Microstructure and High-Temperature Deformation Behavior of Dissimilar Superalloy Welded Joint of DD407/IN718

Welding is an important joining method to fabricate the dissimilar welding integral blisk structure of single crystal and polycrystalline superalloy. The microstructure and properties of the welded joint are the key factors to determine the reliability of the integral blisk structure of dissimilar superalloys. The single crystal superalloy of DD407 and polycrystalline superalloy of IN718 were butt welded by continuous fiber laser system. The evolution of microstructure and composition segregation of the welded joints fabricated under the optimized welding parameters as-welded (AW) and after post weld heat treatment (PWHT) were investigated. The high temperature tensile deformation behavior of the welded joint after PWHT was also examined. The results show that the microstructures of fusion zone (FZ) in the welded joint consist of planar crystal, cellular crystal, columnar crystal and equiax crystal. The difference of the dendrite microstructures between the two sides of the weld centerline is very obvious. In terms of the joint as-welded, the microhardness of the FZ is low and there exists obvious micro-segregation. After PWHT, the micro-segregation has been improved and the microhardness increases significantly in the FZ which is much more than those of both base metals (BMs) of DD407 and IN718 alloys. There exists local hardening zone in the heat-affected zone (HAZ) of DD407 single crystal alloy and narrow softening zone and grain boundary liquation phenomenon in the HAZ of IN718 polycrystalline alloy. The ultimate tensile strength and elongation of the welded joint after tensile test at 650 degrees C are 1111 MPa and 9.42%, respectively. And the tensile specimen of the welded joint fails in the BM of IN718 polycrystalline alloy. The main deformation mode of the laser welded joint at high temperature includes the multi-slips of dislocation in the BM and FZ of single crystal alloy, and the dislocation slip and grain-boundary sliding in the BM of poly-crystalline alloy. The tensile fracture surface is characterized by multi-source cracking, and the dimple and crystal sugar shaped facture surface exist simultaneously in the crack source area, which is a mixed fracture of microvoid aggregation and intergranular fracture. So the tensile fracture mechanism contains micro-void accumulation fracture and inter-granular fracture. The grain boundary liquefaction in the HAZ of IN718 polycrystalline does not affect the short-time high temperature mechanical properties of the welded joints.