Controlling the thickness of the intermetallic seam, which forms in the thermal joining of aluminum alloys with steels, is crucial for achieving high-quality joints, as thick seams may significantly impair the mechanical properties of dissimilar joints. The seam thickness is influenced considerably by the temperature-time cycle of the welding process and by the composition of the aluminum-based filler. Fillers alloyed with Silicon are known to reduce the thickness, as they decrease the growth rates of the common intermetallic layer components η-Fe2Al5 and θ-Fe4Al13, with η-Fe2Al5 usually being the dominant phase. The presence of Si also leads to the formation of additional intermetallic phases at the θ/Al-alloy interface, the αc-Al(Fe,Mn)Si phase. Moreover, Silicon is found to be incorporated in long-range ordered regions of the Fe2Al5 phase. This study investigates the effect of varying silicon content in silicon-rich Al bal.-3Si-Mn and silicon-poor Al bal.-4.5Mg-Mn-Zr (wt.%) fillers during welding on the formation of the intermetallic seam. The intermetallic layers formed during Cold Metal Transfer welding of aluminum-steel sheets using both were researched using Transmission Electron Microscopy - Selected Area Electron Diffraction patterns. The patterns revealed that grains with areas of short-range η-Fe2Al5 and long-range ordered η’’’-Fe2Al5 phase formed. Using Al-3Si-Mn filler, Si has been measured within Fe2Al5 grains by wavelength dispersive X-ray spectroscopy. A detailed analysis by Transmission Electron Microscopy – Electron Dispersive X-Ray Spectroscopy, and Electron Energy Loss Spectroscopy was conducted along regions of short-range and long-range ordered areas of Fe2Al5 grains. It was observed that more Silicon is contained in the long-range ordered η’’’-area, likely exchanging with Al, which suggests the phase formula Fe2(Al,Si)5. These 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. Electron Backscatter Diffraction confirmed αc-Al(Fe,Mn)Si exclusively in the weld sample with high-Si filler at the θ/Al-alloy interface.