The influence of rotational friction welding using the FHI method on the structure of the UFG material in the joint area

Abstract Ultrafine-grained (UFG) materials, characterized by an average grain size below 1 μm and a high fraction of high-angle grain boundaries (HAGBs), offer superior mechanical properties but are highly sensitive to thermal input during joining processes. Conventional rotary friction welding (RFW) methods typically exceed the recrystallization temperature, leading to grain coarsening and degradation of mechanical performance. In this study, a novel friction heat impulse (FHI) method was applied using a prototype RFW machine to join 6-mm-diameter Cu-ETP copper rods with a UFG structure produced via hybrid severe plastic deformation (SPD). The FHI method enables extremely short welding times (< 200 ms), significantly reducing heat input. Microstructural and mechanical characterization of the joints was performed using electron backscatter diffraction (EBSD), microhardness testing, and tensile testing. The results revealed a reduction in average grain size from 0.9 to 0.5 μm in the weld zone, an increase in the HAGBs fraction from 50 to 80%, and the elimination of elongated grains resulting from prior SPD processing. The joint exhibited an ultimate tensile strength of 480 MPa and hardness comparable to the base UFG material (136 HV), indicating no degradation of mechanical properties. These findings demonstrate that the FHI method enables the successful welding of UFG copper without structural or mechanical deterioration, offering a promising approach for future applications involving advanced UFG materials.