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

Rights statement
Awarding institution
  • University of Strathclyde
Date of award
  • 2020
Thesis identifier
  • T16546
Person Identifier (Local)
  • 201594346
Qualification Level
Qualification Name
Department, School or Faculty
  • One of the current research challenges in the aerospace sector is the prediction of microstructural 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 full understanding of this phenomenon, as well as the range of applicability of available microstructural models for industrial forging processes remains open. The main reason for that is the complexity of the thermo-mechanical history and, related to this, the combination of operative microstructural mechanisms. An original methodology using forging trials, based on industrial practices, coupled with a tailored data processing procedure has been designed to link microstructural transformations to thermo-mechanical histories. This provides the ability to investigate the recrystallization (RX) phenomena from a different perspective compared to the available literature. Besides that, data obtained with this methodology gives the ability to benchmark, calibrate and validate existing microstructural models in an industrial setting. Experimental work presented in the research detailed herein was specifically planned to 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 geometry disc forging on the screw press. As part of the proposed methodology, the notion of the zero recrystallization boundary was introduced; experimentally constructed and studied. This helped to analyse the onset of RX and its correlation with the various loading histories. The results obtained have allowed advanced calibration of JMAK-type models and generated an understanding of the range of their applicability. It was found that the range of validity for JMAK models is limited to a specific strain rate and temperature domain and the main reason for 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 the commercial modelling software DIGIMU® as a tool for validation of possible microstructural hypothesis and for interpretation of experimental results is explored. As a part of industrial implementation, the proposed methodology was also used for benchmarking of standard JMAK models embedded in commercial FE software as used by several industrial companies. Lastly, a framework for a new microstructure modelling approach based on the above observations is proposed and a direction for further research outlined.
Advisor / supervisor
  • Bylya, Olga
  • Blackwell, Paul
Resource Type
  • Previously held under moratorium from 6th June 2020 until 24th October 2023.