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A Co-Cr-Mo laser cladding applied on railway wheels is characterised by a combination of EBSD and EDX. A complete pass of the cladding is investigated to achieve a better understanding of the microstructure evolution during laser cladding. A microstructure with columnar grains extending over the whole thickness of the cladding is observed. The grains have a <001>-fibre texture with the fibre axis parallel to the normal of the substrate surface, and a substructure consisting of cells/dendrites. During the cladding process, two different kinds of precipitates form in the cell walls, which can be identified as M6C and a non-equilibrium phase. Furthermore, stacking faults are observed to occur in the cladding grains and are discussed with respect to the laser cladding process.
Electron backscatter diffraction and energy-dispersive X-ray spectrometry were used to investigate the intermixed interface produced during laser cladding of a Co-Cr-Mo alloy on a steel substrate. A multi-component system and rapid solidification conditions together lead to a complex microstructure at the interface. The solidification of the cladding starts with the formation of an interface layer, which is about 75µm in thickness and consists of randomly oriented equiaxed grains of Co-Cr-Fe solid solution and martensite. Orientation analysis of the grains in the interface layer revealed that some grains have a special orientation relationship with the former austenite grains in the heat affected zone but the cladding is not formed by epitaxial growth on the substrate. Intermixing of the materials at the interface is providing a strong bond between the substrate and the cladding. For a grain from the interface layer to emerge as columnar grain in the cladding, it was determined that its <001> crystallographic direction is not supposed to deviate more than 25° from the sample normal direction.
The role of alpha- to epsilon-Co phase transformation on strain hardening of a Co-Cr-Mo laser clad
(2007)
A laser clad CoCrMo alloy has been analysed by the electron backscatter diffraction (EBSD) technique to examine the microstructural evolution during tensile deformation and to study the role of the evolved microstructure on strain hardening of the clad. The allotropic phase transformation from α- to ε-Co, that did not take place during cooling from the solidification temperature, occurs in the form of a strain-induced transformation during plastic deformation. Combined slip on non-parallel planes in parallel bands of α- and ε-Co prevents the formation of cracks and contributes to the strain hardening of the material. Due to twin formation at intersecting ε-Co bands, {1 1 0 1} planes become almost parallel to {1 1 1} planes of α-Co, and facilitate further glide of dislocations. Owing to the strain-induced transformation of α- to ε-Co, the laser clad CoCrMo investigated in this study can accommodate high deformation before cracks are formed at the wavy slip-lines and the precipitates.