Development of ceramics reinforced Inconel 718 composites by selective laser melting

• Main Author: Luu, Duy Nghia
• Other Authors: Zhou Wei
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Materials::Composite materials; Engineering::Materials::Testing of materials
• Online Access: https://hdl.handle.net/10356/162824
• Institution: Nanyang Technological University

Metal matrix composites using Inconel 718 as the base metal are developed using selective laser melting technology. This study's reinforcement particles of choice are micron-TiC, nano-Y₂O₃, and modified nano-Y₂O₃ with nickel coating. The coating was done using a 3-step process: sensitization, activation, and metal coating. Each reinforcement particle type was homogeneously mixed with Inconel 718 powder using low energy ball milling. Laves phase and carbonitride precipitates could be observed in the as-printed samples. In the base metal, cracks were found near the melt pool boundaries and within the heat-affected zones of the newly deposited layer. The cracking occurs along the grain boundaries and interdendritic regions and is associated with remelting of Laves phase; therefore, it was identified as liquation cracking. The mitigation of the cracking was achieved by reducing the layer thickness, which influenced the morphology of the Laves phase due to the significant change in the melt pool shape and grain boundary morphology. The Laves phase morphology and the carbonitride precipitates were also influenced by the reinforcement particle addition. For the oxide composite, the change in Laves phase morphology was due to the combined effect of the chemical inequilibrium at the solidification front and the faster local cooling rate. 1.5 wt.% of Y₂O₃ was found to be excessive as it led to a significantly coarser Laves phase. The coarsening of Laves phase was suppressed using 1.0 wt.% while refinement of the Laves phase could be achieved using the modified Y₂O₃. The size of the precipitates increased with the addition of Y₂O₃ due to the formation of Ti-Nb-Y-N-C-O complex precipitates. However, the formation of these precipitates was suppressed when the modified Y₂O₃ was used. For the carbide composite, the number of carbonitride precipitates was increased significantly due to the increased availability of Ti and C. Laves phase refinement was achieved due to the change in grain boundary morphology and the reduction of Nb availability caused by the formation of carbonitride precipitates. Thus, the addition of TiC at 1.0 wt.% was also effective in mitigating liquation cracking. After heat treatment, the complex precipitates with Y₂O₃ were no longer observed and individual Y₂O₃ and TiN particles were present instead. The carbonitride precipitates were still present and contributed mainly to the higher strength of the composite, except when they formed co-precipitate with Laves or δ phases at 1175 °C solutionizing temperature. Due to the high number of nano-sized precipitates in both composite types, grain coarsening was also reduced substantially due to the Zener pinning mechanism. The addition of TiC particles also contributed to the reduction of solute segregation at the grain boundary and the susceptibility to hot cracking by absorbing the solute materials (Nb and Mo) through Ostwald ripening mechanism. Thus, no segregation was observed in IN718/TiC composite while it was reduced in IN718/Y₂O₃ composite. The combination of finer grains and reduced segregation resulted in ′ and ″ precipitates being distributed more homogeneously. Thus, for the as-printed composites, TiC addition improved the material’s strength while Y₂O₃ addition improved the material’s ductility. However, an improvement in strength was observed in both composite types after heat treatment.