A semi-finished piece of steel-titanium fabricated using the wire + arc additive manufacturing process (WAAM). Optical interference patterns as a result of surface oxide layers produce the iridescent colours. In use the metal would be finished by conventional machining to a smooth surface, as seen on the right of the picture. WAAM deposits layers of metal from a wire in a technique derived from welding. Items produced by this method are subsequently machined to a conventional finish but the time taken to manufacture such pieces from steel, aluminium, titanium, copper and other metals, alloys and composite materials is significantly shorter than using conventional methods, with considerable cost savings. The concept was first described in 1926 but developments in robotic control are creating interest in its possible importance in the manufacture of, for instance, aircraft parts which are conventionally machined from solid blocks of metal.