Weldability of Cast Superalloys - Effect of homogenization heat treatments on hot cracking susceptibility of cast Alloy 718, ATI® 718Plus®, and Haynes® 282®

Abstract: Precipitation hardened Ni- and Ni–Fe-based superalloys are used in high-temperature sections of aero engines owing to their superior mechanical properties compared to those of the other alloys. However, their better mechanical performance is accompanied by its own challenges during the manufacturing process. For the fabrication of hot structural components, instead of the traditional single piece castings, welding is widely employed by joining wrought parts in sections, where high strength is required, and cast parts, where complex geometrical shapes are needed. This can be challenging, as superalloys are known for their lack of amenability to welding. A weld-cracking phenomenon known as “hot cracking”is of concern during their welding. Especially, the cast materials are known to be more prone to cracking owing to the higher extent of segregating phases that remain from the casting process.The present study investigates the weldability of Alloy 718 and two recently developed Ni-based superalloys ATI® 718Plus® and Haynes® 282® with respect to heat affected zone liquation (HAZ) cracking susceptibility. Pre-weld homogenization treatments were performed at 1120 °C and 1190 °C to study the effect of different microstructures on cracking extent. The testing approach consisted of using Varestraint weldability test to assess the HAZ liquation cracking susceptibility and Gleeble thermomechanical simulator for evaluation of hot ductility behaviour. The results revealed that a lower heat treatment temperature at 1120 °C for 4 h was beneficial in minimizing the influence of liquation, and that the grain growth also contributed to lowering the cracking susceptibility in the HAZ. JMatPro simulations and microstructural evaluation on elements such as Nb as solute and precipitate former in Alloy 718 and ATI® 718Plus®, and Mo in Haynes® 282® were found to be important in the liquation mechanism. Secondary ion mass spectroscopy (SIMS) analysis revealed B, which is a strong melting point depressant, to segregate along the grain boundaries in all the three alloys. In addition, in this thesis, different liquation mechanisms were discussed and an explanation for the overall HAZ liquation cracking mechanism for cast superalloys was proposed.