Material adhesion and stresses on friction stir welding tool pins

Abstract

During friction stir welding, polygonal tool pins experience severe stresses and, under certain conditions, loss of functionality due to adhesion of plasticised material on their surfaces. The extent of adhesion is analysed for various pin geometry and welding conditions based on the theory of machining. The effective stresses on the polygonal pins are evaluated following the principles of mechanics. The results show that the polygonal pins with fewer sides can avoid permanent adhesion of plasticised material at higher weld pitch, which is defined as a ratio of welding speed and tool rotational speed. The computed pin geometries for minimum adhesion are compared with the pin profiles recommended by various investigators based on independent experiments. The computed stresses show that pins with larger number of sides will experience lower stresses for any given set of welding variables.

Numerical modeling of friction stir welding using the tools with polygonal pins

“…(4) can therefore be used to estimate the length of contact between each pin side of the polygonal tool and plasticized material in FSW. In case of FSW, the thickness t of the deformed chip matches the layer thickness of the plasticized material around the pin and can be estimated as [22] t…”

“…The pin profiles with the regular polygon shape are preferred in comparison to the complex non-circular shapes because of the ease of manufacturing of the former ones. The relative performance and the longevity of the tools with circular and non-circular pins during FSW were also reported recently [22]. Although many of these studies have indicated the improved performance of the tools with the polygonal pins in comparison to the circular pins, the influences of the polygonal pin cross-section on the peak temperature, and tool torque and forces have been rarely studied using a quantitative numerical model.…”

“…The chips in machining experience plastic deformation and shear fracture along a plane that is inclined at an angle a, which is known as shear angle and estimated as [22,29,30] …”

“…The chip-tool contact length in machining is estimated, presuming that the boundary of plasticized chip region meets the rake face at 45 0 [22,29,30] …”

“…The area of contact between the flat faces of polygonal pins and the plasticized material is estimated based on the principles of orthogonal machining [22]. A three-dimensional steady state heat transfer model of FSW process is developed using finite element method to compute the temperature fields in the workpiece and the tool pin.…”