Microstructural evolution and cracking behavior of Hastelloy X superalloy fabricated by laser directed energy deposition

Abstract

Hastelloy X (HX) block was fabricated by laser directed energy deposition (LDED). Microstructure evolution and cracking behavior of the as-deposited LDED HX were investigated. The results showed that the microstructure of LDED HX was composed of columnar grains with interior fine columnar dendrites, and some of the second phases (Laves, M6C, σ, M23C6) caused by micro-segregation can be observed at the inter-dendrite and grain boundary. From the bottom to the top of the sample, the grain size became coarser and the micro-segregation intensified due to the decreasing cooling rates. The formation of continuous γ-M23C6 eutectic at the grain boundaries led to the hot cracking at the middle and top of the sample. Hot cracking was determined to be caused by a stable liquid film and thermal stress. The liquid film was caused by continuous γ-M23C6 eutectic, and the stability of the liquid film was depended on dendrite coalescence undercooling, ΔTb, which was related to the misorientation angle θ. When the misorientation angle exceeds 11.6°, the dendrite coalescence undercooling, ΔTb> 0. The liquid film was stable at the grain boundary. Therefore, cracking always occurred at high angle grain boundaries (HAGBs). The stress concentration at the grain boundaries was intensified at the middle and top of the sample and provided the driving force for crack initiation and propagation. Carbide particles at the grain boundary promoted a pinning effect on the liquid feeding, which led to the liquation cracking propagating to solidification cracking.