Fatigue performances of FSW and GMAW aluminum alloys welded joints: Competition between microstructural and structural-contact-fretting crack initiation

Abstract The fatigue properties of gas metal arc welded and friction stir welded assemblies made of aluminum alloy AA6061-T6 structural extrusions were examined. The mechanical performances of welded joints were obtained using uniaxial tensile and force-controlled constant amplitude axial fatigue tests. Microstructural and fractographic analyses were conducted to document the influence of the process on microstructure evolution, fatigue crack initiation sites, and propagation mechanisms leading to the final rupture of the assemblies. Microhardness measurements and digital image correlation techniques paired with interrupted tensile tests were also used to investigate the complex heterogeneous local mechanical behavior and to highlight the fact that the crack initiation mechanism was driven by the microstructural state of the joint as well as by the structural-contact-fretting occurring at the notch root. The corresponding fatigue strengths at 2 million and 10 million cycles were evaluated respectively at 10% and 20% higher for friction stir welded assemblies versus gas metal arc welded assemblies. Fractographic analyses revealed that the fatigue cracks were initiated from microstructural features (pores for the GMAW configuration and banded structure on the crown side for the FSW configuration), or from large sub-surface grains in a shallow region below the structural-contact-fretting occurring at the notch root.