A new friction processing technique and its diffusion mechanism is unveiled during welding of aluminium (Al) and copper (Cu), in lap configuration. Cu and Al sheets are subjected to high pressure and temperature conditions in the process zone using a custom setup (see fig. 1(a)). The tool is rotated against a sacrificial top sheet where the parent metals to be joined are clamped firmly in between the top sheet and a steel backing plate. The approach does not involve any direct mechanical stirring [1], significant deformation, or thickness reduction of parent metals. Interdiffusion of copper and aluminium at their contact results in localized melting of metals selectively at the interface, at temperatures near their eutectic point, much below the melting point of either metal (see fig.1(b) for different steps involved in the process). Plunge action of the tool is stopped with continuation of rotation of tool over the consumable top sheet at the onset of melting. Rotating tool is kept at plunge depth where melting starts for a given dwell period (e.g. 10 s) and then retracted. Melt layer thickens in the dwell period at nearly constant temperature. Choice of an appropriate dwell time is crucial to control the melt layer thickness and intermetallic phases formed. Uniform welds are obtained at Al-Cu interface with submicron-featured lamellar microstructures. Joint interface exhibits homogeneous near-eutectic composition. Concentration gradient of Cu in Al parent metal can be considerably reduced for homogenous, eutectic composition in the interface zone. This reduces formation of Cu rich phases on Cu parent metal side. In conventional solid state diffusion, continuous change in Cu concentration gradient across the interface layer results in formation of multiple intermetallic compounds (IMCs) [2] (see Fig. 1. (c)). On the contrary, in the welding processes adopted in this work, localised liquefication selectively at the interface results in uniform Cu concentration in the eutectic region (see Fig. 1. (c)). The driving force for diffusion of Cu into the diffusion interface is drastically reduced restricting formation and growth of continuous IMC layer of Cu rich phases.