Effect of alumina on Fe-Al intermetallics at the SS-Al bimetallic interface fabricated via wire arc directed energy deposition

Abstract

The development of a high-quality stainless steel (SS)-aluminium (Al) bimetallic transitionally graded structure (TGS) via metal additive manufacturing process has been challenging due to the formation of brittle and crack-prone Iron (Fe)- Al intermetallic compounds (IMCs) at the bimetallic interface. However, it has been observed that the addition of ternary alloying elements can significantly improve the metallurgical and mechanical properties of these IMCs. This study investigates the intricate influence of alumina particles on the SS-Al interface, uncovering insights into melt pool forces, tri-layered IMC formation, nano-grained IMC layers, and an inverse Hall-Petch relationship. Three thin-walled SS-Al structures were deposited via wire arc directed energy deposition (WA-DED), incorporating varying alumina content levels (20, 30, and 40 mg/mm). Alumina decomposition released elemental and gaseous oxygen, affecting IMC layer thickness. Oxygen content at the interface increased with alumina concentration (from 20 mg/mm to 40 mg/mm), which in turn altered Marangoni and buoyancy forces, significantly impacting the IMC thickness and respective grain size. Energy dispersive X-ray spectroscopy and electron back-scattered diffraction analyses revealed FeAl, Fe₂Al₅, and FeAl₃ layers, with Fe₂Al₅ layer thickness increasing with alumina content. The nanometric grain size of Fe₂Al₅ and FeAl₃ (∼77.69 nm and 52.68 nm) were observed at the bimetallic interface of lower alumina content (20 mg/mm) which justifies their respective hardness with the inverse Hall-Petch relationship. These findings provide valuable insights into interface dynamics and offer quantitative data for optimising SS-Al structures in diverse applications.