Toshio Enjo

Analysis of electric resistance of bond interface in diffusion welding by constriction resistance.

New equations giving constriction resistance RC as a function of area SM and number density n of true contact spots have been obtained from a model experiment, and applied to the analysis of the electric resistance of the bond interface ΔR for the diffusion-welded joint of titanium. The new equations obtained are described as RC= ρ/√n ⋅S ( √π/2 √S/√SM -1.21) ……(1)for (SM/S) 1/4, where ρ is the resistivity of base metal and S the apparent contact area. It can be seen from these equations that parameter A=(ΔR⋅S/ρ) depends only on (SM/S) and n, if ΔR=RC. Parameter A measured at temperatures from 77 K to room temperature for joints of titanium, however, decreased with a rise in the temperature of measurement, and its temperature dependence became less pronounced with the increase in welding temperature and time. On the other hand, area SM and density n were estimated from fractured surfaces of joints on the assumption that spots where grooves caused by machining of the faying surface were annihilated corresponded to true contact spots. Parameter A estimated from SM and n thus obtained using eqs. (1) and (2) was significantly smaller than that measured at 77 K and rather in good agreement with that measured at room temperature. This result indicates that not all the true contact spots observed on fractured surfaces corresponded to completely bonded spots having electrical properties identical with those of the base metal. The dependence of A on the temperature of measurement can be accounted for by a model that the bond interface consists of three characteristic spots: unbonded spot, completely bonded spot and incompletely bonded spot containing inclusions such as oxide films.

Diffusion welding of Al-Al3Ni eutectic composites.

Unidirectionally solidified Al-Al3Ni eutectic alloy was diffusion welded using various insert-metals. The weldments made with commercially pure copper foil (5μm in thickness) at 630°C for 30min under 6.5kg/cm2 in welding pressure show re-solidified Al3Ni phase in the microstructure at the joint interface, and have the tensile strength equivalent to that of the base metal. The weldments made with 2024 alloy foil (100μm in thickness) at 600°C for 30min under 6.5kg/cm2 in welding pressure have 72% of tensile strength and 81% of Charpy impact value to those of the base metal, and show little Al3Ni phase at the joint interface, but coasening of the phase occurs with an increase in welding temperature. In both cases, partially melted insert-metals play an important role in diffusion welding processes.