Relationship between solidification path, microstructure evolution and solidification cracking behavior of Mg-Al-Ca alloy during TIG welding

Abstract

The high-strength and creep-resistant Mg-Al-Ca-Mn alloys have broad application prospects. However, solidification cracking occurs in these alloys in certain conditions and the origin is still unclear. This work investigated the relationship between the solidification path, microstructure evolution and solidification cracking behavior of the Mg-xAl-2Ca-Mn alloys during tungsten inert gas (TIG) welding. Results show that when the fusion zone's Ca/Al mass ratio ranges from 0.4 to 1.64, solidification cracking occurs at a Ca/Al mass ratio of ∼0.7. As the Ca/Al mass ratio approaches this value, the grain size increases, and the Laves phases are reduced gradually. The early formed Laves phases play an important role in promoting dendrite segmentation, refining grain size and enhancing grain boundaries. When a solidification path delays the formation of Laves phases, the Laves phases will be reduced accompanied by grain coarsening. In such a solidifying microstructure, intergranular cavitation is easy to occur, and the resistance of the semi-solid alloy to crack propagation is severely reduced.