Effect of nitriding and thermo-hydrogen processing on the fatigue behavior of wrought Ti-6Al-4V alloys

Date

2024-1-26

Author

Öztürk, Elif Aybüke

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The Ti-6Al-4V alloys have become the preferred materials in the aerospace industry for load-bearing mechanical parts owing to their high mechanical strength, low density, fatigue resistance, and corrosion resilience. Even without specific processing, these alloys inherently possess high fatigue strength. Nonetheless, secondary heat and surface treatments, particularly in critical aerospace applications, are applied to enhance their fatigue resistance. This study employed a 2-step thermal-hydrogenation process (THP) and a gas nitriding process on specimens to augment fatigue resistance by introducing a refined microstructure and increasing surface hardness, respectively. In addition, to enhance fatigue resistance simultaneous intention of increasing surface hardness and delaying crack formation by inducing residual compressive stresses on the surface through concurrent nitriding was aimed. While the nitrided samples exhibited the highest hardness values, they also displayed the coarsest microstructure and the lowest fatigue resistance due to the elevated heat treatment temperature. On the other hand, the 2-step THP treatment refined the microstructure of the samples and marginally increased hardness and tensile strength. However, ductility decreased in THP-treated samples due to oxidation compared to other specimens. Applied processes also resulted in grain coarsening when applied separately. However, the consecutive application of nitriding and THP processes did not notably alter the ductility compared to the untreated specimen, albeit tensile strength decreased compared to untreated specimen. The 2-step THP process applied after the nitriding process increased the material's ultimate tensile strength (UTS) value compared to that of the only nitrided samples. However, notable differences in fatigue resistance and ductility were not observed after the consecutive application of these two processes. Mechanical tests indicated that thermally treated and surface-treated specimens exhibited tensile and yield strength and ductility values that meet the ASTM standards. However, their fatigue strengths decreased due to grain coarsening in the microstructure and oxidation occurring during the applied processes.

Subject Keywords

Ti-6Al-4V alloy, Gas nitriding, Conventional production, Thermo-hydrogen processing, Fatigue

URI

https://hdl.handle.net/11511/108908

Collections

Graduate School of Natural and Applied Sciences, Thesis