High-temperature ratcheting and low-cycle fatigue failure of a friction stir welding Al-Zn-Mg-Cu alloy

High-temperature ratcheting fatigue behavior of a friction stir welding (FSW) Al-Zn-Mg-Cu alloy was investi-gated. The substructured grains in the weld nugget zone (WNZ) undergo continuous dynamic recrystallization (CDRX) during ratcheting fatigue. The WNZ re-precipitates intragranular eta ' and eta particles and forms the pre-cipitate free zone (PFZ) in the vicinity of the grain boundaries. At high mean stress or stress amplitude, ratcheting failure with obvious necking takes place due to large ratcheting strain. At low stress levels, intergranular fatigue microcracks initiate from grain boundaries of the WNZ due to the local strain incompatibility between the PFZ and grain interiors. The fatigue microcracks coalesce along grain boundaries and propagate via typical striation formation mechanisms. Low-cycle fatigue failure occurs at the WNZ instead of the low hardness zone (LHZ).