A study of process-induced grain structures during steady state and non-steady state electron-beam welding of a titanium alloy

A detailed microstructural characterisation of the emerging weld-line grain structure, for bead-upon-plate welds in Ti-6Al-4V (Ti64) of differing plate thickness, was performed. The microstructure studied was formed during both steady state and non-steady state sections within the weld path, with the non-steady state portion being taken from the end of the plate as the weld bead and heat source overhang the edge of the plate. This allows for the effects of welding process conditions on the microstructural evolution to be determined. The weld pool geometry and 3D tomography of the weld-induced defects have been investigated. Detailed characterisation of microstructure and texture for different welding parameters and for steady and non-steady states have been used to identify physical parameters for the microstructure predictions that are difficult to obtain otherwise. The different states significantly affect the weld crown shape and formation, weld toe, weld bead depth and width. However, the heat affected zone (HAZ) remains unchanged. Regarding the microstructural evolution, both the steady and non-steady states have similar microstructure and texture. No defects were observed in the steady state section of welds, but sub-surface spherical pores have been observed in the non-steady state section of a weld. Finite element modelling to simulate the thermal-metallurgical-mechanical fields within the steady and non-steady state sections of the welds was considered, and the cooling rates predicted within steady state and non-steady sections were interrogated to improve the theoretical understanding of the microstructure and defect formation differences in these Ti64 EB weld regions.