Date

2021-9-08

Author

Özsoy, Andaç

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Selective Laser Melting (SLM) is a metal additive manufacturing process used to produce complex-shaped parts by the fusion of metal powders by a laser heat source. SLM processing of metals offers various advantages such as freedom of design, part consolidation, fast prototyping, weight reduction etc. Stainless steels have been one of the first choices to be implemented in SLM processing. Among these, 17-4 PH stainless steel holds a sweet spot with its corrosion resistance, weldability and high strength at elevated temperatures. Therefore, SLM processing of 17-4 PH stainless steel parts has been of interest since the beginning of this technology. However, parts produced by SLM usually possess different characteristics in microstructure and mechanical properties than conventionally manufactured parts. A columnar microstructure lying along the build direction is usually observed for SLM processed alloys, which causes anisotropic mechanical properties to be attained. Moreover, it was previously shown in literature that the phase constitution of SLM processed 17-4 PH stainless steel differs significantly than that of its conventional counterparts. 17-4 PH stainless steel loses its high strength at temperatures above 400°C due to precipitate coarsening, where alloys such as nickel-based superalloys are preferred despite their high cost. In this study, TiN-reinforced 17-4 PH stainless steel was produced by SLM. It was aimed to utilize nano-sized TiN particles both as inoculants (heterogeneous nucleation sites) to obtain an equiaxed microstructure in as-built condition and as dislocation barriers at elevated temperatures, where strength drops due to precipitate coarsening. Various methods were utilized to incorporate TiN particles into the matrix and SLM process parameters development was conducted for the powder blend. It was observed that direct mechanical mixing is the more feasible choice compared to ball milling. Moreover, ex-situ processing by directly adding TiN particles was found to be a better practice compared to in-situ processing by the addition of pure Ti and its subsequent nitridation during processing, where both cases exhibited similar microstructural features and mechanical properties. Consistent with the literature, it was observed that SLM processing window shifts to higher energy densities with the addition of ceramic particles. Moreover, it was found that smaller point distance values favor continuous melt tracks. TiN-reinforced composites were seen to exhibit a very fine and equiaxed microstructure effectively eliminating directional solidification and consequent anisotropy. In addition, agglomeration of the ceramic particles was observed as a natural consequence of high-temperature processing. Both strength and ductility at room temperature in as-built condition increased significantly for the TiN-reinforced composites. High-temperature mechanical properties of the TiN-reinforced and the control specimens were compared in H900 heat-treated condition. Observations showed a significant increase in strength at 400°C for the TiN-reinforced composites with slight shortcoming in ductility. However, TiN-reinforced composites exhibited slightly lower strength and greatly increased ductility at 600°C. This was shown to be due to dynamic recrystallization phenomenon further favored for the fine-grained structure achieved. It was concluded that TiN-reinforcement can be beneficial in the temperature range where 17-4 PH stainless steel is normally used, yet the physical properties of the matrix start dominating the deformation behavior at high temperatures eliminating the advantages gained by the addition of the reinforcement.

Subject Keywords

Additive Manufacturing, Metal Matrix Composite, Selective Laser Melting, Stainless Steel, Mechanical Properties, High Temperature

URI

https://hdl.handle.net/11511/93039

Collections

Graduate School of Natural and Applied Sciences, Thesis