In the current investigation, AA1050 aluminum rolled sheets were welded using the friction stir welding technique (FSW). The effect of the processing parameters on the welding quality and grain refinement is discussed; hence, four rotation and traverse speeds are used to evaluate the welding characteristics. The heat generated during the welding process has a major effect on the resultant welded joint as well as the mechanical properties and materials quality. The microstructure examination using polarized optical microscopy was employed to inspect the grain size and refinement that occur during the welding process. The microstructure analysis revealed that ultra-fine grains were formed for all investigated samples, but the lower rotational speed significantly improved the grains' reduction by about 63.4% compared to the base alloy. In contrast, the welded stirred zone reduced the microhardness with respect to the base metal by about 24.5 % due to the higher heat generated between the FSW tool and the base welded sheets. The optimum grain size was achieved at 14 mm/min welding speed and 450 rpm tool rotation speed. The prediction model calculates the maximum temperature generated inside the stirred zone; hence as this value is close to (80-85)% from the melting point, the resultant weld quality improved.