<p>This study aims to prevent the possible formation of intermetallic compounds (IMCs) in the weld joints by using Cu/Ni interlayers in rotary friction welded (RFW) AISI2379/AISI420D steel welds. Therefore, Scanning Electron Microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) analyses were carried out to observe the formation with and without interlayered weld joint regions. In EDS analyses, it was observed that in the joints where Ni/Cu interlayer was used, a significant diffusion occurred from the weld zone to the base materials compared to those without interlayer. In the SEM images of the joints with and without the Cu/Ni interlayer, the weld region appeared thinner in samples welded at high rotation speeds and friction times, which is attributed to the material flowing toward the flanges under the combined effect of forging pressure and the temperatures reached. In the weld image without an interlayer, a hard thin line-shaped weld zone was formed. XRD analysis confirmed that this hard line is due to the formation of Fe₃C and the Cu/Ni interlayer acts as a diffusion barrier for Fe and also prevents the formation of Fe₃C intermetallic compounds (IMCs) in the weld zone. In addition, the trends observed in the tensile properties and in the hardness were explained in relation to the microstructure.</p>