Abstract: Microscopic features of TC4 linear friction welded joints were analyzed and the variation of the characteristic parameters with welding time was studied.The results indicated that the joint interface includes the welded zone,the transition zone and the incomplete bonding zone.Fine dynamic recrystallization grains appeared in the welded zone where the joint interface fused completely.Micro pores formed in the transition zone where the joint interface fused partly.In the incomplete bonding zone,metals on both sides of the joint interface were not contacted and grains were out of distortion.Moreover,metal fragments developed in the joint interface.Three-branch structure forms in the joint interface due to the separation of the vibration direction flash and the vertical vibration direction flash.Friction pressure and upsetting pressure are inclined to close the branches,while friction shear force attributes for expanding the branches.