Ti-6Al-4V alloy was subjected to friction stir processing at rotation rates of 400, 800 and 1200 rpm using a polycrystalline cubic boron nitride (pcBN) tool and tool wear at different travel distances was investigated. At high rotation rates of 800 and 1200 rpm, the greatest tool wear, including mechanical and chemical wear, occurred at the initial tool plunge point. Detailed microstructural examinations on the tool plunge point at 1200 rpm by transmission electron microscopy indicated that the "onion ring" structure in the stir zone was caused by a variation in the distribution of TiB particles. Two similar but not identical spatial phase sequences around BN particles, BN-TiB2-TiB-alpha-Ti (N) and BN-TiB2-TiB-transformed beta-Ti (N), as well as Ti2N phase were identified. The reaction mechanism between the tool and the Ti matrix was discussed. Moreover, when the tool wear reached a steady-state condition, the effect of tool wear on the microstructure and mechanical properties of the stir zone was evaluated. A fully transformed beta with a Widmanstatten structure was observed at all rotation rates and the average size of prior grains increased with the rotation rate. The tool wear led to an increment in hardness and tensile strength but a loss of ductility of the stir zone. (C) 2014 Elsevier B.V. All rights reserved.