Material flow visualization and determination of strain rate during friction stir welding

Particle image velocimetry (PIV) technique was adopted to understand material flow and measure strain rate around the tool pin during friction stir welding (FSW). The micro-spherical glass tracers in a transparent viscoplastic material, of almost similar densities, were used as experimental materials. The characteristics of material flow, in particular, flow velocity and strain rate were obtained by following the path of the tracer particles. A rotational zone around the tool pin was observed due to large deformation of the material close to the tool pin. The maximum velocity was noted to be 60% (close to the pin surface) of the pin peripheral velocity, and strain rate was found to be 20 s(-1) (0.6 mm away from the pin periphery) at FSW parameters of 170 rpm and 50 mm min(-1). The strain rate was found to increase from 8 s(-1) to 44 s(-1) with increase in rotational speed from 75 rpm to 425 rpm. Predictive correlations were established for variations of velocity and maximum strain-rate as a function of rotational, traverse speeds and distance away from the tool pin surface. Overall, it was established that PIV technique can be utilized for the understanding of material flow and strain-rate behaviour during FSW. Furthermore, this technique enabled in-situ visualization overcoming drawbacks of other techniques reported in the literature.