Cracking and microstructure formation in laser cladded Ni60 coatings fabricated by circular and annular lasers: a comparative study

Abstract

Laser cladding of Ni60 hard-facing coatings were fabricated using isogenous circular and annular laser beams. The microstructure and cracking susceptibility of these laser-deposited coatings were examined, along with the analysis of temperature and stress fields through finite element (FE) simulation. The comparative study showed that the coatings deposited by the annular laser, at laser power inputs ranging from1500 to 2000 W, exhibited significantly lower cracking susceptibility than those deposited by the circular laser. The FE simulation results reveled a more homogeneous molten pool temperature in the annular laser deposition over 1500 W, which resulted in a lower concentration of stress, lower volume fraction formation of chromium borides, higher volume fraction formation of chromium carbides. Additionally, a more dendritic growth pattern was observed, accompanied by an increase in crystal grain size, which in turn led to a refinement in the chromium carbides crystal dimensions and a decrease in the occurrence of grain boundary formation in annular laser deposited coatings. This structural arrangement attributed to a decrease in hardness and stress concentration, substantially depressing the cracking susceptibility. And therefore, cracking-free coating with acceptable hardness loss was successfully achieved using the annular laser at 2000 W laser power input.