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 mum diam x-ray beam to gather real-time experimental information about the alpha-Ti-->beta-Ti phase transition during weld heating. Six different welding conditions were investigated using SRXRD to experimentally determine the extent of the single phase beta-Ti region surrounding the liquid weld pool. These data were compared to predicted locations of the beta-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 beta-Ti boundary. The differences are attributed to kinetics of the alpha-Ti-->beta-Ti phase transition, which requires superheating above the beta-Ti transus temperature to take place during nonisothermal weld heating. Analysis of the results reveal that the transition to beta-Ti requires an additional 3.96 s (+/-0.29 s) of time and 169 degreesC (+/-25.7 degreesC) of superheat above the beta-Ti transus temperature to go to completion under an average weld heating rate of 42.7 degreesC/s. (C) 2003 American Institute of Physics.