The student Julen Agirre Bikuña obtained the qualification 'OUTSTANDING CUM LAUDE' with the 'INTERNACIONAL DOCTOR' mention

The student Julen Agirre Bikuña obtained the qualification 'OUTSTANDING CUM LAUDE' with the 'INTERNACIONAL DOCTOR' mention

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Thesis title: Development of a thermomechanical tester for intermediate strain rates and phenomenological modelling of microstructural evolution: Application to hot forging of Inconel 625.

Court:

• Chairmanship: Marc Bernacki (CEMEF Mines Paristech)
• Vocal:Pascal De Micheli (Transvalor S.A)
• Vocal:Pello Jimbert Lacha (EHU/UPV)
• Vocal: Iñaki Hurtado Hurtado (Mondragon Unibertsitatea)
• Secretary: Borja Erice Echavarri (Mondragon Unibertsitatea)

Abstract:

The recrystallization phenomena that occur in hot metal forming process determine the resulting microstructure, and thus, the mechanical properties of the final component. Therefore, correct understanding and control of these microstructural transformation phenomena is critical for the optimum design of manufacturing processes at high temperatures. The recrystallization phenomena of metals are well studied in the low strain rate range (<10 s-1). However, higher strain rates typical in high-speed hammer forging processes (100 s-1) have received considerably less attention, despite being of great industrial interest. This is because very few laboratory facilities can accommodate testing at such strain rates and at high temperatures (>1,000ºC). As a result, the recrystallization evolution of very few materials has been characterised under these thermomechanical conditions. To address this gap, an Automatic Thermomechanical Tester (ATMT) was developed during this research project in which a Direct Impact Drop Hammer (DIDH) was incorporated for intermediate strain rate testing. The performance of the novel laboratory machine was evaluated and validated, confirming its suitability for testing at both intermediate strain rates and high temperatures.

Low and intermediate strain rate uniaxial compression tests were then performed in the validated ATMT with Inconel 625 samples and the rheological and microstructural evolution was experimentally characterised. As a result, the dynamic and post-dynamic recrystallization and static grain growth of Inconel 625 was studied under the thermomechanical conditions common in hot hammer forging processes. The data obtained from these tests was used to calibrate well-known JMAK phenomenological models so that the recrystallization phenomena of this nickel-based superalloy were numerically predicted.

The experimental microstructural results of a real industrial component were then compared with the numerical results extracted from an industrial FEM simulation. The results demonstrate the advantages and limitations of the fitted recrystallization models to numerically estimate microstructural transformation phenomena in complex industrial hammer forging processes. The work presented in this thesis sheds light on the unknown microstructural transformation phenomena that occur under thermomechanical conditions that combine both high temperatures and intermediate strain rate deformations.