Abstract
The formation of intermetallic compounds and its effects on the bond strength have been investigated for the friction-welded joint of Ti (commercially pure titanium) to Cu-W alloy (copper-70% tungsten sintered alloy) by being compared with those of Ti to OFC (oxygen free copper). In the joint of Ti to OFC, a mixing layer of Cu and Ti which included brittle intermetallic compounds TiCu4, and Ti3Cu4, was formed. As forge pressure P2 was increased, the thickness of the mixing layer was decreased, since the mixing layer was excluded toward the collar during forge process. The tensile strength of the joint was increased with the rise in P2 ; joints welded at P2 above 250 MPa were fractured in the weld heat affected zone of OFC about 3 mm away from the weld interface, while joints welded at P2 below 200 MPa were fractured mainly in the mixing layer. A similar mixing layer was also observed in the Cu-W/Ti joint. In this case, however, the mixing layer was hardly excluded during the forge process, and consequently the joint was fractured in the mixing layer even at P2 above 250 MPa. A marker experiment using alumina powder was carried out in order to investigate the plastic flow caused by friction welding near the weld interface ; i.e., alumina powder was packed into a hole drilled in one of the specimens, and distribution of alumina powder resulting from the plastic flow during friction welding was observed. The result suggested that plastic flow in the radial direction of the weld interface for Cu-W/Ti joint was insufficient to exclude the mixing layer even at P2=340 MPa. The temperature of the weld interface measured with two-color radiation pyrometer was increased up to 1333 K. At this temperature, the Cu matrix of the Cu-W alloy adjacent to the weld interface may be regarded as melted, since spot analyses with EDX indicated that the Ti content of the Cu matrix within 4 μm from the weld interface was greater than the solidus composition of Cu-Ti system at this temperature. It seem that the melting of the Cu matrix facilitated the formation of the mixing layer and the intermetallic compounds.
