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Title: Towards a micromechanistic understanding of imparted subsurface deformation during machining of titanium alloys
Author: Crawforth, Pete
ISNI:       0000 0004 5346 5135
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2014
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Current surface integrity practice, generally applied by mechanical engineers, characterises macroscopic features such as surface tearing, chip smearing and general deformation of grains in the direction of cutting; with little emphasis placed on subsurface microstructure damage. However, through the exploitation of electron backscatter diffraction (EBSD) it has been possible to show the role microstructure plays during metal removal and further quantify the level of deformation that remains after the component has been machined. From the significant amount of data acquired, it has been possible to construct a mechanistic model, which can predict subsurface deformation during machining. Titanium manufacturers such as Timet are in the process of developing alloys that offer their customers cost savings via an improvement in the material's inherent machinability, whilst offering comparable (or improved) in-service properties. For example, Timetal® 54M (Ti-54M) is currently being marketed as a direct alloy replacement for components that are currently manufactured out of Ti-6Al-4V. The cost benefits for the use of Ti-54M through improved tool wear characteristics at higher machining rates have been documented, however, the reasons for this improvement are still under investigation. Through material supply from Timet UK a significant emphasis of the PhD programme was based on using this new alloy. Further studies have shown the potential deleterious effects of induced deformation features imparted during turning on the important titanium alloy, Ti-834, which due to its good mechanical properties at high temperature is currently used for blades, rings and discs in the compressor stages of an aero-engine. Here machining damage in the form of mechanical twins, which until this point, had not been observed in machined Ti-834 material, provided nucleation sites for silicide precipitation during thermal exposure at 750°C, indicating that creep strength could be locally reduced at the surface. The microtexture developed during the complex multi-step forging route can produce a highly anisotropic billet that has consequences for service performance. All critical aerostructural titanium alloys will be machined following forging and furthermore, will be machined using high-speed practices to meet aircraft build targets. As microtexture heavily influences mechanical performance such as fatigue, there is a growing need to understand how the upstream forging steps influence the machining process and determine the severity of induced microstructure damage. Here machining trials were undertaken whilst using force dynamometers; fluctuations in the recorded force have been subsequently attributed to variation in the workpiece's crystallographic texture. The texture of the billet that has evolved during the primary breakdown forging steps acts as a 'finger print' of the forging process and has a lasting legacy, which can have a significant influence not only on the machining process but also the materials' in-service performance.
Supervisor: Jackson, Martin ; Wynne, Brad Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available