In situ formation of TiC-particle-reinforced stainless steel matrix nanocomposites during ball milling: Feedstock powder preparation for selective laser melting at various energy densities

Abstract

316L–Ti–graphite mixed powders were subjected to mechanical alloying (MA) for milling times ranging from 4 to 35h. After 24h of milling, TiC particles began forming in situ within a steel matrix via MA. The TiC phase formed due to the gradual interdiffusion of Ti into the graphite lattice. Evolutions of the phases, microstructure, and composition of the powders with varied milling times were investigated. The results showed that irregularly shaped particles formed initially; these particles flattened after 10h of milling, coarsened after 24h of milling, and finally became fine and uniform when milled for >24h. Micrographs of the 35-h milled feedstock powder showed 20-nm-diameter TiC nanoparticles uniformly distributed in the 316L matrix. Then, the 35-h milled feedstock powders were processed by selective laser melting (SLM), an additive manufacturing process. The effects of the laser energy density (η) used during SLM on the microstructure, hardness, and tribological behavior of the SLM-processed parts were investigated. TiC particles were uniformly distributed within the steel matrix for all η values. With increasing η, however, the microstructure tended to coarsen because of the slower cooling rate. The applied η also influenced the hardness and wear rate. The hardest samples were obtained at η of 67J/mm3, and the optimum wear resistance was measured at η of 200J/mm3. A continuous, well-adhered strain-hardened tribolayer demonstrated the best wear resistance.