Abstract
A copper-tungsten sintered alloy (Cu-W) containing 70 mass% W (tungsten particle about 3μm in diameter) has been friction-welded to a tough pitch copper (Cu). The friction speed for the welding was 40 s-1, the friction time 0.5 to 2.0s, the friction pressure 50 MPa, and the forge pressure 50 to 340 MPa. The tensile strength of joint increased with an increase in forge pressure P2, approaching a saturated value of 280 MPa at P2 higher than 290 MPa. At P2 below 250 MPa, unbonded area, which decreased with increase in P2, were observed on fractured surfaces. At P2 above 290 MPa, though no unbonded area could be observed, fracture occurred mostly in Cu within 140μm from the weld interface. A microhardness test showed that a weld heat-affected zone formed in Cu had the lowest harndess number at a distance more than 0.3 mm from the weld interface. The location of the lowest hardness number did not coincide with that of the fracture, suggesting that the decrease in the hardness number did not cause the fracture in Cu immediately next to the weld interface. A number of tungsten particles, which were probably picked up by mechanical mixture during the friction process, were observed in Cu within 50μm from the weld interface. On fractured surfaces of Cu side, many tungsten particles were observed at the bottom of dimples. From these results, it can be concluded that the tungsten particle picked up in Cu acts as the nucleation site of fracture and so is responsible for the fracture in Cu immediately next to the weld interface.
