Friction stir processing (FSP) can be employed to modify the grain size and microstructure of a material. In titanium alloys, the refined microstructure achieved during processing can improve the mechanical properties, such as yield stress and fatigue crack initiation resistance. Documenting the microstructural evolution of Ti-5111 (5Al-1Sn-1Zr-1V-0.8Mo) during FSP, as well as simulating the observed microstructure in a Gleeble® 3800 thermo-mechanical simulator can determine the link between strain, strain rate and temperature during processing.
In this study, FSP of Ti-5111 was performed above and below the beta transus temperature allowing for investigation of the microstructural evolution in both conditions. Each processed panel was instrumented with thermocouples to record the thermal histories in the stir zone and adjacent heat-affected zone. Single sensor differential thermal analysis (SS-DTA) was used to determine the alpha-beta transformation during processing. Transverse sections of the processed panels were analyzed using optical and scanning electron microscopy, electron backscatter diffraction (EBSD) and hardness mapping.
FSP produced extreme grain refinement in both processing conditions – reducing the 200-500 micron prior-beta base material grains to 1-20 microns. The stir zone in the panel processed above the transus exhibited a strong transformation microtexture, governed by the Burgers orientation relationship, while the sub-transus panel displayed a shear deformation texture. Vicker’s hardness mapping revealed two distinct hardness regions: the base material and a more uniform and slightly harder stir zone.
The microstructures observed in the FSP panels were simulated using hot torsion testing on a Gleeble® 3800. Ideally, the strain and strain rate data may be used to verify FSP modeling programs of titanium to reduce the parameter selection phases of future friction stir projects. However, strain localization observed during hot torsion testing necessitates a different sample design for titanium alloys to take into consideration the ease of adiabatic shear band formation and the low thermal conductivity.
A continuous cooling transformation (CCT) diagram was constructed for Ti-5111 to separate the effects of deformation and non-equilibrium temperature profiles on the depression of the alpha-beta transformation observed during processing. Similar transformation temperatures were observed during testing, indicating the significant reduction in transformation temperature is independent of the extreme deformation.