A novel amorphous-nanocrystalline interface layer for bonding immiscible Mg/steel by pinless friction stir spot weld with preset nanoscale Fe2Al5 film

Abstract

The absence of a bonding medium is the principal problem of immiscible Mg/Fe system. Obtaining an interface layer at the Mg/steel interface is the key to achieving a metallurgical bonding. However, the type and thickness of the interface layer has a random nature during the welding process of Mg/steel dissimilar metals, which has a large impact on the joint properties. An effective AZ31 Mg alloy/22MnB5 steel joint with 12 μm width Fe2Al5 interface layer is bonded by pinless friction stir spot welding, and obtained maximum shear strength 1.4 kN. In addition, a specific type and thickness of Fe2Al5 intermetallic compound was chosen as the interface layer in this study. Nano-scale Fe2Al5 interface layer prepared by magnetron sputtering method, which achieved a large improvement in Mg/Fe2Al5 coated steel properties with a maximum shear strength 2.4 kN. The shear strength of the Mg/Fe2Al5 coated steel joint was improved by 86.8% after the interface layer thickness was changed from the micron scale to the nanoscale. Nanoscale Fe2Al5 amorphous-nanocrystalline interface layer was contribute to its higher shear strength. The following orientation relationship exists for the Fe2Al5/steel part of the region, (110)Fe2Al5//(101)α-Fe. This study investigated the effect of the mixed structure of nanocrystalline and amorphous interface layers on the interfacial bond strength besides the analysis of nanostructured interface layers. The molecular dynamics simulation indicates that as the thickness of the η-Fe2Al5 interface layer decreases, the stress of the α-Mg/η-Fe2Al5/α-Fe model tends to increase and then decrease.