Commercial pure Ti (CP-Ti) plates with 2 mm in thickness were successfully joined by friction stir welding (FSW) associated with simultaneous backward cooling. The measured processing temperatures were strictly controlled below the α/β phase transformation point. The microstructural characterisation showed that the stir zone has an ultra-refined grain structure with abundant twin boundaries by decreasing processing temperature. The geometric dynamic recrystallisation and twinning-induced dynamic recrystallisation were the main mechanisms of the grain refinement process. The resulting tensile properties of the stir zone illustrate that both the strength and ductility enhanced due to the appearance of abundant twin boundaries. This work also provides an effective strategy to enhance the strength of the stir zone of FSW CP-Ti joint without ductility loss.
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