Post-weld atmospheric contamination of gas tungsten arc deposited welds in commercially pure and Ti-6Al-4V titanium alloys

Titanium alloys are known to exhibit unique weldability issues, particularly atmospheric contamination from interstitial species such as oxygen. The prevention of such impurities from entering the weld region and surrounding heat affected zones is critical for engineering applications as these species are seen to dramatically alter the microstructure and so mechanical properties of the material. Of greatest concern is embrittlement of the weld region. During this study, the influence of inert shielding gas on atmospheric contamination by interstitial species, namely hydrogen, carbon, nitrogen and oxygen, was investigated as a function of post-weld gas shielding temperature. Gas tungsten arc welding (GTAW) was used to compare samples of commercially pure (CP) titanium and Ti-6Al-4V alloy formed by conventional manufacturing methods. The influence of post-weld gas shielding on mechanical properties was quantified ex-situ through microhardness testing with observations of microstructure also made. It is shown that mechanical hardness levels are inversely proportional to the temperature to which continuous post-weld inert gas shielding is maintained. Hence interstitial impurity levels within the weld and heat affected zones are seen to increase with increasing post-weld shielding temperature. This effect was more pronounced in the Ti-6Al-4V material, and may be attributed to the comparatively low thermal conductivity.