Effect of Oxide Film on the Early Process of Diffusion Welding

Fundamental Study of the Early Process of Diffusion Welding by Means of Electric Resistance Measurement (Report 1)

Abstract

The early process of diffusion welding for several kinds of commercially pure metals (aluminum, titanium, iron, copper and silver) has been investigated by means of electric resistance measurement with particular reference to oxide film on faying surfaces. A couple of base metals were placed in vacuo with their faying surfaces in contact at a constant welding pressure and the couple was heated at a constant rate from room temperature to a temperature below melting point. The electric resistance across bond interface ρ was measured during the heating process and analyzed on the basis of the theory of convergence resistance. Results obtained are summarized as follows.
(1) The electric resistance across bond interface ρ for each metal was higher than that of the base metal in the early process of heating and approached that of the base metal as the bond interface was heated. The temperature at which the value of p became to be nearly equal to the resistivity of the base metal was different among the base metals; the temperature was 630°C for aluminum, 900°C for titanium, 850°C for iron, 600°C for copper and 500°C for silver.
(2) The ratios of the initial electric resistance across bond interface (before heating) to the resistivity of the base metals except for silver were rather higher than the value (≈6) estimated from convergence-resist-ance theory on the assumption that the faying surfaces were perfectly clean metallic surfaces. In particular those of the aluminum and titanium which had tenacious oxide film on the faying surface were much higher (10²-10⁵). This fact suggests that the faying surfaces of metals except for silver are not clean metallic surfaces but have a factor which prevents true metal-to-metal contact at the bonding interface.
(3) The electric resistance across bond interface of aluminum, titanium and copper increased largely, as the thickness of oxide film on the faying surfaces increased (the faying surfaces were subjected to hightemperature oxidation before welding). And the temperatures at which the value of p approached the resistivity of the base metal became higher with increasing the thickness of oxide film.
These results described above indicate that the oxide film is one of the most important factors which prevents true metal-to-metal contact at the bonding interface.