Research on the post-weld heat treatment of TIG repair welded joint of sand-cast Mg-Y-RE-Zr alloy

摘要

The heat affected zone (HAZ) usually leads to the softening of the tungsten inert gas (TIG) welded joint due to the formation of network-shaped eutectics under its high heat input. The current study systematically discussed the microstructural evolution of TIG repair welded joints of the sand-cast Mg-Y-RE-Zr alloy during various progressive solution (T4) treatments. Abnormal grain coarsening (AGC) happened after the conventional T4 treatment at 520 °C for 8 h because of the high welding residual stress in the fusion zone (FZ), leading to extremely low elongation of 1.8% of the repaired joint. To avoid AGC, a two-step progressive T4 treatment was proposed without changing the total T4 treatment time. Neither abnormally coarsened grains nor residual eutectics existed by the progressive T4 treatment of 480 °C × 6 h + 520 °C × 2 h. Excellent comprehensive tensile properties were obtained (YS: 241 MPa, UTS: 323 MPa, and EL: 4.0%) after the aging (T6) treatment. DSC and TEM analyses confirmed that the proposed progressive T4 treatment with a lower treatment time at 520 °C can effectively promote the dissolution of the eutectics. The aging hardening was attributed to the precipitation of the evenly dispersed β' phase in the matrix.