On improved fatigue properties of aluminum alloy 5086 weld joints

In this work, we aimed to investigate the fatigue properties of AA5086-H321 and its weld joints. The AA5086-H321 plates were welded in a butt configuration using the friction stir welding. Load-controlled axial sinusoidal cyclic tests were conducted in the ambient conditions, at a frequency 20 Hz, and at different load ratios i.e., R sigma =-1, 0.1, and 0.5. The weld joints, in general, are known to have reduced properties. However, in this study, the base alloy and its weld joint had similar hardness and ultimate tensile strength, but with marginally reduced elongation (similar to 4.4%). The endurance limits of the weld joints were either higher (similar to 28% and similar to 11% at R =-1 and 0.1, respectively) or same (at R = 0.5) as compared to that of the base alloy. The difference in fatigue lives of base alloy and its weld joint kept diminishing with increasing R-ratio. Microscopic examination of fracture surfaces in weld joints revealed three types of crack initiations: voids, inclusions, and tool-marks, and two types of crack propagation mechanisms: intergranular and transgranular. However, in base alloys, only defect-free surface initiation with transgranular crack growth was observed. Mean stress, as expected, had a detrimental effect on fatigue properties, with no definitive control on the type of crack initiation or its propagation mech-anism. An increase in the fatigue lives of weld joints was attributed to the crack growth retardation during the intergranular fatigue crack propagation due to frequent defection of crack path, which was absent in the base alloys. Most of the intergranular facets were observed near the surfaces (top or bottom) of the weld nugget zone and disappeared when the crack front reaches its interface boundary with the heat affected zone. All these as-pects of fatigue damage in microstructurally heterogeneous weld joints and the plausible explanations for the observed mechanical fatigue properties are presented in this study.

Automated summary

This study aimed to investigate the fatigue properties of AA5086-H321 and its weld joints. The weld joints showed similar hardness and ultimate tensile strength as the base alloy, but with slightly reduced elongation. The endurance limits of the weld joints were either higher or the same as compared to that of the base alloy. The fatigue lives of weld joints were increased due to crack growth retardation during intergranular fatigue crack propagation.