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

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 eta-Al5Fe2 is the most observed IMC (followed by theta-Al13Fe4) in almost all processes, such as fusion/solid-state welding and additive manufacturing (AM), since eta-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.

Automated summary

In dissimilar aluminum/steel joints, the formation of Al-Fe intermetallic compounds was found to be influenced by processing conditions and chemical composition. The study revealed that high-pressure processes favor the formation of stable, brittle Al5Fe2 and temperature-favored η-Al5Fe2, which have high thermodynamic driving force and fast atomic diffusivity. Additive manufacturing (AM) was identified as a superior process for achieving desired IMCs in dissimilar materials.