A method for forming a metallic nacelle inlet structure for a turbine engine aircraft propulsion system with an axis, the nacelle inlet structure comprising an inlet lip portion and an outer barrel portion, the method comprising: forming the inlet lip portion; forming the outer barrel portion; arranging the inlet lip portion axially adjacent the outer barrel portion; friction stir welding the inlet lip portion to the outer barrel portion to provide a circumferentially extending weld joint between the inlet lip portion and the outer barrel portion; and perforating the outer barrel portion to provide an array of perforations; disposing a plenum structure radially within the outer barrel portion; and friction stir welding the plenum structure to the outer barrel portion, wherein the plenum structure forms a plenum with at least the outer barrel portion, and the plenum is fluidly coupled with the array of perforations; and wherein: a weld joint between the plenum structure and the outer barrel portion is formed by the friction stir welding of the plenum structure to the outer barrel portion, and the weld joint extends circumferentially between the plenum structure and the outer barrel; or a plurality of weld joints between the plenum structure and the outer barrel portion are formed by the friction stir spot welding of the plenum structure to the outer barrel portion; or the plenum structure is friction stir welded to the outer barrel portion such that a weld joint between the plenum structure and the outer barrel portion and the weld joint between the inlet lip portion and the outer barrel portion are concurrently formed and part of a common weld joint. forming the inlet lip portion; forming the outer barrel portion; arranging the inlet lip portion axially adjacent the outer barrel portion; friction stir welding the inlet lip portion to the outer barrel portion to provide a circumferentially extending weld joint between the inlet lip portion and the outer barrel portion; and perforating the outer barrel portion to provide an array of perforations; disposing a plenum structure radially within the outer barrel portion; and friction stir welding the plenum structure to the outer barrel portion, wherein the plenum structure forms a plenum with at least the outer barrel portion, and the plenum is fluidly coupled with the array of perforations; and wherein: a weld joint between the plenum structure and the outer barrel portion is formed by the friction stir welding of the plenum structure to the outer barrel portion, and the weld joint extends circumferentially between the plenum structure and the outer barrel; or a plurality of weld joints between the plenum structure and the outer barrel portion are formed by the friction stir spot welding of the plenum structure to the outer barrel portion; or the plenum structure is friction stir welded to the outer barrel portion such that a weld joint between the plenum structure and the outer barrel portion and the weld joint between the inlet lip portion and the outer barrel portion are concurrently formed and part of a common weld joint. a weld joint between the plenum structure and the outer barrel portion is formed by the friction stir welding of the plenum structure to the outer barrel portion, and the weld joint extends circumferentially between the plenum structure and the outer barrel; or a plurality of weld joints between the plenum structure and the outer barrel portion are formed by the friction stir spot welding of the plenum structure to the outer barrel portion; or the plenum structure is friction stir welded to the outer barrel portion such that a weld joint between the plenum structure and the outer barrel portion and the weld joint between the inlet lip portion and the outer barrel portion are concurrently formed and part of a common weld joint.