On the fatigue resistance assessment of friction stir welded joints affected by heat input

In view of the fact that composite friction welding parameters cannot independently reflect the service life of automotive joints, the evolution of fatigue behavior caused by process changes was studied by integrating their effects into unified heat input. Taking into account the geometric mutation and mechanical modifications of post-weld Al alloys, the developed hot spot stress estimation model of such joint has the ability to represent the influence of thermal forming process on the alternating singular stress, and is further reflected in the predicted life. The results show that on average, every increase in welding heat input energy by 8515 J led to an improvement in fatigue life of approximately 25.79 %. It was found that fatigue strength benchmark specified by hot spot stress improved the goodness of fit by 29.14 % to 55.94 % over the nominal stress, while increasing the inter-process variation coefficient by 10.77 %, thus exhibiting more accurate failure calibration for this non-uniform spot-welded structure. It was further verified that the life span error predicted using the MC model was 17.67 % to 45.54 % lower than that of the SWT model and closer to the experiment. The fatigue crack propagation modes with respect to load levels were also revealed through fracture morphology. Overall, the study provided a process-regulated fatigue resistance assessment for vehicle joints.