Microstructural Analysis of Linear Friction-Welded 718 Plus Superalloy

Abstract

The microstructure of Allvac 718 Plus (718 Plus; ATI, Pittsburgh, PA, USA) superalloy was examined after linear friction welding (LFW) and after standard postweld heat treatment (PWHT). The liquid phase reaction of second-phase precipitates, which are known to constitutionally liquate during conventional fusion welding, was observed in the thermomechanically affected zone (TMAZ) of the welded material. These phases included MC-type carbides, Ti-rich carbonitrides, and δ phase precipitates. This observation is contrary to the general assumption that LFW is a completely solid-state joining process. However, unlike conventional fusion welding processes that cause heat-affected zone liquation cracking in 718 Plus and many other superalloys, the LFW process did not cause cracking in 718 Plus superalloy despite the liquation of precipitates. This absence of cracking during joining is attributed to the applied compressive stress during the forging stage of the LFW process. Also, no cracking was observed after PWHT, although PWHT resulted in a microstructure that had a nonhomogeneous distribution of precipitates in the weld and the TMAZ.