Understanding the effect of tool geometrical aspects on intensity of mixing and void formation in friction stir process

The reduction of agglomeration of particulate reinforcement in friction stir processing and elimination of joint line remnant in friction stir welding depends on stirring ability of friction stir tool. The magnitude of stirring is indicated by the intensity of material inter-mixing in tool interacted region. Present investigations analyse the influence of geometrical aspects of classical friction stir tool on the intensity of material intermixing, location of high stir regions and void formation during the steady-state phase of friction stir welding. The classical tool was chosen as its geometrical features undergo minimal wear in the harsher processing environment. Investigating material mixing using metallic inserts have their shortcomings due to different flow properties of insert/marker and parent material. Therefore, bi-colour plasticine configuration possessing primary colours was adopted to understand the level of intermixing. The hue attribute of the generated secondary colour was utilised to identify and quantify material mixing. Experimental results revealed that the pin diameter positively influenced mixing and negatively affected void formation. Shoulder size and pin taper angle aided in the closure of void with inferior mixing. The intensity of mixing enhances on selection of tools possessing a shoulder to pin diameter ratio of 2.5. Finite element simulation studies were conducted to understand the reasons for mixing under certain tooling conditions. Since material subjected to stirring is concentrated primarily on a portion of advancing side, components of shear strain, shear strain rates, shear stress and velocity were tracked for this portion of processed material during its interaction with tool pin for one complete rotation.