A low-cost Co-based alloy was employed to fabricate friction stir welding (FSW) tools to weld TA5 alloy under different welding speeds. The tool wear characteristics, tool degradation mechanisms and effects of tool wear behavior on the microstructure and mechanical properties of joints were investigated in detail. Results reveal that the heat input near weld root decreases with the increase of welding speed and the tool wears more extensively under lower heat input when the deterioration rates of tool sizes are considered. Furthermore, the tool wears out mainly by mechanical abrasion near pin tip, leading to a large number of tool particles in mixed regions accompanied by severe material loss. The tool wears less severely near pin root and shoulder, since the adhesion layer caused by adhesive and diffusional wear mechanisms is the main cross-sectional wear morphology, and the interlayer rich in W and Cr between adhesion layer and intact part of tool may protect the tool from serious diffusional wear. The foreign β-stable tool compositions were introduced to the weld, reducing the transus temperature. Therefore, transformed acicular α and retained β phases were detected in contaminated zone (CZ) inside stir zone (SZ) compared with the untransformed equiaxed α grains in non-contaminated zone (NCZ). In addition, the acicular α and β phases result in much higher strength and microhardness than base material (BM). •Friction stir welding (FSW) tools fabricated from Co-based alloy were employed to weld TA5 alloy.•Tool wear characteristics and mechanisms were investigated in detail.•Foreign β-stable tool compositions in weld resulted in phase transformation.•Transformed microstructures in contaminated zone bear much higher strength and hardness than base material.