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PROCESS SIGNATURE AND SURFACE INTEGRITY IN FINISH TURNING OPERATIONS
[Thesis]. Manchester, UK: The University of Manchester; 2019.
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Abstract
A growing emphasis, particularly in the automotive, aerospace and biomedical industry, is the production of machined components characterised by high reliability with maximum safety. This characteristic requires predictability of functional performance over the operating life of the components. The effect of the functional performance is owing to the quality of the machined surface including the metallurgical and mechanical state of the subsurface layer as well as topography. The drive towards quality assurance has led to the need to predict fatigue life and other service life parameters from machining processes. Past studies on the functional performance of the machined surface have suggested a correlation between the residual stresses induced by machining and the resulting fatigue life of the product. By setting proper cutting conditions, it should be possible to obtain longer fatigue life for machined parts. The aim of this study was testing the hypothesis that a process signature could be established and exploited to control the surface integrity of machined components. The focus of the study was to understand the link between cutting conditions, process signals, residual stress and fatigue life in turning AISI 1045 steel. Cutting tools of nose radius of 0.8 mm and 0.2 mm were used in fine cutting at 0.1 mm/rev. The signals and resulting residual stress were studied. To further promote more ploughing and mechanism for the generation of preferential compressive residual stress, these cutting tools were modified to bespoke chamfers. The resulting surface integrity was evaluated after turning with these chamfered tools. The residual stress was evaluated in the circumferential and axial directions. The study was conducted for both dry cutting and cutting with flood coolant. Cutting forces, current and voltage were measured during the cutting tests. These enabled the evaluation of specific cutting force and energy and specific energy requirements. The link between the process signals and residual stress was evaluated. Conditions for generation of minimum tensile strength on machined surfaces or compressive residual stress were determined. Fatigue life testing components were then machined based on the investigated conditions. These were used to study the fatigue life of the tensile and compressive residual stress state components. Findings showed that better surface finish and higher hardness of machined component could be obtained by using a small nose radius tool and the finest finishing cut. This surface integrity property was associated with a mid-range power, energy, force, specific force profile and specific energy profile. Better surface roughness and more extended fatigue life performance were obtained using a 0.8 mm nose radius with cutting fluid application. The work highlighted a possible future to control the generation of compressive residual stress in machining by manipulating tool sharpness in the ploughing regime and maximising the desired output process. Process signatures such as specific cutting energy and cutting forces can be correlated with surface integrity enabling minimisation of tensile residual stress.