Weldability of superalloys alloy 718 and ATI (R) 718Plus (TM) - A study performed by Varestraint testing
Artikel i vetenskaplig tidskrift, 2017
In this study, the old and well-known alloy 718 is compared with the newly developed ATI (R) 718Plus (TM) from the weldability point of view. This is done in order to gain new information that have not been documented and established yet among the high-temperature materials with high strength, oxidation resistance, thermal stability and sufficient weldability, yet. ATI (R) 718Plus (TM) shows a lower sensitivity to hot cracking than alloy 718 with approximately 10 mm total crack length (TCL) difference in Varestraint testing. In the solution-annealed condition at 982 degrees C for 4.5 h followed by air cooling, the crack sensitivity is decreased as compared to the mill-annealed condition. Along the crack path and also ahead of the crack tip, gamma-Laves eutectic is present in both alloys. The microhardness measurements showed similar hardness level of 250 HV in the weld metal of both alloys and even in the parent material of alloy 718. ATI (R) 718Plus (TM) parent metal had hardness of 380 HV and a small increase of less than 50 HV was observed for both studied alloys in the heat affected zone (HAZ). For the same grain size of ATI (R) 718Plus (TM) (8.3 mu m) and alloy 718 (15.6 mu m), the susceptibility to liquation cracking may increase with increasing grain size. With a small grain size, there is a possibility to accommodate more trace elements (B, S, P) due to the larger grain boundary area. The impurity elements were found in relatively small precipitates, typically borides (0.2 mu m), phosphides (0.1 to 0.5 mu m) and carbo-sulphides. The solidification sequence of alloy 718 and ATI (R) 718Plus (TM) is relatively similar, where the liquid starts to solidify as gamma-phase followed by gamma/MC reaction at about 1260 degrees C and then final gamma/Laves eutectic reaction at around 1150 degrees C. Detailed knowledge about weldability of alloy 718 and ATI (R) 718Plus (TM) can be used for material selection.