Ultrasonic assisted second phase transformations under severe plastic deformation in friction stir welding of AA2024

The study explores the effect of ultrasonic vibrations on the dissolution and precipitation of secondary phases in aluminum alloy 2024 under the action of severe plastic deformation in friction stir welding. A method for strain estimation in friction stir welding is proposed to assess the contribution of deformation to microstructural changes. The acoustoplastic effect is estimated which causes an increase in the degree of deformation. It is shown that ultrasonic vibrations give rise to two opposite effects that occur successively in the process of welding: enhancement of the strain-induced dissolution of second phase particles caused by the acoustoplastic effect, and enhancement of their repeated precipitation during ultrasonic aging. One or the other effect will dominate depending on the thermal and deformation conditions determined by the technological parameters of the welding process.