Mechanism of solid state pressure welding

The mechanism of pressure welding in polycrystalline aluminum, copper, silver, and gold was investigated. The role of the oxide film was studied, and it was found that no metal to oxide bonding contributes to the strength of the weld. From scanning electron microscope observations a two-stage model was suggested which could explain the different behavior of the metals studied. The first stage of welding involves the formation of overlapped oxide-free metallic areas; this is controlled by difference on a microscale of the local plastic strain occurring on matching opposite faces of the weld interface, relative hardness of the metal and its oxide film, and mechanical properties of the oxide. The second stage involves plastic flow of the metal to the over-lapped areas--the stress at which this can take place is influenced by the stacking fault energy of the metal--and some relative shear displacement at the points where metal cleaned of oxide comes into contact--this is influenced by surface roughness. The different weldability of different metals is attributed to differences in stacking fault energy, hardness ratio, and plastic properties of the oxide. The weld strength calculated theoretically on the basis of measured welded area was in agreement with measured fracture strength. It was concluded that the strength attained after a given deformation is determined by the fractional welded area at that deformation. 19 figures, 3 tables