In dissimilar welding, interfacial metallurgical bonding typically involves the formation of brittle intermetallic compounds (IMCs). Effective bonding without interfacial IMCs was reported only in a few combinations with high mutual solubility, such as Ag/Au, or in limited specific cases involving Al/steel welding, where interfacial amorphous phases formed. This study reveals a new metallurgical bonding mechanism in friction stir lap welding between immiscible AZ31 Mg alloy and DC01 steel, using high-resolution transmission electron microscopy. Rather than IMCs, AZ31 and DC01 were bonded via a nanometer-thick interfacial layer of Fe(Al) solid solution formed by Al atoms diffusing across the interface from AZ31. This mechanism was found highly effective, as evidenced by the lap shear strength of AZ31/DC01 joint, reaching 101 MPa, which is 78 % of the shear strength of AZ31 base material and 89 % higher than that of pure Mg/DC01 joint. Fracture predominantly occurred in AZ31 stir zone rather than right along interface.

Thickness of the intermetallic compounds (IMC) layer at the interface has a significant effect on the mechanical properties of Mg/Al dissimilar joints. However, the thickness of IMC layer can be only obtained by metallurgical microscopy, which is destructive and has to break down the weld. Therefore, it is crucial to find a reliable approach that can non-destructively predict the thickness of IMC layer in practical application. In the current study, Mg alloy and Al alloy were friction stir butt welded (FSW) under different tool rotation speeds (TRS) to obtain different thicknesses of IMC layers. As the TRS increased from 400 rpm to 1000 rpm, thickness of the IMC layer increased from 0.4 µm to 1.3 µm, the peak welding temperatures increased from 259 °C to 402 °C, and the Z-axis downforces decreased from 10.5 kN to 3.2 kN during welding process. Higher TRS would generally induce higher welding heat input, which promotes the growth of the IMC layer and the softening of base materials. The IMC layer formed through solid-state diffusion and transformation instead of eutectic reaction according to the welding temperature history and interfacial microstructure, and its evolution process was clearly observed by plan view. In order to incorporate the effect of dramatic change of welding temperature which is the characteristic feature of FSW, PsdVoigt function was used to fit the welding temperature histories. A new prediction formula was then established to predict thicknesses of IMC layers with considering sharp welding temperature change. Predicted thicknesses gave good agreement with measured thicknesses obtained experimentally under different welding parameters, which confirmed the accuracy and reliability of the new prediction formula. Based on this prediction formula, the time period of temperature higher than 200 °C during welding was found critical for the thickening of interfacial IMC layers.