Interfacial bonding mechanism and mechanical properties of micro friction stir blind riveting for multiple Cu/Al ultra-thin layers

In this research, bonding mechanisms and mechanical properties were investigated for a newly developed multilayer dissimilar material joining technique, micro friction stir blind riveting (viz. μFSBR), where the multilayer micro joint was made consisting of one Cu layer and three Al layers (each layer having 0.203 mm thickness) connected through Al rivet. Different microscopy techniques were employed (i.e. scanning electron microscopy and transmission electron microscopy) to unearth the bonding mechanisms of the three interfaces (i.e. Al rivet and Cu sheet interface, Cu sheet and Al sheet interface, and Al rivet and Al sheet interface) which revealed intricate yet distinct bonding phenomenon for each interface. A nano-scale diffusion layer in addition to two-way material flow consequential heterogeneous lamellar structures and mechanical interlocking was formed at the Al rivet and Cu sheet interface; Cu particles diffused into a non-uniform thickness thin oxide layer at the Cu sheet and Al sheet interface; and Al sheet experienced grain refinement near the interface close to Al rivet with a nano-scale gap. These observations revealed that both mechanical and metallurgical bonding occurred simultaneously in μFSBR. The formation of different bonding mechanisms resulted in superior bonding strength and absorbed energy compared to conventional micro blind riveting joints, which was further confirmed through layer by layer tensile tests.