For different tool plunge downforces, the optimum FSW conditions of aluminum die casting

alloy were examined. The higher the tool plunge downforce is, the wider the range of the

optimum FSW conditions is. The following three different types of defects are formed,

depending on the FSW conditions.  A large mass of flash due to the excess heat input;

cavity or groove-like defects caused by insufficient heat input; and cavity caused by the

abnormal stirring. As for the abnormal stirring, it is clearly seen that the shape of the

top part on the advancing side in the stir zone is completely different. For this type of

defect, the effect of the tool plunge downforce is small, though the size of the defect due

to insufficient heat input significantly decreases with the increasing downforce.