Mechanism of heat affected zone cracking in Ni-based superalloy DZ125L fabricated by laser 3D printing technique

Abstract

• The heat affected zone cracking initiates due to the re-melting of preexisting intergranular γ/γ′ eutectics and coarse γ′.• The transverse tensile strain/stress causing the initiation and propagation of crack is quantitatively measured.• The micro-size MC carbides are considered to be a possible contributor to the hot cracking initiation. Laser 3D printing is a promising technique to repair damaged Ni-based superalloy components. However, the occurrence of heat affected zone (HAZ) cracking severely limits its applicability. Here we unravel the cracking mechanism by studying the element, phase, defect, and strain distribution around an intergranular crack that initiated from the primary HAZ. Using synchrotron X-ray Laue microdiffraction, we measured high tensile strain/ stress transverse to the building direction in both the primary HAZ and the cladding layers, as well as highdensity dislocations, which resulted from the thermal contraction and rapid precipitation of γ′ phase. The crack initiated because the transverse tensile strain/stress tore up the liquid film formed by the low-melting point preexisting phases in the primary HAZ, such as γ/γ′ eutectics and coarse γ′ precipitates. The incoherent carbide particles were frequently observed near the crack root as local strain concentrators. In the cladding layers, micro-segregation could not be completely avoided, thus the hot crack continued to propagate over several layers with the assistance of the transverse tensile stress. Our investigations provide a useful guideline for the optimization of the 3D printing process to repair Ni-based superalloys with high susceptibility to hot cracking.

Rafting‐Enabled Recovery Avoids Recrystallization in 3D‐Printing‐Repaired Single‐Crystal Superalloys

“…This evidence of a large population of dislocations in many local regions is consistent with the observation in the transmission electron microscopy (TEM) image of Figure 3e, in which the dislocation density is measured to be about 8 × 10 14 m −2 using the line‐intercept method introduced by Norfleet et al28 This value is for a small local region, but from the colored peak width map the dislocation density is inhomogeneous and should be higher in many other regions of the HAZ. Similar to previous reports for cast materials,29 the BM is almost dislocation free (Figure 3f).…”

Multi-scale microstructural investigation of a laser 3D printed Ni-based superalloy

“…Using the line-intercept method adopted by Norfleet et al [24], the dislocation density in the cell interior is estimated to be ~1.4 10 14 m -2 . precipitates in the inter-cellular regions than in the cell interiors [26]. In some of the superalloys like the one studied here, particles with TCP phases are detected [27].…”

Effect of sample orientation on the microstructure and microhardness of additively manufactured AlSi10Mg processed by high-pressure torsion

“…However, the high residual stress, porosity and other defects that often exist in as-manufactured parts cause some concern, which means that some kind of post-processing, e.g. hot isostatic pressing (HIP) and heat treatment before being ready for service [26][27][28].…”