The effect of heat treatment on the fatigue strength of microknurled Ti-6Al-4V

Microknurling, a high pressure surface indentation technique, was devised as an alternative to traditional heat-bonded porous coatings found on many orthopedic implants designed for fixation by tissue ingrowth. Heat-bonded porous coating can cause at the surface of an implant stress concentrations that reduce fatigue strength. However, microknurling may reduce stress intensification without eliminating it. Thus the purpose of this work was to explore surface thermal/mechanical processing of Ti-6Al-4V to improve the fatigue strength of microknurled specimens via the production of a Ti-6Al-4V dual microstructure. The latter consists of a surface layer of equiaxed grains known to be effective against crack initiation and a bulk microstructure of lamellar grains that possesses optimum fatigue crack propagation resistance. Rotating-bending fatigue tests showed that such a microstructure had some benefits, but this was offset by the reduction in compressive strains imparted to the surface by the heat treatments needed to obtain this microstructure.