Theoretical and Experimental Investigation of Hydrogen Effect on Porosity Formation During Electron Beam Welding of Titanium Alloys

Fusion welded titanium alloys are prone to porosity formation, which is one of the current practical problems during electron beam (EB) welding of titanium alloys and hydrogen is considered responsible for the porosity formation [1-3]; but quantitative research to understand hydrogen behaviour and its effect on porosity formation during welding is currently lacking.

In this work experimental characterisation is carried out using high resolution X-Ray tomography, residual gas analysis, and metallographic sectioning; this confirms that porosity formation is associated with hydrogen evolution (Fig 1-3). a coupled thermodynamic/kinetic model has been developed to understand hydrogen transport during welding of titanium alloys, in which the driving force for hydrogen migration is its chemical potential gradient. The model is applied to the case of electron beam welding of Ti-6Al-4V and prediction of hydrogen distribution at joint shows reasonable agreement with reported value in literature (Fig 4).

To clarify the dependence between porosity formation and hydrogen content in the base material, Ti-6Al-4V samples are electrochemically charged to achieve different hydrogen levels prior to welding (Fig 5). The results confirm that strong hydrogen degassing happened at high hydrogen levels, and the surprising result is that porosity can be suppressed when welding is carried out with optimised EB parameters and perfect joint alignment; on the other hand, porosity is exacerbated when a small beam offset (BOF) is employed. The mechanism of BOF on porosity formation is discussed and it would appear that the nucleation rate in the liquid zone at the melting front determines the likelihood of porosity occurrence.

This work provides insights into the mechanisms of porosity formation during the welding of titanium alloys and guidance to aid in its elimination.