The role of process temperature and rotational speed in the microstructure evolution of Ti-6Al-4V friction surfacing coatings

Abstract

Friction surfacing is a solid state technique for depositing metallic coatings. Coating materials are thermo-mechanically processed at high temperatures during deposition. The high degree of deformation involved leads to a dynamically recrystallised fine grained microstructure. For Ti-6Al-4V, the microstructure and mechanical properties of coatings generated by friction surfacing have not been studied yet. The current work focuses on investigating effects of rotational speed on microstructure, grain size evolution and mechanical properties of the coating material. Various rotational speeds in a wide range, exceeding the range of deformation used in many other severe plastic deformation processes, were used to generate Ti-6Al-4V coatings by friction surfacing. Their influence on the thermal cycle and consequently on microstructure formation was revealed. The β grain size is related to the rotational speed and thermal cycle. Grain refinement at low rotational speed was observed, while higher rotational speeds and corresponding increase in maximum temperature led to grain coarsening. Although the peak temperature dominates the grain size evolution, dynamic recrystallisation during friction surfacing counteracts this effect, reducing the grain size by one order of magnitude. The coatings exhibit a hardness ascent about 15% due to martensite formation, high dislocation density and oxide precipitations.