Friction stir lap welding (FSLW) experiment has been conducted on 3mm thick AZ80 extruded sheet and
2.3mm thick AZ31B rolled sheet. The influence of the shape of the taper thread mixing head on microstructure has been
discussed. Pores forming, hooked defects, interface migration, tissue distribution have been analyzed. The organization of
both sides of nugget zone (NZ) is arranged diagonally above, due to the corresponding resultant force along the thread of
pin. NZ was collided to form a cascade "onion ring" by plastic microstructure rotating with thread of the pin and dropping
from concave of mixing head conical body. Cavity defect was concerned with the poor deformation of AZ80 magnesium
alloy placed underneath and relevant organization collision.