Elimination of n phase in WC-Co cemented carbides during laser powder bed fusion by powder coating compensation strategy

The formation of n phase induced by the C-loss for the laser powder bed fusion (LPBF) of WC-Co cemented carbides largely deteriorates the fracture toughness. The current approach of mixing C additive into powder cannot mitigate the n phase formation. This study proposed a new carbon compensation strategy of coating carbon resource on powder surface by fluidized bed chemical vapor deposition to address this issue. C nanoparticles and carbon nanotubes (CNTs) were selectively deposited on WC-Co powder to make uniform C- and CNTs-coated powders by tuning the deposition temperature. Compared with CNTs-coated powder and C-WC-Co powder mixtures, the C-coated powder was more effective in impeding the n phase formation because it had higher reactivity and stronger dissolution ability to compensate the C-loss in the Co-W-C liquid. However, the single-carbon compensation was not enough to eliminate the n phase due to the extreme nonequilibrium characteristics of LPBF, which required secondary heat treatment. The conventional heat treatment procedure of 1000 degrees C for 3 h eliminated the n phase for the C-coated powder but failed for the C-WC-12Co powder mixtures. Because of the absence of n phase, the heat-treated sample made from C-coated powder exhibited the highest transverse rupture strength