Thermal Cycle Effects on Alpha Case Characteristics in Titanium Welds

摘要

Titanium alloys are ever expanding to applications that require out-of-chamber fabrication. Weld color inspection has been, and will continue to be, the primary method used to assess weld quality. Discolored welds indicate that the welding shielding gases did not completely protect the surface of titanium at temperatures above 450℃ (840℉) from the environment. The color of the titanium oxide can be an indicator of the alpha case thickness under controlled conditions like heat treatment. It has been shown that alpha case can adversely affect the surface ductility and in certain cases should be removed completely. The literature lacks data on alpha case formation on welds where a range of thermal cycle conditions exists from the fusion zone through the heat-affected zone (HAZ). The purpose of this investigation is to characterize alpha case formation on welds and relate the weld thermal cycle conditions and oxide color to alpha case thickness, oxygen penetration, surface bend ductility, and fatigue strength. Commercially pure (CP) titanium Grade 2 and Ti-6Al-4V were used to study the effects of thermal cycle conditions on alpha case formation. A Gleeble~(TM) thermo-mechanical simulator produced alpha case samples using thermal cycles that represent different regions of the HAZ. The samples were thermally cycled in atmospheres consisting of a range of argon-air gas mixtures to produce different alpha case conditions. Work to date has demonstrated a significant impact on fatigue life at high cycles when significant contamination was present. Quantitative metallurgical analysis techniques were used to characterize oxide morphology and oxygen penetration depth in these samples.