EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Intermetallic phases forming at the interface between aluminum and steel during thermal joining strongly affect the reliability of multi-material components. Controlling the thickness of the intermetallic layer is crucial for achieving reliable aluminum-steel joints, as thick layers may significantly impair the mechanical properties of dissimilar joints. The thickness is influenced by the temperature-time cycle of the welding process and the composition of the aluminum-based filler material. In particular, fillers containing silicon are known to decrease the thickness by decreasing the growth rate of Fe2Al5 and Fe4Al13, which are two very common intermetallic phases forming in aluminum-steel welding. This study investigates the effect of silicon on the formation of intermetallic Al-Fe-Si phases, particularly on the formation of the recently discovered long-range ordered η’-, η’’-, η’’’- and ηm-Fe2Al5.

The intermetallic layers formed during Cold Metal Transfer (CMT) welding of aluminum-steel sheets using both silicon-rich (Al-3Si-1Mn) and silicon-poor (Al-4.5Mg-Mn-Zr) fillers were investigated using selected area electron diffraction (SAED) patterns. The patterns revealed that grains with domains of disordered and long-range ordered Fe2(Al)5 phase, which is exclusively identified as η’’’, formed. The other ordered structures of Fe2Al5 have not been observed. Using the silicon-rich filler, wavelength dispersive x-ray spectroscopy (WDX) measurements show that silicon solves heterogeneously in the grains hereinafter named Fe2(Al,Si)5. Transmission Electron Microscopy – Electron Dispersive X-Ray Spectroscopy (TEM-EDS) and Electron Energy Loss Spectroscopy (EELS) measurements within the Fe2(Al,Si)5 grains were done along the predetermined regions of disordered and long-range ordered domains. Silicon solves preferentially in the long-range ordered η’’’-Fe2(Al,Si)5 phase. The results indicate a correlation between the presence of silicon and the occurrence of the disordered and the long-range ordered η’’’-Fe2(Al,Si)5 phase.