The Inertia Friction Welding (IFW) process is a high-temperature and high pressure process, with heavy plastic deformation, high power density, fast heating and fast cooling of the weld material. The microstructure produced in the weld line （WL）zones is therefore very different from parent material. A detailed microstructural investigation of the WL zones has been conducted using transmission electron microscopy and scanning electron microscopy. It has been shown that the morphology, energy status and microchemistry of grain boundaries in the WL zones are quite different from those in the parent material. It is also observed that, compared to a bi-modal distribution of intragranular ¢ particles in the parent material, a unimodal distribution of very fine spherical ¢ particles is produced in high density in the WL zones. This work provides a detailed understanding of the physical and chemical changes occurring across the weld line.