A method of manufacturing a multi-alloy aerospace component, wherein the method comprises the following steps: arranging a first metal workpiece against a second metal workpiece, the first metal workpiece being of a first metal alloy and the second metal workpiece being of a second metal alloy, the second metal alloy being different from the first metal alloy, friction stir welding in a first direction from a first side of the first and second workpieces (2, 4) to form a first weld, friction stir welding in a second direction from a second side of the first and second workpieces (2, 4) to form a second weld, characterised in that the first weld extends partially into the workpieces (2, 4) to a first depth and joins partially the first and second workpieces (2, 4) and the second weld extends partially into the workpieces (2, 4) to a second depth and joins partially the first and second workpieces (2, 4), the second side being on the opposite side of the workpieces (2, 4) to the first side, the deepest extent of the first weld being separated from the deepest extent of the second weld by a central region of material that extends all the way between the deepest extent of the first weld and the deepest extent of the second weld, the central region of material including both material of the first metal workpiece and material of the second metal workpiece, and the method comprises the further step of machining away material from the workpieces (2, 4), the machining including machining away material from the central region of material to form a void being open at opposite ends in a direction parallel to the first direction and to convert the first weld and the second weld either side of the void into full penetration welds. arranging a first metal workpiece against a second metal workpiece, the first metal workpiece being of a first metal alloy and the second metal workpiece being of a second metal alloy, the second metal alloy being different from the first metal alloy, friction stir welding in a first direction from a first side of the first and second workpieces (2, 4) to form a first weld, friction stir welding in a second direction from a second side of the first and second workpieces (2, 4) to form a second weld, characterised in that the first weld extends partially into the workpieces (2, 4) to a first depth and joins partially the first and second workpieces (2, 4) and the second weld extends partially into the workpieces (2, 4) to a second depth and joins partially the first and second workpieces (2, 4), the second side being on the opposite side of the workpieces (2, 4) to the first side, the deepest extent of the first weld being separated from the deepest extent of the second weld by a central region of material that extends all the way between the deepest extent of the first weld and the deepest extent of the second weld, the central region of material including both material of the first metal workpiece and material of the second metal workpiece, and the method comprises the further step of machining away material from the workpieces (2, 4), the machining including machining away material from the central region of material to form a void being open at opposite ends in a direction parallel to the first direction and to convert the first weld and the second weld either side of the void into full penetration welds.