1987 Volume 5 Issue 3 Pages 323-329
The copper-tungsten sintered alloy (Cu-W) has been friction welded to the tough pitch copper (CU) and to oxygen free copper (OFC), in order to investigate the effects of copper oxides included in CU on the tensile strength of the joint. The tensile strength of the joint of Cu-W to CU (Cu-W/CU joint), which increased with the rise of the forge pressure P2, was considerably lower than that of the joint of Cu-W to OFC (Cu-W/OFC joint) at P2 below 250 MPa. The low tensile strength of Cu-W/CU joints can be attributed to uncontacted spots which could be observed on the fractured surface of the joint, since no uncontacted spot could be observed for the Cu-W/OFC joint at P2 above 150 MPa. Tensile tests at elevated temperatures and marker experiment using alumina powder show that the difference in the uncontacted spot can be explained by the difference in the degree of plastic flow which facilitated the attainment of intimate contact at the bond interface. On the other hand, at P2 above 290 MPa, the fracture of Cu-W/CU joints occurred in CU immediately next to the bond interface, in contrast to the heat affected zone of OFC for Cu-W/OFC joints, though the difference in their tensile strength became very small. On fractured surfcaes of Cu-W/CU joints, a lot of copper oxides as well as tungsten particles picked-up from Cu-W during the friction process as reported in a previous paper were observed at dimple bottoms, suggesting that they acted as nucleation sites of fracture. In CU adjacent to the bond interface, the copper oxide, which had a preferential distribution in the axial direction of the specimen in the base metal, was distributed preferentially in a direction parallel to the bond interface. The change in the distribution of copper oxides combined with the picked-up tungsten particle seems to cause the fracture in CU immediately next to the bond interface.
