• TRANSPORT AND FORCED DEPOSITION

The weld mass, including attached and detached fragments, is moved by the rotation of the tool into the trailing edge space behind the tool. The motion is from the RS towards the AS and results in deposition of the material. This deposition process involved squeezing of the weld bulk and the fragments. This involves shear and forging-like processes, whereby the fragments are squashed back together in a plastic process. The shear adds a heating effect which softens the material and facilitates their solid state bonding. As the plasticised mass moves closer towards the AS, so the fragments are flattened and squashed up against the AS—this creates the hourglass border. We propose this is the reason for the increasingly fine spacing between the flow arms closer to the AS compared to the centre of the weld. The final result is thin streaks of flow arms, hence the observed interlaced flow.
