In-situ observation of plastic material flow and interfacial condition in friction stir-based processes via particle image velocimetry

Friction stir processes exhibit unique advantages in avoiding solidification-related defects in welding and additive manufacturing applications. These processes involve complex plastic flow of materials, which are critical for the product microstructure and properties but have not been well understood. The lack of quantitative measurements further prevents the quantitative distinction between plasticity versus friction induced heating analysis. This work thus presents an experimental work to visualize and quantify the material flow and frictional behavior under a friction stir spot welding process. Optically transparent polymethyl methacrylate (PMMA) was used as the substrate, and tracer particles were placed at the tool-material interface to visualize the plastic flow. The particle image velocimetry (PIV) algorithm was used to quantify the flow velocity. The experiments, for the first time, directly revealed the stick- and slip-dominant zones on the tool-material interface and their transition along the tool radial direction. The results were compared with a physics-based analytical model to understand the effects of processing conditions on the material deformation mechanisms and plastic flow behavior in the process.