Friction stir additive manufacturing (FSAM) technology is the ideal technique for aluminum alloy additive manufacturing from the perspective of defect control, microstructure regulation, and performance optimization. However, there is limited systematic fundamental research on the aluminum alloy FSAM. This study implemented a consumable-tool-based 5A06 FSAM process. By incorporating carbon nanotubes during the FSAM process, our research investigated its impact on grain refinement and the performance of the additive structure. The results show that the well-formed additive structure is composed of multiple layers of stirred metal. The microstructure of the additive structure of AA5A06 consists of refined recrystallized grains and deformed grains within each layer, while the interface between layers is composed of a finer-grain band, with an average grain size of 6 µm, whose tensile strength ranges from 225 MPa to 260 MPa, with an elongation of 26% to 32%. After the addition of carbon nanotubes, although the grain size was refined to 2 µm, there was no improvement in tensile strength, and the elongation was reduced. The tensile strength now ranges from 225 MPa to 270 MPa, with elongation between 12% and 16%.

This study presents a quasi-in situ observation of the fatigue crack growth behavior in a friction stir welded 2024-T4 joint. The microstructure and fatigue properties of the joint were investigated using electron backscatter diffraction (EBSD) technique, scanning electron microscopy (SEM), and fatigue crack growth tests. The fatigue crack growth behavior of the joint was examined by conducting fatigue crack growth tests with different notch locations. The results show that the sample with the notch in the stir zone (SZ) exhibited the highest resistance to fatigue crack growth, followed by the notched samples of the Advancing side (AS) and Retreating side (RS) weldments. Microstructural observations showed a homogeneous microstructure with a fine grain size in SZ and it was observed that this fine-grained structure significantly enhanced the material’s resistance to fatigue crack growth. The experimental results were further analyzed using the Paris model to provide a quantitative understanding of the crack growth behavior. The study underlines the impact of microstructural characteristics and notch location on the fatigue performance of the weldment. Overall, the quasi-in situ observations and experimental findings contribute to a comprehensive understanding of the fatigue crack growth behavior in friction stir welded 2024-T4 joints.

The reliability of friction stir welded joints is a critical concern, particularly given their potential applications in the aerospace manufacturing industry. This study offers a quasi-in situ observation of the microstructural response during fatigue crack growth (FCG) of a friction stir welded AA2024-T4 joint, aiming to correlate fatigue crack growth behavior with mechanical properties investigated using electron backscatter diffraction (EBSD). Notched compact tension (CT) specimens corresponding to the morphology of the stir zone (SZ), advancing side (AS), and retreating side (RS) were meticulously designed. The findings indicate that the welding process enhances the joint’s resistance to fatigue crack growth, with the base metal exhibiting a shorter fatigue life (i.e., ~10⁵ cycles) compared to the welding zones (SZ ~ 3.5 × 10⁵ cycles, AS ~ 2.5 × 10⁵ cycles, and RS ~ 3.0 × 10⁵ cycles). Crack propagation occurs within the stir zone, traversing refined grains, which primarily contribute to the highest fatigue life and lowest FCG rate. Additionally, cracks initiate in AS and RS, subsequently expanding into the base metal. Moreover, the study reveals a significant release of residual strain at the joint, particularly notable in the Structural-CT-RS (Str-CT-RS) sample compared to the Str-CT-AS sample during the FCG process. Consequently, the FCG rate of Str-CT-AS is higher than that of Str-CT-RS. These findings have significant implications for improving the reliability and performance of aerospace components.