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Title: Understanding the link between machining, microstructure and primary processing for a modern nickel superalloy
Author: Marshall, Kyle
ISNI:       0000 0004 7970 2469
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2019
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Powder metallurgy derived nickel superalloys are often subjected to a range of thermomechanical processes, including extrusion, forging and heat treatment in order to produce the required mechanical properties. In this EngD research program, a modern nickel superalloy was processed using two different powder metallurgy routes, with a view to examining the machining response with respect to changes in upstream forging. The conventional route involved hot isostatic pressing (HIP) followed by forging. Small subscale test pieces were used to first examine the variation in microstructure that occurred with varying strain rates and isothermal deformation temperatures, leading to the development of a two-step forging strategy which allowed successful open-die non-isothermal forging to be produce for the purpose of machining trials. In parallel a powder consolidation technique, known as field assisted sintering technology (FAST), was explored, to produce billets with varying microstructures. This is the first time that the FAST technique has been applied to an advanced nickel alloy powder feedstock. Both the HIP+forged and FAST billets were machined to examine the influence of varying microstructure on cutting forces, tool wear and subsurface damage during high performance machining trials. For the as-FAST material, it was found that coarse grain microstructures exhibited lower cutting forces, however this could not be concluded for the HIP+forged material. In both cases, coarser grain microstructures contained more evidence of microstructural damage including observable shear planes and deformed precipitates, irrespective of the measured cutting forces. Comparing cost and complexity, it is suggested that FAST is a more robust method for producing material when examining the correlation between local microstructure and imparted machining damage during the early stages of alloy development.
Supervisor: Jackson, Martin Sponsor: Not available
Qualification Name: Thesis (Eng.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available