Comparative susceptibility of VDM Alloy 780 and Alloy 718 to weld heat-affected zone liquation cracking: A hot ductility recovery study

Hot ductility recovery determines the alloy's susceptibility to heat-affected zone (HAZ) liquation cracking during welding. In this work, the hot ductility recovery of a next-generation Ni-based superalloy, VDM Alloy 780, was physically simulated using a Gleeble thermomechanical simulator and compared with that of the widely used Alloy 718 to evaluate their relative susceptibility to HAZ liquation cracking. VDM Alloy 780 exhibits a significantly superior hot ductility recovery, resulting in a 25°C narrower brittle temperature range despite a six-fold larger grain size and more extensive supersolidus melting. Enhanced hot ductility recovery in VDM Alloy 780 is attributable to the combined effects of a higher resolidification finish temperature, associated with substantial replacement of Fe by Co and reduced Ti content, and disrupted susceptible (random) grain boundary connectivity due to a larger proportion of coincident-site lattice (CSL) boundaries compared to Alloy 718. These findings highlight the greater HAZ liquation cracking resistance in VDM Alloy 780 compared to Alloy 718, making the new alloy a promising candidate for aero-engine fabrication.