Dissimilar friction stir lap welded joints of AA6061-T6 and Ti6Al4V were successfully welded using suitable parameters, and the effect of tool rotational speed was studied to investigate the influence on mechanical properties, microstructure, and fracture mechanisms. It was observed that tool rotational speed has a great effect on the strength of the welded joint, as it controls the amount of heat input and plasticity at Al/Ti interface as well as the formation of intermetallic compounds (IMCs) and different brittle phases that were formed at the Al/Ti mixture. The maximum shear strength of 3.5?KN was achieved using low rotational speed of 600?rpm and maximum hardness of 384 HV, while minimum shear strength of 2.5?KN was attained at 1000?rpm with minimum hardness of 343.2 HV. Hybrid fracture surface was produced at lower rotation speed which had both brittle and ductile characteristics, while brittle fracture mechanisms were observed at high rotational speed due to the concentration of brittle IMCs layers, TiAl, and TiAl3phases at the interface due to the excessive heat input which causes pores and cracks and thus deteriorating joint strength.
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