Comparison of laser-clad and furnace-melted Ni-based alloy microstructures

Abstract

Optimization of process variables for laser cladding of Ni-based alloys was performed using the pre-placed powder method. The microstructure of laser clad, under optimal processing conditions, and furnace melted, under near equilibrium conditions, Ni-based alloys has been comparatively investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. Comparison of the microstructures of the laser-clad and furnace-melted alloys revealed a remarkable difference. The microstructure of the laser cladding is complex, composed of blocky CrB type chromium carbon borides, orthorhombic structured Cr 7C 3 type dendritic carbides, cellular-dendritic ν-Ni solid solution, different interdendritic eutectics and amorphous phases along grain boundaries. The interdendritic eutectics, either ν-Ni+M 23C 6 or ν-Ni+Ni 3B (Ni 2B), can form depending on the local composition. ν-Ni+Ni 3B stable solidification and ν-Ni+Ni 2B metastable solidification exist simultaneously because of the non-equilibrium rapid solidification involved during the laser cladding. In contrast, the microstructure of furnace-melted Ni-based alloy under near equilibrium solidification is composed of hexagonal structured Cr 7C 3 type carbides with hexagonal prism morphologies, near-equiaxed ν-Ni solid solution dispersed with fine Ni 3Si precipitates, ν-Ni+Ni 3B near lamellar eutectic and near-spherical Ni 3B compound. ν-Ni is the main microstructural constituent in both the laser clad and furnace melted alloys. From the grain size, it was evident that the former is one to two orders finer than the latter.