Mechanism of single centreline grain formation in titanium alloy welds

Abstract

The formation of a single through thickness grain found in the centreline of titanium alloy Ti–6Al–4V thin sheet welds was investigated. Based on the numerical and experimental results, a new mechanism is proposed that considers the dendrite growth velocity and orientation and their influences on the solid/liquid interface temperature. This new mechanism, involving dendrite tip undercooling and competitive growth concepts, can be used to explain the formation and stabilisation of the single grain and grain structure in the weld. It was found that the centreline single grain is stabilised by its dendrites along a favourable 〈100〉 direction and its protrusion into the trailing edge of the weld pool, as a consequence of it growing at a lower velocity with less undercooling than the dendrites in the adjacent grains. In addition, the results suggest that the formation of the single through thickness centreline grain is also dependent on the weld geometry controlled by the welding heat input and travel speed. The macroscopic structure and growth directions of the weld pool must be such that the centreline grain can first become established.