In dry storage, spent fuel is typically stored in casks constructed from neutron absorbing materials (NAMs). The (B4C+Al2O3)/Al composite, which incorporates in-situ amorphous Al2O3 (am-Al2O3) formed on fine aluminum powder as a reinforcing phase, can serve as an integrated structural and functional NAM for dry storage applications. Welding is crucial in the fabrication of these casks. In this study, friction stir welding was performed on (B4C+Al2O3)/Al composite sheets at a welding speed of 50 mm/min and rotation rates ranging from 500 to 1000 r/min. The microstructure of the weld joints was analyzed, and the intrinsic relationship between fracture behavior and microstructure was elucidated. Results showed that defect-free joints were achieved at rotation rates of 500 and 750 r/min, while tunnel defects were observed at 1000 r/min. The ultimate tensile strength of the joint welded at 500 r/min was 205.7 MPa, with a strength efficiency of 82%. Microstructural analysis revealed that the grains within the nugget zones (NZs) coarsened and the Al2O3 network was disrupted due to the welding thermo-mechanical effect, resulting in softening within the NZs. Fracture locations for all three joints were consistently observed at the NZ boundary on the advancing side (AS). Finite element simulations confirmed that cracks propagated along the NZ boundary on the AS, where stress concentration occurred during tensile testing.