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Title: Magnetic field assisted ultra-precision machining for improving machinability of titanium alloys
Authors: Yip, Wai Sze
Degree: Ph.D.
Issue Date: 2018
Abstract: Ultra-precision machining (UPM) is a promising machining technology for manufacturing precise components with nanometric surface roughness and sub-micron form accuracy without the need for a subsequent process. As the machined components generated by UPM are extremely precise, UPM involves various machining and material factors. Those machining and material factors such as high cutting temperature, machining vibration, and tool wear, as well as material swelling, are potentially dominant to the machined surface quality of fabricated components, the complexity of the interaction between each factor adds challenges to UPM.
Titanium alloys are widely applied in the biomedical industry for precise components due to their excellent material properties. However, titanium alloys are difficult to cut materials because of their low thermal conductivity and high sustainability of work hardening at the elevated temperature, causing the localized cutting heat and the machining vibration at the tool/workpiece interface during machining, leading to serious tool wear, a high level of material swelling, and unsatisfactory surface finishing of the machined titanium alloys. Therefore, an evolutional machining technology should be adopted to uplift the machining performances of titanium alloys in UPM. This thesis therefore proposes a novel machining technology that introduces a magnetic field into the UPM process with the aim of overcoming the machining difficulties of titanium alloys encountered in UPM. In the experimental set up, a magnetic field was superimposed onto titanium alloys in single point diamond turning (SPDT) by positioning titanium alloys in between of two permanent magnets. An eddy current damping effect and a magnetic field effect were then induced in the machining processes to positively influence the machining performances by suppressing the machining vibration and enhancing the thermal conductivity at the tool/workpiece interface. The unique benefits of the proposed research can be categorized and summarized as follows: (1) A magnetic field was superimposed onto static titanium alloys during SPDT in order to exert the core influence of the magnetic field, which is the enhancement of thermal conductivity at the tool/workpiece interface. During SPDT in the presence of magnetic field, the magnetic dipolar energy inside the materials is sufficient to overcome the thermal energy. The paramagnetic particles generated at the tool/workpiece interface tend to align along the direction of the external field due to the positive value of magnetic susceptibility of titanium alloys. These aligned titanium alloy particles act as linear chains, which are highly conductive paths for transferring heat and enable the promotion of the fast heat transference along the path of the fluid carrier. Using this principle, SPDT was conducted in between of two permanent magnets for the purpose of uplifting the thermal conductivity of titanium alloys and consequently minimizing the material swelling effect on the machined surface. (2) A magnetic field was superimposed onto rotating titanium alloys in SPDT in order to exert an eddy current damping effect. When a conductive metal rotates within a magnetic field, an eddy current is generated through a stationary magnetic field inside the conductor. The eddy current will create its own magnetic field in the opposite direction of the external magnetic field, it generates a repulsive force called the Lorentz force, which the Lorentz force compensates the vibration displacement of machining system. Using this principle, rotating titanium alloys within a magnetic field in SPDT were subjected to the eddy current damping effect, which functionally counteracts the vibration force and energy from the turning system. (3) SPDT of titanium alloys was influenced by the integral effects of an eddy current damping and a magnetic field. The eddy current damping effect reduces the machining vibration of the rotating workpieces, while the magnetic field effect enhances the thermal conductivity at the tool/workpiece interface in SPDT. Both help to reduce damages to the diamond tool in SPDT of titanium alloys, even in a dry machining environment. The positive influences from the magnetic field on SPDT inspire and facilitate the implementation of ultra-precision manufacturing process in the green and clean directions. The research findings and subsequent works in this thesis contribute to enhance the current machining performances of titanium alloys in UPM by resolving the problems of machining vibration, tool wear, and material recovery. The proposed novel machining technology provides a new cutting approach that not only uplifts the existing precision level of machined products using titanium alloys, but which also contributes to the feasibility of green and clean manufacturing processes.
Subjects: Hong Kong Polytechnic University -- Dissertations
Titanium alloys
Hard materials -- Machining
Pages: xix, 184 pages : color illustrations
Appears in Collections:Thesis

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