Cold weldability of aluminium: Contribution of the mechanical loadings to the formation of metallic bonds

In solid-state welding, temperature generally has an important role (diffusion, recrystallization, etc.) however, a mechanical loading is also necessary for breaking down the thin oxide layer (Al₂O₃) which naturally covers aluminium alloys. As the oxide layers have a melting point of around 2000 °C meanwhile aluminium alloys generally melt at only 600 °C; the heat supply alone cannot lead to the welding and a mechanical loading is always needed. This work aims at understanding the influence of the mechanical loadings on the formation of metallic bonds. To this end a new weldability test has been introduced. A symmetric cyclic shear is run on the welding interface without external heat supply. A tube-shaped sample is cut through its cross-section and undergone both a compression and a cyclic torsion loading. The influence of some parameters has been studied. Among them, surface condition (roughness and cleanliness), torsion angle (low amplitude) and number of cycles are the most significant. The welded joints have been then mechanically tested and microscopically observed (FEG-SEM and EBSD): the joining is achieved on a 1.5 μm thick layer, as things stand, on about 50% of the welding interface. Moreover, a thermomechanical model concludes that the internal intrinsic heat involved in this experimental process is too low to imply a significant increase of temperature in the specimen; thus the joining is only achieved due to mechanical effects of elastoplastic type.