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The pure Zn foils with different thicknesses (0.02,0.05,0.1,0.2 and 0.3 mm) were selected as interlayers to improve the quality of friction stir lap welding joint of 7075-T6 Al and AZ31B Mg dissimilar alloys.The effects of the interlayer thickness on joint formation,microstructure and tensile strength were analyzed.The results displayed that the maximum length of the boundary between stir zone (SZ) and thermo-mechanically affected zone in lower plate was obtained by the addition of the Zn interlayer with 0.05 mm thickness.The Mg-Zn intermetallic compounds (IMCs) were discontinuously distributed in the SZ,replacing the continuous Al-Mg IMCs.The size of Mg-Zn IMCs increased with the increase in the thickness of the Zn interlayer.The maximum tensile shear strength of 276 N mm-1 was obtained by the addition of 0.05 mm Zn foil,which increased by 45.6％ of that of the joint without the Zn foil addition.