•Defect-free friction-stir processed Ti–6Al–4V alloy surface layer was performed.•Thermal cycles under different processing conditions were obtained and analyzed.•Microstructure evolution laws basing on thermal cycle features were demonstrated.•Ultra-refined acicular-α and Martensite-α′ were obtainable by fast cooling rate.•Final fine β-regions with ultra-refined α and Martensite-α′ favored wear property.The friction-stir processing (FSP) method was employed to the surface modification of Ti–6Al–4V alloy applying different processing parameters. The defect-free friction-stir processed surface layers with ~ 2.5 mm thickness, with final microstructures of β-regions with acicular-α and GB-α, or Martensite-α′ phase, were obtainable. The FSP processing thermal cycles, macro/micro-structures, phase evolutions, microhardness measurements, and dry-sliding wear performance of the processed surface layers were investigated. Considering to the basic laws of Ti–6Al–4V microstructure evolutions during the FSP procedures, the relations between processing parameters and produced microstructure characteristics were further elucidated. It was found that a relatively lower processing peak temperature and a fast enough cooling rate benefited the formation of the fine β-regions with ultra-refined acicular-α phase and/or even needle-typed Martensite-α′. The microstructures favored the improvement of hardness and wear performance of the processed surface layers, in comparison with those properties of the base material.