Cracking mechanism and susceptibility of laser melting deposited Inconel 738 superalloy

Highlights: • Not only liquation cracking, but also ductility-dip cracking can be easily induced in the as-deposited Inconel 738 parts. • High heat input and unidirectional scanning can induce long-straight grain boundaries, hence aggravate the cracking. • The cracking is very sensitive to environmental oxygen concentration, thus the inert gas chamber is very necessary. -- Abstract: Laser Melting Deposition (LMD) of Inconel 738 (IN738) superalloy is a promising process for the remanufacturing of gas turbines and aerospace engines, but the cracking has not been thoroughly understood and controlled. This paper conducts a comprehensive study on the cracking behavior by using optical microscope (OM), scanning electron microscopy (SEM), energy dispersion spectrum (EDS), electron backscatter diffraction (EBSD), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The results indicate that liquation cracking which follows the penetration mechanism is the major origin to those cracks. Whereas, at the propagation stage, ductility-dip cracking (DDC) is an important supplement to liquation cracking, especially at the triple junction points of grain boundaries (GBs) or the bottom of already formed liquation cracks. The cracks are very sensitive to GB morphology, the long-straight GBs which result from higher heat input or unidirectional scanning strategy are very vulnerable to cracking. Moreover, GB oxidation always plays an important role in accelerating crack propagation, the local protection from the cladding head is not enough for the LMD of IN738. For these factors, the test specimens which are bidirectionally deposited in an Argon (Ar) chamber with lower heat input achieve better tensile strengthen reaching 1093 to 1116 MPa.