Incredible improvement in fatigue resistance of friction stir welded 7075-T651 aluminum alloy via surface mechanical rolling treatment

[Display omitted] •Two orders of magnitude extension in fatigue lifetime was achieved on the FSW joint.•The enhanced FSW joint is more resistant to fatigue failure than BM in VHCF-region.•SMRT induces the transition of fatigue crack initiation from surface to interior.•Internal fatigue cracks originate at the coarse and incoherent Al18Ti2Mg3 particles. Friction stir welding (FSW) is able to provide high static strength for aluminum alloys, but it always leads to a remarkable loss in fatigue strength. By means of surface mechanical rolling treatment (SMRT), a near-polished, gradient nanostructured surface and high compressive residual stresses were generated on the FSW joint of the 7075-T651 aluminum alloy. Uniaxial fatigue tests showed that an enhanced fatigue property with two orders of magnitude extension in fatigue lifetime was achieved on the SMRT joints, exhibiting failure mode transitions from surface to interior. Transmission electron microscopy analyses showed that the Al18Ti2Mg3 constituent phase was responsible for the internal crack initiation. Furthermore, the enhanced fatigue mechanism and the role of SMRT were discussed. The findings offer a potential pathway to greatly compensate the fatigue resistance deterioration of materials resulting from welding.