Varestraint Weldability Testing of Cast Superalloys

Licentiate thesis, 2018

Precipitation hardened Ni- and Ni-Fe- based superalloys, used in hot structural components for aero engines, are subjected to hot cracking phenomena during manufacturing and specifically in welding. This type of cracking involves metallurgical reactions from the liquid phase in concomitance to weld restraints. Hot cracking is more pronounced during the welding of cast superalloys compared to its wrought counterparts owing to the larger inhomogeneity in the cast material. Cast structural components must therefore undergo a hot isostatic pressing (HIP) treatment prior to welding for closing any pores from the casting process and homogenizing the material. The current study is about the effect of two HIP treatments commonly employed in the aerospace industry on the weldability of the cast Alloy 718. Moreover, the weldability of the recently introduced cast version of ATI® 718PlusTM is also addressed, including the assessment of five different pseudo-HIP treatments and the weldability of the two cast superalloys is compared. Testing was conducted using the Varestraint (Variable-Restraint) weldability testing technique, where test plates were welded by a robot and at the same time bended to achieve external restraint. Welding and test parameters were the same for all the conditions. The hot cracking susceptibility of cast Alloy 718 was found to be worse with increasing HIP treatment temperature. It was found that HIP treatment conducted at 1190°C/4h gave higher crack susceptibility than the one at 1120°C/4h. Cast ATI® 718PlusTM was heat treated using short dwell heat treatments for 4h at 1120°C, 1160°C and 1190°C and long dwell heat treatments for 24h at both 1120°C and 1190°C, respectively. The cracking susceptibility was found to be related to the heat treatment dwell time rather than the temperature. The heat treatment at 4h dwell time exhibited lower amount of cracking than the 24h dwell heat treatment. Considering the comparative heat treatments at 1120°C/4h and 1190°C/4h, the new cast superalloy ATI® 718PlusTM was found to have lower cracking susceptibility than the cast Alloy 718.