Microstructure evolution and mechanical properties of friction stir welded joint of ultrafine-grained dissimilar AA2024/AA5083 composite fabricated by accumulative roll bonding

In this investigation, the dissimilar ultrafine-grained (UFG) composite of AA2024 and AA5083 was made using four cycles of accumulative roll bonding (ARB). The composite was then welded by the friction stir welding (FSW) method. Using the Taguchi technique, welding parameters of tool rotational speed, welding speed, pin geometry, and tool tilt angle were optimized. Optical microscopy, Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM) were used to study microstructural evolution in the ARB processed specimen, as well as in the stir zone (SZ), thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ). Taguchi optimization showed that a Square pin, with a rotational speed of 655 rpm, welding speed of 110 mm/min, and tool tilt angle of 3 degrees were the optimum welding process parameters. The microstructural assessment showed that the SZ consisted of a recrystallized microstructure with an average grain size of about 900 nm. Tensile strength was reduced from 667 MPa in the ARB-processed sample to 403 MPa in the optimized FSW condition. However, yield strength was slightly reduced due to the ultrafine-grains of the weldment. The relatively high strength of the composite was due to solid solution, grain boundaries strengthening, and nano-sized Al20Cu2Mn3 precipitates (T-phase) formation. The presence of precipitates led to a proper level of ductility of up to two times in the FSW structure compared to the UFG structure. The highest mean hardness of the weldment, similar to 132 HV, was obtained which was due to the grain refinement, dislocation generated by the pin movement, and small-size precipitates.