Quasi-in-situ observation of microstructure at the friction interface: Shear deformation, dynamic recrystallization and mechanical responses during friction welding process

Friction based joining processes are monitored and controlled according to the collectable and measurable mechanical responses, such as torque or temperature, during the process. These are a result of the underlying physical microstructural mechanism during the process, where joints are formed under shear deformation (SD) and/or dynamic recrystallization (DRX). To ensure a first quality assessment of the joints during processing, it is critical to precisely investigate the relation between (macro)-mechanical and microstructural responses (SD and DRX). In the present study, the transition from SD to DRX in friction welding has been focused and quasi in-situ observed by ‘stop - action’ rotary friction welding (RFW) experiments coupled with electron back-scattered diffraction (EBSD) analysis using pipe structures, which clarifies the characteristics of the mechanical response. Further RFW experiments with different parameters were conducted to obtain a suitable relation that correlate the DRX transition temperatures to the welding parameters. Thereafter, further ‘stop - action’ RFW experiments were performed on rod structures to investigate the spatial - temporal distribution of SD - DRX at the friction interface and accordingly the friction torque characteristics. The results show that the transition from SD to DRX takes place at the peak torque (PT) and the temperature inflection point (TIP). The TIP of pipe-structure specimens is the critical DRX temperature during FW, which is dominated by friction linear speed. The PT is the threshold that distinguishes the dominating mechanism, SD or DRX, at the welding interface when welding rod structures.