On the formation of modified intermetallics at an Al/Fe interface via segregated Si

Intermetallics (θ and η phases) with high aluminum content (>50 at.% Al) formed at the interface of Fe/Al welded joints significantly reduce the mechanical strength of those joints. Seeking to modify the intermetallics (IM) thus constitutes an opportunity to minimize the risk of brittle failure of the bimetallic joints. In this work, we proposed a method to modify the IM layers by a second diffusion of segregated elements. Firstly, thermomechanical simulation performed using DICTRA® allows estimating the segregation of alloying elements near the interface during friction melt bonding (FMB) of Al/Fe joints. For the experimental part, FMB welded joints composed of DP600 steel and AA6061 were used in this study. FMBed samples with the existing thin θ and η phases were reheated to verify the proposed method via second diffusion. The study revealed that the segregated Si at the interface contributes in a forward diffusion, while other elements (e.g. Mg and Cu) lead to backward diffusion. The experimental observations support that, the joint forms relatively thick θ phase compared to η phase due to the forward diffusion of Si. Moreover, the second heating facilitates the formation of a ternary IM (τ3 - FeAl2Si) at the interface. Using a second diffusion heat treatment may provide an alternate route to change the nature of the intermetallics or the compositions of the IM layer