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Rotary friction welding for medical application

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. 

The objectives were threefold: first to review current joining techniques for this combination; second to review potential interlayer materials and their biocompatibility; and lastly to develop baseline welding parameters for joining Nitinol to stainless using an interlayer. 

Macro of Nitinol (lower) to stainless steel (upper) via an interlayer
Macro of Nitinol (lower) to stainless steel (upper) via an interlayer

The literature search revealed a number of options to be worth exploring.

Two interlayer candidate materials were shortlisted; a precious metal and a refractory metal. An additional refractory material was also examined for its suitability as an interlayer material because it’s known for its weldability by friction to titanium and stainless steel.

Friction welding parameters were applied to three selected combinations and the welds created were assessed by tensile and metallographic testing. 
Conclusions were drawn from what emerged as a successful project: 

  • Most Nitinol to stainless steel joints in manufacture are either mechanical fastenings, with limited design freedom, or performed by fusion welding giving variable performance due to the formation of brittle intermetallics
  • There are only eight basic metals and 160 alloys that are fully biocompatible
  • Continuous drive friction welding can produce high quality joints between Nitinol and 17-4PH stainless steel when an interlayer material is used

From these findings TWI was able to recommend that further work would certainly be worthwhile to assess effects of torque together with interlayer thickness, on corrosion resistance and fatigue performance.

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