Friction stir lap welding of AZ31 and TC4: Mechanical properties and bonding mechanism

The welding between Mg and Ti alloys is challenging, as Mg and Ti do not react with each other and their mutual solubility is also very low. To investigate the bonding mechanism and mechanical properties of the dissimilar joint between Mg and Ti alloys, AZ31 Mg alloy and TC4 Ti alloy were joined together by friction stir lap welding under different tool rotation speeds. High strength joint with lap shear strength of 108 MPa was obtained, which reached 83% of the shear strength of AZ31 base material. Fracture happened both in the AZ31 stir zone and along the AZ31/TC4 interface. Cracking inside the AZ31 stir zone should have taken the main role since 57 to 58% of the fracture surface on the TC4 side was found to be covered by AZ31 residual. The interfacial microstructure was investigated by transmission electron microscopy. AZ31 and TC4 were found to be bonded through a TiAl IMC layer with the thickness of only tens of nanometers. The formation of ultra-thin TiAl layer at the interface could be attributed to the limited supply of Al element to the interface. The formation of interfacial metallurgical bonding was also discussed.