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 degrees 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 degrees C x 6 h + 520 degrees C x 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 degrees C can effectively promote the dissolution of the eutectics. The aging hardening was attributed to the precipitation of the evenly dispersed beta ' phase in the matrix.

Automated summary

This study investigated the microstructural evolution of TIG repair welded joints and the effect of progressive solution treatments on grain coarsening. By proposing a two-step progressive T4 treatment method, abnormal grain coarsening was avoided while maintaining excellent comprehensive tensile properties. The progressive T4 treatment at lower temperatures effectively promoted the dissolution of eutectics and contributed to the aging hardening process by precipitating the evenly dispersed beta ' phase in the matrix.