An Evaluation of the Mechanical Properties and Sulfide Stress Cracking of Additively Manufactured Alloy Ti-6Al-2Sn-4Zr-6Mo for Sour Geothermal Use

Abstract Because of their outstanding corrosion resistance in hydrogen sulfide (H2S) and carbon dioxide (CO2) environments, titanium alloys such as Ti-6Al-2Sn-4Zr-6Mo (Ti-6246) can excel in geothermal sour applications that require high strength. With availability restricting the use of this titanium grade, an additive manufacturing (AM) solution was developed using Laser Powder Bed Fusion (LPBF) and specially ordered powder. AM Ti-6246 specimens, as printed and postprocessed (including stress relieved, hot isostatic pressed, solution annealed, and age hardened), were examined for their mechanical properties, microstructures, and corrosion resistance and further evaluated for sulfide stress cracking (SSC) using NACE test methods. Despite having different microstructures, acicular in AM Ti-6246 Vs. equiaxed in its wrought counterpart, both alloys showed similar mechanical properties under an identical heat treatment. The resistance to SSC of AM Ti-6246 was further compared with that of wrought Ti-6246 in 180,000 mg/L chloride having 1-g/L sulfur at partial pressures of 1,000-psi (6.89-MPa) H2S and 500-psi (3.45-MPa) CO2 at 450°F (232°C). AM Ti-6246 was found to successfully pass SSC testing. With further optimization of the heat treatment, the mechanical properties of AM and wrought alloys were practically matched, with only a slight reduction in corrosion resistance remaining above the typical minimum requirements.