Friction welding of 6061 aluminum alloy similar material was carried out in order to examine the relationship between diameter of base material and heat input. By using the base materials having 10, 12, 14 and 16 mm diameters respectively, experiments were carried out by the brake type friction welding machine under various friction welding conditions. The joint strength was evaluated by tensile test, while the unit deformation heat input in the upset stage was adopted as the heat input. The relationships between heat input and joint strength, and heat input and upset loss were examined respectively. The results obtained indicated that the unit deformation heat input in the upset stage is more related with the joint strength of the welded joints produced with all diameters adapted, and the minimum-limit unit deformation heat input for perfect joint increases with an increase of diameter of the base material, while, the minimum-limit unit deformation heat input per area for perfect joint of base material fracture is almost constant 1.6 × 106 J/m2 s. Moreover it was recognized that the evaluation of the joint strength is possible by the upset loss, therefore, the minimum-limit upset loss for perfect joint is about 5 mm.
