generally improved dry-sliding wear behavior within the investigated composition and test range.