Liquefaction Characteristics and Cracking Behavior of the Grain Boundaries and Interdendritic Regions in Non-Weldable K447A Superalloy During Laser Re-Melting

Abstract

Nickel-based superalloys with high Al + Ti content are considered non-weldable, and hot cracking is a major challenge in their thermal fabrication processes. In this study, the microstructure liquefaction characteristics and liquation cracking behavior of laser-remelted as-cast K447A superalloy have been investigated. In the heat-affected zone (HAZ), the special liquefaction phenomenon of the coarse script carbides consists of cracking, breakdown, and liquefaction successively. The whole liquefaction sequence observed in the as-cast K447A substrate encompasses: IMRs (Ni₇Hf₂-γ + γ–γ′ + M₅B₃-γ), primary γ′, MC-γ, matrix γ, and MC phases. Liquefaction of IMRs during the laser thermal cycle easily forms a continuous liquid film and evolves into liquation cracking. In the remelted zone (RZ), the fine MC particles and IMRs just form isolated liquefaction points. These isolated liquefaction points are interconnected due to micro-cracks generated by thermal stresses and solidification shrinkage stresses. Finally, crack-free remelted specimens have been obtained by process optimization and preheating the substrate. The tensile strength and elongation of the RZ are increased by 32 and 227 pct, which is beneficial for the RZ acting as a buffer layer to relax or inhibit the stress and cracks generated by subsequent additive repair of K447A.

Graphical Abstract