The aim of the present research was to study the effect of friction stir welding parameters such as ( tool rotation and tool transverse speeds) on the joint efficiency of
(6.1 mm) thick weldments of aluminum alloys (7075-T651,2024-T351) , which are used in aircraft industries andare poorly weldable by fusion welding such as (TIG,MIG and Laser) welding .Friction stir welding welds were carried out on a traditional milling machine .Tow tool rotation speeds (900 and 1120 rpm) and three travelspeeds (28,40 and 56 mm/min) were selected for the friction stir welding of each alloy. Thestrength of the weldments has been investigated using the tensile test with standard specimens at room temperature.It was observed that the strength and joint efficiency of the weldments decreased with increasing the tool rotation speed and decreasing travel speed with improvement in ductility.The best weldments joint efficiency of the (2024-T351) alloy is (86.41%) and for (7075-T651) is (73.5%) these were obtained at tool rotation speed (900 rpm) and travel speed (56 mm/min).