Nickel-based superalloys AD730™ and Inconel 718 (IN718) are strategic materials for the aerospace industry, particularly used in turbine disks due to their exceptional mechanical properties at high temperatures. Linear Friction Welding (LFW) is a solid-state joining process especially well-suited to these alloys, as it avoids the metallurgical defects inherent to fusion welding techniques. Reliable numerical simulation of this process requires valid thermomechanical constitutive laws under severe deformation conditions, characterized by strain rates that can reach several tens of reciprocal seconds. However, the data available in the literature for these alloys are limited to moderate strain rate regimes, typically less than or equal to 1 s⁻¹, leaving a major scientific gap for LFW modelling.
This thesis presents an experimental study of the thermomechanical behaviour of AD730 and IN718 at high strain rates, carried out through hot compression tests using a DSI-GLEEBLE 3800-GTC thermomechanical simulator. The tests were conducted at 1120 °C for strain rates ranging from 1 to 10 s⁻¹ and a final strain of 0.6, conditions representative of the highly deformed zones encountered during LFW. The resulting microstructures were characterized using optical microscopy and scanning electron microscopy.
The results show that both AD730 and IN718 exhibit positive strain rate sensitivity over the entire investigated range, with a noticeable saturation of the flow stress beyond 8 s⁻¹, reflecting the competition between strain hardening and adiabatic heating at very high strain rates. The Yield Drop phenomenon, whose origin had previously been debated, is definitively established as being intrinsically physical to the material, independent of experimental conditions. This conclusion is supported by two converging arguments: the reproduction of the phenomenon on two machines with different technologies, and the consistency of its geometric characteristics across the entire strain rate range. Microstructural observations confirm discontinuous dynamic recrystallization as the dominant softening mechanism, with progressive grain refinement as strain rate increases, well described by a Zener Hollomon model validated from 0.001 to 10 s⁻¹. Finally, the Arrhenius-type constitutive model identified in previous work maintains satisfactory accuracy in the 2.5 to 8 s⁻¹ range but reaches its intrinsic limits beyond 8 s⁻¹ due to flow stress saturation, thus requiring more advanced constitutive formulations to accurately describe this transition regime.
These results significantly extend the current understanding of the thermomechanical behaviour of AD730 at high strain rates and provide a solid experimental foundation for the future development of a coupled mechanical–microstructural predictive model dedicated to the simulation of linear friction welding.
| Date | 28 Jul 2026 |
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| Original language | French |
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| Awarding Institution | - École de technologie supérieure
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| Supervisor | Mohammad Jahazi (Supervisor) |
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Pibre, V. (Author),
Jahazi (Supervisor),
28 Jul 2026Student thesis: Master's thesis › Master in Engineering: Engineering