Thesis

Novel experimental methodology for the investigation of recrystallisation during industrial hot forging of Inconel 718

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Awarding institution
  • University of Strathclyde
Date of award
  • 2020
Thesis identifier
  • T16546
Person Identifier (Local)
  • 201594346
Qualification Level
Qualification Name
Department, School or Faculty
Abstract
  • One of the current research challenges in the aerospace sector is the prediction ofmicrostructural changes during the processing of Nickel Superalloys such as Inconel 718.From the literature it emerges that the vast majority of microstructural modelling attempts,focused on the prediction of microstructural transformations and first of all recrystallization,are based on uniaxial tests. The question about the sufficiency of this data for fullunderstanding of this phenomenon, as well as the range of applicability of availablemicrostructural models for industrial forging processes remains open. The main reason forthat is the complexity of the thermo-mechanical history and, related to this, the combinationof operative microstructural mechanisms. An original methodology using forging trials, basedon industrial practices, coupled with a tailored data processing procedure has been designedto link microstructural transformations to thermo-mechanical histories. This provides theability to investigate the recrystallization (RX) phenomena from a different perspectivecompared to the available literature. Besides that, data obtained with this methodology givesthe ability to benchmark, calibrate and validate existing microstructural models in an industrialsetting. Experimental work presented in the research detailed herein was specifically plannedto perform a systematic investigation of the processes with increasing level of complexity:uniaxial laboratory tests, quasi-constant strain rate forging of double truncated cones (DTC)on a hydraulic press, DTC forging on a screw press (high strain rates) and complex geometrydisc forging on the screw press. As part of the proposed methodology, the notion of the zerorecrystallization boundary was introduced; experimentally constructed and studied. Thishelped to analyse the onset of RX and its correlation with the various loading histories. Theresults obtained have allowed advanced calibration of JMAK-type models and generated anunderstanding of the range of their applicability. It was found that the range of validity forJMAK models is limited to a specific strain rate and temperature domain and the main reasonfor unsatisfactory predictions is the overlapping of a few recrystallization mechanisms (e.g.continuous dynamic, discontinuous dynamic and post-dynamic recrystallisation).Unfortunately, decomposition (splitting) of these mechanisms is not trivial. The use of thecommercial modelling software DIGIMU® as a tool for validation of possible microstructuralhypothesis and for interpretation of experimental results is explored. As a part of industrialimplementation, the proposed methodology was also used for benchmarking of standardJMAK models embedded in commercial FE software as used by several industrial companies.Lastly, a framework for a new microstructure modelling approach based on the aboveobservations is proposed and a direction for further research outlined.
Advisor / supervisor
  • Bylya, Olga
  • Blackwell, Paul
Resource Type
Note
  • Previously held under moratorium from 6th June 2020 until 24th October 2023.
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