The friction stir welding test was performed on the 2219C10S aluminum alloy.It was found that there was abnormal aggregation of aggregates in the weld.The aggregated phase was shown as bright white aggregates on X-ray inspection.The main forms were divided into clumps,lines and fingers,and the mechanical properties of the welds in the aggregate phase were basically the same as those of the welds without the aggregate phase.The distribution position of each form of aggregated phase is different depending on the area affected by the welding of the mixing head.The agglomerate aggregate phase mainly existed at the center of the weld or the upper surface of the weld,and the linear and finger-like aggregates mainly existed in the weld center or the position of the weld root.The energy spectrum analysis of the abnormal aggregate phase shows that the main constituent elements of the aggregate phase are Al and Cu,and the atomic percentage is approximately 2∶1.It is determined that the main component of the aggregate phase is Al₂Cu.Analyzing the causes of the aggregate phase,the generation of aggregate phase is mainly affected by the welding heat input and the mechanical agitation of the mixing head.During the welding process,Al₂Cu in the plastic softened metal shows aggregation behavior in the process of filling the cavity behind the stirring needle,solidification with metal,aggregation behavior begins to slow down and ends.