Observations of solidification cracking in Ti alloy weldments

Solidification cracking in weldments normally results from an inability of the partially solidified fusion zone to withstand contractual stresses produced during cooling [1]. The susceptibility of a particular weldment to such "hot cracking" or "hot tearing" has been shown to be dependent on the interrelated metallurgical and thermomechanical characteristics of the weldzone [2, 3]. Observations of solidification cracking and theories explaining the mechanisms involved have been documented for nearly every major alloy system. Studies performed by Savage and Lundin [4] have shown that titanium alloys are highly resistant to solidification cracking, particularly when compared to aluminum and nickel-base alloys. Nonetheless, studies at the Air Force Materials Laboratory have found that, under certain welding conditions, solidification cracking in titantium alloys can occur. This paper briefly describes the morphological characteristics of solidification cracks in weldments on the metastable-/3 titanium alloy Ti-3AI-13V-l lCr and the ot-fl titanium alloy Ti-6AI-6V2Sn. The application of scanning electron microscopy techniques provided information concerning the microscopic characteristics of these weld defects not obtainable with conventional optical light microscopy. Figure l(a) illustrates a longitudinal centerline crack in an autogenous pulsed-current gas tungsten-arc weldment on Ti-3A1-13V11Cr. Cracking in each weld "crater" of the full-penetration weldment appeared to nucleate at or near the trailing edge of the weld pool from an existing crack in the previously solidified region. Fracture of this weldment when stressed transversely in three-point bending occurred at the longitudinal solidification crack after minimal deformation. Observation of the exposed surfaces showed little evidence of mechanical room-temperature fracture, indicating that solidification cracking essentially separated the