Luke Bradley

Bio: Luke Bradley is an academic researcher from Rolls-Royce Holdings. The author has contributed to research in topics: Materials science & Electron backscatter diffraction. The author has an hindex of 2, co-authored 4 publications receiving 12 citations.

An innovative constitutive material model for predicting high temperature flow behaviour of inconel 625 alloy

Abstract: Abstract Inconel 625 nickel alloy with its attractive high-temperature strength, excellent corrosion and oxidation resistance is mainly used for critical applications in demanding environments, in both as-cast and wrought conditions. Hot processing of this alloy is crucial for achieving its tailored mechanical properties due to the significant variation in microstructural changes with varying process parameters like temperature, strain, and strain rate. In this study, isothermal hot compression tests were carried out at temperatures ranging from 900 to 1100 °C, and under strain rates ranging from 0.01 to 1 s −1 . The flow curves revealed three stages of deformation, including a substantial work-hardening stage followed by dynamic recovery and flow softening. Microstructural observations showed the occurrence of discontinuous dynamic recrystallisation (DDRX) as the dominant recrystallisation mechanism during the flow softening. Microstructural analysis suggested that the DRX was more sensitive to the test temperature as compared to the strain rate. An innovative material's constitutive model was developed, by combining Johnson–Cook (JC) and Avrami approaches, to predict work-hardening, dynamic recovery, and flow softening stages of deformation. The predicted flow behavior was in a good agreement with the experimentally measured data. The developed material model was integrated into DEFORM® 3D finite element (FE) simulation software as a user subroutine for the prediction of deformation behaviour in a double truncated cone (DTC) sample. Comparison between the experimentally measured data and the results of FE simulation on the DTC sample showed a very good convergence, indicating the suitability of the proposed material’s constitutive model for large scale simulations. Graphical Abstract

Effect of scanning strategy on variant selection in additively manufactured Ti-6Al-4V

Abstract: Additive manufacturing of the Ti-6Al-4 V alloy is increasingly popular for making complex shaped parts. This alloy undergoes a transformation from the body-centred cubic β phase to the hexagonally close-packed α phase following solidification. There are currently gaps in the understanding of relationships between the processing conditions and the final material microstructure. In particular, the role of α variant selection mechanisms within additively manufactured parts is not well enough understood to assure product quality when varying processing parameters such as the scanning strategy. In this study, Ti-6Al-4 V samples were fabricated via the electron beam powder fusion (E-PBF) process under three different scanning strategies (linear scan, random and Dehoff point fills). Electron back-scatter diffraction revealed that the scanning strategy employed directly affects which variant selection mechanism dominates during the β→α transformation. Faster cooling rates in the linear scan produce microstructures which are influenced heavily by self-accommodation, while the microstructure of the slower cooling random fill strategy is dominated more by prior β grain boundary effects. This, in turn, dictates the microstructural evolution of the material, leading to the prevalence of different microstructural features such as macrozones or intragranular 3-variant clustering. These insights will enable optimisation of processing strategies in additive manufacturing to produce tailored product microstructures.

Dynamic and Post-Dynamic Recrystallization of Haynes 282 below the Secondary Carbide Solvus

Abstract: Thermomechanical processes, such as forging, are important steps during manufacturing of superalloy components. The microstructural development during processing, which controls the final component properties, is complex and depends on e.g., applied strain, strain rate and temperature. In this study, we investigate the effect of process parameters on the dynamic and post-dynamic recrystallization during hot compression of Ni-base superalloy Haynes 282. Specifically, we address the effect of deformation below the grain boundary carbide solvus temperature. During deformation, discontinuous and continuous dynamic recrystallization was observed at the grain boundaries, and particle-stimulated nucleation occurred at primary carbides. Strain rate was determined to be the governing factor controlling the recrystallization fraction for strain rates up to 0.5 s−1 above which adiabatic heating became the dominating factor. Careful examination of the temperature development during deformation showed that the response of the closed-loop temperature control system to adiabatic heating can have important effects on the interpretation of the observed behavior. During a 90 s post-deformation hold, grain growth and an increasing fraction of twin boundaries significantly changed the deformation-induced microstructure and texture. The microstructure developed during post-dynamic recrystallization was mainly controlled by the temperature and only weakly coupled to the prior deformation step.