Weldability and Testing Methodology in Precipitation Hardening Superalloys

Sammanfattning: Usable weldability data is desired in the manufacturing industry, especially within the aerospace industry where fabrication of structural jet engine components are realized. Welding of precipitation hardening superalloys such as Waspaloy®, Alloy 718, ATI® 718Plus™ and Haynes® 282® in particular can lead to solidification cracking in the fusion zone, liquation cracking in the heat affected zone and/or to solid state cracking. This concern requires some kind of weldability testing such as Varestraint testing to improve the fundamental knowledge on how to prevent this type of cracks from occurring. It was found that the micro- hardness for all four alloys is approximately 250HV in the weld metal while the parent metal differs more, 208HV for Haynes® 282®, 243HV for Alloy 718, 340HV for Waspaloy® and 384HV for ATI® 718Plus™. The hardness in the HAZ reaches about 400HV for Waspaloy® and ATI® 718Plus™, while Alloy 718 and Haynes® 282®approach 250-350HV. The grain size is smallest for ATI® 718Plus™ (8.3 µm) and Alloy 718 (16µm) followed by Haynes® 282® (64µm) and Waspaloy® (90µm). Simulation using JMatPro suggested a larger amount of γ' in ATI® 718Plus™ compared to Alloy 718. In Haynes® 282®, the sigma-phase and M6C levels are higher compared with those in Waspaloy®, for which M23C6 was found instead. Based on measurements, system analyses and design of experiment it was concluded that the lowest variation in evaluating weld cracking can be achieved with the method using penetrant combined with the use of one operator. The welding speed affected the variation in weld cracking most followed by current, die mandrel radius and the bending stroke rate. Testing parameters with lowest standard deviation/mean Total Cracking Length (TCL)-values are here found for welding speed of 1mm/s, weld current of 70A, die mandrel radius of 60mm and bending stroke rate of 10mm/s. The compression strains in the lower part of the specimen during the bending at Varestraint testing have no significant impact on the weld cracking. Based on Varestraint testing of Alloy 718 and Waspaloy®, there was similar cracking response at 1.1% to 4.3% augmented strain and if extrapolated downwards the critical strain from crack initiation approach zero. Similar to Alloy 718 and Waspaloy®, it was also found that ATI® 718Plus™ and Haynes® 282® both seemed to have a level of critical augmented strain of around 1% while at the highest strain level of 8.6% Haynes® 282® showed somewhat higher susceptibility values. The lower susceptibility to hot cracking in ATI® 718Plus™ compared to Alloy 718 and Haynes® 282® is supposed to be associated with the smaller grain size of ATI® 718Plus™ despite of its higher hardness. The HAZ liquation cracking in Haynes® 282® seems to be connected to Ti-Mo based MC-type carbides.