Abstract: The influence of four different overlap (5.0 mm, 5.5 mm, 6.0 mm, 6.5 mm) on the interface migration and the effective nuclear size of LY12 aluminum alloy are investigated by friction stir additive manufacturing on LY12 aluminum alloy. Results show that the interface always migration upward, and the amount of interface migration on the retreating side (RS) is higher than that on the advancing side(AS). When the lap thickness smaller than the pin length 0.5 mm, the effective weld nugget size reached a maximum at 7.32 mm, which is 1.22 times the diameter of the pin.