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 . <EOS>