Advanced characterization of intermetallic compounds in dissimilar aluminum-steel joints obtained by laser welding-brazing with Al-Si filler metals

The welding of dissimilar materials such as aluminum and steel is increasingly gaining importance as a strategy to obtain lightweight multi-material assemblies. However, brittle intermetallic compounds (IMCs) are formed at the interface of dissimilar aluminum-steel joints, strongly influencing the mechanical properties of the joints. The identification of such phases is commonly carried out only based on morphology or energy dispersed X-ray spectroscopy (EDS) analysis, potentially leading to incorrect identification. In the present work, a thorough characterization of the IMC layer within dissimilar joints of 6061 aluminum alloy to S235 structural steel is reported. The joints were characterized by electron diffraction-based techniques such as electron backscatter diffraction (EBSD), selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). The EBSD experiments allowed for the identification of a layer of eta phase crystals that grow directly at the top of the steel, followed by another layer of smaller 0 crystals. The SAED and HRTEM analysis allowed for the unequivocal identification of the eta-Fe2Al5 and 0-Fe4Al13 phases. The presence of the ternary Fe4Al17.5Si1.5 phase has been unambiguously identified for the first time within an aluminum-steel dissimilar weld.