During Friction Stir Welding (FSW) process, mass transfer phenomenon was investigated using Smoothed Particle Hydrodynamics (SPH) method. Temperature distribution, strain rates and shear layer thickness were obtained numerically and an appropriate consistency between the model and the experiments was observed. An index was defined as the portion of transferred material from one side to the other side of the weld centerline. The variations of this index respect to the ratio of rotational to welding speeds (speed ratio) examined and showed that the increasing of speed ratio caused decreasing of the mass deposition at the advancing side, suggesting that high value of speed ratio leads to defects on the advancing side of the weld. Furthermore, it was found that high value of tool linear speed causes to unbalanced distribution of material and defects formation on the centerline of the joint. The effects of three used cross-sections (circular, three-fluted and square) for the pin on the material transfer were determined and it was found that the use of pin with square cross-section creates more uniform joining results and is an appropriate select for non-uniform conditions of welding process or existence of uncertainties about the material properties.

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