Microstructure and mechanical performance of dissimilar friction stir spot welded AA2024-O/AA6061-T6 sheets: Effects of tool rotation speed and pin geometry

Nowadays, friction stir spot welding (FSSW) is becoming a serious candidate technology to join aircraft materials such as AA2024-O and AA6061-T6 since the use of FSSW offers significant weight savings compared to a conventional riveting technique. However, some issues, especially related to mechanical properties of FSSW joints still exist. To address these problems, the present investigation aims to study effects of tool rotation speed and pin geometry on microstructure and mechanical strength of dissimilar friction stir spot welded AA2024-O/AA6061-T6 joints. The welding processes were conducted using two different tool pins, namely cylindrical and stepped pins at varying tool rotation speeds of 900 rpm, 1400 rpm and 1800 rpm. Subsequently, experimental works including microstructural observation, microhardness measurements, lap shear tests combined with fractography analysis were conducted. Results showed that an increase in tool rotation speed resulted in microstructural coarsening in stir zone (SZ). In addition, the tool rotation speed together with the pin affected the material flow hence hook characteristics. It was found that the dissimilar friction stir spot AA2024-O/AA6061-T6 welded joint having excellent lap shear failure load (LSFL), typically 4468.4 N was produced using a cylindrical pin at the tool rotation speed of 1400 rpm. Under this lap shear testing, the mode of failure changed from shear to mixed mode failure as the tool rotation speed was increased. It seemed that hook characteristics, grain size and precipitation hardening were the key factors influencing the weld mechanical properties.