[en] Intermixing of bilayer systems induced by swift heavy ions has been clearly evidenced and it was suggested that the energy transferred from the incident ion to the target electron subsystem (electronic energy loss, S_e) could play an important role in the mixing process. However, the mechanism in the intermixing of bilayer systems is still an open question. In the present work, our aim is to try to explain the intermixing in metallic bilayer systems especially the effects induced by S_e. Under the framework of thermal spike model, intense S_e may result in warming up of the target atoms and the intermixing can be attributed to interdiffusion in molten ion tracks where the mixing efficiency correlates to the molten duration τ_m and radius R_m of the ion tracks at the interface. With an extension of the thermal spike model in a three-dimensional space, time dependent size of the molten ion tracks in multilayer systems can be numerically simulated and then the intermixing properties can be qualitatively predicted. For Ni/Ti bilayer systems, as an example, simulations performed with known input parameters suggested that melting could be achieved at the Ni/Ti interface with GeV Ta or U ion irradiation. It implies that intermixing could occur at the Ni/Ti interface even though bulk Ni is insensitive to S_e. Furthermore, the larger the S_e value, the larger the τ_m and R_m values and the larger the intermixing