Formation and influencing mechanism of the intermetallic compound in the friction stir welding of immiscible AZ31 and SPHC steel using aluminium powder as an additive

The primary issue with joining an immiscible magnesium/iron system is the lack of a bonding medium. This research used an aluminium (Al) additive as a bonding medium to facilitate the formation of an interface layer. Immiscible AZ31 magnesium alloy and SPHC low-carbon steel were successfully joined by employing aluminium (Al) powder as an additive in the gap between them with friction stir welding (FSW). The extensive interfacial microstructural analyses confirmed that the aluminium-rich Fe2Al5 intermetallic compound (IMC) formed with a range of 20-25 nm in thickness at the interface between magnesium and iron resulted from the metallurgical reaction between the Al powder additive and base SPHC steel. This IMC phase served as a transitional layer, facilitating the metallurgical bonding between Magnesium and Iron. The tensile strength of the joint was significantly improved by 43%, from 126 MPa without the additive to 180 MPa using the aluminium additive. The formation of the following well-matched interface lattice sites between Fe and Fe2Al5 region was identified: (002)Fe2Al5 // (110)Fe, [110]Fe2Al5 // [ 13]Fe. The intermetallic Fe2Al5 was composed of nanocrystalline and amorphous interface layers. Furthermore, the fracture of the joint occurred at the interface, indicating a brittle mode of fracture behaviour.