a plurality of workpieces are assembled into a stack . two of the workpieces are machined on one surface at predetermined positions from the edges of the workpieces . the workpieces are stacked so that the machined surfaces of the workpieces face away from each other . two workpieces are provided with a stop off material on one of their surfaces to prevent diffusion bonding at predetermined positions . the edges of the workpieces are welded together and a pipe is welded to the stack to interconnect with the stop off material to form a sealed assembly . the workpieces are diffusion bonded together . the integral structure formed is heated and opposite ends of the integral structure are twisted relatively to contour the integral structure to a predetermined shape . the integral structure is heated and the machined portions on the workpieces are isothermally forged and then the twisted integral structure is internally pressurised to superplastically form one workpiece to produce an article , eg a fan blade .