The characterisation and modelling of porosity formation in electron beam welded titanium alloys

Abstract

This thesis is concerned with the porosity formation mechanism during electron beam welding of titanium-based alloys. During the welding of titanium alloys for aerospace engine applications, porosity is occasionally found in the solidified welds. Hence the key factors responsible for porosity formation need to be identified, and guidance to minimise porosity occurrence needs to be provided.

The research conducted in this work is twofold. First, porosity formed in electron beam welded titanium samples is characterised to rationalise the porosity formation mechanism. Second, models based on sound physical principles are built to aid understanding of porosity formation, and to provide predictive capability.

Porosity formed in electron beam welds is characterised using metallographic sectioning, high resolution X-ray tomography, residual gas analysis (RGA), scanning electron microscopy (SEM) and energy and wavelength dispersive spectroscopy (EDS/WDS) analysis. The results confirm porosity formed in electron beam welded titanium-based alloys is associated with gas dynamics; hydrogen is very likely to be responsible for porosity formation. A coupled thermodynamic/kinetic model is proposed to study the hydrogen migration behaviour during electron beam welding process, and then the effect of hydrogen on bubble formation is investigated via quantitative modelling, backed up by targetted experimentation