A method of joining plates of material to form a predetermined structure, the method comprising: assembling a first plate (40) of a material comprising at least one recess (100) against a second plate (80) of material such that the at least one recess (100) of the first plate (40) is in an exterior surface of the first plate (40) on a side of the first plate opposite to the side which faces the second plate (80); bonding a portion (120) of the first and second plates (40, 80) to one another; superplastically forming the bonded plates, wherein the superplastic forming causes the material of the second plate (80) which is opposite to the at least one recess (100) of the first plate (40) to be driven towards the recess (100), and wherein the unbonded portion of the second plate (80) is deflected away from the plate to form the predetermined structure.