Effect of laser boronizing on microstructure and hardness of maraging steel parts manufactured by selective laser melting

Abstract

In the present study, surface laser boronizing technique was applied to improve wear resistance of maraging steel (MSt) parts manufactured by selective laser melting (SLM). Samples for investigation were manufactured of DIN 1.2709 steel powder using Concept Laser M3 equipment. Continuous 1 kW CO2 laser was applied at 0.5 mm laser spot and 500-1500 mm/min laser operating speed, providing power density of 50955 W•cm -2 and heat input between 12.0 and 4.0 J•mm -1 , respectively. Before laser-processing, amorphous boron paste was pre-placed on samples' surface. XPS analysis revealed increase in boron concentrations from ~3.1 wt% to ~5.7 wt% with laser speed increase from 500 mm/min to 1500 mm/min. XRD analysis revealed domination of Fe3B type borides along with presence of FeB, Fe2B type borides and presence of reflections attributable to austenitic and martensitic phases. The microstructure of laser-boronized layers exhibits evolution from fine dendritic boride-based eutectic plus Fe-based solid solution microstructure having ~630-780 HK0.5 hardness (at 500 and 750 mm/min laser speed) to superfine lamellar nanoeutectic (at 1000 and 1250 mm/min; ~1000-1030 HK0.2) and further to submicron-sized boride structure (at 1500 mm/min; ~1770 HK0.2). The obtained hardness is up to three times higher than that of MSt after aging (~600 HK), indicating that laser boronizing technique may be promising in term of the improve of MSt wear resistance.