Microstructures of Inconel 718 alloy subjected to rapid thermal and stress cycle – joint performance and its controlling factors in friction welding of Inconel 718 alloy

Abstract

The microstructures of an Inconel 718 alloy subjected to rapid thermal and stress cycles have been investigated to explain those observed in the friction welded joint of the alloy. The thermal and stress cycles were simulated with a Gleeble thermal and mechanical simulator. It turned out that the microstructural changes caused by the rapid heating cycle at peak temperatures of 1253–1553 K were almost in accordance with those reported in the previous papers about the solidification process of the alloy and phase diagram of Inconel 718 calculated by Thermo-Calc: (1) dissolution of film carbide and needle precipitates of (δ phase at grain boundaries at temperatures from 1253 to 1373 K, (2) dissolution of massive carbide (NbC) at 1373–1533 K, (3) liquation due to the eutectic reaction between γ phase and NbC at 1443–1533 K, and (4) melting of γ phase at 1553 K or higher. In the specimens heated to peak temperatures above 1473 K, γ/Laves eutectic were formed which were not detected in the as-received base metal. When a pressure of 350 MPa was applied during the heating cycle, most of the liquid phase was expelled into the flash, leaving a fine grain zone involving liquid phase areas much narrower than those observed when the pressure was not applied. When the pressure was applied to a specimen involving no liquation areas (peak temperature < 1443 K), a microstructure characterized by jaggy grain boundaries was found at peak temperature below 1253 K and a microstructure that consisted of coarse grain and fine grain distributed along the grain boundary region of the coarse one was found at peak temperature above 1253 K, with the fine grain area increasing with peak temperature up to 1443 K.

Keywords:

• Ni alloy
• Inconel 718 alloy
• thermal cycle
• stress cycle
• friction welding
• liquation
• Laves phase
• niobium carbide