Abstract: Aluminum alloy forgings were found to have linear defects during fluorescence inspection after processing. In order to find out the failure mode and cause, micro observation, microscopic structure examination, hardness testing, energy spectrum analysis, and chemical composition analysis were carried out. The results show that the linear defects are not cracks, but segregation defects of intermetallic compounds, which are metallurgical defects in raw material. Because of improper casting temperature and rate during the smelting process, element concentration and uneven composition occurred. Then linear defects formed during forging. According to the results above, the mechanism, harm and prevention measures of segregation of intermetallic compounds were discussed in detail.