Decoupling grain refinement and precipitation strengthening in CNT-reinforced AA7075 nanocomposite joints via single- and triple-pass friction stir processing: Thermal over-aging overrides Hall-Petch

This study considered the trade-off between grain refinement and precipitation stability in CNT-reinforced AA7075 volumetric nanocomposites through the use of single-pass (S1C) and triple-pass (S 3 C) friction stir processing techniques. Through Raman spectroscopy, it was verified that the CNTs were intact and homogeneously distributed throughout the refined matrix. While the S3C sample performed well in terms of grain refinement (grain sizes of 0.3-1 μm and the highest hardness value of 170 HV attributed to the Zener pinning effect induced by the CNTs), the S1C sample outperformed the S3C sample in terms of ultimate tensile strength (UTS) of 529.8 MPa due to less strain hardening. On the basis of the analytical thermodynamic modeling using the Arbegast model, maximum temperature in the region could be estimated at about 410-430°C. The superior performance of the S 1 C sample was associated with a "thermal over-aging penalty" caused by repeated heat generation during friction stirring in the S 3 C technique. This mechanism is thought to cause the phase transformation from η′ precipitates to η and increase the width of the PFZ, reducing the Hall-Petch effect. EIS and potentiodynamic polarization in 3.5 wt.% NaCl solution revealed that the S 1 C sample exhibited a better electrochemical resistance with a corrosion current density of 0.274 μA/cm 2 compared to the S 3 C sample at 0.390 μA/cm 2 . It could be concluded through EIS analysis that higher thermal exposure during the S 3 C frictioning enhanced grain-boundary sensitization, despite better grain refinement.