Effect of reaction time and sludge formation on the thickness of die soldering reaction layer has been studied in Al-9Si-0.3Mg casting alloy. Ternary ${alpha}_{bcc}-Al_8Fe_2Si$ and ${alpha}_{hcp}-Al_8Fe_2Si$ intermetallic compounds formed at the interface of SKD61 tool steel by interaction diffusion of Al, Fe and Si atoms after 0.5hr and 6hr immersion time, respectively. Binary ${eta}-Fe_2Al_5$ additionally formed at the interface of SKD61 tool steel after 10hr immersion time. Thickness of soldering reaction layer in die surface increased as immersion time increased from 0.5hr to 24hr. Sludge formation was ascertained in the samples which were immersed in the melts more than 10hr. Reaction of die soldering after sludge formation was more accelerated than that of before sludge formation due to a decrease in Fe content, followed by higher diffusion rate of Al in the melt by sludge formation.