Abstract:

Hot tearing is a common and severe defect occurring during solidification of castings. The rational understanding of hot tearing formation mechanism is beneficial to the foundry process design. In the present research, a new developed instrumented “CRC” equipment was applied in characterization of hot tearing in sand cast Mg-5 wt.% Y-4 wt.% RE (WE54) alloy with and without Zr addition. Microstructure observation and thermal analysis were carried out to help analyzing the results. The results showed that hot tearing onset occurs at a relatively low solid fraction (f_s) in WE54 alloy sand castings, which indicates the participation of remaining liquid during hot tearing formation. Microstructure observation of the hot tearing surface also proves the liquid film existence between solidifying dendrites. The contraction strain caused by casting solidification induces the flowing of remaining liquid between solidifying dendrites and results in formation of interdendritic liquid films. These liquid films are separated by sufficient contraction stress and form hot cracks. The addition of Zr in WE54 alloy significantly refines the alloy microstructure and increases the solid fraction at hot tearing onset, both of which result in increasing of the fracture stress of interdendritic liquid film. Thus the hot tearing susceptibility of WE54 alloy is weakened by Zr addition.

Key words: Mg-5 wt.% Y-4 wt.% RE (WE54) magnesium alloy, Sand cast, Hot tearing defect, Instrumented “CRC”