<p>Nickel-based superalloy GH4151 has excellent high-temperature strength, but its high γʹ content and complex compositional and microstructure lead to poor weldability. The microstructure and defect formation mechanism of the joints were analyzed by adjusting the welding pressure of inertia friction welding (IFW) combined with hot deformation experiments. The results show that at low welding pressure, the welding defects are non-uniform grains, harmful precipitates (Laves and γ-γ′ eutectic), and microcracks. The formation of these defects is associated with dynamic recrystallization, the γʹ compositional liquefaction, and the stress concentration caused by carbides. At medium welding pressure, the deformation uncoordination of primary γ′ and carbides at the interface with the matrix, as well as the shear force at the weld interface, led to initial cracks, and the residual stresses generated by the precipitation of γ′ during post-weld cooling led to secondary cracks. Joints with favorable microstructure were obtained at high welding pressure, with uniform and fine grains and tertiary γ′ in the weld zone. Hot deformation experiments show that the rapid dissolution behavior of γʹ under rapid heating and deformation in the IFW process lead to microcracks. Meanwhile, the ultrahigh temperature at the weld interface causes alloy melting and the formation of harmful phases. Reasonably increasing the welding pressure can effectively suppress the liquefaction of γ′, enhance the healing capacity of pores and microcracks, and improve the flowability of plastic metal at the weld interface, thereby reducing the formation of welding defects and obtaining uniform and fine joint microstructure and precipitates.</p>