High-speed friction stir butt welding of 25.4 mm thick 7175-T79 aluminum alloy

•First experimental demonstration of high-speed (>500 mm/min) friction stir welding of thick aluminum plates (>10 mm).•High-speed double-sided FSW possesses yield strength improved by 20–24% over slow- to medium-speed one-sided FSW for a 25 mm thick AA7175 plate.•The joint efficiency of as-welded, double-sided high-speed FSW was about 76%.•Significant grain refinement (grain size 1.3 ± 0.8 μm) was evident at the junction of double-sided friction stir welding (FSW).