Interfacial microstructure and mechanical properties of Fe-36Ni invar alloy GTAW joint

A12 mm thickness invar alloy plate was multi-pass welded using commercial welding wire. Microstructures of the joint were examined by optical microscope (OM) and scanning electron microscope (SEM). The mechanical properties and microhardness of the joint were evaluated. Semi-elliptical band structures were formed in the weld zone, which was related to the interrupted cooling during weld pool solidification process. Ductility dip crack (DDC) was discovered at the grain boundaries of the joint and heated affected zone (HAZ), which is related to sulfur segregation at the grain boundary during the reheating process of multi-pass welding. The tensile strength and elongation of the joint reached 95.8% and 46.4% of the base material, respectively. The fracture position was found on the weld seam.