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

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. •Abnormal grain coarsening occurs in post-weld solution treatment of repaired joint.•A two-step progressive solution treatment was adopted to avoid AGC.•The precipitates in the aging process of the FZ were photographed by TEM.•Heat treatment and failure procedure of different joints is proposed by a physical model.•A desirable combination of strength and ductility is obtained with 241 MPa in YS, 323 MPa in UTS, and 4.0% in EL.