Microstresses and microstructure in thick cobalt-based laser deposited coatings

Abstract

Microstresses in a thick laser clad Co-based coating on steel substrate were investigated with 3D X-ray microscopy using an intense synchrotron microfocused beam. The microstructure was examined with light microscopy and field emission scanning electron microscopy equipped with X-ray energy dispersive spectroscopy and Electron Back Scattering Diffraction (orientation imaging microscopy). Microhardness and scratch resistance variations inside the coating are related to the local microstructure influenced by additional heating and by melt convection during the laser track overlapping. The residual microstrains were accessed with a high spatial resolution defined by the size of the synchrotron microbeam. Type II residual strains and stresses on the level of individual grains and dendrites were analyzed in terms of tensor invariants, hydrostatic and von Mises shear stress, along the depth of a slightly diluted clad track. The upper part of the coating shows a constant spread of hydrostatic stresses between −500 and 500 MPa; towards the bottom of the track the spread of these stresses increases almost linearly with depth. A correlation between the microstructural features and the spread of the hydrostatic microstresses was found. It is concluded that microstresses in individual neighboring grains are inhomogeneously dispersed.

Metallic laser clad coatings: on the processing-microstructure-property relationships

“…However, EBSD signal has to be collected from large number of grains. The presence of strong texture inside the laser clad Co based coatings was already concluded by Olivera et al [16] when measuring the internal microstrains by diffraction of synchrotron radiation. To prove the hypothesis that grains are elongated due to their growth in a direction perpendicular to the solidification front we studied the texture of microstructures formed by different laser beam velocity.…”

“…The presence of such microstructural boundary inside the coating could be less desirable from the point of view of the local sharp change of functional properties of the coating [16]. A comparison of different laser cladding shows that such a sharp microstructural interface is most pronounced in the coating prepared with the highest laser beam scanning speed, e.g.…”

Thick tool steel coatings with laser cladding

“…Usually, residual stresses detected inside the thick Co based laser cladding coatings on both: macro [8] and micro [18] scale levels are strong tensile stresses, due to a substantial shrinkage after the solidification. On the other hand, the massive martensitic transformation in this tool steel material is associated with expansion and therefore it generates compressive stresses, which in this case overcomes tensile stress components.…”

Experimental Study on Surface Characteristics of Laser Cladding Layer Regulated by High-Frequency Microforging

“…According to overall literature (Ref [15][16][17][18][19][20][21], one of the important causes of the generation of cracks is the strong residual tensile stress experienced while the laser cladding layer is in an as-cast state, and there are many internal microcracks, which are not tough enough to endure great tensile stress. Literature (Ref 22,23) pointed out that microcracking in the parts is one of the urgent problems to be solved in laser cladding forming technology.…”

“…In recent years, the residual stress in surface engineering aroused the attention of many scholars; the latest research (Ref [15][16][17][18][19][20][21] showed that the regulation of residual stress state of the coating surface could improve the coating characteristics greatly (including the cracking behavior, bond strength, and wear characteristics, etc.). Mainly, there are two methods of regulating residual surface stress, namely, to optimize parameters of coating preparation or to introduce an external energy into the coating.…”