Cooling rate effects in Ti-6Al-2Sn-4Zr-2Mo weldments

Weldment performance in near-alpha titanium alloys is dependent primarily on the structure and properties of the fusion zone. For gas tungsten-arc welds in sheet materials, the columnar beta grain morphology associated with the fusion zone does not vary appreciably with weld energy input. As a result, the control of mechanical properties in this region can only be accomplished by controlling the transformed-beta microstructure. An early study of continuous-cooling phase transformations in the near-alpha titanium alloy Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) investigated the effect of cooling rate on microstructure of simulated weld specimens. Although the fine scale of the beta decomposition products limited their effective characterization using light microscopy, the investigation did recognize the complex nature of microstructures produced at intermediate cooling rates between about 10 and 1000C per second. A more recent transmission-electron microscopy study of a gas tungsten-arc weldment in Ti-6242 confirmed the complexities associated with both the fusion and heat-affected zone microstructures, and determined these regions to be different combinations of diffusional alpha and alpha-prime martensite. Since cooling rates experienced in the fusion zone of gas tungsten-arc welds in titanium can range from about 5 to 1250C per second, it would be desirable to have a more complete understanding of how cooling rate influences both the weld fusion zone microstructure and mechanical properties. The purpose of this study was to develop this understanding