Forming mechanism of equilibrium and non-equilibrium metallurgical phases in dissimilar aluminum/steel (Al–Fe) joints

Abstract

Forming metallurgical phases has a critical impact on the performance of dissimilar materials joints. Here, we shed light on the forming mechanism of equilibrium and non-equilibrium intermetallic compounds (IMCs) in dissimilar aluminum/steel joints with respect to processing history (e.g., the pressure and temperature profiles) and chemical composition, where the knowledge of free energy and atomic diffusion in the Al–Fe system was taken from first-principles phonon calculations and data available in the literature. We found that the metastable and ductile (judged by the presently predicted elastic constants) Al6Fe is a pressure (P) favored IMC observed in processes involving high pressures. The MoSi2-type Al2Fe is brittle and a strong P-favored IMC observed at high pressures. The stable, brittle η-Al5Fe2 is the most observed IMC (followed by θ-Al13Fe4) in almost all processes, such as fusion/solid-state welding and additive manufacturing (AM), since η-Al5Fe2 is temperature-favored, possessing high thermodynamic driving force of formation and the fastest atomic diffusivity among all Al–Fe IMCs. Notably, the ductile AlFe3, the less ductile AlFe, and most of the other IMCs can be formed during AM, making AM a superior process to achieve desired IMCs in dissimilar materials. In addition, the unknown configurations of Al2Fe and Al5Fe2 were also examined by machine learning based datamining together with first-principles verifications and structure predictions. All the IMCs that are not P-favored can be identified using the conventional equilibrium phase diagram and the Scheil-Gulliver non-equilibrium simulations.

Revealing the Epitaxial Interface between Al13Fe4 and Al5Fe2 Enabling Atomic Al Interdiffusion

“…Several phases can be observed at the interface between fcc Al and bcc Fe: Al 6 Fe, Al 13 Fe 4 , Al 2 Fe, Al 5 Fe 2 , AlFe, and AlFe 3 . , Among them, Al 5 Fe 2 and Al 13 Fe 4 are known to be frequently formed, as they nucleate almost spontaneously. − Due to the low solubility limit (0.012% at 550 °C) and low diffusion coefficient of iron in aluminum ( D = 8.76 × 10 –11 cm 2 /s at 550 °C), the complex Al 13 Fe 4 phase is known to first form at the Al∥Fe interface, which subsequently decomposes into Al 5 Fe 2 at the Fe∥Al 13 Fe 4 interface. A distribution of morphologies and sizes is observed for Al 5 Fe 2 by electron microscopy, but with a preferred growth directionparallel to the normal direction of the (001) plane .…”

Infiltration of aluminum in 3D‐printed metallic inserts

“…The chemical composition, the morphology (whether it is smooth or rough), and the thickness of the intermetallic layer effects the bonding strength [11,12]. Metallic inserts can be fabricated through AM processes since it facilitates the production of complex and interconnected structures such as lattices used for load-transferring applications.…”

“…Regarding the formation of an intimate interface, in the case of dissimilar materials, such as Al and Fe, the formation of intermetallic phases can occur at the interface (e.g., η‐Al 5 Fe). The chemical composition, the morphology (whether it is smooth or rough), and the thickness of the intermetallic layer effects the bonding strength [11, 12].…”