Friction welding of pure tungsten to oxygen free copper with an intermediate layer

Summary

This paper describes an investigation of commercially pure tungsten friction-welded to oxygen free copper with intermediate layers of various metals to improve friction weldability. The intermediate layers used were 20–30 μm thick Ti, Al, Nb, Ni, Fe, and Ag foils. The effects of the intermediate layers on burn-off deformation behaviour and joint performance are examined. The friction welding conditions adopted were: a friction pressure of 50 MPa, forging pressure of 340 MPa, rotational speed of 40 sec⁻¹, and forging time of 6 sec. The friction time was varied in the 1.0–10.0 sec range depending on the type of intermediate layer concerned.

The burn-off rates observed during friction welding with all intermediate layers other than Ag are greater than those found without an intermediate layer. Ti, Nb, and Fe intermediate layers most notably lead to a sharp increase in burn-off rate. All intermediate layers other than Ag become finer or thinner during friction welding and are mixed with Cu to form stratified microstructures. When Ti and Al intermediate layers are used, Ti-Cu and Al-Cu intermetallic compounds are formed in the mixed layer. As the friction time increases, the tensile strength of the joints increases to reach almost saturated values depending on the intermediate layers. The tensile strength of joints with Ti, Nb, and Fe intermediate layers increases more rapidly than that of directly welded joints without an intermediate layer and reaches higher levels. Joints with an Nb intermediate layer most notably fracture in the copper HAZ at a friction time of 4 sec or more, whereas joints with the other intermediate layers fracture in the weld, as in the case of direct welding. The friction torque and heat input found during friction welding with Ti, Nb, and Fe intermediate layers are much greater than those found during friction welding without an intermediate layer, suggesting that the increases in the friction torque and heat input accelerate the burn-off and tensile strength gain of the joints.