A method of joining a first component and a second component, the first component and the second component being made of dissimilar metals, the first component having a lower melting temperature than the second component, the method being characterised by the following: smoothing the faying surface of the second component to a roughness value of Ra < 1 μm and reducing surface oxidation of the second component to a thickness less than 2 nm before joining; inserting a rotating tool having one shoulder portion and one rotating probe portion into the first component, the rotating probe portion is spaced apart from a faying surface of the second component by a standoff distance of 0.05 to 1mm; rotating the rotating tool to generate rapid friction between the second component and material of the first component rotating around the one rotating probe portion at the dissimilar metal interface, the friction being sufficient to generate a layer of quasi-liquid metal and produces shear localization within the quasi-liquid metal at a dissimilar metal interface between the first component and the second component, the quasi-liquid metal being a metal in a liquid-like state at a temperature below the melting point of the first component and the second component, the quasi-liquid metal having a shear strain rate above 1x10 4 s -1; wherein the rotating tool comprises surface features for increasing the surface roughness to enhance the rotational flow of material around rotating tool member, and to enhance locally activated rapid friction between the dissimilar metals; and terminating the application of rapid friction and reducing a processing temperature of the quasi-liquid metal below a crystallization temperature of the quasi-liquid metal within 1 to 5 seconds after generation of the quasi-liquid metal and before 80% of the quasi-liquid metal has crystallized within the quasi-liquid metal thereby joining the first component and the second component along a nanoscale amorphous layer. smoothing the faying surface of the second component to a roughness value of Ra < 1 μm and reducing surface oxidation of the second component to a thickness less than 2 nm before joining; inserting a rotating tool having one shoulder portion and one rotating probe portion into the first component, the rotating probe portion is spaced apart from a faying surface of the second component by a standoff distance of 0.05 to 1mm; rotating the rotating tool to generate rapid friction between the second component and material of the first component rotating around the one rotating probe portion at the dissimilar metal interface, the friction being sufficient to generate a layer of quasi-liquid metal and produces shear localization within the quasi-liquid metal at a dissimilar metal interface between the first component and the second component, the quasi-liquid metal being a metal in a liquid-like state at a temperature below the melting point of the first component and the second component, the quasi-liquid metal having a shear strain rate above 1x10 4 s -1; wherein the rotating tool comprises surface features for increasing the surface roughness to enhance the rotational flow of material around rotating tool member, and to enhance locally activated rapid friction between the dissimilar metals; and terminating the application of rapid friction and reducing a processing temperature of the quasi-liquid metal below a crystallization temperature of the quasi-liquid metal within 1 to 5 seconds after generation of the quasi-liquid metal and before 80% of the quasi-liquid metal has crystallized within the quasi-liquid metal thereby joining the first component and the second component along a nanoscale amorphous layer.