Failure behavior and mechanical properties of aluminum alloy-copper nanoparticle surface region fabricated by friction stir processing

This study investigates the effects of friction stir processing (FSP) and copper (Cu) nanoparticle reinforcement on the structural, mechanical, and corrosion properties of the AA7075 aluminum alloy surface region. Four variants underwent FSP: W (processed without Cu powder), C1 (processed with Cu powder using 5 mm hole spacing, 27 holes, and 0.642 g Cu), C2 (processed with Cu powder using 7.5 mm hole spacing, 18 holes, and 0.428 g Cu), and C3 (processed with Cu powder using 10 mm hole spacing, 14 holes, and 0.333 g Cu). These conditions influenced
the characteristics of the stir zone (SZ), thermo-mechanically affected zone (TMAZ), and heat-affected zone (HAZ).SZ area was largest in W (14.125 mm²) and smallest in C3 (10.348 mm²), with increased Cu content leading to refined grain structures and modified “onion ring” patterns in the SZ. Sample C1 exhibited the most substantial grain refinement (1.7 µm) in HAZ, whereas C3 displayed limited refinement (5.13 µm). Mechanical testing showed Sample C2 had the highest bending strength (1.5 kN) and ultimate tensile strength (UTS) of 460 MPa with 9.6 % elongation. Corrosion resistance improved notably; C1 showed the highest polarization resistance (Rp) at 4.12 kΩ⋅cm², reducing corrosion current density by 64 % compared to W. Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) confirmed that Cu nanoparticles enhance ductility, fracture toughness, and localized corrosion resistance, as seen by the uniform passive layer on C1 and C2. Hardness peaked in C1’s SZ (111 HV), affirming Cu nanoparticles’ role in strengthening AA7075.