The superplastically formed structural assembly includes first and second structural members having facing surfaces. The first and second structural members can include a first outer structural member, a second outer structural member or at least one intermediate structural member. In one embodiment, the first and second structural members include first and second outer structural members. In another embodiment, the first and second structural members include first and second intermediate structural members. The structural assembly includes at least one friction stir weld joint joining the first and second structural members. In one embodiment, the facing surface of the first structural member is at least partially covered with oxide to prevent thermo-compression welding of the first and second structural members adjacent the at least one friction stir weld joint. In another embodiment, the facing surface of the second structural member is at least partially covered with oxide.