This paper describes the effect of the friction welding condition and the weld faying surface properties on the tensile strength of the friction welded joint between pure titanium (Ti) and pure copper (OFC). The joint strength of the joint, which was made with the weld faying surface of the Ti side specimen finished with a surface grinding machine, was investigated. The joint did not have 100% efficiency and OFC side fracture regardless of the friction welding conditions. When the joint was made with a friction pressure of 75 MPa, the joint efficiency was approximately 64% regardless of the forge pressures, and all joints fractured at the weld interface, although that efficiency exceeded that of the joints made with other friction pressures. To improve the joint efficiency, one was made with a Ti side specimen whose weld faying surface was finished by buff polishing. The joint efficiency was increased to approximately 85%, although the joint fractured at the weld interface. Moreover, a joint at a friction pressure of 75 MPa with a forge pressure of 270 MPa or higher was obtained with an OFC side fracture, although it did not achieve 100% efficiency. The fact that the joint did not fracture at the OFC base metal was due to the mechanically mixed layer at the weld interface that depended on the maximum height of the Ti side weld faying surface. To clarify the reason why the joint did not achieve 100% joint efficiency, the tensile strength of the OFC base metal with the addition of various compressive stresses was investigated. When the compressive stress was higher than the yield stress of the OFC base metal, its tensile strength was lower than that without a compressive load. Moreover, the tensile strength along the radial direction of the OFC base metal was also slightly lower than that of the longitudinal direction. Hence, the fact that the joint did not also achieve 100% efficiency was due to the decrease in the tensile strength of the OFC base metal by the Bauschinger effect and the difference of the anisotropic property with as-manufactured condition.

This paper describes the effect of friction pressure on the joining phenomena of friction welds between pure titanium (P-Ti) and pure copper (OFC). When the joint was made at a friction pressure of 60 MPa or lower, the OFC side had deformation from the contact of both weld faying surfaces. Then the joint had generated sparkle from the P-Ti side with increased friction time. The peripheral portion of the P-Ti side had large deformation and was intensely upset when the joint had sparkle. On the other hand, the joint did not have sparkle on the P-Ti side when it was made at a friction pressure of 75 MPa or higher. The deformation of the OFC side was large and was intensely upset, although the P-Ti side was hardly deformed. The joining phenomena had dissimilarity because the difference of the yield stress for each material depended on the temperature in the friction process.

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