Correlation of microstructure and phase formation with molten pool thermal history for in-situ and ex-situ laser cladding of TiC/Inconel 625 MMC coating

Identification of microstructure, hard phases, and intermetallics is crucial for metal matrix composite (MMC) coatings, as they govern overall hardness, and this needs destructive microstructural and phase analysis. However, a clear understanding of how phase formation can be correlated with molten pool thermal history in laser cladding remains highly limited. This study uniquely establishes a process map directly linking phase formation and microstructure with molten pool lifetime values and distinct solidification shelves in the thermal history graphs of in-situ and ex-situ laser-cladded TiC/Inconel 625 MMCs. In addition, the phases and microstructures differ between in-situ and ex-situ cladding. Scanning electron microscope, x-ray diffraction, and energy dispersive spectroscopy analyses reveal that dendrites (carbides of Ti, Nb, and Mo) and dispersoids (Ti-Ni intermetallics) form in all in-situ deposits due to the presence of elemental titanium and graphite. In contrast, in ex-situ deposition, phase formation is primarily governed by molten pool lifetime, as TiC is introduced in the form of ceramic particles. A longer molten pool lifetime, resulting from a slower scan speed (higher line energy), leads to TiC decomposition and subsequent carbide phase formation. In in-situ deposition, unique slope shifts between 1640–1730 and 940–1060 °C are the evidence for the formation of carbides and Ti-Ni intermetallics, respectively. Conversely, in ex-situ deposition, a distinct solidification shelf is observed between 1710 and 1800 °C, indicating carbide phase formation only at lower scan speeds. These shelves are present at a higher molten pool lifetime, indicating a longer interaction time. These differences in phase formation in in-situ and ex-situ cladding also result in different hardness values.