Abstract: In a method of manufacturing a hollow component metallic sheets are diffusion bonded together except where a stop off pattern is applied to the surfaces of the sheets. A gas channel is accommodated in the thickness of the sheets for the entry of pressurised gas under superplastic forming conditions to produce the hollow component. The material defining the gas channel is locally deformed by forging to reduce the height of the gas channel in order that a subsequent fusion welding pass over the deformed area defining the gas channel hermetically seals the gas channel.