Controlling the thickness of the intermetallic seam that forms during thermal joining of aluminum alloys with steels is crucial for achieving high-quality joints since the seam thickness strongly impacts the mechanical properties of dissimilar joints. Next to the temperature-time cycle of the welding process, the seam thickness is significantly influenced by the aluminum-based filler. This study used silicon-rich (Al-3Si-1Mn) and silicon-poor (Al-4.5Mg-Mn-Zr) fillers in a Cold Metal Transfer welding process to join aluminum-steel sheets. The impact of silicon on the intermetallic layer constituents η-Fe₂Al₅ and θ-Fe₄Al₁₃ and the formation of additional intermetallic phases, e.g., α_c-Al(Fe,Mn)Si, at the θ/Al-alloy interface, was investigated. Moreover, areas of short-range ordered η-Fe₂Al₅ and long-range ordered η’’’-Fe₂Al₅ phase were observed.