TWI undertook a project to join a difficult alloy by rotary friction welding using an interlayer. The material was Nitinol, (Nickel Titanium Naval Ordnance Laboratory) an alloy of nickel and titanium, and sometimes known as ‘memory metal’.
Nitinol alloys exhibit two closely related and unique properties: shape memory and superelasticity or pseudo-elasticity. Shape memory refers to the ability of nitinol to undergo deformation at one temperature, then recover its original, undeformed shape upon heating above its transformation temperature. Superelasticity occurs at a narrow temperature range just above its transformation temperature. In this case, no heating is necessary to cause the undeformed shape to recover, and the material exhibits enormous elasticity, some 10-30 times that of ordinary metal.
Rotary friction welding is a solid phase process. No melting takes place, so this process might provide the ideal solution to minimising reduction of desirable properties and any joining difficulties previously experienced using a fusion process.