Improving mechanical properties of friction stir welded AA2024-T3 joints by using a composite backplate

During friction stir welding (FSW) of AA2024-T3 sheets, composite backing plates (CBP) made out of the combination of pure copper and medium carbon steel were employed aiming at enhancing the joint mechanical properties. Three different CBP designs were used. Microstructure and mechanical properties of the welded joints were examined. Results reveal that the grains under CBP have been noticeably refined compared to those under a monolithic steel backing plate typically used in FSW. In addition, using the CBP effectively prevents the dissolution of Guinier-Preston-Bagaryatskii (GPB) zones in the HAZ. Superior strength-ductility synergy was obtained under CBP-3. Those joints exhibit more high-angle grain boundaries, ultrafine grains of about 980 nm, more GPB zones and optimum hardness distribution, resulting in a tensile strength of 442 MPa and an elongation of as much as 16.2%.