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

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 meta...