Microstructural evolution and mechanical properties of dissimilar Al–Cu joints produced by friction stir welding

•Successful dissimilar joining of 5A02 Al and pure Cu by friction stir welding.•A composite-like structure strengthened the weld nugget zone.•Nano-scaled reaction layers at the Al/Cu interface were observed and identified.•Formation mechanism of microstructure in the weld nugget zone was clarified.•Both composite structure and ultra-thin layers enhanced joint strength.

5A02 aluminum alloy and pure copper were joined by friction stir welding (FSW). A defect-free joint was obtained when one of process parameters, i.e. the traverse speed was lowered from 40 mm/min to 20 mm/min. A good mixing of Al and Cu was observed in the weld nugget zone (WNZ). A large amount of fine Cu particles were dispersed in the upper part of the WNZ producing a composite-like structure. In the lower part, nano-scaled intercalations were observed and identified by transmission electron microscopy (TEM). These layered structures were subsequently confirmed as Al4Cu9 (γ), Al2Cu3 (ε), Al2Cu (θ), respectively. Formation of these microstructures caused an inhomogeneous hardness profile. Particularly, a distinct rise in hardness was noticed at the Al/Cu interface. Excellent metallurgical bonding between Al and Cu gave rise to good behaviors in the tensile and bending strength.