Wooldridge, M: Development of SLM Ni-based Superalloys

The adoption of Additive Manufacturing has been slow in the Oil & Gas (O&G) industry due to uncertainty regarding corrosion performance. Ni-based superalloys are valued in the O&G industry for their high strength and good corrosion resistance, but literature to-date has focused primarily on the aerospace grade of 718 rather than the API grade which is tailored for O&G applications. Here, the processing-microstructure-property relationships are developed for two Ni-based superalloys, API 718 and Alloy 945X, produced with Selective Laser Melting (SLM). First, a processing parameter set tailored for achieving full density is developed for Alloy 945X. Through the application of response surface methodology, a model is created which relates laser power, scan speed, and hatch distance in order to efficiently identify successful parameter combinations. Second, processing-microstructure-property relationships are established through tailored heat treatments. Heat treatments are evaluated on the quality of the resulting microstructure, and quasi-static mechanical testing is conducted. Heat-treated SLM API 718 exceeds the published requirements for the tested properties, and heat-treated Alloy 945X meets all requirements with the exception of yield strength. Finally, the stress corrosion cracking (SCC) behavior of these materials is evaluated using slow strain rate testing in a typical O&G environment. Both SLM alloys are determined to be resistant to SCC in the tested environment, with Alloy 945X performing slightly better than API 718. The additive manufacturing method is determined to have no influence on the corrosion behavior of these alloys.