UCLA

There is increasing interest in using selective laser melting (SLM) to produce flight hardware in the aerospace industry. Lower costs, faster lead times, and increased design freedom are among the driving forces for the use of SLM, but its relative novelty and a lack of research into the process-structure-property relationships are inhibiting progress. Inconel 718, one of the most prominent high-temperature alloys used in the industry, is quite popular for use in SLM but the mechanisms by which the microstructure and mechanical properties are affected by the AM process have not been thoroughly discussed in the fully heat-treated state.

Experiments were conducted to analyze how certain process parameters—laser focal shift and the scanning strategy—may affect the microstructure formation and resulting mechanical properties. Heat flow and solidification directionality were found to play dominant roles in microstructural formation, and careful manipulation of these parameters can allow users to create highly anisotropic microstructures. It was also found that the as-built microstructure will heavily influence the fully heat-treated structure in terms of grain size, crystallographic texture, and precipitate distribution.

The mechanical behavior was studied as a function of these process parameters and revealed significant differences among the conditions when tested at an elevated temperature. Room temperature tensile testing did not provide a substantial separation of properties between microstructures. However, characteristics like grain shape and the distribution of precipitates that are determined by the as-built structure were found to play significant roles in the elevated temperature performance. Furthermore, anisotropic structures led to variations in mechanical properties based upon sample orientation. Although differences due to the process parameter variations were found to be substantial, they were insignificant compared to the differences found between SLM and wrought material. A major finding of this work is the sensitivity of SLM IN718 to the environment when testing at elevated temperatures. Elongation increases on the order of 600% were found between wrought and SLM as well as a stark difference in notch ductility; SLM samples consistently fail in the vicinity of a stress concentration where wrought samples remain strong.

Modified heat treatments were performed to address differences between the SLM and wrought microstructures to improve the elevated temperature ductility. Thermal treatments designed to increase the precipitation of the δ phase, minimize the formation of NbC, and increase the overall microstructural homogeneity were performed on SLM material. The changes were evaluated based on the same mechanical tests that revealed environmental sensitivity and were found to improve elevated temperature ductility. However, the failure mechanisms remained the same and substantial oxidation and premature failure were observed in all elevated temperature tests. Although the modified heat treatments were effective in precipitating the δ phase and reducing NbC, further optimization is necessary.

The findings of this dissertation conclude that SLM IN718 given typical thermal treatments as outlined by existing AMS standards do not meet current elevated temperature testing requirements. A susceptibility to gas phase embrittlement mechanisms renders the alloy vulnerable to premature failure in these environments. The information presented here will assist users of SLM IN718 in their understanding of the alloy in the AM state and in the development of appropriate thermal post-processing techniques to achieve the correct level of elevated temperature ductility.