A Study On Hot-tearing Sensitivity Of Mg-Gd-(Y) Alloys

Hot tearing is one of the most common and serious defects and remains a major problem of casting technology.According to the investigation of the causes and propagation of hot tearing during alloys' casting process,it is found that the shrinkage of solidification can cause stress and strain caused by the solidification shrithe formation of shrinkage porosity,it may become the core of the hot tearing;The thermal nkage is the direct cause of the hot tearing.Therefore,the Mg-Gd-(Y)alloys are selected as the reference alloys.The paper aims at studying the solidification shrinkage behavior of castings;semi-solid mechanical properties influence on hot tearing formation;as-cast microstructure of samples and surface morphology of fracture.And then come to the following conclusions:(1)The maximum shrinkage rate and the stress concentration factor k were used as the indexes to evaluate the hot tearing tendency,and the results were compared with that of the CSC criterion,it was found that the results of the two methods are basically consistent.The hot tearing tendency of the Mg-Gd-(Y)alloys is the maximum when the Gd content is 2wt.%;When the content of Gd was higher than 2wt.%,the hot tearing tendency decreased with the increase of Gd content.When the content of Gd exceeds 7%,the cold cracking tendency of the alloy appears;The hot tearing tendency of the Mg-Gd-Y alloys increases with the increase of Y content and then decrease.When Y content is 1wt.%,which means the greatest hot cracking tendency in this composition.(2)The results of the study on the mechanical properties of the semisolid status show that the tension strength increases with solid fraction exponentially;The formation rate of tensile strength increases with the increase of the solid fraction;Y and Gd can reduce the formation rate of tensile strength at the final stage of solidification and stress concentration;The displacement of the fracture was changed to U type with the change of the solid fraction.(3)Hot cracking belongs to the intergranular fracture,and the surface of the fracture is seriously oxidized.The morphology of dendritic morphology was observed by SEM.The liquid films are coated on the dendrites.With the increase of Y and Gd,the liquid film becomes thicker and the second phase increases.Due to the thick liquid films around the grain boundary and the separation of dendrites can be feed by remaining liquid,the separated dendrites will be healed and there is no hot cracking formation.The hot tearing formation mechanism of Mg-Gd-Y,is the result of the liquid film theory and solidification feeding theory played.