In situ synchrotron X-radiography was carried out on Al-13Cu and Al-13Cu-7Si alloys with and without addition of Al-5Ti-1B master alloy.The effects of Si content on grain growth and the solute-suppressed nucleation zone (SSNZ) were quantitatively studied.The average grain size can be refined to 164 μm of Al-13Cu alloy inoculated by Al-5Ti-1B.After addition of 7％ Si content,a large number of TiB2 sites in the melt lost the inoculating ability due to “Si poisoning.” The radius of the dendrite tip curvature of Al-13Cu is more than two times as large as that of Al-13Cu-7Si.The sharper tip is able to disperse the solute more effectively and thus grains can grow more rapidly.This causes that the dendrite arm growth rate with addition of 7％ Si solute is about 2.6 times that of no Si addition.In addition,the solute enrichment in the SSNZ of Al-13Cu is faster,and the area change of SSNZ is slower than that of Al-13Cu-7Si.Thus more nucleation sites in the SSNZ cannot be activated with addition of Si.This study will shed light on the understanding of Si poisoning in casting Al-Cu-Si alloys.