MATERIAL FLOW ANALYSIS IN FRICTION STIR WELDING OR PROCESSING

Abstract

Material flow in friction stir welding (FSW) process is a complex phenomenon, understanding of which plays a significant role in getting defect free weld. Additionally, quantification of the material flow characteristic in terms of strain, strain rate is important to obtain the required material properties in the weld region. Various techniques were used by researchers to understand material flow behaviour. In this work, material flow during FSW was investigated by using three different methods; i) by tracer insert technique using the same tracer and workpiece material, ii) in-situ material flow visualization by a new technique called particle image velocimetry, iii) by computational fluid dynamics. Tracer insert technique: three dimensional material flow and strain accumulation during friction stir welding were investigated by inserting tracer extracted from the workpiece material. Same material for tracer and workpiece allowed accurate strain measurement during FSW. Complete material flow behaviour in three dimensions was obtained by embedding tracers in three orientations; transverse, longitudinal and parallel to the processing direction. Three prominent material movements were deduced from the study. First, rotation around tool pin of the material collected from the leading-advancing side and deposited at the trailing-advancing side. Second, vertical movement of the material from the trailing-advancing side in the cavity created at the back of the traversing tool. The flow occurred under the constraint imposed by the rotating shoulder, pin, and relatively hard surroundings. The constraint imposed by different bodies exerted a torque on the soft material and forced it to flow downward in AS and upward in RS. The surrounding hard material acted as a pivot to the vertical rotation. Third, extrusion and deposition of the material layer by layer from leading side towards trailing side via retreating side. Strain at different locations was determined by measuring the thickness of the deformed tracer. The maximum tensile strain of 2 was measured close to the top surface (near shoulder) and a maximum compressive strain of 0.4 was measured at 1.5 mm depth from the top surface. Particle image velocimetry technique: Particle image velocimetry (PIV) technique has been adopted to understand the material flow and strain rate around the tool pin during friction stir welding (FSW). A transparent non-Newtonian fluid of similar viscoplastic behaviour as metallic material was selected as a modelling material. A negligible amount of micro-spherical glass particles were added in the modelling material, both of similar densities. Hence the physical iv properties of the modelling material remained unaltered. The characteristics of material flow, in particular, flow velocity and strain rate were obtained by following the path of the tracer particles. Further, the results of PIV were validated by computational analysis. A rotational zone around the pin was formed due to large deformation of material close to the tool pin. The maximum velocity was found to be 85% of pin peripheral velocity, at 0.1 mm distance from pin surface at 300 rpm and 50 mm min-1. The maximum strain rate was found 100 s-1, at 0.47 mm distance from pin surface, at 300 rpm and 50 mm min-1 and increase from 20 s-1 to 145 s-1 with an increase in rotation 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 is established that the PIV technique can successfully be utilized for the understanding of material flow and strain-rate behaviour during FSW. Computational fluid dynamics: material flow velocity and strain rate around the tool pin during the friction stir welding process was investigated by computational fluid dynamics approach. The experimental results of PIV study were compared with numerical simulation results. The results from experimental and computational works were in good agreement. Maximum velocity and strain rate were found at the pin surface and have shown linear dependence on rotation speed. The maximum strain rate was found to increase from 48 s-1 to 275 s-1 with increase in rotation speed from 75 rpm to 425 rpm. However, at 0.47 mm from pin surface the strain rate was same as measured in PIV technique.