Low-cycle fatigue behavior of friction stir-welded copper joints

Abstract

Investigation of the fatigue response of friction stir-welded (FSWed) joints is especially important in the design and manufacturing of components with exposure to cyclic loading. In this study, cyclic response of FSWed pure copper joints is investigated in the low-cycle fatigue regime. Microstructural characterizations revealed that FSW introduced a severely deformed microstructure in the nugget zone (NZ). Fatigue response was determined at a strain ratio of 0.1 by varying the total strain amplitude from 0.1 to 0.6%. Cyclic softening was observed for the low strain amplitude of 0.1%, whereas hardening was detected at higher strain amplitudes. The hysteresis loops demonstrated symmetricity along with noticeable linear behavior after the reversals. Typical fractures occurred in the heat affected zone (HAZ) rather than the NZ or the base metal due to grain coarsening of the HAZ. Improved cyclic properties of the NZ along with stable behavior up to 1000 cycles at a total strain amplitude of 0.3% were attributed to its fine and homogeneous microstructure. Moreover, fracture surface analysis demonstrated a ductile behavior represented by dimples in the sample strained at 0.1% in contrast with a brittle fracture surface of the sample fatigued at 0.5% strain amplitude.