Microstructure-Sensitive Fatigue Modeling of Friction Stir Welded Aluminum Alloy 6061

In this work, experiments were conducted to quantify the mechanical properties and microstructure of friction stir welded (FSW) 6061 aluminum alloy butt joints. In particular, strain-control experiments were carried out to characterize the low-cycle fatigue (LCF) performance of the FSW joint and a microstructure-sensitive fatigue model was used to elucidate structure-property relationships. Under fatigue testing, two distinct modes of failure were observed. In the first case, the fatigue cracks initiated and propagated though the heat affected zone, which is due to material softening as a result of the frictional heat generated by the FSW process. The second mode of failure observed was when the fatigue crack initiated and propagated through the stir zone, as a result of inappropriate material mixing. Additionally, results of this study show a strong dependence on the tool rotational rate, where the monotonic tensile and cyclic mechanical properties increased as the tool rate increased. However, experimental results showed that the increase in the mechanical properties were observed to level off or in some case decline as the tool rotational rate continued to increase. This behavior is due to several factors including welding defects and higher frictional heat. In order to further understand the effect of microstructure and welding defects on fatigue behavior, a multi-stage fatigue (MSF) model that incorporates incubation and crack growth regimes was implemented to capture the effect of variation in tool rotational speeds. The MSF model exhibited good correlation to the experimental results, suggesting that the multi-stage approach for modeling fatigue damage in FSW joints is a reasonable approach. Furthermore, the model appears to capture the underlining mechanisms associated with damage in this type of welded joint.