Ductile fracture based joint formation mechanism during friction stir welding

Abstract

Material movement during friction stir welding plays a significant role towards joint formation. However, there is little clarity on periodic or continuous nature of material movement. The objective of this work is to understand the mechanism of joint formation during friction stir welding, by investigating the periodic formation of a cavity and if formed, cavity's role towards the joint formation. A two dimensional numerical methodology is developed to capture the material flow around the tool pin. The plastic limit load model is used to predict the formation of microvoids, growth and coalescence. Friction stir welds are made with AA 1050 at different tool travel speeds, for same tool rotation frequency across experiments. Computed tomography of friction stir welded samples is performed to study the size and morphology of the discontinuities. The chemical constituents, size and distance between inclusions are determined from scanning electron microscopy and energy dispersive spectroscopy of the as-received AA 1050. The numerical model suggests that the stress and strain during friction stir welding of AA 1050 is sufficient for nucleation and growth of the voids at the inclusion sites. Further the voids coalesce to form a cavity in all the cases. The incoming material fills the cavity per revolution. However, in some cases the cavity is partially filled which leads to the discontinuities in the weld. For the welds with discontinuities, the area of unfilled cavity is predicted to be larger in advancing side as compared to retreating side. The discontinuity size increases as the travel speed increases. The tunnel discontinuity is predicted by super-positioning of the discontinuities formed over consecutive passes along the feed direction. The discontinuity size and shape trends compare well against the experimental observations. This work would be helpful towards developing strategies for detecting and mitigating the discontinuities formed during friction stir welding.