In situ observations of phase transitions in Ti–6Al–4V alloy welds using spatially resolved x-ray diffraction.

In situ spatially resolved x-ray diffraction (SRXRD) experiments were used to directly observe the heat-affected zone phases present during gas tungsten arc welding of a Ti-6Al-4V alloy. The experiments were performed at the Stanford Synchrotron Radiation Laboratory using a 250 µm diam x-ray beam to gather real-time experimental information about the α-Ti→β-Ti phase transition during weld heating. Six different welding conditions were investigated using SRXRD to experimentally determine the extent of the single phase β-Ti region surrounding the liquid weld pool. These data were compared to predicted locations of the β-Ti phase boundary determined by calculated weld thermal profiles and equilibrium thermodynamic relationships. The comparison shows differences between the experimentally measured and the calculated locations of the β-Ti boundary. The differences are attributed to kinetics of the α-Ti→[sub -]Ti phase transition, which requires superheating above the β-Ti transus temperature to take place during nonisothermal weld heating. Analysis of the results reveal that the transition to β-Ti requires an additional 3.96 s (± 0.29 s) of time and 169 °C (± 25.7 °C) of superheat above the β-Ti transus temperature to go to completion under an average weld heating rate of 42.7 °C/s. [ABSTRACT FROM AUTHOR]