Localized interstitial element analysis in defect regions of a near alpha titanium alloy fusion weld

Abstract

This study was conducted on a fusion-welded specimen of Ti-6Al-2Nb-1Ta-0.8Mo to identify the cause of extensive cracking and porosity observed in the weld. Localized interstitial element measurements by nuclear reaction analysis techniques established that various regions in the weld and the base metal contained undesirably high concentrations of oxygen, carbon and hydrogen. A 5μm thick surface layer of the as-received base metal, found to be rich in oxygen (16 000 w.p.p.m.), carbon (2700 w.p.p.m.), and hydrogen (425 w.p.p.m.), could have led to the multitude of weld defects observed in this work. In the weld and heat-affected zone, the hydrogen and carbon concentrations near various defects, such as cracks and porosity, were considered normal (i.e. ∼50 w.p.p.m. and 160 w.p.p.m., respectively). There were high oxygen concentrations (∼3000 w.p.p.m.) near weld defects, which were two to three times higher than unwelded base metal. A direct correlation between oxygen content and microhardness showed that crack defect regions were characterized by both high oxygen and hardness, thereby indicating that oxygen played a major role in causing embrittlement and subsequent cracking.