1991 Volume 9 Issue 4 Pages 467-474
The microstructure and bond strength of the friction-welded joint of Al (commercially pure aluminum) to OFC (oxygen free copper) have been investigated in order to understand the formation of intermetallic compounds and its effects on the mechanical properties of the joint. A mixing layer of Al and Cu which showed a fine stratified microstructure was formed in Al adjacent to the weld interface by mechanical picking-up of Cu into Al. In the mixing layer, intermetallic compounds CuAl2, CuAl and Cu9Al4 were detected by X-ray diffraction analyses and TEM. As forge pressure P2 was increased, the width of the mixing layer was decreased and the stratified microstructure became considerably isolated flaky one. The tensile strength of the joint was increased as P2 was increased, and at P2 above 120 MPa the joints were fractured in the heat affected zone of Al. On the other hand, the joints welded at P2 below 80 MPa were fractured mainly in the mixing layer. EDX analyses of the area around cracks suggested that the crack on the tensile test initiated at CuAl in the mixing layer, since the composition around the initiation site of the crack corresponded approximately to that of CuAl. From these results, it can be concluded that the intermetallic compounds, especially CuAl have harmful effects on the mechanical properties of the joint. The increase in the joint strength with rise in P2 can be explained as a consequence of the decrease in the thickness of the mixing layer which includes a number of fine intermetallic compound layers. The formation of mixing layer having the fine stratified microstructure is probably due to the mechanical mixing in the solid state, since the average temperature of frictionn interface estimated from the thermoelectric power between Cu and Al was below the lowest eutectic temperature of the Cu-Al system.