With the popularization of friction stir welding（FSW）, 5083-H321 and 6061-T6 aluminum alloy materials are widely used during the FSW process. In this study, the fatigue life of friction stir welding with two materials, i.e., 5083-H321 and 6061-T6 aluminum alloy, are studied. Fatigue tests were carried out on the base metal of these two materials as well as on the butt joints and overlapping FSW samples. The principle of the equivalent structural stress method is used to analyze the FSW test data of these two materials. The fatigue resistances of these two materials were compared and a unified principal S-N curve equation was fitted. Two key parameters of the unified principal S-N curve obtained by fitting, Cd is 4222.5, and h is 0.2693. A new method for an FSW fatigue life assessment was developed in this study and can be used to calculate the fatigue life of different welding forms with a single S-N curve. Two main fatigue tests of bending and tension were used to verify the unified principal S-N curve equation. The results show that the fatigue life calculated by the unified mean 50% master S-N curve parameters are the closest to the fatigue test results. The reliability, practicability, and generality of the master S-N curve fitting parameters were verified using the test data. The unified principal S-N curve acquired in this study can not only be used in aluminum alloy materials but can also be applied to other materials.
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