Microstructural characteristics of cobalt treated by high-speed laser surface melting under high power

- The morphological characteristic in the melted zone likes a peacock spreading its tail feathers.
- The ultra-fine grains with the dual-phase microstructure are formed in the heated affect zone.
- An abundance of deformation twins are emerged in the stress affected zone.

The microstructural evolution of cobalt (Co) treated by high-speed laser surface melting (LSM) under high power is symmetrical investigated by using electron backscatter diffraction and electron channeling contrast imaging techniques. Four distinctly different microstructural characteristics from surface to substrate are revealed in Co samples treated by LSM under high power: ε martensite plates in the elongated columnar grains for melted zone (MZ); a dual-phase microstructure (γ phase and ε phase) for heated affect zone (HAZ); deformation twins for stress affected zone (SAZ); and recrystallization microstructure for basal metal (BM). The high-speed laser treatment on Co sample shows some remarkable effects: (i) deepening the modified depth in MZ; (ii) obtaining the ultra-fine grains in HAZ; (iii) emerging one new region (SAZ). The micro-hardness reaches the highest value in HAZ due to fine grain strengthening. The metallurgical processes for the different laser-modified regions are symmetrically discussed in this paper.