Selective laser melting of TiB2/316L stainless steel composites: The roles of powder preparation and hot isostatic pressing post-treatment

Selective laser melting (SLM), a promising additive manufacturing technology, was used to fabricate TiB2/316L stainless steel composites. An important factor in manufacturing composites via SLM is feedstock powder preparation. In this work, the powders were prepared by either direct mixing or ball milling. The evolution of constituent phases, microstructural features, and size distribution with milling time was investigated. This research determined that powder particles were both coarsened and refined during the early stages of milling (0-6 h), depending on the milling time. After 8 h of milling, the powders exhibited a wide size distribution and stable spherical morphology, while the average crystallite size of the stainless steel matrix phase significantly decreased to 11.11 nm due to severe plastic deformation. The powders obtained after 8 h of mixing or milling were processed via SLM, and the microstructures exhibited a continuous ring-like structure of uniformly dispersed TiB2 particles. The hardness of the nanocomposite sample prepared from the ball milled powder exceeded that for samples made with the directly mixed powder, but only for those with the highest content of 15 vol.% TiB2 reinforcement particles, due to finer particle sizes and enhancement of the wetting behavior in the molten pool. To increase the final density of these SLM-fabricated components with high hardnesses, a hot isostatic pressing (HIP) post-treatment was applied. The microstructural characteristics of the HIPed samples evolved with HIP holding time from equiaxed grains to segregated regions of coalesced reinforcement particles. In addition, significant drops in hardness and wear resistance values were observed after HIP treatment due to the high-temperature annealing effect. (C) 2016 Elsevier B.V. All rights reserved.