Estimation of fatigue crack growth rate in different zones of friction stir welded AA7039

Abstract

The present research aims to study the fatigue crack propagation and fracture toughness behavior of different zones of friction stir welds of AA7039 alloy, as different zones had varying microstructural and mechanical properties. Compact tension specimens with an initial notch in distinct weld zones parallel to the welding direction were machined using welded plates. Friction stir welding altered the base metal grain structure momentously, refined the grain structure in the weld nugget zone, and coarsen the same in the heat-affected zone. No correlation was observed between grain size and weld zone(s) hardness. Weld tensile properties were slightly inferior to base metal despite significantly higher hardness and alternation of grain structure. The fatigue life was dominated by the cycles consumed in crack propagation than crack initiation. The heat-affected zone had higher quasi-static fracture toughness, energy release rate, and resistance toward fatigue crack growth than other zones, because coarse grains and precipitates acted as effective impediments and slowed crack propagation. The heat-affected zone had a mixed-mode fracture, but the weld nugget zone and thermo-mechanically affected zone had an I mode fracture. On fracture surfaces, fracture characteristics such as microvoid coalescence were apparent.