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Friction stir brazing with ultrahigh rotation speed was applied to 6061 aluminium alloy-pure copper lap joints with the aid of zinc foil.The effects of different shoulder diameters from 7 to 15 mm on the microstructure and mechanical properties of Al/Cu FSB joints were investigated along with the temperature and resistance of the friction tool.The oscillation of forward resistance and lateral force was related to the flow of the plastic metal and contributed to obtain a good appearance during the welding process.From the appearance of the welded joints,it was obvious that the phase difference between the forward resistance and lateral force had a significant influence on the joint characteristics.Obvious scale-like ripples appeared on the weld area when a sharp angle in the phase difference curve existed.Additionally,with a lower axial force and oscillation assistance,a satisfactory joint could be obtained.The results of the shear strength of the brazed joint showed that the shoulder with a 12 mm diameter yielded the highest shear strength.Meanwhile,the zinc foil in the middle melted completely and formed finely dispersed CuZn5 Al-Zn eutectic structures at the Al-Cu interface.