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• Fe 2 Al 5 is the main factor influencing the Fe-Al IMC interface thickness. • Fe 2 Al 5 without Si has obvious (0 0 1) preferential orientation tendency. • Established Fe 2 Al 5 thickness prediction model with calculation error under 10 %. • Si addition is more effective in controlling Fe 2 Al 5 growth compared to energy. Further development of the car lightweight in automotive industry significantly depends on the reliability of the Al/steel dissimilar joint, since more and more Al alloy parts are used in the car manufacturing. It has been proved that the Al/steel dissimilar joint strength can be effectively increased when the intermetallic compound (IMC) growth, especially the Fe 2 Al 5 layer thickness, is controlled at certain level. Because the Fe 2 Al 5 is the dominant phase of the entire IMC layer in the Al/steel dissimilar joint, and cracks usually initiate at this location when damage is conducted. Therefore, for the automotive industry assembly line, the prediction model monitoring the Fe 2 Al 5 layer thickness of each Al/steel weld seam is desired to ensure the dissimilar joint quality and reliability. In the present research, a numerical dynamic growth model based on element diffusion laws is established to calculate the IMC layer thickness of Fe 2 Al 5 phase during the variable polarity cold metal transfer (VP-CMT) welding of aluminum to steel in the function of welding heat input and location. To build up effective connection between the welding heat input and the actual temperature history of Al/steel interface, a finite element analysis (FEA) model is customized for the present VP-CMT process. The Fe 2 Al 5 layer thickness calculation results are further validated using electron back-scattered diffraction (EBSD) characterization under scanning electron microscopy (SEM), confirming a 2.3 % deviation between the model prediction and actual layer thickness. [ABSTRACT FROM AUTHOR]