The effect of hydrogen on porosity formation during electron beam welding of titanium alloys

Titanium and its alloys are prone to hydrogen-assisted porosity formation during welding, but this effect is not yet sufficiently understood. Research aimed at elucidating the behaviour of hydrogen during electron beam welding of Ti- 6Al-4V is presented. 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. To quantify the dependence between porosity formation and hydrogen content in the base material, a hydrogen diffusion-controlled bubble growth model is used to simulate bubble growth in the melt, and thus to make predictions of the hydrogen concentration barrier needed for pore formation. The modeling results are supported up by experimentation on Ti-6Al-4V of different hydrogen levels, achieved by electrochemical charging. The results confirm that vigorous hydrogen degassing happens at high hydrogen levels. But porosity can be suppressed when welding is carried out with optimized welding parameters and perfect joint alignment; on the other hand, porosity is exacerbated when a small beam offset is employed. The influence of beam offset on porosity formation is discussed. It would appear that the nucleation rate in the liquid zone at the melting front determines the likelihood of porosity occurrence.