Asymmetric study on the microstructure and mechanical properties of friction stir welded joints: Finite element simulation and experiment

The local asymmetry in the microstructure of friction stir welded joints in metallic materials is a widespread issue that significantly impacts their mechanical properties. However, the mechanisms underlying this local asymmetry remain unelucidated. This study aimed to investigate the microstructure and mechanical properties of friction stir welded joints in 18 mm-thick aluminum alloy plates. The mechanism underlying the asymmetric local microstructure and mechanical properties was investigated using transmission electron microscopy and finite element simulations. The simulation results revealed that the asymmetric distribution of temperature and equivalent plastic strain between the advancing side and retreating side of the weld led to varied distributions of precipitate phases and dislocation density. Specifically, the peak temperature difference in the transverse direction between the advancing side and retreating side ranged from 11.9 to 35.6 °C, with the advancing side being cooler, while the equivalent plastic strain was slightly higher on the advancing side. Microstructural characterization revealed a decreasing trend in the average volume fraction and size of precipitates on the advancing side in the normal direction. In the transverse direction, the volume fraction of precipitates on the advancing side was two to three times higher than that on the retreating side. Additionally, the geometrically necessary dislocation density was greater on the advancing side, ranging between 0.05 × 10 ¹ ⁴ and 0.20 × 10 ¹ ⁴ m⁻² Theoretical calculations of the strengthening mechanisms indicated that the mechanical property asymmetry between the advancing side and retreating side of the friction stir welded joints was primarily due to dislocation and precipitate strengthening. Mechanical property tests confirmed that the tensile strength and microhardness on the advancing side were significantly higher (by 7–19 MPa and 2–5 HV, respectively) compared to the retreating side, aligning with the theoretical calculations. This study affords valuable insights into friction stir welding technology for metallic materials and provides crucial information and theoretical foundations for optimizing welding processes.