Weldability of Precipitation Hardening Superalloys – Influence of Microstructure

Abstract: Superalloys and in particular the precipitation hardened Ni-based superalloys havealways been used extensively in the hot sections of jet engines. Large hot structuralengine components with complex geometry have preferably been cast as single piececomponents since the large scale vacuum investment casting process becameavailable about fifty years ago. However, a recent trend is to cast smaller pieces whichcan be joined with sheet or forged parts to fabricate structural components. Therationale for this fabrication strategy is the possibility to save weight by the use of higherstrength wrought material, where geometry allows, and join these wrought parts withcast material where complex geometry is needed and where the demand for strength ismoderate. One of the major challenges using this strategy is the obvious fact thatnumerous welds must be made which requires the fundamental understanding, notleast metallurgical, of how different materials may be joined by specific weldingprocesses.The main objective of this research has, for this reason, been to examine and interpretthe weldability of precipitation hardened superalloys from a metallurgical standpoint.Two newly developed superalloys Allvac® 718PlusTM and Haynes® 282® are comparedwith the two well established Alloy 718 and Waspaloy. The understanding of theinfluence of secondary phases such as carbides and δ phase in the microstructure wasaddressed by systematic hot ductility testing (Gleeble) and by weldability testing(Varestraint). The effect of secondary phases were also analysed through practicalwelding as by electron beam welding (EBW), and by gas tungsten arc welding (GTAW).The research showed that all the techniques used (Varestraint testing, Gleeble testing,DSC thermal analysis and welding (GTAW repair and EBW)) in studying the weldabilityindependently provided important knowledge and most importantly that a combination ofthe results from these different techniques were necessary for the understanding of theweldability of these four alloys. From a microstructural point of view it has been possibleto show that δ phase contrary to what has generally been assumed improves theweldability due to its ability to inhibit grain growth and to assist in the healing of cracks.For future research, a new modified weldability testing method was developed where itis possible to perform Varestraint, Transvarestraint and spot-varestraint testing at ramspeeds from 15 to 300 mm/s using GTAW, plasma arc welding and laser welding.