Weldability of Precipitation Hardening Superalloys – Influence of Microstructure

Doctoral thesis, 2011

Superalloys and in particular the precipitation hardened Ni-based superalloys have always been used extensively in the hot sections of jet engines. Large hot structural engine components with complex geometry have preferably been cast as single piece components since the large scale vacuum investment casting process became available about fifty years ago. However, a recent trend is to cast smaller pieces which can be joined with sheet or forged parts to fabricate structural components. The rationale for this fabrication strategy is the possibility to save weight by the use of higher strength wrought material, where geometry allows, and join these wrought parts with cast material where complex geometry is needed and where the demand for strength is moderate. One of the major challenges using this strategy is the obvious fact that numerous welds must be made which requires the fundamental understanding, not least metallurgical, of how different materials may be joined by specific welding processes. The main objective of this research has, for this reason, been to examine and interpret the weldability of precipitation hardened superalloys from a metallurgical standpoint. Two newly developed superalloys Allvac® 718PlusTM and Haynes® 282® are compared with the two well established Alloy 718 and Waspaloy. The understanding of the influence of secondary phases such as carbides and δ phase in the microstructure was addressed by systematic hot ductility testing (Gleeble) and by weldability testing (Varestraint). The effect of secondary phases were also analysed through practical welding 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 weldability independently provided important knowledge and most importantly that a combination of the results from these different techniques were necessary for the understanding of the weldability of these four alloys. From a microstructural point of view it has been possible to show that δ phase contrary to what has generally been assumed improves the weldability 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 it is possible to perform Varestraint, Transvarestraint and spot-varestraint testing at ram speeds from 15 to 300 mm/s using GTAW, plasma arc welding and laser welding.