Laser surface polishing of NiCrSiBC-60WC ceramic-metal matrix composite deposited by laser directed energy deposition process

Deposition of ceramic-metal matrix composite using laser directed energy deposition process presents multi-fold challenges. High melting point ceramic particles often remain partially melted and increase the roughness of the deposit, which essentially requires secondary finishing operation. Besides high surface roughness, the high gradient of thermal and physical properties between ceramic reinforcement and metal matrix introduces cracks in the composite. Therefore, in the present work, the effect of laser surface polishing and substrate heating on improving the surface quality of NiCrSiBC - 60WC ceramic-metal composite deposited by laser directed energy deposition process was investigated. The molten pool thermal history was monitored using an IR pyrometer during laser surface polishing. The effect of rate of heat input on heating rate, cooling rate, molten pool lifetime and peak temperature was investigated and correlated with the surface quality parameters viz. arithmetic surface roughness (Ra) and ten-point height (Rz). A combination of intermediate laser power and scanning speed (600 W and 2000 mm/min) resulted in proper spread of molten pool and rendered better surface finish. The surface roughness (Ra) was found to improve from 19.2 mu m +/- 1.36 to 1.75 mu m +/- 0.20. Further, different orientations of laser polishing (0 degrees, 45 degrees and 90 degrees) with respect to the material deposition direction were examined, and 45 degrees was found to yield better surface finish. Surface cracks were observed for all the cases irrespective of process parameters and cooling rates, which were mitigated by substrate pre-heating.