Effects of cooling rates on microporosity in DC casting Al-Li alloy

During the direct chill (DC) casting process, primary cooling from the mold and bottom block, and secondary cooling from the waterjets produce a concave solid shell. The depth of this liquid pocket and mushy zone not only depends on the solidification range of the alloy but also the boundary conditions such as cooling rates. Al-Li alloys solidify in a long solidification range increasing the susceptibility of porosity nucleation in the semi-solid region. In this study, the effects of cooling rate on the porosity formation were quantified for the large ingot casting using X-ray computed tomography (XCT). By characterizing pore size distributions at four different cooling conditions, the correlation between the mechanical properties at both room and high temperatures and the microstructure features was identified. The constitutive equations were constructed. It is found that increasing the cooling rate reduces the grain size, increases the number density of micropores, and minimizes the number of large pores, thereby improving the mechanical performance. Therefore, long mushy zones and deep liquid pockets in Al-Li alloys can be effectively controlled by controlling the boundary conditions of the DC casting solidification process, thereby obtaining castings with excellent mechanical properties.