The microstructure of inertia friction welding P/M ., a fast heating and cooling process with recrystallization occurring in the welding area. A dramatic change of the microstructure was observed over a narrow welding zone. A significant change of the γ′ distribution was observed across the welding line with very fine γ′ particles precipitated in unimodal size distribution. The fine precipitated γ′ under fast cooling was spherical in shape, which gradually transformed into elliptical, cube shape as the distance from welding line increased. No significant grain distortion was observed in the extensively plastically deformed region due to recrystallization. Compared with the standard post-weld heat treatment, heat treatment with higher temperature could significantly reduce the width of the fine grain area at the welding zone. The volume fraction of the weld line secondary γ′ phase increased from 0% to ~25% when higher temperature heat treatment was applied. Those findings are essential for controlling the microstructure of inertial friction welding component on P/M superalloys.