Fabricating metal matrix composites (MMCs) through laser assisted additive manufacturing (LAAM) has attracted much attention in recent years. This is because the traditional metal components produced by LAAM are usually inferior to the counterparts produced by conventional manufacturing processes, reflected by porosity, lower density, and thus poorer mechanical properties and service performance. Adding reinforcements to metal matrix in LAAM process can alleviate the challenge. Also, for components produced by LAAM processes, post treatment is often required to further strengthen the material, reduce residual stress, or clean off surface for dimensional accuracy. However, research regarding how post treatment affects the microstructure and mechanical properties of LAAM-produced MMCs is still very rare in literature.
In this study, a nano-TiC reinforced Inconel 718 composite is prepared using selective laser melting (SLM) technique. Various post heat treatment processes have been adopted to investigate their effect on final product properties. The motivation is that Inconel 718 is a Ni-based superalloy, whose full potential is explored in heat treatment after manufacturing processes. A composite with 0.5 wt.% nano-TiC addition is prepared. Three levels of solution temperatures at 940, 980, and 1020 °C (for 1 hr) or one level of annealing temperature at 1100 °C (for 1 hr) are adopted for the treatment, combined with the standard two-step aging (720 °C, 8 h, furnace cooling + 620 °C, 8 h, air cooling) on both the MMC and unreinforced Inconel 718 materials. Scanning electron microscopy (SEM) observation is conducted to analyze the microstructure of the composite and understand the reinforcing mechanism. Tensile tests are conducted to evaluate the tensile properties. It is discovered that compared with the pure Inconel 718 by SLM, the Inconel 718-TiC MMC exhibits improved ultimate tensile strength for both as-built and solution/annealing treated conditions. Microscopy observation shows that the dendritic structures of Inconel 718 is remarkably refined by the TiC particles in as-built samples, and grain coarsening is largely inhibited by the TiC particles for solution/annealing treated samples. For both reinforced and unreinforced Inconel 718, dissolving of Laves phase and precipitation of δ phase is observed, but annealing at 1100 °C is not favorable for the formation of δ phase. Aging treatment significantly increases the UTS values for both type of material. Moreover, the strengthening effect of added nano particles becomes less significant in the aged condition, due to the precipitation of γ′ and γ″. Future work includes the process parameter optimization, and further evaluation of other mechanical properties.
Improved plasticity of Inconel 718 superalloy fabricated by selective laser melting through a novel heat treatment process
