Effect of tool eccentricity on the periodic material flow in friction stir welding process

Abstract

The periodic material flow behavior has been widely considered as one of the most crucial characteristics during friction stir welding (FSW) process, but its formation and evolution mechanisms are still not fully understood. Recently, increasing evidences indicate that the original cause of the periodic feature is supposed to be tool eccentricity, rather than geometrical effect of the tool, which is the traditional viewpoint. In this study, numerical simulation based on computational fluid dynamics (CFD) is employed to investigate the effect of tool eccentricity during FSW. A 3D model is first improved for comparison of the thermal-physics responses around the tool in FSW with and without tool eccentricity. Then, a 2D model with refined mesh and volume of fluid (VOF) technique is proposed for detailed material flow during tool rotating, and several tool eccentricity magnitudes are considered. Particularly, the interrelationships between tool eccentricity magnitude, pin orientation, material flow velocity and marker material distribution around the tool are analyzed to elucidate the formation mechanism of the periodic feature as a result of tool eccentricity in detail. It is demonstrated that the tool eccentricity in FSW causes the transformation of material flow velocity around the tool, especially the evident variation of material flow direction, which starts formation of the periodic feature. Moreover, the numerical result is found to be in good agreement with the experimental observation, which verifies the precision of the present method and further confirms the significance of tool eccentricity on the formation of the periodic material flow behavior in FSW.