EFFECT OF ACTIVITY DIFFERENCES ON HYDROGEN MIGRATION IN DISSIMILAR TITANIUM-ALLOY WELDS

The effect of alloy composition on hydrogen activity was measured for seven titanium alloys as a means to determine the tendency for hydroge n migration within dissimilar metal welds. The alloys were: Ti-CP, Ti- 3Al-2.5V, Ti-3Al-2.5V-3Zr, Ti-3Al-2Nb-1Ta, Ti-6Al, Ti-6Al-4V, and Ti-6 Al-2Nb-1Ta-0.8Mo. Hydrogen pressure hydrogen concentration relationshi ps were determined for temperatures from 600-degrees-C to 800-degrees- C and hydrogen concentrations up to approximately 3.5 at. pct (750 wpp m). Fusion welds were made between Ti-CP and Ti-CP and between Ti-CP a nd Ti-6Al-4V to observe directly the hydrogen redistribution in simila r and dissimilar metal couples. Hydrogen activity was found to be sign ificantly affected by alloying elements, particularly Al in solid solu tion. At a constant Al content and temperature, an increase in the vol ume fraction of beta reduced the activity of hydrogen in alpha-beta al loys. Activity was also found to be strongly affected by temperature. The effect of temperature differences on hydrogen activity was much gr eater than the effects resulting from alloy composition differences at a given temperature. Thus, hydrogen redistribution should be expected within similar metal couples subjected to extreme temperature gradien ts, such as those peculiar to fusion welding. Significant hydrogen red istribution in dissimilar alloy weldments also can be expected for man y of the compositions in this study. Hydride formation stemming from t hese driving forces was observed in the dissimilar couple fusion welds . In addition, a basis for estimating hydrogen migration in titanium w elds, based on hydrogen activity data, is described.