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Abstract
Microstructures forming in the friction welding of Inconel 718 alloy have been investigated in order to understand the phenomena occurring during the welding process and to determine the factor controlling the joint performance from a metallographic point of view. In the interfacial zone, liquation microstructures characterized by a eutectic structure consisting of γ and Laves phases, and Nb-rich microstructures along the grain boundary (Nb-rich GB microstructures) were observed, and their amounts increased with the friction time and pressure. The volume fractions of these liquation structures were greater in the flash than in the interfacial zone, suggesting that the liquid phase was preferentially expelled into the flash by friction pressure, compared with the solid phase. Since the liquid phase was rich in Nb, this preferential expulsion of the liquid phase caused the depletion of Nb, a major element for the precipitation hardening of the alloy. The depletion of Nb brought about a decrease in the hardness in the interfacial zone after a post-weld heat treatment for precipitation hardening. Thus, although the friction bonding is a solid state welding process, the liquation occurs in the weld of Inconel 718 alloy having a wide solid–liquid coexisting temperature range, and has a significant influence on the joint performance.