Robotized fiber laser cladding of steel substrate by metal matrix composite powder at cryogenic conditions

The results of investigations on laser cladding of mild steel plates (0.17 wt.% carbon and 1.4 wt.% manganese) by nickel powder with addition 60wt.% of tungsten carbides at conditions of forced cooling by liquid nitrogen are presented. To provide reproducible conditions and high accuracy, tests of cladding were carried out on a robotized stand equippedwith a fiber laser generator and powder feeding system. The symmetrical circular laser beamwith beamspot diameter of 300 μm was applied as the heat source for cladding of composite single stringer beads at different heat inputs and different cooling conditions. Free cooling of the steel substrate was conducted in the ambient air, while the forced coolingwas achieved by partial immersion of the steel plate in liquid nitrogen at approximately-190°C. The results showed that the conditions of cooling have a strong influence on the cladding process, mechanism of a bead formation, quality, microstructure, and geometry of the beads. Laser cladding at forced cooling leads to a favorable finegrained microstructure and increased microhardness of the metal matrix and also, lower dilution. In the case of stringer beads produced at cryogenic conditions, the heat-affected zone in the substrate of the mild steel is hardened with evident traces of bainitic microstructure. However, the heat input of laser cladding must be significantly increased at forced cooling to provide the proper shape of the bead and also to avoid cracks and a lack of fusion the steel substrate.